T23膜式水冷壁鳍片焊缝中温应力松弛裂纹分析

Analysis of Stress-relaxation Crack in the Membrane Water-wall Fin Weld of T23 Steel at Medium Temperature

通过显微金相、扫描电镜、能谱等试验方法分析运行近6年的超超临界机组T23钢水冷壁鳍片焊缝接头裂纹。该裂纹启裂于接头热影响区粗晶区,由粗晶区向管子内壁扩展,最终止裂于细晶区。对裂纹及周围区域进一步分析后发现在晶界析出的Ti(Al)-N化合物促进了孔洞生成,进而发展成楔形裂纹和宏观裂纹,其特征符合应力松弛裂纹。对T23鳍片焊缝在机组运行期间温度场分析表明该裂纹产生在380~520℃之间。通过以上分析得出:T23钢焊接接头在低于575℃再热裂纹敏感温度下限的环境下,会以金属化合物为核心形成孔洞和裂纹,并提出相应的技术措施和建议。

The cracks in T23 steel water wall fin weld joints were analyzed by means of microscopic metallography,scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and so on.The crack specimen was taken from a USC boiler with almost 6 years service time.The crack initiates in the coarse grain zone of the heat affected zone of the joint,extends towards the inner wall of the tube,and finally ends in the fine grain zone.Further analysis on the cracks and their surrounding areas reveals that Ti(Al)-N compounds precipitated on the grain boundaries promote the formation of cavities,and then develop into wedge-shaped cracks and macro-cracks,which have characteristics of stress relaxation cracks.Analysis on the T23 fin weld service temperature field shows that the crack occurs between 380℃to 520℃.Based on the above results,this paper draws the conclusion that the metal compounds will form cavities and cracks in the T23 steel weld-joints under the reheat crack sensitive lower limit temperature of 575℃,and puts forward the corresponding technical measures and suggestions.