Refining The Grain





This is remedied by reheating the piece to a temperature slightly

above the critical temperature of the case, this temperature

corresponding ordinarily to that of steel having a carbon content

of 85 points, When this is again quenched, the temperature, which

has not been high enough to disturb the refined core, will have

closed the grain of the case and toughened it. So, instead of but

one heat and one quenching for this class of work, we have three

of each, although it is quite possible and often profitable to

omit the quenching after carburizing and allow the piece or pieces

and the case-carburizing box to cool together, as in annealing.

Sometimes another heat treatment is added to the foregoing, for

the purpose of letting down the hardness of the case and giving

it additional toughness by heating to a temperature between 300 deg.

and 500 deg.. Usually this is done in an oil bath. After this the piece

is allowed to cool.



It is possible to harden the surface of tool steel extremely hard

and yet leave its inner core soft and tough for strength, by a

process similar to case-hardening and known as pack-hardening.

It consists in using tool steel of carbon contents ranging from

60 to 80 points, packing this in a box with charred leather mixed

with wood charcoal and heating at a low-red heat for 2 or 3 hr.,

thus raising the carbon content of the exterior of the piece. The

article when quenched in an oil bath will have an extremely hard

exterior and tough core. It is a good scheme for tools that must

be hard and yet strong enough to stand abuse. Raw bone is never

used as a packing for this class of work, as it makes the cutting

edges brittle.





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