Effect of thermal cycling on microstructures and mechanical properties of lath and lenticular martensites in Fe-Ni alloys

Effects of thermal cycling on the microstructures and mechanical properties of Fe-15% Ni and Fe-31 % Ni alloys having lath and lenticular martensite, respectively, have been studied. The average width of laths in lath martens ite of Fe-15 % Ni alloy decreased with the increase in number of thermal cy cles, and the width of internal twins in lenticular martensite of Fe-31 % N i alloy also decreased with thermal cycling. The hardness of martensite inc reased up to 3 thermal cycles in Fe-31 % Ni alloy, over which it remained c onstant. On the other hand, the hardness of martensite in Fe-15 % Ni alloy increased up to 1 cycle, and remained nearly constant on further cycles. Th e tensile strength of martensite in Fe-31 % Ni alloy increased up to 3 cycl es, showing a similar tendency of the hardness variation. However, the tens ile strength of martensite in Fe-15 % Ni alloy decreased with increasing th e number of thermal cycles in spite of the increase in hardness with therma l cycling. The reason is that no strain hardening occurred in the thermal-c ycled specimens owing to the brittle fracture due to the segregation of sul fur at prior austenite grain boundaries.