TABLE OF CONTENTS Of all the different types of materials used by the mechanical engineer, steel is still probably the most important, especially in the tonnage used.
Steel is a heat-treatable alloy, primarily an alloy of iron and carbon. The lower limit for a mild steel is about 0.05 C, though the lower limit might be defined as that value below which iron carbide cannot be thrown out of solid solution, which is below 0.01%. The upper theoretical limit is 1.7% which corresponds to the point in the iron-carbon equilibrium diagram beyond which iron carbide cannot be wholly held in solution at any temperature. The lowest carbon alloys are only slightly, and usually only with difficulty, affected by heat treatment, and are termed "irons" rather than "steels".
Other elements are almost always included in ordinary steel, some by design, and some because it is too difficult or expensive to remove them. Excess amounts of sulphur and phosphorous, for example, are quite deleterious. Too much sulphur results in a condition termed hot-shortness, meaning that the steel becomes brittle at high temperatures. Too much phosphorous, on the other hand, causes cold-shortness.
Steel is not a uniform substance like pure gold, but is made up of components--ferrite and cementite. Ferrite is BCC iron in the crystalline form that exists at room temperature in slowly cooled carbon steel. Cementite is a compound of carbon and iron, Fe 3C. When a carbon steel is cooled at a slow rate from a red heat cementite and ferrite...
