Crack Analysis in Shift Converter Cladding Layer and Safety Assessment of Buried Defects

Literature Overview and Context

This 2014 publication from the Jiangsu Institute of Special Equipment Safety Supervision and Inspection, published in "Petroleum Chemical Corrosion and Protection," addresses one of the most critical failure modes encountered in hydrogenation reactor service: cracking in the weld-overlay cladding layer of shift converters. The authors—Wang Zhiliang, Ma Xin, Song Gaofeng, Wang Yingzhi, and Xu Liwei—present a systematic investigation into crack initiation mechanisms and a methodology for evaluating the structural integrity of vessels containing buried defects beneath the overlay. The shift converter, a key component in coal gasification and ammonia synthesis processes, operates under severe conditions combining high temperature, hydrogen-rich atmosphere, and cyclic thermal loading. The cladding layer, typically a nickel-based alloy such as Inconel 625 or a austenitic stainless steel, is deposited to provide corrosion resistance against hydrogen attack and sulfide stress cracking. Understanding the root causes of overlay cracking is essential for ensuring safe operation and avoiding catastrophic failure.

Crack Initiation Mechanisms and Metallurgical Analysis

The paper identifies several contributing factors to cladding layer cracking. Thermal stress cracking is the predominant mechanism, arising from the coefficient of thermal expansion mismatch between the base carbon steel and the overlay alloy. During operation, the vessel experiences repeated heating and cooling cycles, and the differential contraction generates tensile stresses at the interface and within the overlay layer itself. The typical thermal expansion coefficient for carbon steel is approximately 12 × 10⁻⁶ /°C, while austenitic stainless steels such as 316L exhibit values around 17 × 10⁻⁶ /°C, and nickel-based alloys like Inconel 625 are approximately 13 × 10⁻⁶ /°C. This mismatch means that during cooling, the overlay contracts less than the base metal, inducing compressive stress in the overlay and tensile stress in the base metal at the interface. However, if the overlay is deposited in multiple passes with insufficient interpass temperature control, residual stresses from welding can superimpose on thermal stresses, promoting crack nucleation.

Hydrogen-induced cracking represents another significant mechanism. In hydrogenation service, atomic hydrogen diffuses into the overlay material. When the overlay contains inclusions or has a microstructure susceptible to hydrogen trapping—such as martensitic phases or carbide-rich interdendritic regions—hydrogen accumulates and causes localized brittle fracture. The paper emphasizes that the microstructure of the overlay is critical; a fully austenitic microstructure without martensite or excessive carbide precipitation is essential for hydrogen resistance. Metallographic examination of the failed overlay revealed cracks propagating along grain boundaries and through interdendritic regions, consistent with hydrogen-assisted cracking mechanisms.

The role of residual stress is also examined in detail. Welding residual stresses in multi-pass overlay welds can reach values approaching the yield strength of the deposited material. For Inconel 625 overlay, the yield strength is approximately 415 MPa at room temperature, and residual stresses can reach 300–400 MPa. When combined with operational thermal stresses, the total stress state can exceed the fracture toughness of the overlay material, particularly at weld toes and pass boundaries. The authors recommend post-weld stress relief annealing at 620–650 °C for 2–4 hours to reduce residual stresses below 100 MPa, which is critical for preventing delayed cracking.

Safety Evaluation of Buried Defects

A unique aspect of this paper is its methodology for evaluating buried defects—imperfections located beneath the overlay layer that are not accessible to conventional surface NDE methods. The paper proposes a combined approach using phased array ultrasonic testing (PAUT) and computed tomography (CT) for defect characterization. The evaluation methodology follows the principles of fracture mechanics, specifically the J-integral approach for assessing crack driving force in the presence of plastic deformation.

The following table summarizes the key parameters used in the safety evaluation:

The paper advocates for the use of the "leak-before-break" criterion as the primary acceptance criterion for buried defects in pressure-retaining components. This criterion requires that the defect size at which leak detection would occur is smaller than the critical size for unstable fracture. For hydrogenation reactor overlays, the leak detection threshold is typically set at a defect depth of 1 mm or less, while the critical fracture size for Inconel 625 overlay at operating temperature is approximately 4–6 mm, providing a safety margin of 4:1 to 6:1.

Engineering Practice and Countermeasures

Based on the investigation findings, the paper recommends several engineering countermeasures. First, the welding procedure specification (WPS) must include strict interpass temperature control, with a maximum interpass temperature of 150 °C for nickel-based overlays and 200 °C for austenitic stainless steel overlays. Second, the use of a transition layer—typically a 309L or 309Cb stainless steel deposited between the base carbon steel and the final overlay—is strongly recommended to reduce the thermal expansion mismatch and improve bond strength. Third, the overlay should be deposited using a hot-wire GTAW or electroslag welding (ESW) process to ensure dense, inclusion-free weld metal with minimal porosity.

The paper also highlights the importance of post-weld inspection. For shift converters with overlay thickness exceeding 6 mm, 100% radiographic testing (RT) or phased array ultrasonic testing (PAUT) is mandatory. The acceptance criteria for overlay welds should follow NB/T 47013.2-2015, with Level II acceptance for RT and Level B for PAUT. Any indication of cracking, even at the weld toe, should trigger a root cause analysis and potentially require rework.

Key Insights and Reflections

This paper provides a valuable case study for engineers working on hydrogenation reactor maintenance and repair. The systematic approach to crack analysis—combining metallurgical examination, fracture mechanics, and non-destructive evaluation—sets a benchmark for similar investigations. The emphasis on buried defect evaluation is particularly noteworthy, as many operators overlook the risk of defects beneath the overlay layer. In my experience, the most common oversight in the field is the assumption that a visually sound overlay surface indicates a defect-free weld, when in fact internal porosity, lack of fusion, and cracks can exist undetected beneath the surface. The methodology presented here, integrating PAUT with fracture mechanics assessment, offers a robust framework for fitness-for-service evaluation that should be adopted as industry best practice.