Kinetics, mechanism and modelling of microstructural evolution during thermomechanical processing of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel

The paper discusses the kinetics, mechanism and modelling of the microstructural evolution of a 15Cr–15Ni–2.2Mo–0.3Ti modified austenitic stainless steel (alloy D9) during dynamic recrystallization (DRX). The experimental methodology included different hot working operations employing industrial equipment such as forge hammer, hydraulic press and rolling carried out in the temperature range 1,173–1,473 K to various strain levels. The kinetics of DRX has been investigated employing modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. It has been found that the value of Avrami exponent varies in a close range of 1.17–1.34 which implies that D9 exhibits growth controlled DRX. Optical metallography has revealed that nucleation of DRX grains occurred along the prior grain boundaries by bulging mechanism. Microstructural characterization has shown that a significant correlation between microstructural features and processing parameters exists. However, this interrelation is ambiguous and fuzzy in nature. Therefore an artificial neural network model has been developed to predict the microstructural features, namely fraction of DRX and grain size, at different processing conditions. A good correlation between experimental findings and predicted results has been obtained. An instantaneous microstructure, therefore, can be designed in order to optimize the process parameters based on microstructural evolution.