Application of DMAIC to Repair Welding of Vacuum Furnace Structures Made of SS310S Stainless Steel
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Abstract
This research presents a case study on improving the repair welding process for a heat-exposed SS310S stainless steel vacuum furnace, which suffered from persistent hot cracking post-repair. The study applied the Lean Six Sigma (DMAIC) framework to systematically analyze and resolve the issue. The "Analyze" phase confirmed that using the standard ER310 matching filler metal failed due to the formation of a fully austenitic microstructure. This structure is highly susceptible to cracking, especially when welding on the aged (sensitized) base material. The "Improve" phase, therefore, implemented a Composite Filler Metal. This involved using ER309L as buffer layers (Passes 1-3) to establish a crack-resistant Austenite with Ferrite (AF Mode) microstructure. Subsequently, the cover pass (Pass 4) was completed using the ER310 filler metal to restore the original high-temperature properties at the surface. This final ER310 pass did not crack due to the positive metallurgical influence of the underlying ER309L buffer layers. In the "Control" phase, the repair's integrity was supported by one full year of continuous furnace operation with no subsequent cracking or breakdown observed. The field-supported methodology has been established as a Standard Operating Procedure (SOP) for all future SS310S furnace maintenance within the facility. Schaeffler Diagram prediction indicated that ER310 + SS310S at 30% dilution was located in the fully austenitic region or contained very low delta ferrite (<3%), whereas ER309L + SS310S produced an Austenite with Ferrite structure with approximately 8% δ-ferrite. After the improved procedure with an interpass temperature not exceeding 150°C, PT inspection showed no cracking and no breakdown was observed during one year of service monitoring.
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