Reversion of Martensite in 304 Stainless Steel Studied by Dilatometry and Calorimetry

Dilatometry has been used to confirm that the reversion of deformation-induced martensite in Type 304 stainless steel occurs principally sin the temperature range 425 to 650°C, and to show that the reversion is a thermal or not diffusion controlled. An attempt to correlate the change in length of cold-drawn 304 wire samples on heating to 800°C with the martensite content determined from the measured saturation magnetization was only qualitatively successful. The heat of reversion of martensite in cold-drawn wire samples was found by differential scanning calorimetry to be in the range 1800 to 2200 J/mol in cold-drawn 304 wires, in reasonable agreement with values for the heat of formation of martensite in 4340 steel determined by a completely different method. We also observe an exothermic reaction near 650°C of a few hundred J/mol which we ascribe to the recrystallization of cold-worked austenite.