Study On The Residual Stress And Mechanical Property Of Welded Joints Of The Cold-stretching Austenitic Stainless Steel Vessel

Cold-stretching technique can make the austenitic stainless vessel to produce a certain plastic deformation by appropriate high hydraulic test pressure,and thus permits a higher design stress which results in lighter,thinner and less costly vessels.The welded joint is a weak part of the pressure vessel.In this paper,the welded joints of cold-stretching austenitic stainless steel vessel was studied,and the following research work was carried out.The research work of this paper can provide some reference for the practical application of cold-stretching austenitic stainless vessel.The cold-stretching austenitic stainless steel vessel was manufactured and cold-stretching experiment was carried out.Then the mechanical test and microstructure analysis of the basic metal and welded joint with and without 9% elongations stretching were conducted.It is concluded that the yield strength of basic metal and welded joint was increased by 59% and 43% respectively,the section shrinkage is reduced by 29%,and the impact toughness change slightly.Test specimen was cut from the circumferential and longitudinal welded seams,and then mechanical test and microstructure analysis was conducted.Results show that the yield strength of welded joint was greatly increased and the section shrinkage was reduced after cold-stretching.Meanwhile,the mechanical property of the welded joint can meet the relevant standards.In order to study the effect of cold-stretching on the residual stress of welded joint,a numerical model for welding and cold-stretching of austenitic stainless steel vessel was established by ABAQUS finite element software.Results indicate that the residual stress is redistributed after cold-stretching and it is evenly distributed,and the longitudinal and lateral residual stress are greatly reduced.The longitudinal tensile stress at the center of the circumferential weld is reduced from 270 MPa to 70 MPa,and the lateral compressive stress is reduced from 100 MPa to 0 MPa.The longitudinal stress at the center of the longitudinal weld is changed from the tensile stress of 400 MPa to the compressive stress of 200 MPa,and the lateral stress changes less.The residual stresses in the weld joints of the vessel were tested by X-ray diffraction,and results are in good agreement with the numerical simulation resultsThe model of circumferential weld with different strength matching ratios and different weld reinforcements was established.Research indicates that higher strength matching coefficient can lead to greater residual stress of welded seam.When the residual stress and internal pressure of cold-stretching are considered together,the strains of the welded seam are all smaller than the strains of the basic metal under different strength matching coefficients.Higher strength matching coefficient can lead to smaller strain of the welded seam.That is to say,when the strength matching ratio is over match,it can protect the welded seam with weak plastic reserve on condition that the plastic reserve of basic metal is sacrificed.In the process of cold-stretching,the weld reinforcement can contribute to the overall strain greater the weld reinforcement is,smaller the strain of the welded seam is.In other words,to a certain extent,the weld reinforcement can protect the welded seam.