Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca evaluated by temperature gradient type standard ESSO test are the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.
Recently, friction stir welding (FSW) has been widely used in various fields. However, the tool wear and adhesion of the workpiece on the tool surface are serious problems. In the field of cutting tools, hard thin films were coated on the tool surface to improve the life and cutting performance of the tool. To address the problem of FSW, hard thin films such as TiN, TiAlN, TiSiN, TiBON, CrN, CrSiN and AlCrSiN were coated on the welding tools. And the relation between the wettability of these films and the process torque during FSW and quality of the finished surface was examined. The wettability of pure iron and hard thin films was determined using a sessile drop method by focusing on the contact angle. The process torque during FSW was measured through the joining of low carbon steels. The process torque was significantly different depending on the type of the hard thin film. A clear correlation was observed between the wettability and the process torque. The result suggested that the tool performance for FSW can be evaluated by the wettability.
GMA welding under pure argon shielding gas atmosphere (pure argon-GMA welding) is suitable to obtain a high-strength and high toughness welded joint. However, it is difficult to apply pure argon-GMA welding practically to welding structure because of arc instability. In order to perform stable pure argon-GMA welding, duplex current feeding GMA welding has been developed. The duplex current feeding GMA welding consists of primary GMA welding current by constant-voltage power source and secondary current by constant-current power resource. In previous experimental study, it was found that the temperature of a droplet by duplex current feeding GMA welding was higher than the conventional GMA welding. In this study, mechanism and basic characteristics of duplex current feeding GMA welding are investigated by numerical simulation with a simplified duplex current feeding GMA welding model. As a result, it was found that total welding current was largely increased by conducting the secondary current in addition to the primary GMA welding current under the same arc voltage and wire feed speed. Furthermore, the droplet temperature was also increased due to the increase in total welding current.
In GMA welding presently fluctuation of the tip of welding wire has been able to restrain precisely and metal transfer also has been able to regulate periodically, that is stable arc like TIG welding has been realized. Then at this stage reduction of contact tip abrasion is the most important subject to maintain arc stability for long time. In this investigation many factors are evaluated with abrasion amount and new material of contact tip is recommended. Results obtained are as follows. Using uncoated welding wire increases contact tip abrasion rapidly as to Cu coated welding wire. On the contrary other welding conditions concerning contact point have almost no influence. Under the same current the abrasion of contact tip is reduced according as welding wire feeding rate becomes higher. Therefore the abrasion is accelerated violently at the time of arc ignition because of heat accumulation at the contact point by wire stopping and high current which is needed for arc ignition. That is the welding wire which is induced to contact point with room temperature has considerable influence to cool the contact point. As a measure to reduce the abrasion at arc ignition shortening period of high current is effective. To cool the contact point positively thermo-electromotive force was applied. Constantan (Cu-455Ni) has low thermo-electromotive force and high melting point than Fe. So Cu-Ni alloy (Cu-43.3%Ni-0.92%Mn) was selected and examined as the contact tip material. The abrasion of Cu-Ni alloy contact tip becomes less than about two tenth of the abrasion of Cu-Cr contact tip. As the conclusion, the best mixed measures which are effective to reduce the contact tip abrasion are using Cu coated welding wire, wire retract type arc ignition and Cu-Ni alloy contact tip.
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