IX.ON THE SURFACE.
The condition of the surface of steel has much to do with its successful hardening and working.
A slight film adherent to the surface of steel will prevent its hardening properly; the steel may harden under such a film and not be hard upon the immediate surface, and, as in almost every case a hard, strong surface is necessary to good work, it is important that a piece of steel to harden well should have a clean surface of sound steel.
It has been stated already that all bars and forgings of steel have upon the surface a coat of oxide of iron, and immediately beneath this a thin film of decarbonized iron.
Neither of these substances will harden, and in every case where a hard-bearing surface or a keen cutting-edge is desired these coatings must be removed. Polished drill-wire and cold-rolled spring-steel for watches, clocks, etc., should have perfect surfaces, and it is the duty of steel-makers to turn them out in that condition. All black steel, or hot-finished steel, contains these coatings.
In the manufacture of railroad, wagon, and carriage springs it is not necessary or customary to pay any attention to these coatings; the body of the steel hardens well, giving the required resilience andelasticity, so that an unhardened coat of .01 to .001 inch thick does no harm. To all bearing-surfaces and cutting-edges such coatings are fatal.
The ordinary way of preparing steel is to cut the skin off, and this is sufficient if enough be taken off; it happens often that a purchaser, in pursuit of economy and unaware of the importance of this skin, orders his bars or forgings so close to size that when they are finished the decarbonized skin is not all removed, and the result is an expensive tap, reamer, milling-cutter, or some tool of that sort with the points of the teeth soft and worthless.
In small tools ¹/₁₆ inch, in medium-size tools, say up to two or three inches in diameter, ⅛ inch cut off should be plenty; in large tools and dies, especially in shaped forgings, it would be wiser to cut away ³/₁₆ inch.
In many cases sufficient hardness can be obtained by pickling off the surface-scale, but this will not do where thorough hardening is required, because the acid does not remove the thin decarbonized surface. It seems to be impracticable to remove the decarbonized skin by the action of acid, for if the steel be left in the acid long enough to accomplish this the acid will penetrate deeper, oxidizing and ruining the steel as it advances.
Grinding is frequently resorted to, being quicker and cheaper than turning, planing, or milling.
When grinding is used, care must be taken not to glaze the surface of the steel, or if it should be glazed the glaze must be removed by filing or scraping.
In the manufacture of files it is customary to grind the blanks after they are forged and before the teeth are cut.
After the blanks are ground they are held up to the light and examinedcarefully for glaze. Every blank that shows by the flash of light that it is glazed is put to one side; then these glazed blanks are taken by other operatives and filed until all traces of glaze are removed. The file-maker will explain that if this be not done the files when hardened will be soft at the tips of the teeth over the whole of the glazed surface. This inspection and filing of blanks involves considerable expense, and it is certain that such an expense would not be incurred if it were not necessary.
This glaze does not appear to be due to burning, at least the stones are run in water; the blanks are handled by the bare hands of the grinders, and do not appear to be hot.
After pieces are hardened and tempered they frequently require grinding to bring them to exact dimensions. This is usually done on emery-wheels with an abundance of water, and as no temper colors are developed indicating heat it is assumed that no harm can be done.
Just here much valuable work is destroyed.The tempered piece is put on the wheel, in a “flood of water”; the work is rushed, and the piece comes out literally covered with little surface-cracks running in every direction, perfectly visible to the naked eye. Until the steel-worker learns better he blames and condemns the steel.
This result is very common in the manufacture of shear-knives, scissors, shear-blades, dies, etc.
Sometimes too a round bearing or expander-pin is hardened; examined by means of a file it appears perfectly hard; it is then ground, not quite heavily enough to produce surface-cracks, but still heavily, and on a glazed wheel. It is found now that the surface is soft; only a thousandth of an inch or so has been cut off, and the steel iscondemned at once because it will harden only skin deep. Let the file be drawn heavily over the surface and it will be found that the soft surface is only about a thousandth of an inch thick, and underneath the steel is perfectly hard.
Now grind slightly on a sharp, clean wheel and re-harden; the surface will be found to be perfectly hard. Ground heavily again on the glazed wheel, it will become soft, as before. These operations can be repeated with unvarying results until the whole piece is ground away.
These difficulties occur more with emery-wheels than with grindstones, either because emery-wheels glaze more easily than grindstones, or because, owing to their superior cutting powers under any circumstances, they are more neglected than grindstones.
Experience shows that these bad results occur almost invariably on glazed wheels. It is rare to find any bad work come off from a clean, sharp wheel, unless the pressure has been so excessive as to show that the operator is either foolish or stupid.
The remedy is simple: Keep the wheelscleanandsharp.
Many grinders who understand this matter will not run any wheel more than one day without dressing, nor even a whole day if the work is continuous and they have reason to apprehend danger.
Pickling is the placing of steel in a bath of dilute acid to remove the scale. It is a necessary operation in wire-making and for many other purposes, and it may be hastened by having the acid hot.
Sulphuric acid is used generally; it is efficient and cheap. When thinsheets are to be pickled, the acid should not be too hot, or it will raise a rash all over the sheet in many cases. This indicates some unsoundness in the steel, the presence probably of innumerable little bubbles of occluded gases. This is possibly true, yet the same sheets pickled properly and brought out smooth will polish perfectly, or if cut up will make thousands of little tools that will show no evidence of unsoundness.
Steel should never be left in the pickling-bath any longer than is necessary to remove the scale; it seems unnecessary to warn readers that the acid will continue to act on the steel, eat the steel after the scale is removed. When taken from the pickle, the steel should be washed in limewater and plenty of clean running water; but this does not take out all of the acid. It should then be baked for several hours at a heat of 400° to 450° F. to decompose the remaining acid. This is just below a bluing heat, and it does not discolor or oxidize the surface. It is known as the sizzling-heat, the heat that the expert laundry-woman gets on her flat-iron which she tests with her moistened finger.
Acid if not taken off completely will continue to act upon and rot the steel; how far this will go on is not known exactly; for instance, it is not known whether if a block six inches cube were pickled and merely washed, the remaining acid would penetrate and rot the whole mass or not. There must be some relation between the mass of the steel and the power of a small amount of acid to penetrate.
The power of acid can be illustrated on the other extreme: A lot of watch-spring steel is finished in long coils and .010 inch thick; when last pickled, the baking was neglected; the steel is tough, it hardenswell, and when tempered it is springy and strong; by all of the tests it is just right in every coil. It is shipped away and in three or four weeks the spring-maker begins work on it. He reports at once that it is rotten and worthless, it will not make a spring at all, and he is angry. The steel is returned to the maker and he finds the report true: the steel is rotten and worthless. Then by diligent inquiry he finds that the last baking was omitted, and he pockets his loss, sending an humble apology to the irate spring-maker.
Whether the residual acid can ruin a large piece of steel or not need not be considered when the simple operation of baking will remove the possibility of harm.