Effects of minor elements and thermal treatment on EAC of austenitic simulating steels in PWR primary water and implication to IASCC

The effects of minor elements (Si, P and S) and thermal treatment on environmentally assisted cracking (EAC) behavior of austenitic simulated steels in pressurized water reactor (PWR) primary water at 325°C were investigated by performing trapezoidal wave cyclic loading testing on pre-cracked specimens. The basic chemical composition of the steels was 12%Cr-28%Ni, simulating the grain boundary composition of irradiated 304 stainless steel. Results showed that, in solution annealed state, Si raised significantly both intergranular cracking tendency and crack growth rate. Doping only 0.48%Si was able to make the cracking fully intergranular. Si also tended to make the cracking easier to occur in the surface region than in the center of specimens. Only transgranular cracking at relatively low rates took place in the steels doped with P and S respectively. Thermal treatment at 650°C for 20 hours decreased the crack growth rate but did not eliminate intergranular cracking in the high Si doped steel. The stress corrosion cracking (SCC) velocities during trapezoidal wave cyclic loading were estimated. Three possible mechanisms were proposed to explain the Si effect. The present results suggest that radiation induced low Cr/high (Ni, Si) grain boundaries in 304 steel can be susceptible by itself to intergranular SCC in PWR primary water, with segregated S and P having no apparent effects.