The Metallographic Method of Classification
By aid of the microscope it is possible actually to look into the structure of these materials.
TABLE A
IRON AND STEEL PRODUCTION OF THE UNITED STATES FOR 1912
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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.
Machine for Determining Strength of Iron and Steel Bars
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.
No. 198. Photomicrograph of Sand Cast Pig Iron. The Thick Black Lines Are Graphite Flakes
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.
No. 99b. Open-Hearth Iron. Probably the Purest Commercial Iron Product
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.
No. 1d. Section of Wrought Iron Cut Lengthwise of the Bar. Black Patches and Filaments Are “Slag” or “Cinder”
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.
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This should not worry us, however, for we shall not attempt to follow them into their complications.
No. 3b. Steel Containing .10 Per Cent of Carbon. The Carbon is in the Black Patches
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.
No. 22c. Steel with .50 Per Cent of Carbon
No. 36a. Steel with 1.98 Per Cent of Carbon
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.
No. 74. Gray Cast Iron. The Crooked Black Lines Are Graphite Flakes
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.
No. 92d. Semi-Steel, a Stronger Gray Cast Iron
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.
No. 132. Gray Cast Iron 200 Years Old
No. 109. Malleable Cast Iron Before Annealing
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.
No. 35. Malleable Cast Iron Annealed
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.
No. 8. Bar Rolled from Scrap; Contains Both Wrought Iron and Steel
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.