Effect Of Heat Treatment On Welding Residual Stress Of Titanium Alloy Pressure Spherical Shell

With the continuous development of marine resources,more and more attention has been paid to manned submersibles.Because of its low density and high specific strength,titanium alloy has become the first choice material for deep-sea pressure spherical shell manned submersible.The pressure spherical shell is usually welded by two hemispherical shells with a long equatorial weld,and the residual stress near the weld is high,which has a great influence on the structural strength.However,pressure-resistant spherical shell is prone to suffer from stress corrosion cracking under the combined action of residual stress and working stress,which ultimately leads to structural damage.Therefore,it is necessary to understand the distribution of residual stress in the equatorial weld of deep-sea pressure spherical shell and take effective measures to reduce the harmful residual stress.Post-weld heat treatment is a common method to reduce the welding residual stress,which could effectively improve the service life and the bearing capacity of the welded structure.Therefore,it is very meaningful to study the effect of the heat treatment process on the welding residual stress of the titanium alloy pressure spherical shell.The pressure spherical shell of the manned submersible is generally thick and the manufacturing cost is relatively high.Meanwhile,the equatorial weld also belongs to butt welding.Therefore,in this paper,the welding process of Ti80 butt welding flat plate is firstly numerically simulated.Then,on the basis of the consistent results of numerical simulation and test,the residual stress of the equatorial weld of Ti80 pressure spherical shell is numerically simulated.Finally,the effect of different heat treatment processes on the welding residual stress of the equatorial weld of Ti80 pressure spherical shell is studied to obtain a set of heat treatment process methods suitable for the equatorial weld of Ti80pressure-resistant spherical shell.The main research work and conclusions are as follows:(1)Based on the thermal elastic-plastic theory and APDL redevelopment program of ANSYS,the numerical simulation analysis of the welding residual stress of the Ti80 butt welding plate is carried out,and the surface residual stress of the Ti80 butt welding plate is tested based on the non-destructive method.The results show that the transverse residual stress in the direction perpendicular to the weld and longitudinal residual stress along the weld direction are tensile stress,and the longitudinal residual tensile stress is greater than the transverse residual tensile stress.And the numerical simulation results of the residual stress are consistent with the results of the non-destructive testing test;(2)Based on the transient thermal analysis method,the Norton creep theory is introduced,the post weld heat treatment of TC4 butt welded plate is simulated.The welding residual stress is well eliminated after heat treatment.The numerical simulation results are in good agreement with the experimental data in the literature,which verifies the rationality of the numerical simulation method of post-welding heat treatment in this paper;(3)Based on the transient thermal analysis method,different heat treatment processes are used to conduct a numerical simulation study on the post-weld heat treatment of the Ti80 butt welding plate.The results show that the elimination of welding residual stress during the post-weld heat treatment process is mainly due to the creep of the material at high temperature.The release of welding residual stress mainly occurs in the heating stage,the stress release amount in the insulation phase is small,and the residual stress in the cooling phase has rebounded;(4)The numerical simulation analysis of the residual stress and post-weld heat treatment of the equatorial weld of the Ti80 pressure spherical shell is performed based on the thermal elastic-plastic theory.The results show that the vertical weld path and the path along the weld seam of the inner shell of the Ti80 pressure spherical shell are both tensile stress,and the longitudinal residual stress is obviously greater than the transverse residual stress.There is a large residual transverse compressive stress on the vertical weld path of the shell,and there is a longitudinal tensile stress along the weld path.After post-weld heat treatment,the welding residual stress of the equatorial weld and its adjacent area of Ti80 pressure spherical shell has been largely eliminated.