Application and characteristics of low-carbon martensitic stainless steels on turbine blades

410M1 (0.17%C-11.6%Cr-0.18%Nb) and 410M2 (0.17%C-10.2%Cr-0.38%Nb-0.84%Mo-0.2%V-0.05%N) martensitic stainless steels are modified from the basic martensitic stainless steel 410 (0.12%C-12%Cr). They contain Nb and are utilized in the blades of turbines for generating power. This study investigates the heat treatment characteristics, microstructure and secondary hardenability of 410M1 and 410M2. The precipitation hardening of 410 occurs at 400°C but that of 410M1 or 410M2 occurs earlier at 300°C. The peak hardening of 410 occurs at 450°C but that of 410M1 or 410M2 occurs at 500°C. Clearly, addition of Nb improves the mechanical properties of steel at high temperature. Under quenching conditions, 410M1 and 410M2 are lath martensites. 410M2 contains not only Nb but also Mo, V, and N, which improve its secondary hardenability over that of 410M1. From the characteristic chart of quenching and tempering, the tempering softening and the increase in impact toughness of 410M2 are delayed as a high tempering temperature range of 650°C to 670°C is reached. This phenomenon is observed by FE SEM and proves that NbC-carbide with 20^-40 nm are precipitated in the matrix. This investigation studies the effect of alloy design on its toughness, secondary hardenability, microstructure and applications.