1. Basic features
Duplex stainless steel refers to stainless steel with both austenitic and ferrite two-phase structure. Its performance characteristics are between austenitic stainless steel and ferritic stainless steel. The austenitic phase in duplex stainless steel gives the material good toughness and ductility, while the ferritic phase helps to improve the strength of the material and the resistance to stress corrosion cracking. Generally, the proportion of austenite and ferrite in duplex stainless steel is roughly equal, generally accounting for about 40%-60%.
2. The difference between duplex stainless steel and other stainless steel
Organizational structure difference
Compared with austenitic stainless steel, biphase stainless steel has a ferrite phase. Austenitic stainless steel is mainly a single austenitic structure, its crystal structure is face-centered cubic structure, with good cold working properties and welding properties. Duplex stainless steel has more complex properties due to its two-phase structure. For example, in the welding process of duplex stainless steel, due to the different thermal expansion coefficients of ferrite and austenite, more attention needs to be paid to the welding process to avoid welding deformation and cracks.
Mechanical property distinction
Duplex stainless steel has a higher strength than austenitic stainless steel. The yield strength of austenitic stainless steel is generally about 200-300MPa, and the yield strength of duplex stainless steel can reach 400-550MPa. At the same time, the fatigue resistance of duplex stainless steel is also better, in the case of alternating loads, such as in the structural parts of offshore platforms, duplex stainless steel can better resist fatigue damage than austenitic stainless steel.
Corrosion resistance difference
Duplex stainless steel is excellent in resistance to stress corrosion cracking and pitting and crevice corrosion. In an environment containing chloride ions, such as seawater or chemical media containing chloride ions, the corrosion resistance of duplex stainless steel is better than that of austenitic stainless steel. For example, in some desalination devices, duplex stainless steel can effectively prevent corrosion damage caused by chloride ion erosion, while austenitic stainless steel may appear pitting and stress corrosion cracking in this high chloride ion environment.
3: Variety
Standard duplex stainless steel
Such as 2205 (S31803) duplex stainless steel, it is one of the most widely used duplex stainless steel. Its chromium content is about 22%, nickel content is about 5%, molybdenum content is about 3%. 2205 duplex stainless steel has good corrosion resistance, high strength and good welding performance, and can be used in pipelines, containers and other equipment in chemical, petroleum, natural gas and other industries.
Super duplex stainless steel
For example, 2507 (S32750) duplex stainless steel, its chromium content can reach 25%, nickel content is about 7%, molybdenum content is about 4%. Super duplex stainless steel is more excellent in corrosion resistance, especially in harsh corrosion environments, such as acid oil and gas field exploitation, seawater corrosion and strong oxidizing media applications. Its strength is also higher than that of standard duplex stainless steel and can withstand higher pressures and loads.
4. Segmentation by application
Duplex stainless steel for chemical industry
In chemical production, duplex stainless steel is used to manufacture reactors, heat exchangers, pipelines and other equipment. For chemical processes containing corrosive media, such as in the production environment of acidic media such as sulfuric acid and hydrochloric acid, duplex stainless steel can resist corrosion and ensure the long-term stable operation of equipment. Different chemical processes have different requirements for the corrosion resistance and mechanical properties of duplex stainless steel, for example, in the chlor-alkali industry, duplex stainless steel is required to resist the corrosion of chlorine gas and sodium hydroxide solution.
Duplex stainless steel for Marine engineering
Duplex stainless steel is widely used in offshore platforms, ship structures and seawater treatment systems. Due to the corrosive nature of seawater in the Marine environment, which contains a large number of chloride ions, duplex stainless steel can effectively prevent the corrosion of seawater and the attachment of Marine organisms. In the deep-sea environment, duplex stainless steel also needs to withstand high pressure, and its high strength characteristics can also meet this requirement. For example, in the pipeline system of deep-sea oil production platforms, duplex stainless steel ensures the safe delivery of oil and gas while resisting long-term erosion by seawater.
Duplex stainless steel for the food and pharmaceutical industries
In food processing and pharmaceutical equipment, the corrosion resistance and hygiene properties of duplex stainless steel are important advantages. It can prevent food and drugs from being contaminated by metals, and can resist the corrosion of organic acids, salt solutions and other media in the food processing process. For example, in dairy processing equipment, duplex stainless steel is used to manufacture storage tanks, pipelines, etc., to ensure the quality and safety of dairy products.
