Analysis of Leakage Causes on Vessel Flange Overlay-Welded Sealing Surfaces

Literature Overview

This paper by Ding Manfu, Fan Guangpu, and Li Weimin from Shanxi Yangmei Chemical Machinery (Group) Co., Ltd., published in China Chemical Equipment (2019, Vol. 21, No. 4, pp. 26–30), addresses a highly practical and often overlooked problem in pressure vessel and piping engineering: leakage at the sealing surface of vessel flanges that have been overlay-welded with corrosion-resistant materials. The authors systematically analyze the root causes of such leakage, focusing on hardness control of the overlay weld, machining considerations, and the compatibility between the sealing surface hardness and the ring joint gasket. The study is particularly relevant to engineers working in chemical, petrochemical, and power generation industries where flange connections with corrosion-resistant overlays are ubiquitous.

Core Technical Content and Key Findings

The fundamental problem addressed in this paper is that when a corrosion-resistant alloy overlay is deposited on a flange sealing surface, the overlay layer's hardness, surface finish, and metallurgical integrity directly influence the ability of the ring joint gasket to maintain a reliable seal. The authors identify several contributing factors to leakage:

1. Hardness mismatch between the overlay layer and the ring joint gasket. According to API 6A and ASME B16.20, the sealing surface hardness should be at least 30 HB higher than the ring gasket hardness. When the overlay weld hardness is too low or too high, the gasket cannot achieve proper sealing engagement, leading to leakage under pressure.
2. Excessive or insufficient overlay weld thickness. A layer that is too thin may not provide adequate corrosion resistance, while an excessively thick layer can create surface irregularities that compromise the sealing geometry.
3. Machining damage and surface defects. Post-overlay machining can introduce scratches, micro-cracks, or uneven surfaces that act as leakage paths.
4. Inappropriate welding consumable selection. The choice of overlay consumable determines the final hardness, microstructure, and corrosion resistance of the overlay layer.

Hardness Control of Overlay Welded Sealing Surfaces

The paper emphasizes that hardness is the single most critical parameter for ensuring a leak-free flange connection. The following table summarizes typical hardness requirements for common overlay materials and their corresponding gasket compatibility:

The authors note that hardness testing of overlay welds is challenging because the overlay layer is often thin (typically 2–5 mm), and standard Rockwell or Vickers indentation may penetrate into the base metal, yielding inaccurate readings. They recommend using surface hardness testers (e.g., portable ultrasonic or magnetic induction hardness meters) or performing micro-hardness (HV) testing on cross-sections after proper polishing.

Machining and Surface Preparation Considerations

Post-weld machining of the overlay sealing surface requires careful attention to cutting parameters. The authors highlight that:

• Cutting speed should be reduced by 30–50% compared to machining of the base material to prevent work hardening and micro-cracking of the overlay.
• Feed rate should be kept low to avoid excessive plastic deformation of the overlay surface.
• Coolant application is essential, particularly for nickel-based and austenitic overlay materials that are prone to galling and work hardening.
• The final surface roughness should meet ASME B16.5 requirements, typically Ra ≤ 3.2 μm for ring joint faces.

A critical insight from the paper is that the machining process itself can introduce residual stresses that, combined with thermal residual stresses from the overlay welding, may lead to micro-cracking at the overlay-to-base metal interface. This is particularly problematic for duplex stainless steel overlays where the phase balance can be disrupted by excessive heat input during machining.

Engineering Practice Integration

From a practical standpoint, this paper provides a clear diagnostic framework for investigating flange leakage incidents. The following decision tree can be applied:

1. Visual inspection of the flange face for visible defects, corrosion, or mechanical damage.
2. Hardness measurement at multiple points across the sealing surface to verify uniformity and compliance with the minimum differential requirement.
3. Surface roughness measurement to confirm that machining has not degraded the surface quality.
4. Metallurgical examination of cross-sections to check for overlay-to-base metal interface cracking, porosity, or incomplete fusion.
5. Review of welding procedure specifications (WPS) to verify that the overlay welding parameters were within acceptable ranges.

A notable engineering case described in the paper involved a series of flange leaks on a crude oil distillation unit where 316L overlay-welded flanges were used with carbon steel ring joints. Investigation revealed that the overlay hardness was only 110 HB due to improper heat input during GTAW overlay welding, which caused excessive grain growth and softening of the austenitic microstructure. The corrective action involved re-welding with a modified WPS that reduced the arc voltage and increased the travel speed, resulting in overlay hardness of 165 HB and subsequent leak-free operation for over three years.

Study Insights and Implications

This paper, while focused on a specific component, raises broader questions about the interface between welding metallurgy and mechanical design. The authors' emphasis on the hardness differential between the overlay and the gasket underscores the importance of cross-disciplinary communication between welding engineers, design engineers, and maintenance personnel. In many organizations, the welding procedure is developed independently from the mechanical design specifications, leading to situations where the overlay is metallurgically sound but mechanically incompatible with the sealing system.

The study also highlights a common gap in quality control: while overlay welds are routinely inspected for fusion quality, porosity, and corrosion resistance, the final sealing surface hardness is often not verified as a formal acceptance criterion. This paper advocates for incorporating hardness testing into the mandatory inspection plan for overlay-welded flange sealing surfaces, which is a practical and cost-effective recommendation that should be adopted by quality assurance teams.

One limitation of the paper is that it does not extensively discuss the effect of thermal cycling on the long-term stability of the overlay hardness. In service conditions involving repeated heating and cooling, particularly in high-temperature applications, the overlay microstructure can evolve, potentially changing the hardness over time. This aspect warrants further investigation, especially for applications involving start-up and shutdown cycling.

In summary, this paper provides a valuable practical guide for preventing flange leakage through proper control of overlay weld hardness and machining quality. Its recommendations for hardness differential requirements, machining parameter optimization, and inspection protocols are directly applicable to engineering practice and should be integrated into quality control procedures for overlay-welded flange connections.