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Classification of Steel

Steel Sheets

Steel comes in many flavors and varieties, all of which can be classified by a number of different methods, but the most common of theses method is based on chemical composition. This method is generally a measurement of a particular type of steel’s carbon content, or the alloy elements (other than carbon) included in the overall composition.

Carbon Steel

Basic composition:

Iron | Carbon | 1.65% Manganese| .6% (or less) Copper | trace amounts of silicon, sulfur and phosphorus (sulfur and phosphorus are considered impurities and should no more than .05% of the total composition)

The carbon steel family include 50 standard grades, sub-divided into four groups: Low-Carbon Steel (.30% max carbon), medium-carbon steel (.30% – .45% carbon), high-carbon steel (.45% – .75%) and very high-carbon steel (1.50% or more).

Lower carbon steels are more widely used than the higher carbon varieties because the metal is more ductile and easier to weld and machine. As the carbon content increases, the metal’s tensile strength increases. Very high-carbon strength steel is very hard and usually reserved for tools and springs. The hardness factor makes high-carbon steel very difficult to weld, and requires a heat-treatment process prior to, during and after welding to maintain the integrity of the metal.

Alloy Steel

Basic composition:

1.65% manganese or .60% copper, or a guaranteed minimum amount of any other “alloy metal” (which includes nickel, chromium and molybdenum).

 Alloy steels are generally referred to as high-strength low-alloy or high-strength heat-treated alloy. High-strength low-alloy steel is easier to weld than their high-strength heat-treated alloy steel and costs less than higher carbon steel (but are equivalent in strength).

High-strength heat-treated alloy steel is very hard and strong, often used for structural work, and cannot be welded without compromising some of the steels overall integrity.

High-Alloy Steel

This designation refers to any steel containing more than 5% of the alloying metal (primarily chromium and nickel). This type of steel is designed to resist corrosion and intense heat. Stainless steel falls into this category, 18-8 (10% chromium and 8% nickel) being the most common type.

ASIS & SAE Numbering Systems

Most steel is numbered using a four digit figure created by the American Iron and Steel Institute (AISI), also adopted by the Society of Automotive engineers (SAE). The first digit refers the primary alloy, i.e. 1=carbon, 2=nickel etc… The second digit indicates the percentage of the alloy.  The last two digits indicate the amount of carbon in the steel.

ASTM Specifications

The American Society for Testing and Materials (ASTM) also classifies steel using a prefix letter (based on manufacturing method – B indicating open earth and E indicated electric) along with a three digit figure.




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