[Table of Contents]

Plasma and Fusion Research

Volume 10, 1405092 (2015)

Regular Articles

Heterogeneous Precipitation and Mechanical Property Change by Heat Treatments for the Laser Weldments of V-4Cr-4Ti Alloy
Takeo MUROGA, Nam-Jin HEO, Takuya NAGASAKA, Hideo WATANABE1), Arata NISHIMURA and Kenji SHINOZAKI2)
National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
1)
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
2)
Graduate School of Engineering, Hiroshima University, Higashi Hiroshima, Hiroshima 739-8527, Japan
(Received 25 September 2015 / Accepted 6 November 2015 / Published 28 December 2015)

Abstract

Bead-on-plate welds were produced on the 4 mm-thick V-4Cr-4Ti alloy (NIFS-HEAT-2), using a 2.0 kW YAG laser. The post-weld heat treatments (PWHT) were carried out in various conditions. Microstructures, Vickers hardness and Charpy impact properties were obtained for the weld metal after the PWHT. After PWHT for one hour, the hardness increased and after the peak declined with temperature. At 873 K, the hardness increased and after the peak declined with the time of PWHT. Microstructural observation showed that high density of fine precipitates formed homogeneously when the hardness increased, but the precipitate distribution changed into heterogeneous forming islands of developed precipitate aggregates, coincident with decrease in hardness and recovery of impact properties. Optical microscope observations suggested that a cellular structure of precipitate aggregate region was formed by PWHT. Microchemical analysis showed that Ti was enriched in the precipitate aggregate region. Therefore the areal oscillation of Ti concentration with cellular structures formed by melting and resolidification during the welding resulted in the heterogeneous precipitation by the following PWHT. The precipitation in the Ti-rich area purified the matrix of the weld metal and induced the recovery of hardening and impact property degradation. Optimum PWHT conditions were discussed according to the present results.

Keywords

vanadium alloy, laser welding, post-weld heat treatment, precipitation, hardness, Charpy impact property

DOI: 10.1585/pfr.10.1405092

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This paper may be cited as follows:

Takeo MUROGA, Nam-Jin HEOa), Takuya NAGASAKA, Hideo WATANABE, Arata NISHIMURA and Kenji SHINOZAKI, Plasma Fusion Res. 10, 1405092 (2015).