Partitioning Phenomena During the Heat Treatment of Martensitic Stainless Steels

Martensitic stainless steels are an important group of steels for different applications. Their typical hardening consists of austenitizing, quenching and (multiple) tempering (Q&T). As many of these steels contain at least smaller amounts of retained austenite (RA) after quenching, partitioning of carbon and nitrogen from martensite into RA can take place during tempering, changing it from pure Q&T to combined Q&T and quenching & partitioning (Q&P). This contribution provides a systematic overview of such partitioning effects on micro-structure and subsequently on material properties and were investigated with several steel grades and a wide variation in heat treatment parameters. The results demonstrate that partitioning effects dominate over tempering at temperatures up to 500 deg;C. Higher quenching temperatures can increase the RA-content similar to higher austenitizing temperatures. Lower quench rates can reduce RA due to carbide (nitride) precipitation. Rising tempering temperatures enhance austenite stabilization. Larger amounts of RA promote transformation induced plasticity (TRIP), leading to high ductility and tensile strength but reduced yield strength. Increased amounts of RA with sufficient stability raise impact toughness at slightly reduced hardness. Tempering above 500C in contrast promotes carbide and nitride precipitation, resulting in the elimination of the RA and therefore a typical tempering condition.