CLADDING TECHNOLOGY SHANXI CO., LTDIsolation Layer Cladding Application in Pressure Vessel Fabrication
Literature Overview
This study by Bao Wenhong, Chen Hongwei, Wang Cen, Wu Jingwei, Liang Ruifeng, and Zhang Jianxiao, published in 2023, originates from the Gansu Provincial Special Equipment Inspection and Testing Research Institute and Lanzhou Lanchen Heavy Equipment Co., Ltd. The work addresses the critical application of isolation layers in weld overlay processes used for pressure vessel manufacturing. The research was motivated by the practical challenges encountered in fabricating pressure vessels that require corrosion-resistant overlay layers on carbon steel or low-alloy steel substrates, particularly in the petrochemical and hydrogenation reactor industries prevalent in northwest China.
Core Technical Content
The isolation layer in weld overlay serves as an intermediary between the base metal and the functional overlay layer. Its primary purpose is to prevent dilution of the overlay layer by base metal elements and to mitigate the formation of brittle intermetallic compounds at the interface. In pressure vessel applications, the isolation layer typically employs materials such as nickel-based alloys (e.g., Inconel 625, Incoloy 827) or austenitic stainless steels (e.g., 308L, 309L) depending on the specific service environment.
Key Technical Parameters
| Parameter | Typical Value | Rationale |
|---|---|---|
| Isolation layer thickness | 1.0–2.5 mm | Balances dilution control with cost |
| Base metal preheat temperature | 100–150°C | Prevents cold cracking in HSLA substrates |
| Interpass temperature | ≤250°C | Controls grain growth and residual stress |
| Overlay dilution rate | 5–15% | Acceptable for most corrosion service |
| Overlay hardness (HV) | 180–250 | Ensures ductility and crack resistance |
Standards and Codes Reference
The fabrication of pressure vessels with isolation layer cladding must comply with multiple standards simultaneously. GB/T 150.2 governs the design and fabrication of the pressure vessel body, while NB/T 47014 provides the qualification requirements for weld overlay procedures. The weld procedure qualification must demonstrate that the isolation layer achieves acceptable bond strength and that the overlay layer composition meets the specified chemical requirements. ASME Section IX, QW-404.13 through QW-404.15, also provides guidance for weld overlay procedure qualification, particularly relevant for vessels intended for export.
Engineering Practice Analysis
In the fabrication of hydrogenation reactors and high-pressure separators, the isolation layer plays a decisive role in ensuring long-term service integrity. The typical sequence involves: base metal preparation (grinding to a smooth surface with a 60° bevel), application of the isolation layer using GTAW or SAW with nickel-based wire, followed by the functional overlay layer using SAW or FCAW with the specified alloy composition.
A common challenge observed in practice is the formation of chromium carbide precipitation at the isolation layer boundary when the thermal cycle is excessive. This can be mitigated through the following measures:
- Maintaining interpass temperature below 200°C during multi-pass overlay.
- Using low-carbon filler metals (C ≤ 0.03%) for the isolation layer.
- Applying post-weld heat treatment at 1050–1100°C followed by water quenching for sensitized materials.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Undercut at interface | Excessive travel speed | Reduce speed by 15–20% |
| Cracking in isolation layer | High restraint + HSLA base | Increase preheat to 150–200°C |
| Excessive dilution | Insufficient overlap | Increase overlap to 50–60% |
| Tungsten inclusion | Poor GTAW technique | Increase travel speed, reduce arc length |
Integration with Quality Assurance
The inspection regime for isolation layer cladding on pressure vessels typically includes:
- Visual examination (VT) of all overlay surfaces for undercut, porosity, and lack of fusion.
- Magnetic particle testing (MT) or liquid penetrant testing (PT) of the overlay surface for surface-breaking defects.
- Ultrasonic testing (UT) of the bond line between the isolation layer and base metal, per JB/T 4730.
- Chemical analysis of the overlay layer to verify dilution rate and alloy composition.
- Hardness testing across the overlay cross-section to ensure uniformity.
Study Insights and Reflections
The 2023 publication timing of this research is significant, as it coincides with the rapid expansion of the hydrogen energy industry in China, which demands high-integrity pressure vessels with reliable corrosion-resistant overlays. The collaboration between a provincial inspection institute and a major heavy equipment manufacturer reflects the industry's increasing emphasis on evidence-based fabrication practices. The study underscores that the isolation layer is not merely a procedural formality but a critical engineering design element that directly influences the service life and safety of pressure vessels operating in aggressive environments. Engineers should pay particular attention to the thermal management of multi-layer overlay sequences, as improper heat input can compromise both the isolation layer's protective function and the overall structural integrity of the vessel.