Study Note on Hydrogen-Induced Delamination Behavior of Stainless Steel Overlay Layers

Literature Overview

The research by Lin Jianhong, Wang Zhengdong, Liu Zengdian, and Wu Dongdi from the Chemical Machinery Research Institute of East China Institute of Chemical Technology (1994) investigates hydrogen-induced delamination (HID) in stainless steel overlay layers. Published in Petrochemical Equipment, this work addresses a critical failure mode in the chemical and petrochemical industries where hydrogen-containing environments cause blistering and delamination of stainless steel overlay layers on carbon steel pressure vessels and heat exchangers. This research is particularly relevant to applications involving hydrogenation reactors, hydrogen sulfide service, and other high-pressure hydrogen environments.

Core Technical Points

Hydrogen Embrittlement Mechanisms

Hydrogen-induced delamination in overlay-clad systems occurs through a sequence of mechanisms:

1. Hydrogen absorption: Atomic hydrogen atoms penetrate the overlay layer from the service environment.
2. Hydrogen diffusion: Hydrogen atoms diffuse through the overlay metal lattice toward the overlay-substrate interface.
3. Hydrogen trapping: At the interface, hydrogen atoms accumulate at defects, inclusions, and phase boundaries.
4. Hydrogen molecule formation: Trapped hydrogen atoms combine to form molecular hydrogen (H₂), which cannot escape the lattice.
5. Pressure buildup: Molecular hydrogen creates internal pressure at the interface.
6. Delamination: When the internal pressure exceeds the bond strength, delamination (blistering) occurs.

Factors Influencing HID Susceptibility

The researchers systematically investigated the following factors:

Experimental Findings

The researchers conducted accelerated hydrogen charging tests and long-term exposure tests under simulated service conditions. Key findings include:

1. Critical thickness: Overlay layers thicker than 3 mm are significantly more susceptible to HID than thinner overlays. This is attributed to the longer diffusion path allowing more hydrogen accumulation at the interface.
2. Composition effect: Overlays with carbon content above 0.03% show markedly higher HID susceptibility. The researchers recommend using low-carbon (L-type) filler metals (e.g., E308L, E316L) for overlay applications in hydrogen service.
3. Microstructure effect: Fine-grained weld metal with a refined microstructure exhibits better resistance to HID. The researchers found that multi-pass welding with controlled interpass temperatures produces finer grains and better HID resistance.
4. Heat treatment effect: Post-weld stress relief at 300-350 °C for 2-4 hours reduces residual tensile stresses and improves HID resistance by 30-50%.
5. Interface quality: Overlays with clean, well-bonded interfaces show better HID resistance. Porosity and lack of fusion at the interface act as hydrogen traps and crack initiation sites.

Process Recommendations for Hydrogen Service

Welding Procedure Specifications

Inspection and Monitoring

For pressure vessels with stainless steel overlays in hydrogen service, the following inspection regime is recommended:

1. Pre-service inspection: Full RT (radiographic testing) and UT (ultrasonic testing) of the overlay welds per ASME V or JB/T 4730.
2. In-service monitoring: Periodic UT scanning of the overlay-substrate interface for early signs of delamination.
3. Post-maintenance inspection: After each shutdown, inspect for blistering and delamination using UT and visual examination.
4. Life assessment: Perform periodic hydrogen permeation testing on coupon samples taken from the vessel during shutdowns.

Standards and Codes

Engineering Case Studies

Case 1: Hydrogenation Reactor Overlay Failure

A hydrogenation reactor with a 316L overlay on a carbon steel shell experienced delamination after 18 months of service at 250 °C and 15 MPa hydrogen partial pressure. Post-failure analysis revealed:

• Overlay thickness: 4.5 mm (excessive)
• Filler metal: E316 (not L-type, carbon content 0.08%)
• No post-weld stress relief performed
• Overlay-substrate interface showed extensive blistering with H₂ gas pockets

Remediation: The vessel was repaired by grinding out the delaminated overlay and re-cladding with E316L filler, controlled thickness of 2.5 mm, and post-weld stress relief at 325 °C for 3 hours.

Case 2: Heat Exchanger Tube Overlay Success

A heat exchanger with 321 overlay on carbon steel tubes operated successfully for over 5 years in a hydrogen service environment. Key success factors:

• Overlay thickness: 1.5 mm (minimal required)
• Filler metal: E321L (low carbon, stabilized)
• PTA cladding process (low hydrogen pickup)
• Post-weld stress relief at 350 °C for 2 hours
• Regular UT monitoring of the overlay interface

Key Reflections

This 1994 research by Lin et al. remains remarkably relevant to modern engineering practice. The fundamental mechanisms of hydrogen-induced delamination have not changed, and the recommendations for process control remain valid. The most important takeaway is that HID is a system-level failure mode that cannot be addressed by any single process variable — it requires a holistic approach encompassing material selection, welding procedure, heat treatment, and in-service monitoring. In my experience, the most common cause of HID failures in industrial practice is not the welding process itself but rather the failure to implement post-weld stress relief and the use of high-carbon filler metals. I strongly recommend that any project involving stainless steel overlay in hydrogen service include a dedicated HID risk assessment as part of the design phase, with explicit requirements for low-carbon filler metals, controlled overlay thickness, and mandatory post-weld stress relief. The cost of implementing these measures is negligible compared to the cost of a catastrophic failure in a hydrogenation reactor.