Microstructure Evolution Of The 9CrMoCoB Ferritic Heat-resistant Steel During Heat Treatment

The 9-12%Cr ferritic heat-resistant steels have broad prospects in the ultra-supercritical power plant owing to their low thermal expansion,good thermal conductivity and low manufacturing cost.Based on the high Cr ferritic steels,about1.0wt%Co and 0.01wt%B was added in the 9CrMoCoB steel to inhibit the coarsening of M₂₃C₆ particles and martensitic laths,and thus improve its high-temperature stability.Most of the current studies on the 9CrMoCoB steel are concerned with the creep properties,weldability and smelting process,while little attention was paid to the microstructure evolution during hot deformation,tempering and aging.Therefore,the evolution of secondary precipitates in the 9CrMoCoB steel during tempering and long-term aging was studied in this paper,and the influence of hot deformation treatment process on the microstructure evolution of the 9CrMoCoB steel was also analyzed.The following conclusions were obtained:?1?The Arrhenius-type constitutive model and MZA model have been established based on the experimental stress-strain data,and the MZA model was found to be more accurate in predicting the flow stress behavior of the 9CrMoCoB steel during hot deformation.The hot deformation parameters have significant effects on the flow stress and microstructure evolution of the 9CrMoCoB steel.Low deformation temperature and high strain rate leads to the increase of flow stress,while high deformation temperature and low strain rate promotes the dynamic recrystallization.In addition,the combination of high strain rate and low deformation temperature can promote the MX phase precipitation during the tempering process of 9CrMoCoB steel.?2?Four types of precipitated phases were observed in the 9CrMoCoB steel during tempering under 650?,including M₃C,M₂X,M₇C₃ and M₂₃C₆.The M₃C carbides precipitated both in the process of air cooling after austenitization and the early stage of tempering,but its thermal stability was poorer,and would transform into M₂X phase as the tempering time increased.The precipitated sequence of the second phases can be expressed as follows:M₃C?M₂X?M₇C₃?M₂₃C₆.?3?In the 9CrMoCoB steel,the martensitic laths showed a good stability while the M₂₃C₆ carbides coarsened gradually with the increase of aging time under 650?.Moreover,the island Laves phase particles began to precipitate when the aging time was over 500 h,and gradually coarsened with the passage of aging time.