TP347 Single-Layer Surfacing Technology and Performance Evaluation on 15CrMoR Steel

Literature Overview

This study by Li Xiaowei and colleagues from Sinopec Engineering Group Luoyang Technology R&D Center and Sinopec Equipment Anti-Corrosion Research Center, published in Pressure Vessel (2019, Vol. 36, No. 5, pp. 8-13), investigates the feasibility and performance of single-layer TP347 austenitic stainless steel surfacing on 15CrMoR low-alloy steel substrates. Funded by Sinopec Engineering Group Science and Technology Development Project (117008), the research addresses a critical challenge in pressure vessel and heat exchanger manufacturing: achieving corrosion resistance through surface engineering while maintaining mechanical integrity under high-temperature hydrogen service conditions.

Background: The Hydrogen Environment Challenge in Refining

Refining and petrochemical equipment frequently operates in high-temperature hydrogen environments where hydrogen blistering and hydrogen-induced cracking (HIC) pose significant threats to structural integrity. The 15CrMoR steel, commonly used for pressure vessel fabrication due to its excellent high-temperature strength and cost-effectiveness, is particularly susceptible to hydrogen damage in these environments.

TP347 (UNS S34700) austenitic stainless steel, stabilized with niobium, offers excellent resistance to intergranular corrosion and good performance in high-temperature oxidizing environments. However, its application in hydrogen service requires careful evaluation, as austenitic stainless steels can also be susceptible to hydrogen blistering under certain conditions.

The single-layer surfacing approach represents a departure from the more conventional multi-layer surfacing strategy, offering potential advantages in terms of process efficiency, reduced dilution control complexity, and lower manufacturing cost.

Experimental Methodology

The study employs electroslag welding (ESW) as the surfacing process, selected specifically for its low dilution rate characteristic. Two different TP347 welding strips are evaluated, designated as Type A and Type H, to assess the influence of consumable composition variations on hydrogen resistance performance.

The evaluation protocol includes:

1. Chemical composition analysis: Verification of surfacing layer chemistry against TP347 specification requirements
2. Mechanical property testing: Tensile strength, yield strength, and elongation assessment
3. Hardness profiling: Microhardness measurement across the surfacing layer and interface
4. Ferrite content measurement: Ferrite number determination to assess phase balance
5. Intergranular corrosion testing: Standard immersion or electrolytic testing for sensitization evaluation
6. High-temperature high-pressure hydrogen blistering test: Evaluation under simulated hydrogen service conditions with controlled cooling rates
7. Electrolytic hydrogen charging test: Assessment of hydrogen embrittlement susceptibility

Key Results and Technical Analysis

The most significant finding is the differential performance between Type A and Type H surfacing consumables under electrolytic hydrogen charging conditions. Type H demonstrates superior resistance to hydrogen blistering compared to Type A, indicating that compositional variations within the TP347 specification can significantly influence hydrogen damage susceptibility.

Electroslag Welding: Dilution Control Advantage

The selection of electroslag welding for this application is technically justified by several factors:

1. Low dilution rate: ESW typically achieves dilution rates of 5-15%, significantly lower than conventional arc welding processes (20-40%), ensuring that the surfacing layer maintains its TP347 composition with minimal base metal contamination.
2. Thermal input control: The slag pool provides thermal insulation, allowing controlled cooling rates that minimize the formation of detrimental phases.
3. Deposition rate: ESW offers high deposition efficiency, making it economically attractive for thick surfacing layers required in pressure vessel applications.
4. Layer uniformity: The process produces surfacing layers with consistent composition and microstructure across the entire deposit thickness.

Hydrogen Blistering Mechanism Analysis

Hydrogen blistering occurs when atomic hydrogen diffuses into the metal and accumulates at internal defects, inclusions, or phase boundaries, forming molecular hydrogen (H2) that generates sufficient pressure to create internal voids or blisters. The susceptibility to hydrogen blistering depends on:

• Hydrogen absorption rate: Influenced by surface condition, electrochemical potential, and environmental factors
• Hydrogen diffusion rate: Governed by temperature, microstructure, and crystal structure
• Trap density: Carbon inclusions, oxide particles, and phase boundaries serve as hydrogen traps
• Critical hydrogen pressure: The internal pressure required to initiate blister formation

The differential performance between Type A and Type H suggests that compositional differences affect one or more of these factors. Possible mechanisms include:

• Inclusion content: Type H may contain fewer or less severe carbon inclusions that serve as hydrogen traps
• Phase balance: Subtle differences in austenite/ferrite ratio may influence hydrogen diffusion pathways
• Stabilization: Niobium stabilization effectiveness may vary between consumable grades, affecting carbon availability for carbide precipitation
• Sulfur and phosphorus content: These impurities influence inclusion morphology and hydrogen trapping behavior

Engineering Practice Integration

The findings have direct implications for the design and fabrication of pressure vessels and heat exchangers operating in high-temperature hydrogen service:

• Consumable selection: The Type H consumable's superior hydrogen resistance should be specified for critical applications, even if both types meet the basic TP347 chemical specification.
• Process qualification: Electroslag welding should be qualified specifically for hydrogen service applications, with hydrogen blistering testing included in the qualification protocol.
• Cooling rate control: The high cooling rate blistering results indicate that post-weld cooling rate must be controlled to minimize hydrogen damage susceptibility.
• Design considerations: Single-layer surfacing may be sufficient for many applications, reducing manufacturing complexity and cost compared to multi-layer approaches.

Critical Reflections and Study Insights

The single-layer surfacing approach, while offering process simplicity, raises important questions about long-term durability under cyclic hydrogen exposure. The blistering observed under high cooling rates suggests that the surfacing layer may develop microstructural features (such as retained austenite or fine carbide distributions) that become susceptible to hydrogen damage when cooling conditions are not carefully controlled.

The electroslag welding process, while excellent for dilution control, introduces its own challenges:

• Hydrogen pickup: The slag environment can introduce hydrogen into the weld pool, potentially increasing the hydrogen content of the surfacing layer.
• Inclusion formation: Slag inclusions trapped at the surfacing layer surface or within the deposit may serve as hydrogen traps.
• Thermal cycling effects: The high heat input of ESW may promote grain growth and phase coarsening that affects hydrogen diffusion behavior.

The study's emphasis on hydrogen blistering testing represents a significant advancement in surfacing qualification methodology. Traditional surfacing qualification focuses on mechanical properties, chemical composition, and corrosion resistance, but often neglects hydrogen damage susceptibility, which can be a critical failure mechanism in hydrogen service.

Reference Value and Outlook

This study provides valuable guidance for engineers designing surfacing solutions for pressure vessels operating in high-temperature hydrogen environments. The demonstration that consumable selection significantly influences hydrogen blistering resistance underscores the importance of going beyond basic chemical specification compliance and evaluating hydrogen damage susceptibility as a critical qualification parameter. The single-layer electroslag welding approach offers a practical and economical solution for achieving corrosion-resistant surfaces on 15CrMoR pressure vessels, provided that hydrogen damage resistance is properly evaluated and controlled.