The following remarks on the subject of hardening steel, are offered to the Institute as the result of much experience in the regular course of my business, and of essays suggested by some peculiarity, accidentally noticed, and made for my own satisfaction. It is, perhaps, to be regretted, that I have not had leisure to repeat them with a view to greater accuracy of de tail; by some, however, this may be deemed a favour able circumstance, as they are not fortified by any array of numbers, or formulae, and may, therefore, be the more readily discussed, corrected, and amended, for which I am fully aware my best endeavours leave ample room.
The peculiar kind of hardening of which steel is susceptible, depends upon two conditions: first, a sufficient degree of heat (somewhat above the lowest red), which may be termed the hardening heat; and second, sudden cooling. A deficiency of only a few degrees of heat, or an excess of two or three seconds of time, beyond certain limits, will entirely defeat the operation.
The usual method of hardening steel for common purposes is to heat it to the proper degree (the lower the better, provided it be not so low as entirely to fail to harden), and then to plunge it suddenly into cold water. When it is requisite to protect the sur face,from the corroding effects of the atmospheric air, as in engravings, dies of delicate workmanship, &c. it should be imbedded in fine charcoal powder, previously heated to redness, in an iron box, to drive off the evaporable matter, and when sufficiently heat ed, the piece must be removed to the cooling liquid with as little exposure to the air as possible. IF the contents of the box be thrown, with the steel, into oil, so as completely to exclude the air, it will preserve its polish and brightness unchanged.
All articles of steel are more or less liable to be come warped, by rapid cooling, from the unequal contraction of the parts, and many, from the same cause, require the greatest dexterity and skill to pre vent them from breaking in pieces during the opera tion.
Whenever, therefore, the nature of the case admits the use of oil, as a cooling medium, it is safer than water, being much less rapid in its operation. It is obvious, however, that as large masses of steel can with difficulty be cooled, even in water, within the hardening limit of time, only small articles, such as springs, thin blades, &c. can be hardened at all in oil. It is sometimes pretended that oil imparts a degree of toughness to steel hardened in it, just as it would to a bit of horn, or leather, by penetrating its pores; and I believe the patent obtained for the use of it, in hardening a celebrated patent truss, was grounded upon such a supposition.
The danger of breaking increases with the thick ness of the piece, whatever may be its form; and that form is least liable to break, in which there is the greatest freedom of motion, or in which a simulta neous contraction can be effected in all the parts.
In hardening a roller, say two or three inches in diameter, and about the same in length, the first ten dency of the contraction of the surface is to separate it. But this strain being equally divided around the circumference, and the metal being in a yielding state, the only effect, in general, is to enlarge the sur face beyond its original dimensions. The surface thus enlarged immediately becomes hard and fixed; so that the subsequent cooling of the centre reverses the strain upon the surface, tending to compress or shorten it, and that to such a degree, that a segment is often thrown off with great violence, or, when the outer portion has sufficient strength to resist the con tracting force of the centre, that portion in its turn tends to separate, being prevented by the outer part (to which it adheres), from returning to its original dimensions. In this case, a separation at the centre is inevitable, unless a part of the heat be allowed to remain, until the surface be relaxed by tempering, af ter which it may be suffered to cool. When a rent commences at the centre, the parts generally separate with such force, as to sunder the mass, accompanied by a loud report.
It sometimes happens in the breaking of dies, roll ers, &c. (in which the tempering has been omitted) that the effect does not take place until several hours, and even days, after they have been hardened.
Steel is allowed by authors to expand about ith of an inch to the foot, in heating to the hardening point, and to contract, on cooling, about ads of what it had been expanded, provided the hardening effect takes place; otherwise it returns nearly to its original size. Accordingly, I have been in the habit of making al lowances for this enlargement, which is generally found to take place, in a greater or less degree, and for many years held the opinion that it was a necess ary consequence of hardening steel, and that this effect ought to take place, just in proportion to the degree of hardness produced.