针对2种N含量(0.001%和0.03%, 质量分数)的690合金, 采用SEM, TEM并结合EDS分析等手段系统研究了时效处理后合金晶界碳化物和晶界附近的Cr贫化行为, 并定量描述了热处理时间对晶界附近Cr贫化程度的影响. 结果表明, 高N含量合金晶界析出的碳化物之间距离较大, 且碳化物尺寸较小, 随着时效时间延长碳化物逐渐长大. 随着时效时间的延长, 贫Cr区的Cr含量逐渐提高, 高N含量的690合金晶界附近贫Cr区的Cr含量高于低N含量合金, 主要原因是晶粒细化所致.

Nickel-based alloy Inconel 690 (hereinafter called alloy 690) is currently replacing alloy 600 as steam generator tubes in pressurized water nuclear reactors, owing to its excellent resistance to intergranular stress corrosion cracking (IGSCC) and good mechanical properties. The carbide precipitation is a major microstructural characteristic during heat treatment of stainless steels and nickel-based alloys. The carbide precipitation and chromium depletion in the grain boundary of alloy 690 were investigated. The grain size and carbide of alloy 690 with 0.001% and 0.03% (mass fraction) nitrogen contents were observed and analyzed. The extent of chromium depletion in the vicinity of grain boundaries was quantitatively determined as a function of thermal treatment time. The solution treatment of the samples was at 1080 ℃ for 10 min, and then the samples were thermally treated at 715 ℃ for 1~25 h. The results show that the nitrogen addition decreases the intergranular carbide density and the average carbide length but increases its distance. The level of chromium in the depleted regions in alloy 690 with 0.03%N is higher than that with 0.001%N. This is attributed to the beneficial role of nitrogen addition against grain growth and sensitization.