Analysis of Leakage Causes in Overlay-Welded Sealing Surfaces on Vessel Flanges

Literature Overview

This paper by Ding Manfu, Fan Guangpu, and Li Weimin from Shanxi Yanghua Chemical Machinery Group, published in China Chemical Equipment (2019, Vol. 21, No. 4, pp. 26–30), addresses a persistent and costly problem in pressure vessel engineering: leakage at the sealing surface of vessel flanges that have been overlay-welded with corrosion-resistant materials. The authors systematically analyze how hardness measurement and machining operations after overlay welding affect sealing integrity, and provide design guidance for material selection and hardness control to prevent leakage.

Core Technical Problem and Root Cause Analysis

The fundamental issue arises when a flange body made of carbon or low-alloy steel (such as 16Mn or Q345R) requires a corrosion-resistant sealing surface to withstand aggressive process media. Engineers overlay-weld a corrosion-resistant alloy (typically austenitic stainless steel such as 304, 316, or duplex 2205) onto the sealing face using processes such as SMAW or GTAW. However, the overlay weld deposit introduces metallurgical and mechanical incompatibilities that can lead to seal failure. The authors identify several root causes:

1. Hardness mismatch between overlay deposit and sealing ring: The overlay weld deposit, particularly near the fusion zone, may develop hardness values significantly different from the base metal. If the overlay surface hardness is too high (exceeding 250 HV), it can cause excessive wear on the spiral-wound gasket or ring gasket, leading to loss of contact pressure and eventual leakage. Conversely, if the hardness is too low, the sealing surface may deform under bolt-up loads, creating micro-gaps at the gasket interface.
2. Residual porosity and inclusions in the overlay layer: Incomplete fusion, gas entrapment, or slag inclusions within the overlay weld deposit create local weak points on the sealing surface. Under cyclic pressure loading, these defects act as initiation sites for micro-cracking, which propagates to the surface and compromises the flatness and smoothness required for effective sealing.
3. Inadequate surface finish after machining: The overlay weld deposit, even after grinding, often retains a surface roughness (Ra) that exceeds the requirements specified in ASME B16.5 or GB/T 150.1 for flange sealing faces. The typical requirement for spiral-wound gasket service is Ra ≤ 1.6 μm, while for ring-type gaskets (RTJ) with serrated sealing rings, the surface profile must be carefully controlled. Excessive roughness reduces the effective contact area between the gasket and the flange face, concentrating stress at asperities and accelerating gasket relaxation.
4. Thermal distortion of the flange body: The heat input during overlay welding can cause local warping or distortion of the flange, particularly in thinner flanges or those with complex geometries (such as raised face or full-face flanges). This distortion alters the bolt load distribution and creates non-uniform contact pressure across the gasket, leading to localized leakage paths.

Standards and Specifications for Overlay-Welded Flange Sealing Surfaces

The following table summarizes the key standards and specifications relevant to overlay-welded flange sealing surfaces:

The critical insight from the literature is that the hardness of the overlay deposit must be carefully matched to the gasket material. For spiral-wound gaskets (SWG) with stainless steel winding and non-metallic filler, the overlay surface hardness should not exceed 250 HV to prevent gasket winding damage. For ring-type gaskets (RTJ) with serrated sealing rings, the overlay surface should be softer than the sealing ring material to ensure proper embedding and sealing.

Hardness Control and Measurement Challenges

One of the most significant technical challenges discussed in the paper is the accurate measurement of hardness in overlay weld deposits. The overlay layer is typically thin (3–6 mm for single-pass, up to 12 mm for multi-pass), and the hardness profile varies significantly from the fusion zone to the top of the deposit. The authors note that:

• Surface hardness measurement using portable hardness testers (such as Leeb or ultrasonic devices) may not accurately reflect the true hardness at the sealing interface, particularly when the deposit is thin.
• Micro-hardness testing (Vickers HV) is more appropriate for thin overlay deposits but requires careful specimen preparation and is time-consuming for production inspection.
• Hardness mapping should be performed across the entire sealing surface, not just at isolated points, because local variations in hardness can lead to non-uniform gasket contact pressure.

The recommended practice is to perform hardness mapping at a minimum of three locations around the circumference of the sealing face, with additional measurements at the inner and outer diameters. The maximum hardness at any point should not exceed the specification limit, and the hardness gradient from the fusion zone to the surface should be monitored.

Machining Considerations After Overlay Welding

The machining of overlay-welded sealing surfaces requires special attention to several factors:

1. Grinding allowance: A minimum of 2 mm grinding allowance should be provided to remove the outermost layer of the overlay deposit, which may contain porosity, inclusions, or a heat-affected zone with altered microstructure.
2. Cooling during machining: Excessive heat generation during grinding can temper the overlay deposit, reducing its hardness and potentially altering its corrosion resistance. Water cooling or low-speed grinding with generous feed rates is recommended.
3. Surface finish verification: After machining, the surface finish should be verified using a surface profilometer at multiple locations. The Ra value should meet the requirements of the applicable flange standard for the intended gasket type.
4. Flatness verification: The flatness of the sealing surface should be checked using a straightedge and feeler gauge or a coordinate measuring machine (CMM). The maximum deviation should not exceed 0.05 mm per 100 mm of diameter for raised face flanges.

Engineering Practice Recommendations

Based on the analysis in this paper and my own engineering experience, the following recommendations are provided for designers and fabricators:

• Material selection: The overlay alloy should be selected based on the process media, temperature, and pressure conditions. For chloride-containing environments, duplex stainless steel (2205) or super duplex (2507) is preferred over austenitic grades due to superior pitting and crevice corrosion resistance.
• Welding procedure qualification: The overlay welding procedure should be qualified in accordance with ASME Section IX or ISO 15614-1, with specific attention to the hardness and corrosion resistance of the weld deposit.
• Post-weld heat treatment: For overlay deposits made of martensitic or precipitation-hardening alloys, a post-weld heat treatment may be required to achieve the desired hardness and corrosion resistance. However, this must be carefully controlled to avoid over-tempering or sensitization.
• Gasket selection: The gasket material should be compatible with the overlay surface hardness. For overlay surfaces with hardness exceeding 250 HV, a softer gasket material (such as graphite or PTFE) should be used to prevent gasket damage.

Key Questions and Reflections

The paper raises several important questions that deserve further investigation:

1. How does the number of overlay weld passes affect the hardness profile and sealing performance of the overlay deposit?
2. What is the effect of interpass temperature on the microstructure and hardness of the overlay deposit?
3. Can advanced welding processes (such as laser cladding or cold metal transfer welding) reduce the heat-affected zone and improve the hardness uniformity of the overlay deposit?
4. How does cyclic thermal loading affect the long-term sealing performance of overlay-welded flange surfaces?

Study Insights and Implications

This paper provides a valuable contribution to the understanding of overlay-welded flange sealing surface performance. The authors' systematic approach to identifying root causes of leakage and their recommendations for hardness control and machining practices are directly applicable to engineering practice. The emphasis on the interaction between overlay deposit hardness, gasket material properties, and machining quality is particularly noteworthy, as it highlights the importance of a holistic approach to flange sealing design rather than treating each component in isolation. For engineers involved in the design and fabrication of pressure vessel flanges, this paper serves as a practical guide to avoiding costly leakage failures in corrosive service environments.