The Quenching Tank

: CASE-HARDENING OR SURFACE-CARBURIZING
: The Working Of Steel

The quenching tank is an important feature of apparatus in

case-hardening--possibly more so than in ordinary tempering. One

reason for this is because of the large quantities of pieces usually

dumped into the tank at a time. One cannot take time to separate

the articles themselves from the case-hardening mixture, and the

whole content of the box is droped into the bath in short order,

as exposure to air of the heated w rk is fatal to results. Unless

it is split up, it is likely to go to the bottom as a solid mass,

in which case very few of the pieces are properly hardened.






A combination cooling tank is shown in Fig. 38. Water inlet and

outlet pipes are shown and also a drain plug that enables the tank

to be emptied when it is desired to clean out the spent carburizing

material from the bottom. A wire-bottomed tray, framed with angle

iron, is arranged to slide into this tank from the top and rests

upon angle irons screwed to the tank sides. Its function is to

catch the pieces and prevent them from settling to the tank bottom,

and it also makes it easy to remove a batch of work. A bottomless

box of sheet steel is shown at C. This fits into the wire-bottomed

tray and has a number of rods or wires running across it, their

purpose being to break up the mass of material as it comes from

the carbonizing box.



Below the wire-bottomed tray is a perforated cross-pipe that is

connected with a compressed-air line. This is used when case-hardening

for colors. The shop that has no air compressor may rig up a

satisfactory equivalent in the shape of a low-pressure hand-operated

air pump and a receiver tank, for it is not necessary to use

high-pressure air for this purpose. When colors are desired on

case-hardened work, the treatment in quenching is exactly the same

as that previously described except that air is pumped through

this pipe and keeps the water agitated. The addition of a slight

amount of powdered cyanide of potassium to the packing material

used for carburizing will produce stronger colors, and where this is

the sole object, it is best to maintain the box at a dull-red heat.




is necessary.]



The old way of case-hardening was to dump the contents of the box

at the end of the carburizing heat. Later study in the structure

of steel thus treated has caused a change in this procedure, the

use of automobiles and alloy steels probably hastening this result.

The diagrams reproduced in Fig. 39 show why the heat treatment of

case-hardened work is necessary. Starting at A with a close-grained

and tough stock, such as ordinary machinery steel containing from 15

to 20 points of carbon, if such work is quenched on a carbonizing

heat the result will be as shown at B. This gives a core that is

coarse-grained and brittle and an outer case that is fine-grained

and hard, but is likely to flake off, owing to the great difference

in structure between it and the core. Reheating this work beyond

the critical temperature of the core refines this core, closes

the grain and makes it tough, but leaves the case very brittle;

in fact, more so than it was before.