利用SEM、TEM、XRD和EBSD等微观分析手段，研究了核电SA508-3钢马氏体(M)-残余奥氏体(A_R)岛(M-A岛)高温回火转变对冲击韧性的影响机制。结果表明，正火态SA508-3钢中M-A岛呈块状，以A_R为主。M-A岛经650℃高温回火后，转变成铁素体和M₃C碳化物组成的析出相聚集区。沿析出相聚集区边缘分布的粗大M₃C易诱发裂纹萌生而发生解理断裂，导致SA508-3钢低温冲击韧性偏低。进一步研究表明，深冷或回火预处理将M-A岛转变成过渡产物，可改善正火态SA508-3钢650℃高温回火后析出相聚集区中M₃C的尺寸、形态和分布，进而在一定程度上提高了SA508-3钢低温冲击韧性，其中400℃预回火处理效果最佳。这是由于，经400℃预回火后，M-A岛中A_R将完全转变成细小的贝氏体，其内部具有高密度板条亚结构和渗碳体，为M₃C在析出相聚集区内均匀析出提供形核点；同时，400℃预回火也减少了M-A岛边缘位错密度和相变残余应力，避免了650℃高温回火过程中M₃C在原块状M-A岛边缘形核和迅速长大，有利于M₃C在析出相聚集区内均匀弥散分布。

SA508-3 steel is the key structural material extensively used in large components of third-generation nuclear power plants. For increasing the process efficiency of nuclear power plants, extremely thick cross-sectional heavy forgings are necessary for constructing large components for these plants. Owing to thick cross sections, the as-quenched microstructure of the center of heavy forgings is typically granular bainite, composed of bainitic ferrite and martensite (M) and retained austenite (A_R) (M-A) islands. An M-A island is an undesired microstructure that results in the SA508-3 steel having a poor low-temperature impact toughness after conventional tempering at 650^oC. However, it is difficult to tailor the as-quenched microstructure owing to the limited cooling rate during the quenching process. Therefore, the modification of the tempering process is a more feasible method to adjust the microstructure and improve the mechanical properties of heavy forgings. Herein, the decomposition of A_R within M-A islands during different transformation paths and its effect on the mechanical properties of SA508-3 steel have been investigated. The results show that clusters of ferrite and agglomerated M₃C carbides are formed during conventional tempering at 650^oC. These coarse M₃C carbides decorate the boundary of the cluster, reducing the impact toughness of the SA508-3 steel. Accordingly, the size and distribution of these M₃C carbides are tentatively modified by introducing pretreatments at different temperatures before conventional tempering at 650^oC. This modification is because, during pretreatments, A_R first decomposes into various transitional microstructures such as martensite, bainite, or pearlite, which further transform into clusters of ferrite and M₃C carbides during tempering at 650^oC. The results show that 400^oC is the optimal pretempering temperature to improve the impact toughness of SA508-3 steel. Microstructural observations reveal that during tempering at 400^oC, A_R completely decomposes into fine bainite comprising bainitic packets and high-density cementite particles. This provides additional nucleation sites for M₃C carbides inside the clusters during the subsequent tempering at 650°C, avoiding the formation of coarse M₃C carbides distributed along these cluster boundaries.