对地铁转向架用P355NL1钢焊缝金属进行了低温冲击试验，分析了焊缝金属的显微组织和化学成分对其低温冲击韧性的影响，并通过断口形貌观察分析了其断裂机制。结果表明：焊缝金属的显微组织包括先共析铁素体（PF）、侧板条铁素体（FSP）、细晶铁素体（FGF）以及针状铁素体（AF）等；大量的AF使其具有良好的低温冲击韧性；焊缝金属中含有硅、锰、镍等元素，促进了AF的生成并细化了晶粒；焊缝金属的韧脆转变温度为-42.70℃；-20~0℃时焊缝金属的断口形貌以韧窝为主，断裂方式为韧性断裂，断裂机理为微孔聚集型；-40℃时焊缝金属的断口形貌为韧窝和解理面相交织，断裂方式为韧-脆混合断裂；-60℃时焊缝金属的断口形貌已经完全变成解理断口，断口分布着河流状花样和解理台阶，为典型的脆性断裂。

The low temperature impact test was carried out on weld metal of P355NL1 steel for metro bogie, and the effect of microstructure and chemical composition on the low temperature impact toughness of weld metal was analyzed. The fraction mechanism of weld metal was studied by fracture morphology observation. The results show that the microstructure in weld metal included pro-eutectoid ferrite(PF), side plate ferrite(FSP), fine grain ferrite and the acicular ferrite (AF). A large number of AF in weld metal contributed to the good low temperature impact toughness. The alloy elements Si, Mn and Ni of weld metal promoted the formation of the acicular ferrite and refined the grains of weld metal. The ductile brittle transition temperature of the weld metal was -42.70℃. The fracture morphology showed dimples when the temperature was between -20℃ and 0℃, the fraction manner was ductile fracture and the fracture mechanism was microvoid coalescence fracture type. When the temperature was -40℃, dimples and cleavage plane, which was taken ductile and brittle mixed fracture feature, could be seen on the fracture surfaces. When the temperature was -60℃, the fracture feature had been completely turned into cleavage fracture which distributed with river pattern and cleavage step. It was a typical brittle fracture type.