HEATING FOR HARDENING.
A smith can heat an occasional piece for hardening, in his ordinary fire by using care and taking a little time. Where there are many pieces to be hardened, special furnaces should be used.
For thousands of little pieces, such as saw-teeth or little springs, a large furnace with a brick floor, and so arranged that the flame will not impinge on the pieces, is good.
The operator can watch the pieces, and as soon as any come to the right color he can draw them out, letting them drop into the quenching-tank, which should be right under the door or close at hand.
For twist-drills, reamers, etc., a lead bath, or a bath of melted salt and soda, is used. The lead bath is the best if care be taken to draw off the fumes so as not to poison the heaters. Because a bath of this kind is of exactly the right color at the top it is not to be assumed that pieces can be heated in it and hardened without further attention.
Thousands of tools are ruined, and thousands of dollars are thrown away annually, by unobserving men who assume that because a lead bath appears to be exactly the right color at the surface it is therefore just right.
A dark orange color surface may have underneath it an increasingly higher temperature, up to a bright lemon at the bottom, and tools heated in such a bath will have all of the varying temperatures of the bath; then cracked tools, twisted tools, brittle tools, tools too hard at one end and not hard enough at the other, will come out with exasperating regularity.
All of this can be avoided by a simple thorough stirring of the bath, to be done as often as may be necessary to keep it uniform.
In heating toothed tools, taps, reamers, milling-cutters, and the like, care should be taken that the points of the teeth never get above the refining-heat, the dark or medium orange required. It is no easy matter to do this except in a uniform bath, but it must be done. If the teeth are bright lemon, or even bright orange, when the body of the tool is at medium orange refining-heat, the probabilities are that they will shell off from the hardened tool as easily as the grains from a cob of corn.
Even if they are not so bad, if they do not crack off, they will be coarse-grained and brittle; they will not hold a good edge, and they will not do good work. If a long tool, such as a drill, etc., be heated medium orange on one side and bright orange on the other,—a difference of 100° to 200° F.,—and be quenched, it will come out of the bath curved; it must be curved. In quenching a long tool which it is desired to have straight it should be dipped vertically, so as to cool all around the axis simultaneously. If such a tool be dipped sideways, it will come out bent. In heating edge-tools of all kinds it is best to heat first the thicker part, away from the edge, and then when the body has come up to the refining-heat to draw the edge into the fire and let it come up last; as soon as a uniform color is reached quench promptly. If the edge be exposed to the fire in the beginning of the operation, it will almost certainly become too hot before the thicker parts are hot enough.
When a smooth, cylindrical piece is to be hardened, it should be rolled around from time to time while heating, unless it is in a lead bath; if it be left to lie quietly in a furnace until it is hot, it will have a soft streak along the part that was uppermost.
The cause of this is not clear; the fact is as certain as hundreds of tests can make any fact. The experiment can be made by re-heating the piece with the soft streak down; then the original soft streak will come out hard, and another soft streak will be found on top. The changes can be rung upon this indefinitely.
A maker of roller-tube expanders had great trouble with his expander-pins; they cut, and wore out on one side. He tried many makes and many tempers of steel with the same result. He was told to turn his pins over and over as he heated them and his troubles would end. He replied: “Why, of course; I can see the reason and sense in that.” If he did see the reason, he is the only person known, so far, who has done so. His pins worked all right from that time.
In hardening round sections it is necessary to use great care to have the heat perfectly uniform and not too high, because the circular form is the most rigid, offering the greatest resistance to change. For this reason a round piece will be almost certain to split if it be heated above a medium orange, or if it be heated unevenly. Many a round piece is cracked by a heat, or by a little unevenness of heat, that another section would endure safely. A roll with journals is perhaps the most difficult of all tools to harden successfully; the most expert temperers will not be surprised at losing as many as one roll in five.
Engraved dies require to be hardened without oxidizing the engraved face, so that the finest lines will be preserved clear and clean.
This is done by burying the engraved face in carbonaceous material in such a way as to prevent the flame or any hot air from coming in contact with it.
There are many ways of doing this, and many different carbonaceous mixtures are used; one simple, and known to be satisfactory, plan will be explained as sufficient to give any intending operator a good starting-point.
The carbonaceous material preferred is burnt leather powdered—and the older it is the better—until it is reduced to ash, so that the material should be saved after each operation to be used again mixed with enough new material to make up the necessary quantity.
D is the die to be heated; B is an open box about two inches deep and one inch larger each way than the die; L is the burnt leather packed in thoroughly, and as full as the box will hold. The engraved face is down, embedded in the burnt leather, and secure from contact with flame or air.
Sometimes powdered charcoal is used, with or without a mixture of tar, according to the fancy of the operator.
Some operators prefer to have the box so high as to leave only the top surface of the embedded die exposed, but the most successful workers prefer the plan sketched, because they can see more of the die, and so regulate better the even heating.
The die and box are put in the furnace, and the heating is watched, the die being turned and moved about in the furnace so as to obtain a perfectly even heat.
When the right temperature is reached, the whole is withdrawn from the furnace; the die is lifted out of the box and plunged into the water immediately. There must be no delay at this point whatever; a few moments’ exposure of the hot die to the air will result in oxidation and scaling of the engraving.
In heating such a die a furnace should be used. It can be done in a smith’s fire, but it is a hazardous plan, and gives many chances for a failure.
A furnace with an even bed of incandescent coke is good, and such a furnace is very useful for many other purposes.
Where many dies are to be hardened, the handiest appliance is a little furnace with brick floor and lining, and heated by petroleum or gas, so arranged that the flames will not impinge upon the piece to be heated.
Such furnaces are now made to work so perfectly that illuminating-gas is found to be an economical fuel.
For quenching there should be plenty of water. For small dies that can be handled easily by one man a large tub or tank of water will answer if the operator will keep the die in rapid motion in the water.
Running water is the best. A handy plan is to have the inlet-pipe project vertically a short distance through the bottom of the tank, producing a strong upward current which will strike directly against the face of the submerged die.
Some prefer a downward stream; others a side stream; others, again, prefer a shower-bath; and, again, some use side jets.
A very efficient tank has a partition running from a few inches from the bottom to within a few inches of the surface of the water, and so placed as to separate, say, nine tenths of the tank from one tenth. In the smaller compartment there is an Archimedean screw driven at a speed of 200 to 300 revolutions; this drives the water under the partition and out over the top in a violent current. The steel is quenched in the larger space. Where water is an item of expense, this plan is economical, and it is certainly efficient.
An excellent way of quenching large faces, such as anvils, is to have a tank raised twelve to fifteen feet from the floor. In the bottom of the tank is a pipe with a valve, to be operated by a lever. The whole is enclosed in a sort of closet with a door in one side. When the piece is hot, it is placed immediately under the pipe, the door is closed, the valve is opened, and a great body of water is dashed down upon the face that is to be hardened.
A slight modification of this plan is used in hardening armor-plates, where many jets are used to insure even quenching of the large surface. This plan is supposed to be patented, or, more properly, it is patented; but as it is very old and well known the patent should not be allowed to disturb anybody.
Water only has been mentioned so far as a quenching medium, because it is the simplest and the cheapest generally. Oil is used frequently where extreme hardness is not necessary and toughness is desirable. Oil gives a good hardness with toughness, and it is used almost universally for springs, and it is sometimes used to toughen railroad axles and similar work. The oil acts more slowly than water and leaves the piece in more nearly a tempered condition; it is neither so hard nor so brittle as it would be if quenched in water. Straits fish-oil is good and cheap; lard-oil gives greater hardness than fish-oil; mineral oil is too fiery to use safely; but there are mixed oils in the market made expressly for hardening which are cheap and efficient.
If it is desired to get the greatest hardness, brine will harden harder than fresh water; and mercury will give the greatest hardness of all. It is a rather expensive cooling medium.
Acid added to water increases its hardening power; but those who know the effects of acids will be very chary of using them.
As to heating, too much emphasis cannot be given to the importance of even temperature throughout the mass. The illustration of the painted piece mentioned in connection with heating for forging applies more forcibly here. Every piece that is to be quenched should look as if it were covered with a perfectly even coat of paint of the exact tint necessary to give the best result.
All hardening should be done on a rising temperature, because then the grain and strains cannot be greater than those due to the highest heat, and this maximum heat can be watched and kept within limits. If a piece be quenched from a falling temperature, the grain and strains will be those due to the highest temperature, modified slightly by the distance through which it has cooled, and always coarser and more brittle than if quenched at the same heat produced by rising temperature. If by accident a piece gets too hot to be quenched, it should be allowed to go entirely cold, and then be heated again to the right color.
After a piece of steel is hardened it is usually tempered to relieve some of the strain, reduce brittleness, and increase the toughness.
This is done by heating; usually the piece is held over the fire, or in contact with a large piece of steel or iron heated for the purpose, until it takes on a certain color which indicates the degree of tempering that is wanted.
Where great numbers of pieces are to be tempered, a bath is very convenient. Boiling in water produces only a slight tempering sufficient for some purposes. Steaming under given pressure will produce even heating and uniform tempering.
When pieces are quenched in oil, they can be tempered easily and nicely by watching the oil that adheres to them. When the oil is dried off and begins to char, the tempering is good, about right for saw-teeth. If the heat is run up until the oil flashes, the tempering is pretty thorough and is about right for good springs. If the oil be all burned off, there will be little temper left except in very high steel. High steel becomes much harder when quenched than low steel; consequently very high hardened steel may be heated until it begins to show color and still retain considerable hardness or temper, whereas a milder steel, under 90 or 100 carbon, when heated to such a degree will retain no temper, it will be soft.
Saw-teeth, tap, reamer, and milling-cutter teeth, may be drawn, and usually should be drawn, down until a file will barely catch them; then they will do excellent work. Many inexperienced temperers are apt to complain if such tools can be filed at all when drawn to the proper color, forgetful or ignorant of the fact that a file should always contain about twice as much carbon as a tap or reamer, and that if both are drawn to the same color the file must necessarily be the harder. Such men often destroy much good work by trying to get the tools too hard. If a tap-tooth be left file hard, it will be pretty certain to snip off when put to work.