Reasearch On Residual Stress Measurement And Regulation Of Complex Welding Structure In Ocean Engineering

With the number of offshore drilling platforms increasing,fatigue problem of offshore platform structure is becoming more prominent.Statistics show that 60% of the offshore platform structure failures are caused by fatigue,90% of the fatigue fractures occur in welding region,which is due to residual stress exists after welding.Failure occurs because carrying capacity declines in weld area when there is superposition of external alternating load that structure withstand and welding residual stresses.Therefore,to understand the welding residual stress distribution of structure and take stress control measure are very important.This paper based on the review of residual stress test and control methods,taking offshore engineering EQ56 small welded test plate and bearing seat structure as study objects,carried out residual stress measurement,stress control by UIT and other experimental study.Firstly,studied the welding residual stresses distribution of high strength steel of large thickness.Secondly,studied the effect of UIT and different process parameters on residual stress in high strength steel of large thickness.In addition,studied weld toe morphology and microhardness change under appropriate process parameters of impact strength.The welding residual stress test results of small test plate show that high tensile stresses are in weld area,compressive stresses are away from weld zone.The peak of longitudinal tensile stresses is in weld center,while peak of transverse tensile stresses in vicinity of weld toe.There are uniform stress on the surface by impact strength of 10s/cm~2 after UIT.The internal stress distribution curve is "M" type along thickness direction.Surface tensile stresses are converted to compressive stresses,the internal stresses are from compressive stresses to tensile stresses,or tensile stresses increase.The increase of internal tensile stresses or change of compressive stresses to tensile stresses is as a balance to surface compressive stresses.The layer depth of compressive stress in weld surface is about 1~3mm,while 8~10mm in parent material.The compressive stresses distribution is uneven along the layer depth.The welding residual stresses test results of key parts on bearing seat structure show that residual stress values of crescent plate weld and rib plate weld are high.The longitudinal tensile stress is 450~600MPa in crescent plate weld and vicinity of weld,and transverse tensile stress reaches 250~450MPa.The peak of longitudinal tensile stress isabout 500 MPa in rib plate,and transverse tensile stress is about 300 MPa.The residual stresses are compressive away from the weld,the position of maximum compressive stress is about 120 mm away from weld center,about-550 MPa.The tensile stresses are converted to compressive stresses in crescent plate weld by ultrasonic impact on weld and parent material.Compressive stresses are not uneven under impact strength 5s/cm~2,while uniform and close under impact strength 10/cm~2 or 20s/cm~2.The tensile stresses were converted to compressive stresses in crescent plate weld by UIT,tensile stresses are also significantly reduced in non-impact region,the remaining maximum tensile stress value is 150 MPa.UIT has the same effect on free joint as binding joint.The shape of weld toe is from sharp transition to smooth transition under impact strength 10s/cm~2,and surface microhardnesses are increased by 40~60HV0.2.