Cold Working: INTRODUCTION: Cold working of a metal is carried out below its recrystallisation temperature. Although normal room temperatures are ordinarily used for cold working of various types of steel, temperatures up to the recrystallisation range are sometimes used. In cold working, recovery processes are not effective.
PURPOSE OF COLD WORKING: The common purpose of cold working is given as under
- Cold working is employed to obtain better surface finish on parts.
- It is commonly applied to obtain increased mechanical properties.
- It is widely applied as a forming process of making steel products using pressing and spinning.
- It is used to obtain thinner material.
PRECAUTIONS FALLOWED IN COLD WORKING: Cold working leads to crack formation and propagation if performed in excess and it should therefore be avoided. Residual stresses developed due to inhomogeneous deformation cause warping or distortion when the part is released from the tooling and during subsequent machining. Magnitude and distribution of residual stresses should therefore be controlled. Orange-peel and stretcher strains are material related types of roughness defects found on surfaces not touched by tooling. The former can be avoided by using fine grained sheets and latter is minimized by temper rolling or stretching the strip to prevent localized yielding.
CHARACTERISTICS OF COLD WORKING: The main characteristics of cold working are given as under.
- Cold working involves plastic deformation of a metal, which results in strain hardening.
- It usually involves working at ordinary (room) temperatures, but, for high melting point metals, e.g., tungsten, the cold working may be carried out at a red heat.
- The stress required for deformation increases rapidly with the amount of deformation.
- The amount of deformation, which can be performed without introducing other treatment, is limited.
- Cold rolling process generally distorts grain structure.
- Good surface finish is obtained in cold rolling.
- The upper temperature limit for cold working is the maximum temperature at which strain hardening is retained. Since cold working takes place below the recrystallisation temperature, it produces strain hardening.
- Excessive cold working gives rise to the formation and propagation of cracks in the metal.
- The loss of ductility during cold working has a useful side effect in machining.
- With less ductility, the chips break more readily and facilitate the cutting operation.
- Heating is sometimes required.
- Directional properties can be easily imparted.
- Spring back is a common phenomenon present in cold-working processes.
- For relatively ductile metals, cold working is often more economical than hot working.
There is some increase and some decrease in properties of the cold worked part, which are given as under.
Cold working process increases:
• Ultimate tensile strength
• Yield strength
• Fatigue strength
• Residual stresses
Cold working processes decreases:
• Percentage elongation
• Reduction of area
• Impact strength
• Resistance to corrosion
LIMITATIONS OF COLD WORKING
- The cold worked process possesses less ductility.
- Imparted directional properties may be detrimental
- Strain hardening occurs.
- Metal surfaces must be clean and scale free before cold working.
- Hot worked metal has to be pickled in acid to remove scale, etc.
- Higher forces are required for deformation than those in hot working.
- More powerful and heavier equipments are required for cold working.
ADVANTAGES OF COLD WORKING:
- In cold working processes, smooth surface finish can be easily produced.
- Accurate dimensions of parts can be maintained.
- Strength and hardness of the metal are increased but ductility decreased.
- Since the working is done in cold state, no oxide would form on the surface and consequently good surface finish is obtained.
- Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations.
- There is no possibility of decarburization of the surface
- Better dimensional accuracy is achieved.
DISADVANTAGES OF COLD WORKING:
- Some materials, which are brittle, cannot be cold worked easily.
- Since the material has higher yield strength at lower temperatures, the amount of deformation that can be given to is limited by the capability of the presses or hammers used.
- A distortion of the grain structure is created.