Effect of Hydrogen on the Toughness of Stainless Steel Electroslag Overlay Layers

Literature Overview

This 1999 study published in the Journal of the Iron and Steel Research Institute by Liu Hang, Tian Zhiling, Wang Guangsheng, and colleagues from the Institute of Metal Research and Jinling Petrochemical Company examines the influence of hydrogen content on the ductility and toughness of stainless steel electroslag welding (ESW) overlay layers. Funded by the China Petrochemical Corporation Science and Technology Research and Development Program, this work addresses a critical issue in the fabrication of bimetal pressure vessels for the petrochemical industry, where hydrogen-containing environments pose a significant threat to material integrity. The study is particularly relevant to the fabrication of hydrogenation reactors, which are exposed to high-pressure hydrogen at elevated temperatures and are susceptible to hydrogen-induced cracking (HIC) and hydrogen blistering.

Core Technical Points

Hydrogen Sources in ESW Overlay

Hydrogen in electroslag overlay layers can originate from several sources:

Source	Mechanism	Typical Contribution
Flux moisture	Decomposition of water in flux	Primary source
Electrode coating moisture	Adsorbed water on electrode surface	Significant
Surface contamination	Oil, rust, paint on base metal	Variable
Atmospheric moisture	Ambient humidity during welding	Minor
Base metal	Hydrogen trapped in steel during rolling	Minor

In electroslag welding, the flux serves as both a protective medium and a heat source, and its moisture content is the most critical factor controlling hydrogen pickup. The high temperature of the slag pool (approximately 1500 – 1800°C) creates favorable conditions for hydrogen dissolution into the molten metal.

Hydrogen Content and Distribution

The hydrogen content in ESW overlay layers was measured using inert gas fusion methods and typically ranged from 1 – 20 mL/100g depending on the flux moisture control and welding parameters. The hydrogen distribution within the overlay layer is not uniform — it tends to concentrate at the weld interface, at grain boundaries, and near inclusions. This non-uniform distribution is critical because hydrogen-induced damage initiates at locations of highest hydrogen concentration.

Effect on Ductility

The ductility of the stainless steel ESW overlay layers was evaluated through tensile testing, and the following trends were observed:

Hydrogen Content (mL/100g)	Elongation (%)	Reduction of Area (%)	Fracture Mode
1 – 2	40 – 50	60 – 70	Ductile
5 – 8	30 – 40	50 – 60	Mixed
10 – 15	15 – 25	30 – 45	Brittle
15 – 20	5 – 15	15 – 30	Brittle, intergranular

The progressive decrease in ductility with increasing hydrogen content is attributed to hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms. At low hydrogen levels, the material can accommodate the strain energy through localized plastic deformation. As hydrogen concentration increases, the material transitions to a more brittle fracture mode characterized by reduced elongation and a shift from ductile to intergranular fracture.

Effect on Toughness

The Charpy impact toughness of the overlay layers was measured at both room temperature and elevated temperatures. The key findings include:

At room temperature, the impact energy decreased by 40 – 60% as hydrogen content increased from 2 to 15 mL/100g
At elevated temperatures (200 – 400°C), the hydrogen embrittlement effect was less severe but still significant
The ductile-to-brittle transition temperature (DBTT) increased with hydrogen content, indicating a loss of low-temperature toughness

Hydrogen Control Measures

Process Optimization

To minimize hydrogen pickup in ESW overlay layers, the following measures are recommended:

Flux drying: Flux must be dried at 250 – 350°C for 2 – 4 hours before use and maintained at 150 – 200°C during welding. The moisture content of the flux should be controlled below 0.5%.
Electrode coating control: The electrode coating must be free of moisture and contamination. Electrodes should be stored in a controlled environment and heated to 100 – 150°C immediately before use.
Surface preparation: The base metal surface must be thoroughly cleaned to remove oil, rust, and paint. Mechanical cleaning (grinding) followed by solvent cleaning is the most effective method.
Shielding gas: Although ESW uses flux as a shielding medium, the addition of a dry inert gas shroud (argon or helium) at the trailing edge of the slag pool can further reduce hydrogen pickup.
Post-weld baking: A post-weld bake at 200 – 300°C for 1 – 2 hours can diffuse out a significant portion of the dissolved hydrogen, reducing the risk of delayed hydrogen cracking.

Material Selection

The selection of the stainless steel grade for ESW overlay is also important. 316L and 321/347 grades are commonly used for petrochemical applications. The low carbon content of 316L reduces the risk of sensitization during welding, while the niobium stabilization of 321/347 provides excellent intergranular corrosion resistance. The choice of grade should be based on the specific service environment, with consideration given to the chloride content, temperature, and hydrogen partial pressure.

Engineering Practice Implications

In the fabrication of hydrogenation reactors and other high-pressure hydrogen-containing equipment, the control of hydrogen in ESW overlay layers is a critical quality requirement. The following quality control measures should be implemented:

Control Measure	Specification	Frequency
Flux moisture testing	≤ 0.5%	Before each shift
Hydrogen content measurement	≤ 5 mL/100g	Per lot of overlay
Charpy impact testing	≥ 47 J at -20°C	Per qualification
HIC/SSC testing	Per NACE MR0175	Per material lot
Post-weld bake	250°C for 2h	After each overlay section

Key Questions and Reflections

One significant question raised by this work is the long-term effect of hydrogen charging on the overlay layer during service. While the study focuses on hydrogen introduced during fabrication, in-service hydrogen charging can occur through diffusion from the process medium, particularly in high-pressure hydrogen environments. The hydrogen content in the overlay layer may increase over time, potentially leading to delayed hydrogen-induced cracking even if the fabrication quality is excellent. This necessitates periodic in-service inspection and monitoring of overlay layers in hydrogen-containing service.

Another important consideration is the interaction between hydrogen and other defects. Even if the hydrogen content is within acceptable limits, the presence of inclusions, porosity, or lack of fusion at the overlay interface can act as hydrogen traps and crack initiation sites. Therefore, a comprehensive quality assurance program must address both hydrogen control and defect prevention.

Study Insights

This research provides critical insights into the hydrogen embrittlement behavior of stainless steel ESW overlay layers, with direct implications for the safe fabrication of bimetal pressure vessels in the petrochemical industry. The clear correlation between hydrogen content and the loss of ductility and toughness underscores the importance of rigorous hydrogen control measures throughout the welding process. For engineers involved in the design and fabrication of hydrogenation reactors and similar equipment, this work reinforces the need for a systematic approach to hydrogen management that encompasses material selection, process control, quality inspection, and in-service monitoring. The findings also highlight the importance of understanding the fundamental mechanisms of hydrogen embrittlement, as this knowledge is essential for developing effective prevention and mitigation strategies.