Effects of heat treatments on the microstructure and environment-induced cracking of CF8A steel in simulated BWR water

Abstract

The effects of heat treatments on the susceptibility to environment-induced cracking (EIC) of the CF8A stainless steel (SS) in simulated water of a boiling water reactor (BWR) were investigated by slow strain-rate tensile (SSRT) tests. Different microstructures were resulted by subjecting the as-casted CF8A to stress relief treatment (1050 degrees C/2 h, C sample), thermal aging (385 degrees C/20000 h, A sample), recovery annealing (500 degrees C/1 h, R sample) or solution annealing (1200 degrees C/2 h, S sample). The results indicated that after aging treatment (A sample), a decrease in tensile ductility and impact toughness was resulted relative to the un-aged specimens. Recovery annealing (R sample) caused an improved tensile ductility and impact toughness of the embrittled CF8A. Excessive solution (S sample) led to an obvious decrease in tensile strength but increase in ductility and toughness of the CF8A. The EIC susceptibility of the tested samples in simulated BWR water increased in the order: S < C < R < A samples. The cracks in distinct CF8A specimens were found to initiate at and propagate along delta/gamma interfaces in simulated BWR water. The lowered ferrite content and change in ferrite morphology accounted for the superior EIC resistance of the S sample relative to that of the other ones in simulated BWR water.