Heterogeneous Precipitation and Mechanical Property Change by Heat Treatments for the Laser Weldments of V-4Cr-4Ti Alloy

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.