1998 Volume 84 Issue 2 Pages 115-120
Relation between fatigue strength and static strength, and the relation between fatigue crack initiation and microstructure in bainitic microalloyed steels were investigated, using various bainitic steels changed in heat treatment conditions. Results obtained are as follows : (1) Lowering austenitizing temperature raises the tensile strength (σB), 0.2% yield strength (σy) and fatigue limit (σw) of the air-cooled bainitic steels by reason of the refinement of packet size. An increase in cooling rate raises σB, σy and σw of the air-cooled bainitic steels by reason of lowering bainite transformation temperature. (2) The fatigue limit ratios (σw /σB) of the air-cooled bainitic steels are almost the same as those of the tempered martensitic steels and the tempered bainitic steels except the bainitic steels austenitized at a higher temperature (1603K). (3) Under fatigue conditions, linear surface reliefs made of slip bands are found to be generated mainly along bainitic ferrite lath in the air-cooled bainitic steels, and a fatigue crack initiates along a linear surface relief. (4) Coarsening of prior austenite grain size over about 100μm reduces the fatigue limit of the air-cooled bainitic steels. This is because the longer bainitic ferrite laths in coarser prior austenite grains are regarded as small fatigue cracks in the bainitic steels with fine prior austenite grains.
