Plain carbon steels are the steels that contain only carbon and no other major alloying elements. This kind of steel has 3 types which are low-carbon steel, medium-carbon steel and high-carbon steel. They get their special names from their content of carbon.

If the carbon content is lower than 0.25%, it is called low-carbon steel. It cannot be hardened but can be case-hardened. It has these characteristics which are easy machining & forming, comparatively less strength, least expensive, comparatively less hardness and largest quantity produced. In practice use, bolts, nuts, washers, sheet steel, and shafts use low-carbon steel.

Medium-carbon steel contains from 0.25-0.60 percent carbons. This kind of steel is mostly used in situation where greater tensile strength is required. This kind of steel may be hardened for it has higher carbon content. Its characteristics are as follows, hard & strong after heat treating; medium depth of hardness and more expensive than low carbon steel. Tools such as wrenches, hammers, and screwdrivers are drop-forged from medium-carbon steel and later heat treated.

High-carbon steel (also named as tool steel) contains over 0.60 percent carbon, and it can contain as high as 1.7 percent. It has these characteristics: hard & strong after heat treating; depth of hardness increases and more expensive than low & medium carbon steels. Cutting tools, punches, taps, dies, drills and reamers usually use this kind of steel.

Then is the alloy steels. Alloy steel is the steel whose characteristics are determined by the addition of other elements in addition to carbon. Alloy steels have some special characteristics which are not found in plain carbon steels, so alloy steels are widely used in practice. That is to say, sometime we prefer alloy steel to plain steel.

Alloy steel has these following characteristics: it increases in tensile strength, hardness, toughness, antiwear property, corrosion resistance and red-hardness.