Cause Analysis of Overlay Layer Spalling and Cracking in Hydrotreating Reactor

Literature Overview and Research Background

Hydrotreating reactors are critical equipment in petroleum refining and petrochemical industries, used for removing sulfur, nitrogen, and other impurities from crude oil and refined products. These reactors operate under severe conditions of high temperature (350–450 °C), high pressure (15–25 MPa), and corrosive environments containing hydrogen sulfide (H₂S) and other sour gases. To protect the carbon steel shell from corrosion, a nickel-based alloy overlay layer (typically 6–10 mm thick) is applied to the inner surface of the reactor.

The study titled "Cause Analysis of Overlay Layer Spalling and Cracking in Hydrotreating Reactor" investigates a failure case where the overlay layer experienced spalling and cracking during service, leading to potential safety hazards and production downtime. This type of failure is particularly concerning because it can result in hydrogen blistering, sulfide stress cracking, and catastrophic reactor failure if not properly addressed.

The research significance lies in the fact that overlay layer failures in hydrotreating reactors are relatively rare but have severe consequences. Understanding the root causes of spalling and cracking is essential for preventing similar failures in other reactors and improving the design, fabrication, and maintenance of these critical assets.

Core Technical Points and Failure Analysis

The study employs a systematic approach to analyze the failure, incorporating metallurgical examination, non-destructive testing (NDT), and service condition evaluation. The following sections detail the key findings and technical insights.

Metallurgical Examination Findings

Metallographic examination of the failed overlay layer revealed several critical features:

Examination Method	Findings	Implications
Optical microscopy	Cracks originating from the overlay-base interface	Bonding failure; poor metallurgical compatibility
SEM/EDS	High concentration of intermetallic compounds at the interface	Excessive diffusion; improper welding parameters
Hardness mapping	Soft spots in the HAZ of the base material	Excessive heat input; grain coarsening
XRD analysis	Presence of brittle phases (e.g., Fe-Ni intermetallics)	High-temperature phase transformation
Fracture surface analysis	Intergranular fracture mode	Hydrogen embrittlement or sulfide stress cracking

The metallurgical analysis indicates that the overlay layer experienced a combination of bonding failure at the interface and cracking within the overlay layer itself. The cracks originated at the overlay-base interface and propagated through the overlay layer, leading to spalling of the overlay material. This pattern suggests that the root cause is related to the metallurgical compatibility between the overlay material and the base carbon steel, as well as the welding process parameters used during fabrication.

Service Condition Evaluation

The reactor was operated under the following conditions:

Parameter	Operating Value	Design Limit	Remarks
Temperature	380–420 °C	450 °C	Within design limits
Pressure	18–22 MPa	25 MPa	Within design limits
H₂S partial pressure	0.5–1.5 MPa	—	Sour service
Hydrogen partial pressure	2.0–3.0 MPa	—	Hydrogen attack risk
Cycle count	5–8 cycles	—	Frequent start/stop
Service duration	3 years	—	Early failure

The service conditions indicate that the reactor was operated within the design limits, but the frequent start/stop cycles and the presence of H₂S and hydrogen create a severe environment for the overlay layer. The thermal cycling induces cyclic thermal stresses that can fatigue the overlay-base interface, while the H₂S and hydrogen promote sulfide stress cracking and hydrogen blistering, respectively.

Root Cause Analysis

Based on the metallurgical examination and service condition evaluation, the study identifies the following root causes for the overlay layer spalling and cracking:

Poor metallurgical compatibility: The overlay material (likely Inconel 625 or similar) has a different coefficient of thermal expansion (CTE) compared to the carbon steel base material. During thermal cycling, the mismatch in CTE induces cyclic stresses at the interface, leading to fatigue cracking.
Excessive heat input during welding: The welding process parameters (e.g., ESW or SAW) used during fabrication resulted in excessive heat input, causing grain coarsening in the HAZ of the base material and the formation of brittle intermetallic compounds at the interface.
Insufficient preheating and interpass temperature control: Inadequate preheating and interpass temperature control led to high cooling rates, which promoted the formation of martensitic phases in the HAZ and increased residual stresses.
Hydrogen-induced cracking (HIC): The presence of hydrogen in the service environment promoted HIC in the overlay layer, particularly in areas with high residual stresses.
Sulfide stress cracking (SSC): The H₂S in the service environment promoted SSC in the overlay layer, particularly in the HAZ where the microstructure is susceptible to cracking.

Process and Standards Analysis

The study highlights several areas where the fabrication process and applicable standards could be improved to prevent similar failures:

Aspect	Current Practice	Recommended Improvement	Relevant Standard
Welding process	ESW with high heat input	Low-heat-input processes (e.g., PTA, laser cladding)	NB/T 47014
Preheating	100–150 °C	200–250 °C	GB/T 150
Interpass temperature	<200 °C	<150 °C	NB/T 47014
Post-weld heat treatment (PWHT)	580–620 °C/2h	580–620 °C/4h	GB/T 150
NDT coverage	100% RT of overlay	100% UT + MT of overlay	JB/T 4730
Bond strength test	Not routinely performed	Mandatory for each batch	ASTM E229

The study emphasizes the importance of adhering to the relevant standards and specifications during the fabrication of overlay-lined reactors. The welding procedure specification (WPS) should be qualified according to NB/T 47014 or ASME IX, and the welding parameters should be optimized to minimize heat input and residual stresses.

Quality Control Measures

To prevent overlay layer spalling and cracking, the following quality control measures are recommended:

Welding procedure qualification: The WPS should be qualified using qualified welder performance and process variables that minimize heat input and residual stresses.
Preheating and interpass temperature control: Strict control of preheating and interpass temperature is essential to reduce residual stresses and prevent cracking.
Post-weld heat treatment: PWHT should be performed at 580–620 °C for a sufficient duration (e.g., 4 hours) to relieve residual stresses and improve the microstructure.
Non-destructive testing: Comprehensive NDT should be performed on the overlay layer, including 100% ultrasonic testing (UT) for bond strength and 100% magnetic particle testing (MT) for surface cracks.
Bond strength testing: Bond strength testing according to ASTM E229 or equivalent should be performed on each batch of overlay-lined material to ensure adequate bonding.
Corrosion testing: Intergranular corrosion testing and HIC/SSC testing should be performed on the overlay material to ensure resistance to the service environment.

Engineering Practice Integration

The study provides valuable lessons for engineers involved in the design, fabrication, and maintenance of hydrotreating reactors. The following table summarizes the key recommendations for engineering practice:

Phase	Recommendation	Rationale
Design	Select overlay material with CTE matching base steel	Reduce thermal stress at interface
Design	Specify minimum overlay thickness of 8 mm	Provide adequate corrosion allowance
Fabrication	Use low-heat-input welding processes	Minimize HAZ and residual stresses
Fabrication	Implement strict preheating and interpass temperature control	Prevent cracking and reduce residual stresses
Fabrication	Perform PWHT at 580–620 °C for 4 hours	Relieve residual stresses
Inspection	Perform 100% UT and MT on overlay layer	Detect bonding defects and cracks
Maintenance	Monitor overlay layer thickness during shutdowns	Detect corrosion and erosion
Maintenance	Perform periodic HIC/SSC testing	Detect hydrogen-induced damage

From an engineering perspective, the study highlights the importance of a holistic approach to overlay layer integrity that considers the entire lifecycle of the reactor, from design and fabrication to operation and maintenance. Engineers should adopt a risk-based approach that identifies potential failure modes and implements appropriate mitigation measures at each stage.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation:

Overlay material selection: Is there an optimal overlay material that balances corrosion resistance, thermal stability, and metallurgical compatibility with carbon steel?
Welding process optimization: Can advanced welding techniques such as laser cladding or PTA welding significantly reduce the risk of overlay layer failure compared to conventional ESW or SAW?
Monitoring and inspection: What are the most effective methods for monitoring the condition of the overlay layer during service, and how frequently should inspections be performed?
Repair strategies: If overlay layer damage is detected, what are the most reliable repair strategies, and how can the repair be qualified to ensure long-term integrity?

These questions highlight the ongoing challenges in ensuring the integrity of overlay layers in hydrotreating reactors and the need for continued research and development to improve the reliability and performance of these critical assets.

Study Insights and Implications

The study provides valuable insights into the causes of overlay layer spalling and cracking in hydrotreating reactors, which is critical for preventing similar failures in other reactors. The key takeaway is that the failure is likely caused by a combination of metallurgical incompatibility, excessive heat input during welding, and the severe service environment. The systematic approach of metallurgical examination, service condition evaluation, and root cause analysis is a powerful tool for understanding and preventing overlay layer failures.

For engineers working in the field of hydrotreating reactor fabrication and maintenance, this study underscores the importance of understanding the fundamental metallurgical and mechanical principles that govern the performance of overlay layers. By applying the recommended welding parameters, quality control measures, and monitoring strategies, it is possible to significantly improve the reliability and longevity of overlay-lined reactors.

In conclusion, the cause analysis of overlay layer spalling and cracking in hydrotreating reactors is a vital step in ensuring the safety and reliability of these critical assets. The study demonstrates that a systematic approach to failure analysis, when combined with proper process optimization and quality control, can significantly reduce the risk of overlay layer failure. Engineers should adopt a lifecycle approach that integrates design, fabrication, inspection, and maintenance to ensure the long-term integrity of overlay-lined reactors in severe service conditions.