Research On High Temperature Aging And Creep Behavior Of Ferrite-based Heat-resistant Steel

Ferrite-based heat-resistant steel is widely used in the thermal power generation industry.With the development of the industry and the improvement of environmental protection requirements for equipment,new requirements are put forward for the power of generating units.High temperature superheater,reheater,header,steam pipe and other key components need to bear the higher temperature and pressure.Under the condition of high temperature and pressure,the microstructure and properties of materials are deteriorating,which is easy to cause failure accidents and lead to a lot of losses.However,the domestic research on heat resistance started late,and the data of heat-resistant steel degradation process is not complete and the detection methods are less.Therefore,the research on the accumulation of analysis data and the prediction of deterioration degree of heat-resistant steel is particularly important.In this paper,the failure analysis of circumferential weld cracking of 12Cr1MoV steel,which is a typical representative of ferrite based heat-resistant steel,was carried out.The 0-22000h ultra-long aging test and large tonnage creep test were carried out on P91 steel pipe.The aging samples and creep samples with various deformations were tested by metallographic observation,SEM,precipitate composition analysis,precipitation phase statistics and hardness test statistics.The main conclusions are as follows(1)The failure of welded joint of 12Cr1MoV steel is caused by type IV creep cracking.The main fracture mechanism is creep fracture caused by vacancy diffusion under low stress condition.(2)With the increase of aging time,austenite grain boundary and lath grain boundary have the trend of growth and coarsening.During 0-2000h,the precipitates grow rapidly,the hardness decreases,Cr and Mo elements diffuse into M23C6precipitates rapidly,and the content of Fe decreases.During 2000h to 22000h,the material is in a stable state,the precipitates grow slowly,the hardness decreases slowly,and the diffusion rate of elements tends to be stable.In the whole aging experiment,M₂X phase is suspected to be formed in the sample taken about 14000h,but it decomposes quickly.The structure of other precipitates is stable.M23C6 phase gradually transforms to Cr23C6,and MX phase almost has no change.It is found that there are differences between the inner surface side and the outer surface side,and the performance of the inner surface side is worse.(3)For the creep damage of P91 creep samples below 10000 hours,except the deformation near the fracture surface,it is difficult to identify the damage difference by metallographic examination.There is no linear relationship between the size of precipitates and deformation,and the average size of creep precipitates is higher than that of aging experiments.There is a good relationship between hardness and deformation,and the hardness decreases with the increase of deformation.It is found that some creep holes with the size of about 10?m deform along the axial direction,but different from 12Cr1MoV,the creep fracture in this experiment is dominated by dislocation movement.However,the actual creep failure of 12Cr1MoV welded joint is dominated by vacancy diffusion.(4)No matter aging test or creep test,the change of hardness value well reflects the process of material degradation,and the size change of precipitate(M23C6)also has a good corresponding relationship with material degradation,that is,hardness and precipitates can evaluate the deterioration degree of materials.