Effect of heat treatment conditions on the microstructural evolution and mechanical properties in SA508 Grade 3 steel fabricated by PM-HIP

In this study, SA508 Grade 3 steel, representatively employed in reactor pressure vessels, was produced via the powder metallurgy-hot isostatic pressing (PM-HIP) method. The study focused on examining the influence of post-heat treatment parameters on the microstructural evolution and mechanical performance of the material. Heat treatments were conducted by systematically varying the normalizing temperature and tempering duration time. An increase in the normalizing temperature resulted in the coarsening of primary austenite grains (PAG), which induced a microstructural transition from bainitic to martensitic matrix, attributable to the PAG size-dependent variations in hardenability. This transformation was accompanied by a marked reduction in impact toughness, decreasing from 161.9 J to 10.6 J. Furthermore, prolonging the tempering duration time led to the coarsening of M₂C and Mn–Si–N precipitates at grain boundaries. This microstructural evolution was associated with an increase in elongation from 20.2 % to 24.4 %, albeit accompanied by a decrease in impact toughness to 148.7 J.