There are two general kinds of steel:carbon steel and alloy steel. Carbon steel and alloy steel. Carbon steel contains only iron and carbon, while alloy steel contains some other “alloying elements”such as nickel, chromium,manganese,molybdenum,tungsten,vanadium,etc. 

Each kind of steel is divided into several grades as follows:

1. carbon steels

1. low carbon steel; containing from 0.05 to 0.15 percent carbon; this steel is known as machine steel;
2. medium carbon steel containing from 0.15 to 0.6 percent carbon;
3. high carbon steel containing from 0.6 to 1.5 percent carbon; this steel is sometimes called “tool steel”

2. Alloy steels

1. Special alloy steel, such as nickel steel, chromium steel;
2. High-speed steel, also known as self-hardening steel.

Carbon steel are the common steels used in industry. The properties of these steels depend only on the percentage of carbon they contain. Low carbon steels are very soft and can be used for rails, bolts and for machine parts that do not need strength.

Medium carbon steel is better grade and stronger than low carbon steel. It is also more difficult to cut than low carbon steel.

High carbon steel may be hardened by heating it to a certain temperature and then quickly cooling in water. The more carbon the steel contains and the quicker the cooling is, the harder it becomes, because of its high strength and hardness this grade of steel may be used for tools and working parts of machines.

But for some special uses, for example, for gears, bearing, springs, shafts, and wire, carbon steels cannot be always used because they have no properties needed for these parts. Some special alloy steels should be used for such parts because the alloying elements make them tougher, stronger, or harder than carbon steels.

Some alloying elements cause steels to resist rusting (corrosion), and such steels are called stainless steels. Stainless steels contains a high percentage of chromium. Chromium also makes steel harder. Nickel is used in steel to increase strength and toughness. Some alloying elements (such as chromium and tungsten) make the grain of steel finer, thus increasing the hardness and strength of steel, because the finer the grain is, the stronger the steel becomes.

Heat-resistant steel is made by adding some tungsten and molybdenum, while manganese increases the wear resistance of steel. Vanadium steels resist corrosion and can stand shocks and vibrations.

Tools made of high-speed steel containing tungsten, chromium, vanadium, and carbon, may do the cutting at much higher speeds than carbon tool steels.