利用电子背散射衍射 (EBSD) 和显微硬度 (HV) 测量技术, 表征了原始态、锻造固溶处理态和锻造去应力处理态316LN不锈钢 (316LNSS) 的显微组织和残余应变. 利用U型弯曲应力腐蚀评价方法, 研究了3种材料在325 ℃, 3.5%NaOH溶液中的应力腐蚀开裂 (SCC) 行为的差异. 结果表明, 原始态316LNSS的SCC数量最多, 裂纹扩展速率最大, 而锻造固溶处理态316LNSS的SCC敏感性最低; 原始态和锻造固溶处理态的316LNSS在高温碱性溶液中发生明显的沿晶应力腐蚀开裂 (IGSCC), 而锻造去应力态的316LNSS发生混合型SCC; 去应力处理不能有效消除锻造过程中可能产生的条带组织, 不利于316LNSS整体抗SCC性能的提高.

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.