-------------------------------------------------------------------------- | | |Elastic |Ultimate|Percentage. Class by use. | Class by | Per cent | limit |strength|elongation | hardness. | carbon. |lb. per |lb. per |in 8 inches. | | |sq. in. |sq. in. | ------------------|-----------|------------|--------|--------|------------ Boiler rivet steel|Dead soft |0.08 to 0.15| 25,000 | 50,000 | 30 Struc. rivet steel|Soft |0.15 to 0.22| 30,000 | 55,000 | 30 Boiler plate steel|Soft |0.08 to 0.10| 30,000 | 60,000 | 25 Structural steel |Medium |0.18 to 0.30| 35,000 | 65,000 | 25 Machinery steel |Hard |0.35 to 0.60| 40,000 | 75,000 | 20 Rail steel |Hard |0.35 to 0.55| 40,000 | 75,000 | 15 Spring steel |High carbon|1.00 to 1.50| 60,000 |125,000 | 10 Tool steel |High carbon|0.90 to 1.50| 80,000 |150,000 | 5 -------------------------------------------------------------------------- Just why a soft material like carbon (graphite), when added to another soft material like iron, should make the iron harder, has been quite a mystery, and one which has caused a tremendous amount of study. The mutual interactions of these two elements in various proportions and at various temperatures will be discussed at greater length later, especially in Chap. VIII, p. 105. But we may anticipate by saying that some of the iron unites with all the carbon to form a new substance, very hard, a carbide which has been called "cementite." The compound always contains iron and carbon in the proportions of three atoms of iron to one atom of carbon; chemists note this fact in shorthand by the symbol Fe3C (a definite chemical compound of three atoms of iron to one of carbon). Many of the properties of steel, as they vary with carbon content, can be linked up with the increasing amount of this hard carbide cementite, distributed in very fine particles through the softer iron. SULPHUR is another element (symbol S) which is always found in steel in small quantities. Some sulphur is contained in the ore from which the iron is smelted; more sulphur is introduced by the coke and fuel used. Sulphur is very difficult to get rid of in steel making; in fact the resulting metal usually contains a little more than the raw materials used. Only the electric furnace is able to produce the necessary heat and slags required to eliminate sulphur, and as a matter of fact the sulphur does not go until several other impurities have been eliminated. Consequently, an electric steel with extremely low sulphur (0.02 per cent) is by that same token a well-made metal.