Low Transformation Temperature Welding Wire Research Articles

Investigation of the Residual Stress in a Multi-Pass T-Welded Joint Using Low Transformation Temperature Welding Wire.

We investigated whether low transformation temperature (LTT) welding materials are beneficial to the generation of compressive residual stress around a weld zone, thus enhancing the fatigue performance of the welded joint. An experimental and numerical study were conducted in order to analyze the residual stress in multi-pass T-welded joints using LTT welding wire. It was found that, compared to the conventional welded joint, greater tensile residual stress was induced in the flange plate of the LTT welded joints. This was attributed to the reheat temperature of the LTT weld pass during the multi-pass welding. The formerly-formed LTT weld pass with a reheat temperature lower than the austenite finish temperature converted the compressive residual stress into tensile stress. The compressive residual stress was generated in the regions with a reheat temperature higher than the austenite finish temperature, indicating that LTT welding materials are more suitable for single-pass welding.

Control of Weld Residual Stress in a Thin Steel Plate through Low Transformation Temperature Welding Consumables

Low transformation temperature welding (LTTW) consumables are characterized by a low martensite start temperature and a large fraction of martensite forming in the weld. It can efficiently reduce the tensile residual stress because the volume expansion associated with the martensitic transformation compensates for the thermal contraction during cooling. In this work, a LTTW wire, designated as EH200B, was created for the arc welding of advanced high-strength steel thin plates. In comparison to conventional ER70S-3 wires, this LTTW wire generated an opposite distortion pattern. Neutron diffraction measurements along the center thickness of the welded plates showed the maximum residual stress along the longitudinal direction (LD) in the weld region, and the heat-affected zone (HAZ) immediately adjacent to the weld region was reduced from ~330 MPa to below 240 MPa by using the LTTW wire. A finite element (FE) model was developed to predict the residual stress distributions of the plates welded under these two wires. The simulation results showed reasonable agreement with the volume-average neutron diffraction data. Compressive residual stress in the weld region using the LTTW wire was predicted by the FE method. Electron backscattered diffraction and x-ray diffraction measurements confirmed ~90% martensite was present in the LTTW weld. The fatigue life of DP980 steel lap joint panels using EH200B wire nearly doubled that of ER70S-3 wire. This improvement was attributed to the high strength and low LD residual stress in the weld and HAZ immediately adjacent to the weld.

低変態温度溶接材料を用いた溶融金属積層造形材の変形低減法に関する基礎的検討

Current work deals with investigation of the effect of the use of low transformation temperature welding wire (LTT welding wire) on distortion reduction of parts made by wire and arc additive manufacturing technique (WAAM). LTT welding wire and SUS308L wire, which was austenitic stainless steel wire, were used for making WAAM test pieces. Distortion of base plates of test pieces were evaluated by 3D non-contact coordinate measurement system. As a result, longitudinal bending distortion of the base plates of test pieces in which LTT welding wire was partially or fully used was smaller than those of the test piece which was made only by SUS308L wire. Especially, longitudinal bending distortion of the base plates of test pieces which was made only by LTT welding wire was reduced drastically from those of the test piece which was made only by SUS308L wire. However, angular distortion of the base plates were not reduced drastically even if the test piece was made only by LTT welding wire. The difference of the reduction effect of the use of LTT welding wire was discussed by considering the position of neutral plane of each distortion mode.

Fatigue strength improvement of stainless steel using weld toes dressing with low transformation temperature welding wire

In the present study, low transformation temperature welding wire (LTTW), which can induce residual compressive stress around 304L welded joints, has been developed to improve the fatigue performance of welded joints. Procedure of design and chemical composition of weld metal and deposited metal of LTTW are introduced. The microstructure of weld metal of LTTW is composed of low carbon martensite and residual austenite. The residual stress distribution of a single welding bead of LTTW was measured and the result shows that compressive residual stress is generated. In addition, fatigue test was also carried out on no-load cruciform welded joints of 304L stainless steel under three conditions: as welded, 308L dressing and LTTW dressing. The result shows that the fatigue life of LTTW dressing joints (2×106 cycles) is improved by 14–23 times and 3–6 times compared with that of as welded joints and 308L dressing joints.

The applicability of using low transformation temperature welding wire to minimize unwanted residual stresses and distortions

Fusion welding of steel pressure vessels provides one of the most economic methods for creating such structures. However, the highly localised heat distribution, at the fused region gives rise to non-uniform heating/expansion and cooling/contraction of the weld and the surrounding base material, consequently giving rise to residual stresses and distortion. Several techniques are available to minimize welding distortions and residual stresses [1]. One feasible and efficient mitigation technique, that alters the inherent residual stresses developed during welding, includes the use of low transformation start temperature welding wire [2–4]. By altering the filler material and metallurgical transformations developed at the fusion zone and heat affected zone, wielded regions at the weld line can give rise to compressive residual stresses [2]. Consequently by establishing the appropriate chemical composition of the welding wire, a significant reduction in welding distortions can be achieved. This study focuses on establishing the merits of using different filler materials to minimize welding residual stresses and distortion through a thermal-metallurgical-elasto-plastic numerical parametric study. Numerical modelling of butt welded structures using three different filler materials is considered and the applicability of using low transformation temperature welding wire is investigated. The predicted residual stresses and distortions give a significant advance in understanding and control of welding distortion in welded fabrications. The chemical composition of the filler material and ultimately fused zone composition, leading to reductions in residual stresses and distortions is identified.

Analysis of martensite transformation behaviour in welded joint using low transformation temperature welding wire

Low transformation temperature welding (LTTW) wire has been found to improve the fatigue strength in welded joints. In the present study, the temperature dependence of the mechanical properties in a dual phase microstructure of austenite and martensite was estimated using the properties of full austenite and full martensite in numerical analyses. A welding method effective for residual stress reduction and fatigue strength improvement was shown by applying a calculation method under transformation superplasticity and transformation induced plasticity in high strength steel welded joints. With this method, the influence of the welding pass sequence on the residual stress distribution and fatigue strength was examined in a boxing fillet welded joint using LTTW. The transformation tensile residual stress in the weld toe was decreased by sectioned welding, and the fatigue limit by sectioned welding with LTTW improved in comparison with the fatigue limit of a joint welded with conventional wire in the same process.

Analysis of martensite transformation behaviour in welded joints of low transformation-temperature materials

In order to improve fatigue strength in welded joints, low transformation-temperature welding wire has been developed in which residual tensile stress can be reduced. In application of the low transformation-temperature welding wire, the prevention of cold cracking without preheating in high strength steel welded joints is expected and examined from the control of residual tensile stress. However, it is expected that residual stress distribution in a welded joint can be suggested by numerical analysis, because the residual stress cannot be measured simply and non-distractively. In this report, martensite transformation behaviour such as Ms point, transformation expansion, and so on is measured firstly by the Formaster test. And temperature dependence of several mechanical properties was measured in full-austenite and full-martensite microstructures, and temperature dependence of mechanical properties was estimated in dual phase microstructure of austenite and martensite. By these data, numerical analysis was carried out and martensite transformation behaviour was compared with measured and calculated results in a rigid model test. From the comparison, it was suggested that transformation superplasticity had to be considered in numerical analysis. Next, the increase of Ms point due to transformation induced plasticity was guessed from the comparison with measured data by laser speckle measurement and calculated data under transformation superplasticity consideration. From the all results, it was found that the measured transformation behavior and residual stress had good agreement with the calculated results under transformation superplasticity and transformation induced plasticity considerations.

Angular distortion of fillet welded T joint using low transformation temperature welding wire

A newly developed low transformation temperature welding wire, of which the transformation start temperature is lower than that of conventional welding wires, was applied to fabrication of fillet welded T joints. The welding angular distortion and the temperature profile of the weld metal were continuously measured during the welding process. The angular distortion of the fabricated T joint was reduced when the weld metal reached the martensitic transformation start temperature. The residual angular distortion was less with the low transformation temperature welding wire than that with the conventional welding wires. The welding distortion of T joints was calculated by a numerical simulation with consideration of the effect of phase transformation under weld thermal cycles. The welding distortion was reproduced with high accuracy in the numerical simulation. Results of the numerical simulation also determined that there was a direct correspondence between the transformation expansion of the weld metal and the angular distortion.

微量酸素軸流局部添加によるAr-MIGアーク溶接特性

GMA welding under pure Ar shielding gas atmosphere (pure Ar-MIG arc welding) is effective to obtain a high-quality welded joint. However, the pure Ar-MIG arc welding cannot be applied practically because of arc instability. In order to perform stable pure Ar-MIG welding, a new welding process has been required.We analyzed the cause of arc instability by high-speed video camera system. From video images, it was found that long column of liquid metal (CLM) formed at melting wire tip moves irregularly. Therefore, it was expected that unstable MIG arc is due to the behavior of the CLM, although the cathode spots moves irregularly in front of weld pool.Next, new welding process was developed, in which the small amount of oxygen can be added near the melting wire tip through the wire contact tip. In the developed process, the relationship between arc instability and oxygen content mixed in shielding gas was discussed. In the case of the introduction of small amount of oxygen, the length of CLM becomes effectively short and arc becomes stable, but the cathode spot moving area is not decreasing enough. Therefore, it was concluded that pure Ar-MIG arc instability depended on the CLM behavior.Finally, the oxygen content in weld metal was able to reduce to less than 100 ppm in the process, and then high ductility weld metal was successfully obtained with low transformation-temperature welding wire.

Repair of fatigue cracks initiated around box welds using low transformation temperature welding material

In order to improve the fatigue strength of welded structure, compressive residual stress is induced by means of marten site expansion of weld metal using developed low transformation temperature welding wire. Fatigue limit could be improved on repaired box welded joints.

Fatigue Strength Improvement of Lap Welded Joints by Low Transformation Temperature Welding Wire — Superior Improvement with Strength of Steel

The potential improvement in fatigue performance of lap welded joints in 2 mm thick 540MPa and 780MPa class steels from the introduction of compressive residual stress by welding with low transformation temperature (LTT) welding wire was investigated. Comparative fatigue tests in bending at a stress ratio of R=0 on specimens made with conventional and LTT welds confirmed that the fatigue performance of such thin specimens was improved, particularly in the long-life regime. Furthermore, the benefit was greatest in the high-strength steel, the improvement in fatigue strength at 107 cycles being 1.4 times for 540MPa steel and 1.6 times for 780MPa steel.

低変態温度溶接材料を用いた角回し溶接継手の補修による疲労強度向上

In order to improve the fatigue strength of welded structure, compressive residual stress is induced by means of marten site expansion of weld metal using developed low transformation temperature welding wire. Fatigue limit could be improved on repaired box welded joints.

Fatigue strength improvement of box welds using low transformation temperature welding material. Tripled fatigue strength by post weld heat treatment

The fatigue strength of box welds was tripled by using low transformation temperature welding wire and postweld heat treatment (PWHT). The improved fatigue strength was obtained due to the stress ratio effect resulting from compressive residual stress. The low transformation temperature welding material which contains 10 mass % Cr and 10 mass % Ni expands during the final cooling process of welding. This expansion of weld metal induced the compressive residual stress around the welded part. The PWHT at 720°C for 1 hour and subsequent cooling in air induced the transformation from austenite to martensite at the same time for three passes. That is, the volume expansion on three passes by PWHT became large compare with that on final pass in as-welded condition, and the compressive residual stress could be induced efficiently.

低変態温度溶接材料を用いた角回溶接継手の疲労強度向上 溶接後熱処理による３倍化

The fatigue strength of box welds was tripled by using low transformation temperature welding wire and postweld heat treatment (PWHT). The improved fatigue strength was obtained due to the stress ratio effect resulting from compressive residual stress. The low transformation temperature welding material which contains 10 mass % Cr and 10 mass % Ni expands during the final cooling process of welding. This expansion of weld metal induced the compressive residual stress around the welded part. The PWHT at 720°C for 1 hour and subsequent cooling in air induced the transformation from austenite to martensite at the same time for three passes. That is, the volume expansion on three passes by PWHT became large compare with that on final pass in as-welded condition, and the compressive residual stress could be induced efficiently.

Superior fatigue crack growth properties in newly developed weld metal

The fatigue threshold and high growth rate region properties of conventional welded joints were improved by using newly developed low transformation temperature welding wire. The developed weld metal which contains 10 wt% nickel and 10 wt% chromium begins to transform from austenite to martensite at about 180°C and finishes it at room temperature. During the transformation the weld metal expands. This expansion induces a compressive residual stress around the welded part. The stress ratio effect due to this compressive residual stress makes the fatigue crack growth properties of the developed weld metal superior by intensifying the fatigue crack closure.