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.
An integrated tool is presented for the estimation of the microstructure of steels that consist of a mixture of ferrite and martensite following hot deformation processing. The model includes estimation of the effect multi-pass rolling on the austenite size and shape, and its stored energy, information which is then passed on to modules dealing with solid-state phase transformations. Finally, the mechanical properties are estimated using separate modules for strength, toughness and ductility. The integrated model shows good agreement with experimental data.
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 O 2 or CO 2 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.
CO 2 0 30 J TIG 0 100 J 10 11)Ar MIG Ar MIG [ 25 p. 187-195 (2007)]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.
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