Influence Of Annealing Treatment On Microstructure And Properties Of T91 Heat Resistant Steel Joint By Fusion Welding Under The Different Holding Time

P91/T91 heat-resistant steel is a kind of modified martensite alloys steel(9Cr-1Mo).A new type of steel is formed by adding,Nb,N and other strengthening elements.Because of the reduction of S,P and other vulnerable elements,the antioxidant properties of P91/T91 heatresistant steel have been improved.The microstructure is martensite and ferrite,which has good structure stability,high temperature enduring strength and high temperature creep strength.At present,P91 steel has been most widely used in main steam pipeline,the superheater and reheater of plant boiler,heat exchanger of nuclear,boiler tube of oil cracking equipment.However,the welding quality of P91/T91 heat-resistant steel is always the focus of attention.In order to improve the welding quality of P91/T91 heat-resistant steel,the heat treatment process of long period is usually done after welding.Therefore,when microstructure and properties of the joint are ensured,appropriate reducing the holding time can greatly advance the efficiency of welding to save the energy and costs of production.The main research content of this paper is the effect of annealing time on the microstructure and properties of T91 weld seam.At the annealing temperature of 760?,the holding time is as follow: 4h,5h and 6h.At the annealing temperature of 740? and 780?,the holding time is as follow: 5h and 6h.The purpose is to analysis change characteristics about microstructure and properties of T91 heat resistant steel butt joint by TIG welding under different holding time,especially in less than usually holding time conditions.Some factors influencing the change of microstructure and properties of joints are also studied and analyzed under different annealing conditions.This will provide an important research basis for improving the heat treatment process of T91 heat resistant steel during the fusion welding.In this paper,optical metallographic microscope,micro-hardness tester scanning electron microscope(SEM)and X-ray diffractometer(XRD)are applied.Result shows that the microstructure of martensite and retained austenite all changes with the increasing of holding time in weld zone and heat affected zone(HAZ).After the annealing temperature of 760?,the hardness curve of T91 heat resistant steel butt joint by TIG welding of each region tends to slow down after annealing.The hardness of HAZ is about 200HV~240HV,and the hardness of weld zone is about 200HV~250HV.After the annealing temperature of 760?,the overall hardness curve of T91 heat resistant steel butt joint by SMAW welding is increased with the prolongation of annealing time.The highest hardness of the weld zone and HAZ is obtained by annealing 6h.With the increasing of holding time,the fluctuation trend of the overall hardness distribution increases,and the greatest fluctuation trend of the hardness distribution can be obtained by annealed 6h.After the annealing temperature of 760?,the precipitated phase of T91 heat resistant steel butt joint by TIG welding is mainly distributed in grain boundaries,martensitic boundaries and interior in the form of particle or rod in the process of anneal.Partial carbides as Mo C,VC are dissolved by holding 5h.In the process of holding 6h,small dispersed precipitated phases are well distributed in all over of the substrate.It can limit the growth of martensite during further holding temperature by pinning crystal boundary effectively,so the fine microstructure of weld zone is obtained.The microstructure and properties of weld zone by holding 6h are better than holding 4h and 5h,owing to the obviously fine crystal reinforcing.After 760°C+4h annealing treatment,there are a lot of white rod precipitates on the surface of the weld of T91 heat resistant steel butt joint by SMAW welding.The main components of the precipitates are Fe,Cr,Mn,V,Si and so on,and The aggregation of oxides can be found.After 760°C+5h annealing treatment,relatively small white precipitates can be found on the weld surface,and the distribution is relatively uniform.A lot of oxides are dissolved by holding 5h.After 760°C+5h annealing treatment,the precipitated phase is composed of Fe-Cr,Fe3 Si,NiCr-Fe,Fe2 Mn Al and so on.Compared with holding 5h,the microstructure is fine and homogeneous.The second phase particles are dispersed and distributed,and the material mechanics is relatively stable.