Zhuojin Pipe Fitting Co., LtdRepair Technology for Overlay Layer Cracks on Hydrogenation Reactor Circumferential Weld
Literature Overview
This 2022 paper from China Chemical Equipment, authored by Sun Bo of Lanzhou Lanshi Heavy Equipment Co., Ltd., documents the repair of network cracks found in the overlay layer of a hydrogenation reactor circumferential weld during a refinery turnaround. The reactor is a critical high-temperature and high-pressure vessel in the hydroprocessing unit, where the overlay layer provides corrosion resistance against hydrogen attack and sour service. The repair required a detailed analysis of the crack mechanism, development of a reliable repair procedure, and implementation of a comprehensive quality assurance program.
Equipment Background and Service Conditions
Hydrogenation reactors operate under severe conditions that demand robust corrosion protection. The typical operating conditions include temperatures of 350-450 °C, pressures of 15-30 MPa, and exposure to hydrogen gas, hydrogen sulfide, and other corrosive species. The overlay layer, typically a nickel-chromium-iron alloy such as Alloy 6 or Alloy 825, is applied to the circumferential weld between the reactor head and shell to provide resistance against hydrogen-induced cracking and sulfide stress corrosion.
| Operating Parameter | Typical Value |
|---|---|
| Operating temperature | 350-450 °C |
| Operating pressure | 15-30 MPa |
| Hydrogen partial pressure | 10-25 MPa |
| H2S concentration | 0.1-5% |
| Overlay alloy | Alloy 6 or Alloy 825 |
| Overlay thickness | 3-5 mm |
| Inspection interval | 5-10 years |
Crack Analysis and Mechanism
The network cracks observed in the overlay layer were identified as a combination of hydrogen-induced cracking (HIC) and thermal fatigue cracking. The hydrogen attack mechanism involves the dissociation of molecular hydrogen at high temperatures and pressures, with atomic hydrogen penetrating the overlay layer and accumulating at microstructural discontinuities such as grain boundaries, inclusions, and carbide interfaces. This accumulation leads to the formation of microvoids that coalesce into cracks, creating the characteristic network pattern.
The thermal fatigue component arises from the cyclic temperature variations during reactor start-up, shutdown, and load changes. The coefficient of thermal expansion mismatch between the overlay layer and the base metal creates cyclic thermal stresses at the interface, which initiate and propagate cracks over time. The network crack pattern indicates that the cracking was not localized to a single weld defect but was distributed throughout the overlay layer, suggesting a systemic mechanism rather than a local welding defect.
Repair Procedure Development
Repair Strategy
The repair strategy was developed using a systematic approach that considered the following factors:
- Complete removal of all cracked overlay material to eliminate crack initiation sites
- Preparation of a suitable welding groove for re-overlay
- Selection of a compatible overlay alloy with enhanced resistance to hydrogen attack
- Optimization of welding parameters to minimize dilution and residual stress
- Implementation of thorough NDT procedures to verify repair integrity
- Post-repair quality verification including hardness testing and dimensional inspection
Crack Removal and Surface Preparation
All cracked overlay material was removed using a combination of grinding and machining. The removal was extended beyond the visible crack boundaries to ensure complete elimination of the cracked zone. The surface was then prepared to a smooth finish with a Ra value below 3.2 μm to provide a clean substrate for the new overlay. Magnetic particle testing was performed after removal to confirm complete crack elimination.
Re-Overlay Welding Procedure
The re-overlay was performed using GTAW with a modified Alloy 6 or Alloy 825 consumable that contained additional titanium to improve resistance to hydrogen-induced cracking. The welding procedure was optimized to minimize heat input and residual stress:
| Parameter | Specification | Rationale |
|---|---|---|
| Welding process | GTAW with consumable insert | Precise heat control |
| Shielding gas | Argon (99.99%) | Complete protection |
| Wire diameter | 1.6 mm Alloy 6 or 825 | Fine bead control |
| Welding current | 80-120 A | Low heat input |
| Travel speed | 5-10 cm/min | Controlled dilution |
| Preheat temperature | 100-150 °C | Reduce thermal gradient |
| Interpass temperature | Below 150 °C | Limit thermal stress |
| Pass sequence | Center-out symmetric | Minimize residual stress |
| Overlay thickness | 3-5 mm (as designed) | Full corrosion protection |
Post-Weld Treatment and Quality Assurance
After overlay welding, the repaired area underwent post-weld heat treatment at 800-850 °C for 2 hours in a furnace to relieve residual stresses and refine the microstructure. The heat treatment temperature was carefully selected to avoid sensitization of the overlay alloy. Following heat treatment, the following inspections were performed:
- Visual inspection for surface defects
- Magnetic particle testing (MT) for surface and near-surface cracks
- Radiographic testing (RT) for subsurface defects
- Hardness testing to verify overlay composition and heat treatment response
- Dimensional inspection to confirm overlay thickness and coverage
Corrosion Mechanism Analysis
The paper provides a detailed analysis of the corrosion mechanisms at high temperature and pressure that contributed to the overlay failure. The primary mechanisms identified include:
- Hydrogen embrittlement: Atomic hydrogen penetrates the overlay and accumulates at grain boundaries, reducing ductility and initiating cracks
- Sulfide stress corrosion cracking: H2S in the process stream reacts with the overlay to form iron sulfide, which is less resistant to cracking
- Thermal fatigue: Cyclic temperature changes create thermal stresses that propagate existing microcracks
- Creep cracking: At elevated temperatures and high stresses, the overlay may experience creep damage over extended service periods
The interaction of these mechanisms creates a synergistic effect that accelerates crack initiation and propagation beyond what any single mechanism would cause independently.
Study Insights and Reflections
This case study provides valuable lessons for the repair of overlay failures in critical high-pressure vessels. The systematic approach to crack analysis, repair procedure development, and quality verification demonstrates the importance of understanding failure mechanisms before implementing repair solutions. The selection of an enhanced overlay alloy with titanium addition to improve hydrogen resistance is a proactive approach that addresses the root cause rather than simply replacing the failed material. Engineers working on similar hydrogenation reactor repairs should adopt the comprehensive repair procedure outlined in this study, with particular attention to the low heat input welding parameters, symmetric welding sequence, and thorough post-repair inspection protocol. The experience gained from this repair should be documented and incorporated into future maintenance planning for similar equipment to prevent recurrence.