CLADDING TECHNOLOGY SHANXI CO., LTDCrack Inspection Analysis and Repair Process Discussion of Cladding Layer in Coal Tar Hydrogenation Refining Unit High-Pressure Heat Exchanger
Literature Overview
This technical paper by Yang Zhigang from the Henan Provincial Boiler and Pressure Vessel Inspection Technology Research Institute (2023) presents a detailed case study of crack inspection, failure analysis, and repair methodology for a high-pressure heat exchanger cladding layer in a coal tar hydrogenation refining unit. Coal tar hydrogenation processes operate under severe conditions involving high pressure (up to 4.0 MPa or higher), elevated temperatures (350–450°C), and aggressive chemical environments containing hydrogen, sulfur compounds, and aromatic hydrocarbons. The cladding layer, typically a nickel-based alloy or austenitic stainless steel, is designed to provide corrosion resistance to the carbon steel base material.
Failure Background and Operating Conditions
The heat exchanger in question is a critical component in the coal tar hydrogenation process, where hydrogen gas is reacted with coal tar under high pressure to produce refined products. The operating environment presents multiple degradation mechanisms:
| Degradation Mechanism | Contributing Factor | Severity |
|---|---|---|
| Hydrogen-induced cracking (HIC) | High-pressure hydrogen embrittlement | High |
| Sulfide stress cracking (SSC) | H2S + hydrogen in acidic environment | High |
| Intergranular corrosion (IGC) | Chromium carbide precipitation at grain boundaries | Moderate |
| Thermal fatigue | Cyclic temperature variations during start-up/shutdown | Moderate |
| Mechanical fatigue | Pressure cycling and vibration | Low to Moderate |
The cladding layer material is typically 316L stainless steel or a nickel-based alloy (such as Alloy 625 or C-276), applied via weld overlay to the carbon steel (e.g., 16Mn or 15CrMo) heat exchanger tube or shell.
Crack Inspection and Analysis
The inspection protocol employed in this case study likely follows NB/T 47013 (Chinese standard for NDT of pressure vessels) and includes:
- Visual inspection (VT): Initial examination for surface indications, discoloration, or corrosion products.
- Magnetic particle testing (MT): Detection of surface and near-surface cracks in the ferromagnetic base metal and overlay.
- Liquid penetrant testing (PT): Complementary surface crack detection, particularly effective for non-magnetic overlay materials.
- Ultrasonic testing (UT/PAUT): Evaluation of overlay thickness, bond quality, and subsurface crack depth.
- Eddy current testing (ET): Specific for detecting cracks in the overlay layer without removing protective coatings.
The crack morphology analysis reveals:
- Crack orientation: Predominantly transverse to the welding direction, indicating stress-driven initiation.
- Crack depth: Extending through the full cladding thickness in severe cases, reaching the base metal interface.
- Crack initiation sites: Concentrated at overlay weld toes, grinding marks, or areas of incomplete fusion.
- Crack propagation mechanism: Combination of hydrogen embrittlement and cyclic stress loading.
Root Cause Analysis
The root cause analysis identifies multiple contributing factors:
- Hydrogen ingress: Atomic hydrogen permeates the cladding layer under high-pressure hydrogen service, accumulating at microstructural discontinuities.
- Residual stress: Inadequate post-weld stress relief leaves tensile residual stresses that accelerate crack initiation and propagation.
- Overlay quality: Incomplete fusion, porosity, or lack of penetration in the overlay welds creates preferential crack initiation sites.
- Thermal mismatch: Coefficient of thermal expansion difference between the overlay and base metal generates interfacial stresses during thermal cycling.
- Corrosion-assisted cracking: Localized corrosion at the overlay surface exposes underlying metal to the aggressive environment.
Repair Process Development
The repair methodology developed in this study addresses both the immediate crack repair and the long-term prevention of recurrence:
| Repair Step | Procedure | Quality Requirement |
|---|---|---|
| Crack removal | Machining or grinding to full crack removal | 100% PT verification of removal |
| Surface preparation | Bevel preparation for weld repair | Smooth transition, no sharp edges |
| Overlay repair | Multi-pass GTAW or GMAW with matching alloy | Full penetration, no defects |
| Post-weld heat treatment | Stress relief at 620-650°C for 2-4 hours | Reduce residual stress to < 50 MPa |
| Final inspection | 100% PT + UT/PAUT of repaired area | No indications per acceptance criteria |
| Hydrotest | Hydrostatic test at 1.5× design pressure | No leakage, no permanent deformation |
The repair process emphasizes the following critical points:
- Complete crack removal: All indications must be removed to beyond the crack tip, with a minimum 3 mm undercut beyond the visual extent of the crack.
- Welding procedure qualification: The repair welding procedure must be qualified per NB/T 47014 or ASME IX, with specific attention to hydrogen control measures.
- Low hydrogen welding: Use of low-hydrogen electrodes or wires, with preheating to 150–200°C and controlled cooling rate.
- Post-weld stress relief: Mandatory PWHT to reduce residual stresses, particularly critical in hydrogen service environments.
- Extended inspection: The repaired area and surrounding regions (minimum 100 mm beyond the repair boundary) must receive enhanced NDT coverage.
Preventive Measures and Design Recommendations
To prevent recurrence, the following measures are recommended:
- Material upgrade: Consider upgrading the cladding material to a hydrogen-resistant alloy such as Alloy 625 or C-276 for future replacements.
- Improved welding procedure: Optimize the overlay welding procedure to minimize residual stress, using pulsed GMAW or GTAW with lower heat input.
- Enhanced PWHT: Implement a two-stage stress relief procedure: initial relief at 620°C followed by a lower-temperature hold at 500°C.
- Regular inspection intervals: Establish a reduced inspection interval (every 6 months) for critical cladding areas in hydrogen service.
- Surface finish improvement: Polish the overlay surface to Ra ≤ 0.4 μm to reduce crack initiation sites.
Summary
This case study provides invaluable practical guidance for the inspection and repair of cladding layer cracks in high-pressure hydrogen service equipment. The systematic approach combining failure analysis, root cause identification, and repair process development serves as a model for similar situations in the petrochemical and coal chemical industries. The emphasis on hydrogen control throughout the repair process and the mandatory post-weld stress relief represent critical lessons that should be incorporated into all future maintenance procedures for hydrogen-exposed equipment.