采用SEM和TEM研究了4种不同N含量的Inconel 690合金经1080 ℃、10 min固溶及715 ℃热处理后的显微组织演变和晶界微区元素分布,同时测量了合金的层错几率和晶间腐蚀速率。结果表明,相同热处理后,不同N含量Inconel 690合金的晶界M23C6碳化物析出形貌和晶界Cr贫化存在明显差异。随N含量增加,碳化物数量减少,晶界碳化物由连续分布转变为半连续分布,继而转变为离散分布。随N含量增加,Inconel 690合金层错几率先增加,在N含量为100×10-6时达到最大值,随后层错几率降低。此外,N的加入缓解了晶界Cr贫化,提高了合金抗晶间腐蚀能力;但过高N含量导致较多氮化物夹杂。综合考虑,N含量在100×10-6较为适宜。
Inconel alloy 690 is an austenitic nickel-based corrosion resistant alloy with about 30%Cr, which is considered as the most ideal steam generator tubing materials in nuclear power plants because of its superior resistance to intergranular attack (IGA). However, the existence of impurities and the addition of minor alloying elements cause significant difference of carbide morphology, microstructure and chromium depletion of Inconel alloy 690. In this work, the microstructure and grain boundary chemistry of Inconel alloy 690 with four different nitrogen contents have been investigated by SEM and TEM. Stacking fault probability (SFP) and IGA with respect to the microstructure was tested and analyzed. The results indicated that thermal treatment at 715 ℃ following solution annealing (SA) at 1080 ℃ caused a wide range of intergranular carbide morphology with the associated chromium depletion in the vicinity of grain boundaries. With the increasing of nitrogen content, the characters of the carbides ranged from thin continuous bands along boundaries to coarse discrete particles. Stacking fault probability was increased with the increasing of nitrogen content, and the value reached the peak at 100×10-6 of nitrogen content, then it dropped. The corrosion tests showed that moderate nitrogen content alloy performed favorable intergranular attack correlated with the presence of semi-continuous grain boundary carbide and chromium depletion was mitigated. The consequent nitrides were appeared in high nitrogen alloy. So, about 100×10-6 contents of nitrogen in alloy 690 is suitable by synthesis considering of carbides, nitrides and chromium depletion.
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