Tools of large size and for hard usage, such as lathe and planer tools, do not require to be tempered, and should be cooled in thin oil such as lard or kerosene oil. In using kerosene oil care should be used not to let a red-hot part be above the surface of the oil, as it will flash; such a piece should be kept in the bath until cooled to the temperature of the oil. Tools like milling cutters, drills, reamers, taps, are generally tempered at from 400° to 500° Fahrenheit, depending on the size, as small tools of the same kind require letting down a little more on hardness. Furnaces for Tempering. In tempering furnaces, as in all other kinds of furnaces, the really important consideration is to insure that the furnace is so built as to heat the bath uniformly. The furnaces for tempering are practically the same as the ones used in hardening; those are the best which permit of maintaining a constant temperature and have appliances for measuring the heat so that the correct temperature can be attained.
Fig. 191 shows an ordinary type of tempering furnace. The general principles of combustion are the same as for the average oil burning furnace, and the tools to be tempered are laid in the receptacle which is immersed in the oil. Fig. 192 shows a thermometer for indicating the temperature of the oil bath. The thermometer is very much like the ordinary mercury thermometer except that it registers as high as 500° to 700° Fahrenheit.
In Figs. 193 and 194 are shown furnaces for heating, annealing, and carbonizing. The burners enter the combustion chamber underneath the heating oven; the flame strikes a baffle and the tile floor of the oven, and absolute combustion takes place. Heat free from oxidizing properties is produced and is diverted to the heating oven above with a reverberating motion, which produces a uniform circulation; all materials are thoroughly and uniformly heated. The surplus heat is vented through ports on top of the furnace which can be utilized for pre-heating or for tempering. Blowers are usually mounted upon brackets in the rear, or they may be placed in any other convenient location. The air receiver is underneath, and compressed air may be used when reduced to the proper pressure.
Oil- and gas-fired furnaces are a great improvement over coke and coal furnaces, as with the latter it is almost impossible to regulate heat to a desired and given point, to say nothing of the inconvenience of shoveling coal and of cleaning out ashes, while dust and dirt that accumulate from these operations are, to say the least, very disagreeable. Then again, steel heated in such furnaces is more liable to scale or oxidize and to absorb some of the sulphur and other impurities that arise from the combustion. It is a well-known fact that steel absorbs those injurious elements readily at a heat high enough for hardening and working. In a good gas or oil furnace the heat can be very closely regulated, so that the variation is not more than 10 or 15 degrees, which is close enough for most ordinary work.
Although the installation is more costly than that of a gas furnace, oil furnaces are used a good deal for economy by large concerns. A storage tank under ground is generally used together with a pumping system to maintain a pressure sufficient for forcing the oil to the furnace. The oil flows through a simple regulating valve and is immediately atomized with either steam or air at a pressure of from 35 to 100 pounds per square inch. At this point it enters the forcing hood in the form of fine mist where a volume of air at a pressure of about 6 to 15 ounces per square inch carries it along the combustion chamber in which it ignites and burns with an intense heat.
Fig. 203. Cast-Iron Pot.
A good design of burner should have eliminated the troublesome features of sputtering and dripping oil, which cause an irregular flame. The flame should be clear, white, and free from smoke, soot, and dirt.
In Fig. 195 is shown the bottom brick of a semi-muffle furnace. Fig. 196 shows another brick which is called the furnace bridge. Fig. 197 shows the furnace brick for this furnace. The heating chamber is enclosed entirely. Parts to be heated do not come in contact with the furnace flame.
In Fig. 198 is shown a burner without blast connections for use where air blast is not available. This is used by attaching the burner in such a way that air for combustion is drawn into the furnace by the injector effect of the atomized oil. In Fig. 199 is shown an oil burner with blast connections. The supply of oil and the compressed air for atomizing are regulated by one lever. Fig. 200 shows the same burner with natural gas attachment. This burner is just as efficient a gas burner as it is an oil burner. Figs. 201 and 202 show different views of gas burners. An air blast is introduced in the back of the burner.
Fig. 203 shows a cast-iron pot which can be used for a number of things. Fig. 204 shows a cast-iron pot used for a lead pot. Fig. 205 shows a wire basket for immersing work to be tempered in the oil bath, or it may be used as a basket for holding small tools after hardening. Fig. 206 shows a pot for either annealing or casehardening.