X-ray Investigation of Low-Cycle Fatigue in Martensitic Steels

Specimens of 0.38%C steel and Fe-25%Ni alloy were put to test of their low-cycle fatigue, tensile and hardness properties, both in their tempered state. The maximum bending strain on their surfaces under which the fatigue was tested was controlled at between 1.0 and 1.6%. The X-ray diffraction and thin film electronmicroscopy methods and some other techniques were employed for the purpose. The results obtained may be summarized in two phenomena. One is similar to the contrast of annealed metals versus cold-worked metals, and the other is what is considered to characterize the tempered martensite. Both materials in tempered condition present a remarkable variation in microstructure, and the difference in microstructure is almost leveled out only in the low carbon 25%Ni alloy by the fatigue and this phenomenon is similar to the contrast of annealed metals versus cold-worked metals. But in the 0.38%C steel the difference is not leveled out even after the fatigue to failure. The results obtained are not yet adequate enough to explain the correlation between the microstructural variation and the damage fraction in a tempered martensite. The crack initiation and propagation in the 0.38%C steel and 25%Ni alloy are often related longitudinally to the edges of martensite leaves