Drop Forging Dies

: HEAT TREATMENT OF STEEL
: The Working Of Steel

The kind of steel used in the die of course influences the heat

treatment it is to receive, but this also depends on the kind of

work the die is to perform. If the die is for a forging which is

machined all over and does not have to be especially close to size,

where a variation of 1/16 in. is not considered excessive, a low

grade steel will be perfectly satisfactory.



In cases of fine work, however, where the variation cannot be over

0.005 to 0.01 in. we must use a fine steel and prevent its going

out of shape in the heating and quenching. A high quality crucible

steel is suggested with about the following analysis: Carbon 0.75

per cent, manganese 0.25 per cent, silicon 0.15 per cent, sulphur

0.015 per cent, and phosphorus 0.015 per cent. Such a steel will

have a decalescent point in the neighborhood of 1,355 deg.F. and for

the size used, probably in a die of approximately 8 in., it will

harden around 1,450 deg.F.



To secure best results care must be taken at every step. The block

should be heated slowly to about 1,400 deg.F., the furnace closed tight

and allowed to cool slowly in the furnace itself. It should not

soak at the high temperature.



After machining, and before it is put in the furnace for hardening,

it should be slowly preheated to 800 or 900 deg.F. This can be done in

several ways, some putting the die block in front of the open door

of a hardening furnace and keeping the furnace at about 1,000 deg.F.

The main thing is to heat the die block very slowly and evenly.



The hardening heat should be very slow, 7 hr. being none too long

for such a block, bringing the die up gradually to the quenching

temperature of 1,450 deg.. This should be held for 1/2 hr. or even a

little more, when the die can be taken out and quenched. There

should be no guess work about the heating, a good pyrometer being

the only safe way of knowing the correct temperature.



The quenching tank should be of good size and have a spray or stream

of water coming up near the surface. Dip the die block about 3 in.

deep and let the stream of water get at the face so as to play

on the forms. By leaving the rest of the die out of the water,

moving the die up and down a trifle to prevent a crack at the line

of immersion, the back of the block is left tough while the face

is very hard. To overcome the tendency to warp the face it is a

good plan to pour a little water on the back of the die as this

tends to even up the cooling. The depth to which the die is dipped

can be easily regulated by placing bars across the tank at the

proper depth.



After the scleroscope shows the die to be properly hardened, which

means from 98 to 101, the temper should be drawn as soon as convenient.

A lead pot in which the back of the die can be suspended so as

to heat the back side, makes a good method. Or the die block can

be placed back to the open door of a furnace. On a die of this

size it may take several hours to draw it to the desired temper.

This can be tested while warm by the scleroscope method, bearing

in mind that the reading will not be the same as when cold. If

the test shows from 76 to 78 while warm, the hardness when cold

will be about 83, which is about right for this work.