The influence of microstructure on the resistance of low strength ferrous alloys to gas phase hydrogen degradation

摘要

The ferrous microstructure on the near threshold fatigue behavior was determined in air and in 6.9 MPa hydrogen. The fatigue crack growth rate at 1 Hertz and R approximately 0.15 is independent of microstructure and strength level for both an A516-G70 and X-60 linepipe steel at alternating stress intensities (delta K) greater than approximately 15 MP a m1/2. High pressure hydrogen greatly accelerated the fatigue crack growth rate at these delta K's, but did not alter the observed strength or microstructure independences. The fatigue threshold values at one billionth ms were determined for several normalized, and quenched and tempered microstructure in the A516-G70 steel. For delta K 15 MPa m1/2 both an austenitic grain size dependence and a strength level dependence of the near threshold fatigue behavior were observed. The fatigue threshold value increased with increased austenitic grain size and increased yield strength. In addition, the threshold values decreased in a hydrogen environment, the martensitic microstructures showing the largest decrease, normalized microstructure showing the least.