XIII.SPECIFICATIONS.
Specifications should cover three principal points:
Physical properties: Elastic limit; ultimate tensile strength; elongation; reduction of area.
Chemical constituents: Limiting silicon, phosphorus, sulphur, manganese, and copper; all other elements to be absent or mere traces in quantity, except carbon.
Finish and general condition: Fixing limit of variation in size from a given standard; conditions as to pipes, seams, laps, uniformity of grain, and other defects; no red-shortness.
It has been shown inChap. Vthat tensile strength may be had from 46,800 lbs. per square inch to 248,700 lbs. per square inch.
There are published in many transactions and technical periodicals thousands of tests giving elastic and ultimate strength, ductility, etc., so that every engineer can find easily what has been done to guide him as to what he can get.
In almost every case the engineer must be the judge as to the requirements in each; therefore it would be useless to attempt to lay down any fixed rules or limits.
Many engineers adhere to low tenacity and high ductility in the belief that they are securing that material which will be safest against sudden shocks and violent accidental strains.
Theoretically this appears to be correct, but if the statements made in the preceding chapters are credible it is plain that the limit to such safety can be passed, and that in insisting upon too low tenacity and high ductility the engineer may be getting simply a rotten, microscopically unsound material, through no fault of the manufacturer, who has been compelled to overmelt or overblow his steel to meet the requirements, and so reducing the quality of otherwise good material at no saving in cost to himself, and at a considerable cost in quality to the consumer.
Any manufacturer would rather check his melt between 10 and 15 carbon, or stop his blow so as to be sure not to overblow, if he were asked to do so, because it would save him time and expense, and it would yield sounder, better, and easier working steel.
It may not be wise yet for an engineer to fix limits as to blowing or melting, for the reason that neither he nor his assistants would know how to insure compliance, and in attempting to do it they might interfere too far with manufacturing operations and so involve themselves in responsibilities which they ought not to assume.
On the other hand, if they will let the carbon and tensile strength run up a little and reduce ductility slightly, it is safe to say that any manufacturer will be glad of the chance to help them to get the best results, which involve no extra cost.
Boiler-steel and rivet-steel usually suffer the most in this respect. A boiler should be tough, yet it is the belief of the author that boilersmade of the 46,800-lb. steel of which the analysis is given inChap. Vwould not last half as long as boilers made of 65,000-lb. to 70,000-lb. steel when the increased strength was gained by added carbon and no overmelting was allowed.
In thesame tablethe “Crucible-sheet” column gives a mean of 24 tests, and a mean analysis, of boiler-steel which has been in use in 12 boilers for nearly 16 years. The boilers are in perfectly good condition; they have been subjected to severe and very irregular usage, and they have been in every way satisfactory. Only one test-piece of the 24 was mild enough to stand the ordinary bending test after quenching.
That 46,800-lb. steel is remarkably pure chemically; it is unusually red-short. It would appear to some to be an ideal rivet-steel; it would stand a very high heat, it would head well and finish beautifully under a button-set. There is every probability that the majority of rivets driven of that steel would be cracked on the under side of the head, where the cracks would never be discovered until in service the heads flew off.
Rails are usually made of 40 to 45 carbon, tires from .65 up to 80 carbon, crank-pins as high as 70 carbon, with 85,000 lbs. to 95,000 lbs. tensile strength and 12% to 15% elongation.
It is difficult to see how a bridge or a boiler is to be subjected to any such violent usage as these receive daily; and while it is not advised that even 40 carbon should be used in boilers or bridges, although it would be perfectly safe, it does seem to be unreasonable to run to the other extreme to the injury of the material.
For steel for springs, and for all sorts of tools that are to be tempered, there is no need of a specification of physical properties as they are indicated by testing-machines.
The requirement that they shall harden safely and do good work afterwards involves necessarily, high steel of suitable quality.
No engineer should, unless he be an expert steel-maker, attempt to specify an exact chemical formula and a corresponding physical requirement; in doing so he would probably make two requirements which could not be obtained in one piece of steel, and so subject himself to a back down or to ridicule, or both.
On the other hand, he may properly, and he should fix, a limit beyond which the hurtful elements would not be tolerated. Notwithstanding satisfactory machine tests, successful shop-work, and a liberal margin of safety, no steel can be relied upon that is overloaded with phosphorus, sulphur, manganese, oxygen, antimony, arsenic, or nitrogen.
In regard to silicon, it is common to have as much as 20 to 25 points in tires, with 55 to 80 carbon; such tires are made by the best manufacturers, and they endure well. But it is certain that good, sound steel can be made for any purpose with silicon not exceeding 10.
Structural steel can be made cheaply within the following limits:
Steel made within these limits and not overblown or overmelted must be better in every way than steel of
A steel of the latter composition, or with no fixed limits, may be made cheaper than the first by a dollar or two a ton; but for any large lot it is believed that the first specification would be bid to at as low a price as if there were no specification; competition among manufacturers would fix that. At any rate there is no reason why an engineer should refuse to demand fairly pure material when he can do so at little or no extra cost.
Arsenic, antimony, or any other elements should be absent, or < .005.
As there can be no such thing as exact work done, there must be some tolerance as to variation in size. In standard sections, sheets, and plates this is usually covered by a percentage of weight; in forgings or any pieces that are to be machined the consumer should allow enough to insure a clean, sound surface. But it would be unwise to lay down any rule here, because conditions vary; a rolled round bar may finish nicely by a cut of from ¹/₃₂ to ¹/₁₆ of an inch, and so also a neatly dropped forging; an ordinary hammered forging might require a cut of ¼ or ⅜ of an inch; such a forging might be made closer to size at a cost for extra time at the hammer far exceeding the saving of cost in thelathe. These are cases where common-sense and good judgment must govern.
Pipes should not be tolerated if they can be discovered; because a pipe appears small in the end of a bar it is no evidence that it is not larger farther in.
Seams should not be allowed in any steel that is to be hardened; they should be a minimum in any steel, as they are of no possible use; small seams when not too numerous may do no harm in structural or machinery steel, and consumers should be reasonable in regard to them, or else they may have too high prices put upon their work, or too high heat used in efforts to close the last few harmless seams.
Burns, rough, ragged holes in the faces or on the corners, are inexcusable and should be rejected; the steel has been abused, or it is red-short; in either case the ragged breaks are good starting-points for final rupture.
Laps should not be permitted; they are evidences of carelessness; there can be no excuse for them.
Fins are sometimes unavoidable in a difficult shape; for instance, if a trapezoid is wanted, it may be rolled in this form:
or in this:
The consumer must decide which; if he wants sharp angles he must accept the fin and cut it off, or have it cut off by the manufacturer.
Rivet-steel should be tested rigidly for red-shortness, because red-short steel may crack under the head as the steel cools.
Emphasis is laid upon this because engineers will insist upon excessive ductility in rivet-steel, not realizing that they may be requiring the manufacturer to overdose his steel with oxygen to its serious injury.
No sharp re-entrant angles should be allowed under any circumstances where there is a possibility of vibrations running through the mass. All re-entrant angles should be filleted neatly.
No deep tool-marks should be allowed; a fine line scored around a piece by a lathe-tool, or a sharp line cut in a surface by a planing-tool will fix a line of fracture as neatly as a diamond-scratch will do it on a piece of glass.
Indentations by hammers or sledges should be avoided; they may not be as dangerous as lathe-cuts, but they can do no good, and therefore they are of no use.