The Working of Steel | Page 6

described, which allow a skilled chemist to distinguish it without difficulty and unerringly from the other known elements--nearly 100 of them.
Carbon is another chemical element, written C for short, which is widely distributed through nature. Carbon also readily combines with oxygen and other chemical elements, so that it is rarely found pure; its most familiar form is soot, although the rarer graphite and most rare diamond are also forms of quite pure carbon. It can also be readily separated from its multitude of compounds (vegetation, coal, limestone, petroleum) by the chemist.
With the rise of knowledge of scientific chemistry, it was quickly found that the essential difference between iron and steel was that the latter was iron plus carbon. Consequently it is an alloy, and the definition which modern metallurgists accept is this:
"Steel is an iron-carbon alloy containing less than about 2 per cent carbon."
Of course there are other elements contained in commercial steel, and these elements are especially important in modern "alloy steels," but carbon is the element which changes a soft metal into one which may be hardened, and strengthened by quenching. In fact, carbon, of itself, without heat treatment, strengthens iron at the expense of ductility (as noted by the percentage elongation an 8-in. bar will stretch before breaking). This is shown by the following table:
-------------------------------------------------------------------------- | | |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.
Sulphur is of most trouble to rolling and forging operations when conducted at a red heat. It makes steel tender and brittle at that temperature--a condition known to the workmen as "red-short." It seems to have little or no effect upon the physical properties of cold steel--at least as revealed by the ordinary testing machines--consequently many specifications do not set any limit on sulphur, resting on the idea that if sulphur is low enough not to cause trouble to the manufacturer during rolling, it will not cause the user any trouble.
Tool steel and other fine steels should be very low in sulphur, preferably not higher than 0.03 per cent. Higher sulphur steels (0.06 per cent, and even up to 0.10 per cent) have given very good service for machine parts, but in general a high sulphur steel is a suspicious steel. Screw stock is purposely made with up to 0.12 per cent sulphur and a like amount of phosphorus so it will cut freely.
Manganese counteracts the detrimental effect of sulphur when present in the