Selection and Application of Special Alloy Valve Materials

Abstract

This paper examines the characteristics and application requirements of three special alloy valve materials: duplex stainless steel, Monel alloy, and Inconel alloy. It focuses on their use in highly corrosive environments within the petrochemical and coal chemical industries. The analysis covers material composition, structure, forming methods, manufacturing standards, operating conditions, and key considerations for valves made of special alloy. The paper also investigates the current applications of these three valves, comparing domestic and international standards, engineering design specifications, and practical engineering requirements. This study provides a valuable reference for engineers in selecting appropriate valve materials.

Introduction

Valves are essential control components in fluid transmission systems across industries such as petroleum, chemical processing, long-distance pipelines, and municipal engineering. Their primary functions include flow diversion, shutoff, regulation, and backflow prevention. Proper valve material selection is crucial for ensuring the safety and reliability of valves, pipeline systems, and overall plant operations. Valves made of conventional materials such as cast iron, carbon steel, alloy steel, and austenitic stainless steel are widely used in public infrastructure and mildly corrosive environments, where their applications are well established. In contrast, valves made from duplex stainless steel, Monel alloy, and Inconel alloy are designed for highly corrosive environments. These valves are significantly more expensive and therefore require careful selection. A thorough understanding of these metals, along with adherence to sound engineering material selection principles, is essential for cost-effective equipment design and ensuring long-term operational reliability. This paper analyzes the properties and key considerations of special alloys to serve as a reference for engineers in selecting valve materials.

1. Duplex Stainless Steel

Duplex stainless steel has a dual-phase microstructure composed of austenite and ferrite. The two phases are typically present in nearly equal proportions, with ferrite content ranging from 35% to 65%. Duplex stainless steel provides the toughness and weldability of austenitic stainless steel while offering the high strength of ferritic stainless steel.

1.1 Classification and Grades

Duplex stainless steel is primarily made up of Fe, Cr, Ni, Mo, and other alloying elements, with casting grades standardized under ASTM A890/A890M. The ASTM A890/A890M-18a standard classifies cast duplex stainless steel into eight grades: 1B, 1C, 2A, 3A, 4A, 5A, 6A, and 7A, based on differences in chemical composition. The Chinese casting standard GB/T 2100-2017 defines material grades corresponding to those in ASTM A890/A890M.

In the United States, forged duplex stainless steel is standardized according to ASTM A182/A182M-20, which includes 16 grades such as F50, F51, F52, F53, F54, F55, F57, F59, F60, F61, F65, F66, F67, F68, F69, and F71. The Chinese forging standard NB/T 47010-2017, Stainless Steel and Heat-Resistant Steel Forgings for Pressure Equipment, defines six grades of forged duplex stainless steel.

Domestic valve manufacturers primarily follow American duplex stainless steel standards due to industry conventions and the wider variety of American standard grades. The most commonly used grades include F51, F53, F55, and F61 per ASTM A182, as well as 1B, 4A, 5A, and 6A per ASTM A890. Pitting corrosion resistance is evaluated using the pitting resistance equivalent number (PREN = Cr% + 3.3 × Mo% + 16 × N%), which is determined based on the material’s composition and grade.

1.2 Performance and Application

The composition and microstructure of duplex stainless steel are critical factors influencing its susceptibility to stress corrosion cracking (SCC). Austenitic stainless steel is more susceptible to SCC in chloride environments, whereas ferritic stainless steel exhibits better resistance. Smaller ferrite grain sizes enhance resistance to SCC. As a result, duplex stainless steel provides significantly higher resistance to chloride-induced SCC, with a stress corrosion cracking threshold approximately three times greater than that of austenitic stainless steel. In chromium-nickel stainless steel, the ferrite-to-austenite phase ratio is typically evaluated using "chromium equivalent" and "nickel equivalent" calculations. Duplex stainless steel reduces the risk of SCC in austenitic stainless steels, such as 304L and 316L, especially when exposed to chloride-induced stress corrosion, particularly SCC initiated by pitting corrosion. Therefore, duplex stainless steel is commonly used in environments where chloride-induced stress corrosion is a concern, such as catalytic cracking and hydrocracking units in the oil refining industry, PVC and vinyl chloride production units in the petrochemical sector, and wastewater treatment systems in the coal chemical industry.

1.3 Precautions for Use

The σ phase in duplex stainless steel is hard and brittle, and its precipitation significantly reduces the alloy's corrosion resistance. To preserve toughness and corrosion resistance, duplex stainless steel must be rapidly cooled after solution treatment to minimize σ phase formation. The σ phase precipitates when the material is exposed to temperatures between 316°C and 540°C for extended periods. Both national standards and petrochemical industry standards define temperature limits for duplex stainless steel. The maximum continuous operating temperature of duplex stainless steel is lower than that of austenitic stainless steel. Section 6.2 of SH/T 3059-2012 Petrochemical Pipeline Design Equipment Selection Specification specifies that the operating temperature of duplex stainless steel must not exceed 300°C. Appendices C3.2 and C6.9 of GB/T 20801.2-2020 Pressure Pipeline Specification—Industrial Pipeline highlight the risks of 475°C embrittlement and σ phase embrittlement in duplex stainless steel, which must be considered during material selection and design. Hence, duplex stainless steel should be avoided for high-temperature when selecting valves. Additionally, duplex stainless steel valves are typically solution-annealed at 950°C to reduce internal stresses. At temperatures below this, the martensitic structure can begin to precipitate brittle phases. In contrast, austenitic stainless steel is typically heat-treated at temperatures between 600°C and 800°C for stress relief.

2. Monel Alloy

Monel alloy is a nickel-copper (Ni-Cu) alloy, composed of approximately 50% nickel and 30% copper. In Monel alloy, nickel serves as the base metal, providing passivation, while copper is the primary alloying element, enhancing corrosion resistance in reductive conditions such as acidic or saline environments. Additionally, iron (Fe), carbon (C), silicon (Si), and other elements are added to enhance the alloy's mechanical properties and overall performance.

2.1 Classification and Grades

According to ASTM A494/A494M-18a, cast Monel alloys are classified into five primary grades: M35-1, M35-2, M30H, M25S, and M30C. In 2017, China introduced GB/T 35740-2017, which specifies two Ni-Cu casting grades for industrial valve applications: M35-1 and M30C. In practical applications, M35-1 is preferred due to its lower carbon and silicon content, making it ideal for valve casting. The primary forged Monel alloy grades include Monel 400, Monel C, Monel 403, Monel 404, Monel R-405, Monel 406, Monel 411, Monel K500, Monel 501, and Monel 502. ASTM B564 defines the chemical composition and mechanical properties of nickel-copper alloy UNS N04400 (Monel 400), while ASTM B865-04 outlines the specifications for precipitation-hardened nickel-copper-aluminum alloy UNS N05500. Monel 400 and Monel K500 are the preferred grades for corrosion-resistant valve forgings. Monel K500 is strengthened through the addition of aluminum (Al) and titanium (Ti) to Monel 400.

China's NB/T 47028-2012 Nickel and Nickel Alloy Forgings for Pressure Vessels specifies the NiCu30 grade of nickel-copper alloy, corresponding to ASTM N0400 (Monel 400). Additionally, GB/T 26030-2010 Nickel and Nickel Alloy Forgings defines three nickel-copper grades: NiCu30, NiCu30-LC, and NiCu30Al3Ti. However, China has not developed technical standards for nickel alloy forgings used in valves.

2.2 Performance and Application

Monel alloy, a nickel-based alloy, exhibits excellent corrosion resistance to reducing acids, strong alkaline media, and seawater. It is widely used in equipment and valves that handle hydrofluoric acid, hydrochloric acid, chlor-alkali solutions, and other reductive acid environments. Monel alloy exhibits excellent corrosion resistance to hydrofluoric acid, making it suitable for use at various concentrations, including at the boiling point of 10% hydrofluoric acid. In hydrofluoric acid systems, Monel alloy valves are available in two configurations: one with both the valve body and internal components made of Monel alloy, and another with a carbon steel valve body and Monel alloy internals. The choice between these valve types depends on the operating temperature and water content.

Because nickel readily reacts with sulfur and oxygen, Monel alloy is not suitable for sulfur-containing or strongly oxidizing environments. Sections 7.2.4 of GB/T 20801-2020 and 6.2.2 of SH/T 3059-2012 specify the operating temperature limits for nickel-copper alloys in sulfur-containing, oxidizing, and reducing environments. Monel alloy is commonly used for valves and pipelines in coal gasification and other high-pressure oxygen environments due to its excellent flame-retardant properties and high resistance to pressure in oxygen-rich conditions. Table 1 presents the standard pressure ratings and minimum wall thickness requirements for common metals. For example, the instrument root valve trim of a coal gasification unit in northern Shaanxi, operating at 9.8 MPa in a high-pressure oxygen environment, is made of Monel K500 alloy.

2.3 Precautions for Use

Due to differences in manufacturing processes, the chemical compositions of Monel casting and forging alloys in the ASTM standards differ, resulting in no direct one-to-one correspondence between their grades. In comparison to ASTM A494/A494M, China’s GB/T 35740-2017, "Technical Conditions for Nickel and Nickel-Based Alloy Castings for Industrial Valves," offers more detailed specifications for nickel alloy cast valves. GB/T 35740-2017 specifies the following requirements: major repair welding is not permitted for castings, valves must be flanged (welding is prohibited), and valve sealing surfaces must be integral with the body, without the use of cladding alloys. Additionally, the standard imposes more stringent and precise regulations on chemical composition, melting, casting, and quality requirements. Engineers must carefully evaluate the differences between relevant standards when designing systems and procuring valves.

Table 1: Common Metal Exemption Pressure and Minimum Wall Thickness Requirements According to the IGC Doc 13/12/E Standard"

3. Inconel Alloy

Inconel alloy is a nickel-based material with a higher nickel (Ni) content and lower iron (Fe) content than stainless steel. It offers superior resistance to corrosion from hot alkaline solutions and alkali sulfides, as well as exceptional resistance to high temperatures and oxidation.

3.1 Classification and Grades

Compared to U.S. ASTM standards, Chinese standards provide fewer specifications for nickel-chromium-iron alloys. Most domestic valve materials follow U.S. ASTM standards. Table 2 presents a comparison of Inconel alloy castings and forgings.

3.2 Performance and Applications

Inconel 600 is a nickel-chromium-iron alloy. Its high nickel content offers excellent corrosion resistance in reducing environments, while its chromium content improves oxidation resistance compared to pure nickel. Inconel 600 exhibits strong corrosion resistance to high-temperature sulfuric acid and concentrated organic acids, including room-temperature phosphoric and fatty acids. However, it is unsuitable for high-temperature hydrofluoric acid and concentrated hydrochloric acid, and is prone to stress corrosion in high-temperature, strongly alkaline environments. Section 7.2.4 of GB/T 20801-2020 and Section 6.2.2 of SH/T 3059-2012 define the temperature limits for nickel-chromium-iron alloys in sulfur-containing, oxidizing, and reducing environments.

Inconel 625 is a low-carbon nickel-chromium-molybdenum-niobium alloy with excellent resistance to a wide range of corrosive media. It is commonly used in reducing acid environments and applications susceptible to pitting and crevice corrosion. It is extensively used in acidic gas environments for flue gas desulfurization and in high-pressure oxygen pipelines for coal gasification units. Table 2 lists the exemption pressure and minimum thickness requirements for this alloy.

Table 2: Comparison of Inconel Alloy Castings and Forgings

4. Conclusion

Duplex stainless steel, Monel alloy, and Inconel alloy are widely used as specialized valve materials in the petrochemical industry due to their exceptional corrosion resistance and mechanical properties. These alloys are crucial for corrosive environments. However, China’s special alloy valve industry continues to trail behind developed countries in areas such as material processing, component manufacturing, and standardization. Engineers must also evaluate the cost-effectiveness and operational efficiency of special alloy valves. Selecting the appropriate materials in engineering design is essential for ensuring the long-term stability and reliable operation of production equipment.