Mitsuyoshi Nakatani

Influence of Dehydrogenation Heat Treatment on Hydrogen Distribution in Multi-Layer Welds Of Cr-Mo-V Steel

Cr-Mo-V steel normally undergoes a dehydrogenation heat treatment (DHT) of 350 °C × 4 h after welding, to minimize the susceptibility to cold cracking due to residual hydrogen in the weld. For low ambient temperatures or failure to sufficiently heat the weld during the DHT holding period, it is sometimes difficult to maintain and guarantee the 350 °C for 4 h. We therefore surmised that lower temperature DHT over a longer time period could be substituted for the standard DHT conditions and still achieve the same dehydrogenation effect. In this paper, using experimental and numerical methods to measure hydrogen diffusivity and the influence of DHT on Cr-Mo-V steel welds, we demonstrate that even at temperatures as low as 280 °C held over longer time periods, there is an equivalent dehydrogenation effect as in the existing conditions.

Fundamental study of narrow-gap welding with oscillation laser beam

Abstract Laser welding with oscillation laser beams enables control of the heat input distribution. In this study, we attempted to develop a narrow-gap welding process with oscillation laser welding. This process is expected to prevent a lack of fusion because the entire bottom to the groove can be melted by the oscillation laser. As the first step of the study, bead-on-plate welding experiments with an oscillation laser beam were performed to investigate the relationship between the welding conditions and welding results. The experiments revealed that the oscillation conditions strongly affect the welding penetration shape. It was clarified that the wire melting phenomena in oscillation laser welding differed from those of straight (non-oscillation) laser welding. Additionally, narrow-gap laser welding experiments were performed to investigate the relationship between the oscillation conditions and gap width. The results confirmed the effectiveness of oscillation laser welding for narrow-gap welding.

Development of gap sensing system for narrow gap laser welding

Abstract Narrow gap welding with an oscillation laser beam is one of the effective processes for thick plate welding. To put this welding process into practical manufacturing, a groove-sensing system using image processing for narrow gap welding with an oscillation laser beam is used. This developed system uses still images of the weld zone taken by a coaxial CMOS camera. It can recognize the position of the groove wall by analysing the brightness distribution in the still image. It can then control the oscillation width and the laser-irradiated area by calculating the groove width and the groove centre position. Some narrow gap welding experiments were performed to evaluate the performance of the developed system. The results revealed that the developed system is effective for narrow gap welding with an oscillation laser beam. Using this system, the narrow gap groove can be welded even if the groove width has changed during the welding process.

A Semi-Analytical Nonlinear Regression Approach for Weld Profile Prediction: A Case of Alternating Current Square Waveform Submerged Arc Welding of Heat Resistant Steel

The complexity in weld profile caused by abrupt change in polarity in square waveform welding is investigated through the development of a model capable to accurately predict weld profile. A semi-analytical model is conceived wherein characteristic attributes of a composite parabolic-elliptic function, which represent the weld profile, are obtained through nonlinear regression (NLR). The proposed model is demonstrated for its efficacy in the prediction of weld profile over a wide range of welding parameters, vis-a-vis, welding current, frequency, electrode negative (EN) ratio, and welding velocity. The investigation suggests that the center and outer cores of welding arc remains more active during positive and negative polarity, respectively, that leads to distinct macroscopic zones in weld cross section and thus, necessitates a composite profile for representation of weld profile. The intersection of the zones forms a metallurgical notch which the investigation offers a method to estimate and thus control. Unlike the convention continuous arc welding, the waveform arc welding caters welding at higher velocity without compromising the weld penetration and almost abolishing the metallurgical notch as well.

Analysis of Residual Stress for Butt Welding of Thick Plate

There are few reports that evaluated quantitatively the effect of the PWHT (Post Weld Heat Treatment) on the weld residual stress relaxation. We studied the residual stress for butt welding of thick plate. We conducted the experiment and the thermo-elastic-plastic finite element analysis. We became to be able to estimate the residual stress distribution of the butt weld. Also we clarified the effect of the PWHT on the weld residual stress.