We have learned that steel, fundamentally, is an alloy of iron with carbon, i.e., carbon is the characteristic element. We are now to note what often seem to be exceptions to this rule. While in reality steel is just this iron-carbon alloy, there are alloys known as steels to which such strong characteristics are given by elements other than carbon, that carbon seems not to be the defining constituent at all. Indeed, in some of these, the carbon content may be small enough that, judging from our experience with the carbon steel series, we would not expect any such physical properties as some of these alloy steels show.
You remember that in olden days they distinguished between wrought iron and steel by quenching the piece in water from a cherry-red heat. If the piece was hardened and made brittle, by this treatment, it was thereby proved to be “steel.” Also, it is generally known that by annealing a piece of hardened steel, which usually means holding at a cherry-red heat for a time and then cooling slowly, it is made soft.
Then what shall we say concerning a certain one of these new alloys, Hadfield’s manganese steel, which is made very much less brittle and a little softer by quenching, but which refuses absolutely to soften under annealing treatment—in other words, is almost the opposite of what we know as steel in these chief defining traits? The nickel steel which contains 15 per cent of nickel, also, exhibits just these characteristics, being softened by quenching but not by annealing.
Again, while iron, the carbon steels, and even the magnetic oxide of iron which contains only about 72 per cent of the metal, are strongly magnetic, manganese steel which has 85 per cent of iron is so non-magnetic that it is sometimes used in place of brass or bronze where an entirely non-magnetic material is required. The nickel steels with 24 per cent or more of nickel are also non-magnetic though both constituent metals, alone, are strongly attracted by the magnet.
These are some of the things which make a logical classification of the iron family so difficult. Though derived from the steels which we knew and made from the same materials with the exception that a greater amount of one constituent, manganese, is added, or perhaps, in other cases, another element or two, the resulting alloys have markedly different and often contradictory properties.
However, we must not be led astray. In all probability carbon is still the necessary constituent, but much less of it is needed to produce results when the other elements are present. There is no doubt, however, that in “manganese steel” or in “nickel,” “chrome,” “tungsten,” “silicon,” “vanadium,” “titanium,” and other alloy steels, the added element or elements exert very strong modifying influences, and sometimes obscure the influence of the carbon.
In the first place, we better at once dispose of certain of these steels by terming the added element a “scavenger” only. Such usually are “titanium” and “aluminum” steels. These are generally ordinary carbon steels in which a very small amount of titanium or aluminum has been used to rid the alloy of certain gaseous or other deleterious elements. Upon analysis, steels so treated often show no trace of the element which has been added to do the work, all of it having passed into the slag, carrying with it the obnoxious substances, which, had they remained would have injured the quality of the steel. Manganese and silicon which were spoken of in the discussion of the Bessemer process as deoxidizing the metal, also exert just this same influence, though there is usually added of these enough that a certain percentage remains in the finished steel. Vanadium and titanium have a particular affinity for oxygen and nitrogen, and aluminum for oxygen. By chemically combining with these gases in the metal, and through possible other influence, they help to produce sound steel having very good physical properties. Vanadium, however, is much more than a “scavenger” as will be seen later on.
Manganese steel was discovered and highly developed by Robert Hadfield of Sheffield, England, along about 1882. His 11 per cent to 14 per cent manganese steel with about 1 per cent of carbon has such great hardness that it cannot be drilled or cut with tools. In forgings and castings it is used for milling machinery for ore treatment; manganese steel rails inserted around sharp curves and for “frogs,” etc., under severe service conditions outlast ordinary steel rails three or four times; it goes into various rolls and crusher parts, steam and dredge shovels, grab buckets, sand pumps, gears, pinions, etc., which have to resist heavy wear. It is much used, too, as a material for burglar-proof safes. The alloy is far too hard to drill and too tough and strong to be broken. It is said that no manganese steel safe has ever been drilled or forcibly entered.
In forming irregular shapes, manganese steel must be cast and finished by grinding but for ordinary bars and rails it can be rolled. In the “raw” condition it is quite brittle and extremely hard. Quenching from a cherry-red heat greatly toughens it and makes it ductile. Though now it can be dented by a hammer blow and marked with a file, it always is so tough that it cannot be machined with any tool. Ordinary annealing treatment has no softening effect on the alloy.
When alloyed in the steel in certain quantities, silicon gives desirable properties. Steels with from one to two per cent of silicon in the tempered condition are very tough. For this reason the leaves of automobile springs are often made from it. Steels with from 3 to 5 per cent of silicon are much used in electrical appliances because of their improved magnetic properties.
To a certain extent steels containing 3 or 4 per cent of the metal molybdenum, and 1 or 1½ per cent of carbon are used in the construction of permanent magnets. It is said that molybdenum is used in some modern guns, which longer resist the corrosive effect of the powder-gases because of it. A certain amount goes into the high-speed steels where it replaces part or all of the tungsten. Here, however, it has been a disappointment and the amount so used seems to be decreasing rather than increasing.
Steels with about ½ per cent of each of carbon and tungsten are occasionally used for manufacture of springs, and with greater amounts, e.g., ¾ per cent of carbon and 5 or 7 per cent of tungsten, for permanent magnets for which they are claimed to be the best material known. The use of tungsten in the tool steels (other than the high-speed steels) is considerable.
Nickel steel is much used on account of its high strength. The most usual alloy, perhaps, is the one which contains about 3½ per cent of nickel. This is in addition to the carbon which may vary between .15 per cent and ½ per cent. This 3½ per cent of nickel adds several thousand pounds per square inch in strength to the steel, and when tempered, both the strength and toughness are greatly improved.
Nickel steels of these compositions can readily be forged and rolled. They are used for drop forgings, machine parts, engine and automobile parts, in seamless tubes and for bridge members of great span.
Nickel will not rust so it does not surprise one that with 22 per cent or more of nickel the steel is almost immune from ordinary corrosion. Steels containing from 25 to 46 per cent nickel are variously used for resistance wire, for valve stems, valves for motors, etc. The 36 per cent nickel steel is the alloy, “Invar,” which has such slight expansion and contraction with heat and cold that it is used for clock pendulums, watch parts and for parts of measuring instruments.
Forty-six per cent nickel steel is called “Platinite.” It has practically the same rates of expansion and contraction with heat and cold as glass and for this reason it finds extensive use in incandescent electric lamps. Wires of the alloy are fused into the glass bases and connect with the filaments in place of the expensive platinum which formerly was used.
As remarked above, the 13 per cent to 15 per cent nickel-iron alloys soften with quenching but not with annealing. The 15 per cent nickel steel has the highest strength of the nickel-iron-carbon series. Though nickel and iron are each strongly magnetic, alloys of the two which contain 24 per cent or more of nickel are not magnetic.
While “simple” chrome steel is pretty well known as a material for products which require great hardness, such as balls, roller bearings, files, rolls, five-ply safes, stamp shoes, projectiles, etc., and heat-treated chrome-vanadium steels are now extensively used in forged frames and shafts of automobile and other machines, a combination of nickel and chromium gives steels which have been great favorites. With 2 to 3½ per cent of nickel, not over 3 per cent of chromium and ½ per cent of carbon, these steels, when expertly heat-treated, can give “elastic limits” anywhere between 40,000 and 250,000 pounds per square inch, with good freedom from brittleness. They are very largely used for automobile gears, axles, and other parts, for armor plate, for projectiles and for many other purposes.
These alloys are also used for castings.
Within a comparatively short time the chrome-vanadium steels have come to be very largely used, often in place of the chrome and nickel-chrome steels. As vanadium is a “deoxidizer,” whereas nickel is not, the chrome-vanadium steels show fewer imperfections than the nickel-chrome steels and they also roll, forge and machine better.
They are used for automobile frames, shafts, for miscellaneous forged and rolled articles and for heat-treated armor plates. Of this comparatively new material about 90,000 tons were made during 1913, according to a recently issued bulletin of the Department of the Interior.
It is impossible, of course, to even begin to impart any adequate conception of the qualities and great importance of the alloy steels for purposes of construction. As has been shown they are special steels for special purposes and their application is wide. Incorporation of the new element in the alloy imparts peculiar and valuable properties: for example, 12 per cent of manganese, great hardness and toughness; 23 per cent of nickel, non-corrosive properties and great strength; chromium, nickel with chromium or chromium with vanadium, strength and high elastic limit (resistance to distortion) as well as great hardening power when desired, this, of course, the usual hardening through quenching from a cherry-red heat.
Very often instead of the single defining element, a combination of two, three or even four of them is used. Such, of course, are rather complicated steels having combinations of properties as might naturally be expected, though very often these resulting properties are not those which are expected. In fact no one can tell in advance what properties any new combination of metals in an alloy will produce and often new proportions of the same constituent metals give entirely different and unique results.
The only certain method of ascertaining what characteristics and properties a new alloy will have is to develop it and in that way find out.
Description of a special and extremely important class of these alloys, the “high-speed steels,” will serve to show how laborious, slow and expensive a process development of new alloys may be and what unlooked-for results are sometimes obtained.