Effect of Heat Treatments on Impact and Tensile Properties of 450 MPa Ferrite Martensitic Steels
摘要
Yield strength of 450 MPa with impact toughness 100 J (−10 °C) has been the requirement for the many structural application of ferritic martensitic stainless steel in mild corrosive environment. Optimizing the microstructural and mechanical properties of ferritic martensitic stainless steels by performing different heat treatment schedules is necessary to improving strength toughness balance. Four different heat treatment like tempering of the as-rolled steel at 690 °C and 700 °C for 5 hours, full normalizing at 750 °C for 3 hours and holding below Ae1 at 670 °C for 5 hours and inter-critical annealing at 800 °C for 3 hours and holding below Ae1 690 °C for 5 hours was applied in a 409 M variety of stainless steel to understand the effect of heat treatment on the microstructure and mechanical properties of the steel. Microstructures were characterized by optical, SEM, and EBSD. The subcritical tempering at 700 °C of hot-rolled steel provides the better strength-toughness compared to the full/inter-critical annealing treatment followed by holding at respective temperature below Ae1. The full annealing and transformation of γ to α + Cr23C6 is a very slow process and leads to formation of only ~ 5 to 10 pct ferrite, coarse carbide and remaining fresh martensite and increased the dislocation density, thereby the strength increased significantly (~ 30 pct YS, ~ 17 pct UTS in pct increment) with sufficient ductility > 21 pct EL, but the impact toughness decreased to 7 ± 3 J IE at − 10 °C. This significant drop can be attributed to the irregular shaped coarse precipitates formed in between the ferrite-martensitic boundaries during diffusional transformation of austenite to ferrite, that act as crack initiator. The steel also has very high tempering temperature sensitivity, even 10 °C increase in tempering can significantly increase the impact toughness in the 700 °C tempered sample compared to 690 °C sample.