Delamination and Cracking of Overlay Layer in Hydrocracking Reactor

Literature Overview

The article by Guo Jianhua, published in the journal Petroleum Chemical Technology and Economics in 2009, presents a detailed failure analysis of overlay layer delamination and cracking in a hydrocracking reactor at Sinopec Shanghai Petrochemical Company. This is a critical study for engineers in the petroleum refining industry, as hydrocracking reactors are among the most demanding pressure vessels in terms of operating conditions, material requirements, and safety implications. The paper analyzes the root causes of overlay failure and proposes preventive measures to ensure safe and reliable operation of this vital equipment.

Equipment Description and Operating Conditions

A hydrocracking reactor operates under the following severe conditions:

The reactor shell is typically fabricated from a low-alloy steel such as 2.25Cr-1Mo (SA-387 Gr. 22 Cl. 2 or equivalent), with a corrosion-resistant overlay applied to the internal surface to protect against hydrogen attack and sulfidation. The overlay is commonly a nickel-based alloy such as Alloy 625 (IN625) or Alloy 825, applied by submerged arc welding (SAW) or flux-cored arc welding (FCAW).

Overlay Layer Function

The overlay layer serves multiple functions:

1. Hydrogen embrittlement resistance: The nickel-based alloy has a much lower susceptibility to hydrogen embrittlement than the base steel
2. Sulfidation resistance: The overlay resists attack by H₂S and elemental sulfur present in the feed
3. Erosion-corrosion resistance: The overlay protects against the erosive action of high-velocity feed and catalyst particles
4. Thermal barrier: The overlay reduces the temperature gradient at the surface, reducing thermal stress

Failure Modes and Root Cause Analysis

The paper identifies two primary failure modes: delamination and cracking. Each is analyzed in detail below.

Delamination

Delamination is the separation of the overlay layer from the base metal at the fusion boundary. This failure mode is driven by:

• Residual stress: The overlay welding process introduces significant residual tensile stresses in the overlay and at the interface. These stresses can reach 150–250 MPa, which when combined with operating stresses, can exceed the bond strength of the interface.
• Thermal cycling: Repeated heating and cooling during reactor startup, shutdown, and load changes causes differential expansion and contraction between the overlay and base metal, leading to fatigue cracking at the interface.
• Hydrogen ingress: Atomic hydrogen from the operating environment can diffuse through the overlay and accumulate at the interface, causing hydrogen-assisted cracking and eventual delamination.
• Corrosion under the overlay: If the overlay is porous or has micro-cracks, corrosive media can penetrate to the interface, causing underlay corrosion that weakens the bond.

Cracking

Cracking within the overlay layer itself is caused by:

• Thermal stress cracking: Rapid temperature changes during startup or emergency shutdown can generate thermal stresses that exceed the yield strength of the overlay.
• Hydrogen-induced cracking (HIC): Atomic hydrogen diffuses into the overlay and accumulates at inclusions, grain boundaries, or phase boundaries, causing internal cracking.
• Stress corrosion cracking (SCC): The combination of tensile stress and a corrosive environment (H₂S + water) can cause intergranular or transgranular cracking in the overlay.
• Fatigue cracking: Cyclic loading from pressure fluctuations and thermal cycling can cause fatigue crack initiation and propagation in the overlay.

Root Cause Summary

Inspection and Monitoring

The paper describes the inspection methods used to detect and assess the overlay defects:

• Eddy current testing (ECT): Used to detect delamination at the overlay-base metal interface. ECT is particularly effective for this application because it can detect small separations without requiring coupling fluid.
• Ultrasonic testing (UT): Used to measure overlay thickness and detect internal cracks. Time-of-flight diffraction (TOFD) and phased array UT (PAUT) are preferred methods for this application.
• Visual inspection (VT): Conducted during reactor shutdown to identify surface cracks, spalling, and thinning.
• Penetrant testing (PT): Used to detect surface cracks that are visible to the naked eye.
• Metallographic examination: Conducted on samples taken from the reactor to examine the microstructure of the overlay, HAZ, and interface region.

Preventive Measures and Recommendations

Based on the failure analysis, the paper proposes the following preventive measures:

1. Improve overlay welding procedure: Optimize the welding parameters to minimize residual stress. Use a multi-pass welding sequence with alternating directions to balance the thermal input. Apply a buffer layer of pure nickel between the base metal and the corrosion-resistant overlay to reduce dilution and improve bond strength.
2. Post-weld stress relief: Perform a complete stress relief heat treatment after overlay welding. For 2.25Cr-1Mo steel with a nickel-based overlay, the stress relief temperature should be in the range of 550–620°C, held for a sufficient time to allow stress relaxation without over-tempering the base metal.
3. Control hydrogen ingress: Ensure that the overlay is free of porosity and micro-cracks that could allow hydrogen to penetrate to the interface. Use a high-quality shielding gas and clean the base metal surface thoroughly before welding.
4. Implement a regular inspection program: Conduct eddy current and ultrasonic testing at scheduled intervals (typically every 3–5 years, or at each turnaround) to monitor the condition of the overlay.
5. Manage operating parameters: Avoid rapid temperature changes during startup and shutdown. Maintain the hydrogen partial pressure within the recommended range to minimize hydrogen attack while ensuring catalyst activity.
6. Consider overlay replacement: If delamination or cracking is detected, assess the extent of the damage and determine whether repair welding or complete overlay replacement is required.

Engineering Practice Integration

This case study has significant implications for the design, fabrication, and maintenance of pressure vessels in the petroleum and chemical industries. The following points are particularly relevant:

• Overlay welding is not a permanent solution. The overlay layer is subject to degradation over time, and its condition must be monitored throughout the service life of the vessel.
• The interface between the overlay and base metal is the weakest link. Most overlay failures initiate at the interface, and the bond strength of this region is critical to the long-term performance of the overlay.
• Residual stress management is essential. The residual stresses introduced during overlay welding can be as detrimental as the operating stresses, and must be addressed through proper welding procedure design and post-weld heat treatment.
• Hydrogen is a pervasive threat in hydrocracking service. Hydrogen attack, hydrogen embrittlement, and hydrogen-induced cracking are all mechanisms that can degrade the overlay and base metal, and must be considered in the material selection and inspection planning.

Reflections and Recommendations

The paper provides a thorough analysis of a real-world failure case and offers practical recommendations for preventing similar failures. One area that could be further explored is the use of advanced coating technologies such as thermal spray (HVOF) or plasma spray as an alternative to fusion welding for applying corrosion-resistant overlays. These techniques can produce overlays with lower residual stress and better control of the microstructure. Additionally, the development of in-situ monitoring systems that can detect overlay delamination during operation would provide early warning of impending failure, allowing for planned maintenance rather than emergency shutdown.