Ouyang Xin 1, Cheng Lei 1, peng shi 篧1, yuan chunming 2, ma zhiyuan 2, wang jinhua 3, guo dagang 2
(1. Branch of the Science and Technology Research Institute of National Oil and Gas Pipeline Network Group Co., LTD., Tianjin 300457; 2. State Key Laboratory of Metal Strength, School of Materials Science and Engineering, Xi 'an Jiaotong University, Xi 'an 710049, Shaanxi; 3. National Key Laboratory of Green Hydrogen Power, Xi 'an Jiaotong University, Xi 'an 710049, Shaanxi)
[Abstract] : This paper expounds the main research progress of key component materials for centrifugal compressors such as separators, reflux devices, seals, impellers, and bearings, as well as the cutting-edge achievements in hydrogen compatibility studies. It systematically reviews the hydrogen compatibility testing methods and the three hydrogen embrittlement mechanisms of HEDE, HELP, and AIDE, along with their synergistic effects. Finally, it looks forward to the future development trends.
[Key words] : Hydrogen-blended natural gas Centrifugal compressor materials; Hydrogen compatibility Hydrogen embrittlement mechanism
Chinese Library Classification Number: TH452 Document Code: B
Article Number: 1006-2971 (2026) 01-0059-06
1 Introduction
In recent years, the global energy structure has been accelerating its transformation towards low-carbonization. Hydrogen energy, with its zero-carbon emission feature, has become a key energy carrier for achieving the carbon neutrality goal. The International Energy Agency (IEA) predicts that by 2050, hydrogen energy will account for 12% to 15% of global terminal energy consumption [1]. China's "Medium and Long-Term Plan for the Development of the Hydrogen Energy Industry (2021-2035)" clearly states that it is necessary to focus on breaking through the technical bottlenecks in hydrogen storage and transportation and promote the demonstration project of hydrogen blending in natural gas pipelines [2]. Against this backdrop, using the existing natural gas pipeline network for hydrogen blending and transportation has become an important way to reduce the cost of hydrogen energy infrastructure construction. Research shows that when the hydrogen blending ratio is controlled at 10% to 20%, the compatibility of the existing pipeline network materials and equipment is better, and the renovation investment can be reduced by 50% to 70% [3]. However, the high diffusivity and permeability of hydrogen pose a severe challenge to the material performance of the core equipment of the transportation system - the centrifugal compressor. As a key equipment in the hydrogen-blended natural gas transmission system, the centrifugal compressor undertakes core functions such as gas pressurization, energy conversion and stable transmission. This equipment is mainly composed of a stator and a rotor. The stator includes cylinders, partitions, air seals, bearings, etc., while the rotor is mainly made up of impellers, main shafts, thrust discs and couplings, etc. All key components are subject to complex dynamic loads under high-temperature, high-speed and high-pressure conditions. Therefore, the materials used not only need to possess the traditional high strength, good wear resistance and corrosion resistance [4], but also must exhibit good hydrogen compatibility in hydrogen-containing environments to suppress hydrogen intrusion and hydrogen embrittlement [5]. Due to the strong diffsibility of hydrogen, its penetration into metals may lead to weakened local grain boundaries, increased dislocation activity and the formation of microcracks, which poses a severe challenge to the long-term stable operation of equipment. Therefore, the material selection and optimization for each key component of centrifugal compressors have become an important research direction for solving the safety issues of hydrogen blending transportation.
In a hydrogen-doped environment, metallic materials often experience performance degradation due to hydrogen invasion [6], mainly manifested in local hydrogen-induced brittle fracture, accelerated fatigue crack propagation, and reduced fracture toughness, etc. A large number of literatures have shown that under higher hydrogen partial pressure or hydrogen blending conditions, even if the static mechanical properties of the material do not significantly decrease, its fatigue resistance and ductility will drop sharply [7]. The traditional pre-hydrogen filling test is difficult to accurately reflect the hydrogen damage behavior under actual working conditions due to the hydrogen escape effect. Therefore, how to accurately evaluate and predict the hydrogen compatibility of metallic materials in a hydrogen-doped environment has become an important issue that needs to be urgently addressed at present. For this reason, scholars at home and abroad have been constantly exploring various testing techniques, such as online high-pressure hydrogen testing, slow strain rate tensile testing (SSRT), fatigue crack growth testing and low-cycle fatigue testing, etc., aiming to establish a set of testing methods that can truly reflect the performance of materials in a hydrogen environment. Meanwhile, by establishing multi-scale numerical models and in-situ characterization techniques, the diffusion and aggregation behavior of hydrogen within metallic materials are deeply analyzed, providing a theoretical basis for the design of new hydrogen embrittle-resistant materials [8]. In addition, regarding the mechanism of hydrogen embrittlement, there are currently mainly theories such as hydrogen reducing cohesion (HEDE), hydrogen-induced local plastic deformation (HELP), and adsorbed hydrogen causing dislocation emission (AIDE). These mechanisms not only have their own characteristics but also often exhibit coupling effects in practical applications, making it difficult to fully explain the hydrogen erosion resistance behavior of materials with a single theory.
This paper reviews the research progress of key component materials and hydrogen compatibility of centrifugal compressors, with a focus on discussing the materials of each key component of centrifugal compressors and their main research progress, hydrogen compatibility testing methods, influencing factors of hydrogen compatibility, and the theory of hydrogen-induced multi-mechanism coupling damage, with the aim of providing theoretical references for the reliability improvement of hydrogen-blended transportation equipment.
Research Progress on Materials for Centrifugal Compressors
Due to the different functions and stress environments they undertake, the material requirements for each component of a centrifugal compressor also have their own emphases: The separator is required to have excellent casting performance, wear resistance and impact resistance, and gray cast iron or ductile iron is often used. The reflux device is required to have good processability and uniform force-bearing performance, and low-carbon steel or stainless steel is mostly used. Seals not only need to achieve good air tightness, but also ensure wear resistance and corrosion resistance during high-speed operation. In recent years, PEEK composite materials have gradually attracted attention due to their high performance. As the component most directly involved in energy conversion, the impeller not only requires high strength and corrosion resistance, but also has strict requirements for fatigue resistance. Commonly used materials are specific stainless steel or low alloy high strength steel. Bearings, on the other hand, emphasize overall fatigue life and anti-wear performance, and often use materials with special structural designs such as tin-based alloys. The commonly used materials and research progress of each component of the centrifugal compressor are shown in Table 1.
The partition plate is the gas channel for forming fixed components in a centrifugal compressor [23]. It is mostly made of cast iron or ductile iron materials. Among them, QT400-18 ductile iron and 16Mn are more suitable as partition plate materials due to their excellent comprehensive mechanical properties, weldability and wear resistance, and have a very good application prospect.
The return valve is a channel for uniform pressure increase. It is equipped with a certain number of blades inside to improve the gas flow condition and guide the gas flow smoothly into the next stage impeller. It is composed of two partitions and the blades installed between the partitions [24]. The materials for reflux devices are often Q235-A and 0Cr18Ni9. Compared with Q235-A, 0Cr18Ni9 steel has better comprehensive mechanical properties and corrosion resistance, making it more suitable as the material for reflux devices.
The function of the seal is to reduce the gap leakage between the compressor rotor and the fixed components. The PEEK material specifically designed for centrifugal compressors is superior to ZL102, ZL104 and ZL105 in terms of sealing performance, mechanical properties and corrosion resistance. Moreover, the integrated design of metal and plastic also significantly reduces material costs and has a very broad application prospect in the field of sealing materials.
Impellers are classified into three types: open, semi-open and closed. Commonly used materials include X12Cr13 stainless steel, FV520B, KMN steel, etc. Compared with other materials, KMN and FV520B have better strength, toughness and corrosion resistance, and are often used as impeller materials by compressor manufacturers.
Bearings are parts that are subjected to significant forces and have complex and variable loads during the operation of compressors. The material of bearings should have comprehensive properties such as high compressive strength, fatigue strength, sufficient plasticity and toughness, as well as high vibration resistance [25]. Commonly used bearing materials include ZSnSb11Cu6, ZChSnSb11-6, QT500-7, etc. [26]
Research Progress on Hydrogen Compatibility of Materials for Centrifugal Compressors
In hydrogen-blended natural gas transmission systems, centrifugal compressors, as core equipment, have increasingly prominent hydrogen compatibility issues with their materials during service. Hydrogen gas penetrating into the interior of metals often leads to hydrogen embrittlement in the materials, reducing ductility, fatigue life and fracture toughness, which may cause equipment failure or even accidents. For this reason, scholars at home and abroad have carried out a large number of experiments and theoretical studies on the hydrogen compatibility of materials for centrifugal compressors, and have successively proposed pre-filled hydrogen tests, in-line high-pressure hydrogen tests, and slow response variable speed rate tensile tests
A variety of hydrogen compatibility testing methods such as (SSRT), fatigue crack growth test, and low-cycle fatigue test. Meanwhile, by means of in-situ characterization techniques and multi-scale numerical simulation methods, the microscopic processes such as hydrogen diffusion, adsorption, segregation and crack propagation were deeply discussed. At present, the mainstream views on the mechanism of hydrogen embrittlement mainly include hydrogen reducing cohesion (HEDE), hydrogen-induced local plastic deformation (HELP), and adsorbed hydrogen causing dislocation emission (AIDE). However, the latest research shows that under actual working conditions, these three mechanisms often act in coupling, jointly determining the hydrogen embrittlement sensitivity of materials.
3.1 Hydrogen Compatibility Test Method
The commonly used hydrogen compatibility testing methods are shown in Table 2. In the field of hydrogen embrittlement research of materials, traditional pre-hydrogen filling methods have obvious limitations. This method conducts mechanical tests through electrochemical or gas environment pre-filling of hydrogen, but it cannot accurately simulate the synergistic effect of hydrogen and stress under actual service conditions. In contrast, the high-pressure hydrogen environmental test achieved triple similarity in the three dimensions of environment, stress field and hydrogen concentration field to the real working conditions. This type of test usually adopts direct slow strain rate tensile, fatigue crack growth and low-cycle fatigue testing methods under high-pressure hydrogen environment, and combines scanning electron microscope fracture analysis to reveal the damage mechanism of hydrogen on centrifugal compressor materials from macro and micro levels. This dynamic coupling test method can simultaneously observe the interaction between the adsorption, diffusion, and aggregation behaviors of hydrogen and the material deformation process, providing more reliable data support for engineering applications.
3.2 Factors Influencing Hydrogen Compatibility
3.2.1 Environmental Air Pressure
Under high-pressure hydrogen conditions, when the hydrogen concentration in some areas of metallic materials reaches the critical value, their toughness significantly decreases and hydrogen-induced hysteresis fracture occurs [28]. Moreover, the solubility of hydrogen atoms in metals such as iron is directly proportional to the square root of hydrogen pressure [29]. Other studies have shown that within a certain range, increasing the environmental air pressure will promote the further dissolution of hydrogen atoms in metals [30], especially increasing the accumulation of hydrogen atoms at the crack tips inside metals and promoting embrittlement [31].
In recent years, the research on hydrogen embrittlement of metallic materials has attracted much attention from scholars in the field of materials in various countries. ANDREW et al. [32] investigated the fatigue crack growth behavior of two pipeline steel alloys, X52 and X100, under hydrogen pressure ranging from 1.7 to 48MPa. They found that both materials exhibited significantly different fatigue crack growth rates in hydrogen and air environments, with the former being 1 to 2 times higher than the latter
The order of magnitude, and within a certain intensity factor range, this deterioration effect intensifies with the increase of hydrogen pressure. AMARO et al. [33] further investigated the tensile and fatigue crack growth properties of these two pipeline steel alloys in a hydrogen environment.
Studies show that the elongation of the tensile specimens of the two types of steel in a high-pressure hydrogen environment of 13.8MPa is significantly less than that in an air environment, and the toughness loss of the former is significantly higher than that of the latter. Within a certain range of stress intensity factors, the fatigue crack growth rates of the two types of steel increase with the increase of hydrogen pressure. When the hydrogen pressure increases from 1.72MPa to 20.68MPa, the fatigue crack growth rate of X100 steel increases by 2 to 10 times.
It is worth noting that when the hydrogen pressure reaches a certain threshold, the degree of material performance deterioration tends to stabilize. This phenomenon has been further verified in the study of the heat-affected zone of X70 steel welding [34]. At present, further systematic and in-depth research is still needed on the critical threshold of hydrogen pressure causing hydrogen embrittlement in various engineering pipeline steel materials and the mechanism of hydrogen embrittlement sounds.
3.2.2 Hydrogen blending ratio
Hydrogen blending transportation through natural gas pipelines is an effective way for large-scale hydrogen transportation [35-36]. Especially, hydrogen blending transportation through natural gas pipeline systems to provide gas for residential and commercial users is currently recognized as an extremely efficient and feasible strategy. However, in order to minimize the significant decline in the mechanical properties of the metal materials in the pipeline network (such as fracture toughness and fatigue performance, etc.) caused by hydrogen blending, it is first necessary to determine the appropriate hydrogen blending ratio range.
In recent years, scholars at home and abroad have conducted research on the performance of steel in hydrogen-doped natural gas environments with different hydrogen blending ratios. NGUYEN et al. [37-38] conducted a study on the hydrogen embrittlement behavior of X70 pipeline steel in hydrogen-blended natural gas environments. The results showed that in hydrogen-blended natural gas with a total pressure of 5 to 10MPa, the hydrogen embrittlement sensitivity of this material significantly increased with the increase of the hydrogen blending ratio. When the volume fraction of hydrogen reaches a critical value, the material's fracture mode changes from ductile fracture to brittle fracture. AN et al. [39] studied the hydrogen-doped environment of X80 pipeline steel under a total pressure of 12MPa and found that with the increase of hydrogen blending ratio, the fatigue cycle number of notched specimens decreased rapidly (by 20% to 90%), while the crack growth rate of compact tensile specimens increased sharply (by 7 to 14 times). This study further points out that the increase in crack growth rate is the main reason for the reduction in fatigue life of X80 steel. MENG et al. [40] also reached similar conclusions in their study on the influence of a mixture of natural gas and hydrogen with a hydrogen volume fraction of 0-50% on the mechanical properties of X80 pipeline steel at a pressure of 12MPa.
At present, although significant progress has been made in the research on the influence of hydrogen blending ratio on the hydrogen embrittlement sensitivity of steel in a hydrogen-blended environment, which has re-established a new understanding among scholars in the field of materials, and the influence laws and degrees have generally been recognized, there is still no unified conclusion on the range of pressure and hydrogen blending ratio for the safe operation of various pipeline steel materials in a hydrogen-blended environment. In-depth and systematic research is still needed This will be of crucial significance for the strategy of large-scale hydrogen transportation.
3.3 Hydrogen embrittlement Mechanism
When hydrogen is mixed into natural gas pipelines, it may cause risks such as hydrogen embrittlement, hydrogen bubbling, decarburization and hydrogen corrosion in the pipeline materials. Among them, hydrogen embrittlement and hydrogen corrosion pose the greatest risks and are the most harmful. Hydrogen embrittlement and hydrogen corrosion of pipelines are complex processes in which hydrogen forms solid solutions, hydrides, molecular hydrogen and gas products with pipeline metals or additives in metals. These can weaken the bonding force at metal grain boundaries, leading to a decrease in the plasticity of the pipe material and resulting in brittle fracture, micro-cracks or pitting corrosion [41-43]. The factors influencing hydrogen embrittlement, apart from air pressure and hydrogen blending ratio, also include ambient temperature, the strength level of pipeline materials, deformation rate and microstructure, etc.
In fact, the state of hydrogen in the pipeline can also affect its interaction mode with metals, and thus the types of hydrogen embrittlement it causes are also different. Hydrogen embrittlement can be classified into three types based on the differences in hydrogen atom aggregation behavior: internal hydrogen embrittlement (IHE), environmental hydrogen embrittlement (EHE), and hydrogen reaction hydrogen embrittlement (HRE) [44]. Among them, internal hydrogen embrittlement is related to lattice distortion, dislocation pinning and crack nucleation caused by hydrogen atoms entering the metal lattice. Environmental hydrogen embrittlement is related to the formation of hydrogen molecule aggregation areas at the crack tip under high-pressure hydrogen conditions, local stress promoting hydrogen adsorption or surface energy decline, etc. Hydrogen reaction hydrogen embrittlement is related to irreversible reactions with specific elements in the material (such as carbon, alloy elements, etc.). In addition, the mechanism of hydrogen embrittlement is extremely complex. Based on the microscopic mechanism of hydrogen-induced damage, there are currently three major mainstream theories: the theory of hydrogen reducing cohesion (HEDE), the theory of hydrogen-induced local plastic deformation (HELP), and the theory of hydrogen adsorption causing dislocation ejection (AIDE).
3.3.1 Hydrogen reduces cohesion (HEDE) Theory
The HEDE theory holds that hydrogen atoms accumulate at the grain boundaries, crack tips or defects of metals, reducing the bonding force between atoms and leading to brittle fracture of the material under low stress. This theory is particularly applicable to explaining the mechanism of hydrogen-induced cracking of high-strength steel under a high-pressure hydrogen environment [45]. ORIANI [46] proposed through thermodynamic analysis that the segregation of hydrogen within metals would significantly reduce the grain boundary energy, thereby weakening the grain boundary strength. First-principles calculations further verified this view: JIANG and CARTER [47] found that the adsorption of hydrogen at the α-Fe grain boundaries could reduce the interfacial binding energy by more than 30%. NAGUMO et al. [48] confirmed through hydrogen thermal analysis (TDS) that the enrichment concentration of hydrogen at the grain boundaries of steel can reach that of the matrix
Hundreds of times, consistent with the critical hydrogen concentration for brittle fracture. However, the HEDE theory is difficult to explain the local plastic deformation characteristics observed in hydrogen embrittlement. In recent years, multi-scale simulations (such as the coupling of molecular dynamics and continuum mechanics) have shown that hydrogen may simultaneously reduce cohesion and promote dislocation movement through synergistic effects, facilitating the integration of HEDE with other theories [49].
3.3.2 Hydrogen-Induced Local Plastic Deformation (HELP) Theory
The HELP theory holds that hydrogen enhances the dislocation movement ability, leading to the concentration of local plastic deformation and subsequently triggering crack initiation and propagation. BEACHEM [50] found through fracture analysis that there were significant plastic rheological traces at the crack tip in the hydrogen environment, and proposed that hydrogen promoted local slip through the interaction between shielding dislocations.
BIRNBAUM and SOFRONIS [51] further proposed the mechanism by which hydrogen reduces the nuclear barrier of dislocations and observed through in-situ transmission electron microscopy that the proliferation rate of dislocations significantly increased in a hydrogen environment. ROBERTSON et al. [52] confirmed by environmental transmission electron microscopy (ETEM) that hydrogen can promote the cross-slip and climbing of dislocations in stainless steel, leading to local softening. Molecular dynamics simulations show that hydrogen atoms reduce the activation energy of dislocation motion by approximately 40% by lowering the Peierls stress [53]. However, the explanation of brittle fracture under low hydrogen concentration by the HELP theory remains controversial. Some scholars believe that it is more applicable to high hydrogen concentration or dynamic loading conditions [54].
3.3.3 Adsorption Hydrogen dislocation emission (AIDE) Theory
The AIDE mechanism was proposed by LYNCH, who suggested that the adsorption of hydrogen on the crack surface reduces the surface energy required for dislocation emission, promoting the emission of dislocations from the crack tip and leading to crack propagation through alternating dislocation emission and cleavage [55]. XING et Al. [56] calculated through density functional theory (DFT) that hydrogen adsorption could reduce the surface energy at the tip of Al cracks by 35%, significantly promoting dislocation nucleation. ZHAO et al. [57] combined atomic probe tomography (APT) and first-principles calculations to reveal the atomic-scale distribution of hydrogen traps in aluminum alloys and their influence on embrittlement. The AIDE theory successfully explains the formation of the mixed type (dimmer + cleavage) morphology in hydrogen embrittlement fractures, but its neglect of the interaction of bulk hydrogen has been questioned. KIRCHHEIM [58] proposed that AIDE might work in synergy with HEDE/HELP. For instance, hydrogen adsorption promotes dislocation emission (AIDE), while bulk hydrogen further accelerates dislocation movement (HELP).
4 Prospects
The rapid development of hydrogen-blended natural gas transportation technology has put forward increasingly higher requirements for the material performance of centrifugal compressors. Current research has significantly enhanced the mechanical properties and corrosion resistance of key components such as partitions and impellers through alloying design, surface modification and advanced manufacturing technologies. Among them, new composite materials and functionalized coating technologies have demonstrated significant application potential. However, the embrittlement problem of materials in a hydrogen environment remains prominent. The increase in hydrogen partial pressure and hydrogen blending ratio leads to a loss of material toughness and a sharp increase in crack propagation rate. The essence of this is due to the enrichment of hydrogen at grain boundaries and the coupling effect of multiple mechanisms. In the future, efforts should be made to tackle key issues in the development of hydrogen embrittlement resistant materials, the analysis of multi-scale damage mechanisms, and the establishment of an engineering applicability evaluation system. The focus is on exploring new anti-hydrogen embrittlement material systems, combining multiple characterization techniques to reveal the interaction mechanism of hydrogen, stress and environment, and establishing a material performance database and intelligent prediction model covering complex working conditions. Through the collaborative innovation of the three elements of material composition, structure and properties, we promote the development of hydrogen-blended compressors towards high pressure and long service life, providing support for the efficient and safe operation of hydrogen energy infrastructure.
