Partitioning effects during the heat treatment of martensitic stainless steels and its consequences on properties and for practical application

Martensitic stainless steels are an important group of steels for applications as knives, tools & molds and highly loaded parts in the food and plastics processing industry as well as for machinery components. Their typical final heat treatment consists of quenching and, sometimes multiple, tempering (Q&T). As many of these steels contain at least smaller amounts of retained austenite after quenching, partitioning of carbon and nitrogen from the martensite into the austenite can take place during the tempering process, changing it from Q&T to quenching and partitioning (Q&P).

This contribution provides as systematic overview of such partitioning effects on the microstructure like the amount and stability of retained austenite as well as on subsequent effects on material properties such as hardness, toughness, strength and ductility. The various effects were investigated on several steel grades and cover also the effect of variation in heat treatment parameters like austenitizing temperature, quenching rate, quenching temperature, number, duration and temperature of the tempering, respectively partitioning, treatment.

The results clearly show that partitioning dominates over tempering effects at tempering (partitioning) temperatures up to 500°C. Higher quenching temperatures can increase the retained austenite content similar to higher austenitizing temperatures, lower quenching rates can reduce it due to carbide (nitride) precipitation. Rising tempering (partitioning) temperatures up to 400°C enhances the austenite stabilization. Higher amounts of retained austenite with reduced stability promotes transformation induced plasticity (TRIP), providing the possibility to optimized ductility and tensile strength but reduces yield strength. Increased amounts of retained austenite with sufficient stability increases impact toughness at slightly reduced hardness. Increasing the tempering temperature above 500°C in contrast promotes, after a certain nucleation time, carbide and nitride precipitation, resulting in the elimination of the retained austenite and therefore a typical tempering condition.