Overlay Layer Cracking in High-Temperature High-Pressure Separators Beyond Design Life: Cause Analysis and Periodic Inspection Recommendations

Literature Overview and Problem Statement

This literature addresses a critical and increasingly common challenge in the pressure vessel industry: the management of overlay-clad equipment that has exceeded its original design life. The case study focuses on a high-temperature high-pressure gas-liquid separator operating at elevated temperatures (typically 200–400°C) and pressures (10–30 MPa) in oil and gas processing facilities. The vessel, originally fabricated with a nickel-based alloy overlay (such as Stellite 6, Inconel 625, or Hastelloy C-276) applied by submerged arc welding (SAW) or electroslag welding (ESW) onto a carbon steel or low-alloy steel shell, developed cracking in the overlay layer after 15–25 years of service, well beyond the typical 15–20 year design life.

The significance of this topic cannot be overstated. As global energy infrastructure ages, an increasing proportion of pressure vessels and heat exchangers are operating beyond their original design life. The overlay layer, which serves as the primary corrosion and erosion barrier, is particularly vulnerable to degradation mechanisms that accumulate over extended service periods. Understanding the root causes of overlay cracking and implementing effective inspection strategies are essential for safe extended operation.

Root Cause Analysis of Overlay Layer Cracking

The literature identifies multiple contributing mechanisms for overlay layer cracking, which typically manifest as longitudinal cracks parallel to the weld direction or transverse cracks at interpass boundaries. A systematic failure analysis approach combining macroscopic examination, metallographic analysis, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and fracture mechanics assessment reveals the following primary and secondary mechanisms:

Primary Mechanisms

1. Thermal fatigue cracking: Cyclic temperature variations during normal operation and shutdown/startup cycles induce thermal stresses in the overlay layer. The coefficient of thermal expansion mismatch between the nickel-based overlay (typically 13–14 × 10⁻⁶/°C) and the carbon steel base (12–13 × 10⁻⁶/°C) generates interfacial stresses that accumulate over thousands of thermal cycles. At temperatures above 300°C, this effect is amplified by creep-assisted crack growth.
2. Residual stress relaxation and redistribution: The welding process introduces significant tensile residual stresses in the overlay layer, typically in the range of 200–400 MPa. Over extended service periods, these stresses partially relax through creep mechanisms at elevated temperatures, but the redistribution creates new stress concentrations at grain boundaries and inclusion sites within the overlay.
3. Microstructural degradation: Prolonged exposure to high-temperature service conditions causes microstructural evolution within the overlay. For Stellite-type alloys, this includes the precipitation of M23C6 carbides at grain boundaries, which embrittles the matrix and provides crack initiation sites. For Inconel 625 overlays, δ-phase precipitation and subsequent transformation to brittle phases can occur. For Hastelloy C-276, intergranular carbide precipitation reduces ductility and intergranular corrosion resistance.
4. Hydrogen-induced cracking (HIC) and hydrogen blistering: In sour service environments containing H₂S, atomic hydrogen generated by corrosion reactions can diffuse into the overlay layer and accumulate at microstructural discontinuities such as carbide-matrix interfaces, inclusions, and grain boundaries. This leads to blister formation and eventual cracking, particularly in the heat-affected zone (HAZ) of the overlay weld where microstructural heterogeneity is greatest.

Secondary Mechanisms

1. Erosion-corrosion interaction: In gas-liquid separator service, the overlay surface is exposed to high-velocity multiphase flow containing solid particles, droplets, and corrosive chemicals. The synergistic erosion-corrosion attack progressively thins the overlay and creates surface defects that act as crack initiation sites.
2. Intergranular stress corrosion cracking (IGSCC): In the presence of chloride ions (from seawater ingress or process contamination), the overlay layer may be susceptible to IGSCC, particularly if the microstructure has been degraded by carbide precipitation at elevated temperatures.
3. Bond line cracking: Cracking at the interface between the overlay layer and the base material can occur due to the combined effects of thermal cycling, residual stress, and the inherent metallurgical incompatibility between the nickel-based overlay and the carbon steel substrate. The bond line is often the weakest link in the clad system.

Periodic Inspection Strategy and Recommendations

Based on the failure analysis findings, the literature proposes a comprehensive periodic inspection program tailored for overlay-clad vessels operating beyond design life. The inspection strategy follows a risk-based approach aligned with API 579-1/ASME FFS-1 (Fitness-For-Service) principles and incorporates the following elements:

Inspection Frequency and Method Selection

Critical Inspection Zones

The literature emphasizes that inspection efforts should be concentrated on high-risk zones identified through finite element analysis (FEA) and operational experience. These include:

• Nozzle-to-shell transition regions: Where geometric discontinuities create stress concentrations and where overlay welds are most susceptible to cracking
• Weld interpass regions: Where microstructural heterogeneity is greatest due to repeated thermal cycling during fabrication
• High-velocity flow zones: Where erosion-corrosion synergistically thins the overlay
• Thermal cycle hot spots: Areas exposed to the highest operating temperatures, such as the bottom of gas-liquid separators where hot liquid accumulates
• Areas of overlay repair or re-cladding: Previous repair work may have introduced new defects or altered the residual stress state

Engineering Practice and Case Study Insights

A detailed case study in the literature describes a 20-year-old high-pressure gas separator with a 6 mm Stellite 6 overlay applied by SAW to a 2.25Cr-1Mo steel shell. The vessel operated at 280°C and 22 MPa in a sour gas processing facility. During a planned turnaround inspection, PAUT revealed multiple longitudinal cracks in the overlay layer, with the longest measuring 45 mm in length and extending to a depth of 3.2 mm (approximately 53% of the overlay thickness).

Metallographic analysis of a sectioned sample revealed that the cracks initiated at the overlay surface and propagated inward in a transgranular manner, with secondary branching at grain boundaries. EDS analysis confirmed the presence of hydrogen at the crack tips, indicating a hydrogen-assisted cracking mechanism. The overlay microstructure showed extensive M23C6 carbide precipitation at grain boundaries, which had reduced the effective ductility of the overlay matrix and provided preferential crack propagation paths.

The recommended remediation involved complete removal of the damaged overlay by mechanical grinding and replacement with a new overlay applied using plasma transferred arc (PTA) welding with a modified Stellite 6 consumable containing reduced carbon content to minimize carbide precipitation. The PTA overlay was applied in 3 passes with interpass temperature controlled below 150°C, and post-weld stress relief was performed at 700°C for 2 hours. Follow-up inspection confirmed the integrity of the new overlay with no detectable defects by PAUT or MT.

Key Reflections and Implications for Extended Operation

The most important lesson from this literature is that overlay layer integrity is a time-dependent property that degrades predictably with continued service exposure. Engineers must transition from a "fabricate-and-forget" mindset to a proactive integrity management approach that incorporates periodic assessment, trend monitoring, and planned intervention.

Several practical recommendations emerge from the analysis. First, overlay thickness monitoring should be established from the beginning of vessel operation, with baseline thickness measurements recorded during commissioning to enable trend analysis over time. Second, the selection of overlay material should consider not only initial corrosion resistance but also long-term microstructural stability at operating temperatures. For example, low-carbon variants of Stellite alloys or solid-solution strengthened nickel alloys (such as Inconel 625) may offer better long-term performance than high-carbon martensitic overlays in high-temperature service.

Third, the welding procedure for overlay application should be optimized to minimize residual stresses and microstructural heterogeneity. Techniques such as back-step welding, controlled interpass temperature, and post-weld stress relief are particularly important for overlays that will be exposed to thermal cycling. Finally, operators should be trained to recognize the visual and operational indicators of overlay degradation, including surface roughening, color changes, increased vibration, and pressure drop anomalies, which may provide early warning of overlay cracking before it is detected by formal inspection.

In conclusion, managing overlay-clad pressure vessels beyond their design life requires a comprehensive approach that integrates failure analysis knowledge, advanced inspection technology, and systematic integrity management. The cracking mechanisms identified in this literature are well-understood and predictable, and the proposed inspection strategies provide a practical framework for safe extended operation. Engineers should adopt a risk-based approach that balances inspection costs against the consequences of overlay failure, ensuring that critical infrastructure continues to operate safely while deferring costly replacement until truly necessary.