Investigation of precipitation in a new maraging stainless steel

A new type of maraging steel with the composition 12Cr9Ni4Mo2Cu (wt%) was developed by AB Sandvik Steel. According to the analytical electron microscopy (ATEM) investigations preceding this study, the high strengh (3000 MPa) of the material was assigned to a new quasicrystalline phase possessing icosahedral symmetry and of typical composition 35Mo40Fe16Cr2Ni7Si (at%). However, only precipitates after prolonged heat treatment could be successfully studied using ATEM technique. To better understand early stages of precipitation APFIM studies of the steels were therefore undertaken. Materials heat treated at 475°C for 5, 25 min, 4 and 400 h were analyzed. The Ni and Cu-rich precipitates were found to be the major precipitates for the short time heat treatments. In the materials after prolonged aging, Cu-rich areas in direct connection to Ni-rich precipitates were observed. Tomographic atom probe (TAP) investigation was performed to clarify the spatial distribution of elements in and close to the precipitates. A Mo-rich phase was detected in the steel after 4 h aging. The distribution of alloying elements in the observed precipitates is discussed.