Strengthening of Super Invar Cast Steel by Precipitation of Intermetallic Compounds

Abstract

In previous studies, we reported a cryo-annealing process consisting of subzero treatment and subsequent annealing as a novel heat treatment to overcome the low rigidity and strength caused by the coarse solidified <100> texture in super invar cast steel. In the present study, aging was added to the cryo-annealing process of Ti and Si-bearing super invar cast steels for further strengthening. By aging at 723 K after the subzero treatment, the intermetallic compounds L12–Ni3Ti and Ni3Si were finely precipitated within lenticular martensite, but not within untransformed austenite. This is because of the higher driving force for precipitation and faster atomic diffusion in the body-centered cubic matrix of martensite, as well as the high-density lattice defects that acted as preferential nucleation sites. The precipitated intermetallic compounds remained even after the matrix reverted from martensite to austenite, which led to significant strengthening of the reversed austenite. The high-strength reversed austenite contributed to the enhanced strain hardenability in the duplex structure composed of untransformed and reversed austenite, and consequently, the tensile strength was enhanced to over 800 MPa while maintaining an excellent low thermal expansion coefficient. Furthermore, an analysis of the strength mechanism based on the Orowan and Ashby-Orowan models reveals that the intermetallic compounds precipitated in reversed austenite acted as weak obstacles against dislocation motion.