CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Sealing Surface Leakage in Clad Vessel Flanges
Overview and Background
Clad pressure vessel flanges are widely used in chemical, petrochemical, and hydrogenation reactor service where the base material provides structural strength while the overlay layer ensures corrosion resistance at the sealing surface. Despite the inherent advantages of this bimetallic construction, field reports consistently indicate that the sealing face of the overlay layer is one of the most failure-prone areas in the entire pressure boundary. Leakage at these interfaces can lead to process fluid loss, environmental contamination, and catastrophic safety incidents. This study note draws upon published analyses of leakage root causes and integrates them with practical fabrication experience to provide a systematic diagnostic framework.
Root Cause Analysis Using the 5W2H Framework
A structured root cause investigation is essential for understanding why sealing surface leakage occurs in clad flanges. Applying the 5W2H methodology reveals several interrelated failure mechanisms that frequently appear in practice.
| Category | Typical Findings |
|---|---|
| What | Sealing surface cracks, overlay layer delamination, insufficient overlay thickness, surface roughness exceeding spec |
| Why | Residual stress from welding, hydrogen-induced cracking, thermal cycling fatigue, improper welding sequence |
| Where | Near the groove transition zone, at the overlay base metal interface, along the flange face |
| When | During pressure testing, after first thermal cycle, during long-term service |
| Who | Fabrication operator, welding procedure qualification, inspection personnel |
| How | Through improper welding parameters, inadequate post-weld treatment, insufficient NDT coverage |
| How much | Overlay thickness 0.5 mm below specification, surface roughness Ra > 6.3 μm |
Overlay Layer Cracking
Overlay layer cracking is the most frequently reported defect leading to sealing surface leakage. The crack initiation typically occurs at the overlay-to-base metal interface or within the dilution zone where the microstructure transitions from austenitic to martensitic. Several contributing factors have been identified:
- Excessive dilution: When the base metal dilution exceeds 30%, the resulting hard, brittle martensitic zone becomes susceptible to cracking under residual tensile stress. The dilution zone acts as a stress concentrator, and any micro-defects in this region can propagate into the sealing surface.
- Residual stress accumulation: Clad flanges undergo multiple welding passes during overlay application, followed by groove welding and final assembly welding. Each welding operation introduces thermal gradients and residual stresses. If the welding sequence is not carefully planned, the cumulative residual stress in the sealing face region can exceed the yield strength of the overlay layer, initiating cracks.
- Hydrogen-induced cracking: In high-strength overlay materials such as martensitic stainless steels or certain nickel-based alloys, absorbed hydrogen from the welding process can accumulate at microstructural boundaries. The combination of high hydrogen concentration and high residual stress leads to delayed cracking, often appearing hours or days after welding.
Overlay Delamination
Delamination between the overlay layer and the base metal represents another critical failure mode. This is particularly problematic in electroslag welded (ESW) clad flanges where the bond strength between the overlay and base metal is inherently lower than in fusion-welded overlays. The following factors contribute to delamination:
- Insufficient heat input: Low heat input during overlay welding reduces the intermixing between overlay and base metal, resulting in a weak metallurgical bond.
- Surface preparation defects: Oxide films, scale, or contamination on the base metal surface before overlay welding can create a discontinuity at the interface.
- Thermal cycling: Repeated heating and cooling during fabrication and service can fatigue the bond interface, especially if the thermal expansion coefficients of the overlay and base metal differ significantly.
Surface Roughness and Sealing Integrity
The sealing surface quality directly determines the effectiveness of the gasket seal. For spiral wound gaskets, the surface roughness of the sealing face should typically be in the range of Ra 3.2 to 6.3 μm. Excessive roughness causes uneven gasket compression, leading to localized leakage paths. Conversely, a surface that is too smooth may not provide adequate sealing contact with the gasket.
| Surface Condition | Acceptable Ra (μm) | Typical Cause of Deviation |
|---|---|---|
| Spiral wound gasket face | 3.2 – 6.3 | Excessive grinding, uneven overlay deposition |
| Ring joint face | 1.6 – 3.2 | Incomplete machining, overlay surface irregularities |
| Flat gasket face | 6.3 – 12.5 | Acceptable for soft gaskets |
Process Improvement Measures
Based on the identified root causes, several process improvement strategies can be implemented to reduce sealing surface leakage risk:
Welding Sequence Optimization
The welding sequence should be designed to minimize residual stress in the sealing face region. A recommended approach is to complete all groove welding before applying the overlay layer on the sealing face. This ensures that the overlay layer is deposited on a relatively stress-free base, reducing the likelihood of cracking. Additionally, the overlay welding should be performed in a symmetric pattern to balance thermal input and minimize warping.
Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is critical for relieving residual stresses and stabilizing the microstructure of the overlay layer. For austenitic stainless steel overlays on carbon steel or low-alloy steel bases, a stress relief treatment at 550–650°C for 1–2 hours is typically recommended. The PWHT temperature must be carefully controlled to avoid sensitization of the stainless steel overlay, which could lead to intergranular corrosion.
Non-Destructive Testing Requirements
Comprehensive NDT coverage of the sealing surface is essential. The following NDT methods should be applied:
| NDT Method | Purpose | Coverage |
|---|---|---|
| Magnetic particle testing (MT) | Surface cracks, overlay defects | 100% of sealing face |
| Liquid penetrant testing (PT) | Surface cracks, porosity | 100% of sealing face |
| Ultrasonic testing (UT) | Overlay thickness, delamination | 100% of overlay layer |
| Radiographic testing (RT) | Internal defects in overlay | Per specification requirements |
Engineering Practice Reflections
In practical fabrication, the most common oversight is insufficient attention to the sealing face quality during the overlay welding stage. Operators often focus on achieving the required overlay thickness while neglecting surface quality and microstructural integrity. A disciplined approach that integrates welding procedure optimization, thorough surface preparation, controlled thermal input, and comprehensive NDT can significantly reduce the incidence of sealing surface leakage.
The key takeaway from this analysis is that sealing surface leakage in clad flanges is rarely caused by a single factor. Rather, it results from the cumulative effect of multiple process deviations, each of which may be individually acceptable but collectively lead to failure. A systematic, standards-based approach to fabrication quality control is therefore essential. Engineers should adopt a PDCA cycle for overlay welding processes, continuously monitoring process parameters, reviewing NDT results, and refining procedures based on field performance data.