Steel is a combination of iron and carbon, but did you know there are more than 3,500 different grades of steel? A steel’s grade is determined by the amount of carbon, what other alloys it contains, and the way it has been processed.
The Four Types of Steel
Steel is graded as a way of classification and is often categorized into four groups—Carbon, Alloy, Stainless, and Tool.
- Carbon Steels only contain trace amounts of elements besides carbon and iron. This group is the most common, accounting for 90% of steel production. Carbon Steel is divided into three subgroups depending on the amount of carbon in the metal: Low Carbon Steels/Mild Steels (up to 0.3% carbon), Medium Carbon Steels (0.3–0.6% carbon), and High Carbon Steels (more than 0.6% carbon).
- Alloy Steels contain alloying elements like nickel, copper, chromium, and/or aluminum. These additional elements are used to influence the metal’s strength, ductility, corrosion resistance, and machinability.
- Stainless Steels contain 10–20% chromium as their alloying element and are valued for their high corrosion resistance. These steels are commonly used in medical equipment, piping, cutting tools, and food processing equipment.
- Tool Steels make excellent cutting and drilling equipment as they contain tungsten, molybdenum, cobalt, and vanadium to increase heat resistance and durability.
What are the different grades of steel?
Steel grading systems provide a way to categorize steel based on all the different factors that can influence its properties and uses.
For instance, the rate steel is cooled can impact how its molecules are joined together, as can the amount of time the steel is held at several critical temperature points during the cooling process. It’s possible for two steels with the same alloy content to have different grades based on this heat treatment process.
- The ASTM Grading System assigns each metal a letter prefix based on its overall category (“A” is the designation for iron and steel materials), as well as a sequentially-assigned number that corresponds with that metal’s specific properties.
- The SAE Grading System uses a four-digit number for classification. The first two digits denote the steel type and alloying element concentration, and the last two digits indicate the carbon concentration of the metal.
Steel grading standards are widely used by scientists, engineers, architects, and government agencies to ensure quality and consistency of materials. These standards provide a common language to communicate the properties of steel with great specificity, and guide product manufacturers toward proper processing and application procedures.