Case-hardening Treatments For Various Steels

: The Working Of Steel

Plain water, salt water and linseed oil are the three most common

quenching materials for case-hardening. Water is used for ordinary

work, salt water for work which must be extremely hard on the surface,

and oil for work in which toughness is the main consideration. The

higher the carbon of the case, the less sudden need the quenching

action take hold of the piece; in fact, experience in case-hardening

work gives a gre
t many combinations of quenching baths of these

three materials, depending on their temperatures. Thin work, highly

carbonized, which would fly to pieces under the slightest blow if

quenched in water or brine, is made strong and tough by properly

quenching in slightly heated oil. It is impossible to give any

rules for the temperature of this work, so much depending on the

size and design of the piece; but it is not a difficult matter to

try three or four pieces by different methods and determine what

is needed for best results.

The alloy steels are all susceptible of case-hardening treatment;

in fact, this is one of the most important heat treatments for such

steels in the automobile industry. Nickel steel carburizes more

slowly than common steel, the nickel seeming to have the effect

of slowing down the rate of penetration. There is no cloud without

its silver lining, however, and to offset this retardation, a single

treatment is often sufficient for nickel steel; for the core is not

coarsened as much as low-carbon machinery steel and thus ordinary

work may be quenched on the carburizing heat. Steel containing

from 3 to 3.5 per cent of nickel is carburized between 1,650 and

1,750 deg.F. Nickel steel containing less than 25 points of carbon,

with this same percentage of nickel, may be slightly hardened by

cooling in air instead of quenching.

Chrome-nickel steel may be case-hardened similarly to the method just

described for nickel steel, but double treatment gives better results

and is used for high-grade work. The carburizing temperature is the

same, between 1,650 and 1,750 deg.F., the second treatment consisting

of reheating to 1,400 deg. and then quenching in boiling salt water,

which gives a hard surface and at the same time prevents distortion

of the piece. The core of chrome-nickel case-hardened steel, like

that of nickel steel, is not coarsened excessively by the first

heat treatment, and therefore a single heating and quenching will