Kazuo Hiraoka

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

Abstract 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.

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.

Effect of Ms Temperature on Residual Stress in Welded Joints of High-Strength Steels

Thermal stress in the weld joints of high-strength steels after welding receives a great influence of dilatation stress during phase transformation, especially martensitic transformation of the low temperature region in weld metal. In order to quantify the amount of residual stress linked with thermal stress, neutron diffraction analyses were carried out in the welded joints having weld metals with very reduced Ms temperatures. In the butt joint of weld metal with an Ms temperature of about 60 °C, compressive residual stresses of about −400 MPa at the weld centre and of about −75 MPa at the toe were observed. The residual stress distributions were quite different from those of conventional weld metals, which have a tensile residual stress of about 400–500 MPa. The formation mechanism of compressive residual stress and the effect of restraint stress and stress-induced transformation on it are discussed, including previous data. It can be concluded that the reduction amount of residual stress induced by low Ms weld metal is about 11–15 MPa per unit length within about 70 mm weld length.

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

Abstract 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.

Improvement of MIG welding stability in pure Ar shielding gas using small amount of oxygen and coaxial hybrid solid wire

Abstract Metal inert gas/metal active gas (MIG/MAG) welding is generally used in many fields because of its high efficiency. Metal inert gas welding is expected but has not been used in pure Ar shielding gas because of arc instability. Shielding gas including O2 or CO2 has been used to secure the arc stability. However, these gases cause the quality of joints to deteriorate. Therefore, the authors have investigated the case of instability using a high speed video camera system to achieve a stable MIG welding in pure Ar shielding gas. The observation showed that the unstable welding behaviour was due to the instability of a column of liquid metal generated at the wire tip. To shorten the column of liquid metal, the authors developed a welding process which uses a small amount of oxygen and a coaxial multilayer solid wire (coaxial hybrid solid wire) where the melting behaviour can be controlled. A stable welding can be achieved by employing these methods.

Residual Stress Distribution of Steel Welded Joints with Weld Metal of Low Martensite Transformation Temperature

Weld metal with a low martensite temperature may reduce the thermal stress in the low temperature region after welding, and consequently may decrease the residual stress in the weld metal and weld toe. Neutron diffraction analysis made clear that the reduction amount of residual stress is about 400 MPa in the case of a welded joint with no fillet and about 200 MPa in the case of that with a fillet. Reduction of 400 MPa brings about high resistance to weld cold cracking. The residual stress reduction of 200 MPa in boxing fillet weld joints increases fatigue strength by about 40 to 70 MPa. It can be said that the increased amount of fatigue strength is almost one forth of the 200 MPa that is the reduction amount of residual stress.

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.

狭開先化に伴うアーク溶接現象の基本的な特性把握と溶接安定化の提案 : 超狭開先GMA溶接プロセスの開発(第1報)

Narrow gap welding (NGW) joints offers many advantages over conventional welding methods, such as good mechanical properties of joints, high welding efficiency and low residual stress. As the groove gap width becomes narrower, the arc heat input can be reduced and the merits in narrow gap welding increases more. Generally, GMA welding method has been never applied to less than 5 mm groove gap, because it is guessed that it is arc instability and lack of fusion at the groove bottom area occur. In this paper, first of all, arc behavior under narrower gap joints is discussed, and it was concluded that the arc in MIG arc welding irregularly perturbates up-to-downwards along the groove wall under less than 5 mm gap, but CO2 arc was stable under narrower gap. Next, penetrations at the groove bottom area in CO2 arc welding were discussed. Characteristics of bead formation phenomena in CO2 buried arc welding of bead-on-plate were analyzed. From the results, the relationship between hydrostatic potential of molten metal and arc force corresponding with welding current was estimated. Furthermore, the width of gauging region of penetration by arc force was measured and the relationship between the melting width at groove bottom and welding conditions (welding current and welding speed) can be suggested. With these results, numerical simulation model was proposed and the optimum welding conditions to melt the groove bottom area sufficiently and to minimize heat input were searched by numerical simulation. And then narrow gap welding with 5 mm groove gap was carried out using these simulated welding conditions. In the experimental results, the weld bead was obtained without lack of fusion at groove bottom, but the convex surface bead was formed which is disagreeable in multi-pass welding. The new welding process was proposed from numerical simulations in order to prevent this convex bead and to obtain sufficient melting at bottom area. In the new process, the wire extension can be controlled by welding current waveform and then arc regularly oscillated up-to-downwards along the groove wall. In this arc oscillation, arc heating distribution along groove wall led to both sufficient penetration at groove bottom and concave surface bead shape.

GMA Welding Process with Periodically Controlling Shielding Gas Composition. Development of Ultra-Narrow Gap GMA Welding Process. Report 3.

In previous reports, an ultra-narrow gap CO2 gas metal arc welding process has been developed in order to produce excellent welded joints. In the ultra-narrow gap (less than 5 mm gap) welding process, as the welding wire melting behavior was controlled by low frequency pulsated arc current waveform, the arc generating at the wire tip was widely oscillated in the direction of thickness and a good weld bead with large throat thickness was formed. In such a higher efficient welding process, the strong arc force of CO2 arc is essential to obtain sound beads without lack of fusion at a groove bottom, however, the oxygen content of weld metal in CO2 arc welding has to be reduced to obtain high-toughness at weld metal. In this report, a newly welding process is proposed in which an arc is widely oscillated along groove walls and oxygen content of welded metal is reduced as periodically controlling composition of CO2 gas in shielding gas. As an arc length between the wire tip and groove wall is held constant in ultra-narrow I type gap joint, arc current at stable operating point of CO2 arc essentially decreases less than that at stable operating point of Ar arc in constant potential characteristics of power source. It was confirmed from the numerical simulations on both wire melting and electric welding circuit that pulsated current waveform was produced as periodically alternating CO2 arc with MIG arc. Using the developed welding torch locally introducing CO2 gas into Ar+2%O2 shielding gas, the welding was carried out under V-type groove with less than 30 degree in the bevel angle. It was shown that the arc was widely oscillated along V groove walls and sound bead was formed in one side welding. Furthermore, it was confirmed that the oxygen content of weld metal in the proposed process was reduced to the level of that in MIG arc welding, and absorbed energy of weld metal was the same level as the MIG arc welding.

Energy balance in argon-helium mixed gas tungsten (TIG) arcs. Study of characteristics of gas tungsten arc shielded by mixed gases (3rd Report)

Summary The energy balance of an argon-helium mixed gas tungsten (TIG) arc is systematically discussed within the context of the demixing effect proposed in the previous reports in this series. The results obtained may be summarised as follows: The arc voltage is increased by the addition of helium, which has a higher ionisation voltage than argon in the argon arc, there being an especially sharp increase in the pure helium arc. This tendency is due to the fact that the potential in the cathode region clearly increases when helium is added but tends to remain virtually constant at a helium content of more than 75%. It is also due to the fact that the potential gradient of the arc column and the potential in the anode region remain virtually constant up to a helium content of 75%, sharply increasing as 100%He is approached. The heat input absorbed in the anode increases when helium is added to the argon arc, sharply increasing in 100%He. This tendency is due, not to an increase in the amount of energy tran...