Study On Mechanical Characteristics And Microstructure Of TIG Welding On Dissimilar 9% Cr Heat-resistant Steels

Rapid development of national economy has increased energy demand,but actual situation of China's natural resource reserves is rich in coal,less oil and lack of gas.This determines that the power pattern with coal as the main fuel will not change in long time for China.For the purpose of saving energy and reducing emissions,coal-fired power plants need to increase thermal efficiency and steam temperature.China is planning to develop and construct a high-parameter(630?)ultra-supercritical critical unit.Based on comprehensive consideration of performance and cost,9-12%Cr heat-resistant steels are selected for coal-fired unit.G115 new martensitic heat-resistant steel has been listed as the first-choice material for the new generation of ultra-supercritical power plant by the national energy administration.In2018,the Ministry of industry and information technology of the people's Republic of China also listed it as the first of 17 advanced steel materials.G115 steel has more complex components and better performance.If G115 steel is applied in thermal power unit,welding of G115 steel and CB2 steel is an urgent problem to be studied and solved.The property of joint plays a key role in the safe and reliable operation of power plant.Practice has shown that the failure of welded joints is a major form of failure of pressure-bearing components of units,so it is necessary to conduct in-depth research on TIG welded joints of G115/CB2 heat-resistant steels.This paper discussed TIG welded dissimilar 9%Cr heat-resistant steels(G115 and CB2 steel),and good welded joints were obtained with certain welding parameters.In addition,post weld heat treatment(PWHT)was applied to welded joints for improving their microstructure and mechanical properties.Meanwhile magnetic field was added during TIG welding to improve the flow of molten metal.Finally,large-scale welded joint was analyzed from its property and structure.The composition of selected welding wire was the same as that of G115 steel.This work firstly investigated the influence of welding current on microstructure and performances of TIG welded joints of G115/CB2 steel.When welding current were110 and 130 A,un-melted wires in WS and unwelded areas on the back of WS were observed.When welding current reached 150 A,the quality of welded joint became good.When current increased from 130 A to 170 A,the spread of molten welded metal became more sufficient,and the height difference between front and back of WS gradually decreased.WS was composed of quenched martensite,and because of rapid cooling of molten welded metal,there was no precipitation in matrix,which resulted in high hardness of WS.Microstructure of base metals was tempered martensite and M₂₃C₆(M:Fe or Cr)precipitates,their hardness value was the lowest in welded joints.The distance from WS became shorter,the hardness of heat-affected zone(HAZ)became lower,and the largest hardness was measured in coarse-grained HAZ.As welding current increased,tensile property of joints at room and high temperature gradually increased.When welding current exceeded 150 A,mechanical properties of joints had no obvious change.At welding current of 150 A,tensile strength of joint was673 MPa at room temperature,and tensile strength of joint was 309 MPa,at 650?.Combined with the quality and cost of welded joints,the best welding current was 150A.PWHT was mainly performed on joints with welding current of 130 and 150 A,and the temperature of PWHT were 700,730 and 760?,respectively.As PWHT temperature increased,tensile strength of joints firstly increased,then decreased and tensile strength reached maximum at 730?A.After PWHT,microstructure of WS and HAZ transformed from quenched martensite to tempered martensite,and there were a large number of precipitates distributed in the matrix.Additionally,lath boundary of quenched martensite became fuzzy.After PWHT,the hardness of WS became significantly lower,compared with as-weld joint.The high PWHT temperature brought about lower hardness.Compared with as-weld joint,magnetic assisted-joint at 130 A was well formed.More sufficient spread of molten metal could obviously improve the quality of WS.The tensile strength of magnetic assisted-joint was 688 MPa,which was 58 MPa higher than as-weld joint.The fracture mode transformed from cleavage fracture to ductile fracture.For magnetic assisted-joint,the size of grains in WS reduced,and the width of martensite lath also reduced meanwhile became more uniform.For magnetic assisted-joint,its hardness value of WS was higher than that of as-weld joint.Large-scale joint was welded by multi passes,and then experienced PWHT process.From macro perspective,there were no welded defects in large-scale welded joint.The shape of WS was like letter“X”,the width of WS at marginal region of joint was larger than that of central WS.The hardness of WS from one side to the other often changed,showing a zigzag shape.The short-time high temperature performance of joints was tested at 675?/90 MPa,650?/113 MPa and 630?/135 MPa.Their fracture time was 65 hours,87.5 hours and 71 hours,respectively.