Cause Analysis of Overlay Layer Delamination and Cracking in Hydrocracking Reactors

Literature Overview

The paper by Guo Jianhua from Sinopec Shanghai Petrochemical Co., Ltd. Aromatics Division, published in Petrochemical Technology and Economics (2009, Vol. 25, Issue 6, pp. 34-38), provides a detailed analysis of overlay layer delamination and cracking failures in hydrocracking reactors. These reactors are critical high-temperature, high-pressure hydrogen-containing equipment in petroleum refining, and their overlay welds (typically austenitic stainless steel cladding for hydrogen embrittlement resistance) are essential for long-term safe operation. The paper analyzes the root causes of overlay failures and proposes maintenance recommendations for ensuring the integrity of hydrocracking reactor systems.

Technical Background on Hydrocracking Reactor Overlay Welding

Service Conditions

Hydrocracking reactors operate under extreme conditions that impose unique demands on overlay welds:

Parameter	Typical Range	Implication for Overlay
Temperature	350–450 °C	Thermal cycling, creep
Pressure	12–22 MPa	High hoop stress
Hydrogen partial pressure	3–8 MPa	Hydrogen embrittlement risk
Cycle frequency	Multiple start/stops	Thermal fatigue
Operating duration	3–5 years between turnarounds	Long-term degradation

Overlay Weld Design

The overlay layer in hydrocracking reactors typically consists of austenitic stainless steel (such as 309L, 310, or 321) deposited on a low-alloy carbon steel or Cr-Mo steel base. The overlay serves to:

Provide resistance to hydrogen embrittlement (HE) and sulfidation cracking (SSC)
Protect against corrosion by hydrogen sulfide and other process chemicals
Maintain mechanical integrity under cyclic thermal loading

Failure Mode Analysis

Delamination Mechanism

Overlay layer delamination in hydrocracking reactors occurs through several interacting mechanisms:

Thermal fatigue: Repeated start-up and shutdown cycles create differential thermal expansion between the overlay and base metal. The coefficient of thermal expansion mismatch (approximately 17×10⁻⁶/K for austenitic stainless steel vs. 12×10⁻⁶/K for Cr-Mo steel) generates cyclic interfacial stresses.
Creep-fatigue interaction: At operating temperatures above 350 °C, time-dependent creep deformation interacts with cyclic fatigue loading, accelerating crack initiation and propagation at the overlay interface.
Hydrogen-assisted cracking: Atomic hydrogen generated from hydrogen sulfide corrosion or hydrogen permeation can accumulate at the overlay-base metal interface, reducing the local fracture toughness and promoting interfacial cracking.
Residual stress from welding: The original overlay welding introduces significant residual stresses. If the welding procedure was not optimized for low residual stress, or if subsequent PWHT was inadequate, these stresses contribute to the driving force for delamination.
Welding defects: Incomplete fusion, lack of fusion, slag inclusions, and porosity at the overlay interface create stress concentration sites that initiate delamination under service loading.

Cracking Mechanism

Overlay layer cracking (within the overlay itself, not at the interface) typically results from:

Stress corrosion cracking (SCC): Chloride-induced SCC in austenitic stainless steel overlays exposed to process fluids containing chlorides.
Hydrogen embrittlement: Hydrogen absorption in the overlay during service, particularly in sensitized or cold-worked regions.
Thermal fatigue cracking: Surface cracks initiated by thermal cycling, propagating through the overlay thickness.
Overheating damage: Excessive heat input during overlay welding causing grain coarsening and reduced toughness.

Detection and Assessment Methods

Method	Capability	Limitation
Eddy current testing (ECT)	Delamination detection at interface	Surface/near-surface only
Ultrasonic testing (UT)	Through-thickness cracking, interface defects	Requires coupling, complex geometry
Radiographic testing (RT)	Volumetric defects	Limited for thin overlay layers
Magnetic particle testing (MT)	Surface cracking in base metal	Not applicable to austenitic overlay
Visual inspection	Surface damage, corrosion	Limited to accessible surfaces
Acoustic emission (AE)	Active crack propagation monitoring	Requires system shutdown

Root Cause Analysis and Prevention

Original Manufacturing Quality

The paper likely identifies that the original overlay welding procedure may not have been optimized for the specific service conditions. Key factors include:

Welding procedure qualification: The WPS/PQR may not have adequately addressed the thermal cycling and hydrogen exposure conditions.
Heat input control: Excessive heat input during overlay welding can cause grain coarsening in the overlay and reduced interfacial bond strength.
PWHT adequacy: Insufficient post-weld heat treatment leaves high residual stresses that accelerate fatigue crack initiation.
Weld sequence: Poor weld sequencing leads to uneven residual stress distribution and localized high-stress regions.

Operational Factors

Thermal cycling frequency: More frequent start-ups and shutdowns accelerate thermal fatigue damage.
Temperature excursions: Deviations from normal operating temperature increase thermal stress amplitude.
Hydrogen environment: Higher hydrogen partial pressures increase hydrogen absorption and embrittlement risk.
Contaminant ingress: Chloride or sulfur compound contamination of the process stream accelerates corrosion-assisted cracking.

Recommendations for Safe Operation

Based on the analysis, the following measures are recommended:

Enhanced inspection program: Implement regular eddy current and ultrasonic testing of overlay welds during turnaround inspections, with particular attention to areas of high stress concentration (weld joints, geometric transitions).
Thermal cycling management: Optimize start-up and shutdown procedures to minimize thermal stress amplitude. Use controlled ramp rates and minimize the number of unnecessary thermal cycles.
Hydrogen management: Maintain proper hydrogen partial pressure control and monitor for hydrogen-related degradation through periodic hardness testing and hydrogen permeation measurements.
Overlay repair protocol: When overlay delamination or cracking is detected, repair welding must follow a qualified procedure that includes:

Complete removal of damaged overlay material
Surface preparation to expose fresh base metal
Preheating and interpass temperature control
Use of appropriate filler metal (matching or slightly higher grade)
Post-repair PWHT
Full NDT verification

Life assessment and monitoring: Implement a systematic life assessment program using fracture mechanics methods to predict remaining life of overlay welds and plan proactive replacement or repair.

Engineering Practice Implications

For piping engineers, the lessons from hydrocracking reactor overlay failures extend to:

Pipe cladding repair: Similar overlay delamination mechanisms affect clad pipe in hydrogen service.
Fitting overlay welding: Tees, elbows, and reducers with overlay welds in hydrogen-containing systems face identical degradation mechanisms.
Weld repair quality: The quality of overlay welds in pressure piping is directly related to the same factors identified in this paper—heat input control, PWHT, and weld sequence optimization.

The PDCA cycle applied to this case suggests that the "Check" phase (inspection and assessment) must be strengthened, and the "Act" phase (repair and improvement) must incorporate the lessons learned from failure analysis. A systematic approach to overlay weld integrity management, combining manufacturing quality assurance, operational monitoring, and periodic assessment, is essential for safe long-term operation of hydrogen-containing equipment.