EFFECTS OF MECHANICAL WORK.
When an ingot is heated and then hammered, rolled, or pressed hot, its density will be increased, as well as its strength when cold under all strains.
If it be hammered carefully, with heavy blows at first, and with lighter and quicker blows at the last, the grain will become very close and fine; it is called “hammer-refined.”
When down to the so-called cherry red, orange red, great care is needed, and when black begins to show through the red much caution must be used; any heavy blows will crush the grain and produce the dark or black color mentioned before.
Fine-tool makers attach great importance to this hammer-refining; some of the most expert will not have a rolled bar if a well-hammered one can be had. At first thought this would seem to be a mere notion, but the testimony in favor of hammering is so universal among those who know their business that it would seem as if it must be based upon some reason. If it have any scientific basis of fact, it is that the shocks or vibrations of the hammer keep the carbon in more intimate union with the iron, whether it be combination or solution, than either rolling or pressing will do. After considering the phenomena of hardening, tempering, annealing, etc., it may be concluded that there is something in this. It is easy to laugh at and to deride shop prejudices, and there are enough of them that deserve ridicule; again, there are some that will not down, and they compel the scientist to hunt for explanations. But after all, ridicule is dangerous; it is possible that a careful comparison of some of the laws laid down by the highest scientists would tend to excite the risibles. If the hand-worker sometimes flounders in the mud, the scientist is sometimes enveloped and groping in mist.
Hot-rolling produces results similar to those of hot-hammering; it makes the grain finer, increases density, and adds to the strength.
The same precautions are needed in rolling as in hammering. Heavy passes with rapid reduction may be used to advantage while the steel is hot and thoroughly plastic; as the heat falls the passes should be lighter to avoid crushing the grain.
Overrolling, like too much hammering, may be more injurious than too little work; a coarse, irregular structure due to too little work may be rectified and made fine and even by annealing, while if the grain be crushed by overwork the damage cannot be cured by annealing; the annealed grain may appear to be all right, but on testing, the strength will be found impaired.
By care and light passes steel may be rolled safely down to a black heat and be made elastic and springy. It is common to roll spring-steel in this way so that it may be formed into a spring and have all of the properties of a tempered spring without going through the operations of hardening and tempering. This is often desirable for spring-makers, as it saves them considerable expense; but it is hazardous work, because it is so difficult to heat every piece exactly to the same temperature, and secure every time the same number of passes and the same pressure in each. The best roller will get some pieces too hard and brittle, and some too soft and ductile. A careful steel-maker will shun such work.
Cold-hammering, cold-rolling, and cold-drawing reduce specific-gravity and increase tensile, transverse, compressive, and torsional strength. They increase hardness and brittleness, reducing ductility. The hardness due to cold-working is different from that due to hot-work or quenching; the latter operations produce great elasticity as well as hardness.
The hardness due to cold-working might be described as harshness; the steel is not truly springy; of course it will bend farther without permanent set than an annealed piece, but it never has the true spring elasticity. If it be worked far enough to be really springy, it will bear the same relation to a hot-worked spring that a piece of cross-grained, brashy oak bears to a piece of well-seasoned, straight-grained hickory.
The hammering of round sections between flat dies tends to burst the bars in the centre; great care must be used to avoid this, and the most skilful and careful hammermen will often turn out bursted bars. The bursts do not show on the surface; the bars are true to size, round, smooth, and sound on the outside. The safest plan is to hammer in a V-die, or in rounded swedges.
Radial rolling will produce the same results, and it is on this principle that the celebrated Mansmann tubes are made. The explanation seems to be simple, as the following exaggerated sketches will show:
No. 1 has been struck; it is then turned up to position No. 2 and knocked into shape No. 3. The rapid hammering of a bar, turning it a little at a time, must burst it if the blows are heavy enough to deform the whole section. Heavy radial rolling produces the same results.
The concluding pages of this chapter will be devoted to a few examples showing by tests the effects of heat and work upon specific-gravity, tensile strength, elasticity, and ductility; they are not to be taken as fixing exact limits in any case; they are given merely to illustrate the truth of the general properties stated, and to show the wide ranges of strength that are attainable by varying carbon and work.
TABLE I
| Character of Steel. | Ingot Numbers. | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | |
| Carbon | .302 | .490 | .529 | .649 | .801 | .841 | .867 | .871 | .955 | 1.005 | 1.058 | 1.079 |
| Silicon | .019 | .034 | .043 | .039 | .029 | .039 | .057 | .053 | .059 | .088 | .120 | .039 |
| Phosphorus | .047 | .005 | .047 | .030 | .035 | .024 | .014 | .024 | .070 | .034 | .064 | .044 |
| Sulphur | .018 | .016 | .018 | .012 | .016 | .010 | .018 | .012 | .016 | .012 | .006 | .004 |
| Sp. gr. ingots. | 7.855 | 7.836 | 7.841 | 7.829 | 7.838 | 7.824 | 7.819 | 7.818 | 7.813 | 7.807 | 7.808 | 7.805 |
| Sp. gr. bars, | ||||||||||||
| burned, 1 | 7.818 | 7.791 | 7.789 | 7.752 | 7.744 | 7.690 | ||||||
| 2 | 7.814 | 7.811 | 7.784 | 7.755 | 7.749 | 7.741 | ||||||
| 3 | 7.823 | 7.830 | 7.780 | 7.758 | 7.755 | 7.769 | ||||||
| 4 | 7.826 | 7.849 | 7.808 | 7.773 | 7.789 | 7.798 | ||||||
| 5 | 7.881 | 7.806 | 7.812 | 7.790 | 7.812 | 7.811 | ||||||
| cold, 6 | 7.844 | 7.824 | 7.829 | 7.825 | 7.826 | 7.825 | ||||||
| Diff. 6-1 | .025 | .034 | .040 | .073 | .082 | .135 | ||||||
| Mean diff. of carbon | .071 | |||||||||||
The twelve ingots treated here were first selected by ocular inspection for carbons; the carbons were then determined by combustion analyses.
It will be seen that the inspection was correct, and that the mean difference in carbon between consecutive numbers is .007. Between Nos. 7 and 8 there is a difference of only .004; when the analyst discovered this, he asked for a reinspection, not giving any reason for his request. The inspectors made new fractures, examined the ingots carefully in good light, and reported that they erred the first time, that both ingots belonged in the same temper number, but that if there were any difference No. 8 was the harder. It is not claimed that a difference of .004 is really observable.
The contents of silicon, phosphorus, and sulphur show clearly that the controlling element is carbon. This experiment has been repeated a number of times, and always with the same result, showing that there is no uncertainty in this method of separating tempers.
Parts of these ingots were reduced to ¾-inch round bars. The specific gravities of the ingots were taken, showing generally a reduction of sp. gr. for an increase of carbon. No. 3 and 5 are anomalous; an explanation of this could doubtless have been found if a careful investigation had been made, but there was no re-examination.
The sp. gr. No. 6 are of the ¾-inch bars as they came from the rolls; they are all heavier than the ingots except No. 4, and they are of nearly uniform sp. gr.; this is due doubtless to the fact that the higher carbon steels are so much harder than the low-carbon steels that it required much more work to reduce them to the bars, and as hot-working increases density, the densities of the higher carbons were increased more than those of the lower.
The bars were nicked six times at intervals of about ¾-inch and then heated so that the ends were scintillating, ready to pass into the granular condition, and the heat was so regulated as to have each piece less hot than the piece next nearer to the end, the last piece, No. 6, being black and as nearly cold as possible.
It is manifest that this operation is subject to the error of accidentally getting No. 2, for instance, hotter than No. 1, and so on, so that perfect regularity is not to be expected; to obtain a true rule of expansion it would be necessary to make hundreds of such experiments and use the mean of all.
It will be noticed that No. 4 is abnormal in the ingot series, and that the No. 6 piece of No. 4 is abnormal in being lighter than the ingot; probably this No. 6 of No. 4 was hot when it was intended to be cold. Also No. 2 of ingot No. 3 is lighter than its No. 1, showing another irregularity in heating.
Taking the whole list of No. 1 pieces, they are all lighter than their respective No. 6 pieces; the differences of sp. gr. 6-1 are progressive, being only .025 for the No. 3 ingot and .135 for the No. 12 ingot. This shows clearly that expansion due to a given difference in temperature is much greater in high steel than in low steel.
This clears away the mystery of the so-called treachery of high steel, its tendency to crack when hardened. There is no treachery about it; it is very sensitive to temperature, and it must be treated accordingly.
A few examples will now be given to show the changes of tensile strength, ductility, etc., that may be had by differences of carbon, and by differences of treatment, annealing, hardening, and tempering.
TABLE II
| Character of Steel. | O. H. | Crucible Sheet | O. H. | O. H. | O. H. | Crucible Eye-bar, 2″ × 1″. | Crucible Eye-bar, 2″ × 1″. | Crucible Eye-bar, 2″ × 1″. | Crucible ½-in. Drawn Wire. | |
|---|---|---|---|---|---|---|---|---|---|---|
| Carbon | .09 to .12 | .435 | .50 | .60 | .70 | .96 | 1.35 | 1.40 | 1.15 | |
| Silicon | .008 | .014 | .025 | .156 | < .02 | |||||
| Phosphorus | .007 | .050 | .016 | .008 | < .02 | |||||
| Sulphur | .026 | .028 | .028 | .015 | trace | |||||
| Manganese | .055 | .204 | .325 | .24 | < .30 | |||||
| Tensile strength, | ||||||||||
| lbs. per sq. in. | 46800 | 73142 | 84220 | 108800 | 117400 | 124800 | 100733 | 117710 | 141500 | |
| Elastic limit | 30900 | 63560 | 71500 | 69980 | 65000 | 85087 | 69850 | 92420 | ||
| Elongation |  | in 2 | in 1 | 25% | 14.5% | 11.5% | 4.75% | .5% | 7.28 at | 2% |
| in. 41% | in. 42% | 2.85 in 2½ | ||||||||
| Reduction of area | 75.85% | 62.3% | 29.91% | 13.55% | 8.59% | 13.03% | 2.42% | |||
| Fracture |  | |||||||||
| broke | ||||||||||
| silky | in neck | broke | broke | |||||||
| ½ | slight | in head | in | |||||||
| cup | flaw, | close | grip | |||||||
| fine | grain | |||||||||
| grain | ||||||||||
| O. H. is the abbreviation for open hearth. | ||||||||||
| Second column is mean of 24 analyses and 24 tests of boiler-sheets. | ||||||||||
TABLE III
| Cold-drawn Wire, ½-inch Diam. | Tensile Strength, lbs. per sq. in. | Elastic Limit, lbs. per sq. in. | Elongation. | Reduction of Area, per ct. | |
|---|---|---|---|---|---|
| In 3 in. | Per cent. | ||||
| Cold-drawn, broke in grip | 141,500 | 92,400 | .06 | 2.00 | 2.42 |
| Same bar drawn black | 138.400 | 114,700 | .18 | 6.00 | 12.45 |
| “ “ annealed | 98,410 | 68,110 | .30 | 10.00 | 11.69 |
| “ “ hardened and then drawn black | 248,700 | 152,800 | .25 | 8.33 | 19.7 |
Analysis of this bar is given in [Table II] in the last column.
A test of ½-inch wire to show effect of cold-drawing, tempering, annealing, and hardening and tempering. Four pieces were cut from the same bar. It is probable that the first piece would have given a little higher tensile if it had not broken in the grip; it was clamped too tight. The second piece was heated until it passed through all of the temper colors and turned black, technically called “drawing black,” or drawing out all of the temper. It is not quite annealing; the idea was to find the effect of temper-drawing upon a cold-hardened drawn wire.
The effect of this operation was to lower the ultimate and raise the elastic strength, increasing also the ductility.
The third piece was heated carefully to the recalescence-point, and cooled slowly, thus annealing it completely, and giving the normal strength of a bar of this composition.
The fourth piece was heated to recalescence and quenched, hardening and refining it thoroughly; it was then tempered through all of the colors until it turned black; the result shows the enormous potencies there are in the hardening and tempering operations.
The cases given in [Table II] were selected indiscriminately, so as to show better the effect of carbon, as we here have tests of ordinary test-bars, boiler-sheet, small eye-bars, and drawn wire.
The 96-carbon eye-bar and the 115-carbon ½-inch wire are the nearest to the 100-carbon saturation limit mentioned before, and they show the highest strength. The 96-carbon eye-bar had a slight flaw in the fracture, which doubtless caused it to break below its real strength.
The 135-carbon eye-bar broke in the head in a way to indicate that there was some local strain there, due to forging.
These examples are not given as establishing any general law; they are illustrations of what all experience shows to be the fact, that the strength of steel is affected profoundly by the quantity of carbon present, and also by heat and by mechanical work. From 46,800 lbs. to 248,700 lbs. tensile strength per square inch is an enormous range, and these figures probably represent pretty closely the ultimate limits at present attainable.
An inspection of the analyses makes it clear that the other elements present in addition to carbon were not there in sufficient quantity or variety to have had much effect upon the results.