Creep Behavior Of A CrMOV-9%Cr Rotor Steel Weldment Produced By Narrow Gap Submerged Arc Welding

Creep damage and failure in weldments pose major challenges to the development of large-capacity welded rotor for steam turbines.To study the creep behavior of a CrMoV-9%Cr rotor steel weldment produced by narrow gap submerged arc welding,the detailed microstructures were characterized firstly.Meanwhile,the creep properties of subzones in weldment were measured using miniature specimens,and the heterogeneous creep behavior and damage mechanism of multipass weld metal were studied.Also,the effect of metallurgical inhomogeneity on microscopic damage mechanism of weldment was systematically investigated.The main work and conclusions are listed as following:(1)The influence of microstructure of CrMoV-9%Cr rotor steel weldment,produced by narrow gap submerged arc welding,on mechanical properties:Metallographic observation by optical microscope and SEM was conducted to characterize the microstructural evoluation across the welded joint.Micro-hardness tests and tensile tests of mini-sized flat specimen were conducted to gain the mechanical properties of subzones in weldment.Results shown that the mechanical properties of CrMoV-FGHAZ is weaker than that of other zones due to its deteriotated microstructure,but the fracture of all cross-weld tensile specimens is located at CrMoV-BM instead of CrMoV-FGHAZ,resulting from the local mechanical mismatches.(2)In-situ experimental study on creep behavior of subzones in weldment:The creep properties of HAZ at CrMoV side and WM were measured by digital speckle metrology technique with an in-situ facility.In terms of creep strength,the order of different specimens is as follows:WM-C>WM-R>CrMoV-HAZ.Campared to creep curves of WM-C and WM-R,the primary and secondary creep stages of CrMoV-HAZ are negligible due to the coarsened carbides and recovery of microstructure.(3)The effect of inhomogeneous microstructure of weld metal on creep properties:It was found that the multipass weld metal is composed by columnar grain zone(CGZ),equiaxied grian zone(EGZ),recrystallized grain zone(RGZ)and coarse grain heat affected zone(CGHAZ).Compared to the micro-hardness and tensile strength,a remarkable different creep behavior is achieved for different subzones.In addition,the heterogeneous creep deformation is observed,especially in the tertiary creep stage.(4)Microscopic characterization for anisotropic creep damage of subzones in multi-pass weld metal:Microstructural analyses in terms of fracture morphology,creep void distribution,precipates and substructure were conducted for creep ruptured specimens to investigate the effect of heterogeneous microstructure on creep void initation and growth.Results displayed that the change of sampling direction can cause the difference of the grain boundary configuration perpendicular to the loading direction,and the creep resistance can be various when different smapling locations are considered.The coupling of the above two factors is the main cause of the heterogeneous damage.(5)Finite element analysis of multi-pass weldment coupled with anisotropic creep damage:The effects of configuration of welding passes,energy input,and welding velocity on creep life of cross-weld specimen were studied.Results indicated that the influene of the inhomogeneous microstructure of multi-pass weld metal is not obvious at high stress level,while it becomes more remarkable with the decrease of the stress.The creep life of weldment can be extended with intervened arrangement of welding passes.Lower welding velocity and higher energy input may be a candidate option to improve creep properties of the weldment.