Hydrogen-Induced Delamination at the Fusion Zone of Austenitic Stainless Steel Overlay on 2.25Cr-1Mo Steel

Literature Overview

Published in 2009 in the journal Welding by researchers from Fushun Chemical Machinery Equipment Manufacturing Co. and Liaoning Petrochemical University, this paper investigates the delamination of the fusion zone in austenitic stainless steel overlay layers applied to 2.25Cr-1Mo steel. The study was motivated by ultrasonic testing findings of delamination cracks in a hot-wall hydrotreating reactor. The authors used dual-wire submerged arc overlay welding to build up austenitic stainless steel on 2.25Cr-1Mo test plates, then subjected the specimens to simulated reactor operating conditions to reproduce and analyze the delamination phenomenon.

Background: Hydrotreating Reactor Service

Hot-wall hydrotreating reactors operate at elevated temperatures (typically 350-450°C) and pressures (15-30 MPa) in hydrogen-rich environments. The inner surface of the reactor vessel is exposed to high-temperature hydrogen attack (HTHA), which causes decarburization and microcracking of the low-alloy steel shell. To protect against HTHA, the interior is lined with austenitic stainless steel overlay layers (typically 309 or 310 type) that provide a hydrogen diffusion barrier and corrosion resistance.

The fusion zone between the austenitic overlay and the 2.25Cr-1Mo base metal is the most vulnerable region in this composite structure. It is subject to a complex combination of stresses: thermal mismatch during reactor start-up and shutdown, hydrogen ingress from the process side, and residual stresses from the overlay welding process itself.

Delamination Mechanism

The authors identified hydrogen-induced delamination as the primary failure mechanism. The process can be described as follows:

1. Hydrogen ingress: Atomic hydrogen from the hydrotreating process diffuses through the austenitic overlay layer. While austenitic stainless steel is relatively resistant to HTHA compared to low-alloy steels, it is not impervious to hydrogen diffusion.
2. Hydrogen accumulation: At the fusion zone, hydrogen atoms accumulate at microstructural features such as grain boundaries, carbide-matrix interfaces, and microvoids. The fusion zone microstructure typically contains a mixture of austenite, ferrite, and carbides, creating numerous hydrogen traps.
3. Hydrogen embrittlement: The accumulated hydrogen reduces the cohesive strength of the fusion zone, promoting crack initiation and propagation.
4. Delamination: Under the combined action of hydrogen embrittlement and thermal/mechanical stresses, the fusion zone undergoes intergranular or transgranular cracking, leading to delamination.

Welding Process Optimization

The authors proposed optimized welding parameters for the dual-wire submerged arc overlay welding process. The key parameters include:

• Dual-wire configuration: Using two wires simultaneously increases deposition rate and allows for better control of the dilution ratio. The dilution ratio is critical because it determines the composition of the fusion zone—a higher dilution with the 2.25Cr-1Mo base metal reduces the austenite fraction and increases susceptibility to HTHA.
• Heat input control: Lower heat input reduces the width of the heat-affected zone and minimizes the formation of coarse grain structures that are susceptible to hydrogen cracking.
• Interpass temperature: Controlled interpass temperatures prevent excessive grain growth and manage residual stress buildup.
• Post-weld stress relief: A stress relief heat treatment at 620-650°C for 2-4 hours is typically applied to reduce residual stresses, but this must be balanced against the risk of temper embrittlement in the 2.25Cr-1Mo base metal.

Non-Destructive Testing and Examination

The study employed a combination of non-destructive testing (NDT) methods to characterize the delamination:

• Ultrasonic testing (UT): The primary method for detecting subsurface delamination. The acoustic impedance mismatch at the fusion zone/delamination interface produces characteristic echoes.
• Radiographic testing (RT): Used to verify the extent and morphology of delamination cracks.
• Mechanical testing: Tensile and hardness tests on extracted specimens to assess the mechanical properties of the fusion zone under different operating conditions.
• Metallographic examination: Optical and scanning electron microscopy to identify the crack path, microstructural features, and hydrogen-related damage mechanisms.

Engineering Practice Implications

This study has direct implications for the design, fabrication, and inspection of hydrotreating reactors. Several recommendations emerge:

1. Welding procedure qualification: Overlay welding procedures must be qualified with attention to dilution ratio control, fusion zone composition, and post-weld heat treatment parameters.
2. Inspection intervals: UT inspection of the overlay fusion zone should be performed at regular intervals during reactor operation, with particular attention to areas subject to thermal cycling.
3. Operating condition monitoring: Hydrogen pressure and temperature should be monitored to prevent excursions beyond the design envelope.
4. Repair procedures: Any delamination found must be addressed through a qualified repair procedure that includes grinding back to sound material, re-overlay, and re-inspection.

Key Reflections

The hydrogen-induced delamination mechanism identified in this study is particularly insidious because it can progress without obvious external indicators. Unlike external corrosion or mechanical wear, internal delamination at the fusion zone is invisible to visual inspection and can only be detected by UT or other subsurface NDT methods. This underscores the importance of incorporating UT into the regular inspection regime for hydrotreating reactors.

Furthermore, the study highlights the complexity of the fusion zone as a material system. The fusion zone is not a homogeneous material but a gradient region with varying composition, microstructure, and mechanical properties. Understanding this gradient is essential for predicting and preventing delamination. Future research should focus on developing fusion zone compositions that are inherently resistant to hydrogen-induced cracking, possibly through microalloying with elements such as Ti, Nb, or V that promote fine, stable carbide precipitation.

Summary

This paper provides a comprehensive analysis of hydrogen-induced delamination at the fusion zone of austenitic stainless steel overlays on 2.25Cr-1Mo steel, directly relevant to the safe operation of hydrotreating reactors. The systematic approach of reproducing the failure through simulated operating conditions, combined with multi-modal characterization (NDT, mechanical testing, metallography), establishes a robust framework for understanding this failure mode. Engineers involved in the design, fabrication, and maintenance of hydrotreating reactors should incorporate these findings into their welding procedure development, inspection protocols, and operating condition management strategies. The work reinforces the principle that in hydrogen-rich high-temperature service, the fusion zone is the weakest link, and its integrity must be actively managed throughout the reactor's service life.