Steel grades are classified by type and furnace used to produce them. The US has two grading systems, SAE and AISI, which have internationalized. Steel grades have varying physical and chemical properties due to the addition of elements such as carbon, chromium, and manganese. Some grades use a four-digit code, while others use three digits or letters. International standardization compares six regional standards. Example: 1018 carbon steel in the US is C15D in Europe, CK15 in Germany, 040A15 in England, C15 in Italy, and S15 in Japan.
Steel grades are a collection of various steel classifications to define both the type of steel and the process furnace in which it was produced. Two steel grading systems established in the United States are the standards of the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). AISI standards have internationalized and are used in the UK or combined with British and European standards for the same steel grades. Each classification code for a steel grade is usually a four-digit number or alphanumeric code, with AISI codes often adding a letter that also designates the type of furnace used to make the steel. A “C” prefix indicates that an electric arc, oxygen, or open hearth furnace was used, while the “E” prefix indicates that an electric arc furnace was the only type used to make the steel.
There are hundreds of steel grades produced in industry, so a steel grade chart is often used to organize these various metal types. This is because steel has variable physical and chemical properties when mixed with other compounds. The most common types of elements mixed with steel include carbon, chromium, and manganese, but other elements, such as boron, nickel, and silicon, are also often used. Some grade numbers such as those used by AISI will indicate the percentages of compounds in the steel by weight, with grade 1018 containing approximately 0.15% carbon, 0.75% manganese, 0.030% phosphorus, 0.050% sulfur and trace amounts of silicon.
Carbon steel grades in the US begin with a two-number prefix, 10, 11, 12, or 15. This separates them as plain carbon steel, sulfur content steel, sulfur and phosphorus content, or manganese content steel, respectively . These steel grades then numerically progress in the AISI/SAE system, with manganese steels with a prefix of 13, nickel steels with a prefix of 23 and 25, up to silicon-manganese steels with a prefix of 92.
However, some steel grades do not use the four-digit designation, including all European coding schemes. Some grades of American AISI stainless steel also vary, using three-digit numbers starting with 2, 3, 4, or 5. Those starting with the number 3 indicate austenitic steels, which are steel compounds with iron and carbon content. Stainless steel grades starting with the number 4 are martensitic, a more brittle form of austenitic carbon steel, produced by flash quenching or furnace cooling.
International steel grade standardization for all steel grades, from structural steel to mild steel grades, usually incorporates charts comparing six regional standards. These include standards created in the United States, Europe in general and Germany as well as England, Italy and Japan. An example of a steel rating on such a chart would be 1018 carbon steel in the US, which is C15D in Europe, CK15 in Germany, 040A15 in England, C15 in Italy, and S15 in Japan.
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