Hiroto Yamaoka

Determining the BTR by conducting a Trans-Varestraint test using a high-speed camera and two-color pyrometry

The Ni-Fe alloy consumable that applies to the welding of 9% nickel steel for LNG storage tanks is highly susceptible to solidification cracks. To evaluate susceptibilities such as the brittleness temperature range (BTR), a Trans-Varestraint test is generally conducted. However, it is difficult to evaluate the minimum strain value for the BTR and the real temperature at both ends of a solidification crack in a conventional Trans-Varestraint test because these values are measured indirectly. In this study, we propose determining the temperature range by conducting in-situ observations during a Trans-Varestraint test using a high-speed camera and two-color pyrometry so that the temperature range can be measured directly from the temperatures at both ends of the crack. Furthermore, we measured the augmented strain from the time elapsed since the initiation of bending. This method allowed us to successfully measure the augmented strain and the temperature range in the Trans-Varestraint test and to determine the BTR more accurately.

Evaluation of Charpy impact toughness using side-grooved specimen for hybrid laser-arc welds of ultra-high-strength steel

This paper discusses the significance of a narrow hard weld bead, produced by hybrid laser-arc welding, on the Charpy absorbed energy. A parametric finite element analysis with various strength mismatching and bead widths has been conducted on the strain field of side-grooved and normal Charpy specimens. The narrow weld bead promotes the accumulation of plastic strain energy on the base metal side. This feature is more significant in the normal specimen than in the side-grooved specimen. The results show that the method based on the Weibull stress criterion for converting the absorbed energy of the side-grooved specimen to that of the normal specimen can be applied to hybrid laser-arc welds of ultra-high-strength steel. The converted energy is consistent with the absorbed energy measured in the normal specimen. This study indicated that the absorbed energy of side-grooved specimen corresponding to the absorbed energy of the normal V-notch specimen increases with increasing hard zone width. On the other hand, the influence of strength mismatch ratio on the difference of the absorbed energy is rather weak.