Steel For Chisels And Punches

: HARDENING CARBON STEEL FOR TOOLS
: The Working Of Steel

The highest grades of carbon or tempering steels are to be recommended

for tools which have to withstand shocks, such as for cold chisels

or punches. These steels are, however, particularly useful where

it is necessary to cut tempered or heat-treated steel which is

more than ordinarily hard, for cutting chilled iron, etc. They are

useful for boring, for rifle-barrel drilling, for fine finishing

cuts, for drawing dies f r brass and copper, for blanking dies for

hard materials, for formed cutters on automatic screw machines

and for roll-turning tools.



Steel of this kind, being very dense in structure, should be given

more time in heating for forging and for hardening, than carbon

steels of a lower grade. For forging it should be heated slowly

and uniformly to a bright red and only light blows used as the

heat dies out. Do not hammer at all at a black heat. Reheat slowly

to a dark red for hardening and quench in warm water. Grind on a

wet grindstone.



Where tools have to withstand shocks and vibration, as in pneumatic

hammer work, in severe punching duty, hot or cold upsetting or

similar work, tool steels containing vanadium or chrome-vanadium

give excellent results. These are made particularly for work of

this kind.



CHISELS-SHAPES AND HEAT TREATMENT[1]





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.





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 deg.C.

(1,340 to 1,364 deg.F.) according to section. In practice, the first

chisel, is heated to 730 deg.C.; and the second to 735 deg.C. (1,355 deg.F.);

and a 1 in. half round chisel to 740 deg.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 deg.C.

(419 deg.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 deg.C. rise and tested with a file, and at a point between 215 and

220 deg.C. (428 deg.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 deg.C. (1,652 deg.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 1/4 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.