Carbon steels and low alloy steels are designated by a four digit number, where the first two digits indicate the alloying elements and the last two digits indicate the amount of carbon, in hundredths of a percent by weight. For example, a 1060 steel is a plain carbon steel containing 0.60 wt% C.
The standards organization ASTM International produces standards for structural steel used in the construction industry. Those steel alloys have designations which start with A, for example A36, A588, or A514.
In metallurgy, stainless steel is defined as a steel alloy with a minimum of 11.5% chromium content by mass. 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. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch straps.
Stainless steel differs from carbon steel by amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amount of chromium present so that a passive film of chromium oxide forms which prevents further corrosion.
- 100 Series - austenitic chromium-nickel-manganese alloys
- Type 101 - austenitic that is hardenable through cold working for furniture
- Type 102 - austenitic general purpose stainless steel working for furniture
- 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 - free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.
- Type 304 - the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506.
- Type 304L - same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.
- Type 304LN - same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.
- Type 308 - used as the filler metal when welding 304
- Type 309 - better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.
- Type 316 - the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of Type 316L. Also referred to as "A4" in accordance with ISO 3506. 316Ti (which includes titanium for heat resistance) is used in flexible chimney liners, and is able to withstand temperatures up to 2000 degrees Fahrenheit, the hottest possible temperature of a chimney fire.
- Type 321 - similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.
- 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 Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered a more desirable choice in knifemaking than 440A except for diving or other salt-water applications.
- 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.
- 2200 Series
- Type 2205 - 2205 is the most widely used duplex (ferritic/austenitic) stainless steel grade. It finds applications due to both excellent corrosion resistance and high strength.
- 4100 Series - High-Strength Low-Alloy (HSLA) steels, as specified by the Society of Automotive Engineers (SAE). Alloying elements include chromium and molybdenum, and as a result these materials are often referred to as chromoly steel, or cro-mo, or CRMO. They have an excellent strength to weight ratio, are easily welded and are considerably stronger and more durable than standard 1020 steel. While these grades of steel do contain chromium, it is not in great enough quantities to provide the corrosion resistance found in stainless steel.
- Type 4130 - include structural tubing, Ducati Corsa frames, AK-47 receivers, clutch and flywheel components, and roll cages.
The Unified Numbering System (UNS) is an alloy designation system widely accepted in North America. It consists of a prefix letter and five digits designating a material composition. A prefix of S indicates stainless steel alloys, C for copper, brass, or bronze alloys, T for tool steels, etc. The first three digits often match older three-digit numbering systems, while the last two digits indicate more modern variations. For example, Copper Alloy No. 377 (forging brass) in the original three-digit system became C37700 in the UNS System. The UNS is managed jointly by the American Society for Testing and Materials and SAE International, formerly known as the Society of Automotive Engineers. A UNS number alone does not constitute a full material specification because it establishes no requirements for material properties, heat treatment, form, or quality.
Some common materials and translations to other standards:
- UNS K11547 is T2 tool steel
- UNS S17400 is ASTM grade 630, Cr-Ni 17-4PH precipitation hardening stainless steel
- UNS S30400 is AISI 304, Cr/Ni 18/10, Euronorm 1.4301 stainless steel
- UNS S31600 is AISI 316
- UNS S31603 is 316L, a low carbon version of 316
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