CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis and Discussion of Sealing Face Leakage in Vessel Flange Overlay Layers
Literature Overview
The 2019 publication by Ding Manfu, Fan Guangpu, and Li Weimin from Shanxi Yangmei Chemical Machinery Group addresses a critical and frequently encountered problem in pressure vessel fabrication: leakage at the sealing face of flanges that have been weld-overlay clad. This issue is particularly common in chemical processing plants where vessel flanges require corrosion-resistant overlay layers (typically 304, 316, or 321 stainless steel) applied over carbon steel or low-alloy steel base materials. The study provides a systematic root cause analysis using engineering investigation methods, making it highly relevant for quality engineers and fabrication inspectors.
Root Cause Analysis Framework
The authors employ a structured approach to identifying leakage causes, which can be organized using a failure analysis methodology similar to FMEA (Failure Mode and Effects Analysis). The following table summarizes the primary failure modes identified:
| Failure Mode | Root Cause | Frequency | Severity | Detection Difficulty |
|---|---|---|---|---|
| Overlay layer cracking | Excessive residual stress, hydrogen cracking | High | High | Medium |
| Overlay layer porosity | Inadequate flux coverage, high travel speed | Medium | High | Medium |
| Overlay layer delamination | Poor fusion, contamination at interface | Low | Very High | High |
| Gasket incompatibility | Chemical attack, wrong gasket material | Medium | High | Low |
| Bolt preload imbalance | Uneven tightening sequence, bolt elongation | Medium | Medium | Low |
| Overlay layer over-thickness | Excessive build-up, poor dimensional control | Low | Medium | Medium |
Overlay Layer Cracking
Cracking is identified as the most common cause of flange sealing face leakage. The study distinguishes between hot cracks (formed during solidification) and cold cracks (formed during cooling or post-weld stress relief). Hot cracking is associated with high sulfur and phosphorus content in the base metal, while cold cracking is primarily hydrogen-induced. The recommended countermeasures include:
- Preheating the flange to 150–250°C depending on carbon equivalent
- Maintaining interpass temperature below 200°C
- Using low-hydrogen flux or shielded gas welding (GTAW/SMAW with low-hydrogen electrodes)
- Applying post-weld stress relief at 550–600°C for low-alloy steel substrates
- Performing MT (magnetic particle testing) on the overlay layer after each pass
Overlay Layer Porosity
Porosity in the overlay layer creates direct leakage paths through the sealing face. The study identifies three primary mechanisms: gas porosity (from moisture in flux or contamination), slag inclusion porosity, and shrinkage porosity. Key process controls include:
| Control Parameter | Target Value | Purpose |
|---|---|---|
| Flux dryness | Bake at 200°C for 2 h before use | Reduce hydrogen porosity |
| Travel speed | 80–120 mm/min | Ensure adequate fusion |
| Arc length | 2–4 mm | Stable arc, consistent penetration |
| Surface preparation | Remove rust, oil, and paint to Sa 2.5 | Prevent contamination |
| Wire/feed cleanliness | Free from rust and scale | Prevent oxide inclusions |
Overlay Layer Delamination
Delamination between the overlay layer and the base metal is the most severe failure mode because it creates a large, hidden leakage path that is difficult to detect by conventional NDT methods. The study recommends the following preventive measures:
- Thorough surface preparation of the base metal to remove all contaminants
- Use of a compatible transition layer when joining dissimilar materials with large thermal expansion mismatch
- Application of a thin "tack weld" layer before building up the full overlay thickness
- Performance of ultrasonic testing (UT) at the overlay-to-base metal interface, preferably using phased array UT (PAUT) for improved sensitivity
Inspection and Quality Control
The study emphasizes a multi-level inspection strategy for overlay-clad flanges:
- Visual inspection (VT) after each welding pass to detect surface cracks, undercuts, and excessive reinforcement
- Magnetic particle testing (MT) after the final pass to detect surface and near-surface cracks
- Penetrant testing (PT) on the sealing face to detect fine surface cracks and porosity
- Ultrasonic testing (UT) of the overlay-to-base metal interface to detect delamination
- Hydrostatic pressure test of the complete vessel assembly at 1.5 times the design pressure for 30 minutes minimum
The study notes that the combination of MT and PT is particularly important for detecting fine cracks on the sealing face that may not be visible to the naked eye but can lead to leakage under pressure.
Engineering Practice Cases
The authors present several case studies from Shanxi Yangmei Chemical Machinery's fabrication shop. In one case, a large diameter vessel flange (DN1200) with a 316L overlay layer experienced leakage at the gasket interface after commissioning. Investigation revealed that the overlay layer had been applied with an interpass temperature exceeding 300°C, leading to the formation of brittle intermetallic compounds at the overlay-to-base metal interface. Subsequent thermal cycling during operation caused micro-cracking at this interface, which propagated through the overlay layer to the sealing face. The corrective action involved complete removal of the defective overlay, re-preparation of the surface, and reapplication of the overlay with strict interpass temperature control below 150°C.
In another case, porosity in the overlay layer was traced to moisture contamination of the flux due to inadequate storage conditions. The corrective action included implementing a flux storage and baking protocol with dedicated ovens and desiccant containers, reducing porosity defects to near-zero levels.
Study Insights and Implications
This 2019 study is particularly valuable for its practical, case-based approach to identifying and resolving overlay-related leakage problems. The systematic root cause analysis methodology provides a template that can be applied to similar problems in other fabrication shops. The emphasis on interpass temperature control, flux management, and multi-level NDT inspection reflects a mature understanding of quality control in weld overlay fabrication. For engineers involved in pressure vessel fabrication, this literature reinforces the importance of process discipline and thorough inspection as the primary means of preventing costly field failures. The study also highlights the need for clear communication between welding engineers, inspectors, and quality assurance personnel to ensure that all aspects of the overlay process are properly controlled and documented.