INCONEL 690 Strip Electroslag Welding Overlay Process Trial and Overlay Layer Performance Study

Literature Overview

This 1997 study conducted by Zhang Maolong, Yan Changgen, and You Junfu from Shanghai Boiler Works and Shanghai Institute of Testing Technology represents one of the pioneering Chinese research efforts on applying INCONEL 690 as a weld overlay cladding material using the strip electroslag welding (ESW) process. Published in the journal Boiler Technology, this work emerged during a critical period when Chinese power plant engineers were seeking domestic solutions for hydrogenation reactor and high-pressure vessel cladding applications that had previously relied entirely on imported materials and processes. The study addresses the fundamental question of whether INCONEL 690 strip could be reliably applied via electroslag welding to carbon and low-alloy steel substrates while maintaining the required corrosion resistance, mechanical properties, and metallurgical bond integrity.

Core Technical Content and Process Parameters

The electroslag welding overlay process selected for this study is particularly significant because ESW delivers exceptionally high heat input and deposition rates compared to conventional arc welding methods. For cladding applications, this translates to thick overlay layers achievable in relatively few passes, which is economically advantageous for large-diameter pressure vessel components such as hydrogenation reactor shells.

The researchers investigated the effect of welding parameters on the microstructure of the overlay layer, the dilution rate from the base metal, and the resulting corrosion resistance in simulated hydrogenation service environments. The study found that maintaining interpass temperature below 250°C was critical to preventing excessive grain coarsening in the INCONEL 690 overlay, which would degrade both the tensile strength and the intergranular corrosion resistance of the cladding layer.

Microstructure and Dilution Analysis

A key finding of this study was the systematic evaluation of dilution — the percentage of base metal alloying elements dissolved into the overlay layer. In ESW overlay, dilution tends to be higher than in processes such as plasma transferred arc (PTA) or laser cladding because of the large molten pool volume and prolonged liquid residence time. The researchers reported dilution values in the range of 3–8% depending on the base metal composition and welding parameters.

The microstructure of the INCONEL 690 overlay layer deposited by ESW was characterized as columnar dendritic grains growing epitaxially from the fusion boundary. The presence of molybdenum and chromium in the alloy matrix provided solid solution strengthening, while the titanium addition promoted the formation of fine TiC and TiN precipitates that further enhanced strength. However, the researchers noted that excessive dilution from the carbon steel base metal introduced iron and carbon into the overlay, which could lead to the formation of brittle intermetallic phases such as Ni₃Fe and Fe₂₃C at the interface region.

The interface between the overlay layer and the base metal was found to be a fully bonded fusion bond with a distinct but continuous transition zone approximately 0.3–0.8 mm in width. This transition zone showed a gradient in chemical composition from the base metal composition to the INCONEL 690 composition, confirming good metallurgical bonding without unmelted inclusions or microcracks.

Corrosion Resistance Performance

The corrosion resistance testing was conducted using potentiodynamic polarization in 3% NaCl solution and autoclave testing at elevated temperatures simulating hydrogenation service conditions. The INCONEL 690 overlay layer demonstrated excellent pitting resistance with a pitting potential exceeding +0.5 V vs. SCE, significantly superior to the 304 and 316 stainless steel overlays tested for comparison. The combination of high chromium (approximately 29–31%) and molybdenum (approximately 13–15%) content in the overlay provided a robust passive film that was resistant to breakdown even in chloride-containing environments at elevated temperatures.

The study concluded that the INCONEL 690 ESW overlay process was technically viable for hydrogenation reactor applications, provided that the dilution rate was controlled below 5% and the post-weld heat treatment was properly executed to relieve residual stresses and promote precipitate homogenization.

Engineering Practice Implications

From an engineering practice perspective, this study established several important principles that remain relevant today:

• The strip ESW process is economically suitable for thick cladding layers on large-diameter cylindrical components where the deposition rate advantage offsets the higher dilution.
• The use of a consumable strip electrode (rather than a wire electrode) provides better composition control and more uniform chemistry across the overlay width.
• Post-weld stress relief treatment is mandatory for INCONEL 690 overlays on thick-section low-alloy steel substrates to prevent delayed cracking in the heat-affected zone.
• The dilution rate must be monitored through chemical analysis of the overlay layer, and if it exceeds acceptable limits, additional overlay passes must be applied until the required chemistry is achieved.

This research contributed significantly to the domestication of INCONEL 690 cladding technology in China and informed subsequent standards development for nickel-based alloy cladding on pressure vessels. The methodology and findings remain a valuable reference for engineers currently specifying ESW overlay processes for hydrogen service equipment.