Influences of Microstructures of Fatigue Strength of Medium Carbon Microalloyed Steels

Abstract

Influences of the microstructures on the fatigue properties of the medium carbon microalloyed steels were investigated under the rotating bending fatigue tests. Results obtained are as follows: (1) Under fatigue conditions surface relief made of slip bands are found to be generated largely on ferrite in the medium carbon ferrite pearlite steels. The fatigue crack initiates along the surface relief and propagates along the ferrite/pearlite boundary in the early stage of the fatigue crack propagation. (2) Increase in vanadium contents raises the slip band initiation stress and the fatigue strength through strengthening ferrite. Increase in carbon or manganese contents raises the fatigue strength through increasing pearlite volume fraction and refining ferrite grain size. Increase in vanadium contents are more effective to improve the fatigue limit ratio. (3) Coarsening of prior austenite grain size reduces the fatigue limit. This is because the ferrite precipitated along austenite grain boundary are regarded as small fatigue cracks in medium carbon ferrite pearlite steels with fine prior austenite grain size.