Austenitic stainless steel is one of the five classes of stainless steel by crystalline structure (along with ferritic , martensitic, duplex and precipitation hardened [1] ). Its primary crystalline structure is austenite (face-centered cubic) and it prevents steels from being hardenable by heat treatment and makes them essentially non-magnetic. [2] This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen.[ citation needed ] The Incoloy family of alloys belong to the category of super austenitic stainless steels. [3]
There are two subgroups of austenitic stainless steel. 300 series stainless steels achieve their austenitic structure primarily by a nickel addition while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is still a small nickel content, making 200 series a cost-effective nickel-chromium austenitic type stainless steel.
300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type 304, also known as 18/8 or A2. Type 304 is extensively used in such items as cookware, cutlery, and kitchen equipment. Type 316, also known as A4, is the next most common austenitic stainless steel. Some 300 series, such as Type 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion).
Euronorm (EN) number [5] | EN designation | AISI grade [6] | C | Cr | Mo | Ni | Others | Melts at [7] | Remark |
---|---|---|---|---|---|---|---|---|---|
1.4310 | X10CrNi18-8 | 301 | 0.10 | 17.5 | NS | 8 | NS | 1420 | For springs |
1.4301 | X5CrNi18-10 | 304 | < 0.07 | 18.5 | NS | 9 | NS | 1450 | A very common austenitic stainless steel grade |
1.4307 | X2CrNi18-9 | 304L | < 0.030 | 18.5 | NS | 9 | NS | 1450 | Similar to the above but not susceptible to intergranular corrosion thanks to a lower C content. |
1.4305 | X8CrNiS18-9 e | 303 | < 0.10 | 18 | NS | 9 | 0.3 | 1420 | Sulphur is added to improve machinability. |
1.4541 | X6CrNiTi18-10 | 321 | < 0.08 | 18 | NS | 10.5 | Ti: 5×C ≤ 0.70 | 1425 | Same as grade 1.4301 but not susceptible to intergranular corrosion thanks to Ti which "traps" C. |
1.4401 | X5CrNiMo17-12-2 | 316 | < 0.07 | 17.5 | 2.2 | 11.5 | NS | 1400 | Second best known austenitic grade. Mo increases the corrosion resistance. |
1.4404 | X2CrNiMo17-12-2 | 316L | < 0.030 | 17.5 | 2.25 | 11.5 | NS | 1400 | Same as above but not susceptible to intergranular corrosion thanks to a lower C content. |
1.4571 | X6CrNiMoTi17-12-2 | 316Ti | < 0.08 | 17.5 | 2.25 | 12 | Ti: 5×C ≤ 0.70 |
The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series. [8]
Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding.
Heat resisting grades can be used at elevated temperatures, usually above 600 °C (1,100 °F). [9] [10]
They must resist corrosion (usually oxidation) and retain mechanical properties, mostly strength (yield stress) and creep resistance.
Corrosion resistance is mostly provided by chromium, with additions of silicon and aluminium. Nickel does not resist well in sulphur containing environments. This is usually taken care of by adding more Si and Al which form very stable oxides. Rare earth elements such as cerium increase the stability of the oxide film.
EN | EN designation | AISI/ASTM | UNS | C | Cr | Ni | Si | Mn | Others |
---|---|---|---|---|---|---|---|---|---|
1.4878 | X8CrNiTi18-10 | 321H | S32109 | < 0.1 | 18 | 10.5 | - | - | Ti: ≤ 5×C |
1.4818 | X6CrNiSiNCe19-10 | - | S30415 | 0.06 | 19 | 10 | - | - | N: 0.16; Ce: 0.05. |
1.4828 | X15CrNiSi20-12 | 309 | - | < 0.2 | 20 | 12 | 2.0 | - | - |
1.4833 | X12CrNi23-13 | 309S | S30908 | < 0.08 | 23 | 13 | < 0.75 | - | - |
1.4872 | X25CrMnNiN25-9-7 | - | - | 0.25 | 25 | 7 | - | 9 | - |
1.4845 | X15CrNi25-21 | 310S | S31008 | < 0.1 | 25 | 20 | - | - | - |
1.4841 | X15CrNiSi25-21 | 314 | S31400 | < 0.15 | 25 | 20 | 1.8 | - | - |
1.4876 | X10NiCrAITi32-20 | "Alloy 800" | N08800 | < 0.12 | 21 | 32 | - | - | Al: 0.4; Ti: 0.4 |
1.4854 | X6NiCrSiNCe35-25 | "Alloy 353MA" | S35315 | 0.06 | 25 | 35 | - | - | N: 0.15; Ce: 0.06. |
1.4886 | X12NiCrSi35-16 | 330 | N08330 | < 0.15 | 18.5 | 35 | - | - |
Type 309 and 310 [11] are used in high temperature applications greater than 800 °C (1,500 °F).
Note: ferritic stainless steels do not retain strength at elevated temperatures and are not used when strength is required.
Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used.
Grade EN 1.4980 (also known as A286) is not considered strictly as a heat resisting steel in standards, but this is popular grade for its combination of strength and corrosion resistance. [12] [13] [14]
EN No. | EN designation | AISI/ASTM | UNS | C | Cr | Ni | Mo | Others |
---|---|---|---|---|---|---|---|---|
1.4980 | X6NiCrTiMoVB25-15-2 | 660 | S66286 | 0.05 | 15 | 25 | 1.25 | V: 0.3; Ti: 2.0; B: 0.006. |
Condition | Yield stress, min | Ultimate tensile strength, min | Elongation, min (%) |
---|---|---|---|
Solution treated and aged | 590 MPa (86 ksi) | 900 MPa (130 ksi) | 13 |
It is used for service temperatures up to 700 °C (1,300 °F) in applications such as:
Stainless steel, also known as inox, corrosion-resistant steel (CRES) and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum, carbon, nickel and nitrogen depending on its specific use and cost. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen.
Surgical stainless steel is a grade of stainless steel used in biomedical applications. The most common "surgical steels" are austenitic SAE 316 stainless and martensitic SAE 440, SAE 420, and 17-4 stainless steels. There is no formal definition on what constitutes a "surgical stainless steel", so product manufacturers and distributors often apply the term to refer to any grade of corrosion resistant steel.
Martensitic stainless steel is a type of stainless steel alloy that has a martensite crystal structure. It can be hardened and tempered through aging and heat treatment. The other main types of stainless steel are austenitic, ferritic, duplex, and precipitation hardened.
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
AL-6XN is a type of weldable stainless steel that consist of an alloy of nickel (24%), chromium (22%) and molybdenum (6.3%) with other trace elements such as nitrogen.
The weldability, also known as joinability, of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. A material's weldability is used to determine the welding process and to compare the final weld quality to other materials.
An archwire in orthodontics is a wire conforming to the alveolar or dental arch that can be used with dental braces as a source of force in correcting irregularities in the position of the teeth. An archwire can also be used to maintain existing dental positions; in this case it has a retentive purpose.
In materials science, intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum.
The SAE steel grades system is a standard alloy numbering system for steel grades maintained by SAE International.
Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties.
Zeron 100 is a super duplex stainless steel developed by Rolled Alloys. The alloy has excellent corrosion resistance combined with high strength. It typically contains 25% chromium and 7% nickel and 3.6% molybdenum along with copper and tungsten additions. Zeron 100 has a 50–50 austenitic–ferritic structure. It also has greater resistance to chloride pitting, crevice corrosion and stress corrosion cracking than exhibited by the standard 300 series stainless steels.
Incoloy refers to a range of superalloys now produced by the Special Metals Corporation (SMC) group of companies and created with a trademark by the Inco company in 1952. Originally Inco protected these alloys by patent. In 2000, the SMC published a 61-page document entitled "High-Performance Alloys for Resistance to Aqueous Corrosion" highlighting Incoloy, as well as Monel and Inconel products, and their use in fluid environments such as sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, nitric acid, other acids as well as freshwater environments.
SAF 2205, is a Alleima-owned trademark for a 22Cr duplex (ferritic-austenitic) stainless steel. SAF derives from Sandvik Austenite Ferrite. The nominal chemical composition of SAF 2205 is 22% chromium, 5% nickel, 3.2% molybdenum and other alloying elements such as nitrogen and manganese. The UNS designation for SAF 2205 is S31803/S32205 and the EN steel no. is 1.4462. SAF 2205 or Duplex 2205 is often used as an alternative to expensive 904L stainless steel owing to similar properties but cheaper ingredients. Duplex stainless steel is available in multiple forms like bars, billets, pipes, tubes, sheets, plates and even processed to fittings and flanges.
SAF 2507, is a Alleima-owned trademark for a 25Cr duplex (ferritic-austenitic) stainless steel. The nominal chemical composition of SAF 2507 is 25% chromium, 7% nickel, 4% molybdenum and other alloying elements such as nitrogen and manganese. The UNS designation for SAF 2507 is S32750 and the EN steel no. is 1.4410. SAF derives from Sandvik Austenite Ferrite.
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Ferritic stainless steel forms one of the five stainless steel families, the other four being austenitic, martensitic, duplex stainless steels, and precipitation hardened. For example, many of AISI 400-series of stainless steels are ferritic steels. By comparison with austenitic types, these are less hardenable by cold working, less weldable, and should not be used at cryogenic temperatures. Some types, like the 430, have excellent corrosion resistance and are very heat tolerant.
Duplex stainless steels are a family of alloys with a two-phase microstructure consisting of both austenitic and ferritic phases. They offer excellent mechanical properties, corrosion resistance, and toughness compared to other types of stainless steel. However, duplex stainless steel can be susceptible to a phenomenon known as 475 °C embrittlement or duplex stainless steel age hardening, which is a type of aging process that causes loss of plasticity in duplex stainless steel when it is heated in the range of 250 to 550 °C. At this temperature range, spontaneous phase separation of the ferrite phase into iron-rich and chromium-rich nanophases occurs, with no change in the mechanical properties of the austenite phase. This type of embrittlement is due to precipitation hardening, which makes the material become brittle and prone to cracking.