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.     

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.     

Numerical Modeling of Cold Weld Formation and Improvement in Gmaw of Aluminum Alloys

Both mathematical modeling and experiments have been conducted on the formation of cold weld in gas metal arc welding (GMAW) of aluminum alloy 6005-T4. Transient weld pool shape and the distributions of temperature and velocity were calculated by a three-dimensional numerical model. The final weld bead shape and dimensions and peak temperature in the heat-affected zone (HAZ) were obtained. Three techniques were proposed to input more energy at the initial state of welding to improve weld bead penetration. Both the simulation and the experimental results show significantly improved weld bead penetration at the start of welding.     

Thermomechanical modelling of weld microstructure and residual stresses in P91 steel pipe

Abstract

A multipass circumferentially butt welded P91 steel pipe, typically used for high temperature applications in power plants, has been numerically analysed to determine residual stresses, induced by the process of welding, as well as microstructural regions in the weld, caused by thermal cycles. The finite element (FE) method has been applied to simulate residual stresses generated in the weld region and heat affected zone (HAZ), which are then validated by published experimental data. The axisymmetric FE simulation incorporates solid state phase transformation by allowing for volumetric changes and associated changes in yield stress and hardening behaviour due to austenitic and martensitic transformations. The thermal cycles during welding cause different microstructural regions to emerge within the weld metal and HAZ. Columnar and equiaxed microstructural zones have been numerically modelled in the weld region of the pipe. The predicted FE microstructural regions have been corroborated by columnar and equiaxed zones that have been mapped out on a cross-sectional macroimage of the weld.     

Guaranteed fillet weld geometry from heat transfer model and multivariable optimization

Numerical heat transfer models of gas metal arc (GMA) fillet welding do not always predict correct temperature fields and fusion zone geometry. The inaccuracy results, to a large extent, due to the difficulty in correctly specifying several input parameters such as arc efficiency from scientific principles. In order to address this problem, a heat transfer model is combined with an optimization algorithm to determine several uncertain welding parameters from a limited volume of experimental data. The resulting smart model guarantees optimized prediction of weld pool penetration, throat and leg-length within the framework of phenomenological laws. A boundary fitted coordinate system was used to account for the complex fusion zone shape. The weld pool surface profile was calculated by minimizing the total surface energy. Apart from the direct transport of heat from the welding arc, heat transfer from the metal droplets was modeled considering a volumetric heat source. The Levenberg–Marquardt and two versions of conjugate gradient method were used to calculate the optimized values of unknown parameters. An appropriate objective function that represented the difference between the calculated and experimental values of the penetration, throat and leg-length was minimized. The calculated shape and size of the fusion zone, finger penetration characteristic of the GMA welds and the solidified free surface profile were in fair agreement with the experimental results for various welding conditions.     

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.     

不锈钢中厚板激光全熔透焊的有限元模拟

运用有限元计算软件ABAQUS对16mm厚不锈钢板的激光全熔透焊的温度场和应力场进行了模拟.采用一体两面的复合焊接热源模型来刻画激光全熔透焊过程中的热输入特征,以柱状体热源代表焊接小孔传热模式,以2个超高斯面热源代表等离子体/金属蒸气云对熔池的辐射传热模式.结果表明:温度场模拟结果得到了与实验结果相一致的"沙漏状"焊缝;钢板内纵向残余应力最大,横向应力次之,板厚方向横向应力最小;纵向拉应力主要分布在焊缝两侧约25mm的区域内,最大值已超过材料的屈服强度;经测算,钢板焊后的角变形量仅为0.35°,这是由于激光焊接能量输入高且集中、可不用开坡口而一次性将钢板焊透.     

The prediction of LCF properties of HAZ obtained during the welding process of SA 508 Class 2 and DIN 22NiMoCr37 structural materials

The main objective of this investigation has been to study the influence of the heat affected zone (HAZ) on room temperature low cycle fatigue (LCF) properties of two structural steels, type SA 508 Class 2 and DIN 22NiMoCr37. The predicted LCF properties of the HAZ have been related to the corresponding material microstructure obtained under quenched and tempered conditions. The HAZ microstructure has been reproduced by a weld cladding simulation heat treatment and any subsequent heat treatment on the simulated HAZ microstructure has been omitted. Although the thermal cycle introduced by the simulated welding process modifies the structure of the affected region and, as a result, diminishes the monotonic and cyclic ductile properties, it has been found that the cyclic strength properties of the simulated HAZ microstructure have been increased. The Feltner and Landgraf prediction criterion has been applied in order to obtain a reliable estimation of the LCF resistance of the materials tested in this investigation.     

Simulation on the thermal cycle of a welding process by space–time convection–diffusion finite element analysis

Welding plays an important role in manufacturing. But difficulties still exist for simulation of the welding process of large welded structures, due to the limitation of computer capacity, mathematical models and software. This paper is devoted to developing an algorithm that tries to simulate the thermal cycle during welding efficiently and accurately. A space–time finite element method (FEM) is proposed to solve the transient convection–diffusion thermal equation. The method has been applied to the steady-state thermal analysis of welds. A moving coordinate frame (Eulerian frame), in which the heat source is stationary, is used to improve the spatial resolution of a numerical analysis for the thermal cycle of welds effectively, as well as to incorporate the addition of the filler metal naturally. This method is suitable for the thermal analysis in the weld pool or/and weld joint region including starting and stopping transients.     

Effect of welding process parameters on embrittlement of Grade P92 steel using Granjon implant testing of welded joints

Precipitation of Cr-rich carbides, diffusible hydrogen content and heterogeneous microstructure formation across the weldments makes heat-affected zone (HAZ) susceptible to intergranular cracking and makes weldability of creep strength enhanced ferritic (CSEF) Grade P92 steel a critical issue. In the present research work, the Granjon implant test and mercury method (for diffusible hydrogen measurement) have been performed on Grade P92 steel welded specimens to study the effect of welding parameters on diffusible hydrogen levels and their subsequent effect on hydrogen-assisted cracking (HAC). The weld metal was deposited by a shielded metal arc welding process on Grade P92 steel samples using P92 matching filler. The three different welding conditions are used to measure the diffusible hydrogen level in the deposited metal. Granjon implant test was performed to evaluate HAZ HAC susceptibility with similar welding conditions which were used in the mercury method. Lower critical stress (LCS) was also evaluated using the Granjon implant test. The higher susceptibility of CSEF Grade P92 steel welded plate towards HAZ HAC was noticed in case of lower heat input or higher diffusible hydrogen content. However, by considering LCS, fracture mode and diffusible hydrogen content, the weld deposited using the highest heat input (condition III) offers great resistance to HAZ HAC.     

Residual stresses in the welding joint of the nozzle-to-head area of a layered high-pressure hydrogen storage tank

In this study, a sequentially coupling finite element analysis (FEA) program was utilized to simulate the welding temperature and predict the residual stress in the weld and Heat Affect Zone (HAZ) between the nozzle and the head. The results of a numerical calculation indicated that complex residual stresses were generated and concentrated in the weld and HAZ. Due to the existence of the interlayer gap between the two layers of the head, discontinuous stress distributions occurred. There were several sections where there are stress concentrations because of the discontinuous structure. The influences of welding heat input and preheating temperature were also investigated in this study. When the preheating temperature increased, the peak residual stresses in the structure decreased. The welding heat input had little effect on the residual stresses.     

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.     

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.     

SA302C钢热模拟焊接和焊后热处理对热影响区性能的影响

通过热模拟试验研究了SA302C钢在线能量分别为16 200 J/cm和34 650 J/cm时焊缝热影响区粗晶区的组织和性能,并重点比较了不同热处理工艺对粗晶区性能的影响规律.结果表明,在上述焊接线能量范围,热影响区粗晶区的维氏硬度远远高于母材的硬度,粗晶区的主要组成相为马氏体;焊接线能量越高,粗晶区的晶粒尺寸越大.经过热处理后,硬度下降,试样的冲击韧性显著提高;但热处理保温时间对硬度和韧性的影响并不显著.     

Life prediction of repaired welds in a pressurised CrMoV pipe with incorporation of initial damage

Creep damage FE modelling was performed for fully and partially repaired, thick-walled, circumferential pipe weldments, in which initial damage was incorporated into the calculations to take account of the material degradation of the aged materials. The pipe welds were subjected to a realistic internal pressure and axial loading, the latter of which is allowed to vary within the range allowed by design codes. The material properties used are related to a CrMoV weldment at 640 °C. The initial damage distribution was numerically determined using an established procedure. A full post weld heat treatment is assumed to be carried out and the effects of welding induced residual stresses were neglected. The results obtained cover a number of initial damage levels, magnitudes of axial load, and repair excavation depth. On this basis, the sensitivities of the failure life of the repaired welds to these important factors can be evaluated. It was found that both the peak initial damage and the total life are very sensitive to the repair time, particularly when system load is high. The effect of the repair depth for depth: thickness ratios ≥0.5 is generally small for these loadings. There could be a significant benefit if the initial damage in the HAZ of the repair weld, which could be relatively high when the repair time is relatively large, could be reduced by repair welding or by post weld heat treatment.     

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.     

FINITE-ELEMENT MODELING OF HEAT FLOW IN DEEP-PENETRATION LASER WELDS IN ALUMINUM ALLOYS

A two-dimensional finite-element nonlinear transient heat conduction model was developed and used to simulate deep-penetration keyhole laser welds in aluminum alloys. The weld thermal profiles were calculated in an arbitrary reference plane as the laser beam approached and passed the plane. From the calculated thermal profiles, three-dimensional quasi-steady-state shapes of the weld pools were determined. The predicted weld bead shape and dimensions were in good agreement with the experimental results. The experimental laser welds in aluminum alloys contained large amounts of porosity. The model predicted large mushy zones for aluminum laser welds during solidification, which in turn increase the probability of porosity formation by increased bubble entrapment.     

Analysis of residual stresses at weld repairs

In contrast to initial fabrication welds, residual stresses associated with finite length weld repairs tend to exhibit some important invariant features, regardless of actual component configurations, materials, and to some degree, welding procedures. Such invariant features are associated with the severe restraint conditions present in typical repair welding situations. In this paper, residual stress results from several weld repair case studies, using both advanced computational modelling procedures and experimental measurement techniques, are presented and reviewed. From these results, it is evident that weld repairs typically increase the magnitude of transverse residual stresses along the repair compared with the initial weld and that the shorter the repair length the greater the increase in the transverse stress. Also, beyond the ends of the repair the transverse stress sharply falls into compression. For selected cases, predicted stresses are compared with detailed residual stress measurements and the adequacy of finite element simulation procedures is assessed. Welding procedure related parameters (pass lumping, heat input and inter-pass temperature) appear to be more important in analysing weld repairs than in initial fabrication welds. Also great care must be taken when employing simplified two-dimensional cross-section finite element models with applied restraint conditions to simulate the residual stress field at a specific point along the length of a repair.     

Effects of prior damage on the creep failure behaviour of similar and dissimilar welded CrMoV main steam pipes incorporating a partial repair

Creep damage finite element analyses, with the inclusion of “prior damage”, were performed for partially-repaired circumferential welds in a thick-walled, main steam, CrMoV pipe. The repair consists of aged parent material, weld metal and one HAZ region being partially excavated and replaced by new weld metal. The pipe welds were subjected to realistic internal pressure and uniform axial loading, the magnitude of the latter being up to that allowed by design codes. The material properties used are related to those of a CrMoV weldment at 640 °C. It is assumed that a full post-weld heat treatment has been carried out and that the effects of welding induced residual stresses reduce to negligible levels. The results obtained are used to examine the subsequent performance for “similar” and “dissimilar” welds with a range of “repair times” (defined as prior damage levels), magnitudes of axial (system) load, etc. From these results, the failure behaviour of this particular partial repair case was evaluated and discussed.     

The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

The effect of a long post weld heat treatment on the microstructure and mechanical properties of a welded joint in a 0·2%C-1·4%Mn-0·5%Mo pressure vessel steel was studied. Multipass submerged-arc welds were made at a heat input of 1·2 and 4·3 kJ mm⁻¹. Individual microstructural regions observed in the heat-affected zone of the actual weld were simulated. These regions were brittle in the as-simulated condition. Post weld heat treatment for periods of up to 40 h at 620°C resulted in a significant improvement in the Charpy impact toughness. At the same time, a loss of the heat-affected zone and weld metal hardness and transverse weld strenghth occurred. A fracture toughness (J_Ic) of 134 kJ m⁻² was measured in the heat-affected zone of the 4·3 kJ mm⁻¹ welds after prolonged post weld heat treatment. The improvement in weldment toughness with post weld heat treatment was primarily attributed to softening of the structure.