Welding in hydrogen service presents unique challenges due to hydrogen's propensity to cause embrittlement, cracking, and reduced joint integrity. The selection of appropriate filler metals and welding techniques is critical to ensure the longevity and safety of components such as reactors, compressors, and pipelines. Key considerations include minimizing residual stress, preventing hydrogen-induced cracking, and applying post-weld heat treatment (PWHT) where necessary.

### Filler Metals for Hydrogen Service
Filler metals must exhibit low hydrogen content, high resistance to embrittlement, and compatibility with base materials. Common choices include:

- **Low-Hydrogen Electrodes**: AWS E7018 and similar electrodes are widely used for carbon and low-alloy steels. These electrodes are baked before use to reduce moisture content, which minimizes hydrogen pickup during welding.
- **Nickel-Based Alloys**: Alloys such as Inconel 625 and Hastelloy C-276 are preferred for high-pressure hydrogen environments due to their resistance to hydrogen embrittlement and corrosion.
- **Austenitic Stainless Steels**: Filler metals like ER308L and ER316L provide good ductility and resistance to cracking, making them suitable for hydrogen applications.

The chemical composition of filler metals must be carefully controlled to avoid elements that promote hydrogen cracking, such as sulfur and phosphorus.

### Welding Techniques for Hydrogen Service
The choice of welding technique impacts joint quality, residual stress, and susceptibility to hydrogen-related failures.

- **Tungsten Inert Gas (TIG) Welding**:
TIG welding is favored for its precision and low hydrogen input. The use of high-purity argon or helium shielding gas prevents contamination. A key advantage is the ability to control heat input, reducing the risk of excessive residual stress. TIG is often used for thin sections and critical welds in hydrogen compressors and reactors.

- **Laser Welding**:
Laser welding offers deep penetration with minimal heat-affected zone (HAZ), reducing residual stress. It is particularly effective for joining thin materials in hydrogen storage tanks. However, the rapid cooling rates may increase hardness, necessitating PWHT to restore ductility.

- **Electron Beam Welding (EBW)**:
EBW provides high-energy density and deep weld penetration with low distortion. It is used in specialized applications such as aerospace hydrogen components. The vacuum environment eliminates hydrogen contamination risks.

- **Friction Stir Welding (FSW)**:
FSW is a solid-state process that avoids melting, thus eliminating hydrogen pickup. It is suitable for aluminum and copper alloys in hydrogen storage systems.

### Joint Integrity and Residual Stress
Hydrogen service demands stringent control over joint integrity. Poor weld quality can lead to crack initiation and propagation under hydrogen exposure. Key measures include:

- **Pre-Weld Preparation**: Surfaces must be clean and free of contaminants like oil, grease, and moisture. Preheating (150–250°C) is often applied to carbon steels to slow cooling rates and reduce hydrogen diffusion.
- **Controlled Heat Input**: Excessive heat can enlarge the HAZ and increase residual stress. Optimal parameters must balance penetration and thermal effects.
- **Peening**: Mechanical peening can introduce compressive stresses that counteract tensile residual stresses, reducing crack susceptibility.

Residual stresses are a major concern as they can accelerate hydrogen-assisted cracking. Techniques such as interpass temperature control and bead sequencing help mitigate stress buildup.

### Post-Weld Heat Treatment (PWHT)
PWHT is critical for relieving residual stresses and diffusing trapped hydrogen. Common practices include:

- **Stress Relieving**: Heating to 600–650°C for carbon steels reduces residual stresses without significantly altering microstructure.
- **Solution Annealing**: For austenitic stainless steels and nickel alloys, heating above 1000°C followed by rapid cooling restores corrosion resistance.
- **Hydrogen Baking**: Low-temperature baking (200–250°C) for several hours helps diffuse hydrogen out of the weld zone, reducing embrittlement risk.

PWHT parameters must be tailored to material thickness and service conditions. Overheating can degrade mechanical properties, while insufficient treatment may leave harmful stresses.

### Industrial Applications
- **Hydrogen Reactors**:
Reactors handling high-pressure hydrogen require welds with minimal defects. TIG welding with nickel-based fillers is common, followed by PWHT to ensure durability.
- **Compressors**:
Reciprocating compressors for hydrogen service use forged components joined via low-hydrogen processes. Residual stress management is critical to prevent fatigue failures.
- **Pipelines**:
Girth welds in hydrogen pipelines employ mechanized TIG or laser welding for consistency. PWHT is often mandated by codes such as ASME B31.12.

### Standards and Best Practices
Industry standards provide guidelines for welding in hydrogen service:
- ASME Boiler and Pressure Vessel Code (Section IX) outlines qualification requirements.
- ISO 15614 specifies welding procedure qualifications for hydrogen-resistant materials.
- NACE MR0175/ISO 15156 addresses materials for sour (H2S-containing) environments, with relevance to hydrogen service.

Best practices include:
- Using certified low-hydrogen consumables.
- Implementing strict drying and storage protocols for electrodes.
- Conducting non-destructive testing (NDT) such as radiography or ultrasonic inspection to detect flaws.

### Conclusion
The success of welding in hydrogen service hinges on selecting appropriate filler metals, optimizing welding techniques, and applying rigorous post-weld treatments. Industrial applications demand adherence to standards and continuous monitoring to prevent hydrogen-related failures. Advances in welding technology, such as laser and friction stir welding, offer promising solutions for future hydrogen infrastructure.