Finite element simulation of the residual stresses in high strength carbon steel butt weld incorporating solid-state phase transformation

This paper presents a sequentially coupled three-dimensional (3-D) thermal, metallurgical and mechanical finite element (FE) model to simulate welding residual stresses in high strength carbon steel butt weld considering solid-state phase transformation effects. The effects of phase transformation during welding on residual stress evolution are modeled by allowing for volumetric changes and the associated changes in yield stress due to austenitic and martensitic transformations. In the FE model, phase transformation plasticity is also taken into account. Moreover, preheat and inter-pass temperature are included in the modeling process. Based on the FE model, the effects of solid-state phase transformation on welding residual stresses are investigated. The results indicate the importance of incorporating solid-state phase transformation in the simulation of welding residual stresses in high strength carbon steel butt weld.     

Effect of the smart cure cycle on the performance of the co-cured aluminum/composite hybrid shaft

In this work, a smart curing method for the co-cured aluminum/composite hybrid shaft which can reduce the thermal residual stresses generated during co-curing bonding operation between the composite layer and the aluminum tube was applied. In order to reduce the thermal residual stresses generated during co-cure bonding stages due to the difference of coefficients of thermal expansions (CTE) of the composite and the aluminum tube, a smart cure cycle composed of cooling and reheating cycles was applied. The heating and cooling operations were realized using a pan type heater and water cooling system. The thermo-mechanical properties of the high modulus carbon epoxy composite were measured by a DSC (differential scanning calorimetry) and rheometer to obtain an optimal time to apply the cooling operation. Curvature experiment of the co-cure bonded steel/composite strip was performed to investigate the effect of cure cycle on generation of the thermal residual stress. Also, the thermal residual stresses of the aluminum/composite hybrid shaft were measured using strain gauges with respect to cure cycles.

Finally, torsional fatigue test and vibration test of the aluminum/composite hybrid shaft were performed, and it has been found that this method might be used effectively in manufacturing of the co-cured aluminum/composite hybrid propeller shaft to improve the dynamic torque characteristics.     

X65管线厚板控冷残余应力的数值分析

为研究X65管线厚板控制冷却时板厚方向的残余应力,通过开发线性混合热膨胀模型、拓展Avrami相变动力学模型和应用Leblond模型建立了热力耦合有限元模型,用该模型研究了控冷工艺、相变效应对残余应力的影响.结果表明:当上表面层流冷却系数由1 mW/(mm2.K)分别增至2、3 mW/(mm2.K)而下表面维持1 mW/(mm2.K)不变时,上、下表面与心部两侧的平均温差由0℃分别增至9、10℃,上表面一侧的拉应力峰值锐减188 MPa,并使上表面和心部附近的拉、压应力峰向下表面方向大幅迁移,即上下表面的不对称控冷加剧了板厚方向温度和残余应力的不对称分布;实际控冷工艺下,相变效应通过产生拉、压应力峰值分别为779、-454 MPa的应力而显著影响板厚方向的残余应力.     

Analysis of residual stress and distortion resulting from quenching in large low-alloy steel shafts

Abstract

In order to predict residual stresses and distortion caused by quenching in a large low-alloy steel shaft, a computer program based on the finite element method was developed, and uniaxial restraint testing of the transient strain during cooling was carried out. Using the program, the transient stresses occurring during quenching are calculated by a step-by-step procedure, and the effects of transformational behaviour on residual stresses and distortion discussed. The results show that the transient stresses and the transformation affect each other, and that residual stresses and distortion are strongly related to the transformational behaviour.

MST/8     

激光相变硬化残余应力研究

本文主要采用X射线法对激光相变硬化处理后试样表面至内部的残余应力进行逐层测定。并辅以小孔应力释放法予以比较。结果表明:在通常激光相变硬化处理参数下,碳钢的残余应力按正弦波规津分布。表面为压应力,内部为拉应力。其幅值随激光参数减小而减小。二次激光相变硬化处理的残余应力为拉应力。本文对残余应力在激光相变硬化处理热循环过程中的形成进行了解释。在最初升温阶段,表面发生热塑性变形而形成少量压应变。在随后的冷却过程中产生拉应变。当发生奥氏体向马氏体组织转变时,又产生压应变。因此,最终残余应力状态是表面达到的温度,塑性屈服量,组织转变量等因素综合影响的结果。而与是终形成的组织,残余奥氏体量等没有对应关系。     

Eigenspannungen und Mikrostruktur von thermisch getrenntem Stahlblech

Residual stresses and microstructure of thermal cutted steel sheet A research project of Doppelmayr Seilbahnen GmbH and the Swiss Federal Laboratories of Materials Testing and Research (EMPA) was launched to investigate the residual stress depth profiles induced by different thermal cutting methods. The measurements have been performed by X‐ray stress analysis. Pronounced differences in the stress levels at the surface and for the depth profiles have been obtained for the different cutting methods (plasma, microplasma, autogenous (gas), and laser cutting). Near the surface a small region of compression stresses due martensitic or bainitic transformation was found. With increasing depth a transition to tensile stresses occurs, which are caused by the contraction in the heat affected zone (HAZ) during cooling. The highest tensile (237MPa) and compression stresses (‐550MPa) have been obtained for the laser cut samples, while the microplasma cut samples showed the lowest residual stresses (max. 180MPa/‐56MPa).     

Numerical and experiment analysis of residual stress on magnesium alloy and steel butt joint by hybrid laser-TIG welding

Hybrid welding technology has received significant attention in the welding of dissimilar materials recently. While, great welding residual stress and deformation often result by the difference of coefficient of thermal expansion This study describes the thermal elastic–plastic analysis using finite element techniques to analyze the thermo mechanical behavior and evaluate the residual stresses and welding distortion on the AZ31B magnesium alloy and 304L steel butt joint in laser-TIG hybrid welding. A new coupled heat source model was developed which combined by double-elliptic planar distribution, double-ellipsoid body distribution and Rotary–Gauss body distribution model. From the results, it can be concluded that the temperature distribution at the hybrid weld region is exposed to faster rate of heating and cooling in hybrid welding than TIG. Furthermore, compared to the welding stress distribution on the TIG weld, residual stress σ_y is found about 20% higher on hybrid weld joints, and the residual stress on the 304L steel plate is lower than that on the AZ31B magnesium plate.     

Experimental and numerical investigation on thermal fatigue behaviour of 9Cr 1Mo steel tubes

Boiler tubing is subjected to alternate cycles of heating and cooling during operation initiating alternate thermal expansion and contraction. Alternate cycles of differential expansion and contraction causes thermal fatigue of the component. Thermal fatigue causes tube failures, significantly reducing the working life of the tubular components. Boilers have matured into super critical design using T91 grade materials thereby increasing the operating efficiency. The studies involving thermal fatigue of tubes of T91 grades for boiler components in operating conditions are thus an important area. A simple experimental set up has been developed to simulate thermal fatigue conditions in the internal diameter (ID) side of the tube. The work involves both experimental and numerical investigations of the thermal fatigue behaviour by creating a simulated environment and a Finite Element Model (FEM). FEM analyses are carried out based on the decoupled thermal and inelastic stress analyses to compute the total plastic strain range experienced by the boiler tubes. The cyclic spray cooling causes thermal fatigue cracks in the T91 tube. The number of cycles to crack initiation has been obtained from the experimentation and number cycles to failure has been calculated using modified Coffin-Manson relation. The study thus presents a reliable fatigue failure analysis of 9Cr 1Mo steel tubes used in boiler industry.     

Residual stress control of multipass welds using low transformation temperature fillers

ABSTRACT

Low transformation temperature (LTT) weld fillers can be used to replace tensile weld residual stresses with compressive ones and reduce the distortion of single-pass welds in austenitic plates. By contrast, weld fillers in multipass welds experience a number of thermal excursions, meaning that the benefit of the smart LTT fillers may not be realised. Here, neutron diffraction and the contour method are used to measure the residual stress in an eight pass groove weld of a 304?L stainless steel plate using the experimental LTT filler Camalloy 4. Our measurements show that the stress mitigating the effect of Camalloy 4 is indeed diminished during multipass welding. We propose a carefully selected elevated interpass hold temperature and demonstrate that this restores the LTT capability to successfully mitigate residual tensile stresses.

This paper is part of a thematic issue on Nuclear Materials.     

Prediction of welding buckling distortion in a thin wall aluminum T joint

In this paper, local and global welding buckling distortion of a thin wall aluminum T joint is investigated. A thermo-elastic–viscoplastic model is employed to determine longitudinal residual stresses; analysis of thermal model and elastic–viscoplastic (Anand) model are uncoupled. Molten puddle motion (speed of welding) is modeled by using time dependent birth and death element method. Three dimensional nonlinear-transient heat flow analysis has been used to obtain the temperature distribution, and then by applying thermal results and using three dimensional Anand elastic–viscoplastic model, stress and deformation distributions are obtained during welding and after cooling. Local buckling is investigated by analyzing the history of stress and strain relations. Local buckling is assumed to occur at a point if a small change in the magnitude of stress causes large deformation during of the welding process. By applying residual stresses on a structural model and using eigenvalue methods, global buckling instability of the welded structure is determined.     

The effects of solid-state phase transformation upon stress evolution in laser metal powder deposition

To investigate the influences of solid-state phase transformation on stress evolution during multi-pass laser metal powder deposition (LMPD) process, a 3D finite-element (FE) thermo-mechanical model considering phase transformation has been established. The influences of phase transformation such as mechanical properties changes, volume change and transformation induced plasticity (TRIP) are taken into account. Furthermore, the influences of high magnitude stress upon martensitic transformation characteristic temperature and TRIP are considered. The temperature and history (microstructure) dependent material properties used in the present research are obtained by experiments. The stress field during LMPD process is analyzed with and without solid-state phase transformation, respectively. Stress measurement of X-ray diffraction (XRD) method is applied to deposited samples, and the measuring data are compared with the computational predictions. The results show that phase transformation has a dominant effect on the stress evolution, longitudinal residual stresses significantly reduced as a result of solid-state phase transformation. In addition, the effect of stresses on martensitic transformation temperature is important for accurate prediction of residual stresses (stress state after cooling of the clad to ambient temperature). Residual stresses are lower when the phase transformation temperature is reduced.     

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations.     

Thermal Generation of Residual Stress Fields for Purpose of Distortion Minimization

Thermal treatments of steel components with the goal of hardening often result in distortion by releasing the residual stresses which were brought into the specimen during the preceding processing steps. The goal of the presented work is the minimization of this distortion. By generating definite residual stress fields and investigating the resulting distortion, the distortion mechanism can be observed in detail. A flexible and reproducible way to generate such residual stress fields inside a specimen is by means of local thermal treatment with a laser beam. Computer simulations as well as experiments were carried out using an idealized tooth of a gearwheel (finger sample) as a model system. The deformation of the samples due to the laser heat treatment and the stress fields generated inside the samples were determined with respect to different process parameters.     

Residual stresses and fracture mechanics analysis of a crack in welds of high strength steels

This study presented the characteristics of residual stresses in welds of high strength steels (POSTEN60, POSTEN80) whose tensile strengths were 600 MPa and 800 MPa, respectively. Three-dimensional thermal elastic-plastic analyses were conducted to investigate the characteristics of welding residual stresses in welds of high strength steels through the thermal and mechanical properties at high temperatures obtained from the elevated temperature tensile tests. A finite element analysis method which can calculate the J-integral for a crack in a residual stress field was developed to evaluate the J-integral for a centre crack when mechanical stresses were applied in conjunction with residual stresses.The results show that the volumetric changes associated with the austenite to martensite phase transformation during rapid cooling after welding of high strength steels significantly influence on the development of residual stresses in the weld fusion zone and heat-affected zone. For a centre crack in welds of high strength steels where only residual stresses are present, increased tensile strength of the steel, increased the J-integral values. The values of the J-integral for the case when mechanical stresses are applied in conjunction with residual stresses are larger than those for the case when only residual stresses are present.     

Application of low Ms temperature consumable to dissimilar welded joint

Abstract

The welding of dissimilar joints is very common in systems used in oil exploration and production in deep sea waters. Commonly involves welding of low carbon steel pipes with low alloy steel forgings both with inner Inconel clad. The forged steel part undergoes a process of buttering with Inconel or carbon steel electrode before the weld of the joint. The buttering process is followed by a process of residual stresses relief. The conventional way of reducing the level of residual stresses in welded joints is to apply post welding heat treatments. Depending on the size and complexity of the parts to be joined, this can become a serious problem. An alternative technique for reducing residual stresses is to use an electrode that during the cooling process undergoes a displacive transformation at a relatively low temperature so that the deformation resulting from the transformation compensates the contraction during the cooling process, and, although many papers have been published in this direction using Fe–Cr–Ni alloys, most of them report a loss of toughness in the weld metal. Maraging steel is a family of materials with M_s temperature below 200°C and even without the final heat treatment of aging has superior mechanical properties to low alloy steels used in forgings. In this work, forged piece of AISI 4130 was buttered with Maraging 350 weld consumable and subsequently welded to ASTM A36 steel using Inconel 625 filler metal. In addition, the dissimilar base metal plates were welded together using Maraging 350 steel weld consumable. The levels of residual stress, and the toughness and microstructures of heat affected zone and weld metal were investigated.     

Cruciform fillet welded joint fatigue strength improvements by weld metal phase transformations

Arc welding typically generates residual tensile stresses in welded joints, leading to deteriorated fatigue performance of these joints. Volume expansion of the weld metal at high temperatures followed by contraction during cooling induces a local tensile residual stress state. A new type of welding wire capable of inducing a local compressive residual stress state by means of controlled martensitic transformation at relatively low temperatures has been studied, and the effects of the transformation temperature and residual stresses on fatigue strength are discussed. In this study, several LTTW (Low Transformation‐Temperature Welding) wires have been developed and investigated to better characterize the effect of phase transformation on residual stress management in welded joints. Non‐load‐carrying cruciform fillet welded joints were prepared for measurement of residual stresses and fatigue testing. The measurement of the residual stresses of the three designed wires reveals a compressive residual stress near the weld toe. The fatigue properties of the new wires are enhanced compared to a commercially available wire.     

Manufacturing Residual Stress States in Heat Treatment Simulation of Bearing Rings

Residual stresses generated in the different steps of a manufacturing chain are attached great importance to the distortion due to heat treatment. Because the measurement of residual stress states is associated with large experimental work, the use of simulation would be qualified for the examination of the influence of manufacturing residual stresses on distortion. Due to this reason, a simple method for generating realistic residual stress states by the use of artificial thermal strain in a pre‐simulation step is presented. By changing the thermal strain distributions a wide range of residual stress distributions in a component can be generated. A typical residual stress distribution after machining was generated in a bearing ring. The calculated residual stress profile was in good agreement compared to XRD‐measurement. In a second simulation step the local development of stress relieving during heating was observed. Stress relieving can be attributed to local plastic deformation and rearrangement of stresses.     

Prediction of residual stress distributions for single weld beads deposited on to SA508 steel including phase transformation effects

Abstract

The sensitivity of residual stress distributions in bainitic–martensitic steel welds to the transformation strains that arise when austenite decomposes on cooling has been assessed by examining the predictions of three models for a simple bead-on-plate configuration. These cover the following scenarios: case I, no phase transformations; case II, transformations with volume change effects only; case III, transformations with volume change effects and associated Greenwood–Johnson transformation plasticity. Austenite decomposition was predicted by implementing Kirkaldy's reaction rate equations as a subroutine in the finite element code Sysweld, eliminating the need for a continuous cooling transformation diagram. Predicted residual stresses were then compared and rationalised alongside measurements obtained by neutron diffraction and the contour method. It was found that serious errors in predicting the location and magnitude of the peak stresses occurred if transformations were not included, while cases II and III gave similar results generally in agreement with the stress maps. Indeed, the trends in the experimental results were intermediate between cases II and III. Differences between the models and the potential for further improvements are discussed.     

Numerical simulation of transient and residual stresses caused by laser hardening of slender elements

A mathematical and numerical model of progressive laser hardening with regard to influence of thermal field and phase transformation under transient and residual axial stresses field is presented. The thermal field was determined on the basis of the diffusion-convection equation which was solved by a suitable superposition of Greens function. TIT diagrams were chosen as a basis for describing phase transformations during the heating and cooling process. The stresses were determined on the base of model of plastic flow with isotropic hardening. The changes in thermomechanical parameters of material as a function of phase contents and thermal field were considered in the stress calculations.     

FE analysis of residual stress relaxation in a girth-welded duplex stainless steel pipe under cyclic loading

This study intends to characterize the residual stress relaxation in a girth-welded duplex stainless steel pipe exposed to cyclic loading. FE thermal simulation of the girth welding process is first performed to identify the weld-induced residual stresses. 3-D elastic–plastic FE analyses incorporated with the cyclic plasticity constitutive model which can describe the cyclic stress relaxation are next carried out to evaluate reconstruction of the residual stresses under cyclic mechanical loading. The results unveils that considerable reduction of the residual stresses in and around the girth weld occur even after the initial few loading cycles and degree of the stress relaxation is dependent on the magnitude of applied cyclic loading.