利用热处理炉对热轧抗拉强度700 MPa级Nb-Ti微合金钢的激光焊接接头进行了回火处理, 研究了回火温度对焊接接头各个微区显微组织和硬度的影响规律。结果表明, 未处理样品焊接接头各个微区的显微组织分别为焊缝区和粗晶区均为板条马氏体; 细晶区和混晶区均为铁素体和M-A组元的混合组织, 但细晶区的显微组织更为精细; 母材的显微组织为铁素体和沿着铁素体晶界分布的碳化物。当回火温度在400～500 ℃之间时, 焊缝区的硬度变化不明显, 均在310 HV左右, 随着回火温度的升高板条马氏体束之间析出的碳化物逐渐增多, 粗晶区的硬度出现明显的降低, 由未处理样品的350 HV降低至315 HV左右; 当回火温度达到550 ℃时, 焊缝区和粗晶区的硬度均出现陡增, 硬度上升幅度约为35 HV左右, 其主要原因为该区域中固溶的Nb、Ti和C元素发生了二次析出提高了其硬度; 当回火温度超过550 ℃至650 ℃时, 焊缝区和粗晶区的硬度均出现了明显的降低, 主要是由于板条马氏体发生了明显的再结晶。在本文所研究的回火温度范围内(400～650 ℃), 回火温度对细晶区和混晶区的组织和硬度影响不明显。

The laser welded joints of hot rolled 700 MPa tensile strength Nb-Ti microalloy steel was tempered in heat treatment furnace. The effect of tempering temperature on microstructure and microhardness in each zone of welded joints was studied. The results showed: The microstructure of welded joints in absence of heat treatment was: lath martensite in fusion zone and coarse grain zone, ferrite and M-A constituent in fine grain zone and mixed grain zone but finer microstructure was obtained in fine grain zone. The microstructure of base metal was ferrite and carbides along the ferrite grain boundaries. When tempering temperature was 400~500 ℃, hardness change in fusion zone was not obvious, all about 310 HV and the carbides precipitated in martensite lath interval was gradually increased with temperature increased. Hardness in coarse grain zone was obviously decreased, from 350 HV without heat treatment decreased to about 315 HV. When tempering temperature was 550 ℃, hardness in fusion zone and coarse grain zone was increased sharply by about 35 HV. The reason was that the Nb, Ti and C elements in the solid solution were re-precipitated to form (Nb, Ti) C with higher hardness. When tempering temperature was 550～650 ℃, hardness in fusion zone and coarse grain zone was decreased obviously owing to the recrystallization of lath martensite. The effect of tempering temperature (400～650 ℃) on hardness of fine grain zone and mixed grain zone was not obvious in this present paper.