The constituents in the microstructure of steel determine its prop­erties, to a large extent. Some of the important constituents in steel will now be examined.

Ferrite. (Fig. 11-5.) This is a solid solution of a very small percent­age of carbon in iron. It appears at a temperature below 1333F and the iron has a body-centered cubic structure. Ferrite grains appear when the carbon content is less than about.8 percent in plain carbon steel. The number of ferrite grains increases as the carbon content of the steel decreases. It is the softest constituent in steel and as the amount of ferrite increases, the steel becomes softer. In alloy steels, some of the alloying elements may be dissolved in the ferrite as a solid-state solution.

Cementite. (Fig. 11-6.) Iron-carbide (Fe3C), a compound of iron and carbon, is called cementite. It is very hard and wear-resistant. Its composition will be varied when other carbide-forming alloys are

present in the steel. Cementite may appear in several different ways in steel. In Fig. 11-6, it appears as a network surrounding the grains in the region of grain boundaries and also within the grain, where it is in the form of thin plates. Cementite may also appear as round, roughly globular-shaped particles in steel that has been spheroid - ized. In their softened condition, tool steels and other high carbon and alloy steels should be spheroidized because in this condition they are easier to machine.

Pearlite. (Fig. 11-7.) This is a very fine platelike structure consisting of thin plates of ferrite sandwiched together with thin plates of cementite. The structure within the grains in Fig. 1 l-в is pearlite. In Fig. 11-7, the white plates are ferrite and the darker plates are cementite. When the steel is cooled slowly, the pearlite is coarse; it becomes finer as the cooling rate is increased. Pearlite is a very strong and tough structure that adds to the strength and toughness of steel. The amount of pearlite in plain carbon steel increases until a maximum is reached at approximately.8 percent carbon.

Martensite. (Fig. 11-8.) This structure results when steel with a relatively high carbon content is cooled very rapidly. Small amounts of martensite may be obtained by quenching a steel having a lower

Austenite. (Fig. 11-10.) This is the face-centered cubic form of iron occurring in plain carbon steels at temperatures above I333F. It normally is not stable at room temperatures; however, in highly alloyed steels, such as tool steels and stainless steels, it can appear at room temperatures. In steels, austenite can dissolve all of the carbon that is present. This property is important in the heat treatment of steels. At room temperature it has a good tensile strength and a strong tendency to work-harden.

Spheroidite. (Fig. 11-11.) This structure consists of many small spheroidal-shaped particles of cementite dispersed in ferrite. It can be obtained through several different heat treatments. When highly alloyed steel and plain carbon steel are in the unhardened condition, it is the preferred structure because of the marked improvement in machinability obtained.

Bainite is another structure that can occur in steel. It may have either a feathery or an acicular appearance. Bainite is produced by quenching steel from an elevated temperature to some temperature above about 400F (depending on the composition of the steel) and

holding the steel at this temperature for a long time. It does not normally occur as a result of welding.