Sources of Heat Treatment Distortion and Approaches for Distortion Reduction during Quench Hardening Process

Heat treatment of steels is a process of modifying the mechanical properties by solid phase transformations or microstructural changes through heating and cooling. The material volume changes with phase transformations, which is one of the main sources of distortion. The thermal stress also contributes to the distortion, and its effect gets more significant with solid phase transformations, as the material stays in plastic deformation field (TRIP effect). With the basic understanding described above, the sources of distortion from a quench hardening process can be categorized as: 1) nonuniform austenitizing transformation during heating, 2) nonuniform austenite decomposing transformations to ferrite, pearlite, bainite or martensite during quenching, 3) adding carbon/carbide or nitrogen/nitride to the material during carburizing or nitriding, 4) coarsening of carbide in tempered martensite during tempering, 5) stress relaxation from the initial state, 6) thermal stress caused by temperature gradient, and 7) nonhomogeneous material conditions, etc. With the help of computer modeling, the causes of distortion by these sources are analyzed and quantified independently. In this article, a series of modeling case studies are used to simulate the specific heat treatment process steps. Solutions for controlling and reducing distortion are proposed and validated from the modeling aspect. A thin-walled part with various wall section thickness is used to demonstrate the effectiveness of stepped heating on distortion caused by austenitizing. A patented gas quenching process is used to demonstrate the controlling of distortion with martensitic transformation for high temperature tempering steels. The effect of adding carbon to austenite on size change during carburizing is quantified by modeling, and the distortion can be compensated by adjusting the heat treat part size.