Strength and structure under hot-working conditions

Abstract

The main feature of hot working is that extremely large strains are applied to materials at high rates of strain at temperatures above ∼ 0.6Tm, where Tm is the melting temperature in degrees Kelvin. Strength and ductility under these conditions are markedly dependent on both temperature and rate of straining. Although this review is confined to strength and structure during hot working, ductility is intimately related to the deformation processes that govern plastic flow. This aspect has been recently reviewed by one of the authors. These large strains can be achieved with little or no strain-hardening, indicating that dynamic softening processes can operate sufficiently rapidly to balance the strain-hardening processes. In this situation, the structural changes involved can be used to obtain information on the mechanism of deformation. However, as emphasised later, care must he taken in the interpretation of such hot-worked structures, since significant structural changes can occur on holding at the temperature of deformation or on cooling to room temperature. These post-deformation changes can be important in practical situations involving interrupted deformation. Thus, in contrast to extrusion in which the total strain is applied at roughly constant temperature, forging and rolling involve a series of operations in which the total strain is applied in increments at decreasing temperatures.