Effects of forging and heat treatments on stress corrosion behavior of 316ln stainless steel in high temperature caustic solution

The reactor coolant piping in the third generation nuclear power plants of AP1000 is manufactured by integrally forging. Therefore, it is of vital importance to investigate the effects of forging and heat treatments on the stress corrosion cracking (SCC) resistance of 316LN stainless steel (316LNSS), which is the candidate material for the reactor coolant piping in AP1000 nuclear power plants. In this work, electron back scattering diffraction (EBSD) and microhardness measurements (HV) were used to characterize the microstructure and residual strain of the as-received 316LNSS, the forged and solution anneal treated 316LNSS and the forged and stress relief treated 316LNSS, respectively. The average grain size of the as-received 316LNSS was the largest, and the forged 316LNSS followed by solution anneal treatment and stress relief treatment showed no obvious differences on grain size. The as-received 316LNSS exhibited the highest residual strain followed by the forged and stress relief treated 316LNSS and then solution anneal treated 316LNSS. Besides, the residual strain in the as-received 316LNSS concentrated on grain boundaries, while the residual strain in the forged and stress relief treated 316LNSS was characterized by a band-like distribution. The U-bend specimens were utilized to investigate the SCC behavior of the 3 kinds of 316LNSS specimens in high temperature caustic solution. After SCC experiments, the crack morphologies of the 3 kinds of 316LNSS specimens were examined by SEM. Then the macro and micro fracture morphologies were examined by OM and SEM, respectively. Grain morphology, residual strain and grain boundary character distribution near the SCC crack tip of the forged and stress relief treated 316LNSS were investigated using EBSD. The results showed that the forged and solution anneal treated 316LNSS exhibited the lowest SCC sensibility, while the as-received the highest, with the most cracks and the highest growth rate. The as-received and the forged and solution anneal treated 316LNSS showed obvious intergranular cracking, while the forged and stress relief treated 316LNSS showed a mixed cracking mode. The larger average grain size and higher residual strain, especially concentrating on the grain boundaries, were considered to be responsible for the highest SCC sensibility of the as-received 316LNSS. Compared with the forged and stress relief treated 316LNSS, the higher content of coincidence site lattice boundary (CSLB) and lower residual strain contributed to the lower SCC sensibility of forged and solution anneal treated 316LNSS. The stress relief treatment failed to eliminate the band-like microstructure effectively, which disadvantaged the SCC resistance.