Effect of Microstructure and Strength of Low-Alloy Steels on Cyclic Crack Growth in High-Temperature Water

Abstract

The microstructural variability existing in weld heat-affected zones (HAZ) of SA533 Grade B steel has a strong influence on the fatigue crack growth rate of the material in a simulated nuclear reactor environment. In particular, considerably high susceptibility to environmentally enhanced cracking has been noted in the coarse grains of tempered martensite. This phenomenon is considered to be important not only in connection with predicting the cracking of the weld HAZ structures but also in elucidating the mechanism of crack growth enhancement in the high-temperature water environment. The materials used in the study are primarily SA533 Grade B and several other low-alloy steels including SA387, SA542, SA543, JIS SPV 46Q, and AISI 4340. Different heat treatments were given to these alloys to obtain a variety of microstructures and strengths. The results obtained show a general tendency for the crack growth rates of higher-strength materials to be more substantially accelerated in the simulated boiling-water reactor (BWR) water environment as the stress ratio is shifted from 0.1 to 0.5. In order to explore the operation of the cracking mechanism under dynamic loading, slow strain rate tests (SSRTs) were also performed. As with the cyclic crack growth rate tests, the materials with higher yield strengths exhibited highly enhanced crack growth rates and showed Ji values much reduced compared with those obtained in 288°C air. Furthermore, the crack growth rates under cyclic loading are directly compared with those obtained by the SSRT technique in terms of time-based crack growth rate in the water environment and in air by using plots of (da/dt)E versus (da/dt)air where (da/dt)E is the time-based crack growth rate in an aggressive environment and (da/dt)air is the rate in air. A satisfactory correspondence can be obtained between corrosion-fatigue test crack growth rates and those obtained by SSRTs.