Zhuojin Pipe Fitting Co., LtdSubmerged Arc Cladding of Inconel 625 on Flange Sealing Surfaces
Literature Overview
The paper by Li Pei, published in Chemical Equipment Technology (Vol. 42, Issue 2, 2021, pp. 51-54), describes the development and validation of a submerged arc welding (SAW) process for cladding Inconel 625 onto the sealing surfaces of S31603 (316L) flanges. This work addresses a practical manufacturing challenge in the chemical and petrochemical industries, where flange sealing surfaces often require a dissimilar overlay material to achieve compatibility with aggressive process media or to provide a reliable sealing surface for gasket selection.
Core Technical Content
The objective was to produce a high-quality Inconel 625 overlay on the raised face of S31603 flanges using submerged arc welding. The author developed a welding procedure specification (WPS) and validated it through a comprehensive quality assurance program including:
- Macroscopic examination of the weld cross-section
- Penetrant testing (PT) for surface discontinuities
- Chemical composition analysis of the overlay
- Ferrite content measurement
Inconel 625 as a Cladding Material
Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum superalloy with the following key characteristics:
| Property | Value |
|---|---|
| Ni content | Balance (~58%) |
| Cr content | 20-23% |
| Mo content | 5.9-6.7% |
| Nb content | 3.1-4.1% |
| Yield strength (RT) | ~550 MPa |
| Tensile strength (RT) | ~965 MPa |
| Maximum service temperature | ~980°C |
| Corrosion resistance | Excellent in oxidizing and reducing environments |
The selection of Inconel 625 for flange sealing surfaces is driven by its exceptional corrosion resistance in mixed acid environments, high-temperature strength, and compatibility with a wide range of gasket materials.
Process Development and Quality Assurance
Welding Procedure Specification
The SAW cladding process for Inconel 625 on stainless steel flanges requires careful attention to several process parameters:
| Parameter | Specification | Rationale |
|---|---|---|
| Flux type | Low-hydrogen or specialized Ni-base flux | Minimizes hydrogen cracking; reduces dilution |
| Flux coverage | Thick, continuous coverage | Ensures proper shielding and slow cooling |
| Current | 250-400 A (DC) | Controls penetration and dilution |
| Voltage | 24-30 V | Maintains stable arc with proper arc length |
| Travel speed | 150-300 mm/min | Controls heat input and dilution |
| Wire diameter | 1.6-2.4 mm | Appropriate for flange face dimensions |
| Preheat temperature | 150-250°C | Reduces cooling rate; prevents cracking |
| Interpass temperature | <300°C | Maintains overlay properties |
| Post-weld heat treatment | Optional: 850-950°C, 1-2 h | Relieves residual stresses |
Quality Assurance Protocol
The quality assurance program described in the paper follows a systematic approach:
- Visual and macroscopic examination: Verification of weld profile, undercut, and fusion characteristics at the overlay-base metal interface.
- Penetrant testing (PT): Detection of surface-breaking cracks, particularly at the overlay edge and along the fusion line.
- Chemical composition analysis: Confirmation that the overlay composition meets Inconel 625 specification, with attention to dilution effects from the S31603 substrate.
- Ferrite content measurement: Ensuring adequate ferrite content (typically 5-40 F% for austenitic-ferritic weld metals) to prevent hot cracking.
Dilution Control
One of the critical challenges in cladding Inconel 625 on S31603 is controlling dilution from the base metal. The dilution level directly affects:
- The final composition and properties of the overlay
- The corrosion resistance of the cladding layer
- The weldability and crack resistance of the deposit
Typical dilution levels for SAW cladding on stainless steel are 15-30% for the first pass and 5-15% for subsequent passes. The use of a multi-pass technique with a build-up pass followed by a cap pass is recommended to minimize dilution in the final surface layer.
Engineering Practice Implications
Flange Sealing Surface Requirements
In chemical processing applications, flange sealing surfaces must satisfy:
- Corrosion compatibility: The overlay must resist the process fluid to prevent gasket degradation and flange face corrosion.
- Surface finish: Typically Ra 6.3-12.5 μm for spiral wound gaskets; Ra 3.2 μm for ring joint gaskets.
- Hardness: 250-350 HB for compatibility with spiral wound gaskets; 350-450 HB for ring joint gaskets.
- Dimensional accuracy: Conformity with ASME B16.5 or EN 1092-1 dimensional requirements.
Comparison of Cladding Methods for Flange Sealing Surfaces
| Method | Hardness Control | Surface Finish | Cost | Applicable Sizes |
|---|---|---|---|---|
| SAW | Good | Requires grinding | Low | Large faces |
| TIG | Excellent | Good (as-welded) | Moderate | Small to medium |
| HVOF spray | Very good | Excellent | High | All sizes |
| Electroslag | Good | Requires grinding | Moderate | Large faces |
| Plasma transfer | Good | Good | Moderate | Medium to large |
Case Study Considerations
In practice, the application of Inconel 625 to flange sealing surfaces is often driven by specific service conditions:
- Chloride-containing environments: Where pitting and crevice corrosion of austenitic stainless steel is a concern.
- Mixed acid service: Where the flange face is exposed to aggressive process fluids during maintenance or startup.
- High-temperature service: Where the flange face temperature exceeds the recommended limits for the base material.
- Gasket compatibility: Where the gasket material requires a specific overlay hardness or composition for reliable sealing.
Critical Analysis and Reflections
Process Selection Rationale
The selection of SAW for this application is justified by several factors:
- Deposition rate: SAW offers high deposition rates, making it economical for covering large flange faces.
- Penetration control: The flux shielding provides excellent control over penetration depth, which is critical for managing dilution.
- Atmosphere protection: The flux provides complete shielding of the molten pool, eliminating the need for external gas shielding.
- Cost effectiveness: For large-scale production, SAW is significantly more economical than TIG or plasma arc cladding.
However, SAW has limitations:
- Surface finish: The as-welded surface requires machining or grinding to achieve the required surface finish for sealing applications.
- Edge effects: The edges of the overlay are prone to cracking and porosity, requiring careful process control.
- Equipment requirements: SAW requires specialized equipment including flux recovery systems and wire feeders.
Ferrite Control in Inconel 625 Deposits
The ferrite content measurement is particularly significant for Inconel 625 cladding. Although Inconel 625 is a fully austenitic alloy, the dilution with ferritic or duplex stainless steel can introduce delta ferrite into the weld metal. Excessive ferrite can lead to:
- Reduced corrosion resistance in chloride environments
- Increased susceptibility to stress corrosion cracking
- Poor ductility at low temperatures
The target ferrite content for Inconel 625 overlay on S31603 should be maintained below 5-10 F% to preserve the corrosion resistance advantages of the overlay.
Connection to Pipe Fitting Standards
The quality assurance approach described in this paper aligns with requirements found in:
- ASME B16.9: For butt-welding pipe fittings (includes overlay requirements)
- ASME B16.5: For bolted flanges (surface finish and overlay specifications)
- ASTM A403: For wrought austenitic stainless steel fittings
- NB/T 47014: Chinese standard for welding procedure qualification
Summary
This paper presents a practical and well-documented approach to cladding Inconel 625 on S31603 flange sealing surfaces using submerged arc welding. The systematic quality assurance program, including macroscopic examination, penetrant testing, chemical analysis, and ferrite content measurement, provides a robust framework for ensuring overlay quality. The key engineering insight is that dilution control is paramount in determining the final performance of the overlay, and the multi-pass SAW technique with appropriate flux selection and heat input management can achieve acceptable dilution levels while maintaining economic viability. For pipe and fitting manufacturers, this approach demonstrates that high-performance overlay cladding can be integrated into conventional manufacturing workflows with appropriate procedure qualification and quality control measures. The emphasis on ferrite content measurement reflects a sophisticated understanding of weld metal microstructure and its impact on service performance, and this practice should be adopted as standard in all nickel-alloy overlay applications.