Spallation Behavior of Stainless Steel Weld Overlay Layers on Pressure Vessels

Literature Overview and Research Background

Spallation (delamination) of stainless steel weld overlay layers on pressure vessel components represents one of the most critical and costly failure modes in the petrochemical, hydrogen, and ammonia industries. The literature under review investigates the mechanisms, detection methods, and preventive strategies for overlay layer spallation in pressure vessels subjected to cyclic thermal loading, internal pressure, and aggressive chemical environments. The study focuses on austenitic stainless steel (309L/316L) overlays applied to low-alloy steel (SA-516 Gr.70 or 15CrMoR) pressure vessel shells using submerged arc welding (SAW) and electroslag welding (ESW) overlay processes.

Spallation Mechanisms and Failure Modes

Spallation of weld overlay layers is fundamentally a fatigue and corrosion-driven phenomenon that manifests at the overlay-base metal interface or within the dilution zone. The literature identifies three primary spallation mechanisms:

The most prevalent mechanism in pressure vessel service is thermal fatigue cracking at the overlay-base metal interface. The coefficient of thermal expansion mismatch between austenitic stainless steel (α ≈ 17.3 × 10⁻⁶/K) and low-alloy steel (α ≈ 12.0 × 10⁻⁶/K) generates significant shear stresses during thermal cycling. For a vessel experiencing ΔT = 200°C, the differential strain is approximately 1.06 × 10⁻³, which translates to shear stresses of 200–350 MPa at the interface depending on overlay thickness and constraint conditions.

Microstructural Analysis of the Dilution Zone

The dilution zone — the region where base metal alloying elements mix with the weld metal — is the most vulnerable region for spallation initiation. In a 309L overlay on SA-516 Gr.70, the dilution zone can develop a martensitic microstructure when the carbon equivalent exceeds the critical threshold (C_eq > 0.55%). This martensite is susceptible to hydrogen-assisted cracking (HAC) and delayed cracking.

The transition zone with 10–40% base metal dilution is the critical region where the carbon equivalent rises sufficiently to produce retained martensite. This martensite, combined with welding residual stresses (typically 200–350 MPa in the overlay layer), creates conditions favorable for hydrogen-induced cracking when exposed to wet H₂S or hydrogen environments.

Inspection and Detection Methods

The literature emphasizes the importance of systematic inspection protocols for detecting spallation before it progresses to catastrophic failure. The following table summarizes the effectiveness of various NDT methods for spallation detection:

The recommended inspection sequence for pressure vessel overlay layers is: (1) PT or MT for surface-breaking defects, (2) PAUT with a dual-element probe for subsurface spallation, (3) TOFD for quantitative sizing of detected indications, and (4) hydrostatic pressure test to verify structural integrity. For in-service vessels, periodic PAUT surveillance during shutdowns is essential.

Preventive Design and Fabrication Strategies

Based on the spallation mechanisms identified, the literature proposes several preventive measures:

1. Interlayer design: A nickel-based interlayer (e.g., IN718 or 2507) between the base metal and the stainless steel overlay can reduce carbon equivalent in the dilution zone, prevent martensite formation, and provide a ductile buffer zone.
2. Multi-pass overlay with controlled dilution: The first pass should use a high-nickel wire (e.g., ER309L with 23% Ni) to establish a fully austenitic transition. Subsequent passes with the final overlay composition (e.g., ER316L) should maintain at least 1.5 mm of fully austenitic material before the final composition is achieved.
3. Post-weld heat treatment (PWHT): A two-stage PWHT — first at 600–650°C for stress relief of the base metal, followed by 400–450°C for the overlay — reduces residual stresses without causing sensitization of the austenitic overlay.
4. Overlay thickness optimization: Minimum overlay thickness should be 3–5 mm for pressure vessels to ensure adequate protection against corrosion and to provide a sufficient buffer against spallation. Thinner overlays (< 2 mm) are more susceptible to spallation due to higher stress concentration.
5. Surface finishing: Grinding and polishing of the overlay surface to Ra ≤ 0.8 μm reduces stress concentration sites and improves corrosion resistance.

Engineering Case Study

The literature references a case involving a hydrogenation reactor (SA-387 Gr.II Cl.2 with 316L overlay, 6 mm thick, ESW applied) that experienced spallation after 18 months of service in a 7 MPa, 350°C, H₂S-containing environment. PAUT inspection revealed a 150 mm × 80 mm spallation area at the overlay-base metal interface. Metallographic examination confirmed that the failure initiated at the dilution zone where retained martensite had formed due to insufficient nickel content in the first ESW pass. The root cause analysis identified that the ESW consumable composition was not adjusted for the high base metal dilution inherent in electroslag welding (typically 15–25% dilution per pass). The corrective action included applying a nickel-based interlayer (IN718) and re-overlaying with a modified 316L consumable containing 3% additional Ni, followed by a complete PWHT cycle.

Key Questions and Reflections

A significant question arising from this literature is whether the current industry practice of relying primarily on visual and PT/MT inspection is adequate for detecting subsurface spallation. Given that spallation typically initiates at the overlay-base metal interface — a location inaccessible to surface inspection methods — the literature strongly advocates for mandatory PAUT or TOFD inspection of all critical pressure vessel overlays. However, the cost and expertise requirements for PAUT inspection remain barriers to widespread adoption, particularly in smaller fabrication shops.

Another reflection concerns the role of service environment monitoring. Many spallation failures occur in hydrogen-containing environments where hydrogen ingress into the dilution zone accelerates cracking. Implementing hydrogen monitoring systems and periodic hydrogen permeation testing could provide early warning of spallation risk. Additionally, the literature highlights the importance of considering the combined effects of thermal cycling, pressure cycling, and chemical attack — a multi-axial loading scenario that is rarely addressed in standard design codes.

Study Insights and Outlook

This literature provides a comprehensive framework for understanding and mitigating spallation of stainless steel weld overlay layers on pressure vessels. The identification of the dilution zone as the critical failure initiation site underscores the importance of careful consumable selection and interlayer design. The recommended multi-method NDT approach offers a practical inspection protocol that balances sensitivity, cost, and feasibility. For future work, the integration of digital twin technology and real-time monitoring systems for in-service spallation prediction represents a promising direction. Additionally, the development of advanced overlay consumables with inherently low carbon equivalent and high hydrogen resistance — such as high-nickel austenitic or precipitation-hardening alloys — could fundamentally reduce spallation susceptibility in the next generation of pressure vessel designs.