In metal forming (especially in hot working), there are various factors that affect the strength of the metal or alloys.
Here I have given you a short overview of the effect of strain hardening and strain rate on metal forming.
Effect of strain hardening on metal forming
First of all if you don’t know what strain hardening is, then let me tell you something about it.
Strain hardening (also known as work hardening) is a process in which a metal is deformed permanently in order to increase its resistance to further deformation.
In simple words, the material which undergoes strain hardening, increases its strength by plastic deformation.
The strengthening of the metals is due to the dislocations that occur within the crystal structure of the material.
Generally non-brittle metals that are having higher melting points can be strengthened using strain hardening process (or work hardening process).
There are some metals which can not be strain hardened at room temperature (for example, Indium). While many other metals like copper, aluminum, etc can be strain hardened at room temperature.
The effect of strain hardening is beneficial for cold working of metals in which the strength of the metals increases due to its plastic deformation at room temperature.
Thus, due to strain hardening, the strength of the metals increases.
Effect of strain rate on metal forming
Strain rate is the rate at which the metal forming process is carried out.
The main effect of strain rate on metal forming is that it increases the load on forming equipment.
If the rate of forming process increases, the yield strength of the material increases which leads to higher load on the equipment.
There are many factors that affect the metal forming process up to some extent depending upon the temperature at which the metal forming is carried out.
If a metal forming is carried out slowly and below the recrystallization temperature, then such a case can be taken as an isothermal process, in which the effect of temperature change is neglected and only the effect of strain hardening is considered.
And if the metal forming is carried out at a temperature above the recrystallization temperature, then the effect of strain rate as well as strain hardening is considered.
The effect of strain rate and strain hardening is different on different metals at different temperature ranges.
So testing of each metal and alloy at different temperatures is necessary.
At low strain rates, the yield strength increases with increases in strain rate.
But at higher strain rates, it still increases but at a slower rate. This is because of the softening effect due to rise in temperature of the metal.