Steel Hardening for the garage toolmaker
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IN the process of hardening steel for toolmaking, one of the most important factors is the •quenching fluid. This greatly affects results. In the majority of garages, water is the only medium used, but it is well known that for many purposes tools quenched in certain oils require no letting down. A given sample of steel, heated to a certain temperature and quenched in half a dozen liquids will have as many characteris tics.
This is because the various liquids, having different heat conductivities, allow the steel to cool more slowly than ordinary water would. In such works as are not solely devoted to the manufacture of tools, other media than water, or occasionally oil, are rarely used, the requisite degree of hardness and toughness being regulated solely by temperature. However, it must be admitted that cases are continually arising where a knowledge of the properties of other fluids not only would be economical, but also would save much waste and trouble.
As an example of this difference, common salt added -to water considerably increases its heat conductivity and consequent hardening power. In place of salt, soda or sal-ammoniac can be used, but the effects on the steel are milder. Such solutions are particularly useful when hardening milling cutters, for example, because, by their use, there is less likelihood of cracking at the roots of the teeth.
Acid Solutions for Quenching It is also air advantage to add 1 to 2 per cent. of hydrochloric, sulphuric or nitric acid to the quenching solution, but, before so doing, it will be wise to handle samples of the steel first, so as to be able to ascertain exact results, because there is much variation in the qualities of differing steels. Broadly speaking, solutions give better results when used with steels of low carbon content and poor hardening qualities.
If, for some reason, it is advisable to retard the hardening of tools, milk of lime, clay or ordinary whitewash may be mixed with the .quenching water, dr a layer of mineral oil may be allowed to float on the surface. For some tools, oil alone is used for quenching, in which case no further treatment is necessary. Such substances have the effect of forming a film either on the tool or on the water, and do not reduce the con
ductivity of the latter. They thus check, to some degree, the flow of heat from the tool.
There is room for much intensive study in connection with the behaviour of oils. Mineral oil and various vegetable oils give differing results. Paraffin has the greatest effect of any and so on throughout the range of mineral oils. Actually there is little difference between the action of one of the lighter hydro-carbons (such as
paraffin) and -that of water. Melted
tallow gives a considerably greater effect than either water or paraffin, but, generally speaking, the operative will find that the denser the oil or fat employed, the softer and tougher becomes the steel treated. Heavy vegetable oils, for example, linseed or palm oil, give the mildest products of all.
Results can be controlled to a great extent by adrnixtures of these substances. . A mixture of paraffin and palm oil, for instance, will have a markedly different result from that obtained with palm oil alone. In the same way, a tool qUenched in water covered with a film of oil acquires a lower degree of hardness than when quenched in water mixed with salt. By increasing the depth of this layer of oil or by varying the speed with which the tool is passed through it into the water bed, a greater or less degree of temper can be obtained in practice.
Use of Oil-covered Water
Great care must be taken in employing this method of operation and, as in so many things, perfection is only obtainable by constant practice. It is particularly useful in the case of tools which require different degrees of hardness throughout their length.
A little-known point is that all water used for hardening should be boiled and allowed to cool before use, in order that lime and other salts it contains may he thrown down before the operation; otherwise the impurities will be certain to have some effect on the hardening of the steel.
" Letting down "—a term that may be strange to some people—means reheating the outer surface of the metal after it has been quenched, but before the more gradual cooling and consequently greater contraction of the inner core has proceeded so far as to cause cracking.
The effect of this re-heating is to restore some toughness to the hardened surface, and here a curious, characteristic of steel comes into operation. The colour of the metal changes slowly.
First colour to appear is a pale yellow and the external temperature of the metal must not be raised, in " letting down," beyond that at which this colour appears, unless it is intended to combine the processes of tempering and hardening. It will be found in .practice that it. ia seldom necessary, and usually inadvisable, to reheat the surface to any appreciable extent over 200 degrees F.
Much depends on the skill and experience of the operator. I have seen men in a Tyneside works who apparently could judge to a nicety the degree of temper to which they were able to bring a tool merely by watching the colour reached in the letting down process. It is a general •habit in the North to use an ordinary bunsen burner in tempering, bet other districts use a hot lead bath, a muffle or a hardening fire.
If care be taken in the original quenching, a tool may be let down with its own internal heat. Except in the rare cases in which a tool requires the greatest possible hardness, it should be quenched gradually, care being taken not to cool it completely; it should be removed from the bath so soon as it is possible to handle it. Its tempera-, ture, even then, will be considerably lower than that which produces the highest temper on the colour scale, yet the surface of the tool will not have lost all toughness and, therefore, is not likely to crack.
Treatment by Size
The nature of the subsequent treatment depends chiefly, if not entirely, upon the size of the tool. Small tools may be left to cool off in the air, whilst larger ones should be cooled in an oil or hot water bath.
With large tools—and it is this fact that leads to many troubles—there is a hot core which retains its heat for lowg after the exterior is cold and hard. This is the primary cause of cracking in hardening. Furthermore, if the greatest care be not taken, the tool may soften to such an extent that effective tempering is not possible. This is so usual that steel firms should make close researches into the Whole subject and allow workmen every facility to observe results.
Colour charts, -effective as they look, are quite useless in practice. Not one man in 10 is capable of matching colour, and 'were they to depend on that alone some curious results would obtain. My view is that all large tools, such as cutters, reamers and .broaches should be bored throughout their length, so that the quenching fluid should be able to reach the interior.
One of the rules that should be made and enforced in every shop is that 'hot water at a. temperature just below boiling point, but not lower than 200 degrees F., should be used in letting down and that 'the tool should never be returned to the hardening fire as this course leads to uneven heating. The proper course to adopt is to plunge the tool in the liquid and move it about rapidly until it is cool enough, to be touched with -the hand.. Then it should be put into hot water until cold.
If the tool be of large cross section it will be found that its internal heat will suffice to raise its external temperature considerably when it is first placed in the hot bath. When this is the case, the tool . should be quenched for a second time in cold water and then returned to the hot bath to cool.
Both toughness and hardness are thus adjusted to the purpose for which
the tool is required. The operation may be carried out by combining the tempering with the hardening process, so that one heating suffices for both, or they may be performed one after the other. In short, the temperature of the metal should be raised to a degree determined according to the class of steel employed and to the use it is intended to snake of the tool when the operations are concluded.
If the surface of a piece of hardened steel_ be cleaned with emery cloth or other substance, pumice being often
employed, and the metal be then heated to about 400 degrees F., there will appear on the cleaned surface afilm of colour which is at first bright yellow and gradually changes, as heating' is continued, through many shades to pale blue or grey.
If the temperature be raised the changes of colour will appear more rapidly; if it be not raised, hut a steady heat be maintained of 400 degrees F., the' colours will appear more slowly. If the metal be suddenly cooled, when any specified colour appears the tool will have a certain temper, or relation of toughness to hardness, whilst for any given steel treated under the same conditions, each colour wilt always indicate the same temper.
It must never be forgotten, however. that if the nature of the steel or the conditions of treatment vary, the degree' of temper corresponding with various colours will vary also, Other things being equal, steels which are naturally hard will he, colour for colour, harder when tempered than those steels which are naturally soft.
The higher the temperature to which the metal has been raised in the hardening process and the more quickly and completely it has been cooled, the harder it will be when tempered. Another point which the operative should note is that the more slowly the tool is heated in ternpering, the tougher it will be.
In garage practice, when a man decides to combine the operations of tempering and hardening—always a hazardous undertaking—the tool should be allowed to cool in the hardening bath until its core retains, in the judgment of the hardener, little more heat than is required for tempering.
Such tools as taps are cooled off and then brought into contact with hot iron, such as a heated nut or the jaws of a specially shaped pair of tongs. With some it is the practice to temper a large number of small tools in a tray of sand placed over a fire. The most exact method to use is to treat the small tools in a muffle furnace kept at a steady temperature.
Those tools that are subject to heavy blows when in use—chisels, punches and the like—are twice brought to the temper colour, the first colour being rubbed off and the tool reheated till the colour reappears. This double tempering greatly increases the tenacity.