Welding Thermal Cycle Research Articles

Spall Strength of Resistance Spot Weld for QP Steel**Supported by the National Natural Science Foundation of China under Grant Nos 11372149, 11572164 and 11502074, and the K. C. Wong Magna Fund in Ningbo University.

The spall tests under the plane tensile pulses for resistance spot weld (RSW) of QP980 steel are performed by using a gun system. The velocity histories of free surfaces of the RSW are measured with the laser velocity interferometer system for any reflector. The recovered specimens are investigated with an Olympus GX71 metallographic microscope and a scanning electron microscope (SEM). The measured velocity histories are explained and used to evaluate the tension stresses in the RSW applying the characteristic theory and the assumption of Gathers. The spall strength (1977–2784 MPa) of the RSW for QP980 steel is determined based on the measured and simulated velocity histories. The spall mechanism of the RSW is brittle fracture in view of the SEM investigation of the recovered specimen. The micrographs of the as-received QP980 steel, the initial and recovered RSW of this steel for the spall test are compared to reveal the microstructure evolution during the welding and spall process. It is indicated that during the welding thermal cycle, the local martensitic phase transformation is dependent on the location within the fusion zone and the heat affected zone. It is presented that the transformation at high strain rate may be cancelled by other phenomenon while the evolution of weld defects is obvious during the spall process. It may be the stress triaxiality and strain rate effect of the RSW strength or the dynamic load-carrying capacity of the RSW structure that the spall strength of the RSW for QP980 steel is much higher than the uniaxial compression yield strength (1200 MPa) of the martensite phase in QP980 steel. Due to the weld defects in the center of the RSW, the spall strength of the RSW should be less than the conventional spall strength or the dynamic load-carrying capacity of condensed structure.

Effect of Zr addition on toughness and microstructure of coarse grained heat affected zone in Al deoxidised low carbon steel

The effect of Zr addition on the toughness and microstructure of coarse grained heat affected zone (CGHAZ) in Al deoxidised low carbon steel was investigated. In this present work, the specimens (Zr free and Zr added) were subjected to weld thermal cycle with heat input of 54, 80 and 100 kJ cm− 1 at 1350°C peak temperature using thermal simulator. The typical inclusions were characterised by scanning electron microscopy, and the chemical composition of the inclusions was determined by energy dispersive spectroscopy. The nanoscale precipitates were analysed by transmission electron microscopy and energy dispersive spectroscopy. It was verified that the occurrence of Zr in steel mostly exists in the form of oxide inclusions, but a few in the form of solid solution state and Zr containing precipitates when the concentration of Zr is 0.015%. The improvement of CGHAZ toughness is obtained when the heat input is 80–100 kJ cm− 1. The possible reasons about the effects of Zr on the toughness of CGHAZ, including Zr containing inclusions, precipitates and microstructure characteristics, were also discussed.

Effect of Welding Thermal Cycles on Microstructure and Mechanical Properties of Simulated Heat Affected Zone for a Weldox 1300 Ultra-High Strength Alloy Steel

In the present study, the investigation of weldability of ultra-high strength steel has been presented. The thermal simulated samples were used to investigate the effect of welding cooling time t8/5 on microstructure and mechanical properties of heat affected zone (HAZ) for a Weldox 1300 ultra-high strength steel. In the frame of these investigation the microstructure was studied by light and transmission electron microscopies. Mechanical properties of parent material were analysed by tensile, impact and hardness tests. In details the influence of cooling time in the range of 2,5 ÷ 300 sec. on hardness, impact toughness and microstructure of simulated HAZ was studied by using welding thermal simulation test. The microstructure of ultra-high strength steel is mainly composed of tempered martensite. The results show that the impact toughness and hardness decrease with increase of t8/5 under condition of a single thermal cycle in simulated HAZ. The increase of cooling time to 300 s causes that the microstructure consists of ferrite and bainite mixture. Lower hardness, for t8/5 ≥ 60 s indicated that low risk of cold cracking in HAZ for longer cooling time, exists.

Toward Improving the Type IV Cracking Resistance in Cr-Mo Steel Weld Through Thermo-Mechanical Processing

Detailed microstructure characterization of Grade 91 (Modified 9Cr-1Mo, ASTM A387) steel subjected to a thermo-mechanical treatment process was performed to rationalize the cross-weld creep properties. A series of thermo-mechanical processing in the austenite phase region, followed by isothermal aging at temperatures at 973 K to 1173 K (700 °C to 900 °C), was applied to the Grade 91 steel to promote precipitation kinetics of MX (M: Nb and V, X: C and N) in the austenite matrix. Detailed characterization of the base metals after standard tempering confirmed the presence of fine MX dispersion within the tempered martensitic microstructure in steels processed at/and above 1073 K (800 °C). Relatively low volume fraction of M23C6 precipitates was observed after processing at 1073 K (800 °C). The cross-weld creep strength after processing was increased with respect to the increase of MX dispersion, indicating that these MX precipitates maintained during weld thermal cycles in the fine-grained heat-affected zone region and thereby contribute to improved creep resistant of welds in comparison to the welds made with the standard “normalization and tempering” processes. The steels processed in this specific processing condition showed improved cross-weld creep resistance and sufficient room temperature toughness. The above data are also analyzed based on existing theories of creep deformation based on dislocation climb mechanism.

In-situ observation of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by Zr-Ti combined deoxidation

The effect of Zr-Ti combined deoxidation on the grain refinement in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy, transmission electron microscopy, and electron backscattered diffraction analysis. Owing to the Zr-Ti combined deoxidation, a large amount of fine Zr-Ti oxide particles were formed in the steel and retarded the austenite grain growth during simulated welding thermal cycle. The austenite grains were small and uniform. The Mn can diffuse spontaneously from austenite to Zr-Ti oxide inclusion and MnS precipitated on ZrO2, which can form Mn depleted zone in the vicinity of inclusion. The acicular ferrite grains nucleated on intragranular Zr-Ti oxide inclusions in austenite grains grew in different directions and effectively divided the austenite grain into several finer and separate regions at intermediate temperature. The crystallographic grain size became small in the simulated coarse-grained heat-affected zone of Zr-Ti-killed steel due to the effective pinning effect by Zr-Ti oxide particles and acicular ferrite formation.

A study of the softening mechanisms of laser-welded DP1000 steel butt joints

Joint softening occurs in the laser-welded joints of many types of dual-phase steels that are used in automobile structures and weakens the joints' mechanical properties. This paper presents a study of the softening mechanisms in laser-welded joints of DP1000 steel. A Gleeble-3500 thermal simulator was used to simulate the welding thermal cycles of all of the sub-zones of the heat-affected zone (HAZ) of laser-welded specimens of DP1000 steel. The hardness and microstructure of the specimens were then tested and analysed. The results indicate that the softening resulted from two changes in the microstructure. The first change was the transformation of the pre-existing harder martensite into softer tempered martensite and the precipitation of carbides when the DP1000 steel was heated to the tempering temperature during the welding process. The other change was the decrease in the percentage of martensite, while the percentage of softer phases increased because the DP1000 steel was heated to the inter-critical temperature. At this temperature, the ferrite transformed from the martensite and some of the ferrite in the original microstructure were transformed into austenite. During the subsequent cooling process, the austenite was transformed into polygonal ferrite, bainite and martensite-austenite phases, therefore, the percentage of softer phases increased.

Influence of alloy type, peak temperature and constraint on residual stress evolution in Satoh test

The evolution of residual stress in welds is affected by a variety of factors such as joint design, welding parameters, material properties and the possible presence of phase transformations. The Satoh test can be useful as a method to understand differences in welding residual stresses among various materials. This study compares the evolution of residual stress measured in the Satoh test for ferritic and stainless steels. For weld thermal cycles with a peak temperature above Ac3, the ferritic alloys exhibit lower residual stresses than the austenitic alloys due to the well known reduction in stress from the martensitic or bainitic transformation. However, the residual stress levels among these alloys are similar for peak temperatures below Ac3. The application of constraint during both the heating and cooling portions of the thermal cycle affects the final magnitude of residual stress in a way that is not often considered.

Microstructures and mechanical properties of flash butt welded high strength steel joints

Weld thermal cycles, microstructures and mechanical properties of flash butt welded RS590CL steel joints have been investigated. The results indicated that the weld thermal cycles are characterized by high peak temperatures and rapid heating and cooling rates, and the joints included the weld interface zone (WZ), coarse grain zone (CZ), fine grain zone (FZ) and partially recrystallized zone (PZ). The WZ and CZ consist mainly of coarsened upper bainite while the FZ has a fine equiaxed grain structure containing ferrite and pearlite with NbC precipitates. The WZ and CZ have higher hardness compared with the FZ, PZ and base metal. The joint strength (584-611MPa) can match the base metal strength (575–595MPa). The impact toughness of weld interface zone in the joints depends on welding parameters. The excessive flash allowance results in reducing the joint toughness. The reasons for the reduced joint toughness are related to the coarsened upper bainite, retained oxides and joining defects in the weld interface zone. The suitable combination of flash allowance (8–9mm), upset allowance (5.5–7.0mm) and upset pressure (123–160MPa) is beneficial for the improvement of joint toughness.

Numerical Evaluation of Temperature Field and Residual Stresses in an API 5L X80 Steel Welded Joint Using the Finite Element Method

Metallic materials undergo many metallurgical changes when subjected to welding thermal cycles, and these changes have a considerable influence on the thermo-mechanical properties of welded structures. One method for evaluating the welding thermal cycle variables, while still in the project phase, would be simulation using computational methods. This paper presents an evaluation of the temperature field and residual stresses in a multipass weld of API 5L X80 steel, which is extensively used in oil and gas industry, using the Finite Element Method (FEM). In the simulation, the following complex phenomena were considered: the variation in physical and mechanical properties of the material as a function of the temperature, welding speed and convection and radiation mechanisms. Additionally, in order to characterize a multipass weld using the Gas Tungsten Arc Welding process for the root pass and the Shielded Metal Arc Welding process for the filling passes, the analytical heat source proposed by Goldak and Chakravarti was used. In addition, we were able to analyze the influence of the mesh refinement in the simulation results. The findings indicated a significant variation of about 50% in the peak temperature values. Furthermore, changes were observed in terms of the level and profile of the welded joint residual stresses when more than one welding pass was considered.

Effect of nitrogen content on the second phase particles in V–Ti microalloyed shipbuilding steel during weld thermal cycling

The microstructures, mechanical properties, and second-phase particles in the base plate and simulated coarse grain heat affected zone (CGHAZ) of V–Ti microalloyed steels with differing amounts of nitrogen were investigated. A Gleeble-3800, submerged arc welding, and transmission electron microscope were used during this investigation. The results showed that austenite grain size was refined and microstructure and toughness of CGHAZ improved with increasing nitrogen. Furthermore, in the case of low- nitrogen steel, base plate consisted mainly of Ti-rich (Ti, V) (C, N), which were coarsened after welding thermal cycling, thereby leading to coarsening of the austenite grains. In the case of high-nitrogen steels, base plate consisted mainly of Ti-rich (Ti, V) (C, N), V-rich (V, Ti) (C, N), and V (C, N) particles. During welding heating, Ti-rich (Ti, V) (C, N) dissolved only partially, whereas V-rich (V, Ti) (C, N) and V (C, N) particles dissolved completely. Fine Ti-rich (Ti, V) (C, N) and V (C, N) re-precipitated during cooling stage of welding. The sub-50-nm Ti-rich (Ti, V) (C, N) particles present at peak temperature and during cooling, were effective in refining the austenite grain size, by preventing migration of austenite grain boundaries. The nitrogen-enhanced V–Ti steels exhibit excellent toughness for high heat inputs.

Dissimilar ultrasonic spot welding of aerospace aluminum alloy AA2139 to titanium alloy TiAl6V4

The microstructure, hardness, lap shear strength and fracture energy of AA2139–TiAl6V4 spot joints produced by ultrasonic welding were investigated and related to the weld thermal cycle. No obvious intermetallic reaction layer was observed in the AA2139–TiAl6V4 welds, even using transmission electron microscopy. The hardness profile of AA2139 side after welding was studied, demonstrating that the heat introduced by the welding process leads to some softening with partial hardness recovery after natural aging. The effects of welding time on peak load and fracture energy were investigated. The peak load and fracture energy of welds increased with an increase in welding time and then reached a plateau, i.e., maximum peak load 5.3kN and maximum fracture energy 3.7kNmm. In all cases, failure occurred by fracture at the weld interface.

Microstructure and mechanical properties of laser-arc hybrid welding joint of GH909 alloy

In this paper, laser-arc hybrid welding of 10 mm thick low-thermal-expansion superalloy GH909 components was carried out to obtain a joint with good performance. This investigation was conducted using an optical microscope, scanning electron microscope, energy diffraction spectrum and other methodologies. The results showed that weld joints with a desirable wineglass-shaped weld profile can be obtained employing appropriate process parameters. The different grains in between the upper central seam and the bottom seam were associated with the temperature gradient, the pool’s flow and the welding thermal cycle. MC-type carbides and eutectic phases (γ+Laves) were produced at grain boundaries due to the component segregation during the welding process. In addition, γ′ strengthening phase presented in the interior of grains, which kept a coherent relationship with the matrix. The lowest hardness value occurred in the weld center, which indicated that it was the weakest section in the whole joint. The average tensile strength of the joints reached to 632.90MPa, nearly 76.84% of the base metal. The fracture analysis revealed that the fracture mode of the joint was ductile fracture and the main reason for joint failure was as a result of the occurrence of porosities produced in the weld during the welding process.

Influence of Microstructural Inhomogeneity on the Creep Rupture Behaviour of 316LN SS Weld Joints

The fusion zone is usually the weakest link in austenitic steel weld joints and it is usually in this region where damage initiates during creep deformation. The heterogeneity in microstructure of the fusion zone enhances the complexity in understanding the damage evolution during creep exposure. The major source of heterogeneity in the microstructure is the weld thermal cycle, which the weld joint is inevitably subjected to during multi-pass welding. Subsequent thermal cycles are likely to alter the microstructure of the previously deposited fusion zone thereby influencing the mechanical properties of the joints. In this study, the effect of thermal cycle on creep rupture behaviour of 316LN SS weld joint has been studied. Weld joints were fabricated by two welding processes—activated tungsten inert gas (ATIG) and shielded metal arc (SMA) welding. In case of ATIG weld joints, the creep properties of single and dual-pass weld joints were compared and for the SMAW joints, the comparison was made between weld joints made with two different electrode sizes. In ATIG weld joint, the subsequent thermal cycle in the dual pass weld joint improved its creep rupture life. In the case of the SMAW joint, the weld joint made with larger electrode, which was subjected to less number of thermal cycles exhibited better creep properties. The weld thermal cycle induces a ‘thermo-mechanical treatment’ and changes the morphology of delta ferrite in the previously deposited fusion zone which has a synergistic role in dictating the creep rupture life of the 316LN SS weld joints.

Numerical Simulation of the Thermal Cycle of the PAW-MAG Hybrid Welding of Advanced High Strength Steels

Numerical Simulation of the Thermal Cycle of the PAW-MAG Hybrid Welding of Advanced High Strength Steels

Finite Element Simulation of Thermal Field in Double Wire Welding

In this paper, the thermal field of double wire welding is simulated by using ANSYS software. Simulation results were shown that the total heat input (E) is the most significant parameters to change the value of t8/5; By the mean of rationally controlling the proportion of the front arc heat input (E1) in the total heat input (E) and appropriately selecting double wire spacing (L), It is effective means to get the double wire welding thermal cycle. By the way of simulation, it is possible to manage the thermal input in the double welding wires and to control the temperature field and cooling rate that are fundamental for the final joint quality, it is great importance guidance to optimize the double wire welding process parameters.

Structure Formation and Properties of Corrosion-Resistant Steel with Treatment by a Highly Concentrated Energy Source

Structure formation and corrosion breakdown of martensitic class corrosion-resistant steels in an area of thermal action during treatment by highly concentrated energy sources are studied. Heating conditions are reproduced by means of simulating a welding thermal cycle. Corrosion resistance and hardness of the heat-affected zone are studied. A mechanism is revealed for corrosion breakdown of steel EP-56 simulated specimens. Features are established for the effect of heat treatment and welding regimes on resistance of a heated section to corrosion cracking.

Comparison of microstructure and mechanical properties of TIG and laser welding joints of a new Al–Zn–Mg–Cu alloy

Sheets of a new Al–Zn–Mg–Cu alloy with T6 temper were welded using TIG and laser welding method. Microstructure characteristics and mechanical properties of joints were contrastively analyzed. Results show that the width of the fusion zone (FZ) and heat affected zone (HAZ) in laser joint is obviously smaller. The FZs in TIG and laser joints are both made up of dendritic equiaxed grains and lots of coarse particles, the mean grain size is 33.9μm and 6.1μm respectively, and the coarse particles are identified as the T (AlZnMgCu) phase. The two factors of grain size and alloying elements distribution are combined to result in almost the same microhardness of the two FZs. Moreover, the grain morphology, grain size and degree of recrystallization of the two HAZs are almost the same with base metal (BM). Due to the higher heating and cooling rate of welding thermal cycles in HAZ of laser joint, the microhardness is obviously higher. The minimum microhardness of the two joints is obtained in FZ where the fractures always occur in the tensile tests, the ultimate tensile strength of TIG and laser joints is 436.2MPa and 471.1MPa, the elongation of them is about 7.5% and 5.1% respectively.

Artificial Neural Networks to Estimate the Thermal Properties of an Experimental Micro-Alloyed Steel and Their Application to the Welding Thermal Analysis

The effect of welding thermal cycles on the micro-structure and micro-hardness of the heat-affected zone (HAZ) of an experimental micro-alloyed steel was studied. Due to the experimental difficulties involved in acquiring the thermal cycles, these were determined by applying the solutions of Rosenthal’s equations for thick and thin plates. However, to perform this thermal analysis, it requires knowledge of the thermal properties of the micro-alloyed steel; therefore, the implementation of two artificial neural networks (ANNs) was proposed as tools to estimate the thermal conductivity and the heat capacity as a function of the chemical composition and temperature. The ANNs were trained with information obtained from the literature review and then tested with steels that were not used for the training step, but with thermal known properties. A good approximation between the actual and the estimated properties was observed. It was determined that the microstructural characteristics of the welding zone are a function of the thermal cycles, although there is no great difference in micro-hardness. Martensite was not observed in the welding zone; therefore, the welds of this steel, under these welding conditions, could not be susceptible to hydrogen induced cracking (HIC).

Effect of silicon contents on the microstructures and mechanical properties of heat affected zones for 9Cr2WVTa steels

The weldability of 9Cr2WVTa steels with silicon content varying from 0.30 wt.% to 1.36 wt.% was studied to meet the requirement of Generation-Ⅳ nuclear reactor. Samples of enlarged HAZs were fabricated by a thermal-mechanical simulator based on the simulation and measurement of non-equilibrium phase transformation. The content of δ-ferrite in the HAZs increased with the silicon content and the peak temperature of welding thermal cycle. The impact toughness in the HAZs decreased in different degrees when the δ-ferrite exhibits stripe (lower than 4.82%) or blocky types (higher than 4.82%). Post weld heat treatment (PWHT) has a significant role on improving the toughness. Adding silicon content increased the volume of δ-ferrite and therefore, decreased the tensile strength of the HAZs for 9Cr2WVTa steels. Silicon also as solid solution strengthening element increased the tensile strength. The 9Cr2WVTa steel has good weldability when the silicon content is lower than 0.60 wt.%.

Modeling Stress - Strain State in Butt - Welded Joints a fter TIG Welding

In this paper mathematical model was developed for definition of thermal-welding cycle influence on welding deformations distribution in flat samples of austenitic steels after TIG welding and developed recommendations to reduce the welding deformation on o the machinery for welding with a copper backing.