Simulations in multipass welds using low transformation temperature filler material

Transient thermal and residual stress fields in flux-cored arc welds were examined using a finite element (FE) model. Experimental multipass welds were produced using both conventional and low transformation temperature (LTT) filler metals. Temperature-dependent material properties and both convective and radiant heat loss boundary condition have been considered in the FE model. The effects of the transformation temperature and interpass intervals on residual stresses were examined. It was found that compressive longitudinal residual stresses were developed at the weld centreline in the LTT filler metal. A short-time interpass interval causes the weld fusion zone to be above the martensite start temperature allowing the optimal use of the phase transformation effect. The FE model is sensitive to alteration in welding parameters and can satisfactorily predict the residual stress distribution in welded parts.     

Effects of dilution and baseplate strength on stress distributions in multipass welds deposited using low transformation temperature filler alloys

Transformation plasticity can be utilised to control residual stresses in steel welds. This requires special filler alloys that transform at a sufficiently low temperature to compensate for accumulated thermal contraction strains. However, the welding parameters needed to optimise the effect in multipass joints have yet to be established. This topic has been investigated by characterising the residual stress distribution in multipass welds fabricated with different welding alloys and baseplates using neutron diffraction to assess the effects of dilution and baseplate strength. While the use of richly alloyed weld metal does enhance fatigue performance in single pass joints, the extent of stress relief that can be derived from transformation plasticity is reduced due to incomplete martensitic transformation when further layers are deposited. For all cases studied, compressive stresses were measured in the weld metal with balancing tensile stress in the heat affected zone of the plate. The magnitude of the tension was observed to be a function of the strength of the baseplate. Recommendations are also presented for the combination of welding and material parameters that lead to the optimum exploitation of transformation plasticity as a method for boosting the fatigue performance of multipass welded joints.     

Effect of low transformation temperature weld filler metal on welding residual stress

Abstract

The effect of weld filler metal austenite to acicular ferrite transformation temperature on the residual stresses that arise during the gas metal arc welding of a low carbon steel has been examined using a finite element model. It was found that the stress levels in the weld can be tailored by the appropriate selection of the filler metal and compressive, near zero or tensile residual stresses produced. Reasonable agreement was obtained between the model and the stresses measured using neutron diffraction both in welds using conventional and low transformation temperature filler metal.     

Surface residual stresses in multipass welds produced using low transformation temperature filler alloys

Tensile residual stresses at the surface of welded components are known to compromise fatigue resistance through the accelerated initiation of microcracks, especially at the weld toe. Inducement of compression in these regions is a common technique employed to enhance fatigue performance. Transformation plasticity has been established as a viable method to generate such compressive residual stresses in steel welds and exploits the phase transformation in welding filler alloys that transform at low temperature to compensate for accumulated thermal contraction strains. Neutron and X-ray diffraction have been used to determine the stress profiles that exist across the surface of plates welded with low transformation temperature welding alloys, with a particular focus on the stress at the weld toe. For the first time, near surface neutron diffraction data have shown the extent of local stress variation at the critical, fusion boundary location. Compression was evident for the three measurement orientations at the fusion boundaries. Compressive longitudinal residual stresses and tensile transverse stresses were measured in the weld metal.     

In situ analysis of the strain evolution during welding using low transformation temperature filler materials

ABSTRACT

Compared to conventional welding consumables using low transformation temperature (LTT) filler materials is an innovative method to mitigate tensile residual stresses due to delayed martensite transformation of the weld. For the effective usage of LTT filler materials, a deeper understanding of the complex processes that lead to the final residual stress state during multi-pass welding is necessary. Transformation kinetics and the strain evolution of multi-pass welds during welding were investigated in situ at the beamline HEMS@PETRAIII, Germany. Compared to conventional welds, the total strain was reduced and compression strain was achieved when using LTT filler materials. For an optimal use of the LTT effect in the root of multi-pass welds, the alloying concept must be adapted taking care of dilution.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

Pass-by-pass stress evolution in multipass welds

In multipass welding, each successive thermal cycle will introduce local melting, solid state phase transformations, grain growth, grain refinement, recrystallisation and recovery, all of which lead to a complicated stress state. Most stress measurements performed on multipass welded components represent the final residual stress state. Information concerning stress evolution on a pass-by-pass basis is difficult to find. In this investigation, six pass welds were made on high strength quenched and tempered steel sections, and depth resolved strain measurements in two orthogonal directions were carried out after each weld pass using energy dispersive synchrotron X-ray diffraction. The residual stresses were calculated using biaxial Hooke’s law. A thermal–metallurgical–mechanical welding model was constructed and validated with temperature and pass-by-pass stress measurements, which improves the reliability of the model. Cross-sectional stress distributions are presented after each pass, revealing the weld stress development in multipass welds.     

Effect of welding parameters on residual stress in type 420 martensitic stainless steel

Abstract

The effect of welding parameters on residual stress induced by shrinkage of weldment and metallurgical phase transformation in type 420 martensitic stainless steel has been investigated. In this study, type 1018 low carbon steel was adopted as the base metal and type 420 martensitic stainless steel was used for the filler metal during submerged arc welding. The thermal cycles at various locations were recorded and dilatometry was used to examine the martensite phase transformation temperatures. The experimental results show that the residual stress increased with the heat input during welding. Using a higher welding heat input increased the amount of heat going into the weldment and elevated the martensite phase transformation temperature. Residual stresses could not be significantly reduced by increasing preheat (interpass) temperature while welding. Using higher preheat temperature conditions could elevate the equilibrium temperature and the martensite phase transformation temperature and increased the heat input to the weldment.     

On the evolution of local material properties and residual stress in a three-pass SA508 steel weld

In multi-pass welds, the development of residual stress generally depends on the response of the weld metal, heat-affected zone (HAZ) and nearby parent material to complex thermo-mechanical cycles. Here, the evolution of local material properties and residual stress was investigated for each of these zones during the manufacture of a three-pass groove weld in SA508 steel. Residual stress distributions were measured by neutron diffraction for a sample in which only one weld bead had been deposited, and again for a completed three-pass weld. The evolution of material properties was also characterised by measuring local hardness and conducting tensile tests on small coupons that were extracted from the same welded specimens. Overall, the addition of subsequent weld passes resulted in lower peak tensile residual stresses in the weld as a whole, softening of the HAZ, modest cyclic hardening in nearby parent metal, and some softening in the low-carbon weld metal.     

大型电机转子焊接残余应力的数值分析

利用轴对称模型研究了大型电机转子焊接残余应力分布规律 ,探讨了单层单道焊情况下两侧同时焊接、热套、预热等工艺以及三层四道焊情况下焊接顺序对焊接残余应力的影响。研究结果表明 ,磁轭圈与辐板焊接后在焊缝及其周围区域产生较大的三向残余拉应力 ;两侧同时焊接可大大降低径向残余应力 ;热套可降低三向残余拉应力 ,热套后直接焊与热套后先冷却后焊相比效果更佳 ;预热可以降低周向残余拉应力 ;多道焊时径向残余应力主要取决于最后一层 ,尤其是最后一道焊缝 ;两侧的最后一道焊缝同时焊接可显著降低径向残余应力 ,而前面的焊道同时焊接与否并不重要。研究结果为优化生产工艺 ,降低残余应力提供了理论依据     

X80管线钢平板多层对接焊数值模拟

管线钢在实际焊接过程中多采用多层焊,其焊接过程较单层焊更为复杂。利用SYSWELD专业焊接模拟软件,对X80管线钢中厚板平板多层焊焊接温度场及应力场进行数值模拟,研究焊接速度、预热温度及层间温度对焊接温度场和熔深的影响。结果表明,随着焊接速度的增加,焊接温度场的最高温度下降,熔深减小。提高预热及层间温度对温度场无显著影响。残余应力集中在焊缝及近缝的热影响区。最大纵向残余应力出现在打底层的焊缝根部,其峰值大于横向残余应力峰值。     

Evolution of residual stresses in dissimilar steel surfacing layers during process of post-weld heat treatment

Abstract

Residual stress relaxation during post-weld heat treatment (PWHT) is a thermodynamic process, which is affected not only by the heat treatment process, but also by the welding residual stress. In this study, the residual stresses in as welded and heat treated surfacing metal were measured using blind hole and X-ray method. The results reveal that the welding residual stresses are compressive at the surface of the weld and tensile at inner weld. However, after PWHT, the residual stresses at surface and inner weld change to the opposite state. Finite element simulations show that the differences of expansion coefficients between base metal and filler material are the main factor to the changes of stress state. The experimental results verify that the expansion coefficients of base metal and filler materials have been changed greatly after long soaking at high temperature.     

Numerical modelling of temperature distribution during multipass welding of plates

Abstract

Welding is a reliable, cost effective, and efficient metal joining process. Manual metal arc welding (MMAW) is a widely used welding process in industry and multipass welding is very frequently used in the MMAW process. The temperature distribution that prevails during multipass welding affects the material microstructure and hardness of the regions near the weld, and the residual stresses that will be present in the material after cooling to room temperature. These changes will consequently affect the performance of the welded joint. In the present work, a computer model based on the control volume method has been developed to predict the temperature distribution during multipass welding of plates using the MMAW process. To validate the computer model, the temperature distribution was measured experimentally in a 12 mm thickness stainless steel weld pad during multipass welding. The welding parameters were used as input data for the computer model. The computer predictions and the experimentally measured temperature distributions agree well.     

Influence of Filler on the Microstructure,Mechanical Properties and Residual Stresses in TIG Weldments of Dissimilar Titanium Alloys

The influence of titanium alloy (Ti-5Al-2.5Sn) and commercially pure titanium (cpTi) as fillers on dissimilar pulsed tungsten inert gas weldments of Ti-5Al-2.5Sn/cpTi was investigated in terms of microstructure,mechanical/nano-mechanical proper-ties,and residual stresses.A partial martensitic transformation was observed in the weldments for all the welding conditions due to high heat input.The microstructure evolved in the FZ/cpTi interfacial region was observed to be the most sensitive to the proportion of α stabilizer in the filler alloy.Furthermore,the addition of filler alloy improved the tensile properties and nano-mechanical response of the weld joint owing to the increased volume of metal in the weld joint.As compared to the Ti-5Al-2.5Sn wire,the use of cpTi filler wire proved to be better in terms of energy absorbed during tensile and impact tests,tensile strength and ductility of the dissimilar welds.An asymmetrical residual stresses profile was observed close to the weld centerline,with high compressive stresses on the Ti-5Al-2.5Sn side for both the weldments obtained with and without filler wires.This was attributed to mainly the low thermal conductivity of Ti-5Al-2.5Sn.The presence of residual stresses also influenced the nano-hardness profile across the weldments.     

Three dimension simulation analysis of the interpass stress and deformation during multipass welding

It has been widely studied about the final residual stress and deformation in muhipass welding of thick weldments. But there is a lack of a clear understanding of the interrelationship of interpass stress and deformation during multipass welding. In this study, a three dimension numerical model of a sixteen-pass double V-groove welded joint with 50 mm plate is developed to compute the stress field and deformation by using multiple CPU parallel processing technology. The following factors such as the non-linear of temperature, heat radiation, filling of material step by step and so on are considered. Distribution and evolution law of welding stress in the transverse and longitudinal section is analyzed in this paper, and the interpnss stresses are studied also. At the same time the evolution course of angular deformation amount is analyzed, and the experimental results show that the calculated resuhs accord with the measured results of angular deformation.     

Transient thermal stress analysis on rapid post-weld heat treatments applied to flash butt welded rails

Abstract

A finite element (FE) model has been established to estimate the transient thermal stresses developed during rapid post-weld heat treatments (RPWHT) and post-weld quenching (PWQ) of rail flash butt welds. Rapid post-weld heat treatment parameters, such as heat flux intensity, location and heating duration, were varied. Altering the PWQ initiation times was also studied. The sequentially coupled thermomechanical FE model incorporated rate independent plasticity with a constitutive linear kinematic hardening rule. This described the inelastic behaviour of the rail material caused by post-weld thermal cyclic loadings. Representative temperature dependent thermal and mechanical properties including the pearlitic transformation behaviour of rail steel were also used in the FE model. Significant reduction of local tensile residual stresses in the web region of the weld was obtained by RPWHT applied to the entire foot region. RPWHT applied to both the web and the foot regions of the weld resulted in further reductions in vertical tensile residual stresses but a lesser effect on the longitudinal tensile residual stresses. A series of PWQ conditions, which were initiated after completion of the austenite–pearlite phase transformation, had significantly affected residual stresses. Numerical predictions revealed that PWQ initiated after transformation induced higher tensile residual stresses in the web region of the weld with respect to normal cooled welds, without affecting the microstructure.     

Effect of interpass temperature on microstructure,impact toughness and fatigue crack propagation in joints welded using the GTAW process on steel ASTM A743-CA6NM

Mainly due to their great toughness, martensitic stainless steels are used for manufacturing hydraulic turbines. However, these steels have some restrictions regarding regions recovered by welding, mainly due to the formation of non-quenched martensite, which causes a reduction in toughness. Considering repair of hydraulic turbines, there is a great interest in developing welding procedures that increase impact toughness and avoid post-welding heat treatment (TTPS). This study aims to analyse the influence of interpass temperature on microstructure, impact toughness and fatigue crack propagation in multipass welded joints on martensitic stainless steel CA6NM, using AWS410NiMo filler metal and the gas tungsten arc welding (GTAW) process. In the sample with interpass temperature of 80°C, influence of the interpass temperature on the formation of ferrite δ, with intragranular formation in the two-phase δ field, was observed, while in the sample welded at 150°C, the formation of ferrite δδ occurred mainly in the single-phase field. The change in the formation of ferrite δ, with the low interpass temperature, promoted an increase in impact toughness and a decrease in the fatigue crack propagation when compared with the sample welded with a higher interpass temperature. The results obtained indicate that the TIG process is an excellent alternative for the repair of CA6NM steel, with a significant influence from the interpass temperature.     

Prevention of microcracking in dissimilar multipass welds of alloy 690 to type 316L stainless steel by Ce addition to filler metal

Abstract

The microcracking susceptibility in dissimilar multipass weld metals was investigated by a multipass weld test using different type 316L stainless steels with varying P and S contents and using different alloy 690 filler metals with varying Ce contents. The relation between microcracking susceptibility and (P+S) and Ce contents in every weld pass of the multipass weld was investigated. Ductility dip cracks occurred in the compositional range of Ce/(P+S)<0·22, and solidification/liquation cracks occurred in that of Ce/(P+S)>1·1, while no cracks occurred at Ce/(P+S) between 0·22 and 1·1. The ductility dip cracking susceptibility could be improved by adding Ce due to scavenging of impurity elements. Microcracking could be completely prevented in dissimilar multipass weld metals using two kinds of filler metals containing 0·077 wt-%Ce for the weld passes beside the stainless steel base metal (320 ppm P and 183 ppm S) and containing 0·032 wt-%Ce for the other weld passes.     

Effect of dilution on fatigue behaviour of welded joints produced by low-transformation-temperature fillers

ABSTRACT

Low-transformation-temperature (LTT) fillers with various martensitic transformation start (M_s) temperatures were used to produce fillet welds. In comparison with conventional welds, the fatigue strength of the LTT fillet welds offers a significant improvement, with a minimum increase of 145%. Owing to the substantial dilution of base metal, the Cr and Ni alloying elements in the LTT weld metals decrease, resulting in an increase of the M_s temperatures. Therefore, the fillet welds produced using the LTT filler material with the lower M_s temperature (92°C) exhibit a larger compressive residual stress and higher fatigue strength.     

Residual stresses at weld repairs and effects of repair geometry

In this study, an in-depth investigation of repair weld geometry effects on residual stress distributions in terms of both through-thickness membrane and bending content as well as its contribution to fracture driving force is carried out. Some major findings are: A fundamental difference in residual stress distributions between initial and repair welds can be characterised as a significant elevation of membrane and bending content in transverse residual stresses. The dominance of repair-induced residual stresses over those generated by initial welds suggests that initial weld residual stresses can be assumed negligible in repair weld modelling. A weld repair should be designed as long as possible, as narrow as possible and as shallow as possible.