Low alloy steels used in reactor pressure vessel manufacturing often exhibit martensite-austenite (M/A) islands post-quenching. When tempered above 600°C, these islands can decompose into long rod-shaped Fe3C carbides, demonstrating the negative impact of M/A island decomposition on the mechanical properties. To address this issue, the current study investigates a two-step tempering process to better understand the characteristics of M/A island decomposition and its impact on mechanical properties and fracture surface morphology of tensile and impact tested samples. A two-hour pre-tempering treatment at 200°C, 350°C, and 500°C was followed by three hours of final tempering at 650°C. The study reveals temperature-dependent decomposition characteristics of M/A island, as well as the significant impact of tempered microstructure and carbide precipitates on mechanical properties and fracture surface morphology. At 200°C, the M/A island partially decomposes, resulting in long rod-shaped carbide precipitates during final tempering. Meanwhile, at 350°C, the pre-tempering sample showed aggregated carbide precipitates of various sizes at former M/A island locations, resulting in the formation of numerous micro-voids in close proximity during the fracture. The 500°C pre-tempering sample contained coarse spherical carbides, and carbide precipitate clustering had disappeared. Samples pre-tempered at 300°C and 500°C had a higher fraction of ductile dimples, indicating better tensile properties. However, splitting-related decohesion and dimple depth increased as the pre-tempering temperature increased.
Keywords: Reactor pressure vessel, Factography, Decohesion and cracking of precipitates, Martensite–austenite island decomposition, Two-step tempering.