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:

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 steel to an amount at least five times the sulphur content.

Phosphorus is an element (symbol P) which enters the metal from the ore. It remains in the steel when made by the so-called acid process, but it can be easily eliminated down to 0.06 per cent in the basic process. In fact the discovery of the basic process was necessary before the huge iron deposits of Belgium and the Franco-German border could be used. These ores contain several per cent phosphorus, and made a very brittle steel ("cold short") until basic furnaces were used. Basic furnaces allow the formation of a slag high in lime, which takes practically all the phosphorus out of the metal. Not only is the resulting metal usable, but the slag makes a very excellent fertilizer, and is in good demand.

Silicon is a very widespread element (symbol Si), being an essential constituent of nearly all the rocks of the earth. It is similar to carbon in many of its chemical properties; for instance it burns very readily in oxygen, and consequently native silicon is unknown—it is always found in combination with one or more other elements. When it bums, each atom of silicon unites with two atoms of oxygen to form a compound known to chemists as silica (SiO₂), and to the small boy as "sand" and "agate."

Iron ore (an oxide of iron) contains more or less sand and dirt mixed in it when it is mined, and not only the iron oxide but also some of the silicon oxide is robbed of its oxygen by the smelting process. Pig iron—the product of the blast furnace—therefore contains from 1 to 3 per cent of silicon, and some silicon remains in the metal after it has been purified and converted into steel.

However, silicon, as noted above, burns very readily in oxygen, and this property is of good use in steel making. At the end of the steel-making process the metal contains more or less oxygen, which must be removed. This is sometimes done (especially in the so-called acid process) by adding a small amount of silicon to the hot metal just before it leaves the furnace, and stirring it in. It thereupon abstracts oxygen from the metal wherever it finds it, changing to silica (SiO₂) which rises and floats on the surface of the cleaned metal. Most of the silicon remaining in the metal is an excess over that which is required to remove the dangerous oxygen, and the final analysis of many steels show enough silicon (from 0.20 to 0.40) to make sure that this step in the manufacture has been properly done.

Manganese is a metal much like iron. Its chemical symbol is Mn. It is somewhat more active than iron in many chemical changes—notably it has what is apparently a stronger attraction for oxygen and sulphur than has iron. Therefore the metal is used (especially in the so-called basic process) to free the molten steel of oxygen, acting in a manner similar to silicon, as explained above. The compound of manganese and oxygen is readily eliminated from the metal. Sufficient excess of elemental manganese should remain so that the purchaser may be sure that the iron has been properly "deoxidized," and to render harmless the traces of sulphur present. No damage is done by the presence of a little manganese in steel, quite the reverse. Consequently it is common to find steels containing from 0.3 to 1.5 per cent.

Alloying Elements.—Commercial steels of even the simplest types are therefore primarily alloys of iron and carbon. Impurities and their "remedies" are always present: sulphur, phosphorus, silicon and manganese—to say nothing of oxygen, nitrogen and carbon oxide gases, about which we know very little. It has been found that other metals, if added to well-made steel, produce definite improvements in certain directions, and these "alloy steels" have found much use in the last ten years. Alloy steels, in addition to the above-mentioned elements, may commonly contain one or more of the following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium (Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed at more length in Chapters III and IV.

PROPERTIES OF STEEL

Steels are known by certain tests. Early tests were more or less crude, and depended upon the ability of the workman to judge the "grain" exhibited by a freshly broken piece of steel. The cold-bend test was also very useful—a small bar was bent flat upon itself, and the stretched fibers examined for any sign of break. Harder stiff steels were supported at the ends and the amount of central load they would support before fracture, or the amount of permanent set they would acquire at a given load noted. Files were also used to test the hardness of very hard steel.

These tests are still used to a considerable extent, especially in works where the progress of an operation can be kept under close watch in this way, the product being periodically examined by more precise methods. The chief furnace-man, or "melter," in a steel plant, judges the course of the refining process by casting small test ingots from time to time, breaking them and examining the fracture. Cutlery manufacturers use the bend test to judge the temper of blades. File testing of case-hardened parts is very common.

However there is need of standardized methods which depend less upon the individual skill of the operator, and which will yield results comparable to others made by different men at different places and on different steels. Hence has grown up the art of testing materials.

TENSILE PROPERTIES

Strength of a metal is usually expressed in the number of pounds a 1-in. bar will support just before breaking, a term called the "ultimate strength." It has been found that the shape of the test bar and its method of loading has some effect upon the results, so it is now usual to turn a rod 5½ in. long down to 0.505 in. in diameter for a central length of 2-3/8 in., ending the turn with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so the load it bears at rupture multiplied by 5 will represent the "ultimate strength" in pounds per square inch.

Such a test bar is stretched apart in a machine like that shown in Fig. 9. The upper end of the bar is held in wedged jaws by the top cross-head, and the lower end grasped by the movable head. The latter is moved up and down by three long screws, driven at the same speed, which pass through threads cut in the corners of the cross-head. When the test piece is fixed in position the motor which drives the machine is given a few turns, which by proper gearing pulls the cross-head down with a certain pull. This pull is transmitted to the upper cross-head by the test bar, and can be weighed on the scale arm, acting through a system of links and levers.

Thus the load may be increased as rapidly as desirable, always kept balanced by the weighing mechanism, and the load at fracture may be read directly from the scale beam.

This same test piece may give other information. If light punch marks are made, 2 in. apart, before the test is begun, the broken ends may be clamped together, and the distance between punch marks measured. If it now measures 3 in. the stretch has been 1 in. in 2, or 50 per cent. This figure is known as the elongation at fracture, or briefly, the "elongation," and is generally taken to be a measure of ductility.

When steel shows any elongation, it also contracts in area at the same time. Often this contraction is sharply localized at the fracture; the piece is said to "neck." A figure for contraction in area is also of much interest as an indication of toughness; the diameter at fracture is measured, a corresponding area taken out from a table of circles, subtracted from the original area (0.200 sq. in.) and the difference divided by 0.2 to get the percentage contraction.

Quite often it is desired to discover the elastic limit of the steel, in fact this is of more use to the designer than the ultimate strength. The elastic limit is usually very close to the load where the metal takes on a permanent set. That is to say, if a delicate caliper ("extensometer," so called) be fixed to the side of the test specimen, it would show the piece to be somewhat longer under load than when free. Furthermore, if the load had not yet reached the yield point, and were released at any time, the piece would return to its original length. However, if the load had been excessive, and then relieved, the extensometer would no longer read exactly 2.0 in., but something more.

Soft steels "give" very quickly at the yield point. In fact, if the testing machine is running slowly, it takes some time for the lower head to catch up with the stretching steel. Consequently at the yield point, the top head is suddenly but only temporarily relieved of load, and the scale beam drops. In commercial practice, the yield point is therefore determined by the "drop of the beam." For more precise work the calipers are read at intervals of 500 or 1,000 lb. load, and a curve plotted from these results, a curve which runs straight up to the elastic limit, but there bends off.

A tensile test therefore gives four properties of great usefulness: The yield point, the ultimate strength, the elongation and the contraction. Compression tests are seldom made, since the action of metal in compression and in tension is closely allied, and the designer is usually satisfied with the latter.

IMPACT TESTS

Impact tests are of considerable importance as an indication of how a metal will perform under shock. Some engineers think that the tensile test, which is one made under slow loading, should therefore be supplemented by another showing what will happen if the load is applied almost instantaneously. This test, however, has not been standardized, and depends to a considerable extent upon the type of machine, but more especially the size of the specimen and the way it is "nicked." The machine is generally a swinging heavy pendulum. It falls a certain height, strikes the sample at the lowest point, and swings on past. The difference between the downward and upward swing is a measure of the energy it took to break the test piece.

FATIGUE TESTS

It has been known for fifty years that a beam or rod would fail at a relatively low stress if only repeated often enough. It has been found, however, that each material possesses a limiting stress, or endurance limit, within which it is safe, no matter how often the loading occurs. That limiting stress for all steels so far investigated causes fracture below 10 million reversals. In other words, a steel which will not break before 10,000,000 reversals can confidently be expected to endure 100,000,000, and doubtless into the billions.

About the only way to test one piece such a large number of times is to fashion it into a beam, load it, and then turn the beam in its supports. Thus the stress in the outer fibers of the bar varies from a maximum stretch through zero to a maximum compression, and back again. A simple machine of this sort is shown in Fig. 10, where B and E are bearings, A the test piece, turned slightly down in the center, C and D ball bearings supporting a load W. K is a pulley for driving the machine and N is a counter.

HARDNESS TESTING

The word "hardness" is used to express various properties of metals, and is measured in as many different ways.

"Scratch hardness" is used by the geologist, who has constructed "Moh's scale" as follows:

A mineral will scratch all those above it in the series, and will be scratched by those below. A weighted diamond cone drawn slowly over a surface will leave a path the width of which (measured by a microscope) varies inversely as the scratch hardness.

"Cutting hardness" is measured by a standardized drilling machine, and has a limited application in machine-shop practice.

"Rebounding hardness" is commonly measured by the Shore scleroscope, illustrated in Fig. 11. A small steel hammer, ¼ in. in diameter, ¾ in. in length, and weighing about 1/12 oz. is dropped a distance of 10 in. upon the test piece. The height of rebound in arbitrary units represents the hardness numeral.

Should the hammer have a hard flat surface and drop on steel so hard that no impression were made, it would rebound about 90 per cent of the fall. The point, however, consists of a slightly spherical, blunt diamond nose 0.02 in. in diameter, which will indent the steel to a certain extent. The work required to make the indentation is taken from the energy of the falling body; the rebound will absorb the balance, and the hammer will now rise from the same steel a distance equal to about 75 per cent of the fall. A permanent impression is left upon the test piece because the impact will develop a force of several hundred thousand pounds per square inch under the tiny diamond-pointed hammer head, stressing the test piece at this point of contact much beyond its ultimate strength. The rebound is thus dependent upon the indentation hardness, for the reason that the less the indentation, the more energy will reappear in the rebound; also, the less the indentation, the harder the material. Consequently, the harder the material, the more the rebound.

"Indentation hardness" is a measure of a material's resistance to penetration and deformation. The standard testing machine is the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter, is forced into the test piece with a pressure of 3,000 kg. (3-1/3 tons). The resulting indentation is then measured.

While under load, the steel ball in a Brinell machine naturally flattens somewhat. The indentation left behind in the test piece is a duplicate of the surface which made it, and is usually regarded as being the segment of a sphere of somewhat larger radius than the ball. The radius of curvature of this spherical indentation will vary slightly with the load and the depth of indentation. The Brinell hardness numeral is the quotient found by dividing the test pressure in kilograms by the spherical area of the indentation. The denominator, as before, will vary according to the size of the sphere, the hardness of the sphere and the load. These items have been standardized, and the following table has been constructed so that if the diameter of the identation produced by a load of 3,000 kg. be measured the hardness numeral is found directly.