In the early days practically the only steels recognized—certainly the only ones desired—were of the high carbon or hardening variety. These were required for the manufacture of swords and other implements of war, for tools, etc., most of which had to have hard and sharp cutting edges.

When softer and less brittle metal was desired, wrought iron was available, but in all probability high carbon steel was the material most largely used.

Having but the two iron alloys and these of very different properties, it was not difficult to distinguish between them. A piece of metal could be heated to redness and plunged into cold water. If it became glass hard when cooled in this way it was thereby proved to be steel; if still soft, it was iron.

But the problem is not so simple to-day. Medium, mild and yet softer steels, and other alloys which have steel characteristics have appeared and are used in immense quantities. Their advent introduced considerable complication.

It will be well, therefore, before taking up our subject, “Cementation and Crucible Steel,” and the several steels which are to follow, to make sure that we all understand, as well as we may, what is “steel” as defined to-day, what are the best known varieties, and what are their characteristics?

For a rough classification it is safe for us to divide the steel world into four general divisions as follows:

1. The harder, high carbon steels used for tools, dies, etc.

2. The mild and medium steels of which wire, rod, bar, plate, pipe and structural shapes for bridges, ships and “sky scrapers” are made.

3. Alloy steels, to which some metal such as nickel, manganese or chromium gives definite properties and the name.

4. Those other modern materials which are known as “self-hardening” and “high-speed steels.”

The two classes last named are not simple iron-carbon alloys and their properties are less directly derived from and do not so plainly depend upon carbon. Metallurgically, then, they are not steels in the exact former sense of the word; but as they do require carbon—though perhaps in lesser amount, are made by regular steel processes, have most of the characteristics of steel and are used for the same general purposes, they are undoubtedly entitled to the appellation “steel.” However, to distinguish, they are usually termed “alloy steels.”

We are just now concerned only with the steels of classes one and two—the carbon steels. As explained in a previous chapter, these are alloys of iron with not more than 2 per cent of carbon.

Carbon is the element the presence of which confers upon iron the ability to harden when cooled suddenly from a cherry-red heat, as by quenching in water or oil. If the steel contains less than four-tenths of one per cent of carbon it has little or no hardening power under this treatment; but steel with six-tenths of one per cent or more of the element, has the wonderful property of being slightly malleable when in the annealed state, but extremely hard and brittle after this sudden cooling—leads a dual life, so to speak.

At any time, hardened steel may be returned to its former condition of softness by the well known process of annealing, wherein it is reheated to the same cherry-red heat and slowly cooled.

At the will of the blacksmith or metal worker alternate hardening and softening may be repeated a great many times without apparent deterioration.

Various degrees of hardness also, may be obtained according to (1), the percentage of carbon in the steel, and (2), the completeness and suddenness of the cooling.

As considerable brittleness and internal strain in the metal necessarily follow hardening, the hardness is usually “tempered” or “let down” by a careful reheating to a much lower temperature, usually 425 to 550 degrees Fahrenheit. From this temperature a second quenching “fastens” the temper at whatever of the original hardness the steel retains at the temperature chosen by the smith for the second quenching. Much of the brittleness is in this way relieved. The smith calls it “toughening” the steel. Tools so treated are much less liable to break.

The steels that will harden (we will call them “carbon tool steels”), range ordinarily from the .60 per cent carbon variety, used for hammers, cold chisels, etc., to those containing 1.50 per cent of carbon which are selected for razors, scalpels, and other tools requiring high temper. Each one of these many grades is susceptible of a wide variety of temper in the hands of a capable man, who must select his steel and give to it the most desirable temper for the work for which the tool is designed.

Blacksmiths and other tool makers become extremely proficient in judging steels and the proper temperature at which each should be hardened and “drawn” (tempered). They judge temperatures solely by the color of the steel when heated. Every five or ten degree change imparts a slightly different shade as the steel grows hotter in the forge fire or cooler when about to quench.

Observation of a good blacksmith at work and a few minutes’ conversation with him about his “art” will give one greater knowledge and appreciation of the carbon tool steels than volumes of writings concerning them. Along with it will come more respect for the skill of these clever men whose handiwork is never exhibited in salons and about whom the world hears little, though indebted to them for a great measure of its civilization and prosperity.

What and how much would be possible without machines and proper tools?

About sixty years ago steels of much lower carbon content appeared. They have been made softer and softer until we have what we now know as the “mild” steels and even the almost or practically carbonless material which we called “open-hearth iron” or “ingot iron” in a former chapter. These have not the hardening property but they possess softness, ductility and freedom from brittleness which the higher carbon steels always lack. For such real evidences of our Twentieth Century civilization as the great bridges, ships, buildings, etc., they are indispensable, for they are easily cut, bent and otherwise worked into shape, and they combine pliability with sufficient strength for the service intended. Such steels are desirable, for when overloaded they bend before they break, thus giving warning of the danger.

These mild and medium steels are of immense importance industrially. Of the 31,000,000 tons of steel made in the United States during 1912 probably 99 per cent was of the soft and medium varieties.

It has been said that “the exception proves the rule.” Cementation steel is the exception to the rule which we gave in Chapter VI that steel is always melted during its manufacture.

If a thin piece of bar iron be packed in powdered charcoal and heated at low red heat for some time, the metal, after cooling, will be found to have acquired the hardening property. In other words by absorption of carbon it will have become steel with all of the characteristics of that material. Neither the iron nor the carbon by which it was surrounded have melted, yet in some way carbon has penetrated into the iron and if the heating has been sufficiently long, carbon will be found at the center of the bar. But always there will be more carbon in the outer layers of the bar than in those farther inside, i. e., it will be found in diminishing amounts as we approach the center.

Just how and when the cementation process for making steel, to be now described, was discovered is not known. It may have been the result of the non-uniform working of the larger blast furnaces which were developing in Continental Europe during the Thirteenth century. From the German “natural steel” which was probably the steely product too rich in carbon for the wrought iron which they intended to make and much too poor in carbon to be the fluid cast iron which with the growing height and heat of the blast furnace they later did make, may have come the idea. More likely, a piece of thin wrought iron was accidentally left imbedded in glowing charcoal until it had absorbed some carbon.

The first mention of cementation steel appears to have been by an Italian metallurgist, Vannuccio Biringuccio, who, in 1540, described the making of steel by heating billets of soft iron for a long time in molten cast iron. The modern method, the heating of wrought iron in powdered charcoal, was certainly known in the sixteenth century and this method of cementation has been practiced in France, England, Belgium and Germany since the seventeenth century.

Reaumur, the Frenchman, whose process of making cast iron soft by annealing bears his name and is still used in Europe, was the first to study and understand to any extent the cementation process. Publication, about 1722, of his complete directions for cementing iron gave great impetus to the manufacture of steel by this process. Fate, however, was unkind and his own nation, France, by reason of her small production of suitable iron for the work, was unable to profit greatly through his discoveries. Sweden, England and Germany were benefited to a much greater extent.

During the early years many were the secret and wonderful mixtures and compounds offered for this work, but of them all carbon in some form was the only necessary element.

Finely divided or powdered charcoal or bone dust has been mostly used.

Sheffield, England, steel makers, have been very successful in the manufacture of cementation steel. Their usual method is to pack flat strips of best Swedish Walloon iron in charcoal in rectangular stone boxes about four feet wide, three feet high and fourteen feet long. Alternate layers of small-sized charcoal and thin iron bars are piled in these boxes until they are filled, the bars not being allowed to touch one another. When full, top slabs are luted on to the boxes to make them airtight.

Fire is kindled in the firebox below and the heat gradually raised until furnace and boxes are cherry-red in color. This heat is maintained for seven to eleven or more days, depending upon the hardness desired, i.e., the amount of carbon they desire absorbed. The furnace is closed and allowed to cool slowly, which requires another seven or more days.

Upon unpacking the furnace the bars are found to be brittle and of a steely fracture instead of the soft malleable material which was put in. They have become high carbon steel.

Expert workmen are able to judge very closely the hardness of the steel by looking at the fracture and they sort the bars in this way, piling bars of similar hardness together.

Bars thus made show many blisters on the surface and the steel became known as “blister steel” on this account. The reason for these blisters was not discovered until along about 1864, when the well-known English metallurgist, Percy, proved that the blisters were caused by the chemical action of carbon on the slag contained in the wrought iron. The gases formed produced the blistering of the bar. That this is the explanation is proved by the fact that bars of mild steel or iron without slag do not blister.

Blister bars heated to a forging heat and drawn out under the hammer or rolled into bar steel are known as “spring steel”, or “plated bars.”

As in wrought iron manufacture, a cutting to length, repiling, heating, welding and again drawing down by hammering or rolling produces much more homogeneous and reliable steel. Piled and reworked steel of this sort became known as “shear” steel because blades of shears for cropping woolen cloth were always made in this way.

Many of us will recognize in the cementation process an extended “case hardening.” Case hardening is very largely resorted to by iron and steel workers, who in a few hours can give a hardened and long-wearing thin outer layer of steel to a piece of iron or soft steel after it has been forged or machined into the desired shape.

This shear steel was largely made and was quite satisfactory, until, as described before, Huntsman, a Sheffield clock maker, conceived the idea of melting together in a pot or crucible blister bars or bars of shear steel. This he did to equalize the carbon content and give uniformity of product which had never been attainable through the cementation process alone.

From that date (1740) to this the crucible process has undergone only minor alterations and to-day it produces the highest grades of steel which we have. Practically all of the high grade tool steels are produced by this process.

Nor has Huntsman’s form of furnace been greatly changed, as the illustrations prove. Though gas and oil as well as coal are, in many cases, used as the fuel, the general design of the furnace has remained the same.

For a century crucibles were made from clay molded to form, slowly dried and very carefully burned. Usually each steel maker made his own crucibles. They could be used but three times, becoming so thin and tender after use for three batches of steel that they were not safe for a fourth. Graphite crucibles are now very largely used. They withstand the severe heat much better and can be used five or six times. The expense item for either clay or graphite crucibles is a large one.

After filling with small pieces of blister or shear steel the crucibles are entirely surrounded by coal or coke in the furnace pit. The fire is so regulated that the steel is not too quickly melted. Fresh coal or coke must be put in around the crucibles two or even three times.

When he thinks the steel should be molten, the expert attendant known as the “melter” quickly removes the tight fitting cover of the crucible and with an iron rod determines whether any unmelted pieces remain.

After complete melting the steel must be “killed,” else it will boil up in the mold upon pouring and leave a spongy or insufficiently solid “ingot” or block of steel. This “killing” of steel is a rather peculiar phenomenon. It is accomplished by allowing the steel to remain quiet in the furnace for another half hour or so. Undoubtedly the quieting is the result of the escape of the gases or impurities which are contained in the charge, and absorption of the chemical element, silicon, from the walls of the crucible.

We have met this element, silicon, before in our metallurgical journey and we will likely meet it several times again. To the metallurgist it is secondary in importance only to carbon.

When the steel has been properly melted and killed it is ready to pour. An assistant lifts the cover from the melting hole, the “puller-out” seizes the crucible just below the bulge with circular tongs and pulls it from the coke which surrounds it. The slag is skimmed off the top and the steel poured into iron molds forming small “ingots,” usually from 2 to 4 inches square and two feet or more long.

Every part of the process, even the pouring, must be done with extreme skill and care or the product suffers.

After liberation from their molds, the ingots are heated and either rolled or hammered down to the sizes desired for tools, etc.

As stated before, crucible steel necessarily is an expensive material both on account of high labor and crucible costs. For this reason, many have resorted to the process used in the very small way mentioned for the manufacture of Wootz steel—the melting of wrought iron bar or soft steel in a crucible with carbon.

In the Wootz process chopped wood and green leaves were used. Nowadays charcoal is substituted or there is added the proper amount of cast iron to give the desired amount of carbon to the wrought iron or soft steel charged. During the melting the iron takes up the charcoal and alloys with it.

Proper amounts of silicon, manganese, and other beneficial materials are also charged, which become either part of the alloy itself or have a cleaning or fluxing action upon it.

Steels made in this way are practically, though perhaps not quite, as good as steels made by melting together the properly selected cementation bars. The method has come to be very generally used on account of its directness and because it eliminates the long and expensive preliminary cementation process.

When Bessemer and open-hearth steels made their appearance in the market an attempt was made to use them instead of wrought iron as the base for high grade crucible steels. Though seemingly pure enough, apparently purer even than wrought iron, these metals were not able to compete with wrought iron for this purpose. For some reason, not yet satisfactorily explained, these new materials which are made in 15, 35 and 50–ton batches, when used as a base, do not give as high quality tool steel as puddled wrought iron, which is slowly and laboriously made in 500–pound lots. Considerable of these materials are utilized but it is for a somewhat lower grade of crucible steel.

For many years mild steels for castings have been quite largely made by the crucible process. They are among the best but the crucible and labor costs are usually too great to allow crucible steel castings to compete in present markets.