Steels that exceed the element limits for carbon steels. Also includes steels that contain elements not found in carbon steels such as nickel, chromium (up to 3.99%), cobalt, etc. The difference between plain carbon and alloy steels is ambiguous. Both contain carbon, manganese, and silicon. Copper and boron are additives to both classes. Usually high allow steels contain more than 1.65% manganese, 0.6% silicon, and 0,6% copper

EFFECTS OF ALLOYING ELEMENTS

Usually only a small amount of alloying element are added to steels (usually less than 5%). Mostly the purpose is to improve the hardenability and strength corrosion resistance, stability at high/low temperatures, control grain size

Manganese - increases ductility, hardenability, high strain hardening capacity, slightly strengthens, excellent wear resistance

Sulfur - if carefully proportioned can add machinability without imparting embrittlement.

Nickel - increases toughness and impact resistance, good properties at low temperatures. With other alloys imparts excellent corrosion resistance. With certain alloys it has a small thermal expansion and used for sensitive measuring devices. Increase strength with little loss of ductility.

Chromium - if added in large enough amounts can impart corrosion resistance and heat resistance and wear resistance and hardenability. Otherwise for amounts (less than 2.11%) it is used to slightly increase hardenability and strength.

Molybdenum - improves hardenability and increases strength primarily under dynamic and high temperature conditions. Extremely stable at elevated temperatures. It helps to retains fine grain sizes which provides strength and creep resistance at elevated temperatures. Molybdenum carbides are used in hot work tool steels and forging dies to impart hardness even at red heat.

Vanadium - like molybdenum, forms strong carbides at elevated temperatures. Also limits grain size.

Tungsten- used in tool steels to maintain their hardness at elevated temperatures.

Copper - increases the corrosion resistance. Limits have to be controlled or it’ll sacrifice surface quality and hot-working behavior.

Silicon - increases strength without limiting grain size* Used to promote large grain sizes used in magnetic applications. Used in spring steels.

Boron - very important harden ability agent being several hundred times better than nickel, molybdenum and chromium. Used more for low carbon steels. Also improve machinability and cold forming.

*Limits on grain size can effectively increase strength properties like elastic limit, yield point, and impact strength (toughness) with little loss of ductility.

Designation

AISI - American Iron and Steel Institute - general

AISI use a four digit number. The first is the class of alloy specified.

1XXX Carbon steels

2XXX Nickel chromium

3XXX Moybdenum

4XXX Chromium..............etc

2nd number designates the subgroup of the alloy Last two numbers designate the amount of carbon in 0.01%; therefore a 1080 steel has 0.8% carbon.