Effects Of Solid-state Phase Transformation Research Articles

Accurate evaluation on residual stress considering solid-state phase transformation and initial stress in thick weld

During the welding process of thick plates, it is difficult to evaluate the residual stress distribution in thick welds accurately because complicated residual stress is generated by multi-thermal cycles. In this study, a multi-pass butt joint was fabricated using Q690 high strength steel with the thickness of 75 mm, meanwhile experiments were carried out to characterize the microstructure and measure the through-thickness distribution of longitudinal residual stress in a butt-welded joint by the contour method. Then numerical methods were developed to predict the through-thickness distribution of residual stress in butt-welded joints. To evaluate residual stress accurately and efficiently, the effects of solid-state phase transformation (SSPT) and initial stress were taken into consideration, and the parallel computation technology and iterative substructure method were employed. The root-mean-square error (RMSE) was employed to evaluated the prediction accuracy of numerical methods between measurement and numerical results of residual stress. Through comparing the simulated results with experimental data, the validity of the developed numerical methods was verified. Numerical results indicate that the through-thickness distribution of residual stress in butt-welded joints is very complex and peak value of tensile stress can be found in the area adjacent to the heat-affected zone (HAZ). Moreover, the RMSE is significantly decreased from 307 to 8 when considering the effect of SSPT on residual stress in weld vicinity, and the RMSE is apparently decreased from 137 to 11 when considering the effect of initial stress on residual stress in base metal.

Numerically simulated prediction of residual stresses in welding considering phase transformation effects

This work deals with investigating the effects of solid-state phase transformation on residual stresses during the welding of low carbon and high carbon steels. In this study, depending on MSC Marc code, the simulation considers the local microstructure properties changes due to the thermal welding cycles. A sequentially coupled thermal and mechanical 2-D finite element model (FEM) was used. In FEM, phase transformation temperature diagrams are used to anticipate the amount of martensite in the fusion zone (FZ), and heat affects zone (HAZ). The simulation results demonstrated that the residual stress in low carbon steel is not affected by the volume change caused bythe austenite-martensite transformation. In contrarily,the residual stressesin the high carbon steel are considerably influenced by the martensitic transformation.

Thermo-mechanical-metallurgical modeling and validation for ferritic steel weldments

This work explained the effects of solid-state phase transformation (SSPT) on welding residual stress (WRS) evolution in SA508 Gr.3 C1.1(SA508) ferritic steel single-pass and multi-pass weldments. A thermo-mechanical-metallurgical (TMM) model incorporating thermo-elastic-plastic stains and SSPT-related strains was presented. The SSPT kinetics and transformation-induced plasticity (TRIP) models of SA508 steel were established by employing user-defined subroutines developed for ABAQUS. The simulated WRS distribution in the single-pass weldment was compared with previously published neutron diffraction measurement data, and a good agreement was achieved and thus validating the developed TMM model. The microstructural and WRS distribution characteristics of the multi-pass weldment were further investigated based on the validated TMM model. Numerical simulations illustrated how the transformation and heat transfer/cumulative behavior associated with different welding process conditions resulted in the generation of triaxial compressive and tensile stresses in the upper and underlying weld beads at the multi-pass welded joint, respectively. Moreover, an additional simulation model with SSPT but without TRIP was developed to reveal the potential effects of TRIP on WRS distribution. This study numerically provided valuable understanding of microstructure and WRS evolution in air cooling bainitic steel weldments under multiple thermal cycles and constituted a fundamental basis for developing new welding materials to minimize the formation of deleterious tensile WRS by simultaneously combining bainitic and martensitic transformations.

Finite element analysis of residual stress in 2.25Cr-1Mo steel pipe during welding and heat treatment process

The circumferential butt-welded pipe was fabricated using 2.25Cr-1Mo steel and then subjected to local post-weld heat treatment (LPWHT) for releasing residual stress after multi-pass welding. The sectioning method was employed to measure residual stress. Based on SYSWELD software, a thermo-metallurgical-mechanical coupled computational approach was developed to analyses the residual stress distribution on 2.25Cr-1Mo welded pipe. A three-dimensional finite element model and moving heat source model were employed in simulation. The effects of solid-state phase transformation (SSPT), tempering as well as creep were considered simultaneously. The effectivity of computational approach was verified by experimental measurement. The simulation results showed that SSPT could induce an undulating stress profile and large stress gradient in welded zone. The stress gradient could be reduced during LPWHT while there were stress peaks at the edges of heated region. Tempering effect could decrease the residual stress at high-stress region by material softening and yielding. Creep caused a global transformation from elastic strain to plastic strain, which led to a macro-scale stress relaxation. It is necessary to consider SSTP, tempering as well as creep in material model for more accurate simulation results.

Numerical and Experimental Analysis of Dissimilar Repair Welding Residual Stress in P91 Steel Considering Solid-State Phase Transformation

Repair welding is a popular method to repair the cracked zones of pipes in power plants. However, residual stresses induced by repair welding are inevitably generated and have a significant effect on the performance of repair-welded components during service. The purpose of this research is to investigate the effects of solid-state phase transformation (SSPT) on the repair welding residual stress distribution of multi-layer and multi-pass welding in P91 steel. Based on thermal elastic-plastic theory, a three-dimensional thermal–metallurgical–mechanical finite element model considering the SSPT is established, and shielded metal arc welding (SMAW) heat input is modelled as one double elliptical heat source. The effect of phase transformation on residual stress is investigated by considering effects with and without phase transformation in the database of materials. The results show that transverse compressive stress and a low longitudinal stress area are generated in the final pass due to martensite transformation. Compared with different zones along the thickness direction, the effect of SSPT is greater around the outer surface than around the internal fusion zone. The rationality of the developed finite element model is verified by the experimental method, and good agreement is obtained between those results from the numerical analysis and experimental methods.

Residual stress reduction of laser beam welds by use of low-transformation-temperature (LTT) filler materials in carbon manganese steels—In situ diagnostic: Image correlation

Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacements or deformations, as they occur during the cooling down process, in the vicinity of a weld. Observation of the movement of surfaces using a stereo camera setup allows the recording of displacements in three dimensions. Technically, this is achieved by identifying recognizable points on the surface under observation and by tracking their movement via comparisons between several time-shifted images. Visualization of surface contractions, as well as the effects of solid-state phase transformations during the joining process, especially during cooling, is key to understanding and influencing the residual stress state in welds as well to reducing distortion. In this study, the used installation setup, and the methodology for sample preparation, shows the production of information relating to surface displacements in the direct vicinity of the laser weld during the cooling down stage. Different welding parameters were investigated, resulting in different dilution levels for the LTT-Filler material, thereby influencing the distances to the weld pool and temperatures at which transformations took place. In order to provide a point of reference, comparable welds, made with conventional filler wires, were also investigated. The displacements after welding are always lower when using an LTT filler wire when compared to a conventional wire, proving that LTT wires can be used to mitigate distortion during laser beam welding.

Characterising electron beam welded dissimilar metal joints to study residual stress relaxation from specimen extraction

Nuclear power plants require dissimilar metal weld joints to connect the primary steam generator made from ferritic steel to the intermediate heat exchanger made from austenitic steel. Such joints are complex because of the mismatch in the thermal and the mechanical properties of the materials used in the joint. Electron Beam (EB) welding is emerging as a promising technique to manufacture dissimilar joints providing a great many advantages over conventional welding techniques, in terms of low heat input, high heat intensity, narrow fusion and heat affected zones, deeper penetration and increased welding speeds. However before this method can be considered for implementation in an actual plant, it is essential for a careful and a comprehensive outlining of the joint characteristics and the apparent effects on performance during service. In the present study, an EB welded joint was manufactured using austenitic AISI 316LN stainless steel and a ferritic-martensitic P91 steel, without the addition of filler material. Neutron diffraction measurement was conducted on the welded plate to measure the residual stress distribution across the weld as well as through the thickness of the plate. A finite element analysis was conducted on a two-dimensional cross-sectional model using ABAQUS code to simulate the welding process and predict the residual stresses, implementing the effects of solid-state phase transformation experienced by P91 steel. The predicted residual stresses were transferred to a 3D finite element model of the plate to simulate the machining and extraction of a C(T) blank specimen from the welded plate and the extent of stress relaxation is studied.

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

Abstract 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.

Finite element modelling of the inertia friction welding of a CrMoV alloy steel including the effects of solid-state phase transformations

Finite element (FE) process modelling of the inertia friction welding (IFW) between two tubular CrMoV components has been carried out using the DEFORM-2D (v10.2) software. This model has been validated against experimental test welds of the material; this included process data such as upset and rotational velocity as well as thermal data collected during the process using embedded thermocouples. The as-welded residual stress from the FE model has been compared to experimental measurements taken on the welded component using the hole drilling technique. The effects of the solid-state phase transformations which occur in the steel are considered and the trends in the residual stress measurements were well replicated when compared to the experimental data.

Characterising Residual Stresses in a Dissimilar Metal Electron Beam Welded Plate

Dissimilar metal welded components are becoming increasingly common in industrial applications especially in the nuclear sector. Dissimilar metal welding refers to the joining of two materials from different alloy groups. One of the basic requirements of the dissimilar metal welded joint is that the joint strength should be greater than or equal to that of the weakest member and a careful characterisation of the joint is crucial before considering the applicability of the dissimilar metal welded components. The current paper explores the feasibility of an electron beam welded joint between ferritic/martensitic Grade 91 or more commonly known as modified P91 and austenitic 316LN stainless steel, without the addition of any filler material. The residual stress distribution arising from the welding is determined from measurements using neutron diffraction experiment and predictions using finite element analysis. The measured data has been analysed using Rietveld and single peak fits. The finite element analysis was conducted on a two-dimensional cross-sectional model using ABAQUS code, implementing the effects of solid-state phase transformation experienced by P91 steel. The predicted residual stresses are compared with the experimental measurements and conclusions are drawn on the final residual stress distribution.

Prediction of Welding Residual Stress by Using Thermo-Metallurgical-Mechanical Constitute Model

The objective of this paper is to investigate the effects of solid-state phase transformation on welding residual stress in high stress low alloy steel. In this study, based on commercial finite element software, a sequentially coupled thermal, metallurgical, mechanical plane strain finite element model is developed. The main effort is to develop a series of subroutines which consider the heat transfer from welding arc and numerical implementation of a thermo-metallurgical-mechanical constitute equation. The effectiveness of developed computational method is confirmed by a butt welding simulation. Simulation of butt welding demonstrates that the distribution pattern of longitudinal residual stresses perpendicular to weld centerline at the upper surface of weldment has two peak and transformation plasticity has significant effect on the evolution of residual stress.

Finite element simulation of residual stresses induced by the dissimilar welding of a P92 steel pipe with weld metal IN625

Residual stresses induced by the fusion arc-welding of steel pipe joints in power generation plants are a concern to the industry. Residual stresses are induced by the process of welding due to the extreme nature of thermal cycles during the process. Welding is essential in the construction of high-grade steel pipelines, used as a conduit for steam at high temperature and pressure. The integrity and endurance of the welded pipes are necessary for the safe operation in power plants, which may be compromised by the presence of residual stresses. The finite element (FE) method is an effective tool for the prediction of residual stresses in such components, as long as the material behaviour can be accurately modelled. This paper reports the FE simulation of residual stresses, due to the arc-welding of a P92 steel pipe mainly using a nickel-based alloy (IN625) as a dissimilar weld material. The structural analysis part of the FE method of determining the residual stress field in the welded pipe is described and the results presented and discussed. Two user-defined subroutines have been used in the FE structural analysis to simulate the way the different phases of steel evolve during welding, including their differing plastic and hardening behaviour, derived from uniaxial tensile material testing carried out over a wide range of temperature. Thermal-expansion, including the effects of solid-state phase transformations in P92, has also been numerically modelled in the two subroutines, one of which prescribes two phases of P92 steel (tempered martensite and austenite) while the other assumes three phases (tempered martensite, austenite and untempered martensite).

Numerical Simulation of Welding Residual Stress Considering Phase Transformation Effects

The welding processes of steel materials are often accompanied by the occurrence of phase transformation. Volume change caused by phase transformation will affect the history of stress and strain. In this article, taking the welding of Q345 as an example, the effects of solid-state phase transformation on the residual stress were investigated by numerical simulation. The values of thermal strain at different temperatures were set to make the volume change caused by phase transformation equivalent as thermal strain. The simulation contained two cases both considering phase transformation and not. The results show that in both two cases the longitudinal stress distribution in the weld zone has almost the same trend. But in the case without considering phase transformation, there is large longitudinal tensile stress concentrating in the weld and HAZ zone and the maximum value is up to 427MPa in the weld. For transverse stress, phase transformation not only changes the value of the stress, but also alters the sign of the stress in the middle of the weld zone. Experiment was also carried out to measure the residual stress by X-ray diffraction. The result considering phase transformation matched much better with the experimental data. It can be concluded that phase transformation in the process of welding has a significant effect on the residual stress and can not be ignored in the numerical simulation of welding.

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.

Computational modelling of the residual stress evolution due to solid-state phase transformation during welding

Ferrous weldments may experience solid-state phase transformation on cooling of austenite to martensite. It has been known that the volumetric change associated with the metallurgical phase transformation could have a major influence on the residual stress evolution. In this work, residual stress measurements were carried out to investigate the residual stress evolution due to the volumetric expansion in the weld and HAZ. Based on measurement results, this paper presents a three-dimensional one-way coupled thermal–mechanical finite element model which takes into account the effects of solid-state phase transformation on residual stresses. Effects of variations in thermal and mechanical properties due to the phase transformation on the residual stresses are studied. The results show that the volumetric change due to the austenite to martensite phase transformation significantly affects the residual stresses in the transformation region, and the residual stress evolution is not mainly determined by the variation of specific heat but by that in specific volume in the phase transformation temperature range.

FEM prediction of welding residual stress and distortion in carbon steel considering phase transformation effects

The objective of this study is to investigate the effects of solid-state phase transformation on welding residual stress and distortion in low carbon and medium carbon steels. In this study, based on ABAQUS code, a sequentially coupled thermal, metallurgical, mechanical 3-D finite element model is developed. In the numerical simulations, different continuous cooling transformation diagrams are used to predict the fractions of martensite for the fusion zone, the coarse-grained HAZ and the fine-grained HAZ, respectively. Effects of volume change due to austenite–martensite transformation on the final residual stress and the welding distortion are examined. The simulation results revealed that the final residual stress and the welding distortion in low carbon steel do not seem to be influenced by the solid-state phase transformation. However, for the medium carbon steel, the final residual stresses and the welding distortion seem to be significantly affected by the martensitic transformation.

Numerical simulation of P91 pipe welding including the effects of solid-state phase transformation on residual stresses

The methodology of numerically simulating residual stresses in a welded P91 pipe section is described. The finite element (FE) method has been applied to simulate residual axial and hoop stresses generated in the weld region and heat affected zone (HAZ) of an axisymmetric 50-bead circumferentially butt-welded P91 steel pipe, with outer diameter of 145 mm and wall thickness of 50mm. The FE simulation consists of a thermal analysis which is followed by a sequentially-coupled structural analysis. Solid-state phase transformation (SSPT), which is characteristic of P91 steel during welding thermal cycles, has been modelled in the FE analysis by allowing for volumetric changes in steel and associated changes in yield stress due to austenitic and martensitic transformations. Phase transformation plasticity has also been taken into account. Preheat and interpass temperature control has been included in the modelling process. Thermally-obtained temperature contours indicate the size of the weld region, parent metal penetration, and HAZ. Residual axial and hoop stresses have been depicted through the pipe wall thickness as well as along the outer surface of the pipe. The results indicate the importance of including SSPT in the simulation of stresses during the welding of P91 steel.