Effect of martensite volume fraction on the mechanical behavior of an UNS S41003 dual-phase stainless steel

UNS S41003 (410D) steel is a relatively low chromium unstabilized ferritic stainless steel. It has superior mechanical and corrosion resistance than ordinary low carbon steels. Therefore, and for its relatively low cost among specialty steels, it is a strong candidate for replacement of low carbon common steels in many applications. In order to enable new applications for this steel, it is important to ensure good performance in relation to its mechanical strength, so the development of hardening mechanisms without significant loss of ductility is desirable. Dual-phase microstructures are an example that fits in this context, because they tend to increase mechanical strength and are favorable to the fracture toughness and fatigue resistance of steels. This research evaluated the influence of quenching heat treatments after intercritical austenitizing on the microstructure, tensile strength, hardness, fracture toughness (J x Delta a curves) and fatigue crack growth resistance (da/dN x Delta K curves) of a 410D ferritic stainless steel. The used intercritical austenitization temperatures were defined based on its critical temperatures A(c1) and A(c3) measured by dilatometry. Ten different quenching heat treatments were performed, varying the austenitizing temperature and time, in order to obtain dual-phase microstructures (ferrite and martensite) with different volume fractions of constituents. The obtained results revealed that the increase of the austenitizing temperature and time favor the increase of the martensite volume fraction in the microstructure. Higher martensite volume fractions imply greater hardness and mechanical tensile and fatigue strength of the steel, but with loss of ductility and fracture toughness. The best balance among the studied mechanical properties was presented by steel treated at 825 degrees C for 15min, containing 57% of martensite.