Formation and control of the residual δ-ferrite in 9% Cr-HAZ of Alloy 617/9% Cr dissimilar welded joint

Abstract

The formation and control of the residual δ-ferrite in Alloy 617/9% Cr dissimilar welded joint manufactured by the single-layer or multi-layer multi-pass overlaying technique were comparatively investigated. The residual δ-ferrite was identified based on its lower microhardness and carbon content than the surrounding martensite matrix and being free from precipitates. Patch-shape residual δ-ferrite was found in the heat-affected zone (HAZ) of 9% Cr steel adjacent to the fusion line for the single-layer overlaying weldment, while less and smaller residual δ-ferrite could be achieved by employing the multi-layer multi-pass overlaying technique. Attributing to the optimisation of the subsequent thermal input and heat transfer, the technique utilised in this study was proved to be effective to control the formation of residual δ-ferrite.

A Study on Microstructure, Residual Stresses and Stress Corrosion Cracking of Repair Welding on 304 Stainless Steel: Part I-Effects of Heat Input

“…This is because the strip-shaped δ-ferrite in HAZ is the residual ferrite phase at high temperature in the forging process of steel, which can be properly eliminated by solid solution treatment. When the heat input increases, the retention time of the metal near the fusion line becomes longer at a high temperature, which is equivalent to the prolongation of the holding time of solid solution treatment, thus leading to the decrease of strip-shaped δ-ferrite content in HAZ [52]. Figure 10 shows the effect of heat input on the microstructure in the fusion line and the adjacent region.…”

Formation of delta-ferrite and its effect on impact toughness of GTAW weld metals in a 12 wt-% Cr steel

“…For weld metals of 9–12% Cr F/M steels, the effect of non-equilibrium solidification and rapid cooling would most likely cause the formation of residual delta-ferrite (δ) [5,6], which was deemed to deteriorate the impact toughness and raise the ductile-to-brittle transition temperature (DBTT) [5,7]. The δ-phase may also transform into a brittle sigma phase on long-term exposure at elevated temperature, which will deteriorate the creep properties [8].…”