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

Table B—Chemical Composition and Physical Properties
Per Cent Silicon Per Cent Graphitic Carbon Per Cent Per Cent Total Carbon Beginning of Freezing[[2]] Tensile Strength[[3]] Elongation[[4]] Welding Properties Principal Uses
1. Sand Cast Pig Iron 2.50 3.75 .25 4.00 2100 Further Refining.
2. Machine Cast Pig Iron 2.50 3.00 1.00 4.00 2100 Further Refining.
3. Open-Hearth Iron .03 .00 .02 .02 2740 50,000 2½″ Good Bars, pipe, sheet.
4. Wrought Iron Slag .00 .05 .05 2740 52,000 2¼″ Fine Bars, pipe, boiler tubes, stay bolts.
5. Mild Steel .05 .00 .10 .10 2740 55,000 2½″ Good Bars, pipe, wire, sheet, shafting, tubes.
6. Medium Steel .05 .00 .25 .25 2720 65,000 2¼″ Good Bars, plate, structural.
7. Rail Steel .05 .00 .60 .60 2680 90,000 1¾″ Fair Rails, gears.
8. Low Carbon Tool Steel .15 .00 .75 .75 2660 100,000 1¼″ Fair Hammers, cold chisels, saws, springs.
9. Medium Carbon Tool Steel .15 .00 1.00 1.00 2630 120,000 1″ Poor Lathe tools, chisels, drills, dies, springs.
10. High Carbon Tool Steel .15 .00 1.25 1.25 2600 135,000 1⅛″ Slight Lathe tools, chisels, files, saws.
11. Very High Carbon Tool Steel .15 .00 1.50 1.50 2560 124,000 ¼″ Slight Razors, lancets, graving tools, saws for steel.
12. High Steel .00 1.75 1.75 2530 0 None Dies for wire drawing.
13. White Iron .00 2.50 2.50 2460 0 None Dies for wire drawing.
14. White Cast Iron .70 .10 2.65 2.75 2400 41,000 0 None For malleableizing, carwheels.
15. Annealed Malleable Cast Iron .70 2.70 .05 2.75 40,000 ½″ None Railway and agricultural castings, fittings.
16. Cast Iron for Chilled Castings 1.00 1.00 2.00 3.00 2330 35,000 0 None Rolls, gears, brakeshoes.
17. Semi-Steel 1.75 2.80 .40 3.20 2300 35,000 0 None Gears, steam cylinders, valves.
18. Gray Cast Iron 2.00 3.10 .30 3.40 2200 25,000 0 None Machine parts, grates, radiators, valves, soil pipe.
19. Soft Gray Cast Iron 2.50 3.30 .15 3.45 2200 23,000 0 None Stoves, hollowware, pipe fittings, misc.

[2]. In degrees Fahrenheit. For our purposes, the same as the melting points of the alloys.

[3]. Pounds required to pull apart lengthwise a bar one inch square.

[4]. Number of inches that a bar eight inches long will stretch before it breaks.

Iron and Steel Test Pieces and Instruments Used in Measuring Their Size, Elastic Limit and Elongation

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.

Table C—Percentages of the so-called “Metalloids” by Weight and by Volume
Silicon Manganese Sulphur Phosphorus Total Carbon Iron[[5]] Total Metalloids
Wt. Vol. Wt. Vol. Wt. Vol. Wt. Vol. Wt. Vol. Wt. Wt. Vol.
Gray Cast Iron 2.50 10.0 .70 .7 .09 .36 .75 3.2 3.45 12.1 92.5 7.5 26.4
Semi-Steel 1.75 7.0 1.00 1.0 .10 .40 .35 1.5 3.20 11.2 93.6 6.4 21.1
Malleable Cast Iron .70 2.8 .50 .5 .15 .60 .17 .7 2.75 9.6 95.7 4.3 14.3
Cast Steel .30 1.2 .50 .5 .05 .20 .04 .17 .35 1.2 98.8 1.2 3.3
Mild Steel .05 .2 .40 .4 .04 .16 .03 .13 .10 .35 99.4 .6 1.2
Open-hearth Iron .03 .1 .02 .02 .01 .06 .01 .06 .02 .07 99.9 .1 .3
Wrought Iron 1.20 5.0 .08 .08 .01 .04 .15 .65 .05 .17 97.7 2.3 5.9

[5]. These alloys contain small amounts of other elements so these percentages of iron are a little high, though approximately correct. Note the purity (high percentage of iron) of the open-hearth iron.

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.

Exhibition Case Showing Four Well-Known Iron Alloys with Their Metalloids

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.