Simultaneous Optimization of Rigidity and Strength of Super Invar Cast Steel using by Martensitic Reversion

Abstract

Super invar cast steel, Fe–32%Ni–5%Co by mass%, with excellent low coefficient of thermal expansion has disadvantages in the both of Young’s modulus and strength, because of coarse columnar solidification structure having <100> texture. To simultaneously overcome these disadvantages, the variations of microstructure and mechanical properties through the novel heat treatment referred to as cryo-annealing, which is consisting of subzero treatment and subsequent annealing, were investigated in a super invar cast steel. The cryo-annealing promoted fcc-bcc martensitic transformation and then bcc-fcc martensitic reversion. The bidirectional martensitic transformations led to the formation of duplex austenitic structure consisting of untransformed and reversed austenite with a coarse-grained structure similar to solidification structure. Furthermore, it is found that the austenitic structure was varied depending on the annealing temperature of the cryo-annealing; reversed austenite was remained at lower annealing temperature, while it recrystallized to fine-grained structure as increasing annealing temperature. The high-density dislocations in reversed austenite and the randomized orientation of recrystallized austenite contributed to the development of strength and Young’s modulus, respectively. Therefore, the simultaneous development of rigidity and strength is not achieved by single cryo-annealing, but can be achieved by two-cycle cryo-annealing. Increasing the first annealing temperature and lowering the second annealing temperature in the two-cycle cryo-annealing are appropriate to randomize crystal orientation through austenite recrystallization and to make volume fraction of reversed austenite higher, respectively. As a result, Young’s modulus and 0.2% strength were simultaneously optimized.