Effect of Cu addition on solidification, precipitation behavior and mechanical properties in austenitic CrNi–N stainless steel

Abstract

Solidification and segregation behavior of a novel X3CrNiCuN17-6-4 (concentrations in wt.%) cast steel was studied by thermodynamic calculations and corresponding microstructure investigations. In addition, resulting mechanical properties were examined by tensile tests in a temperature range of −60 to 200 °C and compression tests were performed with varying strain rates ranging from 1 s−1 to 10−4 s−1 at room temperature. The investigated steel showed an anomalous temperature dependence of tensile elongation, which is a typical characteristic of high alloy TRIP/TWIP steels. In the as-cast state before heat treatment, a few micrometer-sized copper precipitates were visible, which were exclusively located in the interdendritic regions. A solution annealing was performed at 1050 °C for 30 min to dissolve nitrides and carbides likely to be present in the as-cast microstructure. However, in areas with a high copper content further nanometer-sized copper precipitates were formed after heat treatment. EDS point measurements and EPMA were used to determine the chemical composition of dendrites, interdendritic regions and copper precipitates before and after solution annealing. While the enrichment of chromium and copper in interdendritic regions could be verified by the thermodynamic calculations, the enrichment of nickel in interdendritic regions contradicted the predictions according to Scheil calculation.