What are the types and grades of high performance stainless steel?
“High performance stainless steel” belongs to the stainless steel family, and its corrosion resistance in various corrosive media is obviously superior to standard stainless steel such as 304L (containing 18% chromium and 8% nickel, 18-8) and 316L (containing similar chromium and nickel content as 304, and 2% molybdenum, 18-10-2). They have obvious advantages in pitting corrosion resistance and stress corrosion fracture resistance in chloride medium. By using high content of chromium, nickel, molybdenum and nitrogen to obtain enough corrosion resistance, by using very low carbon content in the production process to maintain the corrosion resistance while taking into account the hot working and welding properties, the above characteristics are achieved. With the development of smelting and refining technology, it is possible to produce very low carbon steel grades and precisely control the composition, thus realizing the commercialization of high-performance stainless steel. These technologies include vacuum melting, electron beam melting, electroslag remelting, as well as the most noteworthy vacuum oxygen decarburization (VOD) and argon oxygen decarburization (AOD) technologies in mass production. Since the 1970s, these high-performance stainless steels have become more and more important in quantity, technology and business. This series of articles introduces high-performance stainless steel to those who have high requirements for material properties (beyond the range of standard stainless steel properties) and want to know the engineering and corrosion properties of high-performance stainless steel.
There are three main types of high-performance stainless steel: Austenitic, Ferritic and duplex (AUSTENITIC FERRITIC) stainless steel. The three kinds of stainless steel have common characteristics, different corrosion resistance and other characteristics. Therefore, its current and potential applications are very wide, each kind of steel and each kind of steel can provide the best performance for a specific need.
High nickel austenitic stainless steel is generally used for strong acid equipment and resistance to chloride pitting and stress corrosion cracking. This kind of steel is often used in flue gas cleaning equipment with acid condensate or acid solution with strong oxidant, such as paper bleaching process. When in-situ fabrication is an important consideration, austenitic stainless steel is more advantageous because it is easy to weld. This kind of steel is widely used in offshore oil and nuclear power plant pipelines with extremely important welding quality. If thin plates are needed, the cost-effectiveness of general ferritic steel grades is the best, so they have been used as tube making materials for a variety of heat exchangers. In the case of strength requirement, duplex stainless steel is usually selected. They are used in pressure vessels of chemical processing industry and are increasingly used in heat exchangers. All the three types of high-performance stainless steel have better resistance to chloride pitting and stress corrosion cracking than 304 and 316 stainless steel in a wide range of applications, so the final material selection of chloride facilities is often determined by the consideration of manufacturing and processing.
Compared with 304 and 316 stainless steel, high-performance stainless steel has more strict technical requirements in metallurgy and processing. This is determined by the nature of the steel itself and the harsh environment in which they are used. In order to use high performance stainless steel successfully, it is necessary to have a thorough understanding of them. This series of articles provides help for selecting the best material in a specific application, and provides guidance for the processing and application of selected steel grades. Due to the complexity of application and the emergence of a large number of steel grades, this series of articles is only a preliminary guide for readers to consult with material manufacturers to fully understand the advantages, limitations and specific requirements of each material.
Classification and grade of high performance stainless steel
High performance austenitic stainless steel
High performance austenitic stainless steel and standard austenitic stainless steel have many common characteristics. In the annealed state, these steels are mainly a single-phase, face centered cubic austenite structure, without magnetism. This kind of structure is characterized by its relatively low yield strength, high work hardening rate and high tensile strength, good plasticity and formability, especially good low temperature toughness, which can not be hardened (or strengthened) by heat treatment.
Compared with standard austenitic stainless steel, in addition to the difference in corrosion resistance, another major difference is that high-performance steel rapidly forms secondary phases at high temperature, which may damage some mechanical properties and corrosion resistance. Therefore, the service temperature of this stainless steel should not exceed 500 ℃. In addition, high temperature operations such as forging and welding should be careful to avoid the formation of these harmful phases.
Table 1 lists some famous forged high-performance austenitic stainless steels, and their common names and UNS grades are indicated for each steel. Many kinds of steel have applied for patents after their initial development, some of which are still valid. The range of chemical composition shown in the table is consistent with the most commonly used ASTM standard a 240. The only content value shown in the table is the “typical value” provided by the manufacturer.
Table.1 Chemical composition of forged high performance austenitic stainless steel, weight%
The steel grades in Table.1 are arranged in the order of increasing Mo, Cr and N content or pre value.
The pre value refers to the number of pitting resistance equivalents, which is defined as pre =% Cr + 3.3% Mo + 16% N,% by weight. In chloride medium, the higher pre value does not fully quantitatively indicate the higher local corrosion resistance. According to the similarity of local corrosion resistance, these steel grades are divided into six groups, and the descriptions of these groups are as follows:
Group A-1 austenitic stainless steel
This group of stainless steel is mainly used for hot high concentration sulfuric acid solution medium. The required corrosion resistance is mainly obtained by high nickel content. 20cb-3 and alloy 825, which contain the same content of chromium and molybdenum as 316 stainless steel, are not superior to 316 stainless steel in resistance to local pitting and crevice corrosion. Their ability to resist chloride stress corrosion cracking increases with the increase of nickel content. They have a long history of application in chemical industry, which requires good resistance to sulfuric acid or stress corrosion cracking.
Group A-2 austenitic stainless steel
Compared with 316L or 317L, the local corrosion resistance of these steels, such as 317lmn, is improved in halide medium, which is achieved by adopting higher molybdenum content and nitrogen content. Nitrogen can not only improve the corrosion resistance, but also stabilize austenite, so it can reduce the amount of nickel and achieve the most economic effect. The performance of these steel grades is better than 316L, and the cost is moderate. They are widely used in the absorption tower of FGD device operating in medium pitting medium.
Group A-3 austenitic stainless steel
This group of stainless steel, such as 904L, containing more than 10% nickel, has comprehensive and balanced stress corrosion cracking resistance to reducing acid and chloride. The addition of copper further increased their resistance to reducing acids.
Group A-4 austenitic stainless steel
This group of stainless steel is usually called 6% molybdenum stainless steel, including AL-6XN, 1925 HMO, 25-6mo and 254 SMO. It was originally designed to resist local corrosion of seawater and pulp bleaching equipment media at room temperature. By adding relatively high content of nitrogen, chromium and molybdenum, the pre value is between 40-43, which can meet the above requirements. This group of steel has high content of nickel, which is helpful to the stability of austenite, so it also has good resistance to general acid corrosion and good resistance to chloride stress corrosion cracking. The steel with high nitrogen content has obvious strength advantage over the steel without nitrogen content. This kind of steel is widely used in seawater pipes, seawater heat exchangers, pulp bleaching equipment, various chemical plants and power plants.
Group a-5 austenitic stainless steel
The main feature of this group of steel is high strength, such as 4565s. It is realized by solid solution strengthening of nitrogen with content of 0.25% – 0.50%. According to the different contents of chromium, molybdenum and nickel, their corrosion characteristics are quite different. For the steel with low nickel content (12% ~ 18%), its chloride stress corrosion cracking resistance is equivalent to that of group A-2 stainless steel.
Group a-6 austenitic stainless steel
This group of stainless steels, such as 654 SMO, represents the highest performance of all high-performance stainless steels at present. They combine high strength with outstanding local corrosion resistance, and have good stress corrosion cracking resistance and acid corrosion resistance. They can resist the stress corrosion cracking of 45% boiling magnesium chloride test solution, and can resist the local corrosion of seawater when the temperature is much higher than room temperature and there are serious cracks. Their local corrosion resistance is close to the best value of nickel base alloy, and their strength is much higher. These new steel grades have outstanding potential to solve the crack corrosion problems of Gasket Joints, seawater treatment systems at higher temperature and many devices operating at high pressure.
High performance ferritic stainless steel
High performance ferritic stainless steel has complete ferrite microstructure besides a small amount of stabilized carbide and nitride. The unique characteristic of this ferrite structure lies in its good resistance to chloride stress corrosion cracking, but its toughness is limited. Large section or grain effect and the precipitation of brittle secondary phase will further reduce its toughness. Due to the limited toughness, these grades of steel are usually not produced into medium and thick plates. The purpose of developing these steel grades is to obtain stress corrosion cracking resistance and pitting resistance superior to 18-8 stainless steel at a price lower than that of high nickel austenitic alloy. They are generally used only for tubular materials or sheets of heat exchangers, and heat treatment will not harden them, but in the annealed state, they show higher strength than many austenitic stainless steels. Table 2 lists the main forged and rolled ferritic steels in the order of increasing chloride pitting resistance.
Table.2 Chemical composition of forged high performance ferritic stainless steel, weight%
F-1 ferritic stainless steel
This kind of stainless steel, such as E-Brite 26-1, has similar resistance to local corrosion as 316 stainless steel, but its resistance to stress corrosion cracking is much better than the latter. This kind of good stress corrosion fracture property makes it suitable for hot concentrated alkali solution and chloride solution.
Group F-2 ferritic stainless steel
This group of stainless steel, including sea-cure, is designed to resist local corrosion of seawater at normal temperature and is widely used in seawater cooling condensers of power plants. Because of their high chromium content, moderate molybdenum and nickel content, they also have good corrosion resistance to strong organic acids and oxidizing or medium reducing inorganic acids. However, the nickel in these steel grades reduces the resistance to chloride stress corrosion cracking and increases the sensitivity to the formation of harmful secondary phases. All these ferritic stainless steels can resist stress corrosion cracking in NaCl test solution, but due to the nickel content of 0.5% – 4.2%, they may not be able to resist stress corrosion cracking in NaCl test solution.
Group F-3 ferritic stainless steel
Like austenitic stainless steel in group a-6, Al 29-4-2 in this group is designed to reach the highest level of comprehensive performance of ferritic stainless steel, which has good local corrosion resistance and acid corrosion resistance.
High performance duplex stainless steel
From the metallurgical point of view, the microstructure of duplex stainless steel is designed to contain approximately the same proportion of austenite and ferrite in the annealed state, which is achieved by limiting nickel to a moderate low content and increasing chromium content to 22% – 26%. The molybdenum content is about the same as 317L stainless steel. Therefore, if the pre value is higher than 30, it is mainly due to the high content of chromium and nitrogen. The two-phase structure shows that the two phases are dominant. Most importantly, these grades have very high strength and workable plasticity and toughness. Because their nickel content is not very high, they combine strength, uniform corrosion resistance and stress corrosion cracking resistance well at a moderate price. Similar to austenitic stainless steels, heat treatment does not harden these two-phase grades. In the process of duplex stainless steel, attention should be paid to avoid the damage of secondary phase and maintain the balance between austenite phase and ferrite phase. In this respect, they are more stringent than austenitic stainless steels. Table.3 shows the main forging and rolling high performance duplex stainless steel.
Table.3 Chemical composition of forged high performance duplex stainless steel, weight%
D-1 duplex stainless steel
There is only one steel grade in this group, namely 2304. Although the corrosion resistance of 2304 is not better than that of standard austenitic stainless steel, it is still regarded as high-performance stainless steel, because like all the second generation duplex stainless steel, the low-carbon and high nitrogen 2304 has better processing performance and corrosion characteristics than the early duplex steel. Compared with 316L or 317L, it has higher strength and better stress corrosion resistance.
Group D-2 duplex stainless steel
This group of duplex stainless steel, especially 2205, is the most useful steel in duplex stainless steel, because it has the advantages of corrosion characteristics, processing and price. They have a variety of adaptability in processing and corrosion resistance, and the stress corrosion cracking resistance is better than 316 stainless steel.
Group D-3 duplex stainless steel
These 25cr duplex stainless steels, such as ferralium255, have higher chromium content than the D-2 steel, so they are more resistant to local corrosion. However, it is generally considered that it can not withstand seawater corrosion in harsh environment. Chromium has good resistance to oxidation-resistant acid corrosion. This group of steel needs a higher content of nickel to balance the higher content of chromium, so the resistance to reductive acid corrosion is also improved. The disadvantage of high chromium content is that it accelerates the precipitation kinetics of harmful phases. Therefore, the processing involving heat treatment needs to accurately control the temperature conditions. In some cases, rapid precipitation kinetics may limit the effective section size.
D-4 duplex stainless steel
This group of stainless steel is the one with the highest degree of alloying in duplex stainless steel, and the corrosion resistance produced by high content of chromium, molybdenum, nickel and nitrogen is the best of all duplex stainless steel, and the strength is higher than any high-performance stainless steel. For this reason, this group of steel is sometimes called super duplex stainless steel. Their resistance to pitting and crevice corrosion is similar to the 6% Mo austenitic stainless steel of group A-4, and their yield strength is more than 550MPa. However, due to their high alloy content, their restrictions on hot working process are even more stringent than those of group D-3 steel.
Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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