Cold working
Cold working is plastic deformation of metals below the recrystallization temperature and is generally performed at room temperature. Some advantages are:
- No heating is required
- Better surface finish and superior dimensional control are achieved
- Strength, fatigue, and wear properties are improved
- Directional properties can be imparted
Disadvantages:
- Heavier forces are required
- Strain hardening occurs (may require intermediate annealing treatment to relieve internal stresses)
- Residual stresses may be produced
For cold working, the ductility and the yield point stress of steel are important. The effect of ductility is shown below:
| Figure37.1: |
Stress strain diagram for low carbon and high carbon steel to understand the suitability of steel for cold working |
In figure 37.1 variation of stress with strain is shown for (A) low carbon steel and (B) high carbon steel. Permanent deformation can not occur until strain is greater than X1. At the other extreme if steel is strained to X4, the metal will fracture. From coldworking point of view the following is important:
- The magnitude of yield stress, which indicates the force required to initiate the permanent deformation and
- The extent of region of strain that is 0 to X4 which determines the extent of plastic deformation
If considerable deformation is required then the tensile properties of steel should be that depicted in figure 37.1A. Greater ductility would be available in the material and less force would be required to initiate and continue the deformation.
High carbon steel which shows stress strain behaviour like figure 37.1B is not suitable for cold deformation but may be suitable for shearing operations
Cold working properties are also affected by the grain size and must be controlled during solidification of steel. Too large and too small grain size have undesirable effects. |
