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C-A-S-T: The Four Types of Steel

Quite often our customers will ask us about the different types of steel we sell, and what to look for when picking steel grades, shapes and sizes. While there are many ways to categorize steel, we find it useful to break steel down into four categories (Carbon, Alloy, Stainless and Tool Steel). In this four-part blog series, (Part 2, Part 3, Part 4) we take an in-depth look at some of the most common categories of steels, what makes them different, and what to consider when deciding which type of steel is right for you. Please note that in the UK Alloy Steel is often called Engineering Steel.

4 Types of Steel

According to the American Iron & Steel Institute (AISI), Steel can be categorized into four basic groups based on the chemical compositions:

  1. Carbon Steel
  2. Alloy Steel
  3. Stainless Steel
  4. Tool Steel

There are many different grades of steel that encompass varied properties. These properties can be physical, chemical and environmental.

All steel is composed of iron and carbon. It is the amount of carbon, and the additional alloys that determine the properties of each grade.


Types of Steel can also be classified by a variety of different factors:

  1. Composition: Carbon range, Alloy, Stainless.
  2. The production method: Continuous cast, Electric furnace, Etc.
  3. Finishing method used: Bright Drawn, Black Steel, Cold Drawn (Cold Finished), Etc.
  4. Form or shape: Bar, Rod, Tube, Pipe, Plate, Sheet, Structural, Etc.
  5. De-oxidation process (oxygen removed from steelmaking process): Killed & Semi-Killed Steel, Etc.
  6. Microstructure: Ferritic, Pearlitic, Martensitic, Etc.
  7. Physical Strength (Per ASTM Standards).
  8. Heat Treatment: Annealed, Quenched & Tempered, Etc.
  9. Quality Nomenclature: Commercial Quality, Drawing Quality, Pressure Vessel Quality, Etc.

Steel Numbering Systems

There are two major numbering systems used by the steel industry in North America, the first developed by the American Iron & Steel Institute (AISI), and the second by the Society of Automotive Engineers (SAE). Both of these systems are based on four digit code numbers when identifying the base carbon and alloy steels. There are selections of alloys that have five digit codes instead. In the UK the British Standards Institute (BSI) has developed British Standards (BS). The European Committee for Standardization (CEN) has the European Norm (EN) standards. Where there is are conflicting standard between BS and EN in the UK BS EN designations are used which identifies a British adoption of a EN standard.

If the first digit is a one (1) in this designation it indicates a carbon steel. All carbon steels are in this group (1xxx) in both the SAE & AISI system. They are also subdivided into four categories due to particular underlying properties among them. See below:

  • Plain Carbon Steel is encompassed within the 10xx series (containing 1.00% Mn maximum)
  • Re-Sulfurized Carbon steel is encompassed within the 11xx series
  • Re -Sulfurized and Re-Phosphorized Carbon Steel is encompassed within the 12xx series
  • Non-Re-Sulfurized High-Manganese (up-to 1.65%) carbon steel is encompassed within the 15xx series.

The first digit on all other alloy steels (under the SAE-AISI system), are then classified as follows:

2 = Nickel steels.

3 = Nickel-chromium steels.

4 = Molybdenum steels.

5 = Chromium steels.

6 = Chromium-vanadium steels.

7 = Tungsten-chromium steels.

8 = Nickel-chromium-molybdenum steels

9 = Silicon-manganese steels and various other SAE grades

The second digit of the series (sometimes but not always) indicates the concentration of the major element in percentiles (1 equals 1%).

The last two digits of the series indicate the carbon concentration to 0.01%.

For example: SAE 4140 indicates a molybdenum steel alloy, containing 1% of molybdenum and 0.40% of carbon.

Carbon Steel

Carbon Steel can be segregated into three main categories: Low carbon steel (sometimes known as mild steel); Medium carbon steel; and High carbon steel.

Low Carbon Steel (Mild Steel):  Typically contain 0.04% to 0.30% carbon content. This is one of the largest groups of Carbon Steel. It covers a great diversity of shapes; from Flat Sheet to Structural Beam. Depending on the desired properties needed, other elements are added or increased. For example: Drawing Quality (DQ) – The carbon level is kept low and Aluminium is added, and for Structural Steel the carbon level is higher and the manganese content is increased.

Medium Carbon Steel: Typically has a carbon range of 0.31% to 0.60%, and a manganese content ranging from .060% to 1.65%. This product is stronger than low carbon steel, and it is more difficult to form, weld and cut. Medium carbon steels are quite often hardened and tempered using heat treatment.

High Carbon Steel: Commonly known as “carbon tool steel” it typically has a carbon range between 0.61% and 1.50%. High carbon steel is very difficult to cut, bend and weld. Once heat treated it becomes extremely hard and brittle.

This article is the first of a four-part series on the different types of steel. Read Part 2 to learn more about Alloy Steel grades and its attributes.

Don’t have time to read the blog?

You can watch our videos below:

Part 1: The Four Types of Steel

Part 2: Carbon Steel

Part 3: Alloy Steel

Part 4: Stainless Steel

Part 5: Tool Steel


Metal Supermarkets


Metal Supermarkets is the world’s largest small-quantity metal supplier with 125 brick-and-mortar stores across the US, Canada, and United Kingdom. We are metal experts and have been providing quality customer service and products since 1985.

At Metal Supermarkets, we supply a wide range of metals for a variety of applications. Our stock includes: mild steel, stainless steel, aluminium, tool steel, engineering steel, brass, bronze and copper.

We carry a wide range of shapes including: bars, tubes, sheets and plates. We can cut metal to your exact specifications.

Visit one of our 8 locations in the United Kingdom today.

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