2001 Volume 41 Issue 4 Pages 350-355
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 martensite of Fe–15%Ni alloy decreased with the increase in number of thermal cycles, and the width of internal twins in lenticular martensite of Fe–31%Ni alloy also decreased with thermal cycling. The hardness of martensite increased up to 3 thermal cycles in Fe–31%Ni alloy, over which it remained constant. 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. The tensile strength of martensite in Fe–31%Ni alloy increased up to 3 cycles, showing a similar tendency of the hardness variation. However, the tensile strength of martensite in Fe–15%Ni alloy decreased with increasing the number of thermal cycles in spite of the increase in hardness with thermal cycling. The reason is that no strain hardening occurred in the thermal-cycled specimens owing to the brittle fracture due to the segregation of sulfur at prior austenite grain boundaries.