Low Temperature Impact Toughness of Weld Metal of P355NL1 Steel Used for Metro Bogie

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.