Investigations on the Effect of Cooling Rate on Quenching & Partitioning (Q&P) in Martensitic Stainless Steels

Quenching and partitioning (Q&P) is a heat treatment used to adjust the retained austenite content in the microstructure. Such heat treatment is used mainly for low-alloyed steels. However, the partitioning effect has an influence on higher alloyed steels also, such as martensitic stainless steels. The typical heat treatment for these steels is quenching and tempering (Q&T). In large-scale tools the cooling rate in the inner area is lower than in the peripheral area, and the central region of the tool might not be cooled down completely to room temperature before the tempering step takes place, resulting in a Q&P instead of a Q&T treatment. This article deals with these effects through dilatometric investigation of steels X40Cr14, "X25CrN13"and "X50CrMoN17-1"at two different austenitizing temperatures and two cooling rates, with a variation of the quenching temperature. It was found that partitioning takes place even at slow cooling rates. However, due to partial pearlite formation and pre-carbide precipitation/coarsening, the retained austenite content may be lower than with rapid cooling. Further, autopartitioning was also detected at slow cooling rates.