CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Sealing Surface Leakage in Clad Flange Overlay Layers
Literature Overview
This paper, published in "China Chemical Equipment" (2019) by Ding Manfu, Fan Guangpu, and Li Weimin from Shanxi Yanghua Chemical Machinery (Group) Co., Ltd., addresses one of the most persistent operational challenges in the pressure vessel industry — leakage at the sealing surface of clad flanges. The authors conducted a systematic root-cause investigation into field failures of weld-overlay flanges used in chemical process piping, integrating metallurgical analysis, welding process review, and service condition assessment. The study is particularly valuable because flange leakage is often misdiagnosed as a gasket problem, when in fact the root cause frequently lies in the integrity of the overlay layer itself.
Root Cause Analysis Framework
The authors applied a structured failure analysis methodology that can be summarized using a modified 5W2H approach:
- What failed: The sealing face of the overlay layer on a carbon steel flange clad with austenitic stainless steel (typically 304 or 316) exhibited localized leakage under operating pressure and temperature.
- Where it occurred: Preferentially at the boundary between the overlay layer and the base metal, and at locations where the overlay thickness was insufficient or non-uniform.
- When it manifested: Typically after several weeks to months of service, often triggered by thermal cycling, pressure fluctuation, or exposure to corrosive media.
- Why it happened: Multiple contributing factors including insufficient overlay thickness, poor bond integrity, corrosion undercut, and improper machining of the sealing surface.
Key Failure Modes Identified
| Failure Mode | Description | Contributing Factor |
|---|---|---|
| Overlay layer cracking | Radial or circumferential cracks in the overlay | High residual stress, improper heat input, HAZ cracking |
| Corrosion undercut | Undermining of the overlay by corrosive medium at the weld toe | Insufficient overlay thickness, poor fusion control |
| Overlay spalling | Detachment of the overlay layer from the base metal | Poor metallurgical bond, thermal fatigue |
| Surface pitting | Localized corrosion pits on the sealing surface | Incomplete coverage, low-carbon steel exposure at surface |
| Machining damage | Disturbance of the overlay microstructure during turning | Excessive machining allowance, improper tool geometry |
Welding Process and Overlay Quality
The paper emphasizes that the quality of the overlay layer is directly governed by the welding process parameters and consumable selection. For austenitic stainless steel overlay on carbon steel flanges, the following process considerations are critical:
Process Parameter Windows
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Heat input | 0.8–2.5 kJ/mm | Low enough to minimize dilution; high enough to ensure fusion |
| Dilution ratio | < 30% (ideally < 20%) | Excessive dilution reduces corrosion resistance of overlay |
| Overlay thickness | ≥ 2.5 mm minimum, 3–5 mm preferred | Must exceed machining allowance and corrosion allowance |
| Number of passes | 2–3 passes minimum | Ensures uniform composition and adequate thickness |
| Interpass temperature | < 150°C | Prevents excessive grain growth and residual stress accumulation |
The authors note that the dilution ratio is perhaps the single most important parameter governing overlay performance. When the dilution exceeds 30%, the carbon equivalent of the overlay layer increases significantly, leading to chromium carbide precipitation at grain boundaries and reduced resistance to intergranular corrosion. This is especially critical for flanges operating in chloride-containing environments, where even minor sensitization can lead to rapid pitting and undercutting.
Consumable Selection
The paper discusses the use of E309L, E316L, and E310L electrodes for overlay welding on carbon steel flanges. E309L provides adequate dilution resistance for moderate service conditions, while E316L is preferred for chloride-containing environments. The choice must be matched to the specific process fluid and operating conditions.
Machining Considerations
A frequently overlooked aspect highlighted by the authors is the effect of post-overlay machining on sealing surface integrity. The overlay layer must be machined to achieve the required flatness and surface roughness for gasket sealing, but excessive or improper machining can:
- Reduce the effective overlay thickness below the corrosion allowance
- Create a work-hardened surface layer susceptible to stress corrosion cracking
- Reveal areas of insufficient overlay coverage where the base metal is exposed
- Generate residual stresses that promote cracking
The recommended machining allowance is 1.5–2.5 mm above the required sealing surface dimension, with final turning performed using carbide tools at low cutting speeds to minimize thermal damage to the overlay.
Engineering Practice Integration
From my experience in pressure vessel fabrication, the lessons from this paper are directly applicable to several common scenarios:
- Flange retrofitting: When existing carbon steel flanges are retrofitted with overlay layers, the base metal condition (hardness, residual stress from previous service) must be assessed before welding. Preheating to 150–250°C is recommended for thick flanges to reduce cracking susceptibility.
- Post-weld inspection: Surface-mounted magnetic particle testing (MT) or penetrant testing (PT) should be performed on the overlay layer before machining to detect surface cracks. Ultrasonic testing (UT) of the overlay-to-base bond should be performed per NB/T 47013 or JB/T 4730 to verify bond integrity.
- Corrosion allowance: The total overlay thickness should account for both the machining allowance and the expected corrosion loss over the vessel's design life. For aggressive service, a minimum of 3 mm overlay with 1.5 mm machining allowance leaves 1.5 mm corrosion allowance, which is often insufficient for long-term service.
Key Questions and Reflections
The paper raises several important questions that warrant further investigation:
- How does the number of thermal cycles during flange installation (bolting, gasket seating) affect the overlay layer integrity?
- What is the optimal overlay composition for flanges exposed to both mechanical wear and chemical corrosion simultaneously?
- Can advanced overlay techniques such as plasma transferred arc (PTA) or laser cladding provide superior sealing surface quality compared to conventional GTAW or GMAW?
These questions reflect the evolving nature of cladding technology and the need to continuously refine our understanding of overlay layer behavior under complex service conditions.
Study Insights and Implications
This paper serves as a practical reminder that the overlay layer on a flange is not merely a surface treatment but a critical functional component that must be designed, fabricated, and inspected with the same rigor as the pressure boundary itself. The failure analysis approach presented — combining metallurgical examination, process parameter review, and service condition assessment — is a model methodology that should be adopted in all overlay-related failure investigations. The emphasis on dilution control, adequate overlay thickness, and careful machining provides actionable guidance for engineers involved in the design and fabrication of clad flanges for chemical process applications.