Cracking Analysis and Repair of Overlay Layers in Hydrocracking Reactor Internal Walls

Literature Overview

This paper by Wen Wei et al. (2024), published in Guangzhou Chemical Industry, presents a detailed analysis of cracking defects found in the internal wall overlay layer of a hydrocracking reactor at PetroChina Dalian Petrochemical Company. The study was conducted by the Shanghai Lanya Petrochemical Equipment Inspection Institute. The work documents the discovery of network-like cracking in the stainless steel overlay layer, identifies the root causes through comprehensive non-destructive and destructive testing, and proposes targeted repair procedures. This case study is particularly significant given the critical nature of hydrocracking reactors in petroleum refining operations and the severe consequences of overlay failure.

Core Technical Content

Defect Description

The inspection revealed two defects in the overlay layer at the lower section of the first cold hydrogen ring (cold hydrogen distribution ring):

The defects were initially detected as paste-like anomalies during visual inspection. Upon grinding and cleaning, the defects revealed a network-like crack pattern, indicating extensive cracking within the overlay layer.

Inspection and Analysis Methods

A comprehensive suite of inspection and analysis methods was employed to characterize the defects and determine their root causes:

Root Cause Analysis

The investigation identified three primary contributing factors to the cracking:

1. Insufficient ferrite content in the austenitic stainless steel overlay: The overlay layer, intended to be a duplex or austenitic stainless steel, had a ferrite content below the required minimum. In austenitic stainless steel overlays, a small amount of delta ferrite (typically 3-10%) is beneficial as it improves hot cracking resistance, reduces distortion, and provides strain-induced martensite transformation that can absorb energy. Insufficient ferrite leads to reduced hot cracking resistance and increased susceptibility to cracking under thermal and mechanical stresses.
2. Hydrogen accumulation: The hydrocracking reactor operates under high hydrogen pressure, and hydrogen can accumulate in the weld metal and heat-affected zone. Hydrogen accumulation can lead to hydrogen embrittlement, reducing the ductility of the material and promoting cracking. The combination of hydrogen embrittlement and insufficient ferrite creates a particularly unfavorable condition for crack formation.
3. Thermal stress superposition: The reactor experiences significant thermal cycling during operation, with the overlay layer subjected to repeated heating and cooling. The thermal stresses generated during these cycles can be superimposed on the residual stresses from the welding process, creating a stress state that exceeds the material's resistance to cracking.

Interpretation of Technical Points

Ferrite Content and Cracking Susceptibility

The ferrite content in austenitic stainless steel overlays is a critical parameter that directly affects cracking susceptibility. The Schaeffler diagram and the DeLong diagram are commonly used to predict the ferrite content based on the chemical composition of the weld metal. For a typical 309L or 310L overlay, the expected ferrite content is 3-15% (in magnetic permeability units, FM).