Evaluation of the Hardening Behavior and Precipitation Kinetics of Precipitation Hardened 13-8+Mo Stainless Steel Using Avrami Relationships and a Global Fitting Procedure

The purpose of the current investigation was to demonstrate the utility of the Avrami relationships and a global fitting procedure by applying it to precipitation hardened (PH) 13-8+Mo stainless steel (SS) hardness datasets taken from literature and calculating overall activation energies (QO) and growth constants (k1 and n′) for precipitation in this system. Additionally, this work evaluated assumptions concerning the significance of heat treatment hold times relative to heating and cooling cycles for short, welding-like, heat treatments using an additivity rule-based model. Data from four sources were evaluated using the global fitting procedure. These sources incorporated a significant range of PH 13-8+Mo SS chemical compositions, included both cast and wrought material, several solution treatment conditions, and a range of aging procedures. The method was applied to each dataset individually as well as all datasets combined and activation energies and growth constants were determined. All fits produced using this technique provided reasonable 1:1 correlations between measured and calculated hardness values. Application of this method to individual datasets produced k1 values between 1.7e3 sec–n′ and 2.0e5sec–n′, n′ values between 0.40 and 0.57, and QO values between 150 kJ/mol and 199 kJ/mol. Application of this method to all the datasets combined produced k1, n′, and QO values of 8.7e4 sec–n′, 0.38, and 234 kJ/mol, respectively. All values were in good agreement with values from literature for either lattice diffusion or dislocation pipe diffusion of the precipitating elements in PH 13-8+Mo SS. Further characterization is required to determine whether these values represent a fundamental significance. However, it was demonstrated that the global fitting procedure provides a reasonable and practical approximation of the hardening behavior for numerous PH 13-8+Mo SS datasets that have been exposed to a range of processing and heat treatment conditions.