Precipitation behavior and hardness response of Alloy 625 weld overlay under different aging conditions

The hardness response of Alloy 625 weld overlay on low alloy steel was evaluated under a range of aging conditions to determine how the properties of the overlay change during postweld heat treatment (PWHT). It was presumed that the hardening compared to the as-welded condition occurs due to the precipitation of gamma double-prime (γ″). To confirm this assumption, an investigation was conducted on weld overlay samples under different aging conditions, using Vickers hardness testing, SEM microstructure characterization, SEM/EDAX analysis, and nanoindentation. The Alloy 625 weld overlay hardness values initially increased but eventually decreased with increasing aging temperature. γ″ precipitation was found in the interdendritic regions of the weld overlay samples. In the overaged condition, dissolution and coarsening of γ″ precipitates occurred. Delta (δ) phase also formed in the interdendritic regions, generally at higher temperatures than for γ″ precipitation. Nanoindentation revealed that the precipitation of γ″ is responsible for the hardening in the interdendritic regions of Alloy 625 weld overlay. The time-temperature-precipitation curves of interdendritic regions are altered from that of wrought Alloy 625 due to higher Nb, Ti, and Mo contents and lower dislocation density. The peak temperatures achieved in service for heating coke drum may result in interdendritic γ″ precipitation which can cause significant hardening of the overlay and promote cracking.