三通护板裂纹失效分析

通过对施焊时开裂的三通护板进行渗透检测、宏微观断口形貌分析、金相检验、显微硬度检验以及力学分析，对三通下护板裂纹产生原因进行分析。结果表明：裂纹成锯齿状，具有层状撕裂特征，为穿晶＋沿晶（少量）混合断裂方式；断面上可见氧化皮，该裂纹为层状撕裂裂纹。裂纹附近存在带状粗大铁素体，裂纹沿带状铁素体扩展；三通下护板生产过程中的焊道没有完全切除，残留焊道处粗大带状铁素体力学性能差，是裂纹产生的直接原因。

Crack Analysis of Tee-joint Sleeve

A tee-joint sleeve was cracked in welding process. The anslysis of penetrant testing, fractograph, metallurgy, micohardness and fracture mechanics were carried out. The results showed the surface had a stepped crack, and was lamellar tearing. The crack propagated perpendicularly to the inner surface and the specimens exhibited transgranular and intergranular fracture, and there are visible oxidition in surface of the crack along the banded coarse ferrit. The coarse ferrite with low mechanical property in the weld bead, which wasn’ t cut off in the fabrication process, was the main reason of the crack.