Study On The Microstructure And Mechanical Performance Of Dissimilar Steel Welded Joint

The consumption of energy is increasing dramatically all over the world with the rapid development of economy and the improvement of living standard.On the other hand,a large number of harmful gases in the process of thermal power generation caused serious environmental pollution.Therefore,it is imperative to improve the power generation efficiency of coal and reduce its impact on the environment.While the thermal power generation efficiency is related to the steam temperature and pressure,which will inevitably improve the material requirements.Rotor is a key component of the steam turbine,the performance of the rotor seriously affects the power generation efficiency as well as the safety of the whole unit.Ultra-supercritical technology improves unit power generation efficiency and also has put forward higher requirements on the properties of rotor materials such as fatigue resistance,creep and impact.Generally,in the manufacturing process of dissimilar rotor steel welded joint,advanced 9Cr steel is used in the high-pressure(HP)condition,while Cr Mo V and Ni Cr Mo V steel is used in intermediate-pressure(IP)and low-pressure(LP)condition.This will not only play their performance advantages of materials,but also reduce manufacturing costs.Taking a series of welding methods into consideration,narrow gap submerged arc welding(NG-SAW)is generally recognized to fabricate the dissimilar welded joint via multi-layer and multi-pass welding process for its high efficiency without losing quality.However,the dissimilar steel welded joints will bring such problems as large organization gradient,uneven mechanical properties,thus affecting the safe operation of welded joints.Therefore,it is very important to study the mechanical properties of dissimilar steel welded joints and joints.In this paper,the dissimilar steel welded joint of 9Cr/Cr Mo V,9Cr/Ni Cr Mo V is taken as the research object,and the influence of microstructure on the fatigue crack growth is analyzed by means of experimental test and microstructure characterization,and the relationship between the Impact toughness of the dissimilar steel joint and the post-weld heat treatment(PWHT)process was studied to provide experimental and theoretical basis for the manufacture and safe operation of dissimilar steel rotor welded joint.Metallographic characterization shows that the microstructure of 9Cr-BM,Cr Mo V-BM and Ni Cr Mo V-BM are lath-martensite,granular tempered bainite and tempered martensite,respectively.The microstructure of WM is tempered bainite composed of the coarse columnar crystals and equiaxed crystal.In addition,the heat affected zone of 9Cr and Cr Mo V are granular tempered martensite structure and high temperature tempered martensite,respectively.Microstructure observation finds that the inhomogeneous microstructure of the dissimilar welded joint is considered as the primary factor resulting in the difference of mechanical properties.Results of temperature-dependent absorbed energy indicated that the fracture appearance transition temperature(50%FATT)for 9Cr/Ni Cr Mo V dissimilar weld metal(WM)was-11 ?,and the corresponding absorbed energy was 68.3 J.At the same time,the room-temperature impact test of joint areas found that 9Cr-HAZ with low and scattered absorbed energies determined by different crack growth paths at 20 ? was the weakest zone of the welded joint which could not satisfy the requirement.By the means of optimizing the post-weld heat treatment(PWHT)process,a dramatically enhanced impact toughness for 9Cr-HAZ was obtained to meet the demand.Characterization of the microstructure revealed that the microstructure evolution from martensitic laths in the previous PWHT metals to a softer ferrite matrix with the supersaturated carbon precipitating from the matrix led to higher toughness in the optimized PWHT materials.Fatigue crack propagation experiments were carried out on 9Cr/Cr Mo V dissimilar rotor steel joints and the results imply that WM shows a better fatigue crack growth resistance than others,while 9Cr-HAZ with narrow size becomes the weaker part of the whole welded joint.With the stress ratio rising from 0.1 to 0.9,the fatigue threshold of Cr Mo V-HAZ has great variable gradient,but smaller changes for 9Cr-HAZ,meeting the design requirements on this condition.Microstructure observation finds that the inhomogeneous microstructure of the dissimilar welded joint is considered as the primary factor resulting in the difference of fatigue crack growth behavior.Based on the observation of crack growth path,it is found that the tempered lath-martensite microstructure of 9Cr-BM could make the crack grow forward in parallel to the laths and then deflect,resulting in tortuous crack and crack branches,which presents an excellent fatigue crack growth resistance,whereas the hard and brittle granular tempered martensite without typical lath structure in 9Cr-HAZ could account for its lower fatigue crack growth resistance.In addition,the obstacles of coarse columnar grains and the ductile equiaxed grains in WM caused the higher resistance of fatigue crack growth.