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Tool Heat-treatment in the Garage

2nd January 1942, Page 20
2nd January 1942
Page 20
Page 21
Page 20, 2nd January 1942 — Tool Heat-treatment in the Garage
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How to Obtain the Best Results in Hardening and Forging, and to Avoid Spoiling Good Tools and Wasting the Initial Work of Making Them

By Basil Barham

fiANUFACTURERS of tool steel are l'ato be excused if they sometimes grumble at the way their products are treated after they leave their hands. It is true that metallurgical science has now grown to such a pitch that the steel niaker can give the tool maker material that is to be relied upon to give the best possible results—if it be properly treated.

But that is exactly the point. Except in those works which are devoted to tool making, tool steel is rarely treated as it should be treated and engineers in charge are liable to leave the control of -this really important department to men who have seldom other than practical experience to guide them. This generally means that all steel is treated alike, and if the results are not as expected, the steel maker is blamed.

Old-fashioned engineers always contend that practical experience is the best teacher, but new steels demand new methods, and it is necessary, in the modern tool-making shop, for experience to be guided to a large extent by theory, and for rule-ofthumb to be displaced by scientific methods.

.Shop Must Be Well Lighted First of all the engineer must con sider the lighting and the equipment of -the tool shop, for, if he does not, it twill be impossible for him to obtain the.reselts he desires. In the various pro,cesses through which, the steel will have, to pass, suitable lighting is of the utmost importance and the light must neither vary nor be too bright if the temperature is to be c,Orrectly judged.

T,J.se of muffle reverberatory furnaces

in tool-making establishments has become general of late years, but there is still a number of shops where only the open Jorge is available. Indeed, Sometime, where suitable muffles are

provided, the smith. prefers to work at

an ppen hearth. It may be said, plainly, that the best results are not to be obtained in that way, as there are many tools, even among those in common use, which it is difficult to forge properly if the open hearth be employed.

It is essential, in tool making, that the steel should be heated uniformly and that the smith should be able to exercise a close control over its temperature. Much steel intended . for important tools has been spoiled by overheating, or by working it at an uheuitable temperature and, once spOileri, it cannot be restored to condition. With an open forge the temperature in different parts of' the fire varies greatly and the power of the smith to control it is much more limited than he is wont to believe, with the coniequence that the risk of spoiling the material is much greater than is usually supposed.

In addition, the danger of injuring .the,.steel by bringing it into direct contact with the blast is real. When this' happens the steel is likely to be

burned, even when the temperature is comparatively low. Another trouble with the ordinary hearth is that sulphur is Often present, the effects of which are most harmful to the steel. Sulphur will combine readily with the metal at high temperatures and in doing so will cause soft spots on the surface of the tool.

It is almost impossible, when heating large pieces in an open fire, to avoid roasting them for a considerable time: Thus, the surface is often decarbonized to such an extent that it isimpossible to harden it, and the tool is rendered useless.

These difficulties can be almost entirely avoided by.thense of a muffle, which, in ;my view, is best. heated by town gas. The last-named has many` things to its advantage, as not only does it give a more uniformheat to the muffle and its walls, but also the heat can be more evenly controlled than if coke fires be used.

Even in the rnuffle furnace, however, the steel can easily be over-heated, so this remark must be taken figuratively. The temperature near the walls that are in contact with the flame is con. siderably higher than in other parts of the furnace, and great care should be taken so that no tool is so placed as to touch the walls, The 'steel to be heated should be laid on firebrick supports and should be turned occasionally.

Heat UnIfOrmly and Gradually When an open fire is used, great care should be taken to prevent the metal from corning into direct contact with the blast. Such contact as is made should be as light as possible. It is a wise precaution to dip the steel in milk of lime or clay water before heating it. Then it should be placed in the cooler portion of the fire until, it is raised to a dull red heat. Having reached this temperature, it may be moved to a warmer portion, but the heating must be done gradually.

During the writer's apprenticeship he saw many good tools, on which much initial labour had been expended, utterly ruined because this precaution had not been taken.

The tool can be taken from the furnace and worked so soon as it reaches the desired temperature. It is advisable that it should be hammered as little as possible, because much working makes a considerable difference to the grain of the material aid may set up internal stresses which are almost certain to cause trouble later.

After the tool has been forged and' before it his been hardened, it must be annealed to remove any such

stresses. With ordinary tools it issufficient to anneal them by re-heating them, gradually, to a bright cherry redness and then covering them with ashes, charcoal, or sand and allowing' them to cool slowly. But those which are intended to take a high temper should be treated in annealing pots or special furnaces.

It is not generally understood that the purpose of the annealing pot is to protect the hot steel from the oxidizing and devarbonizing action of the air. The tool will be best packed in an iron pot with charcoal, hoof parings or meal and clean iron filings. The pot , should then be closed, carefully luted . with clay and afterwards covered with ashes or charcoal and allowed to cool to a handling temperature.

It is desirable to emphasize that the cooling must be gradual and allowed to continue until the pot can be moved away by hand. If this operation has been properly carried out and the annealing correctly executed, the steel will be left very soft, and. of a uniform texture throughout.

Hardening Demands Real Skill Perhaps the most critical operation

in the manufacture of any tool is the hardening, and few things call for

greater skill the workman if the operation is to be successfully accomplished. The heated steel has to be brought to its hardening temperature and then quenched in a cold bath of some liquid, which may be water, oil or sortie othecooling medium. It has been pointed out that the best results can be obtained only when the critical temperature of the steel eted is known exactly.

This varies between 1,300 and 1,501) degrees F. Manufacturers of steel are usually pleased to give full information about it, and they should always be asked as to the exact hardening figure.

Although the open fire may be used for hardening common tools, the muffle furnace, or a bath of molten lead or fusible salts, should rlways be used for finer tools, such as cutters, taps or broaches. Before any operation of a finishing nature' is carried out, the exact temperature of the furnace or bath should be ascertained by means of a pyrometer.

. One of the most common faults of the inexpert topl maker is to heat the steel beyond the critical temperature and, if this be done, the steel is ruined. It may be that the tool or other object being hardene8 has some fine edges or projecting points; these should be coated before hardening with a hardening paste," made of either soot or charcoal formed into a paste with machine oil. This will protect the edges or points from burning and prevents decarbonization.

Care must be taken when hardening high-grade steel that it is not exposed to stress caused by unequal cooling. This will almost' certainly cause it to 'crack.

Should hot steel be suddenly and completely quenched, the free carbon is fixed as carbon and prevented from recombining with the metal as a carbide until the steel is again subjected to heating.

Why Final Tempering Is Needed

We are taught in our textbooks that the fixed carbon acts as a cement in which the metallic elements of the steel are embedded and that it holds them SO rigidly that the metal cannot contract as it would were it allowed to cool slowly. When in this condition, the steel is so hard that it cannot be touched with a file and it will readily scratch glass, but it has lost its elasticity and is so brittle that it is of little use for any purpose until it has been tempered or "let down."

The peculiar fact that the contraction of hardened steel does not correspond with its sudden fall in temperature when quenched, and that its volume is, therefore, greater than that of annealed steel and almost as great as that of steel at its critical temperature, is not even yet fully recognized by all engineers. Nevertheless, it is frOne this very fact that most of the hardener's troubles arise. • Having pointed out that, contrary to the generally accepted view, steel suddenly quenched from its critical temperature does not contract, the writer must stress that its power of contraction is not restored to it by merely reheating it, which merely serves further to increase its volume. Only by reheating it and allowing it to cool slowly is the object achieved.

When a tool of considerable size is heated and quenched, suddenly, in cold water, the surface layers are immediately hardened and -rendered incapable of contraction, but the interior, giving up its heat more slowly, is neither hardened so completely nor so fully deprived of its power of contraction. Should the tool be broken after fracture the effect of these differences on the structure of the metal will be readily apparent, the steel near the centre of the bar having a far coarser grain than that near the surface and

being considerably softer. •

Once. these facts ,are realized, it will be readily understood that the rigid ring of metal must be subjected to great stress in resisting the contractile effort of the softer core. This stress

is, in fart, so great that the metal is often unable to withstand it and cracking results. Cracking may begin either at the surface or internally, according to the nature of the steel, the shape of its cross-section and the dimensions of the piece. The precautions to be taken to prevent it must be left largely to the skill and judgment of the hardener, but much can be done by the designer.

Hardening Methods In Practice

In actual practice, it is seldom necessary to give steel the greatest possible hardness. After hardening a tool must be tempered before it can be used, and tempering is really a partial annealing. Its effect is to decrease the hardness of the metal and to increase its toughness.

In hardening the toughness is decreased and the hardness increased. There would be no need for tempering were the hardening under such exact control that it could invariably be interrupted when the steel had acquired the precise degree of hardness required of it by the purpose for which it is intended. Unfortunately, this cannot always be done, but there is always a wide difference between absolute hardness and the hardness .required for tempering, and this margin must be utilized if cracking is to be avoided.

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