Hardening is performed the more efficiently the more rapidly the quenching is done. In the case of thick objects, however, especially milling cutters, there is risk of cracking, due to the difference of temperature on the outside and in the central body of metal. Rapid hardening is impracticable in such objects. This is the cause of the distortion of long taps and reamers, and of their cracking, and explains why their teeth are often protected with soft soap and other substances.
The presence of the body of heat in a tool is taken advantage of in the work of tempering. The tool, say a chisel, is dipped, a length of 2 in. or more being thus hardened and blackened. It is then removed, and a small area rubbed rapidly with a bit of grindstone, observations being made of the changing tints which gradually appear as the heat is communicated from the hot shank to the cooled end. The heat becomes equalized, and at the same time the approximate temperature for quenching for temper is estimated by the appearance of a certain tint; at that instant the article is plunged and allowed to remain until quite cold. For every different class of tool a different tint is required.
“Blazing off” is a particular method of hardening applied to small springs. The springs are heated and plunged in oils, fats, or tallow, which is burned off previous to cooling in air, or in the ashes of the forge, or in oil, or water usually. They are hardened, reheated and tempered, and the tempering by blazing off is repeated for heavy springs. The practice varies almost infinitely with dimensions, quality of steel, and purpose to which the springs have to be applied.
The range of temper for most cutting tools lies between a pale straw or yellow, and a light purple or plum colour. The corresponding range of temperatures is about 430° F. to 530° F., respectively. “Spring temper” is higher, from dark purple to blue, or 550° F. to 630° F. In many fine tools the range of temperature possible between good and poor results lies within from 5° to 10° F.
There is another kind of hardening which is of a superficial character only—“case hardening.” It is employed in cases where toughness has to be combined with durability of surface. It is a cementation process, practised on wrought iron and mild steel, and applied to the link motions of engines, to many pins and studs, eyes of levers, &c. The articles are hermetically luted in an iron box, packed with nitrogenous and saline substances such as potash, bone dust, leather cuttings, and salt. The box is placed in a furnace, and allowed to remain for periods of from twelve to thirty-six hours, during which period the surface of the metal, to a depth of 1⁄32 to 1⁄16 in., is penetrated by the cementing materials, and converted into steel. The work is then thrown into water and quenched.
| Fig. 1.—Automatic Oil Muffle Furnace. |
A muffle furnace, employed for annealing, hardening and tempering is shown in fig. 1; the heat being obtained by means of petroleum, which is contained in the tank A, and is kept under pressure by pumping at intervals with the wooden handle, so that when the valve B is opened the oil is vaporized by passing through a heating coil at the furnace entrance, and when ignited burns fiercely as a gas flame. This passes into the furnace through the two holes, C, C, and plays under and up around the muffle D, standing on a fireclay slab. The doorway is closed by two fireclay blocks at E. A temperature of over 2000° F. can be obtained in furnaces of this class, and the heat is of course under perfect control.
| Fig. 2.—Reverbatory Furnace. |
A reverberatory type of gas furnace, shown in fig. 2, differs from the oil furnace in having the flames brought down through the roof, by pipes A, A, A, playing on work laid on the fireclay slab B, thence passing under this and out through the elbow-pipe C. The hinged doors, D, give a full opening to the interior of the furnace. It will be noticed in both these furnaces (by Messrs Fletcher, Russell & Co., Ltd.) that the iron casing is a mere shell, enclosing very thick firebrick linings, to retain the heat effectively.
(J. G. H.)