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.     

Microstructure and residual stress variations in weld zone of flash-butt welded railroads

Abstract

In the weld zone and base metal contiguous to the weld zone in flash-butt welded rail, the web of the rail has high tensile residual stresses and the head and base of the rail have compressive residual stresses. The web region is susceptible to failure since most of the weld zone of the rail is coarse grained and has porosity, inclusions and defects resulting from rapid solidification of molten metal entrapment in the weld. Efforts to reduce the amount of these tensile residual stresses require recognition of their causes. In this research, microscopic and macroscopic studies were carried out on vertical and horizontal sections of the weld zone in the head, web and base of the rail. Just after flash-butt welding, the temperature of the web between the current carrying copper electrodes is higher than the temperature of the head and base of the rail. Therefore, by cooling the weld zone to room temperature, the amount of web contraction between the electrodes is higher than the amount of base and head contraction and consequently tensile residual stresses are produced in the web at and near the weld zone. In the head and base of the rail, compressive residual stresses are developed.     

Internal residual stress measurement on inertia friction welding of nickel-based superalloy

The internal residual stress in the narrow inertia friction welding (IFW) welds of FGH96 nickel-based superalloy was measured with the contour method (CM). The as-welded internal hoop and axial residual stresses were obtained after two cuts and the detailed steps of the CM measurement were described. In addition, the hole-drilling method was used to obtain the surface stress. Furthermore, the internal hoop residual stress of a FGH96 superalloy IFW specimen after post-weld heat treatment (PWHT) was also measured with the CM after a single cut. The measured results show that the peak hoop residual stress is not symmetrical about the weld centreline. Axial residual stress on the outer surface at the weld centreline is compressive stress, while tensile stress appears on the inner surface, and it varies linearly along the thickness. The peak values of hoop tensile and compressive stresses decrease dramatically after PWHT.     

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.     

Effects of post-weld heat treatments on the residual stress and mechanical properties of electron beam welded sae 4130 steel plates

The distribution of the residual stresses of electron beam welded SAE 4130 and the effect of stress relief after various post- weld heat treatments (PWHT) were measured using X- ray diffraction. The mechanical properties and microstructure were also examined. Experimental results show that the tensile residual stress increased with the heat input of the electron beam. Most of the residual stresses were relieved by the PWHT at 530 °C for 2 h followed by furnace cooling to 50 °C. The strength of the welds decreased slightly, and the elongation of the welds increased after PWHT.     

Simulating the residual deformation of thin-plate butt weldments without wire filling by contact mode

The residual deformation in thin-plate butt weldments without wire filling was investigated by simulative and experimental methods. The good consistency on the residual deformation between the simulation result and test measurement data indicates that the contact mode can be used to simulate the butt welding process of thin plates without wire filling. The longitudinal residual tensile stress in the weld zone lead to the buckling deformation of thin-plate weldments, and the appearance of buckling deformation would in turn reduce the peak value of longitudinal residual tensile stress. As for the thin-plate butt weldments, the tensile stresses in the weld and its neighboring zone are produced in the post-weld cooling process and have not peak until the temperature of the welds dropped to near room temperature.     

Numerical Investigation on Residual Stresses of the Safe-End/Nozzle Dissimilar Metal Welded Joint in CAP1400 Nuclear Power Plants

The residual stress evolution in a safe-end/nozzle dissimilar metal welded joint of CAP1400 nuclear power plants was investigated in the manufacturing process by finite element simulation. A finite element model, including cladding, buttering, post-weld heat treatment (PWHT) and dissimilar metal multi-pass welding, is developed based on SYSWELD software to investigate the evolution of residual stress in the aforementioned manufacturing process. The results reveal a large tensile axial residual stress, which exists at the weld zone on the inner surface, leads to a high sensitivity to stress corrosion cracking (SCC). PWHT process before dissimilar metal multi-pass welding process has a great influence on the magnitude and distribution of final axial residual stress. The risk of SCC on the inner surface of the pipe will increase if PWHT process is not taken into account. Therefore, such crucial thermal manufacturing process such as cladding, buttering and post-weld heat treatment, besides the multi-pass welding process, should be considered in the numerical model in order to accurately predict the distribution and the magnitude of the residual stress.     

Influence of pre- and post-weld heating on weldability of modified 9Cr–1Mo(V–Nb) steel pipe under shielded metal arc and tungsten inert gas welding processes

Abstract

Welding of modified 9Cr–1Mo(V–Nb) steel pipes has been carried out via shielded metal arc (SMA) and tungsten inert gas (TIG) welding processes. The weld joints have been produced using different preheating temperatures, followed by post-weld heat treatment (PWHT) at various temperatures. The microstructures of the weld and of the heat affected zone (HAZ) of the weld joints have been studied under the optical microscope and correlated with the preheating and PWHT. The average hardness of the weld and different regions of the HAZ, and tensile properties of the weld joints have also been studied and correlated with the preheating and PWHT. The tensile properties of the SMA and TIG weld joints produced using preheating and PWHT at various temperatures are compared and correlated with their microstructures. It is noted that a comparatively high preheating temperature of the order of 573 K is beneficial, and PWHT is necessary to reduce the susceptibility to cold cracking of weld joints of the present steel. The PWHT at 1123 K enhances ductility to fracture, but decreases the tensile strength of the base material, causing fracture of both the SMA and TIG weld joints from this region close to the HAZ. The tensile properties of SMA welds are found to be superior to those of the TIG welds, especially for PWHT at temperatures up to 1023 K.     

Cr-Mo钢管子局部焊后热处理加热宽度准则的确定

采用粘弹塑性有限元方法从应力释放角度讨论了Cr-Mo钢局部热处理下各种优化加热条件.研究表明,局部热处理过程产生的瞬态和残余应力受蠕变性能和温度关系弹性模量等因素的影响.当加热宽度足够大时,局部热处理后的应力与炉中热处理的应力接近.通过与现有标准的比较,从应力释放角度位于焊缝中心5√Rt的加热宽度较为合理.提出了考虑焊接冷却过程Cr-Mo低温膨胀现象的相变模型.给出了焊接残余应力与局部焊后热处理应力的试验结果与模拟结果的对比.     

Effects of annealing on microstructure and properties of linear friction welded dissimilar titanium joints

Linear friction welds of Ti–6Al–4V (TC4, according to Chinese classification) to Ti–6·5Al–3·5Mo–1·5Zr–0·3Si (TC11, according to Chinese classification) were subject to post-weld heat treatment (PWHT) at 650°C×4 h (PWHT1) and at 950°C×1 h+530°C×4 h (PWHT2) under air cooling. In the as welded joint, TC4 had recrystallised sufficiently compared to TC11. After PWHT1, the α grains in the TC4 weld centre zone grew to some extent and many superfine equiaxed recrystallised α grains precipitated along β boundaries on the TC11 side. The grain growth along the weld line in the weld centre zone under PWHT2 was evident. PWHT2 reduced the joint tensile strength due to the coarsening of the microstructure. The joint microhardness decreased after PWHT.     

Distortion control in welding by mechanical tensioning

Abstract

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.     

焊后热处理对输送H_2S介质压力管道维修焊接接头组织性能的影响

采用手工钨极氩弧焊(TIG焊)打底,焊条电弧焊(SMAW)填充、盖面的焊接工艺对输送H_2S介质压力管道(20钢)进行维修焊接,并对其焊接接头进行720～750℃下保温1h的焊后热处理.对热处理前、后焊接接头组织和性能进行分析研究,结果表明:720～750℃×1h热处理使焊缝区与热影响区成针状分布的魏氏组织铁索体晶粒得到细化,并使其呈退化形态;焊缝区、热影响区与母材的显微硬度整体呈下降趋势;焊接接头抗拉强度大幅度提高,断裂均处在远离焊缝的母材上.焊后热处理有效地降低了焊接接头的残余应力,改变了残余应力的分布特征,使其分布趋于"均匀",拉应力大幅度降低.     

Residual welding stresses in laser beam and tungsten inert gas weldments of titanium alloy

Abstract

The residual welding stresses in laser beam (LB) and tungsten inert gas (TIG) weldments of a titanium alloy in thin plate form were investigated experimentally in the present work. A hole drilling technique was used to measure the residual stresses in the weldments. The effects of the welding method and post-weld heat treatment (PWHT) on the residual stresses were analysed. The results show that (i) the residual stress distribution in the LB welded joints is similar to that obtained for traditional fusion welding processes, although the distribution zone is much narrower in LB welding, (ii) the residual stress in the heat affected zone for LB welding is about 100 MPa lower than that for TIG welding, and (iii) PWHT in vacuum greatly relieves the welding residual stress.     

Residual stress engineering in friction stir welds by roller tensioning

Abstract

The authors investigate the efficacy of applying rolling pressure along the weld line in thin butt welds produced using friction stir welding (FSW) as a means of controlling the welding residual stresses. Two cases are examined and in each case, comparison is made against the as welded condition. First, for FSW of AA 2024 aluminium alloy, roller tensioning was applied during welding using two rollers placed behind and either side of the FSW tool. Very little effect was seen for the down forces applied (0, 50, 75 kN). Second, for FSW AA 2199 aluminium alloy, post-weld roller tensioning was applied using a single roller placed directly on the FS weld line. In this case, significant effects were observed with increased loading, causing a marked reduction in the longitudinal tensile residual stress. Indeed, a load of just 20 kN was sufficient to reverse the sign of the weld line residual stress. Only slight differences in Vickers hardness were observed between the different applied loads. Furthermore, unlike some methods, this method is cheap, versatile and easy to apply.     

Recent developments in repair welding technologies in Japan

Abstract

Developments in some difficult repair welding technologies in Japan during the past decade are reviewed. The topics covered include the repair welding of bridges in service, the temper bead method which makes it possible to omit post-weld heat treatment (PWHT) of repaired pressure vessels, the seal welding of a reactor vessel in which stress corrosion cracks were detected, low heat input repair welding of neutron irradiated stainless steel and nickel based alloys, the prevention of solidification cracking in repair welding of aged heat resistant cast steels, the development of welding materials for the mending of single crystal nickel based superalloy turbine blades, underwater repair welding of nuclear reactors, the reduction of residual stresses in repair welding, and an ultrasonic testing method for nickel based weld metals. The local PWHT of creep resistant ferritic steel tubes is also reviewed.     

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.     

残余应力对钢轨焊接接头落锤试验性能的影响

对钢轨窄间隙电弧焊接头热处理前后的残余应力分布进行了测量和比较,结合落锤试验结果和断口形貌,分析了焊接残余应力对钢轨接头落锤试验性能的影响.结果表明,热处理前,钢轨接头的轨腰和轨颚部位存在较大残余拉应力,在落锤冲击载荷作用下容易形成裂纹源,并沿轨腰向母材延伸、扩展、断裂;热处理后,钢轨接头的残余应力最大值下降1/2~2/3,抗冲击载荷能力显著提高,落锤试验时的钢轨接头沿焊缝中心或热影响区垂直断裂.钢轨接头存在较大残余应力时,在落锤冲击载荷作用下,容易在残余应力较大的部位产生裂纹,并加速扩展和断裂.     

Effect of post weld heat treatment on weldability of high entropy alloy welds

The effect of post weld heat treatment (PWHT) temperature on laser beam welds in high-entropy alloys (HEAs) using a cold-rolled cantor system (CoCrFeMnNi) was investigated. Laser welding of low heat input was applied to reduce thermal distortion. The cold-rolled HEA welds indicated larger grain size and inferior tensile/hardness properties as compared to the base metal (BM). By applying PWHT, the welds showed superior hardness to the BM with no variation in the face-centred cubic phase and a decrease in the size and fraction of CrMn oxide inclusions. As the PWHT temperature increased (800–1000°C), the variation in the grain size decreased between the weld metal and heat-affected zone, thus resulting in approximately the same tensile strength and elongation of the transverse welds as compared to the BM.     

Investigations into the tendency to stress corrosion of joints welded in austenitic and duplex steels

Austenitic stainless steel is welded as a cladding on the inner surface of a reactor pressure vessel (RPV) made of low alloy steel. In order to assess the structural integrity of the RPV precisely, the residual stress distribution caused by weld-overlay cladding and post-weld heat treatment (PWHT) is evaluated. Since the cladding layer is very thin compared to the vessel wall, it is necessary to evaluate the residual stress distribution around the weld fusion line, which can be very steep. In this study, cladded specimens were fabricated using different welding methods. Residual stress measurements using both sectioning and deep hole drilling (DHD) methods were then performed to evaluate the residual stress distributions through the weld fusion line. Three-dimensional thermal-elastic-plastic-creep analyses based on the finite element method were also conducted to evaluate the residual stress caused by weld-overlay cladding and PWHT. It was shown that analytical results provided reasonable agreements on weld residual stress with experimental results. It was also clarified that the main cause of residual stress due to welding and PWHT was the difference of thermal expansion between weld and base metals.     

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

Neutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.