Influence of partitioning on mechanical properties and retained austenite stability of martensitic stainless steels

The effect of retained austenite on the mechanical properties of martensitic stainless steels was investigated. Not only the amount of retained austenite is important, but also its stability. Different retained austenite contents were achieved by quenching and partitioning. Tensile tests were performed to obtain elongation and strength values as well as to determine the kP-value, which describes the retained austenite stability. The investigations were carried out on two martensitic stainless steels (X40Cr14 and “X25CrN13”) in order to examine also the influence of nitrogen. The results show that increasing quenching temperatures increase the retained austenite content until a maximum is reached. With increasing quenching temperature, the kP-value also increases, and retained austenite stability decreases. For both steels the kP-value was lowered at the higher partitioning temperature, meaning that the retained austenite was more stable, despite a higher retained austenite content. The nitrogen alloyed “X25CrN13” had similar retained austenite stability as X40Cr14 at the higher partitioning temperature, but retained austenite of X40Cr14 was more stable at the lower partitioning temperature. In general, a reduced retained austenite stability and the corresponding Transformation Induced Plasticity effect led to reduced yield and increased tensile strength as well as increased ductility. Only at highest quenching temperature, due to fresh martensite formation, ductility was reduced.