Steels_ Metallurgy and Applications, Third Edition

Engineering steels 211

linear at carbon contents up to about 0.6% but, depending upon the alloy content
of the steel, the hardness then reaches a plateau and declines at higher carbon
contents. This is due to the depression of the M f temperature below room temper-
ature, leading to incomplete transformation and the presence of retained austenite.
However, because of the marked effect of carbon on hardness, it is important to
bear this point in mind when attempting to compare the hardenability of steels
of different carbon content.
In addition to its effect on hardness, carbon also has a significant effect
on hardenability. This is illustrated in Figures 3.12 and 3.13, which show the
progressive effect of carbon in base steels containing 0.8% Mn and 0.5% Ni
0.5% Cr 0.2% Mo (SAE 8600) respectively. 6 The effect of carbon on the hard-
ness of martensite is evident in both types of steel but the effect on hardenability
is very much more pronounced in the Ni-Cr-Mo steel. However, carbon is
rarely used as a hardenability agent because of its adverse effect on toughness
and its tendency to promote distortion and cracking. In addition, high-carbon
steels are hard and difficult to cut or shear in the annealed condition.
With the exception of cobalt, small additions of all alloying elements will retard
the transformation of austenite to pearlite and thereby increase hardenability.
However, the elements that are most commonly used for the promotion
of hardenability are manganese, chromium and molybdenum but nickel and
vanadium are frequently incorporated for additional purposes. A considerable
amount of work has been carried out, particularly in the United States, to
quantify the effects of the major alloying elements on hardenability and, despite
some complex and interactive effects, the general order of potency has been
established. The published data on this topic have been reviewed very thoroughly

900

800

700

600

o O3 500

>

400

300

200

100

0.88% C

0.76% C

0.66% C

0.59% C

0.41% C

O.3O% C

0.20% C

/ o1~% c

5 10 15 20 25 30 35

Distance from quenched end (mm)

Figure 3.12 Effect of carbon on hardenability of a 0.8% Mn steel (After Llewellyn and

CooP)