Steel AISI: carbon and alloyed

Since the early 1940s in the United States and most countries of the world, the AISI notation system has been used to designate steels.. This system is based on the chemical composition of the steels. It was developed by the American Iron and Steel Institute. (American Iron and Steel Institute – AISI) with the participation of the American Society of Automotive Engineers (Society of Automotive Engineers – SAE).

Over time, the American Iron and Steel Institute ceased to be involved in the development of new steels and the maintenance of their designation system.. Therefore with 1995 year, formally, this designation system began to be called not SAE-AISI, but just SAE. However, since the AISI brand is firmly "stuck" to this steel designation system, then all over the world they continue to use the letters AISI in the designation of steel, as well as a combination of SAE / AISI.

SAE-AISI steel designation system

The designation system for carbon and alloy steels consists of a combination of four numbers and is based on their chemical composition. Sometimes the same AISI steel grades called SAE brands, as well as SAE / AISI brands. However, usually and most often, the letter designation AISI continues to be placed in front of the four digits.. Gotta understand, that for any combinations of literal expressions, the main one is a combination of numbers and sometimes additional letters.

Stainless steels are also designated by the AISI system, but their designation does not consist of four, and from three digits - 2XX, 3XX, 4XX and 5XX. Most Popular Series – austenitic stainless steels – series 3xx and martensitic stainless steels – 4XX series.

For tool steels the AISI system uses completely different designations: one letter and one number, eg, О1.

ANSI Carbon and Alloy Steel Designations

The first number in the designation of AISI steels represents their general category.:
– 1XXX - carbon;
– 2XXX - Nickel;
– 3XXX - nickel-chromium;
– 4XXX - molybdenum;
– 5XXX - chromium;
– 6XXX - chromium vanadium;
– 7XXX - tungsten;
– 8XXX - chromonic nickel molybdenum;
– 9XXX - silicon-manganese.

The second digit in the designation of AISI steels indicates the presence of basic elements, which can affect the properties of steels. The last two digits indicate the carbon content in hundredths of a percent..

Carbon steels AISI 1XXX

Consider for example carbon steel 1018. A zero in the 10XX designation indicates, that the main secondary elements, such as sulfur and phosphorus, have a minimum content in the form of impurities. Sulfur in steels improves their machinability, but in general sulfur, lead, calcium, phosphorus are harmful impurities.

The last two digits indicate the carbon content. For steel 1018 it means, that it is ordinary carbon without the addition of alloying elements and has a nominal carbon content 0,18 %. According to the standards, the carbon range for this steel is 0,15-0,20 % – these are "tolerances" for manufacturing in industrial production.

ANSI High Speed ​​Carbon Steels

Group 1xxx in the AISI system represents carbon steels. This group is further divided into four separate grades of steels.:
– 10xx - ordinary carbonaceous with manganese content up to 1,00 %;
– 11xx - carbonaceous with sulfur additives;
– 12xx - carbon with phosphorus additives;
– 15xx – carbonaceous without additives of phosphorus with a high content of manganese (to 1,65 %).

Let's take a closer look at the designation of the 11XX steel group. The first digit "1" indicates that, what is ordinary carbon steel. The second digit "1" indicates, that the chemical composition has undergone changes in comparison with steel 10XX. Steel,in which the designation contains the first two digits "11" has sulfur additives to improve machinability. Sulfur is added to steel before pouring it into molds. These steels include AISI steels 1113, 1117, 1141.

Letters in the designation of AISI steels

Sometimes additional letters are inserted into the designation of AISI steels between the second and third digits., eg, 11L41, 12L14 or 50V40.

The letter L indicates the presence of lead additives in the steel. Lead, like sulfur, improves cutting of steel. If it is added to steel, in which there is already sulfur, then the machinability increases even more. The letter "B" stands for boron, which is added to low carbon steels to increase hardenability. The letter "H" is added to indicate that, that the steel has a guaranteed depth of heat hardening.