Characterization of segregations and microstructure and their influence on distortion of low alloy SAE 5120 steel

Investigations within the SFB 570 "Distortion Engineering" showed that shafts produced of the low alloy steel SAE 5120 (EN 20MnCr5) changed curvature into a preferred direction after blank hardening. The changes of curvature could be correlated with a macroscopic structure, which was visible in the metallographic cross section of the hardened shafts. Due to a quantitative description of the orientation of the square structure within the metallographic cross section it could be shown that the changes of the orientation of the structure were similar to the changes of the preferred direction of the curvature of the blank hardened shafts. Metallographic structures in different sections became visible because of differences of the local corrosion behavior. Such differences cause in varieties of the local chemical composition or grain size. Therefore the local chemical analysis was characterized by means of EPMA measurements with highest resolution. Further the grain size was described by quantitative image analysis of the former austenite grain boundaries. Additionally, characteristic areas of the metallographic cross section were investigated by FEM measurements due to the fact that the grain size could be affected by elemental precipitations. The laminar EPMA-measurements (mapping) at blank hardened shafts of the element distributions concerning the substantial alloying elements C, Mn and Cr showed a square structure in the element distribution. Best reproduction of the square structure within the metallographic cross section could be reached by moving averages of the highly resoluted mappings and their standard deviations. Different distributions of the former austenite grain boundary were found within characteristic areas of the metallographic cross section. FEM measurements seem to point on the fact that differences of the former austenite grain size distributions could cause in varieties of the character and distribution of elemental precipitations.