Research On Welding Residual Stress Induced By CMT Repairing For P92 Pipes In USC

P92 steel used for pipelines of ultra-supercritical thermal power units was prone to damaging in service.Repair welding was the main method to repair the defects of pipelines made by heat-resistant steel,which had the advantages of low cost and high quality.Welding residual stress had an important effect on the induction of IV-type cracks in the heat affected zone(HAZ).Besides,due to the limitation of space and the surrounding environment during the repair welding,it would be very hard to operate heat treatment after repair welding,which meant that it was difficult to eliminate residual stress after repair welding.The traditional stress testing method could only measure the residual stress value on the surface of welding parts,while the distribution of internal residual stress could be comprehensively evaluated by the numerical simulation method,which may provide a basis for establishing a reasonable repair welding process.The main research contents of this paper are as follows:In this study,the defective parts of the P92 steel pipeline were selectively removed,and the CMT+P welding method was used to repair and weld the removed locations.The stress distribution after the repair welding was measured by X-ray.In addition,the finite element software was used to simulate the residual stress field for repair welding,which was consistent with the results measured by the hole-drilling technique in this experiment and the literature.The influence of welding length,welding width and heat input on residual stress after repair welding were also studied.With the increase of welding length,the transverse residual stress decreased but the longitudinal stress changed little.When welding length increased from 60 mm to 140 mm,the peak value of transverse stress on the welding surface decreased from 372 MPa to 238 MPa,which decreased by 36%.However,the peak value of longitudinal stress was basically maintained around 500 MPa.With the increase of welding width,both transverse and longitudinal welding residual stresses increased,and the influence of welding width on transverse residual stress was greater than that of longitudinal residual stress.When welding width increased from 8 mm to 24 mm,the peak value of the transverse residual stress increased from 220 MPa to 394 MPa on the welding surface,which increased by 79.1%.The peak value of longitudinal residual stress increased from 286 MPa to 480 MPa,which increased by 67.8%.The difference between effect stresses was about 11.3%.The thermal input of repair welding also had a great influence on the residual stress.With the increase of welding heat input,the residual stress in each directions increased in varying degrees.The heat input of repair welding increased from 1800 J/mm to 2800 J/mm,and the peak value of transverse residual stress increased by 24.1% from 220 MPa to 273 MPa on the surface of weldment.While the peak residual stress in longitudinal direction increased by 4.9% from 286 MPa to 300 MPa.By analyzing the distributions of residual stress of repair welding with different welding parameters and by considering the engineering experience,a set of optimal repair welding parameters could be recommended: the welding length was 140 mm,the width of repair welding was 8 mm,and the heat input was 1800 J/mm.