长时间回火(时效)后保持钢材的优异力学性能是全球性技术挑战。本工作对低合金高强钢焊缝金属中的针状铁素体在高温下进行长时间回火，通过OM、SEM-EBSD、HRTEM、冲击和拉伸测试等手段研究其显微组织演变及其对力学性能的影响，同时从材料动力学角度对实验结果进行了分析和讨论。结果表明，经580~700 ℃、1~12 h不同时间回火后，针状铁素体的位错密度及组织尺寸均未发生明显变化，析出相密度随着回火温度的升高而增加，钉扎了晶界和位错，阻碍了针状铁素体的粗化，提高了析出强化作用。640~700 ℃回火时，抗拉强度增加明显且在700 ℃左右时达到峰值。材料动力学分析结果表明，形核率-温度曲线和析出-温度-时间曲线的鼻温都在700 ℃左右，析出相在640~730 ℃时大量形成，析出强化作用明显增强。高密度位错和Mo的添加提高了碳化物形核率且细化了TiC，Mo的添加还提高了动力学曲线NrT和PPT的“鼻温”并使析出开始时间提前，改善了针状铁素体的高温回火力学性能。

Maintaining steel’s high mechanical properties after a long period of tempering (aging) is a worldwide challenge. This study investigates the microstructure and mechanical properties of a low-carbon, low-alloy steel weld metal consisting of acicular ferrite (AF) using OM, SEM, EBSD, TEM, and impact and tensile tests. The precipitation kinetics is used to analyze and discuss the experimental results to study the microstructure evolution and its effect on the mechanical properties of the low-carbon, low-alloy steel weld metal consisting of AF after tempering for a long time in a high-temperature environment. The results show that after tempering at 580-700 ^oC for 1-12 h, the dislocation density and size of the AF showed no noticeable change, indicating that the microstructure of the AF was very stable after high-temperature, long-time tempering. Precipitates increase along with the increase in tempering temperature, pin grain boundaries and dislocations, hinder the coarsening of AF, and thus increasing the precipitation strengthening effect. The tensile strength increased at 640-700 ^oC and peaked at about 700 ^oC. The kinetics curves and calculations indicate that the “nose” temperatures of both the nucleation rate-temperature (NrT) and precipitation-temperature-time (PPT) curves are about 700 ^oC; precipitation occurs in large amounts at about 640-730 ℃ and thus provides a strong precipitation strengthening effect. High-density dislocation and Mo addition increase the nucleation rate and refine the precipitates of TiC. Adding Mo increases the “nose” temperature of the NrT and PTT curves and brings forward the precipitation start time, thus improving the high-temperature tempering mechanical properties of AF.