Steels_ Metallurgy and Applications, Third Edition

220 Steels: Metallurgy and Applications

period, the energizer breaks down to form CO2 which then reacts with carbon in

the charcoal to form CO:

BaCO3 -, BaO + CO2 (1)

CO2 + C -, 2CO (2)

In turn, the CO reacts at the steel surface to form atomic carbon which defuses

rapidly into the austenitic structure:

2CO ~ C + CO2 (3)

The CO2 produced in this reaction then reacts with charcoal, reproducing reac-
tion (2), and the cycle is repeated.
Pack carburizing is generally carded out at a temperature of 925~ and a
case depth of about 1.5 nun can be obtained after carburizing for eight hours
at this temperature. After this treatment, the component is removed from the
carburizing compound and heat treated by various forms of quenching which
will be described later.
Carbudzing in liquid media generally takes place in molten salts (salt bath
carburizing) in which the active constituent is sodium cyanide (NaCN), potassium
cyanide (KCN) or calcium cyanide Ca(CN)2. Oxygen is available at the salt
bath-atmosphere interface and carburizing again takes place via the generation
of CO. However, nitrogen is also liberated from the cyanides and diffuses into
the steel. The amount of carbon and nitrogen absorbed by the steel is related
to the temperature and cyanide content of the bath. With a NaCN content of
50%, the surface concentrations of carbon and nitrogen are of the order of 0.9%
and 0.2% respectively, after treatments involving 2
89 hours at 900~ However,

salt bath carburizing is mainly used for small parts and treatments for
89 hour
produce a case depth of about 0.25 mm.
Gas carburizing is carded out in hydrocarbon gases such as propane (C3H8)
or butane (C4HI0) in sealed furnaces at temperatures of about 925~ Major
strides have been made in the control technology of the process and, since the
1950s, gas carbudzing has become the most important method of case hardening.
During the heating-up stage, the component is surrounded by an inert or reducing
atmosphere. This atmosphere is referred to as the carrier gas, which is commonly
an endothermic gas but may also be nitrogen based. On reaching the carburizing
temperature, the furnace atmosphere is enriched to the required carbon level
(carbon potential) by the addition of hydrocarbons which generate CO and carbon
is absorbed at the surface of the steel. The carbon potential is controlled by
varying the ratio of hydrocarbon to carder gas but a surface carbon content of
0.8-0.9% is generally employed. A four-hour treatment at 925~ will produce a
case depth of about 1.25 ram.
Following the carburizing operation, the components are subjected to hardening
heat treatments, involving different forms of quenching:

9 Direct quenching - quenching directly from the carbudzing temperature
9 Single quenching- allowing the component to cool to a temperature of about
840~ from the carburizing temperature before quenching