Fractographical observations of low alloy steels in high temperature water were carried out, and the responsible mechanisms for corrosion fatigue crack growth were discussed. The fracture surface of a low sulfur content steel was wholly occupied by transgranular ductile cracking characterized by striation patterns. In a medium sulfur content steel, on the other hand, quasi-cleavage cracking characterized by both striation and river patterns was observed. The crack growth in the medium sulfur content steel was abruptly accelerated at ΔK≅19MPa·m1/2. This acceleration was caused by hydrogen embrittlement associated with dissolution of MnS. Fourier power spectrum analysis was found to be a very powerful tool for the striation spacing measurement, especially when the striation patterns were made indistinct by derusting treatment. Based on the linear summation hypothesis with consideration for fracture area fractions, the quasi-cleavage crack growth rate in the medium sulfur content steel was derived.
