PHYSICAL PROPERTIES.
It has been shown in [Chap. V] that 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 boilers made of the 46,800-lb. steel of which the analysis is given in [Chap. V] would 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 the [same table] the “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.