Steels_ Metallurgy and Applications, Third Edition

222 Steels: Metallurgy and Applications

Table 3.2

Grade C% Si% Mn% Cr% Mo% V% Al%

709M40 0.4 0.25 0.85 1.05 0.3

722M24 0.24 0.25 0.55 3.25 0.55

897M39 0.39 0.25 0.55 3.25 0.95

905M39 0.39 0.25 0.55 1.6 0.2

0.2

1.1

purposes), the grades of steel shown in the Table 3.2 are identified specifically
as being suitable for nitriding.
Given the low temperature involved in the process, nitriding can be carded
out after the conventional hardening and tempering treatments have been applied
to through-hardened steels.

Carbonitriding (nitro-carburizing)

Carbonitriding can be regarded as a variant of gas carburizing in which both
carbon and nitrogen are introduced into the steel surface. This is achieved by
the introduction of ammonia gas into the carburizing atmosphere which cracks,
liberating nascent nitrogen. In some respects, the term carbonitriding may be
misleading in that it is completely different from the nitriding process that takes
place at low temperatures in the ferritic state and the term nitro-carburizing
might be more appropriate. However, the process is carded out at temperatures
of the order of 870"C and the case depths are lower than those produced by gas
carbudzing, e.g. a four-hour carbonitriding treatment at this temperature produces
a case depth of about 0.75 mm.
The introduction of nitrogen produces a significant increase in the hardenability
of the case region such that high surface hardness levels can be produced in steels
of relatively low alloy content.

Induction hardening

In the processes described above, the hardness of the surface is increased by
modifying the chemical composition of this region. In induction hardening, the
composition of the material is unchanged but the surface is hardened by selective
heat treatment. This is achieved by induction heating but a less controlled effect
can be produced by the direct impingement of an oxy-acetylene torch in the
process of flame hardening.
This process is generally applied to steels containing 0.30-0.50% C which give
hardness values in the range 50-60 HRC. The steels may be C-Mn or low-alloy
grades and induction hardening is carded out in the normalized or quenched and
tempered condition, depending upon the section size or the properties required
in the core.

Relative merits of surface treatments

In presenting a detailed review of gas carburizing, Parrish and Harper la examined
the benefits of this process in relation to other surface-hardening treatments such