Zhuojin Pipe Fitting Co., LtdDissimilar Steel Welding and Surfacing in Hydrogenation Equipment
Literature Overview
The paper by Chai Xiangdong, published in Petrochemical Equipment Technology in 2019 (Vol. 40, No. 3, pp. 57–62), provides a comprehensive discussion of dissimilar steel welding and surfacing challenges in hydrogenation equipment. Written by an engineer at Sinopec Guangzhou Engineering Co., Ltd., this work draws on extensive industrial experience to address one of the most critical welding challenges in the petroleum refining industry: the prevention of hydrogen blistering (hydrogen debonding) in surfacing layers of hydrogenation reactors, separators, and associated equipment. The study examines the interplay between dissimilar steel welding characteristics, carbon migration, hydrogen concentration at interfaces, and the selection of surfacing methods, providing practical guidance for engineers designing and executing surfacing procedures for hydrogen service equipment.
Hydrogen Blistering Mechanism
Hydrogen blistering, also known as hydrogen damage or hydrogen-induced cracking, is a degradation mechanism that occurs in low-alloy steels exposed to high-pressure hydrogen environments. The mechanism involves the dissociation of molecular hydrogen at the steel surface, followed by atomic hydrogen diffusion into the steel matrix. The atomic hydrogen accumulates at microstructural defects such as inclusions, grain boundaries, and phase boundaries, where it recombines to form molecular hydrogen. The high internal pressure of molecular hydrogen (up to 1000 MPa) causes the formation of blisters, which can coalesce and lead to catastrophic failure.
In dissimilar steel welds and surfacing layers, the risk of hydrogen blistering is significantly elevated due to several factors:
| Factor | Effect on Hydrogen Blistering | Mechanism |
|---|---|---|
| Interface stress | Increases susceptibility | Residual stress provides driving force for hydrogen diffusion to defects |
| Carbon migration | Increases susceptibility | Carburization of low-carbon steel side creates hard, brittle zones prone to blistering |
| Hydrogen concentration gradient | Increases susceptibility | Dissimilar interfaces act as hydrogen traps |
| Microstructural mismatch | Increases susceptibility | Different thermal expansion coefficients create interface stresses |
| Inclusion distribution | Increases susceptibility | Stringer inclusions at interface provide hydrogen recombination sites |
The carbon migration phenomenon is particularly important in dissimilar steel welds. During welding and subsequent heat treatment, carbon diffuses from the high-carbon steel side to the low-carbon steel side, creating a decarburized zone on the high-carbon side and a carburized zone on the low-carbon side. The carburized zone has a higher hardness and lower toughness, making it more susceptible to hydrogen blistering. The severity of carbon migration depends on the carbon potential difference between the two steels, the welding procedure, and the post-weld heat treatment schedule.
Surfacing Method Selection
The study provides valuable practical guidance on surfacing method selection for hydrogenation equipment. The author's experience indicates that the surfacing method significantly influences the interface microstructure and, consequently, the susceptibility to hydrogen blistering. The recommended approaches are:
| Application | Recommended Method | Key Advantages |
|---|---|---|
| Single-layer surfacing | Electrode strip submerged arc surfacing | Low dilution, good penetration, uniform heat input |
| Double-layer surfacing | Electrode strip SAW + electrode strip electroslag surfacing | Combined benefits of both processes |
| Low-alloy steel to Cr-Mo steel | Low-dilution electrode strip SAW | Minimizes carbon migration and interface stress |
| High-pressure hydrogen service | Multi-pass with interpass temperature control | Reduces residual stress and hydrogen concentration |
The electrode strip submerged arc welding (ES-SAW) process is particularly well-suited for hydrogen service surfacing because of its low dilution rate, deep penetration, and uniform heat input. The electrode strip provides a large volume of overlay material relative to the arc energy, resulting in low dilution and a compositionally faithful overlay. The deep penetration ensures good fusion with the base metal, while the uniform heat input minimizes the thermal gradient that drives carbon migration.
For double-layer surfacing, the combination of electrode strip SAW for the first layer and electrode strip electroslag welding (ES-ESW) for the second layer provides an optimal balance of fusion quality and deposit quality. The SAW first layer ensures good fusion with the base metal and establishes a sound interface, while the ESW second layer provides a dense, uniform deposit with minimal defects. The electroslag process operates at a higher temperature than SAW, which helps to homogenize the deposit and reduce residual stress through thermal cycling.
Dissimilar Steel Welding Considerations
Dissimilar steel welding in hydrogenation equipment typically involves joining carbon steel or low-alloy steel (such as SA-516 Gr.70 or SA-387 Gr.11/Cl.2) to chromium-molybdenum steels (such as SA-387 Gr.22/Cl.1 or SA-387 Gr.91). The key challenges include:
| Challenge | Description | Mitigation Strategy |
|---|---|---|
| Thermal expansion mismatch | Different coefficients of thermal expansion create residual stresses | Use low-residual-stress welding procedures |
| Carbon migration | Carbon diffuses from high-Cr side to low-C side during welding and PWHT | Limit PWHT temperature and duration |
| Dilution | Base metal dilution changes overlay composition | Use low-dilution surfacing processes |
| Hydrogen embrittlement | Residual hydrogen in weld metal causes delayed cracking | Post-weld bake-out at 200–300 °C |
| Creep compatibility | Different creep rates at service temperature | Match creep strengths of dissimilar steels |
The selection of welding materials for dissimilar steel joints is critical. The welding consumable should be selected to match the lower creep strength of the two steels, typically the carbon or low-alloy steel side. This ensures that the weld metal is not the weakest link in the joint at elevated service temperatures. Common welding consumables for these applications include low-alloy steel electrode strips such as SA-182 F22, F91, or custom compositions designed to minimize carbon migration.
Engineering Practice and Case Studies
In practice, the surfacing of hydrogenation equipment requires a comprehensive approach that integrates material selection, welding procedure qualification, non-destructive examination, and post-weld heat treatment. The following sequence is typically followed:
- Substrate preparation: Surface preparation to remove contaminants, oxide, and scale. The surface should be cleaned to a minimum Sa 2.5 grade (near-white metal blast) to ensure good fusion and minimize hydrogen sources.
- Welding procedure qualification: A qualified welding procedure specification (WPS) must be developed and qualified in accordance with applicable codes such as ASME Section IX, AWS D10.9, or API 935. The qualification should include mechanical testing, metallographic examination, and hydrogen blistering testing.
- Surfacing execution: The surfacing should be performed by qualified welders following the qualified WPS. Critical parameters include preheat temperature, interpass temperature, welding speed, and arc travel direction. The preheat temperature is typically 150–250 °C for low-alloy steels and 250–350 °C for Cr-Mo steels.
- Post-weld heat treatment: PWHT is essential to relieve residual stresses and reduce hydrogen concentration. The PWHT temperature and duration must be carefully controlled to minimize carbon migration. For low-alloy steels, a PWHT at 620–680 °C for 1 hour per 25 mm thickness is typical. For Cr-Mo steels, a higher temperature (700–750 °C) may be required, but this increases the risk of carbon migration.
- Non-destructive examination: The surfacing must be inspected using appropriate NDE methods. Magnetic particle testing (MT) is used to detect surface and near-surface cracks, while radiographic testing (RT) or ultrasonic testing (UT) is used to detect subsurface defects such as porosity, lack of fusion, and inclusions.
- Hydrogen blistering testing: For critical applications, the surfacing should be tested for hydrogen blistering susceptibility using methods such as the NACE TM0177 or ASTM G129 hydrogen blistering test. This test exposes the material to a high-pressure hydrogen environment and evaluates the formation of blisters and cracks.
Key Questions and Reflections
The study raises important questions about the long-term performance of surfaced hydrogenation equipment. The interaction between hydrogen blistering, creep, and fatigue is complex and not fully understood. In service, the material is subjected to cyclic thermal and mechanical loading combined with continuous hydrogen exposure, and the combined effect of these factors on the surfacing integrity is difficult to predict. Long-term surveillance programs that include periodic NDE and material testing are essential for ensuring the continued integrity of hydrogen service equipment.
The study also highlights the importance of understanding the specific service environment. Not all hydrogen environments are equally damaging. The severity of hydrogen damage depends on the hydrogen partial pressure, temperature, and the presence of contaminants such as sulfur compounds that can accelerate damage. Engineers must carefully characterize the service environment and select surfacing materials and procedures accordingly.
The role of residual stress in hydrogen blistering is another area that deserves more attention. Residual stresses from welding can be tensile at the surface, which provides a driving force for hydrogen diffusion to the surface and subsequent blistering. Stress relief through PWHT is important, but the effectiveness of stress relief depends on the PWHT temperature, duration, and cooling rate. Incomplete stress relief can leave significant residual stresses that compromise the surfacing integrity.
Study Insights and Conclusions
This paper provides valuable practical guidance for engineers working with dissimilar steel welding and surfacing in hydrogenation equipment. The key insights are that the surfacing method significantly influences the susceptibility to hydrogen blistering, that low-dilution processes such as electrode strip submerged arc welding are preferred for single-layer surfacing, and that the combination of electrode strip SAW and electrode strip electroslag welding is an effective approach for double-layer surfacing. The paper emphasizes the importance of understanding the interplay between carbon migration, interface stress, and hydrogen concentration in determining the hydrogen blistering susceptibility of dissimilar steel welds. For engineers designing and executing surfacing procedures for hydrogen service equipment, this paper serves as an important reference for making informed decisions about material selection, process selection, and quality control. The practical experience and engineering judgment presented in this paper are invaluable for ensuring the safe and reliable operation of hydrogenation equipment in petroleum refining and petrochemical processing.