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Stainless Steel

Stainless steel 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.
Stainless steels have high resistance to rust and corrosion many natural and man-made environments; however, it is important to select the correct type and grade of stainless steel for the particular application.

Types of stainless steel

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc.
Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.
Stainless steels are also classified by their crystanlline structure:
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.
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.

Stainless steel grades

200 Series - austenitic chromium-nickel-manganese alloys
Type 201 - austenitic that is hardenable through cold working
Type 202 - austenitic general purpose stainless steel
300 Series - austenitic chromium-nickel alloys
Type 301 - highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
Type 302 - same corrosion resistance as 304, with slightly higher strength due to additional carbon.
Type 303 - easier machining version of 304 via addition of sulfur and phosphorus.
Type 304 - the most common grade; the classic 18/8 stainless steel.
Type 304L - the 304 grade but specially modified for welding.
Type 309 - better temperature resistance than 304
Type 316 - the second most common grade (after 304); for food and surgical stainless steel uses
Type 321 - similar to 304 but lower risk of weld decay due to addition of titanium.
400 Series - ferritic and martensitic chromium alloys
Type 405 - a ferritic especially made for welding applications
Type 408 - heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409 - cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
Type 410 - martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
Type 416 - easy to machine due to additional sulfur
Type 420 - Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.
Type 430 - decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.
Type 440 - a higher grade of cutlery steel, with more carbon in it, which allows for much better edge retention when the steel is heat-treated properly. It can be hardened to around Type 446 - For elevated temperature service
500 Series - heat-resisting chromium alloys
600 Series - martensitic precipitation hardening alloys
601 through 604: Martensitic low-alloy steels.
610 through 613: Martensitic secondary hardening steels.
614 through 619: Martensitic chromium steels.
630 through 635: Semiaustenitic and martensitic precipitation-hardening stainless steels.
Type 630 is most common PH stainless, better known as 17-4; 17% chromium, 4% nickel.
650 through 653: Austenitic steels strengthened by hot/cold work.
660 through 665: Austenitic superalloys; all grades except alloy 661 are strengthened by second-phase precipitation
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