Overlay Layer Spallation and Cracking in Hydrocracking Reactor — Root Cause Analysis and Mitigation

Literature Overview

Guo Jianhua's paper, published in Petrochemical Technology and Economics (2009, Vol. 25, No. 6, pp. 34–38), provides a detailed root cause analysis of overlay layer spallation and cracking defects observed in a hydrocracking reactor during long-term operation. Hydrocracking reactors are among the most demanding pieces of equipment in petrochemical plants, operating under high temperature, high hydrogen partial pressure, and high total pressure conditions. The overlay weld layer, typically a corrosion-resistant alloy deposited on the carbon steel or low-alloy steel base, serves as the primary barrier against hydrogen damage and sulfide stress cracking. When this overlay fails, the consequences can be catastrophic.

Defect Description and Classification

The paper identifies two primary defect modes:

1. Spallation (delamination) — separation of the overlay weld layer from the base metal at the weld fusion line, resulting in loss of corrosion protection.
2. Cracking — both transverse and longitudinal cracks within the overlay layer and at the overlay-base metal interface.

Root Cause Analysis

Using a systematic 5W2H approach, the root causes can be categorized as follows:

What (Defect Mechanism):

• Thermal mismatch between the overlay alloy and the base metal generates high residual stresses during cooling after welding.
• Hydrogen from the process gas diffuses into the weld zone and base metal, causing hydrogen embrittlement and blister formation at the fusion line.
• Sulfide stress cracking (SSC) initiates at grain boundaries in the overlay weld and heat-affected zone (HAZ) under the combined action of residual stress and sulfide-containing process fluids.

Why (Contributing Factors):

• Inadequate post-weld heat treatment (PWHT) temperature or duration, leaving high residual stresses in the overlay weld.
• Excessive welding heat input leading to coarse grain growth in the HAZ and overlay layer.
• Insufficient preheating during welding, promoting martensitic transformation in the HAZ.
• Long-term cyclic thermal loading during reactor startup and shutdown operations.

How (Detection and Evaluation):

• Ultrasonic testing (UT) is the primary method for detecting spallation at the fusion line.
• Magnetic particle testing (MT) can detect surface and near-surface cracks in the overlay layer.
• Radiographic testing (RT) provides evidence of volumetric defects but is less sensitive for planar delaminations.

Mitigation Strategies and Recommendations

Based on the root cause analysis, the following engineering recommendations emerge:

1. Weld procedure optimization: Reduce welding heat input by using multi-pass techniques with smaller filler wire diameters. Preheat the base metal to at least 200°C for carbon steel substrates and 150°C for low-alloy steel substrates to minimize cooling rates.
2. PWHT enhancement: Apply PWHT at a temperature of at least 580°C for a minimum of 2 hours per 25 mm of thickness (following ASME Section VIII Division 1 requirements), with adequate soaking time for stress relief.
3. Overlay alloy selection: Select overlay alloys with lower carbon equivalent (CE) to minimize hardness and susceptibility to hydrogen cracking. Consider multi-layer overlay designs with a transition layer to reduce thermal mismatch.
4. In-service monitoring: Implement regular UT scanning of the overlay layer at critical locations, particularly at weld ends, start/stop points, and areas of high thermal gradient.
5. Process condition control: Maintain hydrogen partial pressure and temperature within design limits, and monitor for any process excursions that could accelerate hydrogen damage.

Engineering Practice Case

In a typical hydrocracking reactor repair scenario, a spallation defect of 120 mm × 8 mm was detected at a longitudinal seam weld overlay layer during a scheduled turnaround. The repair involved complete removal of the overlay weld, base metal HAZ grinding to a depth of 2 mm below the original weld toe, application of a new multi-layer overlay weld with reduced heat input (1.5 kJ/mm), and PWHT at 600°C for 6 hours. Post-repair UT inspection confirmed full fusion and no defects. This case underscores the importance of thorough defect removal and careful weld procedure control during repair operations.

Study Insights and Implications

This paper serves as an important case study for engineers working on high-pressure hydrogen-containing equipment. The root cause analysis methodology employed is directly applicable to similar overlay weld failures in other petrochemical and refinery applications, including hydrotreaters, reformers, and hydrogen compressors. The key takeaway is that overlay weld integrity in hydrogen service is a multifactorial problem requiring integrated control of welding procedures, post-weld treatment, materials selection, and operational parameters. A failure in any one of these areas can lead to overlay layer degradation and potential catastrophic loss of containment.