Hydrogen-Induced Peel Cracking at Stainless Steel Weld Overlay Interfaces and Its Prevention

Literature Overview

This paper by Ling Wenjun, published in Chemical Machinery in 1990, addresses a long-standing and critical quality problem in stainless steel weld overlay fabrication: hydrogen-induced peel cracking (also known as hydrogen blistering or hydrogen-induced cracking) at the interface between the overlay layer and the carbon steel substrate. This phenomenon has historically caused significant production losses in the manufacture of bimetal clad plate, overlay-welded heat exchanger tubes, and lined pressure vessels. The paper represents one of the early systematic investigations into this defect mechanism in the Chinese welding community.

Core Technical Content

Defect Mechanism

Hydrogen-induced peel cracking occurs when atomic hydrogen generated during the welding process diffuses into the substrate and becomes trapped at the weld interface. The trapped hydrogen combines to form molecular hydrogen (H2) at inclusion sites, grain boundaries, or phase boundaries, creating internal pressures sufficient to cause separation of the overlay from the substrate. The cracking typically manifests as a thin, continuous separation along the fusion boundary, often not detectable by conventional visual or magnetic particle inspection.

Hydrogen Diffusion Behavior

The study highlights that hydrogen diffusion rates differ significantly between stainless steel and carbon steel. Carbon steel, particularly those with higher carbon and manganese content, has a higher hydrogen diffusivity and lower hydrogen permeability, making the substrate side more susceptible to hydrogen accumulation. The interface acts as a hydrogen trap due to the abrupt change in crystal structure (FCC austenitic overlay vs. BCC ferritic substrate) and the presence of carbide precipitation at the fusion boundary.

Prevention Measures

The paper proposes a multi-layered prevention strategy:

1. Rigorous pre-weld cleaning: All substrate surfaces must be free of oil, rust, and moisture. Solvent cleaning followed by grinding to bare metal is mandatory.
2. Low-hydrogen consumables: Use of low-hydrogen electrode types (e.g., E309L with hydrogen content <5 mL/100g) or gas-shielded processes with pure argon shielding.
3. Interpass temperature control: Maintaining interpass temperature below 150 °C to allow hydrogen escape between passes.
4. Post-weld baking: Applying a post-weld bake at 200–300 °C for 2–4 hours to promote hydrogen diffusion out of the weld zone.
5. Substrate pre-heating: Pre-heating the substrate to 200–350 °C to slow cooling rates and provide additional time for hydrogen escape.

Process Comparison for Hydrogen Sensitivity

Engineering Practice Integration

In the context of bimetal pressure vessel fabrication under GB/T 150 and ASME VIII Div.1 requirements, the prevention of hydrogen-induced peel cracking is not merely a quality consideration but a safety-critical requirement. The following engineering practices have been established based on this research:

• NDT requirements: For critical cladding applications, phased array ultrasonic testing (PAUT) or TOFD at the overlay/substrate interface should be specified in the quality plan. Standard contact ultrasonic testing may miss interface separation defects due to poor acoustic coupling at the separation plane.
• Witness coupons: Each production batch should include witness coupons that undergo bond strength testing (shear or peel) after the full PWHT cycle. Acceptance criteria per ASTM A263 typically require a minimum bond strength of 90% of the lower tensile strength of the cladding or substrate material.
• Hydrogen embrittlement testing: For applications involving sour service (H2S environments), hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC) testing per NACE MR0175/ISO 15156 should be performed on the overlay material.

Key Questions and Reflections

The 1990 publication predates modern understanding of hydrogen trapping in weld microstructures. Contemporary research has revealed that nano-scale carbide precipitates at the fusion boundary can act as reversible hydrogen traps, while micro-voids and oxide inclusions serve as irreversible traps. This distinction is important for understanding why some weld overlay configurations are more susceptible to peel cracking than others, even under identical process conditions.

Furthermore, the role of residual stress in promoting hydrogen-assisted cracking deserves greater attention. Compressive residual stresses at the interface can reduce crack driving force, while tensile residual stresses can synergistically interact with hydrogen to initiate and propagate peel cracks. Residual stress measurement (by X-ray diffraction or hole-drilling method) should be incorporated into the quality assurance program for critical cladding applications.

Study Insights and Outlook

This paper, though published over three decades ago, remains highly relevant to current engineering practice. The fundamental mechanisms of hydrogen-induced peel cracking have not changed, and the prevention strategies proposed continue to form the basis of modern welding procedure specifications for stainless steel overlay. The key lesson for engineers is that hydrogen control in cladding fabrication requires a holistic approach encompassing consumable selection, surface preparation, process parameter optimization, and post-weld treatment. No single measure is sufficient; rather, a defense-in-depth strategy is essential for achieving reliable cladding integrity in demanding service conditions.