Deformation process for materials

Deformation is a process to transform solid materials from one shape into another. The initial shape is simple and the material is plastically deformed between tools, or dies, to obtain the desired final geometry. Deformation process, like machining, have been core of modem mass production, because they involve metal flow and it do not actually depend on a long-term metallurgical rate processes.

Cold work

A metal is said to be cold worked if the metal is processed below the re-crystallization temp. of the metal mechanically thus the grains of that work are in a complete distorted condition after plastic deformation in that work is completed.

During cold working residual stress are build up. As their presence is usually undesirable a suitable heat treatment is generally carried out to neutralize these unwanted stresses and restore the metal configuration to its original structure

The various types of cold working operations are:

1. Drawing: Itis an operation in which the cross-sectional area of a bar or any rod or any wire is reduced by pulling it through a die opening.

2.Bending:It is a sheet-metal operation which is defined as the straining of the metal around a straight axis. In this process, the metal inside of the given plane is compressed, while the metal on the outside of the is stretched outwardly .

3. Shearing: It is a sheet-metal cutting operation along a straight line between two cutting edges.

4.Hobbing: It is a deformation process in which a hardened steel form is pressed into a soft steel (or other soft metal) block.

5.Peening: It is the process of working a metal's surface to improve its material properties, usually by mechanical means such as hammer shots or blow, by blasting with shot peening or blasting of light beams with laser peening.

6.Cold Extrusion: It is a compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. Process can be likened to squeezing toothpaste out of a toothpaste tube This method is done at room temperature or near room temperature.

Strain hardening and Residual stress

Work (or strain) hardening is defined as a structural phenomenon that results in an increase in hardening and strength of a metal or any work when subjected to plastic deformation at temperature lower than the re-crystallization temperature ,When any (work) material is subjected to plastic deformation ,external work is to be done, a certain amount of this work done on material is stored as strain energy. This stored energy in a crystal results in strengthening or work hardening of work .

Study has shown, small portion of the stress-(usually measured 10%) is stored in the form of residual stresses within the structure. The value of unwanted stresses increase the total energy of the system. Thus developed residual stresses generated by plastic deformation may not always be required to us and can be released by a heat treatment process called as stress-relief annealing. In some cases however residual compressive stresses at surface of work is generally created to increase their mechanical properties.

Properties versus degree of cold work

Work hardening is a phenomenon which reduces ductility and plasticity. so this process is used in many manufacturing processes example (rolling of bars and drawing of tubes). During this process,T ensile strength, hardness, and yield strength are increased, while the value of ductility is decreased. Although both strength and hardness increase, however, the rate of change of both is not the same.

Treatment after Cold Work

Annealing

As a process of cold working, the value of hardness, electrical resistance, and tensile strength increase, while ductility decreases. There is also a large increase in the number of dislocations, and certain planes in the crystal structure are extremely distorted.

Full annealing is defined as the process by which the distorted cold worked lattice structure is changed to a form which is strain free. It is carried through the application of heat. Annealing process is carried out entirely in the solid state and is usually followed by slow cooling in the furnace from the desired temperature. The process of annealing may be divided into three stages:

1. Recovery

This is first stage and primarily a low temperature process, and the property changes during this stage produces no appreciable change in microstructure or the properties, such as yield strength, hardness, tensile strength, and ductility of material. The principal objective of recovery process is the relief of internal stresses due to cold working process.

This low temperature treatment of materials in the recovery stage is known as stress relief annealing or process annealing.

2. Re-crystallization

As the temperature of the recovery stage is reached, very tiny new nano-crystals appears in the microstructure. So formed new crystals have the same composition and lattice , crystal structure as the original un-deformed or true grains The grains are not elongated but are uniform in dimensions, symmetric.

As a result of re-crystallization stage, there is a significant drop in yield strength, hardness ,tensile strength and a large increase in the ductility of the material.

3. Grain Growth

During this stage the value of tensile strength, yield strength and hardness continue to decrease but it is at a less rate than the re-crystallization stage. The considerable change observed during this stage is characterized by the growth of the grain and regaining the original grain size, original configuration.

Hot work process and its types

Hot working process refers to work processes where metals are first plastically deformed above their re-crystallization temperature. Hot working process actually being above the re-crystallization temperature, this allows the material to re-crystallizes after deformation. This process is important because re-crystallization phenomenon keeps the materials from strain hardening, which solely maintains the yield strength and hardness low and ductility high after the end of process.

The various hot working operations are:

1. Rolling: It is defined as a deformation process in which the thickness of the work is decreased by compressive forces maintained by two opposing rollers.

2.Forging: It is also a hot working deformation process such that work is compressed between two dies with high pressure, using either impact or increasing pressure to form the part. A machine that applies impact an impact (sudden) load is called a forging hammer, conversly machine that applies gradual pressure is known as a forging press.

3. Pipe Welding: it is the process of joining metal pipes by heating them at certain high temperature near melting point and joining them together.
4. Rotary piercing: It is also a hot working metal working process for producing thick-walled seamless tubing.

4.Hot Extruding: It is a hot working process, which means this process is done above the work's re-crystallization temperature. Work is forced through the die opening at elevated temperature.

Some of the advantages of hot-working process are given below:

1. Decrease invalue of yield strength, so it becomes easier to work and uses less energy or force.
2. Increase in ductility of work.
3. high temperatures increase diffusion which can remove or reduce chemical in homogeneities.
4. Pores or holes may reduce in size or get completely closed during deformation.

Comparison between Cold Working and Hot Working

Cold Working

1.It is done at a temperature below the value required for re-crystallization, so no appreciable recovery takes place during deformation process.

2. Hardening of work is not eliminated as working is done at a temperature below re-crystallization, so work always contains strain hardening.
3. Cold working phenomenon decreases the value of elongation, reduction area and impact values.
4. Embrittlement do not occur due to less diffusion and there is no reaction of oxygen at lower temperature.
5. Surface decarburization (loss of carbon from surface) in steels does not occur.

Hot working

1. It is done at a temperature above re-crystallization temperature, so it can be considered as a simultaneous occurrence of deformation and recovery process both.
2. Hardening due to plastic deformation is completely removed by recovery and re-crystallization. This is true, however, only if the rate of crystallization is higher than rate of deformation.
3. Mechanical properties like elongation, reduction of area and impact values are improved.
4. Reactive metals get extremly embrittled by oxygen and hence they must be protected from the action of oxygen by using inert atmosphere.
5. Surface decarburization is familier to occur at higher temperatures unless protected by a proper inert atmosphere.