TEMPERATURE IN METAL WORKING

Classified into two as Hot Working & cold Working Hot Working: Deformation under conditions of temperature& strain rate such that recovery processes take place simultaneously .  Strain Hardening & distorted grain structure produced by deformation are very rapidly eliminated by the formation of new strain free grains as result of Recrystallisation .  ( Above Recrystallization Temperatures )  Very large Deformation possible because recovery process keep pace with the deformation.  Hot working occurs at an essential constant flow stress & because the flow stress decreases with increasing temperature.  Energy required is much less for hot working than for cold working.  Cold Working : Deformation carried out under conditions where recovery processes are not effective .  ( below Recrystallization temperature )  Strain hardening is not relieved in cold working.  Flow stress increases with deformation . Therefore the total deformation that is possible without causing fracture is less for cold working than for hot working .  Effect of cold working relieved by annealing . It is imp to realize that the distribution b/w cold working & hot working does not depend o any arbitrary temperature of deformation.  For most Alloys Hot working operations must be carried out at an relatively high temperature in order that a rapid rate of Recrystallisation be obtained.  Lead & tin : recrystallizes rapidly at room temperature after large deformation so that it constitutes the Hot working .  Tungsten @ 1100C in the hot working range for steel constitutes cold working because the high melting metal has a Recrystallisation temperature above this working temperature . Temperature of W/p in metal working depends on 1) The intial temperature of the tools & material. 2) Heat generation due to plastic deformation . 3) Heat generated by friction at the die /material interface 4) Heat Transfer between the deforming material & the dies & surrounding environment. For a Friction less deformation process the maximum increases in temp is Td = Up / ρ c = σϵβ/ρ c Where Up = the work of plastic deformation per unit volume ρ = Density of w/p c = Specific heat of W/p β = fraction of deformation work converted into heat . ( remainder is stored with the material as energy associated with the defect structure . The temperature increase due to friction is given by Tf = µpυ A Δt / ρ c V µ= Friction coefficient at material / tool interface p = stress normal to interface υ = velocity at the material / tool interface A = Surface area at the material / tool interface Δt = time interval of consideration V= volume subjected to the temperature rise .  Temperature is highest at the material / tool interface where friction generates the heat and its fall off towards the inside the w/p and into the die. Neglecting the temp gradient & consider the deforming material to be a thin plate between a work piece initially at To & the die at a temperature T 1. . The average instantaneous temp of the deforming material @ Interface is given by T = T1 + ( T0-T1) exp ( -ht/ρcδ) Where h= heat transfer coefficient between the material & the dies δ = material thickness between the dies . ( Equation represents the variation of the average material temperature during cooling of the material , which assumed to be a thin plate cooled between two die surfaces . ) It does not include the temp increase due to deformation & frictions. Average material Temperature at a time t is Tm = Td +Tf + T