An investigation of phase transition on the microstructural characteristic and creep behavior for the SA508 Gr.3 steel used for nuclear reactor pressure vessels

The purpose of this work was to describe and discuss the effect of phase transition on the creep behavior of SA508 Gr.3 steel, used in Gen III nuclear reactor pressure vessels (RPVs). The phase transition will occur in the RPV steel, once the strategy of in-vessel retention (IVR) exerts its effect in severe accidents. The microstructural evolution of the steel during phase transition was revealed, and the creep tests were conducted at 650-850 °C, to investigate the effect of phase transition on the creep behavior. The microstructural characteristics in the heat-treated and crept specimens were analyzed by scanning electron microscopy coupled with transmission electron microscopy. Results show that: the phase transition takes place at 700-800 °C with the formation of austenite, dissolution of cementite, coarsening of M2C carbides and subgrains. The creep tests indicate that the phase transition significantly affects the creep rupture life. Through detailed microstructural examination, the predominant mechanism in determining the creep rupture life of the steel before and after phase transition was unveiled. The work performed will provide basic date to understand and implement the IVR strategy.