Engineering steels 207Examples of both types of presentation are shown in Figures 3.5 and 3.6. A
feature of the temperature-time presentation is the insertion of the cooling curves
used to generate the CCT diagrams and the hardness developed after each cooling
rate is shown in a circle at the end of the cooling curves. This type of presentation
was favoured by Cias 2 of Climax Molybdenum who compiled a series of CCT
diagrams for medium-carbon alloy steels. Temperature-bar diameter plots were
used by Atkins 3 of British Steel in a major publication of CCT diagrams, coveting
a wide range of carbon and alloy steels.
HardenabUity testing
The term hardenability is used in various ways to describe the heat treatment
response of steels, employing one or other of interrelated parameters such as
hardness and microstructure. When evaluated by hardness testing, hardenability
is often defined as the capacity of a steel to harden in depth under a given
set of heat treatment conditions. What must be emphasized in this definition
is the fact that hardenability is concerned with the depth of hardening or the
hardness profile in a component, rather than the attainment of a specific level
of hardness. Using microstructure as the control parameter, Siebert et al. 4 have
defined hardenability as:
'the capacity of a steel to transform partially or completely from austenite to some percentage
of martensite at a given depth when cooled under some given condition'Whereas the hardenability of a steel can be determined from continuous-cooling
transformation diagrams, the construction of such diagrams is both
time-consuming and expensive and therefore more economical methods were
required for the measurement of hardenability. A number of hardenability tests
were developed in the 1930s but the best known and most widely used is the
Jominy end quench test. Developed in imperial units, the Jominy specimen is
a cylinder 102 mm (4 in) long • 25.4 mm (1 in) diameter with a flange at
one end. It is usual to normalize the material to be tested prior to machining
the specimen in order to eliminate variations in the hardening response that
might be introduced by differences in microstructure in the as-rolled condition.
The specimen is heated to the appropriate austenitizing temperature and then
transferred quickly to a fixture which suspends the specimen above a tube through
which a column of water is directed against the bottom face. The arrangement
is shown in Figure 3.7 and it should be emphasized that the water flow is
tightly specified and controlled in order to produce a consistent quenching effect.
Whereas the quenched end of the specimen experiences a rapid rate of cooling,
the effect diminishes along the length of the specimen to give a value approaching
air cooling at the other end. When the quenching operation is complete, fiats are
ground at diametrically opposed positions on the specimen to a depth of 0.38 mm
to remove decarburized material and provide a suitable surface for hardness
testing. This can involve either Vickers (HV) or Rockwell (HRC) hardness testing
at intervals of about 1.5 mm for alloy steels or 0.75 mm for carbon steels. A
typical Jominy hardenability curve is shown in Figure 3.8, which reflects