Inconel 625 Submerged Arc Welding Overlay for Flange Sealing Surfaces

Literature Overview

This technical article, published in Chemical Equipment Technology in 2021 by Li Pei from Xi'an University of Architecture and Technology, addresses the manufacturing challenge of applying Inconel 625 overlay to flange sealing surfaces using submerged arc welding (SAW). Flange sealing surfaces are critical components in high-pressure and high-temperature chemical processing systems where gasket integrity determines joint reliability. The use of Inconel 625—a nickel-chromium-molybdenum superalloy—on carbon or low-alloy steel flanges provides excellent resistance to corrosion, pitting, and crevice corrosion while maintaining the structural strength of the base material.

Technical Background and Design Considerations

Inconel 625 is a solid-solution strengthened superalloy containing approximately 62% Ni, 21% Cr, 9% Mo, with minor additions of Nb and Al. Its key properties include:

Property	Value
Density	8.44 g/cm³
Melting range	1310–1330 °C
Tensile strength (RT)	690 MPa
Yield strength (0.2%)	310 MPa
Elongation	40%
Creep strength at 650 °C	~100 MPa (10⁵ h)
Corrosion resistance	Excellent in oxidizing and reducing acids

For flange sealing surfaces, the overlay thickness is typically 2–5 mm, with the critical requirement being a smooth, flat surface finish (Ra ≤ 1.6 μm after machining) and uniform metallurgical bond strength. The sealing surface must withstand gasket seating stresses, thermal cycling, and the corrosive process medium.

SAW Process for Flange Overlay

Submerged arc welding is selected for flange overlay due to its high deposition rate (5–15 kg/h), deep penetration, and excellent arc stability. However, SAW presents unique challenges for overlay applications:

Process Parameters

Parameter	Recommended Value	Rationale
Arc current	350–500 A	High current for deep penetration and good bonding
Arc voltage	28–35 V	Controls bead width and dilution
Travel speed	200–400 mm/min	Balances deposition rate and dilution
Flux type	Low-hydrogen basic flux (e.g., HJ431)	Minimizes hydrogen cracking risk
Flux coating	Pre-coated or semi-automatic	Ensures consistent flux coverage
Shielding	Flux blanket + trailing gas (Ar or Ar+CO2)	Prevents surface oxidation

Key Technical Challenges

Dilution control: SAW inherently has high dilution (30–60%), which can compromise the corrosion resistance of the Inconel 625 overlay. Strategies to minimize dilution include:

Using a multi-pass approach with the first pass at lower current to establish a clean bond, followed by subsequent passes at optimized parameters
Employing a "build-up" technique where the first pass uses a consumable insert or backing ring to reduce base metal melting
Applying a pre-weld machining step to remove surface contaminants and establish a clean starting surface

Residual stress and distortion: Flanges are relatively thin-walled components (typically 20–80 mm thickness for standard pressure ratings). SAW overlay introduces significant residual stress that can cause warping, particularly for raised-face flanges. Preheating to 150–250 °C and post-weld stress relief at 650–700 °C for 2 h per 25 mm thickness are standard practices.

Surface quality: The SAW process produces a rough surface (Ra 25–50 μm as-welded). Post-weld machining is mandatory to achieve the required sealing surface finish. The machining allowance must be sufficient (minimum 1.5 mm) to ensure complete removal of the SAW weld bead surface irregularities.

Quality Control and Inspection

For flange sealing surface overlay, the following inspection regime is recommended per ASME Section IX and API 934:

Visual inspection (VT): 100% inspection of all weld surfaces for cracks, undercut, excessive reinforcement, and surface defects.
Liquid penetrant testing (PT): 100% of the overlay surface after machining to detect surface-breaking cracks and porosity.
Magnetic particle testing (MT): 100% of the overlay surface for subsurface defects within 3 mm of the surface.
Ultrasonic testing (UT): Spot check (10–20% of welds) for lack of fusion at the bond interface.
Hardness testing: Verify hardness of the overlay (typically 200–250 HB for Inconel 625 as-welded) and the HAZ.
Chemical analysis: Confirm overlay composition meets ASTM B625 specifications.

Engineering Practice Insights

In industrial practice, the most common failure mode for Inconel 625 overlaid flanges is intergranular corrosion (IGC) in the overlay layer. This occurs when the overlay is sensitized (Cr carbide precipitation at grain boundaries) during the welding thermal cycle. To prevent sensitization:

Maintain interpass temperature below 150 °C for multi-pass welds
Use a post-weld solution heat treatment at 1050 °C followed by water quench if the service environment is aggressive (e.g., chloride-containing media)
Consider using Inconel 625 with Nb stabilization (weld metal) to tie up carbon and prevent Cr23C6 precipitation

Another practical consideration is the matching of the overlay to the gasket material. For spiral wound gaskets with Inconel 625 winding, the flange overlay should also be Inconel 625 to prevent galvanic corrosion. For graphite-filled gaskets, the overlay hardness should be controlled to avoid gasket crushing.

The study's contribution to engineering practice is the demonstration that SAW overlay is a viable and economical alternative to strip cladding or machining of Inconel 625 rings for flange sealing surfaces. The SAW process offers higher productivity than GTAW or plasma arc methods, making it suitable for batch production of standard flange sizes.

However, the technique requires careful process control, particularly regarding dilution and surface finish. The recommendation is to develop a qualified welding procedure specification (WPS) per ASME IX or ISO 15614-1, with procedure qualification testing including bond strength, hardness profile, and corrosion testing (ASTM A262 Practice E for intergranular corrosion).