CLADDING TECHNOLOGY SHANXI CO., LTDHydrogen-Induced Delamination Cracks at Stainless Steel Cladding Interfaces and Their Prevention
Literature Overview
The paper by Ling Wenjun, published in Chemical Machinery in 1990, addresses a critical failure mode encountered in the fabrication of stainless steel-clad carbon steel pressure vessels and heat exchangers. The study focuses on hydrogen-induced delamination cracks (HIC) that form at the interface between the cladding layer and the base metal during and after welding overlay operations. This work is particularly significant because it was among the early systematic investigations into this failure mechanism in the Chinese chemical equipment industry, where hydrogen service and aggressive chemical environments are common.
Core Technical Analysis
Failure Mechanism
Hydrogen-induced delamination cracks at the cladding interface arise from the combination of several factors. During welding overlay processes such as submerged arc welding (SAW) or gas tungsten arc welding (GTAW), hydrogen from the welding arc, flux, or moisture in the base metal diffuses into the weld metal and the cladding layer. The austenitic stainless steel cladding layer (typically 304 or 316 grade) has a high affinity for hydrogen due to its face-centered cubic crystal structure, which provides numerous interstitial sites for hydrogen dissolution.
When residual stresses from welding combine with dissolved hydrogen, localized microvoids form at the interface. Under sustained stress or cyclic loading, these microvoids coalesce and propagate along the interface, resulting in delamination cracks. The crack morphology typically exhibits intergranular features in the stainless steel layer adjacent to the interface, with a characteristic "fish-mouth" appearance in cross-section.
Key Contributing Factors
| Factor | Description | Typical Threshold |
|---|---|---|
| Hydrogen content in weld metal | Total diffusible hydrogen from flux, wire, and base metal | > 5 mL/100g increases risk significantly |
| Residual stress | Longitudinal and transverse residual stresses from welding | Tensile stress > 200 MPa at interface |
| Base metal hydrogen embrittlement | Hydrogen trapped in the carbon steel base metal | HIC-sensitive microstructures (inclusions, banding) |
| Interface metallurgy | Dilution, carbide precipitation, brittle phases | Fe-Cr intermetallics at interface |
| Post-weld cooling rate | Rapid cooling traps hydrogen in solid solution | > 50°C/s increases susceptibility |
Prevention Strategies
The paper proposes a multi-layered prevention approach:
- Hydrogen control in welding consumables: Use low-hydrogen fluxes (e.g., basic fluxes with moisture content below 1% for rutile-type fluxes) and ensure proper flux baking at 250-300°C for 2 hours before use.
- Preheating and interpass temperature control: Preheat the base metal to 150-250°C depending on thickness to slow cooling rates and allow hydrogen diffusion during welding. Maintain interpass temperature between 150-250°C.
- Post-weld heat treatment (PWHT): Perform a hydrogen bake at 200-300°C for 2-4 hours immediately after welding, or incorporate a complete PWHT cycle at 550-650°C for stress relief.
- Interface preparation: Ensure the cladding layer is applied in multiple thin passes (typically 2-3 passes) with thorough cleaning between passes to minimize dilution and hydrogen pickup.
- Base metal selection: Use HIC-resistant base steels with controlled inclusion morphology and sulfur content below 0.02% when operating in hydrogen-containing environments.
Engineering Practice Implications
In practical fabrication of hydrogenation reactors and high-pressure hydrogen vessels, the interface between the stainless steel cladding and the carbon steel base metal is a critical inspection zone. Non-destructive testing methods such as ultrasonic testing (UT) with contact probes at the interface, and radiographic testing (RT) with appropriate exposure geometry, are essential for detecting delamination cracks.
The study's findings remain highly relevant today. Modern codes such as ASME VIII Div.1 and GB/T 150 require specific qualification of cladding weld procedures, and the understanding of hydrogen-induced cracking mechanisms continues to inform qualification requirements in ASME IX for cladding welds.
Study Insights
This 1990 paper demonstrates that the fundamental understanding of hydrogen behavior in welded joints was already well-established in the Chinese welding community. The systematic approach to identifying contributing factors and proposing preventive measures reflects a mature engineering methodology. The emphasis on hydrogen control through consumable selection and thermal management remains the cornerstone of modern cladding weld practice.
Summary
The prevention of hydrogen-induced delamination cracks at stainless steel cladding interfaces requires a comprehensive approach that addresses hydrogen sources, thermal management, and metallurgical control. The key insight from this literature is that no single measure is sufficient; rather, a combination of low-hydrogen consumables, appropriate preheating, controlled cooling, and post-weld heat treatment forms an effective defense-in-depth strategy against this failure mode.