HEAT-TREATMENT OF INVESTMENT CAST PH 13-8 MO STAINLESS-STEEL .2. ISOTHERMAL AGING KINETICS

The hardening response of investment cast PH 13-8 Mo stainless steel h as been evaluated by hardness measurements following aging in the temp erature range normally specified for this alloy (510-degrees-C to 593- degrees-C). A new relationship between fraction transformed and hardne ss was developed, and analysis of the data in terms of the kinetics of precipitation, in a manner similar to that frequently applied to othe r precipitation-hardenable martensitic steels, yielded low time expone nts and a low value for the apparent activation energy. The values of the time exponents were 0.49, 0.37, 0.56, and 0.53 at 510-degrees-C, 5 38-degrees-C, 566-degrees-C, and 593-degrees-C, respectively, and that for the apparent activation energy was 139 kJ/mole. As has been propo sed for other maraging type steels, these estimates suggest that beta- NiAl precipitates along or near dislocations and that growth of the pr ecipitates is dominated by dislocation pipe diffusion. However, these predictions were neither supported nor refuted by transmission electro n microscopy (TEM) because of difficulties in imaging the beta-NiAl pr ecipitates at the aging times and temperatures used. Further, analysis of the data using the formalism of Wert and Zener for the growth of p recipitates with interfering diffusion fields indicated that the estim ates of fraction transformed from hardness data are not fully appropri ate for maraging type steels. Consideration of the nature of the Avram i analysis and the electron microscopy results suggests that other phe nomena, including dislocation recovery and reversion of martensite to austenite, occur at rates sufficient to convolute the Avrami analysis. It is further suggested that these results cast doubt on the fundamen tal implications of previous analyses of precipitation kinetics in age -hardening martensitic steels. Although the Avrami analysis was found not to provide a tenable description of the precipitation kinetics, it does provide a reasonable methodology for portrayal of the hardening response of PH 13-8 Mo stainless steel.