Some Mechanistic Observations on the Crack Growth Characteristics of Pressure Vessel and Piping Steels in PWR Environment

The fatigue crack growth behavior of A533B and A508 pressure vessel steel and Types 304 and 316 steels used in reactor coolant piping has been studied in a pressurized water reactor environment at 288 C (550 F). The influence of stress ratio (P_min/P_max), frequency, ramp times, specimen orientation, and material microstructures were included in the study. While none of the materials showed evidence of static crack growth in the environment, the ferritic steels did show an enhanced fatigue crack growth rate at test frequencies of 5 cpm and lower. Based on fractographic examinations, the enhanced growth rate is not the result of environmentally induced intergranular or cleavage modes of crack propagation, instead, striation spacing measurements were found to agree with the macroscopic crack growth rate. This indicates that a time dependent environmental interaction exists which introduces a frequency dependent enhancement of the mechanically developed striations. Crack growth experiments using hold times have also confirmed the absence of any superimposed contribution of static crack growth components. The deleterious effect of hydrogen on the fatigue crack growth rate of pressure vessel steels was established by testing in a hydrogen sulfide environment.