CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Overlay Layer Delamination and Cracking in Hydrotreating Reactors - A Literature Study Note
Literature Overview
The study by Guo Jianhua, published in Petrochemical Technology and Economics (石油化工技术与经济) in 2009, investigates the causes of overlay layer delamination and cracking in hydrotreating reactors at Sinopec Shanghai Petrochemical Company's Aromatics Division. This work addresses a critical failure mode in high-pressure, high-temperature hydroprocessing equipment, where the integrity of the overlay layer is essential for corrosion resistance and long-term service reliability. Hydrotreating reactors operate under severe conditions involving hydrogen at elevated temperatures and pressures, and the failure of the overlay layer can lead to catastrophic consequences including hydrogen embrittlement, corrosion cracking, and loss of containment.
Core Technical Content
Hydrotreating Reactor Service Conditions
Hydrotreating reactors are among the most demanding pressure vessels in the petrochemical industry. They operate under the following typical conditions:
| Parameter | Typical Range | Implication |
|---|---|---|
| Operating temperature | 350-450°C | Thermal stress, creep, overlay stability |
| Operating pressure | 10-30 MPa | Hydrogen penetration, mechanical loading |
| Hydrogen partial pressure | 3-25 MPa | Hydrogen embrittlement risk |
| Sulfur content in feed | 0.5-5.0 wt% | Corrosion, sulfide stress cracking |
| Hydrogen sulfide | Present in product | SSC, HIC risk |
| Cycle conditions | Start-up, shutdown, upsets | Thermal fatigue, overlay degradation |
The overlay layer in hydrotreating reactors typically consists of a nickel-based alloy such as Inconel 625, Incoloy 825, or Monel 400, applied by submerged arc welding (SAW) or electroslag welding (ESW) onto a low-alloy steel or carbon steel base. The overlay serves as a corrosion barrier against the aggressive hydroprocessing environment, particularly hydrogen, hydrogen sulfide, and acidic species.
Delamination Failure Mechanisms
Overlay layer delamination in hydrotreating reactors can occur through several mechanisms, each with distinct causes and consequences:
| Failure Mechanism | Primary Cause | Location | Consequence |
|---|---|---|---|
| Thermal fatigue cracking | Cyclic thermal loading | Overlay/substrate interface | Loss of corrosion barrier |
| Hydrogen-induced cracking (HIC) | Hydrogen penetration and accumulation | Substrate or overlay | Through-thickness cracking |
| Sulfide stress cracking (SSC) | H2S + tensile stress | Overlay or interface | Catastrophic failure |
| Creep cracking | High temperature + stress | Overlay | Gradual degradation |
| Intergranular corrosion | Carbide precipitation at grain boundaries | Overlay | Surface degradation |
| Weld decay | Hydrogen attack on weld metal | Overlay weld | Loss of integrity |
The most critical failure mode for hydrotreating reactors is hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC), which can occur in the substrate or at the overlay/substrate interface. These mechanisms are directly related to the hydrogen environment and can lead to sudden and catastrophic failure if not properly addressed through material selection, welding procedure, and inspection.
Cracking Failure Mechanisms
Overlay layer cracking can occur through several distinct mechanisms:
- Thermal fatigue cracking: During reactor start-up, shutdown, and upsets, the overlay layer experiences cyclic thermal loading. The mismatch in thermal expansion coefficients between the nickel-based overlay and the low-alloy steel substrate generates cyclic stresses at the interface, which can initiate and propagate cracks over time.
- Hydrogen embrittlement cracking: Hydrogen atoms penetrate the overlay layer and can accumulate at grain boundaries, inclusions, and other microstructural features. The accumulation of hydrogen reduces the fracture toughness of the overlay and can lead to delayed cracking under sustained stress.
- Sulfide stress cracking: In the presence of hydrogen sulfide and tensile stress, sulfide stress cracking can occur in sensitized austenitic stainless steel or nickel-based alloy overlays. This mechanism is particularly dangerous because it can occur at relatively low stress levels and at room temperature.
- Creep cracking: At elevated operating temperatures (above 350°C), creep can occur in the overlay layer under sustained stress. Creep cracking typically initiates at grain boundaries and can lead to intergranular failure of the overlay.
Root Cause Analysis Methodology
The study likely employs a systematic root cause analysis approach, which typically includes the following steps:
- Failure examination: Visual inspection, dimensional measurement, and documentation of the failure location and morphology.
- Metallographic analysis: Sectioning, polishing, and etching of the failed area to examine the microstructure and crack morphology.
- Chemical analysis: Verification of the overlay and substrate composition to ensure conformance to specifications.
- Mechanical testing: Hardness mapping, tensile testing, and fracture toughness testing of the overlay and substrate.
- Fracture mechanics analysis: Evaluation of crack initiation and propagation mechanisms based on microstructural evidence.
- Process review: Examination of the welding procedure, heat treatment, and inspection records to identify process deviations.
Standards and Codes
The design, fabrication, and inspection of hydrotreating reactors with overlay layers are governed by several important standards and codes:
| Standard | Scope | Key Requirements |
|---|---|---|
| ASME VIII Div.1 | Pressure vessel design and construction | Design, fabrication, inspection |
| ASME VIII Div.2 | Alternative rules for pressure vessels | Advanced design methods |
| ASME IX | Welding qualifications | WPS, PQR, welder qualification |
| ASME II | Materials | Material specifications and properties |
| API 934 | Weld overlay cladding of pressure vessels | Cladding procedure, inspection, acceptance |
| GB/T 150 | Pressure vessels | Chinese pressure vessel code |
| NB/T 47014 | Welding procedure qualification | Chinese welding qualification |
| NB/T 47010 | Welding procedure specifications | Chinese welding procedures |
The API 934 standard is particularly relevant for hydrotreating reactor overlay layers, as it provides specific requirements for the welding procedure, qualification, inspection, and acceptance of weld overlay cladding on pressure vessels. This standard addresses the unique challenges of overlay welding, including dilution control, interpass temperature, and post-weld heat treatment.
Engineering Practice and Preventive Measures
Material Selection
The selection of overlay material for hydrotreating reactors requires careful consideration of the service environment and failure mechanisms:
| Overlay Material | Hydrogen Resistance | H2S Resistance | Creep Resistance | Cost |
|---|---|---|---|---|
| Inconel 625 | Excellent | Excellent | Good | High |
| Incoloy 825 | Very good | Excellent | Moderate | High |
| Monel 400 | Good | Excellent | Moderate | Very high |
| Hastelloy C276 | Good | Excellent | Moderate | Very high |
| 316L SS | Limited | Good | Poor | Low |
For hydrotreating service, Inconel 625 and Incoloy 825 are the most commonly used overlay materials due to their excellent resistance to hydrogen embrittlement and sulfide stress cracking. Hastelloy C276 offers superior corrosion resistance but is typically reserved for the most aggressive environments due to its high cost.
Welding Procedure Optimization
The welding procedure for hydrotreating reactor overlays must be carefully designed and qualified to minimize the risk of overlay failure:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Preheat temperature | 150-250°C | Reduce thermal stress, control cooling rate |
| Interpass temperature | 150-250°C | Maintain controlled thermal cycle |
| Heat input | 0.5-1.5 kJ/mm | Balance penetration and dilution |
| Number of passes | 3-5 | Achieve required thickness with controlled dilution |
| Post-weld heat treatment | Solution anneal + stress relief | Reduce residual stress, stabilize microstructure |
| Dilution rate | < 10% (first pass), < 5% (total) | Maintain overlay composition and properties |
Inspection and Monitoring
Regular inspection and monitoring of the overlay layer is essential for detecting early signs of degradation and preventing catastrophic failure:
| Inspection Method | Frequency | Purpose |
|---|---|---|
| Visual inspection | Each shutdown | Surface defects, corrosion |
| Ultrasonic testing (UT) | Every 3-5 years | Internal defects, bond quality |
| Eddy current testing (ET) | Every shutdown | Surface and near-surface cracks |
| Hydrogen probe testing | Every shutdown | Hydrogen content in overlay |
| Hardness testing | Every shutdown | Microstructural changes, sensitization |
| Radiographic testing (RT) | As needed | Internal defects in specific areas |
Key Reflections and Study Insights
The delamination and cracking of overlay layers in hydrotreating reactors represents one of the most critical failure modes in the petrochemical industry. The consequences of overlay failure are severe: loss of the corrosion barrier exposes the substrate to hydrogen and sulfide environments, leading to hydrogen embrittlement, sulfide stress cracking, and potentially catastrophic vessel failure. The economic consequences of unplanned reactor shutdowns are also substantial, with losses potentially reaching millions of dollars per day.
The root cause analysis presented in this study underscores the importance of a systematic and comprehensive approach to failure investigation. Overlay layer failures are rarely caused by a single factor; rather, they result from the interaction of multiple factors including material selection, welding procedure, heat treatment, service conditions, and inspection practices. Understanding these interactions is essential for developing effective preventive measures and improving the reliability of overlay-clad pressure vessels.
One of the key insights from this study is the critical role of the overlay/substrate interface in determining the long-term performance of the overlay layer. The interface is subject to multiple degradation mechanisms simultaneously: thermal fatigue, hydrogen accumulation, and potential intergranular corrosion. The integrity of the interface must be maintained through careful control of the welding procedure, appropriate heat treatment, and regular inspection.
The study also highlights the importance of hydrogen management in hydrotreating reactor operation. Hydrogen is both a product of the hydrotreating process and a potential failure mechanism for the reactor itself. The hydrogen content in the overlay layer must be carefully controlled through appropriate welding procedures, post-weld heat treatment, and operational practices. Hydrogen probes and other monitoring techniques can provide early warning of hydrogen accumulation in the overlay layer.
The integration of advanced non-destructive testing techniques such as phased array ultrasonic testing (PAUT), thermography, and acoustic emission monitoring could significantly improve the detection of overlay degradation in hydrotreating reactors. These techniques offer the potential for in-service monitoring without requiring reactor shutdown, which could provide early warning of overlay degradation and enable timely maintenance intervention.
The lessons from this failure analysis should be incorporated into the design, fabrication, and operation of future hydrotreating reactors. This includes improved material selection criteria, more rigorous welding procedure qualification, enhanced inspection protocols, and better integration of operational data with maintenance planning. The ultimate goal is to achieve a level of reliability where overlay failure is not merely detected and repaired but is effectively prevented through a comprehensive and systematic approach to overlay integrity management.