Study On The Creep Rupture Behavior And Microstructure Evolution Of Modified 9Cr Heat Resistant Steel Welded Joint

For the conventional thermoelectric industry,to improve its thermal efficiency and decrease the CO2 emissions,we need to improve the steam temperature and pressure,while higher requirements are also put forward to the materials used in the thermoelectric equipment at the same time.At present,because of their good comprehensive mechanical properties,9-12%Cr heat resistant steels are widely applied in thermoelectric equipment.For example,CB2 heat resistant steel,which is based on the European COST project,now is the main material for steam turbine valve,and welding is the main method used to join the valve and pipe.In the actual operation,the main properties of safety and reliability of the welded joint depend largely on its creep rupture perfoemance.So it is important to study the creep rupture behavior and microstructure evolution at high temperature and reveal the creep rupture mechanism of CB2 steel welded joint,which is meaningful to evaluate and predict the life of welded joint and understand the failure mechasim of welded joint systematically.In this paper,a series of creep rupture tests are carried out on CB2 steel welded joints under 90-170 MPa at 620?.The life equation of welded joint is obtained as logtr=25.80-10.89log? and the creep rupture strenth is estimated as ?(10)5620 =75 MPa based on the experimental analysis.Besides,it is found that the welded joints exhibit different rupture mechanisms under different stress conditions.Under the stress above 130MPa,the welded joints fractured in the over-tempered zone(OTZ),and the fracture is crater-like with severe necking and a lot of dimples on the fracture surface.Under the low stress below 130 MPa,the fracture is in the weld metal(WM)with zig-zag shape with almost no deformation,and the surface shows dim dimples.Based on the microstructure characterization,in the process of high stress and short-term creep,there are a lot of large creep cavities in OTZ,and the cavities are elongated and deformed through the grains and martensite laths due to the high stress.M23C6 carbides are coarsening.For low stress and long-term creep,a serious softening occurred in WM,and more and smaller creep micro-cavities formed along the boundaries of grain and martensite lath,which has the tendency to connect into micro-cracks to cause intergranular fracture.Besides,coarse Laves phase particles formed during the long-term process,consuming the solid solution strengthening element Mo.The martensite lath structure also degenerates and the dislocation density sharply decreases.What's more,the precipitation of Laves phase and their effect on creep rupture properties were analyzed combined with the microstructure characterization and kinetic calculation.Laves phase are(Fe,Cr)2Mo type intermetallic compounds with elongated and polygonal shapes,mainly distributing in the prior austenite grain boundary.The Avrami exponent of Laves phase is1.02,indicating that Laves phase nucleate by diffusion-controlled mechanism,mainly nucleating at the interface with reduced nucleation rate.Laves phase grow by the grain boundary diffusion controlled Ostwald ripening model,and will swallow and consume the nearby small particles to form larger particles.Early precipitated Laves phase have some pinning effect on the grain boundary and dislocations.With the prolonged progress,Laves phase are seriously coarse,and the pinning effect is weakened,which is detrimental to the microstructural stability.Finally,the internal connection between failure transition of welded joints and the stress condition,regional microstructure and its evolution is found based on the failure analysis.The short-term rupture is considered to be mainly controlled by stress and deformation.Lower deformation resistance caused by relatively less amount of grain boundaries in OTZ,and softening caused by the twice temperings,lead to the short-term rupture in OTZ under high stress.For low stress and long-term rupture,there are martensite lath structure degrading,the dislocation density decreasing,and the Laves phase coarsening in WM,which facilitate the micro-cavities to form in WM,leading to the long-term creep rupture in WM.