Understanding Principles of Quench Distortion of Carburizing Steel Parts using a Sigle Side Carburized Almen Strip and Computer Modeling

Quench hardening of steel components is a complex problem because multiple factors are involved in the process, including the part geometry, quench uniformity and phase transformations, etc. The quench hardening of carburized steels is considered as a black box in general by the heat treatment industry mainly due to the complexities of microstructural changes, which are affected by the variation of chemical composition for a given steel grade, carbon profile from case to core, form and size of carbides, and quenching rate. To simplify the problem, a single side carburized Almen strip made of Pyrowear 675 is used to investigate the effect of phase transformations on distortion in this study. The Almen strip is low pressure carburized (LPC) for only one side by copper plating all the other surfaces. The conventional process for Pyrowear 675 is applied, including carburizing, double normalizing, quench hardening, deep freeze and tempering. The temperature gradient in the Almen strip during high pressure gas quench (HPGQ) is small due to its thin section thickness, and the thermal stress is negligible. The volume (strain) change caused by carburizing and phase transformation is considered as the main cause of bow distortions observed at different processing stages. Material models and material data required to describe the part responses during the heat treatment process steps are described. Computer modeling by DANTE is used to analyze the collected experimental data and demonstrate the principles of distortions and residual stresses.