Stainless steel
Stainless steel (other names anti-corrosion steel, corrosion-resistant steel, corrosion-resistant steel, corrosion-resistant steel, corrosion-resistant steel, noble stainless steel; rather informally inox [from Fr.], antikoro, stainless steel, older anticorro [last three by trade mark "Anticorro" of the Poldi smelter, patented in 1910]; incorrectly stainless steel [according to the hist. trade mark "Nerez"], incorrectly stainless steel) is a steel highly alloyed with chromium, nickel, manganese and possibly other additives (silicon, copper, molybdenum, tungsten, niobium, nitrogen and the like).
It is a corrosion-resistant steel. It is an important construction material for equipment for the chemical industry, the food industry, and especially for nuclear technology.
Breakdown by composition:
chromium hardenable (12-17% Cr, up to 1% C)
chromium ferritic (above 17% Cr, below 0.1 C)
austenitic (above 18% Cr, above 8% Ni, below 0.1% C)
two-phase austenitic-ferritic (above 20% Cr, 3-6% Ni, below 0.1% C).
Anti-corrosion steels are chromium alloys with iron, containing 12-30% chromium, up to 30% nickel or up to 24% manganese with a certain amount of molybdenum, silicon, copper, titanium, niobium, nitrogen and the like (a few percent at most). Chromium ensures the passivity of these alloys and is therefore a decisive element for achieving corrosion resistance. Stainless steels are prone to local types of corrosion (pitting, crevice, intergranular, corrosion cracking) in some environments. However, these can be excluded by the appropriate selection of steel for the given conditions. Even though chromium, nickel, manganese and other alloying elements are in relatively large quantities in stainless steels, the basic element is still iron and its alloy with carbon, i.e. steel. We divide stainless steels according to their chemical composition and structure into the following basic groups:
• austenitic,
• ferritic,
• martensitic (hardenable),
• austenitic – ferritic (duplex),
• superalloys
Quality
AISI 301 Anti-corrosion flexible material used in the temperature range up to 300 °C. Application: material with high strength, suitable for the automotive industry, springs, spring tools.
AISI 303 Austenitic chrome-nickel steel, very machinable, alloyed with sulphur, which allows processing on automatic machines. Deteriorated mechanical properties and reduced resistance to corrosion (higher sulfur content), but the values are better than for the material
AISI 430 F, correspond to material AISI 430. Material unsuitable for cold working and welding (flows during welding). This grade is only available in bars. Application: Suitable for the production of nuts, screws, fittings, for the production of rotary parts, machined on automatic machines, if better corrosion resistance than AISI 430 is required.
AISI 304 Austenitic chrome-nickel steel is the most widely used type of stainless steel with very good corrosion resistance, cold formability and weldability. Resistant to water, water vapor, air humidity, edible acids, weak organic and inorganic acids. After welding sheet metal up to a thickness of 6 mm, it is resistant to intercrystalline corrosion even without additional heat treatment (low carbon content). Approved for temperature stress up to 300°C, titanium stabilized steel should be used at higher working temperatures.
AISI 321. Well polishable and malleable by deep drawing, bending and rounding. During machining, it is necessary to work with sharp tools made of high-alloy high-speed steel, carbide or carbide alloys due to the tendency to solidification. Arc weldability when using all welding methods is good, not suitable for flame welding. Application: In the engineering and nuclear industry, in architecture, in transport equipment, in the food industry, in the pharmaceutical and cosmetic industry, in the construction of chemical apparatus and motor vehicles, in the production of surgical instruments, sanitary equipment, objects and devices for the household and objects of art.
AISI 304L Austenitic, chromium-nickel steel with increased resistance to intercrystalline corrosion (extremely low carbon content). Sheets over 6 mm thick, even if they were welded under less favorable conditions, do not need to be additionally modified. The material is weldable, very well polished and well cold formable. Machining conditions are similar to AISI 304. Use: similar to AISI 304, but cold processing is better.
AISI 304 PS/DDQ Anti-corrosion material suitable for deep drawing. Special malleable properties are achieved by easy modification in the chemical composition and heat treatment in the rolling phase. Use: dishes, sinks, sinks.
AISI 309/310 Heat-resistant stainless steel materials with high chromium and nickel content. Application: refractory devices with temperatures up to 1150 °C.
AISI 316 Austenitic acid-resistant chromium nickel molybdenum steel with increased corrosion resistance (increased molybdenum content). After welding sheet metal up to a thickness of 6 mm, it is resistant to intercrystalline corrosion even without additional heat treatment (low carbon content). It is approved for temperature stress up to 300 °C. The material is weldable, very well polished and can be machined by deep drawing, bending and rounding. Machining conditions are similar to AISI 304. Application: Wherever structural parts, devices and tools with the required medium strength are exposed to the action of water, water vapor and air humidity, in the engineering industry, in the construction of turbines, pumps, in the production of fittings, household items and devices, sports equipment, medical and surgical devices, etc.
AISI 316L Austenitic acid-resistant chromium-nickel-molybdenum steel, with a carbon content of up to 0.03%, highly resistant to acids and corrosion, with a slight tendency to pitting in chlorine-containing solutions. The steel is very well weldable, without the need for additional heat treatment after welding, with high resistance to intercrystalline corrosion, can be polished to a mirror shine and is extremely malleable (bending, circling, deep drawing, pressing, etc.). Use: Structural parts, devices and apparatus of the chemical industry with high chemical stress (presence of chlorides), in the production of hot water treatment equipment and structural parts that come into contact with seawater.
AISI 316Ti Austenitic acid-resistant chromium nickel molybdenum steel, stabilized with titanium additive. Higher acid resistance, higher yield strength and strength, no risk of deep local corrosion (molybdenum additive). After welding thicker dimensions, no heat treatment (stabilization with titanium) is necessary. It is not possible to achieve a high gloss (titanium additive). Formability by deep drawing, folding and rounding is good. Machining conditions are similar to AISI 304. Arc weldability when using all welding methods is good. Flame welding is not possible. Use: Structural parts, devices and apparatus of the chemical industry with the required resistance to halogens and non-oxidizing acids. A higher yield strength in tension brings advantages in mechanically more stressed constructions (construction). Typical examples of use are technologies for the production of pulp, viscose yarn, textiles, dyes, fertilizers, plastics and fuels, the photographic and pharmaceutical industries, and the petroleum industry.
AISI 321 Austenitic chrome-nickel steel, stabilized with titanium additive, after welding resistant to intercrystalline corrosion without additional heat treatment even when welding thicker dimensions. It can be used for pressure vessels up to a temperature of 400 °C, as a scale-resistant steel it can be used up to 800 °C. The addition of titanium does not allow the steel to be polished to a high shine. Formability by deep drawing, folding and rounding is good. Machining conditions are similar to AISI 304. Arc weldability is good when using all welding methods. Flame welding is not possible. Use: Chemical resistance similar to AISI 304 material, similar possibility of use if high gloss is not required. Suitable for structures with material thickness greater than 6 mm, which cannot be heat treated after welding. A higher yield strength in tension brings benefits for mechanically more stressed devices, structural parts and connecting elements. It is used in all branches of the food and chemical industry.
AISI 409 Similar to AISI 430 material with titanium addition, more resistant to aggressive environment. Use: Inserts for chimneys.
AISI 410, 420, 420F Anti-corrosion material, hardenable with increasing carbon content, after hardening it is possible to achieve higher hardness. Application: for components with higher strength, springs, pistons, screws and knives.
AISI 430 Ferritic 17% chrome steel with good corrosion resistance, very good polishability, deep elongation and bendability. When welding at temperatures below 20 °C, it tends to become brittle. The machinability is comparable to alloy case-hardened steels. As with other soft materials, it is necessary to take into account the poorer removal of chips. For sheet thicknesses above 3 mm, cold forming is improved if the material and the tool are heated to 100-300 °C. Even if the material is weldable, embrittlement and a reduction in corrosion resistance appear in the weld area. Application: Very diverse, thanks to its resistance to water, water vapor and humidity, weak acids and alcohols, suitable for the production of household electrical appliances, in the hospitality industry, in the production of food and beverages, in architecture, in the production of furniture, in medical technology, in the chemical industry, etc. . It is characterized by relatively good resistance to corrosion and good workability.
AISI 430 F Processable stainless automatic steel, with 17% chromium content and molybdenum additive, alloyed with sulfur (improvement of machinability), unsuitable for cold processing and welding. Reduced resistance to corrosion and ductility (high sulfur content). Application: for the production of rotating parts on machines without increased requirements for corrosion resistance.
AISI 439 – 441 Ferritic chrome steel with a Cr content of 16 – 18% and stabilizing elements of Ti or Nb. Improved features compared to A430. Good weldability and machinability. Higher corrosion resistance. Use: application as material A430. In some applications, a replacement for A304 (design, furniture sector...).
AISI 904L Super-austenitic steel got its name from its high resistance to corrosion in a very aggressive environment. Material with low carbon content, high content of molybdenum (4.5-5%) and chromium (20-21%), excellent resistance to interstitial corrosion even in the presence of chlorine. Copper 1.2-1.8% and Ni 25% guarantee good resistance to stress corrosion. Use: in the paper, chemical industry, production of sulfuric acid, phosphoric acid, production of fertilizers.
Duplex Austenitic-ferritic steel - referred to by the common name Duplex, due to the coexistence of two phases: an austenitic-type phase and a ferritic-type phase, their chromium content varies between 18-26% and nickel 4.5-6.5%. Microcrystalline austenitic-ferritic structure that gives the material high resistance to pitting. Applications: heat exchangers, tanks and containers for liquids with a high chlorine content, desalination tanks, seawater cooling equipment, PVC production, fertilizers, acid tanks, offshore oil and gas industry.
Nickel superalloys Based on an austenitic structure, which represents a better distribution of atoms capable of guaranteeing high mechanical resistance even at high fractions of the casting temperature. Alloys of this type maintain a high surface physico-chemical stability, excellent resistance to corrosion and a long service life. Application: aviation, cosmonautics, nuclear, thermal energy, chemical equipment, petrochemical equipment, shipping industry.