Distortion Control during Welding

Abstract

The local material expansion and contraction involved in welding result in permanent deformations or instability i.e., welding distortion. Considerable efforts have been made in controlling welding distortion prior to, during or after welding. Thermal Tensioning (TT) describes a group of in-situ methods to control welding distortion. In these methods local heating and/or cooling strategies are applied during welding. Additional heating and/or cooling sources can be implemented either stationary or in a transient state. In static methods, a pre-set temperature distribution is imposed on the workpiece, while in transient methods, the temperature depends on the position and the time. The mechanisms of distortion reduction in thermal tensioning are complex. The complicated nature of welding stress and strain fields is increased by the large number of parameters involved in thermal tensioning. Type, intensity and characteristics of the additional heating and/or cooling sources play an important role in the development of the stress and strain fields during welding with thermal tensioning. The positioning of the additional sources with respect to the welding centre line and the welding torch are other critical parameters involved in thermal tensioning. In this work the focus is on dynamic method using heating strategies (welding with additional heating) and can be classified into two types, Transient Thermal Tensioning and Side Heating. If the additional heaters are located close to the welding torch and contribute to the thermal field of the welds, the process is called transient thermal tensioning. If there is no interference to the thermal field of the weld, the process is called side heating. In this study, the thermal, the microstructural and the mechanical fields for both conventional welding and welding with additional heating were investigated by means of numerical models and by experimental methods including temperature, distortion and residual stress measurements and by microstructural investigations. The usage of both experimental and numerical work has provided valuable insight into welding with additional heating. Three case studies were defined for laboratory tests (with different materials: AISI-316L, DP600 and AH36 steels) and one case study was set for industrial implementation (AH36 steel). The experiments consisted of two tracks: conventional welding and welding with additional heating. Welding parameters were selected to obtain a sound weld. The parameters concerning the additional heating sources were systematic investigated. The conditions where deformation was reduced, referred to as 'experimentally obtained minimum distortion' were used for further study and to validate numerical models. For the experimentally obtained minimum distortion (for different materials), temperature was measured using thermocouples at the underside of the plate during conventional welding and welding with additional heating. The out-of-plane distortions of the plates before and after welding and welding with additional heating were measured by means of the digital image correlation method. The microstructure of the weld metal, the heat affected zone of the weld (HAZ-welding), the base metal and the heated area beneath the burners (HAZ-welding with additional heating) were studied at a cross section perpendicular to the weld and in the middle of the plate for all materials. Residual stress measurements were performed on AISI-316L and DP600 steel plates by means of neutron diffraction (ND) at the Paul Scherrer Institute. At the Laboratoire Leon Brillouin, the residual stress profiles of AH36 plates were also measured by ND. It was found from the experiments that transient thermal tensioning using the burners applied in this study cannot reduce welding distortion (with the materials and experimental conditions employed in this study), while side heating can successfully reduce out-of-plane deformation for the selected materials. In order to apply transient thermal tensioning, the heating source should be localized, for example by laser heating. In side heating, for all of the materials studied, it was found that the closer the burners were to the weld centre line the higher deformation obtained. Moreover, the trend in distortion as a function of the burner positions (leading, parallel or trailing) relative to the welding torch was non-linear. The results of distortion measurements indicated that the distortion of the plates is less sensitive to this parameter. Experiments showed that side heating temperatures in the range of 200-400 C, defined at the underside of the plate, can reduce deformations for all materials. Higher temperatures cause severe plastic deformation and with lower temperatures no visible change will occur. The best temperature for welding with additional heating depends on many factors such as the position of the burners, the thermal and the mechanical material properties, the clamping system around the weld, the area heated by the burners, the geometry of the plate, the welding process and so on. It was seen that for all materials, the thermal field around the welds is not changed by additional heaters. The introduction of the additional heat by the burners is limited. Although at the top surface of the plate, higher temperatures are obtained. This causes minor changes in microstructure and mechanical properties. For example, the areas beneath the burners in DP600 showed a lower micro-hardness than the base metal. For AH36 steel plate, the top surface of the heated areas beneath the burners showed a re-crystallized microstructure, while the microstructure of the underside surface is not affected. It was seen that the maximum tensile residual stresses in the HAZ (of welding) for both the conventionally welded plates and AISI-316L plates welded with side heating were similar. This was also true for the maximum compressive stresses. Welding with side heating induces tensile stresses beneath the burner positions in the order of 200 MPa. For DP600, the maximum tensile stresses at the weld and HAZ (of welding) for conventional welds and welds with side heating were again similar and there was tensile stress peaks at the areas beneath the burners. In the region beneath and close to the burners, compressive residual stresses are reduced in welding with side heating compared with those of conventional welding of AH36. However, the tensile stress peak was not observed in AH36. Finite element models were constructed to simulate and investigate the thermal, the microstructural and the mechanical fields in both conventional welding and welding with additional heating (both for side heating and transient thermal tensioning). The assumptions made in the high temperature material properties, plastic strain resetting, modelling of clamps and the additional heaters resulted in some discrepancies between the models and the measurements. For the conventional welding process, close matches between the temperature, residual stress and distortion measurements and the numerical predictions were observed. The main sources of deviation in the thermal modelling of both conventional welding and welding with side heating are related to the thermal material data at elevated temperature and the heat transfer coefficients. The essential feature of the welding with side heating is the creation of a temperature peak at the location of the burners. The temperature distribution in the weld zone and the HAZ (of welding) remains unchanged. The effect of phase transformations on residual stress and distortion was studied only for DP600. The phase fractions were validated. Although, the predicted phase fractions were in an acceptable range compared to the experiments, it was found that the model with solid state phase transformations predicts an out-of-plane deformation 9% lower than the model excluding the effects of the transformations and closer to the measured values. However, the risk of computational instability and divergence is high when phase transformations are included; therefore for other cases, the microstructural field was ignored. The out-of-plane deformation of the workpiece after conventional welding and welding with side heating was predicted and the distributions were quantified using different criteria: i) a distortion index, ii) an out-of-plane deformation contour and iii) a scan along a line perpendicular to the weld centre line. For all materials the predictions of conventional welding are in a good agreement with the experimental measurements. The predicted residual stresses are close to the measured values for both conventionally welded plates and plates after side heating. The characteristic of side heating from a numerical point of view is the creation of tensile residual stresses at the location of the burners (even for AH36 steel, tensile peaks were observed in the numerical results for the regions beneath the burners). The predictions of the numerical models developed in this study showed a good agreement with the experimental results and were used to study the mechanisms responsible for distortion reduction when additional heating (side heating or transient thermal tensioning) is used. The results of the simulations indicate that the responsible mechanism of distortion reduction for transient thermal tensioning using leading and parallel burners is the reduction of the transient compressive stresses along the weld centre line. In transient thermal tensioning using trailing burners, the responsible mechanism of distortion reduction is related to the increase of the transient tensile stresses along the weld centre line to the yield point and yielding of the weld metal during cooling. In side heating, it was found that the transient stresses along the weld centre line are not influenced by the stresses generated by the side heaters. The area with tensile residual stresses (at the position beneath the burners) tends to increase the strain required for out-of-plane deformation. The redistribution of final stresses (stresses formed due to the welding process in combination with the stresses formed due to the additional heating) reduce the final deformation of the plate. During side heating, there are three regions with tensile peaks, two more than in conventional welding. The total width of tensile stress regions is increased due to side heating and the critical buckling load will be larger than that of conventional welding. In other words, side heating reduces buckling deformation by increasing the critical buckling load and not by the reduction of compressive stresses. The implementation of side heating during welding of large plates in the shipbuilding industry showed that side heating can significantly reduce welding distortion.