CHISELS-SHAPES AND HEAT TREATMENT[1]
[Footnote 1: Abstract of paper by HENRY FOWLER, chief mechanical engineer of the Midland Ry., England, before the Institution of Mechanical Engineers.]
In the chief mechanical engineer's department of the Midland Ry., after considerable experimenting, it was decided to order chisel steel to the following specifications: carbon, 0.75 to 0.85 per cent, the other constituents being normal. This gives a complete analysis as follows: carbon, 0.75 to 0.85; manganese, 0.30; silicon, 0.10; sulphur, 0.025; phosphorus, 0.025.
The analysis of a chisel which had given excellent service was as follows: carbon, 0.75; manganese, 0.38; silicon, 0.16; sulphur, 0.028; phosphorus, 0.026. The heat treatment is unknown.
FIG. 83.—Forms of chisels standardized for the locomotive shops of the Midland Ry., England.
At the same time that chisel steel was standardized, the form of the chisels themselves was revised, and a standard chart of these as used in the locomotive shops was drawn up. Figure 83 shows the most important forms, which are made to stock orders in the smithy and forwarded to the heat-treatment room where the hardening and tempering is carried out on batches of fifty. A standard system of treatment is employed, which to a very large extent does away with the personal element. Since the chemical composition is more or less constant, the chief variant is the section which causes the temperatures to be varied slightly. The chisels are carefully heated in a gas-fired furnace to a temperature of from 730 to 740°C. (1,340 to 1,364°F.) according to section. In practice, the first chisel, is heated to 730°C.; and the second to 735°C. (1,355°F.); and a 1 in. half round chisel to 740°C., because of their varying increasing thickness of section at the points. Upon attaining this steady temperature, the chisels are quenched to a depth of 3/8 to 1/2 in. from the point in water, and then the whole chisel is immersed and cooled off in a tank containing linseed oil.
The oil-tank is cooled by being immersed in a cold-water tank through which water is constantly circulated. After this treatment, the chisels have a dead hard point and a tough or sorbitic shaft. They are then tempered or the point "let down." This is done by immersing them in another oil-bath which has been raised to about 215°C. (419°F). The first result is, of course, to drop the temperature of the oil, which is gradually raised to its initial point. On approaching this temperature the chisels are taken out about every 2°C. rise and tested with a file, and at a point between 215 and 220°C. (428°F.), when it is found that the desired temper has been reached, the chisels are removed, cleaned in sawdust, and allowed to cool in an iron tray.
No comparative tests of these chisels with those bought and treated by the old rule-of-thumb methods have been made, as no exact method of carrying out such tests mechanically, other than trying the hardness by the Brinell or scleroscope method, are known; any ordinary test depends so largely upon the dexterity of the operator. The universal opinion of foremen and those using the chisels as to the advantages of the ones receiving the standard treatment described is that a substantial improvement has been made. The chisels were not "normalized." Tests of chisels normalized at about 900°C. (1,652°F.) showed that they possessed no advantage.
Tools or pieces which have holes or deep depressions should be filled before heating unless it is necessary to have the holes hard on the inside. In that case the filling would keep the water away from the surface and no hardening would take place. Where filling is to be done, various materials are used by different hardeners. Fireclay and common putty seem to be favored by many.
Every mechanic who has had anything to do with the hardening of tools knows how necessary it is to take a cut from the surface of the bar that is to be hardened. The reason is that in the process of making the steel its outer surface has become decarbonized. This change makes it low-carbon steel, which will of course not harden. It is necessary to remove from 1/16 to ¼ in. of diameter on bars ranging from 1/2 to 4 in.
This same decarbonization occurs if the steel is placed in the forge in such a way that unburned oxygen from the blast can get at it. The carbon is oxidized, or burned out, converting the outside of the steel into low-carbon steel. The way to avoid this is to use a deep fire. Lack of this precaution is the cause of much spoiled work, not only because of decarbonization of the outer surface of the metal, but because the cold blast striking the hot steel acts like boiling hot water poured into an ice-cold glass tumbler. The contraction sets up stresses that result in cracks when the piece is quenched.