Steels_ Metallurgy and Applications, Third Edition

224 Steels: Metallurgy and Applications

produced from 50 kg experimental casts of steel. The specimens were carburized
at 925"C for 5
89 hours at a carbon potential of 0.8%, furnace cooled to 840"C
and then cooled at various rates to room temperature.
It was shown that hardenability has a marked effect on dimensional change,
as illustrated in experimental casts of steel containing 1-3% Cr. The Jominy
hardenability curves for these steels are shown in Figure 3.19 and the dimen-
sional changes that occurred on oil quenching are illustrated in Figure 3.20.
As the hardenability of the steels is increased, progressive contractions occur
in the OD and B D and these are compensated by increases in thickness. This
pattern of results was also repeated in other series of steels of different alloy
content. As the alloy content and hardenability are increased, the transformation
temperatures are depressed from those involving upper bainite to lower bainite
and, ultimately, martensite. With regard to distortion, the consequence of this
progression is the increasing volume change on transformation. Therefore, the
progressively greater movement that results from increasing hardenability is asso-
ciated with the progressive increase in volume on transformation. However, such
an explanation does not address the reason for the irregular changes in shape that
occur in these specimens but these are obviously associated with the complex
stress situation that arises from the differential cooling rates that take place in a
disc or component of limited symmetry.
One departure from the general trend between hardenability and distortion was
observed in the case of boron-treated steels which behaved in an entirely different
manner from boron-free steels of comparable hardenability. A satisfactory expla-
nation for the unique behaviour of boron steels has not been proposed but they
differ from boron-free steels in the following respects:

1. The case hardenability of a boron steel is lower than that of a boron-free steel
   of the same core hardenability.


2. Boron-treated steels will tend to have a higher Ms-Mf transformation range
   than boron-free steels of comparable hardenability.

50 r'- 3.03% Cr

40 2.55% Cr

~ I \

n- 2.05% Cr

-1- \ ~__

30

1.03% Cr 1.50% Cr

Figure 3.19

20 I I I I I ---7

0 10 20 30 40 50

Distance from quenched end (mm)

Effect of chromium on Jominy hardenability (After Lleweilyn and Cook 15)