Non-technical chats on iron and steel, and their application to modern industry

We have arrived at the parting of the ways. From the vast beds of iron ore, the coal fields and coke ovens, and from the quarries of limestone, all roads have led to the blast furnace. This we have visited and we now know how pig iron is made.

From this point the several paths diverge. One by one we are to follow them to get acquainted with the interesting country which they traverse and the regions to which they lead. However, before choosing any one of these paths for our first trip, it will be to our advantage to pause, to study for a moment our position and get a general view of the country ahead of us. We should know the relative locations and importance of the places we are going to visit, for only by getting a comprehensive idea of the general plan of this ferrous (meaning iron) world, can we understand to the best advantage the position of each of the main products, wrought iron, steel, cast iron, malleable iron, etc., and acquire a satisfactory knowledge of them. In the last chapter pig iron was called the intermediate stage between the ore and the finished iron product, and such the sketch given shows it to be. It is only as an intermediate product that pig iron has value, for nowhere in the commercial world has it a purpose except as a material to be chemically and structurally transformed into other materials which may themselves be used without further transformation. It is desirable that we fully realize the position of this very important semi-raw material, pig iron, before passing on to the study, one at a time, of the refined ferrous products which are susceptible of direct use in our commercial life.

From Table A (page 71) it will be seen that from blast furnace metal several well-known products are formed by the various processes of refinement. Each of these methods of purification may be said to result in a certain general composition and structure which give to the material formed its character and properties.

The Metallographic Method of Classification

By aid of the microscope it is possible actually to look into the structure of these materials.

By cutting in any direction through a piece of metal or alloy, polishing the surface exposed very clean and smooth and then etching (corroding) slightly with acid, the exposed grains of the metal may be seen when sufficiently magnified. Not only may the grains of metal be seen, but certain of the other constituents which are present are visible. Photographs, also, can be taken by attaching a camera to the microscope. This method of analysis, which is known as “metallography,” has proved as valuable an aid to the metallurgist as it proved to the geologist when applied to the study of rocks and geological specimens.

The metallography of wrought iron, cast iron, malleable iron and steel differentiates them to us with considerable accuracy, as is shown by a glance at the accompanying photomicrographs, as photographs taken through the microscope are called.

For our immediate purpose of gaining a general knowledge of the relative positions of these products, this method of analysis probably cannot be excelled.

After the processes of manufacture of these materials have been taken up one by one in later chapters, the photomicrographs will be even better understood than now, as will the differences of chemical composition and physical properties of the alloys, such as strength, hardness, brittleness, forging quality, etc. The photomicrographs are given at this time to show that the materials are structurally very different and to aid in the general classification.

To make them comparative, all have been taken at the same magnification of seventy diameters; i.e., the microscope has made everything shown just seventy times as large as it actually was in the alloy.

As stated before, the alloy pig iron normally contains from 3 to 5½ per cent of carbon. This was absorbed during the journey through the blast furnace. As long as the iron was molten all of this carbon was in the “combined” form; i.e., in chemical combination with the iron itself. Cold iron, however, cannot retain in the chemically combined form as much carbon as does molten iron, so, during the solidification and cooling of the alloy, more or less of its carbon was precipitated, i.e., thrown out of solution and from chemical combination with the iron, the amount depending mainly upon the speed of the cooling. It appeared then as the “free” carbon (crystalline graphite) which remained distributed throughout the alloy and may be seen as the jet black flakes in photomicrograph No. 198.

Every pure metal is supposed to be composed of crystals or grains which would have been of the true cubic form if the severe internal pressure during solidification and cooling had not distorted them.

Photomicrograph No. 99b represents quite well a pure metal. It is that of an extremely mild steel made by special methods in the open-hearth steel furnace. It is so highly refined that it can hardly be called steel at all but is often called “open-hearth iron” or “ingot iron.” It is probably the purest iron on the market in commercial quantities to-day. While in the chemical laboratory iron of considerably greater purity can be made, for a commercial product this is remarkably pure, seldom containing more than ¼ per cent of elements other than the metal, iron.

The boundary lines of the crystals or grains may be plainly seen. Each grain should show practically white. The dark parallel lines, the dots, and the grayish portions result from inequalities in the polishing and etching.

After noting the appearance of photomicrograph No. 99b, which is of a nearly pure iron, one need have no difficulty in realizing that pig iron and the steels are alloys and not simple metals. The truth is that of all alloys some of the well-known iron products which we are studying are by far the most complicated, much more so than are the nonferrous alloys, which include the brasses, bronzes, babbitts, German silver, etc.

This should not worry us, however, for we shall not attempt to follow them into their complications.

The important point just now is to observe the crooked black flakes of crystalline graphite in this photomicrograph No. 198. It is largely because of these flakes of brittle, soft graphite that pig iron and the cast irons are so fragile. One has no difficulty in realizing that these flakes, which cut in every direction through the metal, make it structurally weak, especially toward a sudden blow.

No. 1d shows a typical section of wrought iron cut lengthwise of the rolled bar. It will be noted that, as in photomicrograph No. 99b, there are no graphite flakes.

In the process of wrought iron manufacture practically all of the carbon of the original pig iron is burned out, leaving little besides the iron itself and some viscous cinder or slag. In the rolling or hammering out of the resulting white-hot “bloom,” the slag enclosures which remain after the squeezing process are extended lengthwise through the bar. Upon observing with the microscope any prepared section of wrought iron which has been cut lengthwise of the bar the filaments of slag may be plainly seen, all parallel or practically so. When such filaments of slag can be discerned in a longitudinal section it is practically an absolute indication that the material in question is wrought iron.

Photomicrograph No. 3b is that of mild steel containing .10 per cent (⅒ per cent) of carbon. Here we have neither the graphite flakes of No. 198 nor the slag filaments of No. 1d. We can plainly see the boundary lines of the grains. The irregular dark patches which are evenly distributed throughout are the defining features of steel. In what might be called a chemical-mechanical combination, these dark patches contain all of the small percentage of carbon which gives to carbon steel its definite properties.

During the manufacture of this alloy all but a small amount of carbon is eliminated by burning it out, as happens with wrought iron. But the steel is molten or fluid when finished and the slag which has been formed floats on top and is also eliminated, which does not occur with wrought iron, which is thick and pasty at its finishing temperature.

Therefore, steel contains no graphite and no slag but has only the small percentage of carbon which was purposely put back to give to it its valuable properties.

Photomicrograph No. 22c is typical for steel which contains .50 per cent (½ per cent) of carbon. The irregular patches containing the carbon are much larger and more frequent. It will be seen, therefore, that through metallography the various iron alloys, to a considerable extent, may be analyzed as well as classified.

Photomicrographs Nos. 74 and 92d represent soft and stronger grades of ordinary gray cast iron respectively. It will be noted at once that both much resemble pig iron in structure, as of course they should, for simple remelting in the cupola does not effect much modification in composition or structure.

Occasionally castings are made from molten iron direct from the blast furnace, but such practice is not very satisfactory and is little done. It forms probably the only exception to the statement made above that pig iron has no useful purpose in the commercial world except as something to be transformed by some refining process into another material.

The remelting for the well-known cast iron is usually of such selected brands of pig iron and cast scrap as will produce cast iron best adapted to the purposes intended. The resulting alloy still contains 3 per cent or 3½ per cent of carbon.

No. 132 is interesting. It is a photomicrograph of a piece of cast iron which is approximately 200 years old. From the standpoint of the metallurgist it is the same as other photomicrographs of cast iron, the difference in appearance resulting probably from casting conditions, etc.

Malleable cast iron is made much as is gray cast iron except that it is brought to such a composition that sections of castings made from the melt show a white fracture when broken. Castings of gray iron of course show a gray fracture. The former are extremely hard and are as brittle as glass until they have gone through a careful long anneal or heat softening process. By maintaining them at cherry-red heat away from air for sixty hours or more and cooling slowly, they become “malleable”; i.e., not brittle at all but capable of considerable distortion without cracking.

From the viewpoint of malleability, malleable iron may be considered to occupy a position between gray cast iron and steel.

Photomicrographs Nos. 109 and 35 are those of malleable iron before and after the annealing treatment. The former shows the typical structure of white cast iron, while the latter plainly shows the minute lumps of pure carbon and the surrounding grains of pure iron metal, the two having become divorced through the annealing process. Note that the large amount of carbon in this, the “temper carbon” form, does not make the alloy brittle through cutting of the grains as does the crystalline graphite form of carbon.

The above gives very briefly the most essential points in the classification of the irons and steels from the structural standpoint. True it has not entered into the vast complications of the higher carbon or tool steels which are those which will take a “temper”; i.e., the tool steel’s quality of hardening by quenching in water from a cherry red heat. The simpler points of these will be taken up later. But enough has been given that we now understand something of the relative positions of the great main products, some reasons therefor, and their general structures.

It has been seen that their micrographs quite definitely differentiate them and probably no one will have difficulty in recognizing and naming such specimens. As testing this it may be interesting to note photomicrograph No. 8. Two different iron alloys made up the material from which this photomicrograph was taken. Apparently the bar was one made by heating and rolling together scrap metals. Such material is on the market. The photomicrograph shows that some of the scrap used was wrought iron and some of it mild steel of about .08% carbon.

Classification by Chemical Analysis and by Physical Tests

So far not much has been said about composition except that upon it to a great extent depends the structure and physical properties of the alloy. Composition and physical characteristics as well as structures are necessary to a fair

understanding of the subject. There is given therefore, a table showing approximate comparative values of chemical compositions and physical properties of the alloys under discussion.

It should be distinctly understood that the figures given in Table B are approximate only and are intended to be average, or rather, perhaps, typical.

There are all sorts of conditions which in practice modify the figures given in the table, and criticism may be maintained justly against some of the too specific statements which here it was necessary to make. The classification is given with considerable hesitation and only because, arranged in this way, it brings out existing relationships which otherwise would escape notice and which we cannot afford to overlook.

But please do not gain the impression that these alloys are divided into distinct classes. There are no dividing lines at all. One group merges into the next so gradually that it is impossible to tell where the one ends and the other begins.

It is to be hoped that no one will make himself miserable by trying to digest these rather formidable figures of Table B all at one sitting. They are given mainly for comparison and for reference. It is suggested that after noting carefully the similarities and differences to which attention is called, they be reserved until the processes of manufacture of the various alloys are taken up one by one. Reference to these figures on those occasions should be profitable.

The main points to be noted at this time are:

1. Open-hearth iron is practically pure iron, having no constituents or slag inclusions which materially affect its properties.

2. Wrought iron, for all practical purposes, is pure iron except for its content of slag. It is the only one of the iron family which does normally contain slag.

3. Neither open-hearth iron nor wrought iron contains carbon in appreciable quantities.

4. The distinguishing and active element of the steel family is carbon. With increase of carbon the hardness of the alloy increases as does its tensile strength, but the ductility (elongation or stretch) decreases.

5. Other conditions being equal, the more carbon the alloy contains the more easily it melts; i.e., at lower temperature. So the purer irons such as open-hearth iron, wrought iron, and mild steel (i.e., steel with low carbon, usually under 0.15 per cent) have relatively high and the cast irons lower melting points.

6. In the cast irons, the carbon occurs not in one only but in two different forms; i.e., as graphitic carbon, commonly called graphite (Gr. C.) and the combined form (C. C.). The sum of these is usually between 3.00 per cent and 3.50 per cent. It is not so much the total amount of the carbon that causes the differences in structure and physical properties which have been noted in 1 and 2 and in 16 and 17 above, as it is the relative proportions in which these two varieties occur.

7. No one knows just when, with increase of carbon, steel ceases to be steel and becomes white cast iron. There is no definite dividing line either in chemical or physical properties. The changes are extremely gradual throughout the scale. Aided by the microscope, modern physical chemistry has disclosed the fact that alloys of iron with carbon “freeze” from molten to solid condition according to two different laws. The change from one to the other occurs somewhere between 1.7 per cent and 2.2 per cent of carbon as is described in Chapter XXII. This is our only basis for calling alloys with less than 2 per cent of carbon, steels, and those with greater amounts, cast irons.

8. For our immediate purposes the other “metalloids” or constituents are of secondary importance and will not be taken up now. From this it must not be understood that they can be slighted by the metallurgist and furnace man in his work. They cannot. Every one of them is of importance and must be accounted for in the final product or trouble results.

Volumetric Analysis of the Iron Alloys

There is a way in which we may visually get a very intimate idea of the relative composition of these alloys.

The cabinet of which a photograph is given is partitioned into four sections. Each one of these contains a bar and six specimen jars. As you may or may not be able to read from the labels, the bars, all of exactly the same size, are soft cast iron, semi-steel (a stronger cast iron), annealed malleable iron and cast steel. The six jars above each bar contain the exact amounts of the various constituents other than iron which are in the bar beneath.

As none of the constituents except manganese are as heavy as iron, their volumes per unit of weight are correspondingly greater. Putting it into approximate figures we have the percentages by weight and by volume shown in Table C.

This means, of course, that of the cast iron plates of your cook stove or steam or water radiators fully one-quarter (26 per cent by volume) is not iron at all but brittle substances of little or no strength. These elements, silicon, sulphur, phosphorus, and carbon, are commonly called “metalloids.” While the first three named are not in “free” form in the alloy and therefore allow of some doubt as to just the space they require, we have good reason to suppose that the figures given are not far from correct.

With such a volume of weakening constituents and particularly with the graphite flakes cutting through and separating the iron grains as the photomicrographs show, can one wonder that cast iron is fragile—more so than steel or wrought iron?

To sum up, naming only the most familiar alloys and the two or three qualifying features of each which stand forth with particular boldness, we have:

Pig Iron—Very High Carbon. Brittle.

Gray Cast Iron—High Carbon. Brittle.

Malleable Cast Iron—High Carbon. Made Malleable by Annealing.

Wrought Iron—Slag. Little or no Carbon. Very Malleable without Annealing.

Mild Steel—Very Low Carbon. No Slag. Very Malleable without Annealing.

Carbon Tool Steel—Medium Carbon. No Slag. Of Medium Malleability.

So steel which is not called “iron” at all is a very pure metal in comparison with gray and malleable cast iron and usually has a larger percentage of the chemical element, iron from which all are derived, than has the well-known wrought iron itself.