Simulation of Segregation-Induced Distortion During the Heat Treatment of a Case-Hardening Steel SAE 5120

Segregations are chemical inhomogeneities which form during continuous casting. At the subsequent rolling and forming these segregations were deformed strongly. During the final heat treatment segregations can induce a distortion of work-pieces. Due to physical reasons segregations are unavoidable during continuous casting. The position and direction dependency of micro- and macro-segregations within the rolled bar were characterized by using EPMA and metallography. By dilatometry the position and direction dependency of the strain and transformation behavior was determined. From these experiments a model for the influence of segregations on the distortion during heat treatment simulations was development. The segregation-induced distortion after the heat treatment was determined at simple cylindrical shafts (?20 mm x 200 mm) to avoid a geometry-induced distortion. Heat treatments were slow cooling within the furnace, blank hardening and case hardening. A heat treatment as symmetric as possible was applied to avoid a heat treatment-induced distortion. The length change and the curvature of the shafts were analyzed to characterize the distortion. It is shown that the simulation results can reproduce the experimental results of the shafts, anisotropic volume change as well as curvature. The agreement between simulation and experiment shows, that the anisotropy in the strain due to segregations explain the observed distortion and that for prediction of distortion of industrial work-pieces the anisotropy in the volume change has to be regarded. One heat of SAE 5120 was used for all experiments for the determination of segregation-induced distortion. Additionally a segregation-free spray-formed SAE 5120 was used for comparison.