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| Stainless Steel |
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| What is Stainless Steel? |
| In metallurgy, stainless steel is defined as an iron-carbon alloy with a minimum of 11.5 wt% chromium content. Stainless |
| steel does not stain, corrode or rust as easily as ordinary steel (it "stains less"), but it is not stain-proof. It is also called |
| corrosion resistant steel when the alloy type and grade are not detailed, particularly in the aviation industry. |
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| There are different grades and surface finishes of stainless steel to suit the environment to which the material will be |
| subjected in its lifetime. High oxidation-resistance in air at ambient temperature is normally achieved with additions of a |
| minimum of 13% (by weight) chromium, and up to 26% is used for harsh environments. The chromium forms a passivation |
| layer of chromium oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, which means that the metal |
| remains lustrous. It is, however, impervious to water and air, protecting the metal beneath. |
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| In addition, Nitronic alloys (trademark of Armco, Inc.) reduce the tendency to gall through selective alloying with manganes |
| e and nitrogen. Nickel also contributes to passivation, as do other less commonly used ingredients such as molybdenum |
| and vanadium. |
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| Types of stainless steel |
| Austenitic, or 300 series, stainless steels comprise over 70% of total stainless steel production. They contain a maximum |
| of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at |
| all temperatures from the cryogenic region to the melting point of the alloy. A typical composition of 18% chromium and 10% |
| nickel, commonly known as 18/10 stainless, is often used in flatware. Similarly, 18/0 and 18/8 are also available. |
| Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice |
| corrosion due to high molybdenum contents (>6%) and nitrogen additions, and the higher nickel content ensures better |
| resistance to stress-corrosion cracking over the 300 series. The higher alloy content of superaustenitic steels makes them |
| more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications. |
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| Ferritic stainless steels are highly corrosion-resistant, but less durable than austenitic grades. They contain between 10.5% |
| and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminium or titanium. |
| Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. Martensitic stainless steels are not as |
| corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be |
| hardened by heat treatment. Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-< 2%), |
| and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. |
| It is also known as series-00 steel. |
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| Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be |
| precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% |
| chromium and 4% nickel. There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel if possible |
| in new projects, as it is estimated that 2% of the U.S. GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike |
| Fighter is the first aircraft to use a precipitation-hardenable stainless steel?Carpenter Custom 465?in its airframe. |
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| Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although |
| in commercial alloys, the mix may be 40/60 respectively. Duplex steels have improved strength over austenitic stainless steels |
| and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. |
| They are characterised by high chromium (19?28%) and molybdenum (up to 5%) and lower nickel contents than austenitic |
| stainless steels. |
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| Stainless steel finishes |
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| Grade |
Ftatures |
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Usages |
| 304 |
| Most widely used steel grade. Good corrosion |
| resistancd. Themal resistance, low-temperature |
| strength and mechanical properties. Good drawability |
| such as deep drawing, bending and does not |
| harden during heat treatment Non-magnetic. |
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| Facilities in chemical, food and dairy industries, |
| sinks, interior piping, hot-water boilers, vessel |
| parts, medical instruments, building materials. |
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| 304L |
| Low carbon Ni-Cr steel, normal conditions, it has |
| corrosion resistance that is similar to 304 grade. |
| But, excellent resistance to inter-granular corrosion |
| after welding and stress relieving. |
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| Machinery and tools used in the chemical coal, |
| textile, neclear energy and petroleum industry that |
| required high inter-granular corrosion resistance, |
| buinding materials, heat resistant parts that are |
| difficult to implement heat treatment on |
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| 316 |
| Excellent corrosion resistance, pitting corrosion |
| resistance and high temperature strength by adding |
| Mo & Ni 2~3%. Useful in severe/harsh conditions. |
| Excellent drawing hardening. Non-magnetic. |
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| Sea water equipment, equipment for chemicals, |
| paper, dye, acetic acid, fertilizer, photo and food |
| industry and construction in coastal areas, ropes, |
| nuts and bolts part. |
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| 316L |
| Low carbon Ni-Cr streel type with adding Mo. It has |
| the normal properties of 316 grade, but plus excellent |
| inter-granular corrosion resistance after welding and |
| stress relieving. |
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| Pulp, paper, chemical fiber, dye, mesh, phosphoric |
| acid manufacturing facilities. |
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