Steels_ Metallurgy and Applications, Third Edition

Engineering steels 223

as nitriding and induction hardening. Their main findings can be summarized as
follows:

9 Carburizing- capable of producing a wide range of case depths and core
strengths and providing good resistance to bending and contact fatigue. The
main drawback of the process is the distortion that occurs due to the thermal
gradients induced by quenching from the austenitic range.
9 Nitriding - produces relatively shallow hardening, e.g. a highly alloyed grade
such as 897M39 (3
88 % Cr-Mo-V) has an effective case depth (at 500 HV) of
only 0.35 mm after nitriding for 80 hours. However, nitriding is a distortion-
free process that produces a surface which is resistant to scuffing and adhesive
wear.
9 Induction hardening - capable of producing a wide range of case depths using
a range of compositions. Capable of producing similar contact fatigue and wear
resistance to case carburizing and the process produces little distortion.

Distortion in case-carburized components

When transformable steels are heat treated, the volume changes that occur
during heating and cooling (quenching) are not completely complementary and
a component will exhibit a small change in shape compared with its original,
pre-heat-treated condition. The term distortion is widely used to describe such
changes in shape and represents a significant problem in the production of
precision engineering components such as automotive gears. In such components,
slight inaccuracies in shape lead to irregular tooth contact patterns which can
result in problems ranging from a high level of noise in a gearbox or back axle
to an overload situation which produces premature fatigue failure. The effect
is therefore very important commercially but, given that dimensional change is
inevitable under fast-cooling conditions, the approach to the problem is one of
control and consistency of response rather than elimination.
In the mid-1960s, Murray n4 published work on the effects of composition on
distortion in carburizing steels, using the Navy C specimen. This consisted of
a split ring in which the dimensional change is measured by the degree of gap
opening after heat treatment. Whereas some interesting results were obtained in
both the UK and United States with this specimen, it had two major limitations:

1. The unrestrained nature of the split ring could result in gap openings of up
   to 1.2 mm which were very much larger than the dimensional changes that
   occur in automotive gears.


2. The effective section size of the specimen was small which made it unsuitable
   for the investigation of steels with medium to high hardenability.

Llewellyn and Cook n5 therefore designed a new specimen for the investigation

of distortion which was washer-shaped with an outer diameter (OD) of 132 mm,

bore diameter (BD) of 44 mm and a thickness (T) of 22 mm. These dimensions

represented a compromise between the section size that was considered typical

of a medium-size truck gear and the limiting size of specimen that could be