The rate at which austenite decomposes to form ferrite, pearlite and bainite is dependent on the composition of the steel, as well as on other factors such as the austenite grain size, and the degree of homogeneity in the distribution of the alloying elements. It is extremely difficult to predict hardenability entirely on basic principles, and reliance is placed on one of several practical tests, which allow the hardenability of any steel to be readily determined.
The rate at which austenite decomposes to form ferrite, pearlite and bainite is
dependent on the composition of the steel, as well as on other factors such as
the austenite grain size, and the degree of homogeneity in the distribution of
the alloying elements. It is extremely difficult to predict hardenability entirely
on basic principles, and reliance is placed on one of several practical tests,
which allow the hardenability of any steel to be readily determined.
The Grossman test
Much of the earlier systematic work on hardenability was done by Grossman
and coworkers who developed a test involving the quenching, in a particular
cooling medium, of several cylindrical bars of different diameter of the steel
under consideration. Transverse sections of the different bars on which hardness
measurements have been made will show directly the effect of hardenability.
In Fig 1, which plots this hardness data for an SAE 3140
, 0.55-0.75% Cr
, 0.40% C
oil-quenched from 815‹C, it is shown that the full martensitic hardness is only
obtained in the smaller sections, while for larger diameter bars the hardness
drops off markedly towards the center of the bar. The softer and harder regions of
the section can also be clearly resolved by etching.
In the Grossman test, the transverse sections are metallographically examined to
determine the particular bar, which has 50% martensite at its center. The diameter
of this bar is then designated the critical diameter D0. However, this
dimension is of no absolute value in expressing the hardenability as it will obviously
vary if the quenching medium is changed, e.g. from water to oil. It is therefore
necessary to assess quantitatively the effectiveness of the different quenching media.
This is done by determining coefficients for the severity of the quench usually
referred to as H-coefficients. The value for quenching in still water is set at 1,
as a standard against which to compare other modes of quenching.
Using the H-coefficients, it is possible to determine in place of D0, an
ideal critical diameter Di which has 50% martensite at the center of
the bar when the surface is cooled at an infinitely rapid rate, i.e. when H = ‡.
Obviously, in these circumstances D0 = Di, thus providing
the upper reference line in a series of graphs for different values of H.
In practice, H varies between about 0.2 and 5.0, so that if a quenching experiment
is carried out at an H-value of, say, 0.4, and D0 is measured, then the
graph can be used to determine Di. This value will be a measure of
the hardenability of given steel, which is independent of the quenching medium
Fig.1: Steel Ni-0.75Cr-0.4C. Hardness data from transverse sections through
water-quenched bars of increasing diameter
The Jominy and quench test
While the Grossman approach to hardenability is very reliable, other less
elaborate tests have been devised to provide hardenability data. Foremost amongst
these is the Jominy test, in which a standardized round bar (25.4 mm diameter,
102 mm long) is heated to the austenitizing temperature, then placed on a rig
in which one end of the rod is quenched by a standard jet of water (Fig.2).
Fig.2: The Jominy test: A - specimen size; B - quenching rig
This results in a progressive decrease in the rate of cooling along the bar
from the quenched end, the effects of which are determined by hardness measurements
on flats ground 4 mm deep and parallel to the bar axis (Fig. 3). A typical hardness
plot for a En 19B steel containing 1% Cr, 0.25%
Mo and 0.4% C, where the upper curve represents
the hardness obtained with the upper limit of composition for the steel, while the
lower curve is that for the composition at the lower limit. The area between the
lines is referred to as a hardenability or Jominy band.
Additional data, which is useful in conjunction with these results, is the hardness
of quenched steels as a function both of carbon content and of the proportion of
martensite in the structure. Therefore, the hardness for 50% martensite can be
easily determined for a particular carbon content and, by inspection of the
Jominy test results, the depth at which 50 % martensite is achieved can be
Fig.3: The Jominy and quench test
The Jominy test is now widely used to determine hardenabilities in the range
Di = 1-6; beyond this range the test is of limited use.
The results can be readily converted to determine the largest diameter round
bar which can be fully hardened. Fig. 4 plots bar diameter against the Jominy
positions at which the same cooling rates as those in the centers of the bars
are obtained for a series of different quenches. Taking the ideal quench (H = ‡)
the highest curve, it can be seen that 12.5 mm along the Jominy bar gives a
cooling rate equivalent to that at the center of a 75 mm diameter bar. This
diameter reduces to just over 50 mm for a quench in still water (H = 1). With,
for example, a steel which gives 50 % martensite at 19 mm from the quenched end
after still oil quenching (H = 0.3), the critical diameter D0 for a round rod will
be 51 mm.
The diagram in Fig. 4 can also be used to determine the hardness at the center
of a round bar of a particular steel, provided a Jominy end quench test has been
Fig.4: Equivalent Jominy positions and bar diameter, where the
cooling rate for the bar center is the same as that for the point in the
For example, if the hardness at the center of a 5 cm diameter bar, quenched in still
water, is required, Fig. 3 shows that this hardness will be achieved at about 12 mm
along the Jominy test specimen from the quenched end. Reference to the Jominy hardness
distance plot, then gives the required hardness value. If hardness values are required
for other points in round bars, e.g. surface or at half radius, suitable diagrams are
available for use.