Simulation of the Precipitation Kinetics of Maraging Stainless Steels 17-4 and 13-8+Mo During Multi-pass Welding

Recent work has shown that the strength of fusion welds in alloys 17-4 and 13-8+Mo can be restored by re-precipitation that occurs during subsequent thermal cycles associated with multi-pass welding. The purpose of the current investigation is to develop a more detailed understanding of the precipitation kinetics of these materials during times and temperatures representative of welding thermal cycles. Peak aged samples of each alloy were subjected to a series of short isothermal holds at high temperatures using a Gleeble thermomechanical simulator. Hardness measurements were then recorded to estimate the precipitate dissolution. Additional secondary heating experiments were performed and hardness measurements were recorded to estimate the extent of precipitate growth. The hardness data were then used in combination with the Avrami equation and strengthening considerations to develop a relationship between hardness and fraction transformed of the strengthening precipitates. Light optical microscopy and X-ray diffraction were performed on all samples to determine the evolution of the matrix microstructures. The matrix microstructure had minimal effect on the hardness, while the strengthening precipitates were the primary factor affecting the hardness. Apparent activation energies for precipitation, determined through conventional Arrhenius rate analysis at constant fraction transformed, were calculated as 175 and 132 kJ/mol for 17-4 and 13-8+Mo, respectively. Global fitting of the data, based on a simplifying assumption for the temperature dependence of the Avrami rate constant, was also performed in order to improve the practical utility of the analysis, and provided good correlations between estimated and measured hardness over the full range of times and temperatures. Although additional microstructural characterization is needed to understand the significance of these relationships, the kinetic parameters determined from global fitting are useful for practical estimation of strength restoration procedures in multi-pass fusion welds.