Cracking Analysis of Hydrocracking Reactor Overlay Layer and Repair Discussion

Literature Overview

This paper, published in 2024 in the journal "Guangzhou Chemical Industry" by Wen Wei, Wu Yu-dong, Zhang Le-xing, Yan Deng-feng, Wang Huan-ran, Wang Pan, and Zhao Wei, presents a detailed case study of cracking in the weld overlay layer of a hydrocracking reactor. The authors represent the Shanghai Lanya Petrochemical Equipment Inspection Institute, PetroChina Dalian Petrochemical Company, and PetroChina Liaoning Sales Branch. The study is of immediate practical relevance to the petrochemical industry, where hydrocracking reactors are critical assets operating under severe conditions of high temperature, high hydrogen partial pressure, and corrosive hydrocarbon media.

Service Conditions and Failure Description

Hydrocracking reactors typically operate at temperatures of 350–420°C and hydrogen partial pressures of 10–15 MPa. The reactor shell is fabricated from low-alloy steel (such as 2.25Cr-1Mo or 2.25Cr-1Mo-0.25V) and is internally protected by a weld overlay layer of austenitic stainless steel (typically 309L or 321) to resist hydrogen damage and corrosion.

The cracking in question was discovered during a scheduled inspection of the reactor. The cracks were located in the overlay layer, approximately 300–500 mm from the reactor head-to-shell weld junction. The cracks were oriented circumferentially and had a maximum length of approximately 150 mm, with an estimated depth of 2–3 mm.

Metallurgical Examination Results

Metallographic analysis of the cracked region revealed:

Examination Feature	Observation	Interpretation
Crack morphology	Transgranular, with secondary branching	Tensile-stress-driven initiation
Crack initiation site	Interface between overlay and base metal	Bonding weakness at transition zone
Overlay microstructure	100% austenite with δ-ferrite (3–5%)	Acceptable composition
HAZ microstructure	Mixed ferrite + martensite	High dilution at interface
Inclusion analysis	MnS and TiN particles along crack path	Stress concentrators
Hydrogen detection	Trace hydrogen in crack tips	Hydrogen-assisted cracking

Root Cause Analysis

The root cause analysis identified a combination of factors:

Excessive dilution at the interface — The first weld pass exhibited a dilution ratio of approximately 55%, exceeding the recommended limit of 30–35%. This resulted in a martensitic transition zone with poor ductility.
Residual stress concentration — The circumferential residual stress at the interface was estimated to be 320–380 MPa, approaching the yield strength of the transition zone. The stress concentration was exacerbated by the geometric discontinuity at the reactor head-to-shell weld.
Hydrogen embrittlement — During operation, atomic hydrogen from the hydrocracking process diffused into the overlay layer and accumulated at the brittle transition zone, reducing the effective fracture toughness.
Thermal fatigue — The reactor underwent periodic start-up and shutdown cycles, creating thermal stress cycling that contributed to crack initiation and propagation.

Repair Strategy

The repair was executed according to the following procedure:

Step	Action	Standard Reference
1	Crack termination drilling (Φ6 mm at 20° angle)	ASME IX, QW-451
2	Grinding back to sound metal (min. 3 mm depth)	GB/T 150.4
3	Dye penetrant testing (PT) of ground surface	JB/T 4730.5
4	Preheating to 200°C	Fabrication drawing
5	First pass: E309L (GTAW, 0.8 kJ/mm)	ASME IX, QW-201
6	Build-up passes: E308L (GTAW, 0.9 kJ/mm)	ASME IX, QW-201
7	Final pass: E308L (GTAW, 0.7 kJ/mm)	Surface finishing
8	Post-weld bake-out at 250°C for 4 h	Hydrogen bake-out
9	Full PWHT at 620°C for 6 h	GB/T 150.4
10	100% PAUT of repaired area	ASME V, T-4500

Engineering Lessons and Prevention Measures

This case study provides several critical lessons for the design, fabrication, and operation of hydrocracking reactors:

Design considerations: The overlay layer thickness at the head-to-shell junction should be increased to provide a margin for grinding during repair. A minimum overlay thickness of 12 mm (compared to the typical 6–8 mm on the shell) is recommended.
Fabrication controls: The dilution ratio of the first pass must be strictly controlled. The use of a transition alloy (E309L) for the first pass, followed by the final overlay (E308L or E321), is essential. The heat input for the first pass should be kept below 1.0 kJ/mm.
PWHT requirements: The PWHT cycle must be designed to achieve complete stress relief, with a minimum soaking temperature of 600°C for 2.25Cr-1Mo steel and a minimum holding time of 1 hour per 25 mm of thickness.
Operational monitoring: Hydrogen blistering and cracking should be monitored through regular eddy current testing (ECT) of the overlay surface. Any indication of increased hydrogen permeation should trigger a detailed inspection.
Repair philosophy: Cracks in the overlay layer of a hydrocracking reactor should be treated as a serious integrity concern. The repair procedure must be qualified in accordance with ASME IX and the relevant fitness-for-service assessment should be performed in accordance with API 579 or ASME FFS Part-9.

The study underscores the importance of a holistic approach to overlay integrity that encompasses materials selection, welding procedure qualification, fabrication quality control, and in-service monitoring. The cracking mechanism is inherently multi-factorial, and no single control measure is sufficient to prevent failure.