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.
The objective of this research is to reveal relationship between melt flows and spatter reduction by angle of incidence and defocusing distance in partial-penetration welding of a SUS304 stainless steel plate with a 6-kW power laser beam. In welding speeds from 50 mm/s to 250 mm/s, underfilled weld beads with spatters were obtained more than 150 mm/s. Accoriding to three-dimensional X-ray transmission in-situ observation of melt flows at 150 mm/s in welding speed with tungsten carbide (WC) tracers, the melt flews achieved approximately 2.3 m/s in speed and made convex molten-pool surface behind a keyhole inlet grow higher, resulting in spattering over 0.1 mm in the diameter. A 2-mm inner defocusing distance or a 20-degrees angle of advance decreased the number of spatter over 0.1 mm in the diameter by half or one third in comparison with that at focal point and zero angles. The X-ray transmission images demonstrate that the appropriate defocusing distance and angle of incidence made the speed of the melt flow decrease and the melt flow behind a keyhole inlet circulate, which led to not only suppressing the convex surface but also improving the frequency that the convex surface went back to the molten pool.
Fatigue strength decreases by thickness effect, therefore the improvement treatments of the fatigue strength are important as the structure consisting of the thick material. Post weld treatments are one of useful method to improve fatigue strength. However, the thickness effect on welded joint improved by post weld treatments is not clear because the experimental data is insufficient for quantitative evaluation. The object of this study is to evaluate thickness effect on the fatigue strength which is improved by post weld treatments to reflect it in fatigue design guideline; therefore in transverse no-load carrying cruciform joint which is one of the main welded joints of the ship structure, the improvement effect of the fatigue strength by post weld treatments is evaluated. In this report, the result of HFMI (High Frequency Mechanical Impact treatment) and Shot peening are evaluated with the result of the as-welded condition and conventional Burr grinding by nominal stress approach. In addition, the fatigue strength is evaluated using the equivalent stress approach by the modified MIL-HDBK-5 method that both the stress concentration of weld toe and the weld residual stress are separated from nominal stress. Good correlation between the fatigue test results was achieved using modified version of the equivalent stress approach defined in MIL-HDBK-5. With the equivalent stress that was adopted in this study, both the stress range and the effect of maximum stress including the residual stress are considered. It is considered that both number of cycles to crack initiation and that to failure are evaluated by equivalent stress approach integrally.
To understand the cause of compressive residual stress in welded joints, we analyzed by numerical analysis the effect of welding pass sequence using Low Transformation Temperature (LTT) welding materials, on residual stress around the weld toe of boxing fillet welded joints. It was determined by numerical analysis that the produced compressive residual stress and the influence of the stiffeners are reduced in the equivalent position of the weld toe in a fillet welded joint because of the influence on behavior of the stiffener in the weld being due to residual stress distribution around the weld toe. The residual stress reduction method of extending the length of the welded bead and releasing the weld toe from the stiffener, similar to the concept of discarding a bead to reduce tensile residual stress, was effective in fillet welded joints. Numerical analysis of the relationship between residual stress around the weld toe and width of the weld bead in the bead-on-plate welding model, clarified that compressive residual stress can be introduced around the weld toe by having a wide width weld bead. In addition, a fully penetrated welded joint was very effective for causing compressive residual stress around the weld toe.