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This research was conducted by Yi Liu, who was a visiting PhD student in the advanced manufacturing lab from tongji university, supervised by Junying Min, thanks to a grant from the China Scholarship Council. His work at ETH was supported by Michael Tucker and Prof. Markus Bambach.

Abstract: Austenitization is significant for understanding the microstructure and residual stress evolution in additive manufacturing of non-austenitic steels. However, the accurate modeling of austenite transformation of additively manufactured parts is rarely reported. In this work, a new kinetic model is proposed to describe the diffusional austenitization of additively manufactured 17-4 PH martensitic stainless steel. The proposed kinetic model is based on the classic Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, and incorporates a new austenite grain growth model that accounts for the maximum austenite grain size and the effective driving force for growth. Experimental results obtained through dilatometry show that the proposed kinetic model is able to fit and predict the austenite transformation curves across a wide range of heating rates. This model accurately predicts the saturated austenite fraction by maximum austenite grain size and facilitates the understanding of the effect of heating rate on diffusional austenite transformation behavior. The findings imply that the drag force restricting the maximum austenite grain size originates from the initial martensitic microstructure. The transition from the diffusional austenitization to massive or displacive phase transformation at the heating rate of 100 °C/s is identified for additively manufactured 17-4 PH martensitic stainless steel.

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