Research On Residual Stress Distribution In 304 Stainless Steel Girth Weld Pipe Overlaied With SNi6625 Alloy And Optimization Of Overlay Dimension

Residual stress with complex distribution will occur during multi-layer welding of stainless steel pipes.The tensile residual stress is very easy to lead to stress corrosion cracking in the girth weld and heat affected zone on the inner wall of stainless steel pipe with a corrosive medium.Therefore,it is an effective way to prevent stress corrosion cracking and improve service safety performance of welded components of stainless steel pipe by eliminating the harmful tensile stress even inducing compressive stress.The current common methods to mitigate residual stress(such as heat treatment and vibration aging)have limited effect on the residual stress on the inner wall of girth weld pipe.Weld overlay is a common technology to effectively transform the internal tensile stress into compressive stress in girth weld and strengthen the girth weld.It is of great significance for the weld overlay design and the service perforamnce evaluation of stainless steel pipes to study the stress distribution characteristics of girth welds after overlay and the effect of overlay dimension on the stress distribution in girth weld.In this thesis,a 304 stainless steel girth weld overlay pipe(?273×28mm)for the nuclear power plant was manufactured.The surface stress of the girth weld overlay pipe was measured by the hole-drilling method and the full map distribution of internal stress was measured by the contour method.The stress in the girth weld overlay pipe was simulated by the finite element method with an axisymmetric model and the rationality of the simulated results was verified by the experimental ones.The stress evolution in the girth weld overlay from the butt welding to the overlaying was studied based on the finite element method.And the effects of the thickness and width of the overlay on the internal stress distribution of the girth weld overlay pipe were studied.The stress measurement results in the girth weld overlay pipe show that the surface stress of the overlay and its adjacent region is tensile.The hoop stress is compressive towards the inner wall of the girth weld and tensile near the outer surface with a compressive stress depth of about 70% wall thickness from the inner surface and a peak compressive stress of-380 MPa.The axial stress from the inner wall to the surface of the overlay presents a distribution trend of tensile stress-compressive stress-tensile stress with a compressive stress range of about 75% wall thickness of the pipe and a peak compressive stress of-130 MPa(The inner wall is tensile stress).The hoop and axial stresses in the overlay are both tensile.The simulated stress evolution in the girth weld overlay pipe shows that the hoop stress in the girth weld is tensile stress after butt-welding,the axial stress from the inner wall to the outer wall presents a distribution trend of tensile stress-compressive stress-tensile stresss-compressive stress and the range of axial compressive stress is 40% of the wall thickness.Overlay welding can transform the hoop stress of the girth weld into compressive stress,while it has little effect on the axial stress.After the overlay is completed,the hoop compressive stress range of wall thickness of the pipe is about 75%and the peak compressive stress reaches-340 MPa,the axial compressive stress range of wall thickness of the pipe is about 65% and the peak compressive stress is-120 MPa.The numerical simulation results of the girth weld overlay show that girth weld overlay can transform hoop stress into compressive stress and the thickness of overlay has a great effect on the hoop residual stress in the girth weld;the axial compressive stress at the inner wall can only be obtained by increasing the overlay width.When the overlay thickness is over 20mm,the hoop tensile stress will appear at the inner wall near the girth weld,and the axial tensile stress zone near the girth weld will be enlarged.The most suitable overlay dimensions for the studied girth weld overlay are 14mm in thickness and210mm in width.