Numerical Simulation Of Coupled Multi-fields In Post Weld Heat Treatment Of Submergrd Arc Welded Thick-wall Workpiece By Medium Frequency Induction Heating

The medium frequency induction heating technology used in the post weld heat treatment(PWHT)of submerged arc welded(SAW)thick-wall workpieces could significantly reduce the residual stress aroused during the welding process,thereby tendencies of distortion and cracking of welded structure would decrease to some degree.By combining experiments with numerical simulation,the coupled thermal and stress fields of thick-wall workpieces are analyzed during SAW process and preheating or PWHT heated by the medium frequency induction heating technology.As a result,effects of welding and induction heating parameters on residual stress fields in welded and post weld heat treated workpieces are carefully studied.Firstly,the temperature and stress fields of thick-wall workpieces during SAW process are carried out on the basis of ANSYS.A numerical model of coupled thermal and stress fields for thick-wall workpieces welded by SAW is established using the double ellipsoid heat source.Transient temperature variation in workpieces is predicted by the element birth and death method,and the curves of temperature variation with time at different points in workpieces are obtained as a consequence.Stress field in welding process and residual stress distribution after welding are obtained by using the elastic-plastic theory.Distribution mechanism of residual stress at different directions is also predicted.The results show that the maximum residual stress exists in fusion zone,and residual stresses in heat affected zone(HAZ)and base metal rank next.The maximum values are 234 MPa,188 MPa and 50 MPa,respectively.Tests on temperature variation in welding process and residual stress after welding are performed,while the experimental results indicate a good agreement with the simulated ones.Secondly,an electromagnetic-thermal-stress coupling model is built to analyze the electromagnetic field,temperature field and stress field of a SAW workpiece during PWHT by medium frequency induction heating,with focuses on distribution of residual stress after PWHT.The results illustrated that the PWHT can significantly relax the residual stress in welded joints,with the fusion zone is of the largest decreasing magnitude.After PWHT,the maximum residual stress in fusion zone,HAZ and base metal are reduced to 142 MPa,134 MPa and 36 MPa,respectively.The post-weld residual stresses and residual stresses after PWHT in welded joints are measured by the blind-hole method,and the test results agree well with the simulated ones.Finally,on the basis of the established electromagnetic-thermal coupling model,the transient temperature field of a large diameter 321 stainless steel cylinder is investigated during its preheating process by induction heating.Temperature distribution of workpiece at cases with different current size and frequency in induction coil and in different induction heating modes are studied carefully.By adjusting the current parameters and induction heating modes,the preheating demands that 120±10°C must achieved in heating area after induction heating are satisfied.The predicted results showed that the optimized induction parameters are by using 10 KHz and 750 A current with heating 10 s between intervals of 20 s stop.