Welding , Hardfacing , Cladding and Cutting of metals

ID PTA welding torch minimum Dia 50 mm, Weld bead height 3 mm.

noreply@blogger.com (arcraftplasma) — Tue, 04 May 2021 07:15:00 +0000

PLASMA TRANSFERRED ARC HARD FACING AT ARCRAFT PLASMA

Ø Arcraft is the name to turn to for innovative.

Plasma technology.

Ø With Plasma Transferred Arc we provide high productivity, high quality of powder deposits and significantly lower cost compared to other processes for Hardfacing, Cladding and Reclamation even in the most challenging area such as valves, valves seat of internal combustion engine, accessories for ship, petroleum and power generation and many more Industries where components are subjected to heavy wear, abrasion, impact loads, protect from corrosion and many such applications. By depositing High cost alloys and metals on comparatively low cost metal parts only on surfaces requiring the required mechanical and metallurgical properties a huge saving on cost is the benifit to customers.

Ø PTA is also used for reclamation of worn jobs extending the life of components and giving a huge savings on cost.

Ø Argon is basically used for arc plasma supply, powder transport and molten material shielding. It produces a very high quality deposit offering optimal protection with minimal dilution or deformation of the base material.

PTA SET UP

Ø CNC based fully automatic operation.

Ø 12” TFT LCD Touch-Screen Display.

Ø Pre-Program facility for all parameters like:

-Gas Pre-flow / Post-flow time, Powder flow rate.

-Weld current, Up-slope / Down-slope time.

-Rotation speed, Boom step-over,

Number of bands / cycle.

-Oscillation width, Oscillation speed, Dwell time.

Ø Choice of Positioners / Job Manipulator:

-Weight capacity from 100 kg to 2500 kg.

-Face plate diameter from 250 mm to 2000 mm.

Ø Choice of Column & Boom: Stroke length from 500 mm to 2000 mm.

Ø Choice of ID as well as OD Torches.

Ø Suitable for Circular, Straight, Screw, Ball Valve deposits.

We design, develop and manufacture Custom built machines as per requirement of the customer.

PTA ID TORCH (50mm ID)

l Minimum ID: 50mm

l Maximum Current: 100 A

l Duty Cycle : 100%

l Torch Length: 500- 750mm

PTA 50mm ID torch video

PTA ID TORCH (75mm ID)

l Minimum ID: 75mm

l Maximum Current: 200 A

l Duty Cycle : 100% @ 150 A

l Torch Length: 500 - 750mm

l Consumables:

l Nozzle, Ceramic, Tungsten Electrode

PTA 75mm ID torch Video

Comparisons of 75mm and 50mm ID PTA torch

Features of PTA ID torch

Torch length can be vary from 300mm to 1000mm depending on requirement

Small PTA ID torch has a diameter of 37mm. It can do ID hard facing in small pipes from 50mm ID and above.

Straight as well as angularly bent torch as per job requirement.

Designed for rapid heat dissipation which result in increased nozzle life.

Nozzle of the torch is designed to provide efficient cooling for application requiring Pre heat temperature. It is achieved by providing bigger groove inside the nozzle.

Complete encapsulation of the torch’s gas and water services tubes.

Precise powder delivery.

Efficient powder delivery with 4 holes at specified degree

Fully sealed internal powder feed to eliminate losses and contamination.

Better Arc Control by using ceramic for centering of the tungsten electrode.

Improved Service life

Deposition rate is high at 100 Amps

Torch is designed for 100 Amps @80% duty cycle

High quality of brass and copper used for rigid construction of the torch body.

We do manufacture PTA OD TORCHES also.

l Maximum Current: 350 A

l Duty Cycle : 100%

l Consumables:

l Nozzle, Ceramic, Tungsten Electrode

APPLICATIONS

Ø Down hole drilling tools.

Ø High wear piping components / slurry transport.

Ø Ground engagement equipment for the oil sands / mining industries.

Ø Extruding machine screws, valves, valve seats of internal combustion engines (motorcar, marine, locomotive etc).

Ø Equipment for mining, crushing, rolling, road building and tunneling.

Ø Areas where traditional wire based overlays such as chromium carbide are not providing significant wear-life.

Ø Moulds and forging dies, Pulp and paper industry equipment, Agricultural equipment.

Ø Parts for nuclear & chemical plants etc.

BENEFITS

Ø Reduces Cost - Restoring a worn part to "as new" condition generally costs between 20-70% of a brand new replacement part.

Ø Prolongs Equipment Life - Service life increases of 3 to 10 times are common with properly overlaid parts.

Ø Reduces Downtime - Parts last longer and fewer shutdowns are required.

Ø Less Spare Parts Inventory - There is no need to keep numerous spare parts when worn parts can be rebuilt.

Ø High Quality Deposit - It produces a very high quality deposit offering optimal protection with minimal dilution or deformation of the base material.

Ø Precise Coating - It is a process that deposits very precise coatings of perfectly controlled alloys on mechanical parts that are subject to intense wear, significantly extending their service life.

Ø Protection - Corrosion, thermal shock and abrasion.

Ø Metal buildup / Reclamation - Certain alloys are made to rebuild a part to a required dimension while others are designed to be a final overlay that protects the work surface.

APPLICATIONS

Ø Down hole drilling tools.

Ø High wear piping components / slurry transport.

Ø Ground engagement equipment for the oil sands / mining industries.

Ø Extruding machine screws, valves, valve seats of internal combustion engines (motorcar, marine, locomotive etc).

Ø Equipment for mining, crushing, rolling, road building and tunneling.

Ø Areas where traditional wire based overlays such as chromium carbide are not providing significant wear-life.

Ø Moulds and forging dies, Pulp and paper industry equipment, Agricultural equipment.

Ø Parts for nuclear & chemical plants. etc.

THANK YOU

ARCRAFT PLASMA EQUIPMENTS PVT. LTD.
Address : 124, Diamond Industrial Estate,
Ketkipada, Dahisar (East), Mumbai- 68.

Ph : 0091-22-28965890/5745/3247.
Fax: 00-91-22-28966418 .

Email:arcraftplasma@gmail.com
Website : www.arcraftplasma.com

SS304 filter mesh welded with micro plasma

noreply@blogger.com (arcraftplasma) — Mon, 12 Apr 2021 09:33:00 +0000

Piping welding symbols

noreply@blogger.com (arcraftplasma) — Thu, 12 Nov 2020 10:40:00 +0000

Plasma melting furnace table top model for experiments

noreply@blogger.com (arcraftplasma) — Fri, 06 Nov 2020 07:04:00 +0000

We have designed and manufactured a plasma melting furnace which can be used for metallurgical experiments making metals , alloys etc.

One of our customer approached us with processed ore in powder form which he wanted to melt using the plasma process to form copper ingot.

We took up the challenge and designed and manufactured the system the video of which you can see here.

The picture of the system is here

The finished ingot ( machined ) picture is here

This is a 10 kw system which can be used for experiments in metallurgical applications in R&D institutions, metal and alloy manufacturing companies, metallurgical labs in Engineering colleges and many more applications of interest to engineers and scientist for research in material science and metallurgy.

Please write to us for more information.

arcraftplasma@gmail.com

www.arcraftplasma.com

Why choose PTA Hardfacing? ( Plasma Transferred Arc )

noreply@blogger.com (arcraftplasma) — Thu, 10 Sep 2020 09:05:00 +0000

Most components, for instance this engine Intake and Exhaust valves have few surfaces exposed to extreme wear. During this case the tip and seat.

The most popular materials for exhaust valves, however, are austenitic chrome steel alloys like 21-2N and 21-4N. Though austenitic chrome steel can handle high temperatures very well, the steel is softer than martensitic steel at lower temperatures and can't be hardened by heat treatment. To enhance wear, a tough material could also be welded to the tip and therefore the valve stem.

So you would like to overlay the tip and seat with a hard material for an extended life. Cobalt based alloys give a hardness upto 38 HRC and more but is costly. If you create the entire part out of this material it'll be very costly for the end customer.

21-2N alloy has been around since the 1950s and is an austenitic chrome steel with 21 percent chromium and a couple of percent nickel. It holds up well for general available valve applications and costs less then 21-4N because it contains less nickel.

21-4N is additionally an austenitic chrome steel with an equivalent chromium content but contains almost twice the maximum amount nickel (3.75 percent), making it a costlier alloy. 21-4N is typically considered to be the premium material for performance exhaust valves. 21-4N steel also meets the “EV8” Society of Automotive Engineers (SAE) specification for exhaust valves.

SAE classifies valve alloys with a code system: “NV” is that the prefix code for a low-alloy valve, “HNV” may be a high alloy valve material, “EV” is an austenitic valve alloy, and “HEV” may be a high-strength valve alloy.

So the part is formed of 21-2N alloy at 28 HRC less expensive than cobalt based alloys. But the end performance as far as tip and seat wear resistance is concerned is the equivalent of cobalt based alloys of 38 HRC.

Cost of cobalt based alloys and 21-2N / 21-4N will differ across countries so we'll not specify it here.

So you can first calculate the saving when this part is made in millions all over the world. There are hundreds of components across the engineering spectrum requiring hard facing only on mating parts and not the whole surface both internal and external surfaces.

We supply options for both Internal and Surface hard facing,surfacing, reclamation applications with equipment using the latest technology.

For more details write to us at sales@arcraftplasma.com

visit our Web Site : www.arcraftplasma.com

noreply@blogger.com (arcraftplasma) — Mon, 07 Sep 2020 09:19:00 +0000

Diaphragmn for welded bellows. SS 304L Dia 31.8mm OD , ID 19mm thickness 0.1mm each.

Welded internal and external both to demonstrate technology.

Diffusion Bonding

noreply@blogger.com (arcraftplasma) — Sat, 13 Jun 2020 07:46:00 +0000

Diffusion fastening is generally used for metallurgically incompatible systems.

It can also be used for similar metals. Conjugation surfaces to be joined are brought together whereas actual contact space of the components is extremely tiny.

The applying of pressure at hot temperature can deform all the asperities peaks and valleys (soft points) leading to plastic deformation.

The process relies on the very fact that although metal surfaces seem smooth visually once examined beneath a magnifier they're truly quiet rough.

When heat and pressure is applied it leads to

1. Reduction in yield strength (between soft points).

2. Exaggerated creep particularly close to the conjugation surface.

This collapses all the asperities (soft points) increasing metal to metal contact

following which lower points on the surfaces then acquire contact and fuse along.

This sequence repeats till diffusion across the complete joint surface of the components are coalesced and grain boundaries are fashioned leading to a metallurgical bond.

Oxides and different contaminants are squeezed out of the joint throughout the fastening method leading to a metallurgically sound joint.

3. Voids disappear as grain boundary plays major role than deformation.

The welded surface has identical physical and mechanical properties as per the base material.

Once bonding is complete, the joint is also tested using tensile testing for example.

The diffusion bonding method is ready to provide prime quality joints wherever no separation or porousness exists within the interface.

Diffusion bonding permits the manufacture of high exactitude elements with advanced shapes.

The diffusion bonding technique will be used for either similar or dissimilar materials, and is additionally vital in processing composite materials.

The method isn't very difficult to approach and also the price to perform the diffusion bonding isn't high. Diffusion bonding is primarily used to produce involved forms for the natural aerospace and nuclear industries.

Since this type of bonding takes a substantial quantity of your time compared to different welding techniques such as explosion bonding, components are created in small quantities. Most are specially fabricated.

However, because of completely different needs, the desired time might be reduced.

In a trial to scale back fastener count, labour prices, and part count, diffusion bonding, in conjunction with superplastic forming, is additionally used once making advanced sheet metal forms.

Multiple sheets are stacked atop each other and bonded in specific sections.

The stack is then placed into a mould and gas force per unit area expands the sheets to fill the mould.

In many military aircraft diffusion bonding can facilitate to permit for the conservation of expensive strategic materials and the reduction of producing prices.

For more information

email :arcraftplasma@gmail.com

www.arcraftplasma.com

noreply@blogger.com (arcraftplasma) — Thu, 11 Jun 2020 06:32:00 +0000

Advisory – You ask. Experts answer

We undertake turnkey contracts for welding SS316 L pipes in the pharma industry. We are facing a problem of discolouration in the root of the weld despite our best efforts of purging with argon gas. Any suggestions?

Do ensure you are using welding grade argon gas. For specialised applications argon hydrogen mixtures are available. The cause of discolouration and oxidation in welds is the presence of atmospheric gases. To make sure oxygen and moisture is not present invest in oxygen analysers and dewpoint meters.

Use the correct gas flow and pressure. Too much or too little will both create problems? Use proper purging dams. Many choices are available water-soluble papers and pastes, collapsible disc barriers, rubber gasket dams, thermally disposable barriers, inflatable bladder dams and more.

We manufacture industrial knives for paper cutting mills. The material of construction is SS 316L. The blade is ground on one side and the other side is flat. Can you suggest ways to increase the working life?

The debris of the cutting operation impacts the flat side of the knife and causes a swale to be eroded in the metal, which supports the edge. As the swale deepens the edge is poorly supported and rolls over to render the blade inefficient. A thin tungsten carbide coating will prevent the swale formation and extend the life of the blade.

What are the precautions to be taken when welding Titanium?

As compared to other metals Titanium requires a great deal of shielding on both the face and backside of the weld. All weld zones at temperatures above 650°C must be protected from air contamination. The most critical area is the weld puddle. Oxygen, nitrogen and hydrogen from air diffuse into molten titanium very rapidly. The zone next to the weld puddle is also subject to surface contamination at this temperature. Contaminated surfaces have low ductility and their presence may lead to premature failure in service. Good quality shielding gas should be used. There should be no turbulence in the flow of the shielding gas. Proper coverage of shielding gas should be ensured by using trailing shields. Contamination can also occur by oxidation of the filler wire, dirt on the filler wire or surface being welded. In welding of Titanium all these points have to be considered.

We have a critical application where the customer has asked us to use bimetallic welded sheet, i.e., stainless steel and aluminium. Could you please let us know where we can procure such sheets?

You would have to use the explosion bonding process to manufacture these sheets. Sheets are also available with traders. You may contact one of these sources: Triton Alloys (info@tritonalloysinc.com), IDL Explosives (info@idlind.com), Giridhari Explosives Pvt Ltd (giridhariexplo@gmail.com) and Explofab Metals India Pvt Ltd (sales@ explofab.org)

Can you give some simple methods that can be used for hardness testing in the workshop?

Simple Hardness testing Methods: The file and scratch test is a quick procedure for checking hardness. This method is most frequently used by the workmen to check the machinability of the metal. This method involves scratching or trying to cut the surface of the metal with a pointed object of known hardness. This practice just gives the hardness of the material on the outer surface only.

File Hardness Test Data:

Brinell Hardness	File Action
100	File bites into surface very easily
200	File removes metal with slightly more pressure
300	Metal exhibits its first real resistance to the file.
400	File removes metal with difficulty
500	File just barely removes material
600	File slides over surface without removing metal, file teeth are dulled

Simple Hardness Tester

It is possible for welders to make their own set of hardness testers, by heat treating ¼ inch steel electrodes to give a graduated series of hardness. The rods are labelled with their respective Rockwell C Hardness. A sharp point is then ground on the end of each rod, taking care not to soften the steel by overheating. By scratching these scratch testers one at a time over a different metal surface the welder can determine which rod has hardness about the same metal being scratched. If the sharp point of the scratcher digs and leaves a scratch mark on the metal this indicates that the hardness of the scratcher is more than that of the metal being tested. If it does not leave any scratch and the scratcher is slipped from the surface of metal then this indicates that the hardness of the metal being tested is greater than that of scratcher being used.

For any welding and inspection queiries please contact us at email :
arcraftplasma@gmail.com

see our website : www.arcraftplasma.com

___________________________________________________________________________

Protection of Weld Bead

noreply@blogger.com (arcraftplasma) — Sat, 20 May 2017 10:11:00 +0000

What is the need for protection of the welding bead?

Metals show a natural affinity to gases like N2, O2 and H. In a weld bead the presence of these gases can result in various phenomena.

During welding if these dissolved gases do not escape or are not eliminated before solidification of the weld metal it results in porosity.

It is also possible for these gases to react with metal at high temperatures to form oxides, nitrides and hydrides. These will be present in the weld metal as inclusions.

Other than the obvious porosity and inclusions presence of gas has an effect on the mechanical properties of the weld. The Graphs below shows the effect of Nitrogen and Oxygen in the weld bead.

Effect of Nitrogen

Effect of Oxygen

The different welding processes have an affinity to certain gases as shown in the graph below.

% of O2 & N2 in weld bead of various welding processes

Depending on the welding process being used the solution to these issues are the use of inert (argon, helium), inactive gases (C02) as a shielding gas so that the liquid weld metal does not come into contact with atmospheric gases. This is the case in Tig, Mig, and Plasma welding

In the case of processes like SMAW, SAW, Electroslag welding suitable fluxes are used during the welding process. These fluxes contain elements which react with the oxides & nitrides to form slag. Slag being lighter then metal floats to the top of the weld pool and eliminates the oxides and nitrides.

In processes like electron beam welding and diffusion bonding the weld metal is protected by a vacuum.

Conclusion is depending on the welding process being used the method of protection of the weld bead has to be selected accordingly.

Visit us : www.arcraftplasma.com Email us: salesarcraft@gmail.com

Quality in welding and fabrication

noreply@blogger.com (arcraftplasma) — Thu, 16 Feb 2017 04:54:00 +0000

Without welding almost all modern technical achievements would not exist. Welding when correctly carried out simplifies assemblies and reduces the size of the jobs as compared to other methods of joining.

Welding has aided the development of products from diverse industries which touch every part of human life.

The key word for safe and proper functioning of components and assemblies is the maintenance of an effective Quality Management system during the manufacturing process.

Quality is a perceptual, conditional and somewhat subjective attribute and may be understood differently by different people, consumers may focus on the specification quality of a product / service, or how it compares to competitors in the marketplace. Producers might measure the conformance quality or degree to which the product / service was produced correctly.Support Personnel may measure quality in the degree that a product is reliably maintainable or sustainable simply put; a quality item (an item that had quality) has the ability to perform satisfactorily in service and is suitable for its intended purpose.

There is live aspect of quality in a business context:

Producing – providing something
Checking - confirming that something has been done correctly
Quality Control – controlling a process to ensure that the outcomes are predictable.
Quality Management – directing an organization so that it optimizes its performance through analysis and improvement.
Quality Assurance – obtaining confidence that a product or service will be satisfactory (normally performed by a purchase).

In the manufacturing industry it is commonly stated that “Quality drives Productivity”. Improved productivity is a source of greater revenues, employment opportunities and technological advances, however, this has not been the case historically, and in the early 19^th century it was recognized that some markets such as those in Asia, preferred cheaper products to those of quality, most discussion of quality refer to a finished part, wherever it is in the process. Inspection which is what quality insurance usually means, is historical since the work is done. The best way to think about quality is in process control. If the process is under control, inspection is not necessary.

In this context quality assurance should be ensured right from the stage of raw material and consumable purchase. Quality Management system should be the assurance that the job will be done the right way the first time. It should not be the case that inspection is done at the completion of the job to find out defects and reject the job.

The whole team of manufacturing department, Quality control, suppliers and vendors should work as a team to ensure success.

However, there is one characteristic of modern quality that is universal. In the past, when we tried to improve quality, typically defined as producing fewer defective parts, we did so at the expense of increase cost, increased task time, longer cycle time, etc. We could not get fewer defective parts and lower cost and shorter cycle times, and so on. However when modern quality techniques are applied correctly to business, engineering, manufacturing or assembly processes, all aspects of quality – customer satisfaction and fewer defects / errors and cycle time and task time / productivity and total cost, etc. must all improve or, if one of these aspects does not improve, it must at least benefit Quality (Product Performance)
Cost efficiency (Low Product Price)
Dependability (Reliable; Timely Delivery Of Order To Customer)
Flexibility (Responding Rapidly With New Products Or Changes On Output Volumes)

Quality, especially in manufacturing and diverse industries, is regulated tested and certified by diverse methods. Models and standards are provided to test the quality. For instance, the fitness for use (FFU), concept introduced to help test the quality of various types of electrical and electronic equipment, including household appliance and video / audio equipment.

In the welding industry various national and international organisations have established codes and standards specifying testing methods, welding process, certification of welders and welding inspectors. Following these standards and a proper Quality management system will ensure the best results to the satisfaction of all stakeholders.

Email us: salesarcraft@gmail.com Visit us: www.arcraftplasma.com

Disclaimer

Special Welding Processes

noreply@blogger.com (arcraftplasma) — Sat, 28 Jan 2017 08:45:00 +0000

High Energy beam Welding
Electron Beam Welding

A high-velocity, narrow-beam electron converting into heat to produce a fusion weld in a vacuum (Multiple degrees of vacuum).
From foil to plate as thick as 150mm.
Very small heat effected zone
Power density PD = f1EI/ A

Laser Beam welding

A high-power laser beam as the source of heat to produce a fusion weld without a filler material
Due to the high density energy on a small focused area, narrow and deep penetration capability
Pulsed beam for spot-weld thin samples
Continuous beam for deep weld and thick sample
e.g.: Gillette Sensor razor

Solid-State Welding

No filler metals but w/o local melting with either pressure-alone or heat and pressure.
Intimate contact is necessary by a through cleaning or other means.

Solid-state Welding Processes

Forge welding – Samurai sword
Cold welding – high pressure
Roll welding
Hot-pressure welding
Diffusion welding at 0.5Tm
Explosive welding: Mechanical locking commonly used to bond two dissimilar metals, in particular to clad one metal on top of a base metal over large areas
Friction welding: Friction to heat
Ultrasonic welding: Oscillatory shear stresses of ultrasonic

Email us: salesarcraft@gmail.com Visit us: www.arcraftplasma.com

3 Reasons Why You Have To Search for PQR, WPS and WPQ

noreply@blogger.com (arcraftplasma) — Sat, 28 Jan 2017 05:52:00 +0000

The other day, I was talking to one of my prospective clients (structural fabricator to EN 1090-2), and we were discussing the search and management of welding documents such as Procedure Qualification Records (PQR), Welding Procedure Specifications (WPS) and Welder Certificates (WPQ). He tells me this:
“I asked one of my clients, a really big company, how did they search for PQR, WPS and Welder Certificates, in order to avoid qualifying new procedures and welders. He told me that when a new project arrives, they just qualify everything they need for that project”
Both of us agreed that this is completely unsustainable for smaller companies. Qualification of PQR could cost up to 3000 €. If you have 5 projects each year, and you need to qualify 3 PQR for each of them. This amounts to 45000€. If you have a good database of PQR from previous projects, that is 45 thousand that you could save.

So what is the key issue here?

The problem is that welding documents have too much information (and it is important that they do), and each project for your company also has a lot of information, such as pipe or plate sizes, specific project requirements as well as construction code requirements.
This means that engineers will have to do a lot of cross-checks and information matching, to assure that your company is qualified for the job (or that you know exactly what you need to qualify. Just think that you need to be qualified to weld those thicknesses, those diameters, those base materials, with those filler materials, and so on.

And why is this an issue?

Well, no matter how good an engineer is, he is still only human, and doing a lot of information checking and verification will tire him and could eventually cause some mistakes, which could make him flag a specific weld as not being qualified (when, in reality, it is), or worse, making him flag a specific weld as being qualified (when it isn’t). Both will add increased cost to the company.

In the first case, by qualifying an overlapping PQR (i.e. you already had it qualified, but due to mistake on the verification process, you qualify it again), which costs 1000 to 3000 €, not accounting for the time wasted, welder’s salary, and material costs.

Also in the first case, qualifying an overlapping welder certificate, which costs up to 500 €. It is a less extreme scenario but the more you get of these, the harder it will be to requalify them after 6 months or 2 years.

In the second case, by your clients mandating that you stop all work until the weld or welds that were mistakenly flagged as being qualified, are in fact qualified. This is not so easy to quantify because it depends on the project and company size, but it could go into the hundred thousands of euros or even millions.

While this may seem like an extreme situation, it has happened before, and while it is not a frequent one, it is always a hassle and a headache when a situation like this happens.

How can you avoid this altogether?

You should work towards having a populated database of all your Procedure Qualification Records (PQR), Welding Procedure Specifications (WPS) and Welder Certificates, with all the essential variables that are relevant to the codes you work to in a spreadsheet or a dedicated welding management software. You can find the most important variables to keep track of here.
All in all, you should improve your ability to search for your documents, this will allow you to respond to any auditing inspector’s requests easily and quickly.

About WeldNote

WeldNote is a welding management software that allows you to quikcly search for your Procedure Qualification Records, Welding Procedure Specifications and Welder Certificates in bulk - search for everything you have qualified for every different weld you’ll make in a project. It also writes welding procedure specifications and welding books easily and based on your qualified PQR.

By,

Tiago Pereira

(CEO at WeldNote)

Reference:
http://weldnote.com

Email us: salesarcraft@gmail.com Visit us: www.arcraftplasma.com

Diaclaimer

Physics of Welding

noreply@blogger.com (arcraftplasma) — Wed, 25 Jan 2017 11:57:00 +0000

Coalescing Mechanism:

Fusion via high-density energy
Process plan to determine the rate at which welding can be performed, the size of the region and power density for fusion welding

Powder density (PD):

PD = P/A
P = power entering the surface, W (Btu/sec); and
A = the surface area, mm2 (in2)

With too low power density, no melting due to the heat conducted into work

With too high power density, metal vaporizes in affected regions

Must find a practical range of values for heat density

In reality, pre & post-heating and non-uniform
For metallurgical reason, less energy and high heat density are desired.

The estimated quantity of heat:

Um = KTm2
where K=3.33x10-6

Heat waste:

Heat transfer efficiency, f1, between heat source and surface

Heat problem:

Oxyfuel gas welding is inefficient while Arc welding is relatively efficient.
Melting efficiency, f2 , due to the conduction of a work material

Conduction problem:

Al and Cu have low f2

Net Heat Available for Welding:

Hw = f1 f2 H

Balance between energy input and energy for welding:

Hw = UmV

Rate Balance:

HRw = UmWVR

= f1 f2 HR = Um AwV

Where WVR = Volume rate of Metal Welded

Approximate Power Densities and Efficiency

Welding process	W/mm2	(Btu/sec-in2)
Oxyfuel	10	(6)
Arc	50	(30)
Resistance	1,000	(600)
Laser beam	9,000	(5,000)
Electron beam	10,000	(6,000)

Arc Welding Process	f1
Shield Metal Arc Welding	0.9
Gas Metal Arc Welding	0.9
Flux-cored Arc Welding	0.9
Submerged Arc Welding	0.95
Gas Tungsten Arc Welding	0.7

Email us: salesarcraft@gmail.com Visit us: www.arcraftplasma.com

Terminology and typical Dimension for V - Joints

noreply@blogger.com (arcraftplasma) — Sat, 31 Dec 2016 07:33:00 +0000

Typical Dimensions
Bevel Angle	30 to 35
Root face (mm)	1.5 to 2.5
Root gap (mm)	2 to 4

Nondestructive Examination (NDE) and Responsibility

noreply@blogger.com (arcraftplasma) — Fri, 25 Nov 2016 07:41:00 +0000

It’s time for engineers, fabricators and inspectors to recognize that how they interact on a construction project vis à vis nondestructive examination (NDE) of weldment, can affect overall confidence in service performance. Nondestructive examination specifically, magnetic particle (MPI), liquid penetrant (LPI), ultrasonic (UI) and radiographic (RI) inspection techniques cannot stand apart from design and quality construction; rather, it must be regarded as part of the process that aims for design and construction quality control.

Cast of Characters

On every project, there are three key players: the engineer, the fabricator and the inspector.
The engineer is primarily responsible for the design of a welded assembly and is the individual acting on behalf of the project client. He or she makes all decisions that modify the original design, approves changes proposed by the fabricator or accepts the inspector’s report on NDE results.
The fabricator refers to the contracting company that performs the welded fabrication; this term may be used for either shop or site welding operations.
The inspector is the individual responsible for supervision of NDE operations, and the one who will communicate with the fabricator and engineer about the results of the NDE.

What Engineers Need to Know

Engineers sometimes misunderstand the strengths and weaknesses of NDE, and mistakenly believe all welds, if made in accordance to welding standards, will provide the same level of integrity. By this way of thinking, if a weld is to be inspected only by unaided visual methods and is deemed acceptable, it would be just as sound if examined by a subsurface NDE method such as UI. The problem lies with the definition of integrity and a lack of understanding of not only NDE but also the economics and business of fabricating metal.

To an engineer, “integrity” means the ability of a weld to perform its service function. But, in truth, integrity can only be determined once the weld is actually put into service, well after fabrication and inspection is completed. NDE is, therefore, an indirect method of assessing integrity by determining homogeneity. The techniques of NDE let us know the extent to which a weld has failed to achieve homogeneity with the base metal being connected or, more specifically, the degree to which the weld and base metal have failed to form a continuous, uninterrupted mass, free of internal (e.g., porosity) and external (e.g. Undercut) flaws.

Although a “perfect” weld is ideal, it is not required in most circumstances to provide adequate service function.

Welding specifications typically permit a variety of flaw types, sizes and frequencies. Some, such as porosity, have relatively little effect on load resisting adequacy; others, such as incomplete fusion, can have a significant impact.

The knowledgeable engineer, familiar with the fabrication business, will realize contract bid requirements will materially affect the way a fabricator tries to minimize these internal and external flaws. If welds are only to be inspected with surface inspection methods, whether unaided visual, MPI or LPI, fabricators will take the measures necessary to ensure the surface satisfies these visual or NDE acceptance criteria. This may include using experienced welders or small electrodes for the root and capping passes, as well as careful grinding and/or the use of GTAW toe remelting.

Sometimes, the fill passes may not receive the same degree of cleaning, or less experienced welders may be used. This may simply be a case where the fabricator is making optimum use of time and available resources to comply with the weld quality standard involved. Employing qualified, or prequalified, weld procedures, in addition to using qualified welders on a job, will minimize the size and frequency of subsurface flaws. However, a method to investigate the internal volume of a weld may be needed to maintain confidence that major flaws have been avoided.

If a subsurface quality standard needs to be determined, then the contract needs to call for a subsurface method, at least for critical connections or some randomly selected percentage of total welds. This will ensure the fabricator devotes special attention to these welds, employing the top welders and taking extra care in preparing joints prior to welding and cleaning the weld while in progress. Naturally, this extra level of precaution will cost more, but it will add confidence that the weldment has achieved the necessary degree of homogeneity.

In the United States, there is a provision in the AWS D1.1 Structural Welding Code Steel stating if the client wants additional NDE other than the contracted visual inspection, then the client must absorb all of the subsequent costs, including the cost of repair for flaws that are outside the acceptance levels for subsurface NDE. This appears to be reasonable; after all, the client should have specified this NDE in the original bid documents, which would have allowed the fabricator to devote more stringent quality control standards to the welds in question. If these welds fail to live up to this new, higher standard, the fabricator can hardly be blamed.

However, the D1.1 code adds another clause, which requires the fabricator to absorb all the costs if this new NDE reveals “an attempt to defraud” or “gross non-conformance.” This last clause prompts all kinds of arguments about what these words mean. Naturally, the client, who expects sound work, may argue that degree of non-conformance with the higher NDE flaw acceptance criteria is “gross” or “an attempt to defraud.”

Realistically, neither the code nor the D1.1 Structural Welding Committee will ever attempt to define what is “an attempt to defraud” or “gross nonconformance,” but future imbroglios can be avoided if some percentage of random subsurface NDE is specified in the contract, just to keep the fabricator on his toes and to ensure extra care is devoted to critical joints. But engineers have to realize not all welds require the same level of attention in order to adequately perform their intended service function. Trying to make all welds achieve the highest quality standard only drives up the cost of fabrication.

Finally, it is worth noting that the use of quality docu

NDE Techniques to Use

The following is a partial list of recommendations using NDE techniques:
Weld and Load Types Appropriate for MPI or LPI

Fillet welds:

Complete or partial penetration groove welds in compression or shear.
Complete or partial penetration groove welds in low static tension or fatigue loads and structures with low criticality and/or high redundancy.
Complete or partial penetration groove welds in high static tension or fatigue loads and structures with high criticality and/or low redundancy.

Recommended test frequency:

Random testing for substantially less than the total weld length.

Weld and Load Types Appropriate for UI or RI

Complete penetration groove welds in high static tension or fatigue loads and structures with high criticality and/or low redundancy.
Complete penetration groove welds in low static ten-sion or fatigue and structures with high criticality with low redundancy and materials with low or unspecified toughness.

Recommended test frequency:

100% manual inspection of welds or some random percentage for automated NDE.
These weld or load types are not meant to be definitive, but serve to highlight typical industry practice. Notice that MPI or LPI would be applied to complete penetration groove welds, even if these would also justify UI or RI. This is because such welds, when subject to high tensile loads in critical applications, would need to be checked for potential crack starters on the surface, which UI in particular can have trouble detecting.

20 • INSPECTION TRENDS

Quality systems such as ISO 9001 and its myriad variants may offer some security in this regard, but engineers should be wary of relying exclusively on paper empires. It is the actual implementation of written quality systems that determines effectiveness and, for this reason; third party inspection teams are frequently used.

Picking the Right Technique

Judgment is needed to decide which weld types may be appropriate for various NDE methods if economy, as well as integrity for all interested parties, is to be achieved. Service function and weld geometry can provide some answers. Service function will involve issues such as criticality of the connection (consequences of load-carrying failure), redundancy (alternative load paths), loading type (cyclic tension, static compression), stress level (sub yield) and brittle fracture resistance (is toughness specified?). The geometry of a weld can determine the effectiveness and practicality of using some NDE methods since some weld types are more conducive to reliable flaw detection than others.

Groove welds, for example, can be inspected with either UI or RI. However, partially penetrated groove welds will always provide flaw-like indications or images at the unfused weld roots. Unless proper NDE procedures are written, such “false” flaws can result in high reject rates. Fillet weld geometry creates reflective problems with UI and image problems with RI. However, the use of partial penetration groove and fillet welds is usually limited to non-fatigue, non-tensile or low tensile applications, so the need for confidence in subsurface quality is not great. Since the concern is usually restricted to detecting surface flaws, MPI or LPI are the preferred NDE techniques in this case.

Likewise, engineers need to specify to what extent welds of a particular type need to be inspected. Logically, welds that are subject to low stresses or are in compression or are in highly redundant structures could have less coverage than welds subject to high tensile stresses in low redundancy structures. It is also important to con-sider the advantages of automated NDE over purely operator controlled NDE; generally, NDE operated by humans is considered less reliable than automated, computerized NDE.

The use of ultrasonic energy to detect flaws is a tried and true method that does have drawbacks, the biggest being its reliability in flaw detection. Everyone has heard horror stories about UI detecting rejectable flaws, only to discover upon excavation the flaw was acceptable or nonexistent. This may undermine confidence in UI to some degree; because it is also true reject able flaws will also go undetected. The reasons for these false alarms or missed flaws can vary from operator error to poor calibration to malfunctioning equipment. But, as long as we understand and accept that no NDE method is guaranteed to detect 100% of all rejectable flaws, we can appreciate the wisdom of safety factors, redundant designs and specified toughness in critical connections. These all assist in bolstering our confidence in overall performance.

Various welding standards use different approaches to UI, each with its inherent limitations to accurately size flaws. UI that uses beam reflection amplitude only can undersize flaws significantly, while UI that employs beam boundary techniques are somewhat more reliable, but still not foolproof. Even more sophisticated techniques, such as time off light diffraction, are not a guarantee of accuracy, so engineers need to be prudent when using NDE derived flaw dimensions.

Radiographic inspection is, of course, an indirect, visual method of detecting and sizing flaws, but its weakness is flaw orientation. Planar flaws normal to radiation may go undetected. Naturally, planar flaws, such as incomplete fusion or penetration, are the more significant ones from a fracture mechanics standpoint; for this reason UI is preferred since its strength is detecting planar flaws normal to acoustic energy. Additionally, UI is cheaper than RI and does not involve safety hazard issues. For these reasons RI is not generally preferred for structural work, although many contracts will specify RI in addition to UI for critical connections.

What Fabricators Need to Know?

A fabricator’s primary focus is to deliver a product that meets the standard of quality required in the client’s contract. If NDE is employed, it is to verify the standard has been achieved. Additionally, the client may request a third party inspector to peer over the fabricator’s shoulder during fabrication to further ensure quality.

But fabricators have to be aware of their obligations vis à vis NDE. If a fabricator knows a critical structure is being bid for visual inspection only, do they remain silent at the bidding stage or anticipate that the client may get wise and require NDE later? Some fabricators may be reluctant to tell the engineer what their responsibilities are, but this should be addressed at the bid stage if potentially nasty, litigious struggles over “gross nonconformance” are to be avoided.

Many welding standards will use different flaw acceptance limits for different load types, so it is imperative this information be conveyed to the fabricators (and inspectors who may work for the fabricator or the engineer). This can be done in a number of ways, but probably the most direct method is by indicating on the design drawings whether a connection is subject to shear, compression or tension. It must be understood that this does not refer to the load in the weld proper, but the load sense in the attaching members. This is an important distinction to make for fillet welds, which are always considered to be loaded in shear.

Fabricators also need to be aware of how NDE can affect integrity. For example, using prods for MPI can cause arc strikes on base metal. While the fabricator may be diligent in removing arc strikes caused by welding, they may not show the same concern for prodinduced strikes. Both can have potentially harmful hardening effects on steel.

WINTER 2001 • 21

The fabricator also needs to be aware NDE is an integral part ographed, it is particularly important for of the NDE complications that can arise a schedule to be established in order to from the selection of certain welding of the three-part process minimize the safety hazards details.

Inspectors should also resist the temp fabricators to specify the type of detail, involving design andtation to supplant the engineer in accept-after indicating the design requirement construction quality or rejecting detected flaws of weld type (typically for complete penetration Inspectors are primarily reporters of etration welds.) For example, complete such information, who should, of course, penetration groove welds made with control provide their assessment of acceptability

Steel backing that needs to be ultrasonically inspected will produce a large number of false indications arising from the small air gap between the un-fused portion of the backing and the base metal. Too often, inexperienced UI operators will report these as rejectable flaws. This can be avoided if removable non fusible backing is used or a back gouged, two-sided weld is employed. Naturally, economics will play a decisive role in determining which detail will be employed, but it behooves all parties to be aware of the potential NDE problems associated with a selected detail. Other problems associated with NDE include the residual magnet-ism produced by MPI, which may affect subsequent welding or even the service function of the part. Appropriate degaussing techniques may need to be implemented.

The most important aspect of fabricator responsibility lies in the area of quality control, i.e., the methods used to ensure that a given quality standard is being conformed with in practice. Quality control adds another layer to an engineer’s confidence that, even if some major flaws go undetected, the overall system has been soundly fabricated and the consequences of localized substandard quality are minimal. Naturally, this does impose on the engineer the need to verify in some fashion that a quality control system is in place and functioning.

What Inspectors Need to Know?

Nondestructive examination personnel have education, training and experience in the esoterica of their craft. They know the basic physics and fundamental topics associated with the various methods. They will work according to procedures established by supervisory personnel to adequately examine the prescribed weldments. And it is these supervisory personnel typically the inspector or people who report to the inspector who will need to work closely with engineers and fabricators.
Inspectors must insist on information regarding the tensile or compressive nature of the loads on inspected welds when the applicable acceptance standards require this. Frequently engineers fail to provide this, and the temptation of the inspector is to assume a load type rather than seek the engineer’s input; this should be avoided.
Inspectors should work with fabricators before fabrication or erection even begins in order to determine the accessibility of welds during the construction process. This is particularly true for welds that will be enclosed after final assembly is complete. Sometimes artificial access, such as plate cutouts, has to be provided for tight and congested areas. For details that are to be ready to the fabricator. However, the fabricator is free to try to get the engineer to accept the flaw and should not feel hamstrung by an inspector’s opinion. The engineer should be the final arbiter of any disputes that arise.
As mentioned earlier, fused steel backing can become the source of disagreement as a result of the air gap interface. The good inspector recognizes this potential and writes a procedure that does not reject these indications but instead predicts them. When included in the report, the engineer may decide to accept these indications as innocuous or insist on removal of backing, re inspection or some other alternative.
Similarly, if an inspector sees that partial groove welds are to be subjected to UI, a procedure must be developed that recognizes the reflectors at the not fully penetrated weld root so that these are not grounds for automatic rejection.
If an inspector reports unacceptable flaws to the fabricator, there are two options: a) fix it or b) get the engineer to accept the flaw. Naturally, most engineers will opt for A, but often conditions will warrant exploring B. This forces the engineer to evaluate structural integrity based on a flaw’s existence. Such fitness for purpose evaluations requires the engineer to evaluate information about flaw dimensions, location, and orientation to determine how confident he or she is of the flaw parameters reported by NDE.

Why NDE is Necessary ?

Can NDE, in and of itself, guarantee anything about a product? Can it make up for inadequate design? Can it even detect all aspects of poor construction? The answer to all three questions is “No.” But what NDE can do is enhance overall confidence in the product’s capacity to do its intended function; it is an integral part of a process involving design and construction quality control.
“Confidence” is a word that has been used throughout this article, because this unquantifiable emotion is the very basis for all the conservative design, construction and inspection standards in use today. Compliance with all of these cannot guarantee performance, but each project phase can add layers of confidence to the end objective service performance. Nondestructive examination is the last and perhaps most visible layer, but it cannot stand in isolation from design and quality construction. Only when all three project disciplines cooperate and communicate effectively can maximum confidence be obtained.

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FAQ's on Powder Metallurgy & Electroforming

noreply@blogger.com (arcraftplasma) — Fri, 18 Nov 2016 11:19:00 +0000

How to prevent SS316 butterfly valves seats from galling during service?

You should go for overlay of the valve seat with Nickel using PTA or any other Thermal spray process.

We have an engine seat repair job. We do not know the base metal as it is required in preparation for reclamation welding?

A material testing lab or an NDT company can get the testing done using the PMI method.
With Positive Material Identification (PMI) the alloy composition, and thus, the identity of materials can be determined. Positive Material Identification is particularly used for high-quality metals like stainless steel and high alloy metals.
Common PMI techniques are XRF (X-ray Fluorescence) or OES (Optical Emission Spectroscopy).
OES is preferred to XRF technology because of OES’s superiority in the measurement of light elements in metals, such as carbon and aluminium. OES is the only reliable way to measure carbon outside of the laboratory, which commonly needs to be measured in materials like stainless steel, magnesium, and silicon.

What is Powder Metallurgy?

Manufacturing is in a perpetual state of economic and technological change. Production process which can shape a finish component directly from raw material presents the ideal in manufacturing economy. The elimination of all secondary operations is the ultimate, though perhaps unattainable, goal.
The techniques by which close-tolerance products can be produced directly from powder metal, without the benefit of machining, are called powder metallurgy. The powder metallurgy techniques include the production of acceptable powdered metal. The design and production metal, and the heat treatment, or sintering of the briquettes.
The process of using iron powder in manufacturing gained importance during the 1920s. The process grew rapidly during World War II. Germany was short of copper and used porous iron as a substitute. The powder iron was first produced by the milled and sponged processes. Economic consideration, however, demanded a less expensive process. The result was that research developed a method of production powder iron by disintegrating a stream of molten iron. One method accomplished this disintegration by using a rotating water-cooled disk, while another method used a jet of air, steam, or water. The iron powder thus produced was used not only for powder metal parts but also for coating welding rods, since 1947 the addition of powder metal to a flame for cutting and scarfing has added impetus to the demand for and uses of, powder metals.

Q. What are all the different available Metal powders?

Iron Powders are presently in the greatest demand. However, numerous other metals are available in powder form. Here is a list of the metals that are now available in powder form:

Aluminium
Copper
Molybdenum
Steel
Brass
Graphite
Nickel
Tin
Bronze
Lead
Nickel
Silver
Tungsten
Cobalt
Magnesium
Stainless steel
Zinc

Powder metallurgy development has done much to improve manufacturing, efficiencies, and possibilities.
Materials such as tungsten carbide, titanium carbide, tantalum carbide and their variations in form of alloys are common and are used extensively as cutting tool materials as also in welding overlay applications.
Materials specified for application in rockets, aircraft engines, and other high or low-temperature devices could not be machined if these hard metals for cutting tools were not available. The cutting tools of this type are made from a powder that was properly processed, pressed, high pressures and finally sintered or heat treated.
Welding hard facing alloys are available in powder form for use in Plasma Transferred arc (PTA) laser, HVOF, and other Thermal spray applications.

Q. What is Electroforming?

Electroforming is the processing of a part by the electro-deposition of metal on a tool having a predetermined shape, Size, accuracy, and finish. The tool is called a mandrel or matrix. The electro-deposited metal is built up to the required thickness and the mandrel is removed leaving the desired from without need for further finishing. Electroforming finds the application in the processing of parts having intricate internal shapes, the extra-thin wall section of higher precision requirements relative to dimension and finish which prove to be difficult, expensive, or even impossible to manufacture in any other way.
Electroforming is preferred over conventional process be-cause it can produce accurate dimension and high surface finish of 1 or 2 in., and because it is more economical. Many combinations of metals, such as silver, copper, iron, nickel, gold, rhodium and chromium, are used in the electroforming process. Specific metals are applied to give the finished part special properties, such as high conductivity, strength, corrosion, resistance and wear resistance.
Electroforming parts can be produced to tolerances of 0.001 in. on production runs, or even to closer tolerances if specially handled. The components are not limited to a minimum wall thickness, and depositions can be made up to 0.125in., or even to 0.375 in. with special handling. Also, there is practically no limited to the shapes that can be formed. The internal stresses of electroformed components are often less than those for similar components that are cast, forged, spun, or injection molded; and the physical properties are about equal to, or better than, those for the same metals formed by these other processes.

Q. What are the applications of Electroforming?

Electroforming is used in the electronics, plastics, automotive, aircraft, and general consumer products industries. Molds and dies for plastics are electroformed, as are also electrotypes for printing, air intakes for jet aircraft, and tips and leading edges of helicopter blades and other aircraft parts.
Spherical, elliptical and parabolic reflectors are electroformed in nickel and copper with a rhodium or aluminized reflecting surface. The bulkheads of large fuel tanks for missile are also electroformed. Since weight-saving is such an important requirement in aerospace applications, the high strength of certain thin wall deposits permits many uses of electroforming. Radar horns have nickel deposits that produce a stress-free deposit with a tensile strength of 60,000 to 110,000 psi and yet have adequate ductility for the application.
The parts are “grown on” in the electrodeposition process, thus eliminating the possibility of heat distortion in thin walled sections which is a common fault of the conventional brazing technique. The joining of thin wall sections is accomplished by assembling the parts in sections on a mandrel and completing the joint by electroforming the base metals.
The electrodeposition of a layer of pure copper to microwave transitions and transducers results in the component having good accuracy, finish, strength and electrical conductivity.

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Schematic Representation of Weld Defects

noreply@blogger.com (arcraftplasma) — Wed, 16 Nov 2016 07:15:00 +0000

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Judging the Temperature of Steel by its Colour

noreply@blogger.com (arcraftplasma) — Tue, 15 Nov 2016 08:18:00 +0000

Temperature Colour	^oC	°F
Pale Yellow	200	392
Straw Yellow	220	428
Yellow brown	240	464
Bluish purple	270	518
Violet	285	545
Pale blue	300	572
Blue	325	617
Luminious Colour
Faint dark red	590	1094
Cherry red (dark)	650	1202
Cherry red (medium)	700	1292
Red	760	1400
Light red	815	1499
Orange	930	1706
Pale orange lemon	1000	1832
Lemon	1150	2102
Yellow	1260	2300

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Calculation of Filler Material Consumption

noreply@blogger.com (arcraftplasma) — Fri, 11 Nov 2016 11:53:00 +0000

The number of pounds of welding electrode or welding wire necessary to complete a given joint may be calculated by the formulas:

P = WL / E

where:

P = Pounds of electrode or wire required

W=Weight per foot of weld metal

L = Length of weld (feet)

E= Deposition efficiency

WEIGHT PER FOOT OF WELD METAL:

Calculating the weight of weld metal requires that we consider the following items:

Area of the cross-section of the weld.
Length of the weld.
Volume of the weld in cubic inches.
Weight of the weld metal per cubic inch.

In the fillet weld shown below, the area of the cross-section (the triangle) is equal to the half the base times the height, the volume of the weld is equal to the area times the length, and the weight of the weld then, is the volume times the weight of the material (steel) per cubic inch.

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Piping fittings

noreply@blogger.com (arcraftplasma) — Sun, 30 Oct 2016 12:10:00 +0000

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Wearplates Applications

noreply@blogger.com (arcraftplasma) — Mon, 24 Oct 2016 15:44:00 +0000

Wear plates, wear strips and wear parts are used to protect and minimise abrasion, erosion, wear and impact in mechanical systems. In the industrial applications high level of abrasion, impact, or heat causes wear and tear on mechanical components of low hardness steel plates. Bearing in mind the cost factor of exotic high hardness metals the solution is to manufacture bi metallic plates where the hard side of the plate is exposed to the harsh working conditions.

This kind of plates brings down the cost of fabrication and operation cost tremendously. The proper selection, manufacture or purchase of wear strips, plates, and parts requires an understanding of the customers’ application requirements. This is a highly specialized business and many of the alloys, compositions of metals used in hard facing the wear plates are proprietary with a huge variety of products available to choose from.
Wear plates are usually manufactured using the Open arc process, MIG, FCAW, and SAW. Wear plates are composite plates having a highly alloyed wear resistant Chromium carbide , tungsten carbide , Nickel and other hard metals in a ferrous matrix used for surfacing on a low carbon steel base.

Chromium carbide based wear plates have a low coefficient of friction; Tungsten carbide wear plates are extremely wear resistant. Depending on the base metal used to form a matrix these compositions can be corrosion resistant also.
These plates are manufactured by automated gantry type systems for fast and mass production. Plates are normally clamped to the base to prevent distortion. Other than this the manufacturers have fine-tuned the proper sequence of laying the weld beads to prevent distortion because of the high heat input and continuous overlay during the production process.

Automatic hard facing process results in high quality, hard, tough and highly wear resistant surface. The mild steel base facilitates the plates use for easy forming, welding or bolting during fabrication to the component surfaces.
Wear plates, strips, and parts are also made of basalt or ceramic liners which protect manufacturing machinery from abrasion or impact in the cement industry for example. These wear pads or linings are highly resistant to abrasive effects during bulk processing of grain, salt, limestone, coal, different ores, fly ash etc.
Wear plates are designed to work in harsh environments and find applications in mining, power generation, steel, cement, coal-fired power station, oil & gas, sugar industry etc. Applications are in truck beds, attachments to diggers, bulldozers and earth movers and other heavy earthmoving applications, bearings systems, foundry mold slides and gas turbines.

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Automatic Welding of storage tanks with all position variable speed carriages using the MIG process

noreply@blogger.com (arcraftplasma) — Mon, 24 Oct 2016 10:27:00 +0000

With the large amount of welding required in the construction of storage tanks, contractors, all over the world, including India, are resorting to automatic welding techniques to increase productivity, improve quality, and to meet the stiff deadlines. The acute shortage of skilled welders makes it more of a mandatory necessity to automate the welding process.
The objective of this paper is to give an overview of the types of automatic welding equipment being used worldwide for the fabrication of large storage tanks. The author has stressed more on the application of the equipment than the design, joint configuration, metallurgy, consumables etc. The paper has been “punctuated” with drawings and photographs of tanks being welded by the programmable, variable speed, all position welding carriages.

TYPES OF AUTOMATIC WELDING EQUIPMENT USED FOR TANK WELDING

The automatic welding equipment used for storage tank fabrication falls mainly into four categories:

Electro-gas welder for vertical welding on shell butt welds.
“3 O’clock” Submerged Arc Girth Welder for welding of circumferential seams.
Programmable, variable speed, all position welding carriages (used with the MIG welding process) for welding of vertical and circumferential seams of tanks, wind girders, floor plates, floor plates to vertical shell welding, etc.
Auxiliary equipment i.e. Mechanical high speed bevellers, Automatic gouging equipment, etc.

ELECTROGAS WELDER

The Electro-gas welder is an extension of the MIG process, and is designed for carrying out vertical welds in a single pass. It is an efficient process except that it can lead to high heat input and unrefined grain structure in special steels, not to mention distortion.
The arc creates a molten puddle between two sets of copper shoes (one on either side of the weld seam), which climb up (powered by a motorized trolley) at the same speed as the vertical travel carriage. These copper shoes are water cooled, and “freeze” the molten metal, creating a full penetration weld.
The equipment is a “dedicated type” (can only be used for this type of application), and may require long periods of inactivity till the next tank job. It would be useful for large contractors who have a steady flow of contracts to ensure minimum idle time of the equipment.
It can only be used for the vertical welds (3G) on storage tanks.

THE "3 O'CLOCK" GIRTH STORAGE TANK WELDER

This equipment uses the submerged arc process. It can be a single or dual head type, where the latter is used with one head on one side of the circumferential seam, and the other head on the other side, so that the seam can be welded from both sides simultaneously. One, or both the heads are suspended from a motorized, variable speed trolley (providing the welding speed) running on the top edge of the tank shell. The heads are lowered by a telescopic structural arrangement so as to reach the various tiers of the shells in “jack-up” tank construction.
The equipment welds at high speeds, and reduces the number of passes.
It is the most popular equipment used by large tank fabricators and contractors. However, it has its own disadvantages, such as:
The equipment is dedicated, and once the job is over it may require long periods of inactivity till the next job!
This is a high speed and efficient process, but, at the same time, uses high currents, which means high heat input. This can lead to coarse grain structure on certain heat sensitive materials. It is very prone to slag inclusions if the welding parameters are not optimized.
The equipment is very bulky, and has a high capital cost. It uses dedicated power sources. This equipment is normally used by large fabricators of tanks, who have continuous jobs in hand to ensure minimum idle time of the equipment between projects.
It can only be used for welding the circumferential seams (2G) of storage tanks.

PROGRAMMABLE, VARIABLE SPEED, ALL POSITION WELDING CARRIAGE

Fig 1. Programmable variable speed, all position carriage with oscillator

The programmable, variable speed, all position carriages run on rigid or flexible tracks fitted with magnets, which can be mounted parallel to the vertical and circumferential seams of the weld.
These “all position” sophisticated carriages have controls for oscillation of the welding torch i.e. “dwell”, left- centre - right, programmed arc start, speed control, bead width and oscillation speed control, store and recall programmes etc.
A MIG welding torch is clamped onto the torch holding device which can position the torch at any angle. Vertical and horizontal rack arms and rack boxes are provided on the carriages for making minor adjustments for maintaining a consistent arc length, and following the weld seam. Idlers can be hooked on to the carriages, facilitating the towing of wire feeders.
Since all the functions are automatic, the operator has to only ensure that the arc length is maintained, and the MIG welding torch follows the weld seam.
These carriages can be used with any conventional MIG welding equipment, hence lending it flexibility.
There is very little, or virtually no idle time, as these carriages can be used for almost any type of welded fabrication.
The same carriages can be used for cutting “in situ”. An oxy-acetylene machine torch has to be mounted on a special cutting torch holder which provides up and down movement to the torch by means of a rack and pinion arrangement to ensure a consistent gap between the torch tip and the work piece. The torch can be tilted at various angles for bevelling operations.
The programmable variable speed, all position carriages can be used for almost all the continuous welding applications on storage tanks.

AUXILIARY EQUIPMENT

Mechanical High Speed Portable Bevelling Machine:

Fig 2. Portable, mechanical, high speed beveller

All the advantages, such as increased arc-time and high deposition rates that the variable speed carriages ‘family’ can provide, will be ‘undone’ if the edge preparation is poor, leading to bad joint fit-ups.
High-speed portable bevelling machines are commercially available. They can accurately bevel carbon steel, alloy steel, stainless steel, titanium and aluminium etc; at speeds up to 10 ft/min at different bevelling angles on plates. Plates up to 50 mm thickness can be accommodated by the beveller. Bevelling is carried out with special cutters.
This process is a noiseless and vibration free shearing process.
Mechanical bevelling, being a non-thermal process, has the important metallurgical advantage of not inducing any thermal effects like an HAZ, nor causing any thermal distortion.
No skill is required for operation.
The machine is portable, and can be easily wheeled to the job without the help of cranes.
The beveller follows the edge while bevelling without the help of the operator.
A spring loaded base allows the machine to run on an undulating floor.
The adjustment of the root face is with the help of a graduated scale on the machine.
No further grinding is required prior to welding.
Bevellers are also available for upside down bevels. For large scale production, one beveller can bevel the topside of the plate, while a second one follows by bevelling the under side of the plate. This way a double ‘V’ can be achieved without turning over the plate.

AUTOMATIC CARBON ARC GOUGING

Fig 3. Automatic gouging system

This portable automatic gouging equipment can be interfaced to a programmable variable speed, all position carriage, and an automatic arc gouging torch. Arc gouging at high speeds can be carried out, resulting excellent and consistent grooves. Since the carbon arc gouging process is a non-oxidation process, the gouged grooves (with an accuracy of 0.25 inches), do not require grinding before welding.
The various diameter electrodes are copper coated carbon electrodes (male and female types). One electrode can be push fitted into the other one, hence making the gouging process to be continuous. The arc length is electronically controlled, and the electrode feed speed is controlled automatically by the arc length.

CERAMIC BACKING STRIPS

Fig 4a. Ceramic backing strip

Fig 4b. Ceramic backing strip fixed on a butt joint

Fig 4c. Weld based on reverse side of butt weld after the removal of ceramic backing strip

With the variable speed carriages and oscillators capable of high deposition welding, it is imperative to eliminate the expensive and slow process of back chipping/gouging, and re-welding, by using ceramic backing strips for one side welding.
For sound welds and excellent radiography results, the inherent characteristics of the ceramic backing strips, such as; halogen-free, non-hygroscopic (resistance to moisture pick-up), neutral, fibrous materials, granular density etc., the refractory nature of the material to withstand high welding heat as a heat absorbing metal bar, the quality of the adhesive tape to resist preheat temperatures, and a host of other positive characteristics to increase productivity, has made this product very popular with fabrication workshops and shipyards.
These non-metallic weld backings are available in several segmented shapes and sizes, to suit almost any application requirement. The ceramic tiles are normally available in 2 ft. strips that are held in position by a heat resistant adhesive coated aluminum tape.

WELDING OF STORAGE TANKS WITH PROGRAMMABLE VARIABLE SPEED, ALL POSITION WELDING CARRIAGES

The main areas for automation in the welding of large storage tanks are:

VERTICAL WELDING OF SHELL PLATES.
WELDING OF CIRCUMFERENTIAL TIERS OF SHELLS.
TANK FLOOR WELDING BUTT/LAP WELDING.
SHELL TO FLOOR PLATE (INSIDE AND OUTSIDE) FILLET WELDING.
DOME TO CURB ANGLE, WIND GIRDERS, ETC.
ROOF WELDING.

VERTICAL WELDING OF SHELL PLATES

Fig 5a. Vertical 3G Welding with the programmable variable speed, all position carriage welding of vertical seam

Fig 5b. Horizontal 2G welding with the programmable variable speed, all position carriage welding of circumferential seam

Fig 5c. Floor welding 1G with programmable variable speed, all position carriage

Fig 5d. 2F Fillet welds with the programmable variable speed, all position carriage welding of tank shell to floor plate/ annular plate

These shell plates vary from 2m to 2.5 meters, and have thicknesses; generally ranging from 6 mm to over 60 mm. Up to 15 mm plate thickness, the joint preparation is normally a single ‘V’ butt preparation, with an included angle of 60º.
The root run is welded by the MMAW process, or by MIG. The subsequent passes are MIG welded (FCAW 1.2 diameter wire) with the programmable variable speed, all position carriage, at welding currents of 180 to 200 amps, and an average carriage speed of 10cms/min to 150cms/min.
A highly skilled welder is not required for operation of the programmable, variable speed, all position carriage, as it welds automatically, oscillating (weaving) the torch as it climbs up the track, which is fixed parallel to the vertical weld seam by means of magnets (and vacuum cups on nonferrous weldments).
After automatic welding one side of the shell, the root run is gouged out on the other side of the plate, and the gouged area re-welded manually.
For plates above 16 mm thickness, normally a double ‘V’ is required with a “⅓ - ⅔” weld preparation, having an included angle of 60º on both sides.
The root run is welded manually by MMAW or the MIG process. The variable speed carriages are used for the rest of the passes.
The inside root run is gouged out, and the groove welded manually, or automatically with the variable speed carriage, depending on the thickness of the plate and the number of passes required for the carriage to offset the “set up” time.
Manual back-gouging of the root run is a very slow and monotonous process. This can be totally eliminated by the use of ceramic backing strips which facilitate welding from one side only, allowing the formation of a bead on the other side. The weld satisfies all the metallurgical and radiographic requirements laid down by the respective codes.
Ceramic backing strips, when used with automatic welding carriages, can increase welding deposition rates tremendously, resulting in reduction of welders.

WELDING OF CIRCUMFERENTIAL TIERS OF SHELLS

Fig 6a. Vertical 3G welding on storage tank with programmable variable speed, all position carriage

Fig 6b. Vertical 3G welding inside 9% Nickel storage tank, with programmable variable speed, all position carriage

Flexible tracks fitted with magnets are placed parallel to the circumferential joint which is to be welded. The variable speed carriage (with an idler to carry the MIG wire feeder) run on these tracks with a MIG welding torch at a preset welding speed.
These carriages can have oscillators fitted on to them, for facilitating weaving of the weld to increase weld deposition rates.
To increase productivity on large diameter storage tanks, 3 to 4 variable speed carriages can run simultaneously on one track around the circumferential seam, hence increasing productivity three to four fold. The circumferential seam of a tank can be divided into 4 quadrants, with one carriage and an operator in each quadrant.
The welding can be carried out in a mix of stringer passes and oscillation, in order to control the heat input.
The root run is gouged out from the inside by the carbon arc gouging process, or grinding/back chipping, and the gouged groove welded manually, or, by automatic welding, depending on the number of passes required.
Ceramic backing strips can also be used (as explained earlier) to prevent back gouging/grinding. Welding automation, with programmable carriages and ceramic backing strips increase productivity substantially as the arc time lost by the grinding of the root is eliminated.
The same carriages can be used for automatic butt welding of the vertical seams of shell plates, shell to floor annular plate fillet welding, floor butt/lap welding, roof plate welding etc.

TANK FLOOR WELDING BUTT / LAP WELDING

The floor plates are normally lap joints. In the case of butt joints, a backing strip is an integral part of the joint.
The lap or butt joints on the floor plates can be welded by the same variable speed carriages by positioning the tracks on the magnets, parallel to the longitudinal joints to be welded.
The process is fast, clean, and the welds are of a very high quality.
Weld metal deposition rates of 4Kgs/hr on lap joints, and, up to 7Kgs/hr for butt joints can easily be achieved depending on the thicknesses of the joints, or the size of the lap welds.

SHELL TO FLOOR PLATE (INSIDE AND OUTSIDE) FILLET WELDING

Fig 7a. Circumferential 2G welding on Storage Tank with the Programmable Variable speed, all position carriage

Fig 7b. 3 no’s. Programmable Variable Speed, all Position carriages welding simultaneously

Fig 7c. Circumferential 2G Welding inside a Storage Tank with 3 no’s programmable variable speed, all position carriages

These fillet joints (the outside joints of the shells are normally beveled, as they are thick plates) can be welded by mounting the flexible tracks on the vertical shell parallel to the fillet welds, and the carriage running on the tracks with the MIG torch positioned for fillet welding. The torch can also be programmed for oscillation, if required.
A similar procedure can be used with the same carriages for welding the inner vertical shell to the floor plate or the annular plate.
The welds are consistent (with minimal stops and starts) and of high quality.

DOME TO CURB ANGLE, WIND GIRDERS, ETC.

The wind girders consisting of angles or channels can be welded to the shell automatically with the programmable carriages in a similar manner to the floor plate to the vertical shell i.e. The tracks will be mounted along the circumference of the vertical shell, and the torch placed in position for welding the continuous fillet weld on the angle/channel to be welded on to the shell. These can also be welded (running fillets, intermittent fillets and lap welds) with the trackless magnetic base fillets welders.

ROOF WELDING

Depending on the tank configuration, the roof plates are normally lap welded to each other.
These can easily be welded with the same programmable, variable speed carriages, by placing the track longitudinally parallel to the weld seam.
The dome of the roof, which is welded to the shell (again, depending on the type of tank), can be welded with the same programmable , variable speed carriages, by placing he tracks on the circumference of the shell plate and positioning the torch to follow the weld seam.
The same carriages can be used for oxy-acetylene cutting with one or two torches, in all positions of the plane. They give consistent, excellent cuts, and can also be used for beveling operations.

BENEFITS OF USING THE PROGRAMMABLE VARIABLE SPEED, ALL POSITION CARRIAGE

Fig 8a. Weld quality - Down hand 1G Welding with the programmable variable speed, all position carriage

Fig 8b. Weld quality – Vertical 3G Welding with programmable variable speed, all position carriage

Fig 8c. Weld quality – 2G root run with the programmable variable speed, all position carriage

Fig 8d. Weld quality – 2G final run with the programmable variable speed, all position carriage

Fig 8e. Weld quality – 2F welding with programmable variable speed, carriage

Excellent consistent x- ray quality welds with controlled heat input and reduced HAZ.
Programmable, hence the parameters can be preset as per the WPS.
Highly productive and flexible. It can be used on a variety of applications.
It is very simple to use, and operator friendly.
No need of skilled welders for operation.
Can be used for butt, fillet, lap, and welding in “all positions”.
Same equipment can be used for “all position” cutting with oxy-acetylene, plasma and water-jet cutting processes. It can be used with multiple torches for cutting operations in all positions
The microprocessor based controlled carriages with tacho-feedback controls, can tow wire feeders up to 45kgs in any position with no change in the pre-set carriage speed.
For large tanks, 3-4 carriages can be used in tandem, to speed up production.
The carriages can be used on rigid tracks or flexible tracks, which can be bent to take the shape of the profile of the tank.
9% Nickel tanks double walled tanks are beyond the scope of this paper. It is a fact that these carriages have been used for welding them. These carriages can be used for any type of
Storage Tank i.e. LPG bullets, Spheres, Vessels etc. they can also be used for welding aluminum, stainless steel, and other non- ferrous materials by using vacuum suction cups for holding the track on to the shells.

AUTOMATIC WELDING OF SPHERES

Spheres have been successfully welded by the programmable, variable carriage oscillator combos. A few self explanatory photographs of the carriages welding the inside and outside of the spheres are shown.
When used with ceramic backing strips, the elimination of back chipping/grinding helps substantially in increasing weld deposition rates, and reduction of man power. It is common practice to weld Spheres with these carriages and ceramic backing strips.

CONCLUSIONS

It is evident that welding and cutting automation is the answer to the fabrication of Storage Tanks and Spheres. With the projects getting larger, and with tight completion schedules, it is getting more and more difficult for contractors to mobilize large numbers of skilled welders. Not only, there is a shortage of welders, it is their migratory tendencies of changing jobs that create hold- ups and delays in projects.
A typical example below clearly highlights the need for automation, due to its high efficiency, and the high quality of welding, with minimum flaws. It provides the answer to achieving low man hours per ton, (which every project manager looks for), and quick turn-around cycles without compromising on high quality. One can see from the foregoing paragraphs the flexibility of these programmable, all position variable speed carriages. They can be used for welding almost all the long joints on the tanks with excellent, consistent bead appearance and x-ray quality welds at high deposition rates, and at welding ‘arc-on’ time cycles up to 80%.
The efficiency and productivity of the programmable, all position, variable speed carriage, using the MIG process against the MMAW process on a 22m thick and 2.5 metre long vertical joint (3G) on a storage tank is shown below:

Comparison of MMAW versus MIG welding with the programmable, all position, variable speed carriage in the vertical (3G) position:

Wire diameter: 1.2mm, Flux Cored Wire.
Considering a shift to be of 8 hrs.
Arc time for MMAW, MIG and Mechanised welding are taken as 18%, 30% and 80% respectively.
A manual welder burns approximately 70-80 of 4 mm rods/shift. = 2kgs weld metal.
A MIG welder deposits 2.6 Kgs weld metal/ hour.
A MIG welder deposits 6.25 Kgs of weld metal / shift at 30% arc time.
Mechanised FCAW deposits 16.6 Kgs of weld metal / shift at 80% arc time.

Summary of deposition rates:

A MMAW welder deposits 2.0 Kgs of weld metal in an 8 hour shift (18% arc time).
A MIG welder deposits 6.25 Kgs of weld metal in an 8 hour shift (30% arc time).
Mechanised FCAW deposits 16.6 Kgs of weld metal in an 8 hour shift (80% arc time).

By

B. Malkani

M.AWS, FEL.I.INST.W. - Technical Consultant, Gullco International Ltd.

bishmalkani <bishmalkani@gmail.com

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Pipe Thickness Chart

noreply@blogger.com (arcraftplasma) — Thu, 20 Oct 2016 19:33:00 +0000

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Destructive testing methods after Welding

noreply@blogger.com (arcraftplasma) — Mon, 17 Oct 2016 10:17:00 +0000

INTRODUCTION

To produce quality weld joints, it is necessary to keep an eye on what is being done in three different stages of the welding

Before welding such as cleaning, edge preparation, baking of electrode etc. to ensure sound and defect free weld joints
During welding various aspects such as manipulation of heat source, selection of input parameters (pressure of oxygen and fuel gas, welding current, arc voltage, welding speed, shielding gases and electrode selection) affecting the heat input and so melting, solidification and cooling rates besides protection of the weld pool from atmospheric contamination
After welding steps, if any, such as removal of the slag, peening, post welding treatment

Selection of optimal method and parameters of each of above steps and their execution meticulously in different stages of production of a weld joint determine the quality of the weld joint. Inspection is mainly carried out to assess ground realties in respect of progress of the work or how meticulously things are being implemented. Testing helps to: a) assess the suitability of the weld joint for a particular application and b) to take decision on whether to go ahead (with further processing or accept/reject the same) at any stage of welding and c) quantify the performance parameters related with soundness and performance of weld joints. Testing methods of the weld joint are broadly classified as destructive testing and non-destructive testing. Destructive testing methods damage the test piece to more or less extent. The extent of damage on (destructive) tested specimens sometime can be up to complete fracture (like in tensile or fatigue testing) thus making it un-useable for the intended purpose while in case of non-destructive tested specimen the extent of damage on tested specimen is either none or negligible which does not adversely affect their usability for the intended purpose in anyways.
Weld joints are generally subjected to destructive tests such as hardness, toughness, bend and tensile test for developing the welding procedure specification and assessing the suitability of weld joint for a particular application. Visual inspection reflects the quality of external features of a weld joint such as weld bead profile indicating weld width and reinforcement, bead angle and external defects such as craters, cracks, distortion etc. only

Destructive Test

TENSILE TEST

Tensile properties of the weld joints namely yield and ultimate strength and ductility (%age elongation, %age reduction in area) can be obtained either in ambient condition or in special environment (low temperature, high temperature, corrosion etc.) depending upon the requirement of the application using tensile test which is usually conducted at constant strain rate (ranging from 0.0001 to 10000 mm/min). Tensile properties of the weld joint are obtained in two ways a) taking specimen from transverse direction of weld joint consisting base metal- heat affected zone-weld metal-heat affected zone-base metal and b) all weld metal specimen as shown in Fig.1 (a, b).

Fig.1.a.Transverse Section of Weld

Fig.1.b.All weld specimen

Tensile test results must be supported by respective engineering stress and strain diagram indicating modulus of elasticity, elongation at fracture, yield and ultimate strength (Fig.2). Tests results must includes information on following point about test conditions.

Type of sample (transverse weld, all weld specimen)
Strain rate (mm/min)
Temperature or any other environment in which test was conducted if any
Topography, morphology, texture of the fracture surface indicating the mode of fracture and respective stress state

Fig.2.Typical stress stain diagram for AA 7039 in as received (BM) and friction stir prcoessed (PM) condition

BEND TEST

Bend test is one of the most important and commonly used destructive tests to determine the ductility and soundness (for the presence porosity, inclusion, penetration and other macro-size internal weld discontinuities) of the weld joint produced using under one set of welding conditions. Bending of the weld joint can be done from face or root side depending upon the purpose i.e. whether face or root side of the weld is to be assessed. The root side bending shows the lack of penetration and fusion if any at the root. Further, bending can be performed using simple compressive/bending load and die of standard size for free and guided bending respectively (Fig.3&4). Moreover, free bending can be face or root bending while guided bending is performed by placing the weld joint over the die as needs for bending is better and controlled condition as shown in Fig. 4.

Fig.3.Schematics of free bend tests

Fig.4.Schematics of guided bend tests a) face bend and b) root bend.

For bend test, the load increased until cracks start to appear on face or root of the weld for face and root bend test respectively and angle of bend at this stage is used as a measured of ductility of weld joints. Higher is bend angle (needed for crack initiation) greater is ductility of the weld. Fracture surface of the joint from the face/root side due to bending reveals the presence of internal weld discontinuities if any.

HARDNESS TEST

Hardness is defined as resistance to indentation and is commonly used as a measure of resistance to abrasion or scratching. For the formation of a scratch or causing abrasion, a relative movement is required between two bodies and out of two one body must penetrate/indent into other body. Indentation is the penetration of a pointed object (harder) into other object (softer) under the external load. Resistance to the penetration of pointed object (indenter) into the softer one depends on the hardness of the sample on which load is applied through the indenter.
All methods of hardness testing are based on the principle of applying the standard load through the indenter (a pointed object) and measuring the penetration in terms of diameter/diagonal/depth of indentation (Fig.5). High penetration of an indenter at a given standard load suggests low hardness. Various methods of hardness testing can be compared on the basis of following three criteria 1) type of indenter, 2) magnitude of load and 3) measurement of indentation.

Fig.5.Schematic diagram showing indentation using different indenters corresponding to different hardness test methods

Penetration due to applied normal load is affected by unevenness on the surface and presence of hard surface films such as oxides, lubricants, dust and dirt etc. if any. Therefore, surface should be cleaned and polished before hardness test. In case of Brinell hardness test, full load is applied directly for causing indentation for measuring hardness while in case of Rockwell hardness test, minor load (10 kN) is applied first before applying major load. Minor load is applied to ensure the firm metallic contact between the indenter and sample surface by breaking surface films and impurities if any present on the surface. Minor load does not cause indentation. Indentation is caused by major load only. Therefore, cleaning and polishing of the surface films becomes mandatory for accuracy in hardness test results in case of Brinell test method as major load is applied directly.
Steel ball of different diameters (D) is used as an indenter in Brinell hardness test. Diameter of indentation (d) is measured to calculate the projected area and determine the hardness. Brinell hardness test results are expressed in terms of pressure generated due to load (P). It is calculated by the ratio of load applied and projected contact area. Load in the range of 500 to 3000 kg can be applied depending upon the type of material to be tested. Higher load is applied for hardness testing of hard materials as compared to soft materials.

In case of Rockwell hardness test first minor load of 10 kg is applied and then major load of 50-150kg is applied on the surface of the work-piece through the indenter and the same is decided by scale (A, B, C and D) to be used as per type of material to be tested. Minor load is not changed. Out of mainly scales, B and C scales are commonly used. Different indenter and major load are required for each scale. Steel ball and diamond cone are two types of indenters used in Rockwell testing. B scale uses hardened steel ball and major load of 90kg whereas C scale uses diamond cone and major load of 140kg accordingly hardness is written in terms of HRB and HRC respectively.
Vickers hardness test uses square pyramid shape indenter of diamond and load ranging from 1 to 120 kg. Average length (L) of two diagonals of square indentation is used as a measure of hardness. Longer is average diagonal length lower is hardness. Vickers hardness number (VHN) or diamond pyramid hardness (DPH) is the ratio of load (P) and apparent area of indentation given by the relation:

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New Welding Procedures for repairing H13 and P20 Tool Steels

noreply@blogger.com (arcraftplasma) — Wed, 12 Oct 2016 11:25:00 +0000

ABSTRACT

AISI H13 and P20 are tool steels commonly used in the manufacturing of polymer injection moulds. P20 is provided by manufactures in the quenched and tempered condition while H13 steel can be used both, in the annealed condition and quenched and tempered state.
Welding technologies are frequently applied to repair polymer injection moulds, namely due to changes in design and alterations due to use.
The application of welding technology in plastic injection mould repair is not simple due to the mould material itself (high alloy steels), to the complex shapes with reduced thicknesses, small tolerances, and hard-to-reach-areas. Welding tool steels moulds is also extremely demanding regarding the mechanical/metallurgical know how and the welder’s skills.
The aim of the present paper is to investigate TIG welding in this particular application, the metallurgical behaviour of AISI P20 and H13 tool steels during welding thermal cycles, and to define welding procedures to repair these tool steels.

INTRODUCTION

Polymer injection moulds frequently require some form of repair by welding: (a) during the manufacture, by errors in machining or by some change in the design of the part to be injected and (b) During the service, by the incidence of failures in the mould. Fusion welding is frequently used to repair worn or cracked surfaces and to modify the shape of existing tools. However, the application of this repair technology is not simple due to complex shapes, hard-to-reach areas, thin blades and small sizes within specified tolerances. Welding in moulds is extremely demanding in what regards the mechanical/metallurgical know how and the welder’s skills. Mould materials are, generally, high alloy steels and the weld deposits must be done in complex geometries, with reduced thicknesses and small tolerances, and access is often difficult.
Despite the fact that equipment’s and filler materials for tool steels welding are available in the market, there is a lack of know-how relatively to the weldability and welding procedures of tool steels (the scientific work published for sheet welding does not contemplate moulds specific requirements).
The required quality of the weld is not only related to the suitable mechanical properties, but also to the weld and the heat affected zone (HAZ) behaviour when finishing processes are applied to provide a uniform and adequate texture to the mould cavity.
The correct use of welding technologies in mould repair can help mould makers to obtain high quality welds and will allow them to implement and spread their use, facilitating re-utilization of the mould with the related cost savings.
The purpose of the present study is to analyze the influence of welding parameters on the metallurgical behaviour of AISI P20 and H13 tool steels, as well as welding procedures for mould repair.

EXPERIMENTAL PROCEDURE

Welds were made with TIG welding process in two base materials (AISI P20 and H13 tool steel). These tool steels are commonly used in the manufacturing of polymer injection moulds. P20 steel was provided in the quenched and tempered condition, with hardness between 30 and 34 HRC, and H13 steel was tested in, both annealed and quenched and tempered states, (with the latest having an hardness between 50 and 52 HRC). The welds were made using a filler wire recommended by the steel manufactures. The composition of the base metals and the filer wires are indicated in table 1.

Table 1. – Chemical composition of base metal and filer wires.

	C	Mn	Cr	Mo	V	Si	W	Cu	Ceq
P20 base metal	0.4	1.5	1.9	0.2	-	-	-	-	1.07
H13 base metal	0.4	-	5.3	1.4	1.0	1.0	-	-	1.94
P20 filer wire (ø 1mm)	0.09	0.95	1.15	0.47	-	0.62	-	0.15	0.58
H13 filer wire (ø 1mm)	0.32	0.29	2.25	-	0.54	0.29	4.12	-	0.93

TIG welding is characterised as a low heat input processes that lead to good quality welds. Beads on plate were made on 24mm thick steel, using four levels of welding current and two welding speeds, as indicated in table 2. Two preheating temperatures were used (250ºC and 350ºC, for 2 hours) and in some trials no pre-heat was done, to account for its influence on the hardness of the weld after welding. The wire feed speed was adjusted in each case to produce satisfactory welds.

Table2. – TIG welding parameters (arc voltage=11V)

Current [A]	Welding speed [cm/min ]	Heat Input [KJ/Cm]
90	15	1.98
100	15	2,20
110	15	2.42
120	15	2.64
90	7	4.24
100	7	4.71
110	7	5.81
120	7	5.65

Figure 1 shows the sequence of bead on pates adopted in this study

Figure 1– Illustration of the single welds welding procedure

Multipass welds were also made with the purpose of investigating the effects of repeated welding thermal cycles in the hardness and microstructure of the welds, using two levels of welding current, 90, and120A, and two welding speeds, 7 and 15cm/min, as shown in figure2.
After welding, some samples were subjected to a tempering post-heat treatment to evaluate the possibility to obtain hardness values in the heat affected zone (HAZ) and weld metal (WM) similar to base metal (BM).

Figure 2 – Ilustration of Multipass welding

After welding the plates were sectioned and samples were prepared for metallographic analysis by grinding, polishing and etching (Nital 2% was used as the etching agent). Hardness profiles in the different regions of the welds were measured using a Vickers microhardness tester with a load of 1 Kgf in selected points as shown in fig 3.

Figure 3 - Illustration of the hardness measurements.

RESULTS AND DISCUSSION

TIG weld deposits made with H13 and P20 tool steels show good superficial homogeneity, without detectable superficial defects, as shown in fig 4. The weld dimensions of TIG deposits width varied between 3.3mm and 6.3mm, while the depth varied between 0.8 mm and 1.9mm, being the higher values associated to higher heat inputs. After etching the samples, it was observed that the heated affected zone was smaller than 1.8 mm.

Figure 4 - TIG weld deposits.

Relatively to the microstructure, differences were observed between P20 and H13 tools steels, so results will be presented separately.

P20 steel:

Microstructural observations and hardness evolution of the samples show that for single weld TIG deposits the Base Metal (BM) has a tempered martensite microstructure, the weld metal (WM) and the Heated affected zone (HAZ) are constituted by martensite with high hardness levels, as shown in fig 5.

Figure 5 - SEM photomicrography of a P20 TIG Weld, showing: a) Base metal, b) Weld metal and c) heated affected zone.

Figure 6 - Mean hardness of TIG single pass welds, for different welding conditions a) with a 250ºC preheating temperature; b) with a 350ºC preheating temperature

The higher values of hardness in the heated affected zone, close to the fusion line, are due to the thermal modification in that region, which consisted of the re-austenitisation of the steel, as well as dissolution of Cr-rich carbides due to the high peak temperature reached followed by martensite formation on rapid cooling.
The weld metal has, however, lower hardness, when compared with the heated affected zone, due to the higher dilution effect and to the lower hardness of the filler metal and the possibly of the presence of retained austenite.
In figure 6 it can also be seen that within the range of parameters used, hardness values are relatively independent of the heat input, and that the variation of preheating temperature does not promote, also, significant changes in the hardness profile.
Figure 7 illustrates the hardness profiles, measured in a horizontal line parallel to the surface, in multipass welds, at one milimeter from the surface. It is observed that the average hardness values measured in the weld metal multipass samples were significantly lower than those of the single pass deposit, due to the tempering of the martensite by subsequent stress relive effects.

Figure 7 - Hardness profile multi pass TIG welds, measured in a horizontal line parallel to the surface, at 1mm from the surface.

However, it is worth to mention that in order to produce a uniform hardness profile it is necessary to make a post-weld heat treatment, as these results indicate that the effect of multipass welding is not sufficient to lower the harness to values correspondent to the hardness of the base metal (300 HV). For the welds which were post-weld heat treated (tempered), it was possible to obtain a mean hardness of 300 HV in both weld metal and heated affected zone, as observed in fig. 8.

Figure 8 - Hardness profile of single, P20 TIG welds after post-weld heat treatment.

H13 steel

In H13 tool steel similar results were observed, indicating the presence of martensite in the heated affected zone and weld metal. However, as far as H13 is concerned the hardness profile shows a region in the heated affected zone, with lower hardness comparatively to the base metal, as shown in fig. 9. This is the result of an over-tempering effect. Unlike P20 tool steel, where cracking can arise in regions of high hardness in the heated affected zone, that could be reduced to values equal to the base metal by a tempering treatment after welding, for H13 tool steel the only way to reduce the hardness is through a complete post-heat treatment of quenching and temper.

Figure 9 - Mean hardness of H13 TIG single pass welds, for two welding conditions.

In order to explain the regions of lower hardness in the heated affected zone of the H13 tool steel, the tempering diagrams from the steels manufactures, as shown in fig. 10, were used. After the quenching and tempering treatment, H13 tool steel presents a range of hardness between 50 and 52 HRC (base metal hardness), which, according to the diagram indicates a tempering at around 550-600ºC. Therefore, material regions that during welding reach a temperature higher than the above mentioned and lower than the austenitization temperature, will suffer an over-tempering, with an inherent decrease in hardness.

Figure 10 - Tempering diagrams a) H13; b) P20

Relatively to the multipass welds, similar results were obtained, where the tempering effects give rise to areas with lower hardness comparatively to the base metal, which can affect the structural integrity of the mould, as illustrated in fig 11.

Figure 11 - Mean hardness of H13 TIG multipass welds, made with a 350ºC preheating.

Figure 12 represents the evolution of hardness for TIG welds made with no pre-heating and with a 350ºC preheating. It can be seen that pre heating has little influence on the hardness profile, being the average hardness slightly lower for the samples that were pre-heated. However, preheating has a beneficial effect in service lifetime since it reduces residual stresses and prevents cracking in service.

Figure 12 - Mean hardness of H13 TIG single pass welds, with no pre heating and 350ºC preheating, for 120A and 7cm/min

Some welding trials were also made in the annealed conditions followed by a quenched and tempered treatment. From the results obtained it can be observed from fig 13, a uniform hardness distribution along the weld metal, heated affected zone and base metal, with a value similar to the base metal. This result indicates that when welding the H13 steel in the annealed state it is possible to obtain after quenching and tempering a similar structure to the base metal. Despite the composition of the filler material being slightly different, its behaviour to heat treatments was similar.

Figure 13 - Mean hardness of TIG single pass welds, made in the annealed condition, which were than quenched and tempered and welds made in the quenched and tempered state.

CONCLUSIONS

Based on the experimental investigation, the following conclusions can be drawn:

P20 tool steel:

The welds present regions with high hardness values compared to the base metal, corresponding to martensitic microstructures. These occur using or not pre-heating. However, in order to avoid risk of material cracking during welding, preheating can be done. In multipass welds, overlapped regions were tempered by repeated welding thermal cycles, lowering the hardness values. However, this procedure did not lower the hardness to base metal values.
The post tempering treatment made in some welded samples has shown that it was possible, with a post-welding heat treatment, to obtain a uniform hardness profile across the weld, even in case of using a filer wire with a composition different from that of the base metal.

H13 tool steels:

The results were similar to the P20 tool steel, indicating the presence of martensite in the heated affected zone and the weld metal. However the heat affected zone has a lower hardness comparatively to the base metal, due to an over-tempering effect.
Opposite to P20 tool steel, where a uniform hardness in weld can be obtained by a tempering treatment after welding. For the H13 tool steel a complete post-heat treatment of quenching and tempering is required.
Based on these results the following procedures should be used in order to obtain good quality welds:
Before welding, the material should be pre-heated, to avoid risk of failure. A pre-heat temperature of 250ºC, during 2 hours, is sufficient to prevent thermal shock without altering material structure.
During welding the heat-input should be adjusted to the dimensions of the zone to repair. For bigger areas to repair, higher heat inputs should be used, in order to increase deposition rate.
After welding a post tempering treatment should be done using a temperature between 600-650ºC for a period of time of 2 hours.
Mould welding repair, if properly done, leads to good mould properties and improved performance. Plastic injection moulding companies and mould makers need to use correct moulding procedures to achieve good results and increase the reliability of the mould, leading to costs savings.

ACKNOWLEGEMENTS

This work was supported by the POCI 2004/FCT/Portugal.

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