Effects of martensite as a hard secondary phase on fatigue crack growth properties in heavy steel plate

The authors reported that the steel with an elongated and banded martensitic structure in a ferrite matrix has significantly high resistance to fatigue crack growth in the thickness direction. In this paper, the effects of area fraction, hardness, and aspect ratio of martensite with similar martensitic morphology and distribution on fatigue crack growth properties are investigated. As a result, the fatigue crack growth rate decreases with increasing hardness, area fraction, and aspect ratio of martensite. This corresponds to enhancement of crack deflection and branching. This crack path can be attributed to two mechanisms concerned with the increase of resistance and the decrease of driving force of crack growth. First, martensite constrains plastic deformation of ferrite at the crack tip. Second, transformation expansion of martensite changes the internal stress in the microstructure. The reasonability of these two mechanisms is confirmed experimentally.