Background
As we all know, fastener products include thousands of specifications, in order to ensure safety or consider the influence of temperature, harsh environment or other dangerous places, often put forward higher requirements for the safety, durability and surface quality of fastener steel, that is, steel must meet the conditions of stress corrosion resistance or heat resistance and high strength resistance. Therefore, stainless steel and heat-resistant steel have been widely used in fastener manufacturing.
gb/t 3098.6-2023 "Mechanical properties of fasteners - Stainless steel bolts, screws and studs", GB T 309815-2023 "Mechanical Properties of Fasteners - Stainless Steel Nuts" recommended national standard, issued by the State Administration for Market Regulation and the Standardization Administration of the People's Republic of China on May 23, 2023, and implemented on December 1, 2023.
The new standards are revised to adopt ISO 3506-1:2020 "Fasteners - Mechanical properties of corrosion-resistant stainless steel fasteners - Part 1: Bolts, screws and studs with specified groups and performance levels" (English version) to replace GB T30986-2014；and modifications to adopt ISO 3506-2:2020 "Fasteners – Mechanical properties of corrosion-resistant stainless steel fasteners – Part 2: Nuts with specified groups and performance classes" (English version) instead of GB T309815-2014。
The new standards are part 6 and part 15 of GB T3098 "Mechanical Properties of Fasteners", which stipulate the mechanical properties of different types of stainless steel fasteners, and provide a basis for the design, manufacturing, acceptance inspection and use of fasteners.
Status quo
In today's industrial production process, high-pressure vessels are one of the indispensable equipment. However, due to the influence of factors such as high temperature, high strength and high concentration, metal materials are prone to corrosion, which can lead to container failure and even accidents. Therefore, it is necessary to conduct in-depth research on stainless steel and heat-resistant steel materials in pressure vessels.
At present, relevant research work has been carried out in various countries around the world, among which the research in the United States focuses on the analysis of corrosion mechanisms and influencing factors in different types of pressure vessels; Research in Germany focuses on exploring new corrosion-resistant materials and their application methods; The United Kingdom pays more attention to improving the safety performance and life of pressure vessels; And some research results have been achieved in China. With the advancement of materials and manufacturing processes, the revised GB T30986-2023 and GB T309815-2023 The performance requirements of duplex stainless steel materials and higher strength grades are introduced to meet the requirements for high-strength design under corrosive conditions.
Duplex austenitic- ferritic stain less steels, with a matrix of both austenitic and ferritic two-phase structures (where the content of less phases is at least greater than 25%) can be magnetized, generally strengthened by cold working. Duplex stainless steel not only retains the excellent toughness and welding performance of austenitic stainless steel, but also has the advantages of high strength of ferritic stainless steel.
Duplex stainless steel grades
The main components of duplex (austenitic-ferritic) stainless steel are Fe, Cr, Ni, Mo and other elements, and the casting grade of duplex stainless steel implements ASTM A890 A890M standard. In the ASTM A890 A890M-18A standard, cast duplex stainless steels are available in 8 grades, including 1B, 1C, 2A, 3A, 4A, 5A, 6A, and 7A, depending on the chemical composition. China's standard for duplex stainless steel castings GB T 2100-2017 also has material grades corresponding to the ASTM A890 890M standard.
The main executive standard for forged duplex stainless steel in the United States is ASTMA182 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. China's stainless steel forging standard NB T47010-2017 "Stainless steel and heat-resistant steel forgings for pressure equipment" stipulates six forged duplex stainless steel grades. Due to the habit of use and the abundance of American brands, at present, domestic valve fastener products still mainly use the American duplex stainless steel standard, among which, the commonly used grades are mainly ASTM A182 standard F51 F53 F55 F61, ASTM A890 standard 1B 4A 5A 6A. The pitting resistance and pitting susceptibility coefficient of duplex stainless steel (pren=cr%+3.)3%MO+16%N), and the PREN value is related to the composition of the material, and the pitting resistance is also different for different material grades, different compositions, and different pitting resistance.
On the basis of the GB T20878-2007 "Stainless Steel and Heat-resistant Steel Grades and Chemical Compositions" national standard, in order to meet the market demand standards, the GB T20878-2024 to be released next year has added 85 stainless steel grades and chemical compositions, of which 20 duplex (austenitic-ferrite) stainless steel grades have been added, and 9 new ones have been added, see Table 1, which expands the application range of the fastener market.
Table 1 Duplex (austenitic-ferritic) stainless steel grades and their chemical compositions.
Duplex stainless steel properties and applications
The composition and structure of stainless steel play a key role in stress corrosion crack susceptibility (SCC). Austenitic stainless steels are susceptible to SCC in chloride solutions, while ferrite has better resistance to SCC. The smaller the ferrite grains, the better the SCC resistance, so duplex stainless steels have higher chloride SCC resistance, and the threshold value of stress corrosion cracking is about 3 times that of austenitic stainless steels. In Cr-Ni stainless steels, the ratio of ferrite phase to austenite phase in the solution structure is often calculated and evaluated using "chromium equivalent" and "nickel equivalent".
Duplex stainless steel solves the danger of SCC generation of austenitic stainless steels such as 304L and 316L under CL-stress corrosion, especially in the stress corrosion rupture occasion caused by pitting. Therefore, duplex stainless steel is widely used in many occasions of CL-stress corrosion, such as catalytic cracking and hydrocracking units in the oil refining industry, PVC units and vinyl chloride production units in the petrochemical industry, and fasteners for sewage treatment units in coal chemical industry.
Heat treatment process matters
Duplex stainless steel is hard and brittle in the structure, and its precipitation directly affects the corrosion resistance of duplex steel. In order to avoid the degradation of toughness and corrosion resistance, duplex stainless steel solution heat treatment requires fast cooling to shorten the phase precipitation process. Duplex steel will precipitate phase after a long stay in the range of 316 540, and the national and petrochemical industry standards have made relevant provisions on the use temperature of duplex steel materials, and the upper limit of continuous use temperature of duplex stainless steel is lower than that of austenitic stainless steel. SH T3059-2012 "Petrochemical Pipeline Design Equipment Selection Specification" No. 6Section 2 stipulates that the operating temperature of duplex stainless steel should not be higher than 300. gb/t20801.2-2020 Appendix C3 of "Pressure Piping Code for Industrial Piping".2 and Appendix C69. It is proposed that attention should be paid to the brittleness and phase embrittlement of duplex steel 475 when designing and selecting materials, so when selecting fasteners, duplex steel should not be applied in high-temperature occasions.
In addition, duplex stainless steel fasteners are generally annealed with 950 solution annealing to relieve stress, because below 950, the martensitic structure will gradually precipitate brittle phase, which is different from austenitic stainless steel, which is usually heat treated at 600 800 to relieve stress.
Screw Jun's experience and summary
Duplex stainless steel is a very good corrosion-resistant material because of its high strength and excellent mechanical properties. However, due to its relatively high **, it is usually only used for special occasions in practical applications. If it is necessary to store or transport pressure vessels containing acidic or alkaline solutions for a long time, stainless steel fasteners can be selected as the main material. When selecting a material for stainless steel fasteners in a pressure vessel, there are issues such as its scope of application, cost-effectiveness, and interaction with other materials need to be considered. The best choice can only be made after taking into account a variety of factors.
Screw Jun believes that there has been some progress in the research of stainless steel and heat-resistant steel materials at home and abroad, but there are still some problems that need to be further solved. Future research directions should pay more attention to how to improve the service life and safety performance of fasteners? At the same time, it strengthens the work combined with practical engineering practice to provide more reliable technical support for industrial production.
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