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

In recent years,the continuous development of thermal power to the direction of high efficiency,energy saving,environmental protection,put forward higher standards and requirements for the power plant boiler.As an important component of superheater and reheater in power plant boiler,need metal materials with different properties,therefore a large number of dissimilar steel welded joints appear in these parts.Because the chemical composition,microstructure and mechanical properties of metal materials used for dissimilar steel welding are not the same,the early failure is easy to occur.At present,there is relatively little research on the reason of the failure of dissimilar steel welded joints,which is necessary to carry out related research work.In this paper,for the explosion,leakage,cracking and other failure phenomena of T91/TP347 H,T22/TP347 H,12Cr1MoV/TP347 H welded joints in operation state,to study mechanical properties of room temperature and high temperature and microstructure,identify the reasons for failure of dissimilar steel welding joints,and provide a theoretical basis for the safe operation of the thermal power units.The main research contents are as follows:(1)To carry out room temperature mechanical properties test of T91/TP347 H,T22/TP347 H and 12Cr1MoV/TP347 H welded joints,tensile fracture location at room temperature are close to the fusion line,the fracture side of the joint near the fusion line should focus on the observation and analysis.The notch sensitivity test of T22/TP347 H welded joints shows that the influence of the pre-fabricated notch on the welded joint is not strong,the joints are not sensitive to the two kinds of defects.(2)To carry out high temperature mechanical properties test of T22/TP347 H and 12Cr1MoV/TP347 H welded joints,oxidation notched exists at the low alloy side of samples,the tensile fracture location is located at one side of the low alloy steel base metal,The fracture mechanism of high temperature short time tensile fracture of welded joint is discussed by using the theory of stress triaxiality,the mechanical properties of the low alloy side of the two kinds of joints are relatively weak,the oxidation gap of the existing size has little influence on the tensile properties of the joints at the high temperature short time test.The T22/TP347 H high temperature endurance strength about 97 MPa after running for 105 hours is derived from the high temperature endurance strength test results,higher than the standard requirements(?81MPa).(3)Microstructure analysis of T91/TP347 H,T22/TP347 H,12Cr1MoV/TP347 H welded joints,the T91/TP347 H welded joint found no abnormalities;the microstructure spheroidization grade of T22/TP347 H and 12Cr1MoV/TP347 H welded joints is up to level 2.5~3.0,the oxidation notch depth of the outer wall of the low alloy side fusion line is greater than the inner wall,the depth is about 40~170 ?m.EDS analysis indicated that the chemical elements of T91/TP347 H weld seam upper layer and bottom is different;the "kind of martensite" is located on the low alloy steel side fusion line has a bad effect on the mechanical properties of the T22/TP347 H and 12Cr1MoV/TP347 H welded joints.EDS analysis revealed that the precipitates are mainly M23C6 and NbC and so on.Carbon element determination showed that the carbon migration on both sides of the fusion line is the main reason for the peak or valley value of micro hardness test.