CLADDING TECHNOLOGY SHANXI CO., LTDDelamination Behavior of Stainless Steel Weld Overlay Layers on Pressure Vessels
Literature Overview
This paper, authored by Cheng Zhe-wen, Lu Hao, Wang Kai-yun, and Xu Meng-jia from the School of Materials Science and Engineering at Shanghai Jiao Tong University (published in 2016 in the journal "Hot Working Technology"), addresses a critical failure mode encountered in clad and weld-overlay pressure vessels: the delamination (spalling) of stainless steel overlay layers from the base metal. The study is significant because delamination represents one of the most insidious and costly defects in pressure vessel fabrication, as it often does not manifest until the vessel is in service or during hydrostatic testing. The authors employ metallurgical analysis, mechanical testing, and process parameter evaluation to identify the root causes and propose preventive measures.
Core Technical Findings
The research systematically examines the metallurgical interface between the stainless steel overlay (typically 304L or 316L) and the carbon steel or low-alloy steel base. The key findings revolve around three interrelated failure mechanisms:
- Interfacial bonding quality — The dilution ratio at the interface between the base metal and the overlay determines the chemical composition of the transition zone. Excessive dilution (>50%) can lead to the formation of brittle intermetallic phases such as sigma phase (FeCr) and martensite, which drastically reduce ductility and promote cracking.
- Residual stress accumulation — Each weld pass introduces thermal residual stresses. In multi-pass overlay welds, the superposition of these stresses can exceed the yield strength of the interface, creating conditions favorable for delamination initiation.
- Hydrogen-induced cracking (HIC) — Absorbed hydrogen during welding, particularly in thick-section overlays, can accumulate at the interface and cause delayed cracking. This is especially relevant for vessels operating under hydrogen service conditions.
Key Parameters and Process Windows
| Parameter | Recommended Range | Risk Threshold |
|---|---|---|
| Dilution ratio at interface | < 30% (first pass) | > 50% (high brittleness risk) |
| Interpass temperature | 100–150°C for austenitic overlay | > 250°C (grain growth, sigma phase) |
| Heat input per pass | 0.8–1.5 kJ/mm for GTAW | > 2.0 kJ/mm (excessive dilution) |
| Preheat temperature (base metal) | 50–100°C | < 30°C (cold cracking risk) |
| Post-weld stress relief | 550–650°C for 2 h per 25 mm thickness | Incomplete PWHT (residual stress) |
Metallurgical Analysis and Interface Characterization
The authors emphasize the importance of metallographic examination of the weld interface. A well-bonded overlay interface should exhibit a smooth, continuous transition with no visible cracks, pores, or unmelted base metal. The transition zone typically shows a gradient from ferritic base metal through a mixed microstructure (ferrite + martensite) to the fully austenitic overlay.
The critical observation is that delamination frequently initiates at the first weld pass interface, where the dilution is highest and the metallurgical incompatibility is greatest. The authors recommend using a transition weld metal (such as E309L or E309MoL) for the first pass to reduce dilution effects, followed by the final overlay layer of E308L or E316L.
Defect Analysis Using FMEA Approach
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Interfacial cracking | 10 | 4 | 5 | 200 | Use transition alloy, control dilution |
| Sigma phase formation | 8 | 3 | 4 | 96 | Limit interpass temperature, reduce heat input |
| Hydrogen-induced delamination | 9 | 3 | 3 | 81 | Post-weld bake-out, low-hydrogen consumables |
| Incomplete fusion | 8 | 3 | 5 | 120 | Proper joint preparation, technique training |
Engineering Practice Implications
In engineering practice, delamination of overlay layers is often discovered during hydrostatic testing or during non-destructive examination (NDT) of the completed vessel. The consequences are severe: the vessel must be cut open, the affected area ground back to sound metal, and the overlay re-applied. This results in significant schedule delays and cost overruns.
Based on the findings of this paper, the following engineering practices should be adopted:
- Pre-weld preparation: The base metal surface should be ground to a minimum depth of 2 mm to remove any segregated inclusions or decarburized layers. The ground surface should be free of rust, oil, and scale.
- Weld sequence optimization: The overlay weld sequence should be designed to minimize residual stress concentration. A "back-step" or "skip" welding pattern is preferred over a continuous run.
- Post-weld heat treatment (PWHT): A complete PWHT cycle is essential to relieve residual stresses. For vessels subject to hydrogen service, an additional hydrogen bake-out at 200–250°C for 2–4 hours should follow the PWHT.
- NDT coverage: 100% ultrasonic testing (UT) in accordance with JB/T 4730 or ASME V should be performed on all overlay welds. Phased array UT (PAUT) provides superior detection capability for interfacial defects compared to conventional contact UT.
Study Insights and Reflections
This paper serves as an important reminder that the integrity of a weld overlay is determined not only by the final surface quality but by the cumulative metallurgical history of every pass. The delamination phenomenon is fundamentally a multi-scale problem: it involves macro-scale residual stresses, meso-scale microstructural evolution, and micro-scale hydrogen diffusion. Addressing it requires a holistic approach that integrates welding process control, materials selection, and post-weld treatment.
The paper's emphasis on the dilution ratio as the primary control parameter is particularly valuable for practicing engineers. In many fabrication shops, the focus tends to be on surface appearance and thickness compliance, while the subsurface metallurgy receives less attention. This study demonstrates that a systematic approach to interface control — including the use of transition alloys, controlled heat input, and proper post-weld treatment — can virtually eliminate delamination failures.