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

Abstract

Specimens of 0.38% C steel and Fe-25% Ni alloy were put to test of their low cycle fatigue properties, tensile and hardness, both in their quenched and tempered states. The maximum bending strain on their surfaces under which the fatigue was tested was controlled at 1.0∼1.6%. The X-ray diffraction and thin film electron microscopic 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 martensite.
The low carbon 25Ni alloy in tempered condition presents remarkable variation in microstructure, but the differentiation is almost leveled by the fatigue, the phenomenon that is similar to the contrast of annealed metals versus cold-worked metals. The differentiation in microstructure is kept up in the 0.38% carbon steel in spite not only of tempering but also of low cycle fatigue process.
The results obtained are not yet adequate enough to explain the correlation between the microstructural variation and the damage fraction in martensitic steels. The crack nucleation and fracture line in the 0.38% carbon steel and low carbon Ni alloy are often related longitudinally to the edges of the martensite leaves.