Residual Stress Analysis Of Thick Walled Pressure Vessels Based On Inherent Strain Method

As a special-engineering equipment,thick wall pressure vessel plays an important role in aerospace,automobile,ship,bridge building,petrochemical,nuclear power engineering and other engineering.Welding is a complex way of material connection which includes the heat,metallurgical reaction and physical mechanics,,and it is widely used in the preparation of pressure vessel.The residual stress resulted from the welding process seriously affects the reliability of pressure vessels,which may lead to the stress corrosion failure,as well as the leakage accidents of pressure vessels.Therefore,it is of great significance to study the distribution of welding residual stress in the thick wall pressure vessel,as well as put forward appropriate stress prediction and protection plan,for the evaluation of structural safety.In this paper,the inherent strain theory was applied to predict the residual stress in the center symmetric J groove welded member of thick wall pressure vessel.The theoretical analysis and finite element calculation were carried out to solve the difficulties in the numerical analysis of the inherent strain of the thick wall pressure vessel,including the acquisition and application of the inherent strain of the welded joint and the calculation of the residual stress.On the mentioned above basis,as for the welding heat input and welding joint thickness were the main factors that affect the inherent strain,and the welding speed was the main factor affecting the welding heat input.Therefore the influence of the welding speed and the joint thickness on the inherent strain were studied,and the empirical database of the inherent strain was created by the mathematical fitting method.The main contents were as follows:Firstly,focusing on the center symmetric J groove welded structure of a thick wall pressure vessel,the three-dimensional inherent strain distribution of thick wall pressure vessel was obtained by the inverse calculation of inherent strain,and it was compared with the results of plastic strain calculated by the thermal elastoplastic finite element methods,which verified the accuracy of calculated values of the inherent strain.The calculated results showed that the radial and circumferential inherent strain in the weld area of thick wall pressure vessel was shrinkage strain and reached the peak value.The radial and circumferential inherent strain decreased to zero with the increase of distance from the weld,The axial inherent strain induced larger tensile strain In the middle of weld area,and the axial inherent strain gradually turned to be shrinkage strain with the increase of distance from the weld,and the axial inherent strain gradually approached to zero at the place far away from the weld.Secondly,three dimensional welding residual stress distribution of a thick wall pressure vessel was obtained by elastic calculation of inherent strain.The calculated values of the inherent strain method,the calculated values and the measured values of thermal elastic-plastic method,originated from the surface heat transfer tube,the inner wall and the outer wall surface of the thick wall pressure vessel wall,were compared and analyzed,and the three results exhibited the identical regularities of distribution.Thirdly,the effects of welding speed and thickness of welding joint on the inherent strain of the top surface of pressure vessel were studied.It was found that the increase of welding speed might lead to the decrease of welding line energy,and thus reduced the inherent strain of the joint produce.The increase of pressure vessel wall thickness led to the increase of structural stiffness,which would inhibit the welding deformation and decreases the inherent strain.Finally,inherent strain corresponding to different structure thickness,welding speed and point coordinates was fitted by a data analysis software.Mathematical models were established,which proved the feasibility of establishing the empirical database of inherent strain.