Effect of Stainless Steel Overlay Transition Zone Microstructure on Hydrogen-Induced Delamination

Literature Overview

The paper by Xu Ying, Yao Shoushan, Wang Lan, Sun Yongjian, Wang Zhengdong, and Wu Dongdi from Shanghai Jiao Tong University and East China University of Science and Technology examines the critical role of the transition zone microstructure in stainless steel overlay welding on its susceptibility to hydrogen-induced delamination. Published in Materials Science and Engineering in 1993, this work is foundational for understanding hydrogen embrittlement mechanisms in bimetallic weld overlay systems, particularly relevant to hydrogen service pressure vessels and nuclear applications.

Core Technical Content

Hydrogen-Induced Delamination Mechanism

Hydrogen-induced delamination (HID) is a failure mode in which atomic hydrogen diffuses to the interface between the overlay layer and the base metal, accumulating at microvoids, inclusions, or grain boundaries and causing separation of the overlay layer from the substrate. This phenomenon is particularly problematic in high-pressure hydrogen service, where atomic hydrogen can be generated through the dissociation of molecular hydrogen at the metal surface.

The transition zone, which is the region where the overlay material and base metal have intermixed during welding, plays a critical role in HID susceptibility. The microstructure of this zone includes:

• Fully fused zone: Where the overlay and base metals have completely mixed, forming a composition gradient from overlay to base metal.
• Partially fused zone: Where the base metal has partially melted and mixed with the overlay material.
• Heat-affected zone: Where the base metal has been heated but not melted, undergoing microstructural changes.

Microstructural Factors Influencing HID

The authors identify several microstructural features that promote or inhibit hydrogen-induced delamination:

Experimental Findings

The study demonstrates that the transition zone microstructure is strongly influenced by the welding parameters, particularly the heat input and travel speed. Higher heat input produces a wider transition zone with a more gradual composition gradient, which can either improve or worsen HID susceptibility depending on the specific alloy system.

For 304 stainless steel overlay on carbon steel substrates, the authors find that:

• Low heat input (10-15 kJ/mm): Produces a narrow transition zone with sharp composition gradient. Hydrogen tends to accumulate at the interface, increasing HID risk.
• Moderate heat input (15-25 kJ/mm): Produces a wider transition zone with a more gradual composition gradient. Hydrogen can diffuse more easily through the gradient, reducing accumulation at the interface.
• High heat input (25-35 kJ/mm): Produces an overly wide transition zone with potential for excessive dilution, which may compromise the corrosion resistance of the overlay layer.

Hydrogen Trapping and Diffusion

The authors emphasize that hydrogen trapping at microstructural features such as carbides, inclusions, and grain boundaries is a key mechanism in HID. The trapping efficiency of these features depends on their size, shape, and distribution. Fine, uniformly distributed carbides can act as reversible traps, temporarily holding hydrogen and reducing its diffusivity, while large, irregular carbides can act as irreversible traps, promoting hydrogen accumulation and void growth.

Engineering Practice Integration

Application to Hydrogen Service Pressure Vessels

Hydrogen-induced delamination is a critical concern in the fabrication of pressure vessels for high-pressure hydrogen service, such as hydrogenation reactors, hydrogen storage tanks, and hydrogen piping systems. The ASME Boiler and Pressure Vessel Code Section VIII Division 2 includes specific requirements for hydrogen service vessels, including restrictions on the carbon equivalent of the base metal and the use of weld overlay for corrosion resistance.

The study's findings have direct implications for the selection of welding parameters and post-weld heat treatment for hydrogen service overlay welds. The following practices are recommended:

• Preheating: Preheat the base metal to 150-250°C to reduce cooling rate and minimize hydrogen trapping.
• Low hydrogen electrodes: Use electrodes with low hydrogen content (< 5 mL/100g) to reduce hydrogen generation during welding.
• Post-weld baking: Bake the weld at 200-300°C for 2-4 hours to allow hydrogen to diffuse out of the weld metal.
• PWHT: Apply post-weld heat treatment at 600-700°C to relieve residual stress and promote hydrogen diffusion.

Inspection and Quality Control

Detecting hydrogen-induced delamination is challenging because it may not be visible on the surface. The following NDT methods are recommended:

FMEA Analysis of HID

Applying FMEA methodology to hydrogen-induced delamination:

• Failure mode: Delamination of overlay layer from base metal under hydrogen pressure.
• Effect: Loss of corrosion protection, potential for catastrophic failure.
• Cause: Hydrogen accumulation at interface due to microstructural features.
• Detection: Difficult; requires advanced NDT methods.
• Risk priority: High, due to safety implications.
• Countermeasures: Optimize welding parameters, apply PWHT, implement rigorous inspection procedures.

Key Questions and Reflections

The most significant finding of this research is that the transition zone microstructure, rather than the overlay layer composition, is the primary determinant of HID susceptibility. This challenges the conventional approach of focusing solely on the overlay material selection and suggests that greater attention should be paid to the welding process parameters that control the transition zone microstructure.

Another important reflection is the role of residual stress in promoting HID. The authors note that residual tensile stress at the interface can significantly increase hydrogen accumulation and delamination risk. This has implications for the design of overlay welds, where stress-relieving welds or backstitch welding techniques may be beneficial in reducing residual stress.

Study Insights and Implications

This research provides a fundamental understanding of the mechanisms governing hydrogen-induced delamination in stainless steel overlay welds, which is essential for the safe design and fabrication of hydrogen service pressure vessels. The key insight is that the transition zone microstructure must be carefully controlled to minimize hydrogen trapping and promote hydrogen diffusion away from the interface.

For engineers involved in bimetal pressure vessel fabrication, the recommendations include:

• Develop welding procedures that produce a transition zone with fine grain structure and minimal carbide stringers.
• Implement rigorous hydrogen control measures, including low hydrogen electrodes, preheating, and post-weld baking.
• Apply post-weld heat treatment to relieve residual stress and promote hydrogen diffusion.
• Use advanced NDT methods, such as PAUT, to inspect for subsurface delamination.

In conclusion, the effect of stainless steel overlay transition zone microstructure on hydrogen-induced delamination demonstrates that careful control of the welding process and post-weld treatment is essential for ensuring the integrity of overlay layers in hydrogen service applications.