class="center c17">Grades

Approximate carbon range Common uses Extra soft
(dead soft) 0.08-0.18 Pipe, chain and other welding purposes; case-hardening purposes; rivets; pressing and stamping purposes. Structural (soft) (medium) 0.08-0.18 Structural plates, shapes and bars for bridges, buildings, cars, locomotives; boiler (flange) steel; drop forgings; bolts. Medium 0.20-0.35 Structural purposes (ships); shafting; automobile parts; drop forgings. Medium hard 0.35-0.60 Locomotive and similar large forgings; car axles; rails. Hard 0.60-0.85 Wrought steel wheels for steam and electric railway service; locomotive tires; rails; tools, such as sledges, hammers, pick points, crowbars, etc. Spring 0.85-1.05 Automobile and other vehicle springs; tools, such as hot and cold chisels, rock drills and shear blades. Spring 0.90-1.15 Railway springs; general machine shop tools.

CHAPTER II

COMPOSITION AND PROPERTIES OF STEEL

It is a remarkable fact that one can look through a dozen text books on metallurgy and not find a definition of the word "steel." Some of them describe the properties of many other irons and then allow you to guess that everything else is steel. If it was difficult a hundred years ago to give a good definition of the term when the metal was made by only one or two processes, it is doubly difficult now, since the introduction of so many new operations and furnaces.

We are in better shape to know what steel is than our forefathers. They went through certain operations and they got a soft malleable, weldable metal which would not harden; this they called iron. Certain other operations gave them something which looked very much like iron, but which would harden after quenching from a red heat. This was steel. Not knowing the essential difference between the two, they must distinguish by the process of manufacture. To-day we can make either variety by several methods, and can convert either into the other at will, back and forth as often as we wish; so we are able to distinguish between the two more logically.

We know that iron is a chemical element—the chemists write it Fe for short, after the Latin word "ferrum," meaning iron—it is one of those substances which cannot be separated into anything else but itself. It can be made to join with other elements; for instance, it joins with the oxygen in the air and forms scale or rust, substances known to the chemist as iron oxide. But the same metal iron can be recovered from that rust by abstracting the oxygen; having recovered the iron nothing else can be extracted but iron; iron is elemental.

We can get relatively pure iron from various minerals and artificial substances, and when we get it we always have a magnetic metal, almost infusible, ductile, fairly strong, tough, something which can be hardened slightly by hammering but which cannot be hardened by quenching. It has certain chemical properties, which need not be 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:

Class by use.	Class by hardness.	Per cent carbon.	Elastic limit lb. per sq. in.	Ultimate strength lb. per sq. in.	Percentage elongation in 8 inches.
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