The Microstructure Evolution Of RAFM Steel During High Temperature Thermal Cycle

Reduced activation ferritic/martensitic（RAFM）steels are considered as the preferential candidate material for application in structural materials of fusion reactors,due to their excellent thermal physical properties,such as the low radiation swelling coefficient and the low thermal expansion coefficient.However,the creep properties of RAFM steels are poor at high temperature,since premature failures will occur in the heat affected zone of welded joints,which is referred as type IV cracking.To provide the theoretical experimental foundation and study the crack initiation behavior and its mechanism of IV type,RAFM steel was heated to various thermal cycle temperatures to study the microstructure evolution of the heat affected zone of RAFM steels.The microstructures of specimens upon single thermal cycle temperature and secondary thermal cycle temperature were examined using optical microscopy（OM）,scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The results indicate that:With the peak temperature of the single and secondary thermal cycle increasing from 900°C to 1200°C,the austenite grains,M₂₃C₆ and MX precipitates of RAFM steels gradually coarsen.But the austenite grains,M₂₃C₆ and MX precipitates in the secondary thermal cycle are more refined than that in the single thermal cycle due to the structural heredity of the steel.On the other hand,the M₂₃C₆ and MX precipitates of RAFM steels upon single thermal cycle temperature and secondary thermal cycle temperature gradually coarsen and dissolve into the matrix with the peak temperatures increasing.The M₂₃C₆ precipitates dissolve completely at 1100°C while the MX precipitates dissolve completely at 1200°C.The evolution of two types of precipitates has a significant effect on the size of austenite.Based on the Zener pinning model,the effect of precipitate evolution on austenite grain size is quantified.It is found that the coarsening and dissolution of M₂₃C₆ and MX precipitates leads to the decrease in pinning pressure on grain boundaries,facilitating the rapid growth of austenite grains.In addition,with the increase of the peak temperature of the single and secondary thermal cycle,the amount of austenite phase increases first and then decreases,and reaches the maximum at 1000°C.The austenite phase fraction is also affected by the coarsening and dissolution of precipitates.