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