Masanobu Toyoda

Angular distortion of fillet welded T joint using low transformation temperature welding wire

Abstract 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.

Full-Scale Measurements on Hull Response of a Large-Container Ship in Service

In order to investigate hull responses of post-Panamax container ships in the actual sea, full-scale measurements on hull responses of a post-Panamax container ship in service were conducted. In linear wave domain, the probability density distributions of hull responses obtained by full-scale measurements were compared with the Rayleigh distributions to check on the range of the applicability, and comparisons with the long-term distributions of the longitudinal stress obtained by full-scale measurements and the direct structural analyses based on the wave loads analyzed by using the linear 3D Rankine source method were made to verify the accuracy. In non-linear wave domain, the measured longitudinal stresses showed the asymmetry of vertical bending moment. The long-term distributions of hull responses, which have the high harmonic components, obtained by full-scale measurements were compared with the numerical results analyzed by using non-linear methods to investigate the non-linearity on hull responses of container ship.Copyright

Study of residual stress reduction in welded joints using phase transformation behaviour of welding material. studies on numerical simulation of temperature, microstructure, and thermal stress histories during welding and their application to welded structures (2nd report)

Abstract The residual stress generated in welded structures is well-known to exert a strong influence on features such as brittle fracture,1 buckling,2 stress corrosion cracking,3 fatigue,4 etc. It is therefore important to develop appropriate measures for evaluation of the residual stress distribution at structural locations requiring prevention of deformation or fracture and for suitable reduction of residual stress in response to the weld configuration being used.

Safety of Mega Container Ship Focusing on Brittle Crack Initiation and Arrest Behavior of Heavy Thickness Plate

The enlargement of containership requires the higher tensile strength steel and thicker plate around the hatch coaming more than the prescribed classification rules. These call the concerns about the safety of brittle crack initiation and arrestability of hull structure. It is known that increasing of stress and thickness of plates will reduce the toughness of base metal and welding joints. Therefore, the authors conducted to assure the toughness of base metal and welding joint for new higher tensile strength steel of 460MPa yield strength (YP460) and 60mm thickness. It has been considered that weld assembled steel plate structure has some arrestability at intersections of plates, though there are no prescription on the rules. And it was reported that very thicker mother plate has not enough toughness to arrest brittle crack recently. The authors investigated the structural arrestability by model testing and FE analysis and established it. Furthermore, structural details for intersections were also established. Both preventing the brittle crack initiation and arresting the brittle crack of weld structure were described, focusing on developing higher yield strength and heavier thickness of YP460 for containership in this paper.© 2008 ASME

Numerical simulations of weld hardness distributions in ultra-narrow gap GMA welding. Evaluation of weld hardness distributions by numerical simulations of welding thermal cycles and microstructural phase fractions (1st report)

Welded joints used in some circumstances may generally face the problem of loss of joint performance due to welding heat input as well as welding deformation and welding residual stress. For example, ultra-fine grained steel welds may sustain softening in regions heated in a slightly lower temperature range than the Ac 1 transformation point in such a way that zones with a lower strength than the base metal strength may possibly arise. The heat-affected zone of recently developed ultrafine grained steels may also sustain softening due to grain coarsening, so that when such steels are used for fabrication of actual structures, they risk facing the problem of being satisfactorily able to maintain joint performance. Heat input reduction is an effective measure that may be taken to control these problems. Heat input reduction, however, implies loss of welding fabrication efficiency and it is therefore important to achieve lower heat input and higher efficiency as simultaneous objectives in welding fabrication. Ultra-narrow gap GMA welding has been developed from this perspective. Ultra-narrow gap GMA welding seeks to achieve higher welding speed and lower heat input through narrowing the welding groove gap to such an extent as to make weaving in the plate width direction unnecessary. Groove width narrowing generated problems such as arc instability phenomena or poor groove root penetration are satisfactorily overcome through the current, voltage and wire feed rate during welding being suitably controlled, and through the welding wire being oscillated in the plate thickness direction. The calorific value of the arc can be suitably dispersed through welding wire oscillation in such a way that the deposited metal throat thickness can be increased. During application of ultra-narrow gap GMA welding to fabrication of actual structures, however, it is necessary to clarify joint performance-related factors, such as the temperature profiles and strength properties of welds during consideration of the effects of welding wire oscillation. Ultra-narrow gap GMA welding is envisaged particularly for application to welding of steels, typified by ultra-fine grained steels that risk loss of weld properties due to the welding thermal cycle, and it is therefore imperative to undertake thorough evaluations of joint performance during consideration of welding thermal cycles and associated changes in material properties. Through application of numerical simulation techniques to solve the foregoing problems, it is expected to be able to make global joint performance evaluations that consider material properties and the temperature fields in which welds are formed. This study, as a first step towards joint performance evaluation of ultra-narrow gap GMA welded joints, uses detailed measurements of temperature profiles during welding alongside numerical simulations of welding thermal cycles and microstructural phase fractions to develop a technique for estimation of weld hardness distributions as an important feature of ultranarrow gap GMA welds.

Effect of unwelded length on behaviour of brittle crack arrest in T-joint structure

In the large heat input weld joint of heavy gauge steel plate used for large container carriers, a brittle crack possibly propagates straight along a weld joint without diverging to the base metal. This phenomenon is discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at the T-joint embedding unwelded face, the effect of the unwelded face on behaviour of brittle crack propagation/arrest in the T-joint structure was investigated and analysed. The ESSO test of the T-joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into T-joint of hatch coaming and strength deck. The test results showed that the brittle crack arrested at the T-joint embedding the unwelded face, and the brittle crack arrested easier, the longer the unwelded face. The results of static FEM analysis showed that the stress intensity factor of the brittle crack was increased by the unwelded face, until the brittle crack reached a flange. However, when the brittle crack propagated into the flange, the stress intensity factor of the brittle crack was decreased by the unwelded face. The crack-arrest effect of the unwelded face appears after the brittle crack propagates into the flange.

Numerical simulation of transformation-induced microscopic residual stress in ferrite-martensite lamellar steel

The effect of transformation-induced microscopic residual stress on fatigue crack propagation behavior of ferrite-martensite lamellar steel was discussed. Fatigue tests of prestrained and non-prestrained specimens were performed. Inflections and branches at ferrite-martensite boundaries were observed in the non-prestrained specimens. On the other hand, less inflections and branches were found in the prestrained specimens. The experimental results showed that the transformation induced microscopic residual stress has influence on the fatigue crack propagation behavior. To estimate the microscopic residual, a numerical simulation method for the calculation of microscopic residual stress stress induced by martensitic transformation was performed. The simulation showed that compressive residual stress was generated in martensite layer, and the result agree with the experimental result that inflections and branches were observed at ferrite-martensite boundaries.