Engineering steels 221 9 Reheat quenching- cooling to room temperature and then reheating to a
temperature of about 840"C before quenching
9 Double reheat quenching- reheating first to about 900"C and quenching to
produce a fine-grained core, followed by a second quenching treatment from
a lower temperature, such as 800"C, in order to produce a fine-grained case
region.
In case carburizing, single quenching is generally preferred to direct quenching
because it reduces the thermal gradients in the steel, thereby minimizing
dimensional movement or distortion. This topic will be discussed in the next
section. Reheat quenching is employed in pack carburizing and also in gas
carburizing if machining operations must be carried out before the final hardening
treatment. Double reheat quenching is now virtually obsolete since satisfactory
grain refinement can now be obtained in the core and case of modem fine-grained
steels in the shorter quenching practices.
The type of quenching medium employed, e.g. oil or polymer, depends on the
mechanical properties required but, as illustrated shortly, the quenching rate may
have to be controlled very carefully in order to minimize dimensional move-
ment. Following the quenching operation, carburized components are generally
tempered in the range 150-200"C for periods of 2-10 hours in order to produce
some stress relief in the high-carbon martensitic case.
Carburizing is used extensively in the automotive industry for the treatment
of shafts and gears. It is also an important process in the production of large
bearings in which the required level of hardness and fatigue resistance cannot be
achieved in through-hardening grades such as 1.0% C 1.5% Cr (SAE 52100).
Nitriding
As its name suggests, nitriding involves the introduction of nitrogen into the
surface of a steel but, unlike carburizing, it is carried out in the ferritic state
at temperatures of the order of 500-575"C. However, like carburizing, it can
be performed in solid, liquid or gaseous media but the most common is that
involving ammonia gas (gas nitriding) which dissociates to form nitrogen and
hydrogen:
2NH3 ~ 2N + 3H2Nascent, atomic nitrogen diffuses into the steel, forming nitrides in the surface
region.
In salt bath nitriding, mixtures of NaCN and KCN are employed and
the holding times are rarely longer than two hours, compared to periods of
10-100 hours in gas nitriding. A variation of salt bath nitriding is the Sulfinuz
process which involves the addition of sodium sulphide (Na2S) to the bath. This
results in the absorption of sulphur into the steel as well as the introduction of
carbon and nitrogen which is characteristic of conventional salt bath nitriding. The
presence of sulphur in the surface of the component improves the anti-frictional
behaviour and also the corrosion resistance of the steel.
Nitriding is carried out on steels containing strong nitride-forming elements
such as AI, Cr and V and in BS 970 (Wrought steels for mechanical and allied