Hot and cold working; temperature and strain rate

Hot working is plastic deformation above the recrystallisation temperature (about $0.5T_m$ in kelvin; hot working is usually done at $0.5$-$0.75T_m$). Cold working is deformation below it (usually near room temperature), so the metal strain hardens.

Differences

Effect of temperature

Flow stress falls as temperature rises because of thermal softening and recrystallisation. Above the recrystallisation temperature the strain-hardening exponent is nearly zero, and the forming load drops by a large factor. Too high a temperature causes burning or grain growth.

Effect of strain rate

Flow stress depends on strain rate $\dot{\varepsilon}$ as

$$\sigma = C\,\dot{\varepsilon}^{\,m}$$

where $m$ is the strain-rate sensitivity exponent: about 0.01 or less for cold working and 0.05-0.4 for hot working. In hot working, a faster press or hammer therefore needs a higher load. The strain rate in forging is $v/h$ (ram velocity divided by the instantaneous height).

Effect of friction

• Friction between the work and the die opposes the flow at the interface; it raises the forming force and power.
• In upsetting, it causes barrelling and a "friction hill" of pressure that is highest at the centre.
• In rolling, friction pulls the metal into the roll gap, but too much friction raises power and roll wear. The condition for entry is $\mu \ge \tan\alpha$.
• It is controlled by lubricants (graphite, oil, glass) and by the die finish.

Friction therefore has the most influence when the contact area is large compared with the thickness, as in flat rolling of thin sheets and forging of thin discs.