CLADDING TECHNOLOGY SHANXI CO., LTDInconel 625 Submerged Arc Cladding of Flange Sealing Surfaces
Application Context and Technical Requirements
Flange sealing surfaces are critical components in piping systems operating under high pressure and high temperature conditions, particularly in the petrochemical and power generation industries. Inconel 625, a nickel-chromium-molybdenum superalloy, is widely used for cladding flange sealing surfaces due to its excellent resistance to corrosion, oxidation, and stress corrosion cracking in aggressive environments. Li Pei from Xi'an University of Architecture and Technology published a study in Chemical Equipment Technology (2021) on the submerged arc welding (SAW) process for Inconel 625 cladding of flange sealing surfaces, addressing the specific challenges associated with this application.
The requirements for flange sealing surface cladding are stringent. The overlay must be free of defects such as cracks, porosity, and incomplete fusion, as any defect in the sealing surface can lead to leakage under pressure. The overlay thickness must be sufficient to provide the required corrosion resistance while maintaining the dimensional tolerances of the flange. The surface finish of the overlay must be suitable for gasket sealing, typically requiring a smooth and uniform surface after machining.
Process Design and Parameters
Submerged arc welding is well-suited for Inconel 625 cladding due to its high deposition rate, low spatter, and good arc stability. The process uses a consumable electrode wire coated with a flux, which melts to form a slag that protects the weld pool from atmospheric contamination. For Inconel 625 cladding, a matching Inconel 625 wire is used, and the flux is typically a rutile-type or basic-type flux designed for stainless steel and nickel alloy welding.
| Parameter | Typical Value | Rationale |
|---|---|---|
| Welding current | 200-400 A | High deposition rate, adequate penetration |
| Arc voltage | 25-35 V | Stable arc, adequate heat input |
| Travel speed | 150-300 mm/min | Controls dilution and cooling rate |
| Wire diameter | 1.6-3.2 mm | Balances deposition rate and flexibility |
| Flux type | Rutile or basic | Low hydrogen, good slag coverage |
| Preheat temperature | 50-150°C | Reduces thermal cracking tendency |
| Interpass temperature | <200°C | Prevents excessive grain growth |
The dilution of the base metal into the Inconel 625 overlay is a critical concern. Carbon steel base metals typically have a carbon content of 0.2-0.3%, which is significantly higher than the maximum carbon content of 0.1% specified for Inconel 625. Excessive dilution can reduce the corrosion resistance of the overlay by increasing the carbon content and forming chromium carbides at grain boundaries. To minimize dilution, a multi-pass approach is recommended, with the first pass providing the bond and subsequent passes building up the overlay thickness with minimal dilution.
Multi-Pass Cladding Strategy
| Pass | Function | Dilution | Key Consideration |
|---|---|---|---|
| First pass | Bond to base metal | High (30-50%) | Adequate penetration for fusion |
| Second pass | Transition layer | Medium (10-20%) | Composition transition |
| Third pass | Overlay buildup | Low (<10%) | Maintain Inconel 625 composition |
| Final pass | Surface finish | Minimal | Smooth surface for machining |
The first pass is the most critical because it establishes the metallurgical bond between the base metal and the overlay. Inadequate fusion at this interface can lead to delamination during service or during subsequent machining operations. To ensure proper fusion, the first pass should be deposited with sufficient heat input and with the arc traversed across the full width of the sealing surface. The surface should be thoroughly cleaned of rust, paint, oil, and other contaminants before welding to ensure a clean interface.
Quality Control and Inspection
Quality control of Inconel 625 SAW cladding on flange sealing surfaces involves multiple inspection stages. Visual inspection is performed after each pass to detect surface defects such as undercut, excessive reinforcement, and slag inclusion. Magnetic particle testing (MT) is used to detect surface and near-surface cracks, while ultrasonic testing (UT) can be used to detect subsurface defects and measure overlay thickness.
The overlay thickness must be verified by ultrasonic thickness measurement. The minimum thickness is typically specified by the design code or the project specification, commonly ranging from 3 mm to 6 mm for flange sealing surfaces. The thickness must be uniform across the sealing surface to ensure consistent gasket sealing. After welding, the overlay is machined to the final dimensions and surface finish. The machining process must be performed carefully to avoid excessive heat generation that could affect the microstructure of the overlay.
A common defect in SAW cladding of flange sealing surfaces is porosity caused by flux contamination or inadequate shielding. Porosity in the sealing surface can lead to leakage and must be repaired by grinding out and re-depositing the overlay. Another concern is cracking in the overlay, which can be caused by excessive restraint, inadequate preheat, or hydrogen pickup from the flux. To prevent cracking, the flux must be properly dried according to the manufacturer's recommendations, and the interpass temperature must be controlled to prevent hydrogen absorption.
Practical Insights and Recommendations
The SAW process for Inconel 625 cladding of flange sealing surfaces is a well-established technology, but its successful application requires attention to several key factors. The wire and flux must be compatible and properly stored to prevent moisture absorption. The welding parameters must be optimized to balance deposition rate, dilution, and microstructure. The base metal surface must be thoroughly prepared to ensure a clean and oxide-free interface. Post-weld heat treatment is generally not required for Inconel 625 overlays, but if the base metal requires PWHT, the overlay temperature must be limited to avoid sensitization. Engineers should also consider the long-term performance of the overlay under thermal cycling and mechanical loading, as the thermal expansion mismatch between Inconel 625 and carbon steel can lead to residual stresses and potential cracking. A systematic approach to process development, including weld procedure qualification, trial builds, and comprehensive inspection, is essential for ensuring the reliability of Inconel 625 cladded flange sealing surfaces.