某核电站安全端在异种钢焊接后，其18MnD5低合金母材和E309L不锈钢堆焊层沿熔合线发生开裂。通过宏观分析、化学成分分析、力学性能试验、金相检验及微观分析等方法对该安全端焊后熔合线的开裂原因进行了分析。结果表明：由于焊接热输入过小，熔池温度不够，冷却速度过快，从而产生了马氏体淬硬组织，使得熔合线处产生应力集中，造成安全端18MnD5低合金母材和E309L不锈钢堆焊层之间发生开裂。同时主裂纹引起了不锈钢焊缝沿晶二次开裂，冷却速度过快加快了二次裂纹的萌生和扩展，从而加速了安全端熔合线的开裂。

The 18MnD5 low alloy base metal and E309L stainless steel surfacing layer cracked along the fusion line after dissimilar steel welding of a safety end. The cracking causes of the fusion line after welding were analyzed by means of macro analysis, chemical composition analysis, mechanical property test, metallographic examination and micro analysis. The results show that due to the small welding heat input, insufficient bath temperature and fast cooling rate, the martensite hardening structure was produced, which made the stress concentration at the fusion line and caused the cracking between the 18MnD5 low alloy base metal and the E309L stainless steel surfacing layer of the safety end. At the same time, the main crack caused the secondary intergranular cracking of the stainless steel weld, and the rapid cooling rate accelerated the initiation and propagation of the secondary crack, thus accelerated the cracking of the fusion line at the safety end.