ROBERT GIBSON GRISWOLD

In this chapter we touch for the first time the domain of special tool steels. The small hand tools treated in the first two chapters were all made of what is commonly known as ordinary tool steel, which is technically termed a "high carbon steel " and can be hardened in water or oil.

Lathe tools, however, are usually made of a special self-hardening steel that will not bear water, but must be hardened in a current of air. The reason fur this is that they are in general harder than a carbon steel and hold their edges better; in fact, some of them will hold their edge while red hut under the chip. It is this quality which makes the new high-speed steels so valuable. With them the cutting speeds are materially increased, which lessens the cost of production.

These steels are made by what is known as the Crucible method. For those unfamiliar with the process a short description will be given. It may be well to add that the structural steels are made by what is known as either the Open Hearth or the Bessemer process. In the former the steel is melted in a large furnace and poured from there into large ingot moulds. In the Bessemer method the steel, after melting in a furnace, is poured into a huge barrel-shaped receptacle through which a current of air is forced. In this receptacle, called the converter, the chemical changes take place, after which the molten metal is poured into molds as above. These are the two methods mostly in use today for producing steel in large quantities. The product might generally be termed a mild steel.

But the crucible steels are much more costly, owing to the greater cost of production. In the crucible process the materials used to make the steel are either bar iron, puddled iron or blister steel. The crucibles in which the steel is melted are made of clay or graphite, holding anywhere from 50 to 100 lbs. These crucibles are charged eold. The pieces of iron are placed in the crucible and thoroughly packed with charcoal, mixed with a little manganese, and occasionally a little ferryocyanide of potassium or common salt is mixed with it. The crucible is then covered and placed on the coals in a hot furnace.

After a period of three hours the cover is lifted, the charge having melted by that time and the melter determines upon examining the condition of the charge, the duration of the " killing " period, generally about 45 minues. During this period the metal is becoming tranquil and is absorbing silicon from the walls of the crucible, the latter element preventing blow holes. At the proper time, determined by the experience of the melter, the crucible is removed from the furnace and the molten charge skimmed of its slag, after which the metal is cast or "teemed " by pouring into split ingot molds.

These ingots are graded and converted into bars of various sizes. Owing to the faet that pure materials are used in making up the charge, and that it is protected from the sulphurous gases from the fuel, this crucible steel furnishes the finest grades for cutlery and machine tools. The Bessemer and open hearth steels are very much inferior to crucible steel, the latter having a low percentage of phosphorous, low sulphur, no iron oxide, less gases, high silicon and high carbon.

The crucible steels are graded about as follows, the grades becoming better towards the bottom of the list:

Die. Contains about .75 percent carbon, has little temper, is difficult to burn and welds easily.

Set. Contains about .875 per cent carbon, has little temper, easily welded and burns with difficulty.

Chisel. Contains about 1.00 percent carbon, has a fair temper and is not readily burned.

Spindle. Contains about 1.125 per cent, carbon, welds with difficulty, has hard temper and not very readily burned.

Saw-file. Contains about 1.375 per cent carbon, hard temper and welds with some difficulty.

Razor. Contains about 1.5 per cent carbon, is very easily burned, has very hard temper and welds with extreme difficulty.

The effect of different elements on steel will be of interest at this place:

Carbon. Up to 1.5 per cent, increases tensile strength and raises the elastic limit; the welding power and malleability are decreased.

Silicon. Increased hardness, tensile strength and compression strength, homogenity and prevents blowholes, but too much makes steel brittle. From 3 to 5 per cent should be about the range.

Sulphur causes hot shortness, that is, the metal is brittle when hot, both under the hammer and rolls.

Arsenic. Same effect as sulphur.

Phosphorous. Causes cold shorteness. It makes steel hard and liable to break, but increases the elastic limit and reduces elongation.

Manganese. Prevents hot shortness and blow holes, removes or offsets the effect of sulphur; elongation, toughness and tensile strength are increased by its presence, but too much renders the steel brittle when cold, especially after quenching.

Copper. Causes red shortness.

Tin. Renders steel not forgable or ductile either while hot or cold.

Tungsten. Renders steel very hard.

While tempering was spoken of in the first article, it will be well to speak of the process of hardening and tempering more fully at this place. The combined processes of hardening and tempering steel give to it the necessary hardness combined with the requisite toughness. It takes more than mere hardness of cutting edge to make a good tool. A tool may be so hard that it will readily scratch glass and yet be totally un fit for turning even 6oft metals like lead and copper. This Is due to fact that the process of hardening renders the steel very brittle, while the subsequent process of tempering reduces, or grades, the degree of hardness and at the same time imparts to the steel a certain degree of toughness, which property enables the tool to hold the hardened edge without cracking off readily.

The tool, after forging to shape, is hardened by heating to a cherry red and plunging it into a bath of water, brine or oil, for a short distance above the cutting edge. The tool should be moved about in the cooling medium to prevent the clinging of steam or vapor bubbles, which would cause soft spots. The portion of the tool above the surface of the bath will remain at a dull red heat for some little time.

The hardened end is now quickly polished on a smooth surface by rubbing it with a strip of emery cloth glued to a flat stick. The heat in the stock of of the tool will now begin to creep into the quenched hardened portion by conduction and increase its temperature. As this temperature increases, the oxides will begin to cover the polished surface in distinct bands or waves, advancing in a regular procession as given in the table of tempering colors in first chapter, pale yellow, straw yellow, brown yellow, light purple to a dark purple, and finally a blue. As the desired color reaches the cutting edge, the tool is suddenly plunged beneath the water or oil and constantly kept in motion. This fixes in the steel the definite degree of hardness determined by the color, and the process has rendered the steel tough enough to stand up to its work. The entire process requires practice and thoughtfulness more than anything else to render one proficient There are many tools that will not stand such treatment. The above, of course, applies only to carbon steels. The .self hardening steels should never touch water while hot. They are peculiarly air hardening steels.

One of the oldest and best carbon steels is the Jes-sop's steel. It hardens at a low red heat. In forging do so at a full red heat, but be very careful not to overheat, and avoid a strong blast. Heat uniformly, allowing time for the heat to reach the centre, and turn the tool in the fire so that one side will not be heated more than another. The fire should be of good size and free from sulphur. Do not try to forge steel at a dull heat. This should be used only at the finish. Stubb's steel is probably well known in the form of drill rods. Its treatment is the same as that for carbon steels.

Under the head of self-hardening steels come the Mushet, Jessop's Self-Hardening, Novo, Sanderson's, H. S. H. and C. L. makes. The Taylor-White process and Special steels require special treatment, the method being secret and sold only on shop rights.

In the case of the self-hardening steels, they are forged at a full red heat. To harden they are generally heated to a bright heat and set in a cool, dry place. They are never subjected to the action of water. The Jessop's self-hardenihg steel is heated to a white heat for hardening, as it cannot be burned. As a matter of fact, each special steel is generally accompanied by a label or card, giving directions for working and treating.

In the case of special formed cutter, large dies and taps and large hollow mills, the heating for tempering is generally done in hollow rings or between hot plates or some such method, to insure even, regular heat.

Large tools should be ground on a wet wheel to prevent the heat drawing the temper. Especially is this true with lathe tools. Use as fine a wheel as possible for finishing the edge, which should be smooth and keen.