Low Transformation Temperature Welding Research Articles

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.

Determination of Residual Stresses in Low Transformation Temperature (LTT -) Weld Metals using X-ray and High Energy Synchrotron Radiation

Crack and fatigue resistance are relevant evaluation criteria for welded joints and are decreased by tensile residual stresses resulting from the welding and cooling process, while compressive residual stresses can have a positive influence on the characteristics mentioned. In order to generate compressive residual stresses, a set of post weld treatment procedures is available, like shot peening, hammering, etc. These procedures have the disadvantage that they are time and cost extensive and have to be applied after welding. As another point, such technologies can only produce compressive stresses at the top surface, i.e. can only contribute to the reduction of the risk of cracks initiated at the surface, like fatigue cracks. A chance to generate compressive stresses over the complete weld joint during the welding procedure is offered by the so-called Low Transformation Temperature (LTT -) filler wires. Compared to conventional wires, these materials show lower phase transformation temperatures, which can work against cooling-related tensile stresses, resulting from respective shrinkage restraint. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Welds produced with different LTT — filler wires have shown different levels and distributions of the resulting residual stresses depending on the specific transformation temperature. The transformation temperatures are determined by temperature measurement. Classical X-ray diffraction as well as diffraction methods using high energy synchrotron radiation have been used for residual stress analysis. By means of high energy synchrotron diffraction in reflection mode residual stress depth gradients can be determined non-destructively. The phase selective nature of the diffraction measurements enables the simultaneous determination of the phase specific residual stresses of all contributing crystalline phases within one experiment. The application of white beam diffraction implies recording of a multitude of diffraction lines within the energy range of the provided energy spectrum of the white beam. By this means phase specific residual stress depth distributions up to distances of 150 μm below the surface can be analysed for steel using the energy dispersive set-up of the HMI-beamline EDDI at the Bessy site, Berlin, providing an energy range between 20–150 keV. As a side effect quantitative phase analysis can be carried out using white energy dispersive diffraction e.g. the determination of the content of retained austenite in the weld.

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

ABSTRACTArc 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.

Development of new low transformation temperature welding consumable to prevent cold cracking in high strength steel welds

A low transformation temperature (LTT) welding consumable has been developed to prevent cold cracking in high strength steel welded joints without preheating. In the LTT welded joint, the residual tensile stress is reduced by martensitic expansion of weld metal formed by the LTT consumable. In the weld cracking tests, cold cracking in the LTT weld metal is successfully prevented under high restraint conditions, but cold cracking occurs at very low joint restraint strength in case the weld metal is fully martensitic. Chemical compositions of the consumable are designed to retain austenite in martensite in the newly developed weld metal to absorb the diffusible hydrogen into the austenite to prevent cold cracking. In the newly developed LTT weld metal, cold cracking is almost fully suppressed without preheating under every joint restraint condition.

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.

低変態温度溶接材料利用による溶接部の変態挙動の定量評価の試み

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 pre-heating 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 welded joint can be suggested by numerical analysis, because the residual stress cannot be measured simply and non-distractively.In this report, martensite transformation behavior such as Ms point, transformation expansion and so on is measured firstly by 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 behavior was compared with measured and calculated results in side 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 the good agreement with the calculated results under transformation superplasticity and transformation induced plasticity considerations.

低変態温度溶接材料を用いた高張力鋼溶接継手の大気腐食挙動

When low transformation-temperature welding consumables are applied to welding of high strength steels, there is chemical mismatch between high strength base metal and the Cr-Ni system low transformation-temperature weld metal. The chemical mismatch may affect galvanic corrosion of welds in outdoor steel structures such as a bridge.In this study, atmospheric corrosion behavior was investigated for chemically mismatching weld joints formed by low transformation-temperature welding consumables.Corrosion depth profile at weld interface was measured by using laser microscope after outdoor exposure test of a year and the measured profiles were analyzed by plotting on Gumbel probability paper.Results of measurement and analysis showed that corrosion depth of base metal was independent on Cr content of weld metals and distance from weld interface.

ESPIシステムの溶接への適用

Behavior of displacement at welded joints during welding is an important factor for reliability of the joints. But in-situ measurement of the behavior is very difficult because contact method such as strain gauge is inapplicable in high temperature. In this study, ESPI (Electronic speckle pattern interferometry) system is applied to in-situ measurement of displacement increment distribution in SUS304 steel, 9%Ni steel, and low transformation temperature welding (LTTW) material during GTA welding. Qualitative tendency of measured results agrees with numerical simulation. Expansion behavior of phase-transformation is detected in 9%Ni steel and LTTW material during cooling. Moreover ESPI system enabled measurement of displacement increment at uranami bead surface and surrounding area of molten pool. These results show that ESPI system is applicable to measurement of displacement distribution at welded joints during welding process.

高張力鋼の低温割れを防止する新低変態温度溶接材料の開発

There has been growing interest in high strength steel with a tensile strength higher than 800 MPa, which would be ideal for weight reduction and better performance of steel structures. However, there are two major problems in the application of high strength steel to structures. One is that the fatigue strength of welded joints is far lower than that of the base metal, and the other is that the weld metal and the heat-affected zone (HAZ) are likely to have cold cracking. These problems must be solved before high strength steel can be widely used in steel structures. As a solution to the problem of cold cracking, a reduced preheating type of high strength steel has recently been developed in which cold cracking in the HAZ is reduced by reduction of alloy components (lower Ceq, Pcm) without loss of strength. However, since the resistance to cold cracking of the weld metal has not been improved, preheating of the weld metal is still necessary, and the problem remains toward its wider use. Recently, Ohta et al. developed a low transformation-temperature welding consumable, in which compressive residual stress is induced by transformation expansion of martensitic transformation of weld metal at a low temperature near room temperature, and demonstrated that this consumable improves the fatigue strength of welded joints. We considered that the reduction of tensile residual stress in the low transformation-temperature welding consumable might also be effective in decreasing the cold cracking and that this consumable would be useful in manufacturing high performance welded joints with improved fatigue strength and cold-cracking resistance. In the present study, we examined the effects of low temperature-transformation weld materials on the prevention of cold cracking in high strength steel. Firstly, the y-shaped weld cracking tests demonstrated that reduction of residual tensile stress induced by martensitic transformation expansion of weld metal was effective in reducing cold cracking. Secondly, the effects of the degree of joint constraint on cold cracking in low transformation-temperature weld materials were examined by the y-shaped and H-shaped weld cracking tests. The cracking ratio was high at low degrees of joint constraint and low at high degrees of joint constraint. Two causes were considered: one was that the reduction of tensile residual stress by transformation expansion is higher at a higher degree of joint constraint, and the other was that the weld metal of martensitic structure alone is sensitive to cold cracking. According to the above investigation, we attempted to develop a weld material with high resistance to cold cracking at different degrees of constraint. To maintain the effects of reduction of tensile residual stress by transformation expansion to reduce the amount of diffusible hydrogen and the sensitivity of cracking, which are other causes of cracking, we designed and developed a 2-microstructure phase weld metal (martensite + retained austenite) by modifying the low temperature-transformation weld material to obtain a lower Ms point. We examined the effects of the degree of joint constraint on cold cracking, and confirmed that the cracking rate of the modified rod was almost 0% at all degrees of constraint, and its resistance to cold cracking was high.

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.

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

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

Fatigue properties of longitudinal attachments welded using low transformation temperature filler

In welded structures, fatigue properties are normally considered to be independent of mean stress owing to the presence of tensile residual stresses with a magnitude close to yield level. Introduction of compressive residual stresses by use of low transformation temperature welding consumables has been reported in the literature to have a positive influence on the fatigue strength. In this investigation new low transformation temperature welding consumables have been evaluated with respect to static strength, impact toughness, residual stresses and fatigue properties. Welds were produced in the base material Domex 700 MC, an extra high strength cold forming steel with minimum yield strength of 700 MPa. The improvement of fatigue strength was found to be 25 - 90% at 2 × 106 cycles.

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.

疲労 低変態温度溶接材料による自動車用薄鋼板重ね溶接継手の疲労強度向上

The fatigue strength of lap welded joints of thin steel plate for automobile use was improved by using low transformation temperature welding material which induced the compressive residual stress around welded part. The transformation of welding material from austenite to martensite begins around 200°C and finishes around room temperature. The expansion of weld metal due to the transformation was constrained by the surrounding base metal, and the compressive residual stress is induced in the as welded condition. The mean stress effect due to this compressive residual stress improved the fatigue strength.

低変態温度溶接材料を用いた角回し溶接継手の疲労強度向上

The fatigue strength of developed box welds was improved about two times of the conventional box welds. The improved welds used the low transformation temperature welding material which contains 10% Cr and 10% Ni. The expansion of the developed welding material was 0.55% due to the transformation from austenite to martensite. This expansion induced the compressive residual stress around welds. On low stress range condition, the stress ratio effect by this compressive residual stress avoids the fatigue crack initiation at the weld toe. On high stress range condition, the fatigue crack initiated from weld toe and the fretting was observed on the fractured surface. While in the conventional welds, no trace of fretting was observed.

Fatigue strength improvement of box‐welded joints using low transformation temperature welding material

The fatigue strength of developed box welds was improved about two times of the conventional box welds. The improved welds used the low transformation temperature welding material which contains 10% Cr and 10% Ni. The expansion of the developed welding material was 0.55% due to the transformation from austenite to martensite. This expansion induced the compressive residual stress around welds. On low stress range condition, the stress ratio effect by this compressive residual stress avoids the fatigue crack initiation at the weld toe. On high stress range condition, the fatigue crack initiated from weld toe and the fretting was observed on the fractured surface. While in the conventional welds, no trace of fretting was observed.

低変態温度溶接材料を用いた箱型断面溶接部材の疲労強度向上

The box section welded members were made by using low transformation temperature welding material without preheating and postweld heat treatment. The expansion of weld metal due to the transformation from austenite to marensite around room temperature induced the compressive residual stress of-159 MPa along the weld line. The fatigue strength at 2×106 cycles was improved about 1.3 times as large as that of a usual box section welded member. The blowholes of which size was up to 4.5 mm did not induce the fatigue crack in the compressive residual stress field.