The influence of material uniformity on the heat treat distortion of a steel ring

The effects of material uniformity on the heat treatment distortion of a carburized 1Cr-0.3C steel ring have been investigated. The ring dimensions were 76 mm outside diameter and 6.4 mm wall thickness. This geometry was mathematically modeled, changing the composition and related material properties in localized regions to determine the effect that segregation has on the shape during heat treatment, as follows: (1) The geometry was two-dimensional, representing one fourth circumference, with a mesh of 16 elements in the thickness direction, and 16 elements along the circumferential direction. The surface layer and subsurface layers of elements on both surfaces were 0.5%C and 0.4%C, respectively, representing a case-carburized layer. Elements inside the case were "core" elements which were varied in composition. (2) The ring surface temperature history was determined experimentally, was used for all surface nodes. (3) The temperature histories of interior nodes were calculated via the FEM code ABAQUS. (4) A separate mathematical model was developed to determine the effects of small composition changes on transformation behavior, primarily for core elements with compositions differing from base. (5) The transformation behavior was incorporated into coefficient of thermal expansion versus temperature tables. (6) The effects of transformation plasticity were accounted for by means of the "material property modification" method. The material model was elastic-linear plastic, assuming fully kinematic hardening. (7) The stress displacement solution at each node was calculated by numerical solution of the incremental virtual work equation in the FEM code ABAQUS.; A series of models were run to simulate the effects of localized material composition variation. The results show that local variation of carbon composition in the range of 0.01 to 0.02 weight percent can cause heat treatment distortion as large as {dollar}{lcub}sim{rcub}120 mu{dollar}m, depending on the severity and spatial distribution of the segregation present. The mechanism by which chemical segregation causes distortion is the imbalance of transformational stresses during quenching. In general, positive segregation near the quarter radius region of the ring appears to be most effective for creating a large shape change.