Effect of solid-solution temperature on the microstructure of Fe-Ni based high strength low thermal expansion (HSLTE) alloy

The influence of solid-solution temperature on the dissolution of carbide precipitates, the average grain size and the micro-hardness of the austenite matrix in an Fe-Ni based high strength low thermal expansion (HSLTE) alloy was investigated to obtain the proper temperature range of the solid-solution process. The XRD analysis, microstructure observations, and the theoretical calculations showed that the Mo-rich M2C-type precipitates in the Fe-Ni based HSLTE alloy dissolve completely at about 1100°C. The average grain size of the studied alloys increases from 14 to 46 μm in the temperature range of 1050 to 1200°C. The microhardness of the matrix decreases first for the sake of solid-solution treatment, but then increases later with increasing solution temperature because of the solution strengthening effect.