Question

Which of the following processes reduces the residual stresses within a steel structure?
1. Hot forming
2. Cold forming
3. Heat treatment
4. Welding

Answer 1

Correct Answer - Option 3 : Heat treatment

Explanation:

Heat treatment and its purpose

• The properties of steel depend upon its composition and its structure.
• These properties can be changed to a considerable extent, by changing either its composition or its structure.
• The structure of steel can be changed by heating it to a particular temperature, and then, allowing it to cool at a definite rate.
• The process of changing the structure and thus changing the properties of steel, by heating and cooling, is called ‘heat treatment of steel’.
• Heat treatment is mainly used to relieve the internal stresses within a metal structure. 

• A few important heat treatment processes are:

  1. Annealing
  2. Normalising
  3. Hardening
  4. Tempering

Hot working or Hot forming:

• Plastic deformation of metal carried out at a temperature above the recrystallization temperature is called hot working.
• Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming. This is called recrystallization.
• In hot working, the temperature at which the working is completed is critical since any extra heat left in the material after working will promote grain growth, leading to poor mechanical properties of a material.
• In comparison with cold working, the advantages of hot working are
  • No strain hardening
  • Lesser forces are required for deformation
  • Greater ductility of material is available, and therefore more deformation is possible
  • Favourable grain size is obtained leading to better mechanical properties of a material
  • Equipment of lesser power is needed
  • No residual stresses in the material

Cold Working:

• Plastic deformation of metals below the recrystallization temperature is known as cold working
• It is generally performed at room temperature
• In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength