Pressure Dependence of Yielding and Associated Volume Expansion in Tempered Martensite.

The stress-strain behavior of quenched and tempered AISI 4310 and 4330 steels was examined in tension and compression under superimposed hydrostatic pressures up to 160 ksi 1104 Mnsq m. The results showed that yield and flow stresses were sensitive to hydrostatic pressure, but that work hardening and the magnitude of the strength-differential S-D effect between compression and tension were relatively intensitive. Measurements of the volume changes occurring during plastic deformation at atmospheric pressure showed that the plastic volume expansion was larger than that measured in materials which show a smaller S-D effect. They also showed, however, that the plastic volume change was a small portion of the total volume expansion during plastic deformation. The observations on pressure dependence of yielding were used to derive a specific yield condition for these steels, and the observations on volume expansion were then used to test the normality of the plastic-strain-rate vector to this yield surface. Contrary to the widely accepted theory, the results suggest that the total strain-rate vector, rather than the plastic portion, is normal to the yield surface. Author