The effect of heat treatment on secondary phase formation in dissimilar metal welds

Abstract

Ferrite numbers (FN) of the welds were measured and compared to the values predicted by the WRC-1992 diagram, revealing that the diagram over-predicted the FN for the 308L and 2209 welds. Electron-probe microanalysis (EPMA) was performed on the as-welded samples to study the micro- and macrosegregation between the dendrites and across the weld fusion zone. Micrographs of the welds were obtained using light optical microscopy and hardness measurements were taken using Rockwell Hardness B. The major findings of this study are that (1) the WRC-1992 diagram inaccurately predicts the solidification method for rapidly cooled gas metal arc welds, (2) sensitization of the 308L welds causes a maximum hardness at 6-hours at 750°C, indicating carbide embrittlement, (3) sigma phase rapidly grows along the interdendritic regions of the 904L, 625, and 2209 welds increasing the hardness by embrittlement, and (4) the 1205°C heat treatment initially dissolves ferrite and stabilizes austenite; NiO forms after the 24-hour heat treatment in the 308L, 904L, and 625 welds, stabilizing ferrite in the Ni depleted weld metal after this exposure.

This study proves that a bead-on-plate dissimilar metal gas metal arc weld, with ~40% dilution can be made using a volumetric feed rate greater than the maximum rate predicted in literature studies. The base material used was 316L plate and the filler metals studied with their matching base material in parentheses were ER308L (304L), ER385 (904L), ERNiCrMo-3 (625), and ER2209 (2205). While the dilution is much greater than the 0% dilution predicted, the weld shows a lack of mixing in the weld pool, micro- and macrosegregation, and a lack of a heat affected zone. All of these properties can be attributed to the low heat input produced by the welding input parameters. The as-welded material was heat treated at 750°C and 1205°C for 1-hour, 6-hours, and 24-hours to understand the precipitation and dissolution of secondary phases with relation to the thermodynamic predictions.