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Steel Fabrication

2012-07-23T12:20:00.001+05:30

Metal fabrication :- Metal fabrication is the fabrication of metal by cutting, bending, and assembling (by welding, binding with adhesives,riveting, threaded fasteners,etc.) processes.

Structural steel and sheet metal are the usual starting materials for fabrication, along with the welding wire, flux, and fasteners that will join the cut pieces.

1.FABRICATION PROCEDURES

1. Workshop Layout

Fabricators range from small general firms to large specialized producers with different facilities at their disposal. In either case the fabrication must always be organized in such a way that the material will pass through a one-way system from receipt to final dispatch. A flow cart, as indicated in the main areas of activity in a modern fabrication shop; the specific activities for a simple steel beam can also be organized as a production line.

Most fabrication shops are equipped with overhead travelling cranes, sometimes remotely controlled from the shop floor. Mechanized conveyor systems are common in the larger shops. They can greatly reduce handling costs.

2. Material Handling and Preparation

Material is taken into temporary stock in such a way that it can be easily identified and moved. Some companies stack the material for easy access and move it by using cranes equipped with chains and hooks. Other companies use a high degree of automation in their material handling, using cranes on conveyors with magnetic lifting devices; Slide 4, for example, shows a travelling Goliath Magnet Crane with the capacity to lift both plates and sections. Computerised records hold details of member sizes, lengths, weights and steel quality, all related to an identification mark.

3. Templates and Marking

Steel may be marked directly by hand with scribe lines and hole centres; nowadays, however, in most shops pre-programmed automatic plant is in use. Traditionally, full-sized templates, made of timber or heavy cardboard, were used to mark the steel for cutting and for centre popping where holes were to be drilled.

4. Sawing Line and Rolled Sections

The rolled sections are in most cases sawn to length, the other options being mechanical cutting or flame burning. Three types of saws are available to the fabricator:

Circular saw;
Band saw;
Motor operated hacksaw.

5.Drilling and the Beam Line System

The traditional method of drilling involves three operations:

Marking the position of the holes to be drilled;
Moving the member to the drill by crane, by conveyor, or by other means;
The actual drilling of the hole, using for instance, a radial drilling machine (radius about 1,5 metre).

Like the sawing line, this system is controlled by computer programs; some machines are equipped with multiple drilling heads enabling them to drill several holes simultaneously in each axis.

New twist drills are currently available which are capable of higher speeds and greater efficiency as follows:

Coolant fed drills, giving a threefold increase in drilling speed.
Titanium nitride coated drills, enabling a six-fold speed increase.
Carbide tipped drills with exceptionally high cutting speeds.

6.Punching

Punching holes in steelwork is much faster, and therefore less costly, than drilling; its use, however, is generally limited to predominantly statically loaded structures with limited thickness, or to secondary members, unless HSFG bolted connections are used or the holes are reamed out to a larger size. The maximum thickness where punching is applicable depends on the material grade and quality.

7.Pressing and Forming

For the modern fabricator the most important application of plate forming and pressing is to add to the available range of rolled sections. The trapezoidal shaped trough used to stiffen bridge decks, is a very good example. Other examples are the circular sections of larger than standard dimensions.

8.Methods of Welding

Three welding processes are most commonly used in modern fabrication shops:

Manual Metal-Arc Welding for fittings and for some profile and positional welding.
Metal Active Gas Welding (MAG) and Cored Wire Welding with and without gas.
Submerged Arc Welding for fully automatic processes; particularly useful for heavy welding in the flat or horizontal-vertical position and for the long-run welds in plate and box girders.
uses laser welding, etc..

9.Machine Operations

Most fabrication shops are equipped with facilities for edge planing, for end milling and for surface machining of plate.

Unacceptable levels of hardness at the edge of the plate, often caused by burning, can be removed by planing.

End planing of members is used to get a higher standard of squareness than can be achieved by sawing. Optical laserbeam methods are used to align the axis of the member to the cutting head.

Surface machining is only necessary for special bearing surfaces and sometimes for the slab base plates of columns.

10. Fabrication Tolerances

Modern fabrication shops have accurate dimensional control over fabricated sections and have no problems in cutting the rolled material to length. The main problem is coping with the deviations in the sections and plates received from the steelmills. Euronorm (CEN) and ISO standards give dimensional tolerances for rolled sections, plates and flats, hollow sections and angles respectively. The fabricator will use bending rolls to straighten the material and to "square" flanges of beam sections at critical connection points. As already mentioned, the control of distortion due to The details and the connections must be designed in such a way that the tolerances will be met within the limits of good workmanship.

11.Inspection and Quality Control

Quality Control should commence with the designer and continue through the preparation of drawings and material procurement; maintaining the quality during the entire production process will depend heavily on the fabrication details and on the material obtained.

The larger fabricators have their own Quality Control Department, which will create and maintain a QC-manual, describing the method of operation throughout the fabrication process. The Quality Control Department will liaise with the shop management to make sure that all workers have the skill required for the job on hand and that welders are qualified to undertake the prescribed welding procedures.

Regular checks are necessary to ensure that:

All materials can be checked against specifications.
Material is checked for laminations.
Welding electrodes are identifiable.
Welding electrodes are stored in the required conditions.
Welding procedures are being followed.
Welding is being inspected during the process.
Correct procedures are in operation for tightening HSFG bolts.
Identification marks are clear and visible.
All equipment is maintained correctly.

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2011-05-26T20:39:00.000+05:30

CRYSTAL IMPERFECTIONS

INTRODUCTION- The perfectly regular crystal structures that have been considered up to now are called ideals crystal in which atoms were arranged in a regular way. An ideal crystal can be described in terms a three-dimension ally periodic arrangement of points called lattice and an atom or group of atoms associated with each lattice point called motif:

Crystal = Lattice + Motif

However, there can be deviations from this idealistic.

These deviations are known as crystal imperfection. These imperfections affect the properties of crystal, such as mechanical strength, chemical reaction, electrical properties, etc. to a great extent.

TYPES OF IMPERFECTION:-

1. Point Imperfections

2. Line Imperfections

3. Surface And Grain Boundary Imperfections

*Defects*	*Dimensional*	*Examples*
Point defects	0	Vacancy
Line defects	1	Dislocation
Surface defects	2	Free surface, Grain boundary

(1):- POINT IMPERFECTIONS: - In some cases, when atoms missing or in irregular places in the lattice (vacancies, interstitial, substitution).These imperfections are always present in crystals and their presence results in a decrease in the free energy. The number of defects at equilibrium concentration at a certain temperature can be compound as;

n = Ne^-Ed/kT

n = Number of imperfections

N = Number of atomic sites per mole

K = Boltzmann’s constant

Ed = the free energy required to from the defect

T = Absolute temperature

(a)- Vacancies- A lattice position that is vacant because the atom is missing. It defect may arise due to increased thermal energy causing individual atoms to jump out of their position of lowest energy. The thermal vibrations of atoms increase at high temperatures. The vacancies may be single or two or more may condense into a di-vacancy or tri-vacancy. Vacancies exist in a certain proportion in a crystal at thermal equilibrium, leading to an increase in randomness of the structure.

The equilibrium number of vacancies formed as a result of thermal vibrations may be calculated from thermodynamics:

Where Ns is the number of regular lattice sites, k_B is the Boltzmann constant, Q_v is the energy needed to form a vacant lattice site in a perfect crystal, and T the temperature in Kelvin (note, not in ^oC or ^oF).

Using this equation we can estimate that at room temperature in copper there is one vacancy per 1015 lattice atoms, whereas at high temperature, just below the melting point there is one vacancy for every 10,000 atoms.

(b)- Interstitial - An atom that occupies a place outside the normal lattice position. It may be the same type of atom as the others (self interstitial) or an impurity interstitial atom.

(c)-Substitutional Impurity- It may be produce compositional defects in the crystal structure. When impurity in the form of the foreign atoms occupies lattice sites where regular atoms are missing, they produce substitutional impurity.

(d)- Frankel Defect- A frankel defect is closely related to interstices. An ion displaced from the lattice site into an interstitial site is called a Frankel Defect. Closed packed structures have fewer intestacies & Frankel Defects because additional energy is required to force the atom into a new position.

(e)-Schottky Defect- This is closely related to vacancies and is obtained when an atom or ion is removed from a normal lattice site and replaced by an ion on the surface of the crystal. Both vacancies and Schottky Defects facilitate atomic diffusion.

(2):- LINE IMPERFECTIONS / DISLOCATIONS: - A linear

Disturbance of an atomic arrangement, which can very easily occur on the slip plane through the crystal, is known as line dislocation. It is two dimensional line defects & it may also be concluded that be is the region of localized lattice disturbance separating the slipped and unslipped regions of a crystal.

These are formed in the process of solidification of metal and mainly in their plastic deformation of strain hardening. Yield point, creep and fatigue and brittle fracture.

Causes of dislocation are:

1. thermal stress or external stresses causing plastic flow

2. crystal growth

3. phase transformation

4. Segregation of solute atoms causing mismatches.

There are two types of dislocation:

(a) Edge Dislocations

(b) Screw dislocations

(a) Edge Dislocations:- An edge dislocation formed by adding an extra partial plane of atoms to the crystal. The poison of the dislocation line is marked by the symbols ┴ and ┬ indicating the involvement of extra planes from the top (positive sign) and bottom (negative sign) of the crystal, respectively. The vertical line of the symbol ┴ points in the direction of the dislocation line in the extra partial plane. The dislocation line is a region of high energy than the rest of the crystal. The lattice above the dislocation line is in a state of compression, whereas below this line, the lattice in tension.

To describe the size and the direction of the main lattice distortion caused by a dislocation we should introduce so called Burgers vector b. To find the Burgers vector, we should make a circuit from atom to atom counting the same number of atomic distances in all directions. If the circuit encloses a dislocation it will not close. The vector that closes the loop is the Burgers vector b. Dislocations shown above have Burgers vector directed Perpendicular to the dislocation line.

(a) Screw Dislocations:- the formation of a screw dislocation

by a perfect crystal and a plane cutting part way through it

are also shown. The geometry of the screw dislocation has an interesting effect on the solidification process

A Screw dislocation has its displacement or Burger vector

Parallel to the linear defect but there is a distortion of the plane. In this the atoms are displaced in two separate plane perpendicular to each other and the distortion follows a helical or screw path, both right hand and left hand screw are possible. In these types of dislocation, shear stresses are associated with adjacent atom and extra energy is involved along the dislocation. A screw dislocation does not exhibit climb motion.

These effects of a screw dislocation are of great importance:

1. The force required to form and move a screw dislocation probably somewhat greater than that required to initiate an edge dislocation.

2. Plastic deformation is possible under low stress, without breaking the continuity of the lattice.

3. Screw dislocation causes distortion of the lattice for considerable distance from the center of the line and takes the from of spiral distortion of the planes. Dislocation of both types (combinations of edge and screw) are closely associated with crystallization as well as deformation.

Dislocation of both types (Mixed/partial dislocations)

In general, there can be any angle between the Burgers vector b (magnitude and the direction of slip) and the line vector t (unit vector tangent to the dislocation line)

b ^t ÞEdge dislocation b ççt ÞScrew dislocation

(3) SURFACE IMPERFECTIONS: - these defect are two –dimensional and due to a change in the stacking of atomic planes on or across a boundary. Such imperfection may include external & internal defects (grain boundary, tilt boundary, twin boundary etc.)

External free surface: - the external surface of the material is an imperfection itself because the atomic bonds do not extend beyond it since these surface atom are not entirely surrounded by other, they posses higher energy than internal atoms. Surface atoms have neighbors on only one side while atoms inside the crystal have neighbors on both sides.

Internal Surface:-

(a) Grain Boundary:- Polycrystalline material comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called grain

Boundaries.

Surfaces and interfaces are reactive and impurities tend to

Segregate there. Since energy is associated with interfaces, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

Add caption

(b) Tilt Boundaries: - Low angle grain boundary is an array of aligned edge dislocations. This type of grain boundary is called Tilt Boundary (consider joint of two wedges)

Transmission electron microscope image of a small angle tilt boundary in Si. The red lines mark the edge dislocations; the blue lines indicate the tilt angle.

(c) Twin Boundary: - Space imperfections which separate two like orientations and look like mirror image of each other are called Twin Boundary.

Low-energy twin boundaries with mirrored atomic positions across boundary may be produced by deformation of materials. This gives rise to shape memory metals, which can recover their original shape if heated to a high temperature. Shape-memory alloys are twinned and when deformed they untwine. At high temperature the alloy returns back to the original twin configuration and restore the original shape.

(d) Stacking faults: - This type of imperfection may arias where there is only a small dissimilarity between the stacking sequences of close packed planes in FCC and HCP. It is possible for one atom layer to be out of sequence relative to the atom of the layer above and below, giving a fault.

For example, the stacking sequence of an ideal FCC crystal may be described as ABC ABC.... but the stacking fault may change the sequence to ABC ACAB.

REFRENCE

· Material science by G.K. Narula,V.K.Gupta,

· Lecture 5 of Rick Holt, Queen’s University, Canada,

· From internet,

· From classnotes,

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