Abstract
Successive observation of fatigue crack initiation process was conducted on six types of thermally simulated HAZ microstructures to clarify the fundamental reason of fatigue life difference between coarse grain (CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. As a result, regardless of microstructure, fatigue cracks initiated and propagated along shear stress plane when the crack is smaller than prior austenite grain size. In addition, this shear mode fatigue crack growth of CGHAZ were much faster than that of FGHAZ and categorized as microstructurally small fatigue cracks. Although prior austenite grain size of crack initiation life of the FG and CGHAZ were comparable, the remaining fatigue life after the grain size crack initiation of the CGHAZ become shorter than that of the FGHAZ as a function of grain size difference. As a result, total fatigue life of CGHAZ becomes shorter than that of FGHAZ. Therefore, relatively larger part of CGHAZ's fatigue life is consumed by shear mode microstructurally small fatigue crack growth process.
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