CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure in Stainless Steel Overlay Transition Zone and Its Effect on Hydrogen-Induced Delamination
Literature Overview
The research by Xu Ying, Yao Shoushan, Wang Lan, Sun Yongjian, Wang Zhengdong, and Wu Dongdi, published in 1993 in Materials Science and Technology, addresses a critical and historically significant problem in stainless steel overlay welding: hydrogen-induced delamination (HID) at the transition zone between the overlay layer and the base metal. The study was conducted jointly by Shanghai Jiao Tong University and East China University of Science and Technology. This topic remains highly relevant in modern practice, particularly for hydrogenation reactors, petrochemical equipment, and hydrogen storage vessels operating under conditions where atomic hydrogen can be generated.
Core Technical Content
Mechanism of Hydrogen-Induced Delamination
Hydrogen-induced delamination occurs when atomic hydrogen atoms, generated by corrosion reactions or cathodic charging in the service environment, diffuse into the weld metal and accumulate at specific microstructural features within the transition zone. In stainless steel overlay welds deposited on carbon steel substrates, the transition zone is characterized by a complex mixture of austenitic overlay material, ferritic carbon steel, and dilution-affected martensitic or duplex regions. These regions contain microstructural heterogeneities—such as grain boundaries, phase boundaries, and inclusions—that act as hydrogen traps.
The driving force for hydrogen accumulation is the difference in hydrogen diffusivity and solubility between the various phases present in the transition zone. Carbon steel (ferrite) has higher hydrogen diffusivity than austenitic stainless steel, but the dilution-affected zone often contains martensite, which has a body-centered cubic (BCC) structure with significantly higher hydrogen diffusivity than the face-centered cubic (FCC) austenite. Hydrogen atoms migrate preferentially along high-diffusivity paths and become trapped at interfaces where the diffusivity drops abruptly.
Microstructural Features of the Transition Zone
The transition zone in stainless steel overlay welds deposited by submerged arc welding (SAW) or gas metal arc welding (GMAW) typically exhibits the following microstructural characteristics:
| Zone | Typical Microstructure | Hydrogen Trap Sites | Susceptibility to HID |
|---|---|---|---|
| Overlay layer (pure) | Austenite + delta ferrite | Grain boundaries, inclusions | Low |
| Transition zone (high dilution) | Martensite + retained austenite | Phase boundaries, dislocation networks | Very High |
| Transition zone (moderate dilution) | Duplex (ferrite + austenite) | Grain boundaries | High |
| Base metal HAZ | Fine-grained ferrite/pearlite | Grain boundaries, inclusions | Moderate |
The most critical region for HID is the high-dilution transition zone where martensite forms due to the dilution of austenite-stabilizing elements (Ni, Mn) by the carbon steel base metal. Martensite provides numerous dislocation networks and phase boundaries that trap hydrogen effectively. When the trapped hydrogen concentration exceeds a critical threshold, hydrogen-assisted crack initiation occurs at these trap sites, leading to blistering and delamination parallel to the weld interface.
Experimental Findings
The study demonstrates that the susceptibility to HID is strongly correlated with the microstructure of the transition zone. Welds deposited with higher dilution ratios exhibit more martensite in the transition zone and show significantly higher susceptibility to hydrogen-induced blistering and delamination. The presence of sulfide inclusions (MnS) along the interface further exacerbates the problem, as these inclusions serve as additional hydrogen traps and crack initiation sites.
The researchers found that controlling the dilution ratio to below 20-25% significantly reduces the amount of martensite in the transition zone and improves resistance to HID. This can be achieved by using multi-pass welding with careful control of each pass's heat input, employing a backing layer of a more austenite-stabilized composition, or selecting overlay consumables with higher Ni content.
Standards and Testing Requirements
The resistance to hydrogen-induced delamination is evaluated according to several testing standards:
| Standard | Test Method | Criteria |
|---|---|---|
| NACE MR0175 / ISO 15156 | Hydrogen blistering test | No blisters ≥ 1 mm |
| API 934 | Weld overlay qualification | HID resistance verification |
| ASTM G174 | Hydrogen embrittlement of steels | Load-bearing capacity retention |
| NB/T 47014 | Welding procedure qualification | Qualification welding and testing |
For pressure vessels and equipment intended for service in sour environments (containing H2S), compliance with NACE MR0175 or API 934 requirements is mandatory. The weld overlay procedure must demonstrate resistance to hydrogen-induced delamination through appropriate testing.
Engineering Practice and Countermeasures
In engineering practice, several strategies are employed to mitigate HID risk in stainless steel overlay welds:
- Consumable selection: Using overlay consumables with higher Ni content (such as ER309L or ER310) to maintain austenitic structure even at moderate dilution levels.
- Multi-pass strategy: Depositing a transition layer of a more austenite-stabilized composition before the final overlay layer to reduce the dilution effect on the final layer.
- Welding parameter control: Using lower heat input and higher travel speed to minimize dilution and reduce the width of the high-dilution transition zone.
- Post-weld heat treatment: Applying a solution treatment or tempering cycle to transform martensite to retained austenite or fine tempered martensite, which reduces hydrogen trapping.
- Surface preparation: Ensuring the substrate surface is free from scale, rust, and contaminants that could act as hydrogen sources during welding.
Key Reflections and Study Insights
This 1993 study remains remarkably relevant to modern engineering practice. The understanding that microstructural heterogeneity in the transition zone drives hydrogen-induced delamination has been incorporated into contemporary welding procedure design for hydrogenation reactors, sour service equipment, and nuclear applications. The key insight is that the dilution-controlled transition zone microstructure is the primary determinant of HID susceptibility, and that multi-pass overlay strategies with controlled dilution are more effective than single-pass deposits. Engineers designing overlay weld procedures for hydrogen-containing environments must consider not only the corrosion resistance of the final overlay layer but also the microstructural integrity of the transition zone under hydrogen attack conditions. The interplay between welding process parameters, dilution, microstructure, and hydrogen behavior represents one of the most complex challenges in overlay welding engineering.