HARDENING HIGH-SPEED STEEL
In forging use coke for fuel in the forge. Heat steel slowly and thoroughly to a lemon heat. Do not forge at a lower heat. Do not let the steel cool below a bright cherry red while forging. After the tool is dressed, reheat to forging heat to remove the forging strain, and lay on the floor until cold. Then have the tool rough ground on a dry emery wheel.
FIG. 85.—Be sure to have a full yellow heat at the dotted line. Remember this is a boring mill tool and will stand out in the tool-post, and if you do not have a high thorough lemon heat, your tool will snap off at the dotted line. (Ninety-five per cent of all tools which break, have been forged at too low a heat or at a heat not thorough to the center.)
FIG. 86.—Keep your high lemon forging heat up. If you forge under a steam hammer, take light blows. Do not jam your tool into shape. Put frequently back into the fire. Never let the high lemon color go down and beyond the dotted line.
For built-up and bent tools special care should be taken that the forging heat does not go below a bright cherry. For tools ¾ by 1½ or larger where there is a big strain in forging, such as bending at angles of about 45 deg. and building the tools up, they should be heated to at least 1,700°F. Slowly and without much blast. For a ¾ by 1½ tool it should take about 10 min. with the correct blast in a coke fire. Larger tools in proportion. They can then be bent readily, but no attempt should be made to forge the steel further without reheating to maintain the bright cherry red. This is essential, as otherwise the tools crack in hardening or while in use.
FIG. 87.—Be sure that the tool is absolutely straight at the bottom, so as to lie flat in the tool-post.
FIG. 88.—This is the finished forged tool, and let this grow cold by itself, the slower the better. It is well to cool the tool slowly in hot ashes, to remove all forging strain. You can now grind the tool dry on a sharp emery wheel. The more you now finish the tool in grinding, the less there is to come off after hardening.
In hardening place the tool in a coke fire (hollow fire if possible) with a slow blast and heat gradually up to a white welding heat on the nose of the tool. Then dip the white hot part only into thin oil or hold in a strong cold air blast. When hardening in oil do not hold the tool in one place but keep it moving so that it cools as quickly as possible. It is not necessary to draw the temper after hardening these tools.
FIG. 89.—This tool is ground, ready for hardening. Never harden from the forging heat.
FIG. 90.—Heat the nose of the tool only up to dotted line, very slowly and thoroughly to an absolutely white welding heat, so that it shows a trifle fused around the edges, and be very sure that this fusing has gone thoroughly through the nose, otherwise the fusing effect will be taken off after the second grinding. Note the difference of the nose between this and Fig. 86.
FIG. 91.—Shows unnecessary roasting and drossing. Such hardening requires a great amount of grinding and is not good. After hardening grind carefully on a wet emery wheel, and be sure that the wheel is sharp with a plentiful supply of water. Do not force the grinding, otherwise the cold water striking the steel heated up by friction, will crack the nose. Be sure that the grinding wheel is sharp.
In grinding all tools should be ground as lightly as possible on a soft wet sandstone or on a wet emery wheel, and care should be taken not to create any surface cracks, which are invariably the result of grinding too forcibly. The foregoing illustrations, Figs. 84 to 91, with their captions, will be found helpful.
Special points of caution to be observed when hardening high-speed steel.
Don't use a green coal fire; use coke, or build a hollow fire.
Don't have the bed of the fire free from coal.
Don't hurry the heating for forging. The heating has to be done very slowly and the forging heat has to be kept very high (a full lemon color) heat and the tool has to be continually brought back into the fire to keep the high heat up. When customers complain about seams and cracks, in 9 cases out of 10, this has been caused by too low a forging heat, and when the blacksmith complains about tools cracking, it is necessary to read this paragraph to him.
Don't try to jam the tool into shape under a steam hammer with one or two blows; take easy blows and keep the heat high.
Don't have the tool curved at the bottom; it must lie perfectly flat in the tool post.
Don't harden from your forging heat; let the tool grow cold or fairly cold. After forging you can rough grind the tool dry, but not too forcibly.
Don't, for hardening, get more than the nose white hot.
Don't get the white heat on the surface only.
Don't hurry your heating for hardening; let the heat soak thoroughly through the nose of the tool.
Don't melt the nose of the tool.
Don't, as a rule, dip the nose into water; this should be done only for extremely hard material. It is dangerous to put the nose into water for fear of cracking and when you do put the nose into water put just 1/2 in. only of the extreme white hot part into the water and don't keep it too long in the water; just a few seconds, and then harden in oil. We do not recommend water hardening.
Don't grind too forcibly.
Don't grind dry after hardening.
Don't discolor the steel in grinding.
Don't give too much clearance on tools for cutting cast iron.
Don't start on cast iron with a razor edge on the tool. Take an oil stone and wipe three or four times over the razor edge.
Don't use tool holder steel from bars without hardening the nose of each individual tool bit.
Air-hardening Steels.—These steels are recommended for boring, turning and planing where the cost of high-speed seems excessive. They are also recommended for hard wood knives, for roughing and finishing bronze and brass, and for hot bolt forging dies. This steel cannot be cut or punched cold but can be shaped and ground on abrasive wheels of various kinds.
It should be heated slowly and evenly for forging and kept as evenly heated at a bright red as possible. It should not be forged after it cools to a dark red.
After the tool is made, heat it again to a bright red and lay it down to cool in a dry place or it can be cooled in a cold, dry air blast. Water must be kept away from it while it is hot.
CHAPTER XI
FURNACES
There are so many standard furnaces now on the market that it is not necessary to go into details of their design and construction and only a few will be illustrated. Oil, gas and coal or coke are most common but there is a steady growth of the use of electric furnaces.
FIG. 92.—Standard lead pot furnace.
Typical Oil-fired Furnaces.—Several types of standard oil-fired furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig. 93 is a vertical furnace with a center column. This column reduces the cubical contents to be heated and also supports the cover.
FIG. 93.—Furnace with center column.
A small tool furnace is shown in Fig. 94, which gives the construction and heat circulation. A larger furnace for high-speed steel is given in Fig. 95. The steel is supported above the heat, the lower flame passing beneath the support.
For hardening broaches and long reamers and taps, the furnace shown in Fig. 96 is used. Twelve jets are used, these coming in radially to produce a whirling motion.
FIG. 94.—Furnace for cutting tools.
FIG. 95.—High-speed steel furnace.
Oil and gas furnaces may be divided into three types: the open heating chamber in which combustion takes place in the chamber and directly over the stock; the semimuffle heating chamber in which combustion takes place beneath the floor of the chamber from which the hot gases pass into the chamber through suitable openings; and the muffle heating chamber in which the heat entirely surrounds the chamber but does not enter it. The open furnace is used for forging, tool dressing and welding. The muffle furnace is used for hardening dies, taps, cutters and similar tools of either carbon or high-speed steel. The muffle furnace is for spring hardening, enameling, assaying and work where the gases of combustion may have an injurious effect on the material.
FIG. 96.—Furnace for hardening broaches.
FIG. 97.—Forging and welding furnace.
FIG. 98.—Semi-muffle furnace.
FIG. 99.—Muffle furnace.
Furnaces of these types of oil-burning furnaces are shown in Figs. 97, 98, and 99; these being made by the Gilbert & Barker Manufacturing Company. The first has an air curtain formed by jets from the large pipe just below the opening, to protect the operator from heat.
FIG. 100.—Gas fired furnace.
FIG. 101.—Car door type of annealing furnace.
Oil furnaces are also made for both high- and low-pressure air, each having its advocates. The same people also make gas-fired furnaces.
Several types of furnaces for various purposes are illustrated in Fig. 100 and 101. The first is a gas-fired hardening furnace of the surface-combustion type.
A large gas-fired annealing furnace of the Maxon system is shown in Fig. 101. This is large enough for a flat car to be run into as can be seen. It shows the arrangement of the burners, the track for the car and the way in which it fits into the furnace. These are from the designs of the Industrial Furnace Corporation.
Before deciding upon the use of gas or oil, all sides of the problem should be considered. Gas is perhaps the nearest ideal but is as a rule more expensive. The tables compiled by the Gilbert & Barker Manufacturing Company and shown herewith, may help in deciding the question.
| Heat units per thousand cubic feet 1,000,000 | |
|---|---|
| Natural gas | 1,000,000 |
| Air gas (gas machine) 20 cp | 815,500 |
| Public illuminating gas, average | 650,000 |
| Water gas (from bituminous coal) | 377,000 |
| Water and producer gas, mixed | 175,000 |
| Producer gas | 150,000 |
Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the following comparisons may evidently be made. At 5 cts. a gallon, the equivalent heat units in oil would equal:
| Heat units per thousand cubic feet at $0.375 | |
|---|---|
| Natural gas | at $0.375 |
| Air gas, 20 cp | at 0.307 |
| Public illuminating gas, average | at 0.244 |
| Water gas (from bituminous coal) | at 0.142 |
| Water and producer gas, mixed | at 0.065 |
| Producer gas | at 0.057 |
Comparing oil and coal is not always simple as it depends on the work to be done and the construction of the furnaces. The variation rises from 75 to 200 gal. of oil to a ton of coal. For forging and similar work it is probably safe to consider 100 gal. of oil as equivalent to a ton of coal.
Then there is the saving of labor in handling both coal and ashes, the waiting for fires to come up, the banking of fires and the dirt and nuisance generally. The continuous operation possible with oil adds to the output.
When comparing oil and gas it is generally considered that 4½ gal. of fuel oil will give heat equivalent to 1,000 cu. ft. of coal gas.
The pressure of oil and air used varies with the system installed. The low-pressure system maintains a pressure of about 8 oz. on the oil and draws in free air for combustion. Others use a pressure of several pounds, while gas burners use an average of perhaps 1½ lb. of air to give best results.
The weights and volumes of solid fuels are: Anthracite coal, 55 to 65 lb. per cubic foot or 34 to 41 cubic feet per ton; bituminous coal, 50 to 55 lb. per cubic foot or 41 to 45 cubic feet per ton; coke, 28 lb. per cubic foot or 80 cubic feet per ton—the ton being calculated as 2,240 lb. in each case.
A novel carburizing furnace that is being used by a number of people, is built after the plan of a fireless cooker. The walls of the furnace are extra heavy, and the ports and flues are so arranged that when the load in the furnace and the furnace is thoroughly heated, the burners are shut off and all openings are tightly sealed. The carburization then goes on for several hours before the furnace is cooled below the effective carburizing range, securing an ideal diffusion of carbon between the case and the core of the steel being carburized. This is particularly adaptable where simple steel is used.