Single-Layer TP347 Surfacing Technology and Performance Evaluation for Hydrogen Blistering Resistance

Literature Overview

This paper by Li Xiaowei and colleagues from Sinopec Refining Engineering Group and the China Petrochemical Corporation Equipment Anti-Corrosion Research Center investigates single-layer TP347 stainless steel surfacing technology on 15CrMoR steel using electroslag welding (ESW) as the deposition process. Published in Pressure Vessel Technology (Vol. 36, No. 5, 2019, pp. 8-13), the study was supported by a Sinopec Refining Engineering Group Science and Technology Development Project (117008). The research focuses on evaluating the performance of single-layer TP347 surfacing for resistance to high-temperature high-pressure hydrogen blistering, a critical failure mechanism in hydrogen-containing pressure equipment in the petroleum refining industry.

Engineering Background and Technical Challenge

High-temperature high-pressure hydrogen (HTHPH) service is common in petroleum refining processes such as hydrocracking, hydrotreating, and reforming. In these environments, atomic hydrogen diffuses into the steel, combines at internal defects or interfaces, and forms molecular hydrogen gas bubbles that cause blistering, cracking, and eventual catastrophic failure. This is known as high-temperature hydrogen attack (HTHA) or hydrogen blistering, and it is a major concern for pressure vessel and piping integrity in refineries.

The base material 15CrMoR is a chromium-molybdenum alloy steel commonly used for pressure vessel construction due to its good strength and creep resistance at elevated temperatures. However, 15CrMoR has limited resistance to HTHA, particularly at temperatures above approximately 200°C and hydrogen partial pressures above 1.0 MPa. The addition of a corrosion-resistant overlay layer such as TP347 stainless steel can provide a barrier against hydrogen ingress and improve the overall resistance of the component to HTHA.

TP347 Material Characteristics

TP347 is a stabilized austenitic stainless steel containing approximately 18-21% Cr, 9-13% Ni, and 0.7-1.1% Nb (niobium). The niobium stabilization prevents chromium carbide precipitation at grain boundaries during high-temperature exposure, thereby maintaining intergranular corrosion resistance. This makes TP347 particularly suitable for high-temperature applications where sensitization would otherwise compromise the material's corrosion resistance.

The key properties of TP347 relevant to HTHA resistance include:

Electroslag Welding Process for Single-Layer Surfacing

The selection of electroslag welding (ESW) for single-layer surfacing is significant because ESW offers a lower dilution rate compared to conventional arc welding processes. The electroslag process involves the passage of electric current through a molten slag pool, which generates the heat required for melting. The slag acts as both a heat source and a shielding medium, creating a unique welding environment that results in lower dilution of the base metal into the deposit.

For single-layer surfacing, achieving low dilution is critical because it ensures that the overlay composition closely matches the nominal TP347 composition, retaining the essential Cr, Ni, and Nb content required for corrosion resistance. Conventional processes such as submerged arc welding or flux-cored arc welding typically have dilution rates of 30-60% for single-layer surfacing, which would significantly reduce the Cr and Ni content in the deposit. The ESW process, with its lower dilution rate, enables single-layer surfacing to achieve adequate overlay composition without the need for multiple layers, simplifying the manufacturing process and reducing production costs.

Performance Evaluation Results

The study evaluated two types of TP347 welding strips (designated as Type A and Type H) for single-layer surfacing on 15CrMoR steel. The key performance results are summarized as follows:

The most significant finding is the difference in hydrogen blistering resistance between the two TP347 welding strip types. Under high-temperature high-pressure hydrogen testing at lower cooling rates, both samples performed adequately without blistering. However, at higher cooling rates, Type A samples experienced blistering while Type H samples remained intact. Similarly, in electrolytic hydrogen charging tests, Type A samples showed blistering while Type H samples did not. This indicates that the Type H welding strip produces a surfacing layer with superior resistance to hydrogen blistering, particularly in the fusion zone region.

Hydrogen Blistering Mechanism and Interface Analysis

The hydrogen blistering mechanism in surfaced components involves several stages. First, atomic hydrogen is generated at the metal surface through corrosion reactions or catalytic effects. Second, atomic hydrogen diffuses through the metal lattice to internal sites such as inclusions, interfaces, and grain boundaries. Third, atomic hydrogen combines to form molecular hydrogen (H2) at these sites, creating internal pressure that leads to blister formation. Fourth, under sustained pressure, the blisters can grow and coalesce, leading to cracking and loss of structural integrity.

The resistance to hydrogen blistering depends on several factors: the hydrogen permeability of the overlay material, the cleanliness of the overlay-base metal interface, the presence of internal defects that serve as hydrogen trapping sites, and the residual stress state. The superior performance of Type H welding strip suggests that its composition or microstructure results in a fusion zone with fewer hydrogen trapping sites or better resistance to hydrogen permeation. This could be attributed to differences in inclusions, grain structure, or residual stress state between the two welding strip types.

Standards and Code Considerations

The evaluation of TP347 surfacing for HTHA resistance must comply with relevant standards and codes. The applicable standards include API 941 for high-temperature hydrogen attack resistance, NACE MR0175/ISO 15156 for materials in H2S-containing environments, and ASME B31.3 for process piping design. The hydrogen blistering testing methodology should follow established procedures such as those specified in ASTM G119 or similar standards for hydrogen blistering resistance evaluation.

The intergranular corrosion testing is particularly important for TP347 because, despite its niobium stabilization, improper welding or heat treatment can still lead to sensitization in localized regions. The study confirms that both TP347 samples exhibit good intergranular corrosion resistance, which is essential for ensuring the long-term integrity of the surfacing layer in high-temperature service.

Key Questions and Reflections

The study raises several important questions for engineering practice. First, the specific composition and microstructural differences between Type A and Type H welding strips that account for the difference in hydrogen blistering resistance are not fully detailed. Understanding these differences is essential for selecting the appropriate welding consumable for HTHA-critical applications. Second, the study does not report on the long-term creep behavior of the surfacing layer under sustained high-temperature hydrogen exposure, which is important for pressure vessel applications where components are subjected to cyclic loading.

Another consideration is the effect of the electroslag welding process parameters on the hydrogen blistering resistance of the surfacing layer. The ESW process involves high heat input and slow cooling, which can influence the microstructure and inclusion content of the deposit. Optimizing the ESW parameters to minimize hydrogen trapping sites and maximize overlay quality is essential for achieving the best HTHA resistance.

Study Insights and Implications

This research demonstrates that single-layer TP347 surfacing using electroslag welding is a viable technology for improving the HTHA resistance of 15CrMoR pressure vessel components. The lower dilution rate of ESW enables single-layer surfacing to achieve adequate overlay composition, simplifying the manufacturing process. The finding that welding strip type significantly affects hydrogen blistering resistance highlights the importance of consumable selection in surfacing technology development. For engineers designing pressure equipment for HTHA service, this study provides practical guidance on the use of single-layer TP347 surfacing and underscores the critical role of welding consumable quality in achieving the desired performance. The results also suggest that the fusion zone between the overlay and base metal is a critical region for hydrogen blistering initiation, and that optimizing the fusion zone composition and microstructure is essential for maximizing HTHA resistance.