Temperature distribution and residual stresses due to multipass welding in type 304 stainless steel and low carbon steel weld pads

Welding is a reliable and efficient metal-joining process widely used in industry. Due to the intense concentration of heat in the heat source of welding, the regions near the weld line undergo severe thermal cycles, thereby generating inhomogeneous plastic deformation and residual stresses in the weldment. Plates of different thickness are used in industry and these plates are normally joined by multipass welding. In a multipass welding operation, the residual stress pattern developed in the material changes with each weld pass. In the present experimental work, thermal cycles and transverse residual stresses due to each pass of welding have been measured in the weld pads of AISI type 304 stainless steel and low carbon steel with 6, 8 and 12 mm thickness. X-ray diffraction method was used for residual stress measurements. The welding process used was the Manual Metal Arc Welding (MMAW) process. In this paper, the peak temperatures attained at different points during deposition of weld beads in stainless steel and low carbon steel weld pads are compared. The residual stress patterns developed, the change in the peak tensile stress with the deposition of weld beads, and the relation between the peak temperatures and the residual stresses in the weld pads are discussed.     

Thermo-mechanical modelling of a single-bead-on-plate weld using the finite element method

This work describes a three-dimensional thermo-mechanical finite element analysis of a single weld bead-on-plate of austenitic stainless steel performed as part of the NeT programme. The overall aim is to validate the use of finite element (FE) weld simulations to accurately predict residual stress states for use in the assessment of welded components. Here, the final residual stresses in the plate are predicted, which can later be verified through comparison with measured distributions. A one-way coupled analysis is carried out with the thermal and mechanical problems solved in sequence. For the thermal analysis, two approaches are adopted to simulate the welding process. In one case sections of the weld bead's elements are sequentially deposited, while in the other the whole bead is deposited simultaneously. A moving heat source is used to simulate the torch traversal over the weld bead in both cases. Predicted thermal profiles, residual stresses and equivalent plastic strains, due to the welding process are presented at certain locations in the plate.     

NeT bead-on-plate round robin: Comparison of transient thermal predictions and measurements

The members of the European network NeT have undertaken parallel round robin activities measuring and predicting the residual stresses generated by laying a single Tungsten Inert Gas (TIG) weld bead on an AISI Type 316L austenitic stainless steel flat plate. This is a strongly three-dimensional configuration with many of the characteristics of a repair weld. The round robin finite element predictions of transient temperatures and the extent of the melted zone are compared with thermocouple measurements made during welding, and with the results of destructive metallography. The most reliable thermocouple positions for calibrating the global heat input are identified. The actual achieved weld efficiency, and hence the heat input, are deduced from the response of these far-field thermocouples. As a result, best practice recommendations are made for finite element simulation of welding thermal transients.     

Residual stresses in austenitic stainless steel primary coolant pipes and welds of pressurized water reactors

Surface and through thickness residual stress measurements were performed on an aged cast austenitic-ferritic stainless steel pipe and on an orbital TIG weld representative of those of primary coolant pipes in pressurized water reactors. An abrasive-jet hole drilling method and a block removal and layering method were used. Surface stresses and through thickness stress profiles are strongly dependent upon heat treatments, machining and welding operations. In the aged cast stainless steel pipe, stresses ranged between −250 and +175 MPa. On and near the orbital TIG weld, the outside surface of the weld was in tension both in the axial and hoop directions, with maximum values reaching 420 MPa in the weld. On the inside surface, the hoop stresses were compressive, reaching −300 MPa. However, the stresses in the axial direction at the root of the weld were tensile within 4 mm depth from the inside surface, locally reaching 280 MPa.     

Robustness analyses of numerical simulation of fusion welding NeT-TG1 application: “Single weld-bead-on-plate”

This study contributes to the NeT-TG1 European Network formed in 2002. The aim of this study is to predict, by numerical simulation, the residual stresses generated in a test plate by the fusion welding process. The experiment consisted in the deposit of a weld bead along the longitudinal centre-line of an austenitic 316L plate using an automatic Tungsten Inert Gas (TIG) welding process with 316L filler material. During and after thermal cycle, a large quantity of measurement data is obtained that serve to develop a comparison with the results of different numerical models. The comparative thermal and mechanical analysis allows assessment for the general ability of the numerical models to describe the structural behaviour. The importance of the heat-input rate and material characteristics is also investigated. The residual stresses were predicted by the finite element method using the program Code_Aster of EDF and SYSWELD of ESI-GROUP. Finally the numerical results are validated by comparison with experimentally measured data.     

Hydrogen induced cold cracking studies on armour grade high strength,quenched and tempered steel weldments

Quenched and tempered (Q&T) steels are prone to hydrogen induced cracking (HIC) in the heat affected zone after welding. The use of austenitic stainless steel (ASS) consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase. The use of stainless steel consumables for a non-stainless steel base metal is not economical. Hence, alternate consumables for welding Q&T steels and their vulnerability to HIC need to be explored. Recent studies proved that low hydrogen ferritic (LHF) steel consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. In this investigation an attempt has been made to study the influence of welding consumables and welding processes on hydrogen induced cold cracking of armour grade Q&T steel welds by implant testing. Shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes were used for making welds using ASS and LHF welding consumables. ASS welds made using FCAW process offered a higher resistance to HIC than all other welds considered in this investigation.     

异种钢环缝自动TIG焊接工艺研究

低合金钢（铁素体钢）与镍基合金（不锈钢）锻件之间的异种钢对接焊缝越来越多的应用于换热设备的设计和制造过程中.自动TIG窄间隙焊接工艺已经成为异种钢对接焊缝的主要工艺方法.研究自动TIG焊接工艺,对焊接材料的选择、母材性能参数及焊接工艺等进行了介绍,通过焊接评定试验对焊接工艺进行试验论证和总结,并成功应用于换热设备异种钢环缝的焊接.     

MODELING OF TRANSPORT PHENOMENA IN A GAS METAL ARC WELDING PROCESS

A numerical study of three-dimensional heat transfer and fluid flow in a moving gas metal arc welding (GMAW) process is performed by considering various driving forces of fluid flow such as buoyancy, Lorentz force, and surface tension. The computation of the current density distribution and the resulting Lorentz force field is performed by solving the Maxwell equations numerically in the domain of the workpiece. The phase change process during melting and solidification is modeled using the enthalpy-porosity technique. Mass and energy transports by droplet transfer are also considered through a thermal analysis of the electrode. The droplet heat addition to the molten pool is considered to be a volumetric heat source distributed in an imaginary cylindrical cavity within the weld pool ("cavity" model). This nature of the heat source distributed due to the falling droplets takes into account the momentum and thermal energy of the droplets. The numerical model is able to capture the well-known "finger" penetration commonly observed in the GMAW process. Numerical prediction regarding the weld pool shape and size is compared with the corresponding experimental results, showing good qualitative agreement between the two. The weld pool geometry is also found to be dependant on some key parameters of welding, such as the torch speed and power input to the workpiece.     

NeT bead-on-plate round robin: Comparison of residual stress predictions and measurements

The members of the European network NeT have undertaken parallel round robin activities measuring and simulating the residual stresses generated by laying a single Tungsten Inert Gas (TIG) weld bead on an AISI Type 316L austenitic stainless steel flat plate. This is a strongly three-dimensional configuration with many of the characteristics of a repair weld. The round robin finite element predictions of weld residual stresses are compared with each other in order to identify the effects on the predicted residual stresses of material hardening model, global heat input, mechanical and thermal boundary conditions, and the handling of high temperature inelastic strains. Comparison with the residual stress measurements then leads to the optimum choices for these variables.     

An investigation of heat transfer and fluid flow on laser micro-welding upon the thin stainless steel sheet (SUS304) using computational fluid dynamics (CFD)

A transient three-dimensional model is numerically developed using the method of computational fluid dynamics (CFD) to characterize some thermal phenomena and characterization of heat transfer and fluid flow in laser micro-welding by considering the heat source and the material interaction leads to rapid heating, melting and thermal cycles in the heating zone. The application of developed thermal models has demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter, have considerable effects on the peak temperature and resulted weld pool. The heat source model is consisted of surface heat source and adaptive volumetric heat source that could be well represented the real laser welding as the heat penetrates into the material. In the computation of melt dynamics, mass conservation, momentum and energy equations have been considered to compute the effects of melt flow and the thermo-fluid energy heat transfer. The simulation results have been compared with two sets of experimental research to predict the weld bead geometry and solidification pattern, which laser welds are made on thin stainless steel sheet (SUS304). The shape comparison describes those parameters relevant to any changes in the temperatures and melt dynamics are of great importance on the heat distribution and formation of weld pool during laser micro-welding process. The fair agreement between simulated and experimental results, demonstrates the reliability of the computed model.     

Finite element simulation of welding residual stresses in martensitic steel pipes

Abstract

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.     

Numerical and experimental study of arc and weld pool behaviour for pulsed current GTA welding

A finite element model is introduced in this paper to describe the coupling between the welding arc and the weld pool dynamic in pulsed gas tungsten arc welding. The cathode, arc-plasma and melting anode regions are taken into account. The unified time-dependent model describes the heat transfer, fluid flow and electromagnetic fields in the three regions. The originality of the numerical model is its ability to treat the arc and weld pool time evolution under pulsed current welding in a unified formalism, taking into account eddy current in the weld pool. The case of thin plates with fully penetrated weld pools is also handled.To validate the predictions of the model, an Infra-Red camera is used to film the dynamic of the weld pool surface. Then an image processing algorithm permits to get the time evolution of the weld pool width directly from the film. The numerical model is applied to the 304 stainless steel welding, and the computed results show that the predictions are in fair agreement with the experimental results.     

Optimal parameters for pulsed gas tungsten arc welding in partially and fully penetrated weld pools

In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation.The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model.     

3D modelling of a multi pass dissimilar tube welding and post weld heat treatment of nickel based alloy and chromium steel

A dissimilar tube welding is performed between the nickel based Alloy617 and creep resistant steel VM12 using the former as the weld material. SYSWELD welding software is used to model the thermal and mechanical analysis. A readily available thermal history is used to calibrate the heat source input for the thermal analysis to generate the adequate thermal cycle by fitting the welding velocity, heat intensity factor of the GOLDAK heat source and the length of molten zone. The transient temperature field is then incorporated as the input for the mechanical analysis to obtain the residual stresses in which the phase transformation of the materials during welding is taken into account. Subsequently, the weld materials are characterized by using the Norton’s creep law to determine the Norton parameters based on relaxation experiments. The residual stresses generated after the multi pass welding by SYSWELD is transferred into ABAQUS as the initial condition for the post weld heat treatment (PWHT) simulation. The simulations show that the residual stresses reduce in magnitude but still present even after PWHT.     

EFFECT OF VARIOUS DRIVING FORCES ON HEAT AND MASS TRANSFER IN ARC WELDING

Abstract

In this study the heat transfer and fluid flow of the molten pool in stationary gas tungsten arc welding using argon shielding gas were investigated. Transporting phenomena from the welding arc to the base material surface, such as current density, heat flux, arc pressure, and shear stress acting on the weld pool surface, were taken from the simulation results of the corresponding welding arc. Various driving forces for the weld pool convection were considered: self-induced electromagnetic, surface tension, buoyancy, and impinging plasma arc forces. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Because the fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation. The numerical model was applied to AISI304 stainless steel and compared with the experimental results.     

Hydrogen absorption of different welded duplex steels

In this study, hydrogen absorption and storage was investigated for various high-alloyed ferritic–austenitic duplex stainless steels. On account of the specific transformation and solidification behaviour, respectively, of duplex stainless steels as compared to single-phase ferritic and austenitic steels, special conditions have to be considered concerning hydrogen absorption which may ultimately lead to microstructure-dependent hydrogen-assisted weld metal cracking. Hydrogen absorption during welding may occur via the shielding gas, moisture from the surroundings or via the welding filler material. As a contribution to the interpretation and prediction of hydrogen-induced cracking in welded duplex stainless steels, the actual hydrogen absorption via the arc as well as the weld metal hydrogen diffusion was investigated in a duplex stainless steel DSS (1.4462) and in a lean-duplex stainless steel LDS (1.4162). Isothermal heat treatment using carrier gas hot extraction enabled quantification of the amounts of hydrogen trapped in the respective microstructures. The total hydrogen concentrations were found to be nearly identical. Trapped hydrogen was however observed to be dependent on the material and on the microstructure condition. The influence of hydrogen on the mechanical properties of the weld metal was characterized with the help of tensile tests. In addition, hydrogen embrittlement was detected in scanning electron microscopic analyses.     

A Reduced Numerical Model for the Thermit Rail Welding Process

In this article, a reduced numerical model for the heat transfer in a commonly used Thermit rail welding procedure is presented. A geometrically reduced calculation domain was deduced from the welding system consisting of rails, weld material and mold. The geometrical domain is restricted to heat transfer in the rail web. Unsteady heat conduction in base rail and weld regions undergoing melting and solidification are modeled using the finite difference method. Therefor the consecutive periods of the process are described by specified initial and boundary conditions: preheating, tapping time, pouring and the final cooling. The solid-liquid phase change occurring during pouring and cooling is described using the enthalpy method. Thermal radiation between rail and mold surfaces is considered. Validation is carried out against results of models using computational fluid dynamics and solidus temperature isothermal positions in micrographs of longitudinal weld cuts from experiments. A sensitivity analysis was performed for the reduced model. The temperature of the liquid steel melt and the specific heat of the rail steel have the largest impact on the fusion zone width whereas mold material properties show negligible influences. The calculated width of the final fusion zone agrees within a deviation of 16% to experimental results.     

Optimising residual stresses at a repair in a steam header to tubeplate weld

Following the discovery of incorrect weld metal in the steam side shell to tubeplate weld in a type 316H stainless steel superheater steam header, a repair strategy had to be determined. The strategy adopted was to remove the incorrect weld material, which extended around the full circumference, by machining from the inside of the header, followed by rewelding from the inside using an automatic welding process and localised post-weld heat treatment. Due to concern over potential reheat cracking of the repair after return to service, a considerable amount of residual stress modelling was carried out to support the development and optimisation of a successful repair and heat treatment strategy and thus underwrite the safety case for return to service.     

Genetic algorithm for optimisation of A-TIG welding process for modified 9Cr–1Mo steel

Abstract

It is essential to set up the activated tungsten inert gas (A-TIG) welding process parameters to produce the desired weld bead geometry and heat affected zone (HAZ) width in modified 9Cr–1Mo steel weld joints. Therefore, it becomes necessary to develop a tool for optimisation of A-TIG welding process. Genetic algorithm (GA) based model has been developed to determine the optimum process parameters. In this methodology, first independent ANN models correlating depth of penetration, weld bead width and HAZ width with current, voltage and torch speed respectively were developed. Then, GA code was developed in which the objective function was evaluated using the ANN models. There was good agreement between the target and actual values of bead geometry and HAZ width obtained using the GA optimised process parameters. Thus, a methodology using GA has been developed for optimising the A-TIG process parameters for modified 9Cr–1Mo steel.