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.     

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.     

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.     

Analysing the influences of weld size on fatigue life prediction of FCAW cruciform joints by strain energy concept

The effect of weld size on fatigue life of flux cored arc welded (FCAW) cruciform joints containing lack of penetration (LOP) defect has been analysed by using the strain energy density factor (SEDF) concept. Moreover, new fracture mechanics equations have been developed to predict the fatigue life of the cruciform joints. Load carrying cruciform joints were fabricated from ASTM 517 ‘F’ grade steel. Fatigue crack growth experiments were carried out in a vertical pulsar (SCHENCK 200 kN capacity) with a frequency of 30 Hz under a constant amplitude loading (R=0). It was found that the crack growth rates were relatively lower in the larger welds fabricated by the multipass welding technique than the smaller welds fabricated by the single pass welding technique.     

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.     

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.     

Thermal analysis of a wet-disk clutch subjected to a constant energy engagement

A combined theoretical and experimental thermal analysis is conducted on the plates within a wet clutch for one engagement. An analytical model using the separation of variables technique is developed to simulate the temperature rise due to the non-conservative friction and relative motion between the steel plates and friction plates of the clutch. A complimentary numerical model is also developed to compute the temperature distribution in the steel plate. The experiment performed included a wet clutch instrumented with thermocouples and installed in a power-shift transmission where the temperature rise during one clutch engagement was measured. The total energy is then estimated by accounting for system inertia, torque and rotating speeds. Finally, the temperature rises predicted by the analytical and numerical models are validated with the experimental data.     

Thermal Stresses and Distortion Developed in Mild Steel DH36 Friction Stir-Welded Plates: An Experimental and Numerical Assessment

Welding processes involve localized heating which in turn give rise to thermal stresses and distortion. Friction stir welding (FSW) is a solid state joining process where temperatures below melting are experienced. Nonetheless, some degree of thermal heating and consequently thermal stresses develop at the joint. This study aims to quantify the stresses developed in friction stir welding of mild steel DH36 plates, through an experimental and numerical investigation. The temperatures and transient strains developed during FSW, are experimentally measured and used to validate thermo-elastoplastic numerical models. These models are used to investigate the evolution of thermal stresses and distortion for different welding parameters.     

低合金调质高强钢焊接工艺研究进展

对国内低合金调质高强钢焊接工艺的研究情况进行了综述，在高强钢焊接的各个控制点都得到了比较丰富的经验和总结，认为伴随高强钢焊接出现的热影响区软化、脆化和冷、裂纹方面应采取控制线能量、预热、根据不同要求匹配不同级别焊材等措施，还应对焊材管理、施焊环境、顺序等作出更严格的要求是能够得到性能优良的焊接接头的。对今后低合金调质高强钢更多更广泛的应用提供了参照和借鉴并指出了工艺控制的重点和关键点。     

Comparative analysis of TIG welding distortions between austenitic and duplex stainless steels by FEM

In this study, thermal stress analyses were performed in the tungsten inert gas (TIG) welding process of two different stainless steel specimens in order to compare their distortion mode and magnitude. The growing presence of non-conventional stainless steel species like duplex family generates uncertainty about how their material properties could be affected under the welding process. To develop suitable welding numerical models, authors must consider the welding process parameters, geometrical constraints, material non-linearities and all physical phenomena involved in welding, both thermal and structural. In this sense, four different premises are taken into account. Firstly, all finite elements corresponding to the deposition welding are deactivated and, next, they are reactivated according to the torch’s movement to simulate mass addition from the filler metal into the weld pool. Secondly, the movement of the TIG torch was modelled in a discontinuous way assuming a constant welding speed. Thirdly, the arc heat input was applied to the weld zone using volumetric heat flux distribution functions. Fourthly, the evolution of the structural response has been tackled through a stepwise non-linear coupled analysis. The numerical simulations are validated by means of full-scale experimental welding tests on stainless steel plates. Finally, the results and conclusions of this research work are exposed.     

Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate

Clad plates are widely used in the construction of corrosion resistant equipment. During the repair of clad plates, residual stresses are generated and influence the structure integrity. This paper uses the finite element method (FEM) to predict the residual stresses in a repair weld of a stainless steel clad plate. The effect of repair width on residual stresses has also been investigated by numerical simulation. Due to the material mismatching between clad metal and base metal, a discontinuous stress distribution has been generated across the interface between clad and base metals. The peak residual stress occurs in the heat affected zone (HAZ) of the base metal, because the yield strength of the base metal is larger than that of the clad metal. With an increase in repair width, the residual stresses are decreased. When the repair width is increased to 24 mm, the residual stresses in the weld have been decreased greatly and the peak residual stresses have been reduced to less than the yield strength. Therefore, the recommended repair width should not be less than 24 mm, which provides a reference for optimizing repair welding technology for this stainless steel clad pate.     

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.     

Performance of Power Plant Spray Cooling Modules and Systems

The detailed three-dimensional local interaction model developed previously for spray units is used here to predict the cooling performance of both a pass of sprays coaxially aligned with the wind, and a multipass spray system. Since the model predicts the local changes in temperature and humidity of the air-vapor phase as well as droplet trajectories and temperatures, it is capable of determining the degree of interference between adjacent spray units as a function of upwind conditions and spray spacing without recourse to empirical interference factors. Model predictions are compared with droplet cooling and wet-bulb temperature data for spray passes consisting of units of the Power Spray Module. The thermal efficiency of a pass of sprays is examined by use of a theoretically generated dimensionless performance chart.. Such charts are utilized in a computer model of a multipass spray canal system. The model shows excellent agreement with system data. System design and performance charts are presented for industrial use.     

Thermal and electrical effects of basbars on Li-Ion batteries

Keeping temperature under control is critical for batteries to work effectively. When the safe operating range is exceeded, both the lifetime of batteries decreases and undesirable situations may occur such as fire. In this study, the effects of basbars on battery modules are examined. It is shown that the most intense current transitions pass through the shortest path connecting the batteries. When the distance between the batteries is increased, cooling is effective and the temperature of the batteries is expected to decrease. However, this is not the case in our study. It is observed that the temperature of the battery modules increases due to the increased length of the basbar. For this reason, it is emphasized that basbars are a parameter that cannot be ignored with respect to the temperature of battery modules. In this study, six different materials (ie, silver, copper, gold, nickel, steel and titanium) are used as basbars when trying to achieve the best results. For each basbar material, the maximum temperature values and discharge curves reached by the battery module are found. The results suggest that silver is the best basbar material. For the silver material, the temperature distributions of the battery modules, the maximum temperature value received by the module and the discharge curves are found for different C ratios.     

Single and double pass solar air heaters with wire mesh as packing bed

The thermal performances of single and double pass solar air heaters with steel wire mesh layers are used instead of a flat absorber plate are investigated experimentally. The effects of mass flow rate of air on the outlet temperature and thermal efficiency were studied. The results indicate that the efficiency increases with increasing the mass flow rate for the range of the flow rate used in this work between 0.012 and 0.038 kg/s. For the same flow rate, the efficiency of the double pass is found to be higher than the single pass by 34–45%. Moreover, the maximum efficiencies obtained for the single and the double pass air collectors are 45.93 and 83.65% respectively for the mass flow rate of 0.038 kg/s. Comparison of the results of a packed bed collector with those of a conventional collector shows a substantial enhancement in the thermal efficiency.     

Fracture toughness of 15X2HMFA steel and its welds

The generalized experimental fracture toughness results of industrial semi-products, made from Cr---Ni---Mo---V steel and intended for VVER-1000 reactor pressure vessel manufacture, are presented. On the base of test results of nine steel heats having a common purity (sulfur to 0.018%; phosphorus to 0.015%; and copper to 0.20%) and of the same number of heats having a higher purity (sulfur to 0.012%; phosphorus to 0.012%; and copper to 0.06%) the fracture toughness temperature dependences were constructed. These differ appreciably from the reference curves for this material according to PNAE G-7-002-86. The temperature dependences K_1c for 15X2HMFA steel welds were also obtained. By this, for welds produced using manual arc welding and the types RT-45A and RT-45B electrodes as well as electroslag welding with Sv-16X2HMFTA wire and OF-6 flux, these dependences were constructed on the limited number of welded samples, as these welding methods are, practically, not used in the manufacture of such reactors. The fracture toughness assessment for welds produced by submerged arc welding was carried out for two variants of production processing (Sv-10XGNMAA wire and AH-17M flux; Sv-08XGHMTA wire and NF-18M flux), which were applied in the fabrication of circumferential welds for a VVER-1000 reactor.     

TRANSIENT VARIATIONS OF THERMAL STRESSES AND THE RESULTING RESIDUAL STRESSES WITHIN A THIN PLATE DURING WELDING PROCESSES

Variations of thermal and residual stresses are investigated inside a thin mild steel plate during welding processes. The temperature distribution is determined analytically using Green's functions. Transient thermal stresses developed within the plate are computed numerically. The resulting residual stresses, which remain after cooling of the plate, are found based on a method presented originally by Tall (L. Tall, Welding Journal, vol. 43, pp. 10–23, 1964). It is found that welding speed and heat source intensity are the main factors that affect the residual stress formation in the plate.     

A review of factors affecting toughness in welded steels

This review paper discusses the local toughness at various positions within a weldment in plain carbon, carbon-manganese, microalloy and low-alloy ferrite steels. Single pass weldments in such steels, which are austenitic where temperatures during welding exceed the critical values for that steel but which transform to other phases at lower temperatures, can be divided into six zones—subcritical heat-affected zone (HAZ), inter-critical HAZ, grain-refined HAZ, grain-coarsened HAZ, partially molten zone and weld metal.Microstructural and compositional factors which affect toughness are qualitatively reviewed for each of these zones, in both as-welded and heat-treated conditions. Some areas which require further research are pointed out.     

Mechanical behaviour of HTR materials: Developments in support of defect assessment,structural integrity and lifetime evaluation

Mod 9Cr–1Mo steel (T91) is a candidate material for pressure vessels and for some internal structures of GCR (Gas Cooled Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr–1Mo steel has been undertaken at CEA in order to characterize the behaviour of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue and creep loadings. Concerning the GCR, complementary studies are conducted in order to validate the existing methods (developed for the fast reactors) and to get new experimental data on Mod 9Cr–1Mo steel. Moreover, if the geometry and the loadings of a standard CT specimen allow performing a 2D analysis, the case of industrial loadings appears much more complicated, notably because of surface defects which propagate and present shapes that can be considered as half ellipse. Therefore, in the frame of the defect assessment methods validation, the LISN undertakes both standard tests on CT specimens to determine the propagation laws and bending tests on large plates under high temperature fatigue and creep loadings. These components present an initial semi-elliptical surface notch normal to the loading direction and its initiation and propagation are studied.