Iron and Steel
The internal structure of steel almost wholly dependent upon the exact relationship of the iron and carbon. The carbon is in chemical combination with the iron as iron carbide (Fe3C), called cementite. In steels containing 0.85% carbon, the cementite forms a perfect mixture with pure iron(called ferrite) present. This mixture is called pearlite because of its resemblance in appearance to mother of pearl. Pearlite is a mechanical mixture of six parts of ferrite to one part of cementite. Steels with less than 0.85% carbon are composed of pearlite and excess ferrite. Practically all aircraft steels are of this type. On the other hand, tool steels which contain more than 0.85% carbon are composed of pearlite and excess cementite.Pearlite is normally a laminated structure consisting of alternate layers of ferrite and cementite. In some cases pearlite has a granulated appearance and is called granular pearlite. If steel is cooled very slowly through the critical range, laminated pearlite, which is the most stable form, will result. Pearlite is relatively strong, hard and ductile. It has a tensile strength of over 100,000 p.s.i., an elongation of approximately 10%, and maximum hardening power. This latter point is extremely significant. It means that the greatest hardness from heat treatment is obtained by steel containing 0.85% carbon. It is also true that starting with low-carbon steel, greater hardness is obtainable as the carbon content increases and approaches 0.85%. This point is important when selecting a steel to give great strength and hardness after heat treatment. Ferrite is pure alpha iron in steel carbon steels. In alloy steels containing nickel, molybdenum or vanadium, these alloying elements are in solid solution in the ferrite. Ferrite is very ductile and has a tensile strength of about 40,000 p.s.i. It should be noted that it imparts these properties to low-carbon steels of which it is the major constituent. Ferrite does not have any hardening properties. Cementite is iron carbide. It has very hard and brittle and produces a hardening quality on steels of which it is a part. Austenite, the same name given to steel when it is heated above the critical range, consists of a solid solution of cementite in gamma iron. It is stable only when maintained at a temperature above critical range. It will, however, attain perfect homogeneity if sufficient time is allowed. The grain size of steel, it has been found, is smallest just above the critical range, and it is a known fact that the smallest grain size will give the strongest and best metal. For this reason, when steel is heated for subsequent hardening or working, its temperature is kept just above the upper critical point for the time necessary to insure through heating of the metal. Pure iron(FERRITE) is a soft metal having a crystalline structure like a map with fields separated by hedges. In pure state, it is so soft that while machining it would tear badly giving a poor finish. To obtain pure iron is also a difficult process because during melting it is not easy to get rid of certain elements for which iron has great affinity. Therefore pure iron is of theoretical interest only.