When a steel tool is cutting at a high speed, or doing other work of a severe character, great heat is generated by friction. If the tool becomes over-heated the temper is drawn and the tool breaks down. Carbon tool-steels fail at a much lower temperature than high-speed steels or certain special alloy-steels.

Therefore, in considering whether a carbon tool-steel may be used for any particular purpose, this is the principal point to watch. If the work to be performed by the tool will not generate much heat, then a carbon tool-steel may be used. Rock - drills, blacksmiths' tools, chisels, files, hammers, etc., are all tools in which carbon tool-steel is efficient.

It is difficult to standardise the numerous steels required for the cheaper types of tools, because of the fine variations in hardness and quality necessitated by differing kinds of work on the one hand, and competition between tool-makers on the other. Work done upon one metal may require a steel of great hardness, while exactly the same work upon another metal may be just as well done with a milder steel.

Messrs. Edgar Allen & Co., Ltd., brand their carbon tool-steels with one of four quality marks, each of which can be supplied in six degrees of hardness (temper), 1 to 6. No. 1 is the hardest. The different uses for the various tempers is as follows :-

Temper No. 1 (1-35% carbon). Suitable for extra-hard planing-, slotting-, and turning-tools, drills, etc.

Temper No. 2 (1-20% carbon). Suitable for lathe-tools, drills, and small cutters.

Temper No. 3 (1-05% carbon). Suitable for large turning-tools, cutters, taps, reamers, drills, shear-blades, punches, blanking tools, etc.

Temper No. 4 (0-90% carbon). Suitable for cold chisels, blades for hot shearing, hot sets, taps, special miners'-drills. etc.

Temper No. 5 (0-75% carbon). Suitable for chisels, sets, blacksmiths' tools, blades for cold shearing, etc.

Temper No. 6 (0-60% carbon). Suitable for boiler-makers' tools, hammers, miners' tools, etc.

Treatment

All tool-steel should be heated slowly, evenly, and thoroughly before it is forged ; and for hardening it should be reheated also slowly and evenly. The result of quick, uneven, and partial heating is to set up strains in the steel, and to render it very liable to crack when plunged into water.

Carbon tool-steels should be hardened at the lowest heat that will ensure the required hardness. When steel has been over-heated, the fracture is coarse, and shows bright specks : whereas properly hardened steel presents a close uniform fracture, similar to that of china.

Too much cold hammering after a tool is formed destroys the structure of the steel, and the cutting edge invariably fails after such improper treatment.

Heating And Forging

The steel should be heated slowly and thoroughly for Tempers Nos. I, 2 and 3, to cherry-red heat (825° C.) and for Tempers 4, 5 and 6, to bright re-heat (950° C), then forged to the shape required, and allowed to cool.

Normalising

After being forged the steel should be normalised before the hardening process is attempted. Normalising unifies the crystalline structure, and removes strains. Heat the steel to 840°-860° C. for Tempers Nos. 1 and 2 ; to 800°-820° C. for Tempers Nos. 3 and 4 ; and to 800°-830°C. for Tempers Nos. 5 and 6. Mold long enough for the heat to penetrate uniformly and then cool off freely in air.

Hardening

Re-heat the steel as uniformly as possible for :-

Temper Nos. 1 and 2 to dull-red heat (780° C).

Temper Nos. 3 and 4 dark cherry-red heat (800° C), and

Temper Nos. 5 and 6, cherry-red heat (825° C), and quench in water.

a practice which has given good results is, when quenching, not to allow the steel to fall below the boiling-point of water, and immediately to proceed with the tempering process. a hardened tool can have the hardening-strains removed by immersing in a bath of boiling water, and it will not alter its hardness to any marked degree.

The following table of approximate colours and temperatures may be used as a guide when a pyrometer is not available.

Tempering

Temper in the usual manner, i.e., rub the part to be tempered with a piece of emery cloth or sand-stone, so that the surface is clean enough to show the colours. The tool is then heated to the hardening-heat and the cutting-edge quenched in water and quickly withdrawn so as to leave sufficient heat in the body of the tool to re-heat the part to be tempered. When the proper colour has reached the part to be tempered, the tool is again quenched. In the case of milling-cutters, etc. the cutter can first be hardened and then warmed up on a plate till the desired colour is arrived at, then quenched again in water.

The following table shows the approximate temperatures of the different colours.

Annealing

Heat the steel in a closed receptacle to 700°-750° C, allowing sufficient time for the heat, to penetrate uniformly, then allow to cool off slowly in the furnace.

Round Carbon Cast Steel Drill-Rods, Or Silver Steel

Edgar Allen silver steel i- specially useful for such tools as borers, punches, reamers, twist-drills, watch and clock parts, electrical work, dental tools, taps, etc.

Heal-Treatment Instructions

To water-harden. Heat very gradually to a dull red (700° C.f 1292° F.) and place vertically into water, moving the steel slowly until cold.

To oil-harden. Heat slowly to a red heat (700° C, 1292° F.), plunge in oil, linseed or olive preferred, and move rapidly until cold. Overheating does not increase the hardness to any appreciable degree. The best heat is the lowest at which the steel becomes glass-hard. Little tempering is then required. A hardening temperature is about 680° C. (1250° F.).

From the above it is clear that to get good results from tool-steel a definite treatment is required, depending upon the steel composition, and its intended purpose. From this it follows that the best treatment for a piece of tool-steel of unknown make or composition can only be told by trial and error, and therefore, for exact results, no piece of unknown origin should be used.

Fig. 196.

Steel Analysis By The Spark

Fig. 190 shows how sparks from various grades of steel held against a grindstone differ : A = magnetised steel (orange colour) ; B = highspeed steel (dark red) ; C = manganese steel (white) ; D = steel containing much carbon (white) ; E = another high-speed steel ; F, G and H = hard, semi-hard and soft steels (white or straw yellow). (" English Mechanics.")