Submerged Arc Overlay Welding of Inconel 625 on Flange Sealing Faces

Literature Overview

This technical paper, published in Chemical Engineering Equipment Technology (2021, Vol. 42, Issue 2, pp. 51-54) by Li Pei from Xi'an University of Architecture and Technology, addresses the practical challenge of applying Inconel 625 overlay to flange sealing faces using submerged arc welding (SAW). The work focuses on a specific product requirement where the sealing face of an S31603 (316L) stainless steel flange must be clad with Inconel 625 to meet corrosion resistance and sealing performance specifications.

Core Technical Content

The application of Inconel 625 overlay to flange sealing faces is a common requirement in the chemical processing, petrochemical, and power generation industries where aggressive media (acids, chlorides, high-temperature oxidizing environments) demand superior corrosion resistance at gasket contact surfaces. Inconel 625 (UNS N06625, equivalent to GH625 in Chinese standards) is a nickel-chromium-molybdenum superalloy with excellent resistance to pitting, crevice corrosion, and stress corrosion cracking.

Why Submerged Arc Welding for Flange Cladding

The selection of SAW for this application is driven by several factors:

• High deposition rate, which is critical for covering large flange sealing face areas efficiently.
• Excellent shielding provided by the flux, minimizing oxidation of the weld metal.
• Good process stability and repeatability for production environments.
• Ability to achieve thick, uniform overlay layers in a single operation.

Welding Procedure Qualification

The author developed and qualified a welding procedure using the following verification methods:

Process Analysis and Technical Considerations

Heat Input Management

One of the most critical aspects of SAW overlay welding Inconel 625 on stainless steel substrates is controlling the heat input to manage dilution. Excessive heat input leads to high dilution of the base metal into the overlay, which can:

• Reduce the corrosion resistance of the weld metal by introducing excessive carbon and chromium carbide formation.
• Increase the ferrite content beyond acceptable limits, promoting 400°C sensitization and intergranular corrosion.
• Alter the microstructure away from the desired austenitic-ferritic duplex balance.

Typical SAW parameters for Inconel 625 overlay on stainless steel flanges include:

Ferrite Content Control

The ferrite content measurement is particularly important for Inconel 625 overlays. According to ASTM A396 and related specifications, the weld metal should contain 15-35% delta ferrite to:

• Prevent solidification cracking in the fully austenitic weld metal.
• Provide adequate ductility and toughness.
• Resist hot cracking during welding.

However, excessive ferrite (>35%) can lead to:

• Reduced corrosion resistance due to chromium depletion at ferrite-austenite boundaries.
• Potential for 400°C sensitization and intergranular corrosion.

Engineering Practice Integration

For flange manufacturers and piping contractors, this work provides a practical template for qualifying SAW overlay procedures. Key engineering considerations include:

• Surface preparation: The flange sealing face must be ground to bare metal with a minimum depth of 1.5 mm to ensure proper bonding and remove any mill scale or contamination.
• Edge preparation: A suitable groove profile (typically V-groove or J-groove) must be machined to ensure adequate penetration and fusion.
• Post-weld machining: The overlay must be machined to the required sealing face geometry (RF, RTJ, or flat) after welding, with a minimum remaining overlay thickness of 3.0 mm to maintain corrosion protection.
• Heat treatment: A solution treatment at 1,150-1,200°C followed by rapid cooling may be required to restore full corrosion resistance, depending on the service conditions.

Key Questions and Reflections

The paper does not explicitly discuss the residual stress state of the overlay, which is a significant concern for flange components subjected to bolt preload and internal pressure. Residual stresses from welding can combine with operational stresses to promote fatigue cracking or stress corrosion cracking, particularly in chloride-containing environments.

Additionally, the paper does not address the compatibility of the Inconel 625 overlay with the specific gasket materials used in the flange assembly. For example, graphite gaskets may require a specific surface roughness (typically Ra 3.2-6.3 μm) on the sealing face, which must be achieved during post-weld machining without exposing the base metal.

Another important consideration is the cost-benefit analysis of SAW overlay welding versus alternative methods such as laser cladding or electroslag welding for flange applications. While SAW offers high deposition rates, it may not achieve the same level of dilution control as laser cladding, which could be critical for high-corrosion service applications.

Study Insights and Implications

This work provides a practical, production-oriented approach to qualifying SAW overlay welding procedures for Inconel 625 flange cladding. The combination of macro examination, penetrant testing, chemical analysis, and ferrite measurement provides a comprehensive quality assurance framework that can be adapted for other overlay welding applications. For engineers involved in flange specification and procurement, understanding the dilution control and ferrite management aspects of this process is essential for ensuring long-term service integrity in aggressive chemical environments. The qualification methodology presented here can serve as a template for similar overlay welding procedures on other flange materials and geometries.