Steels_ Metallurgy and Applications, Third Edition

216 Steels: Metallurgy and Applications

The interaction between boron and carbon was also investigated in SAE 8600
base steels and the Jominy hardenability curves for boron-free and boron-bearing
versions of these steels are shown in Figures 3.13 and 3.16. These figures also
indicate the formation of retained austenite at carbon contents in excess of 0.8%.
However, the interaction between boron and carbon is illustrated in Figure 3.17
where the hardenability criteria examined are the Jominy distances which coin-
cide with decreases in hardness of 50, 100 and 125 HV below that obtained
at the quenched end of the specimen (Hmax). This figure shows that boron
produces a marked increase in hardenability at a carbon content of 0.2% but
the effect is steadily reduced to zero as the carbon content is increased to a
level of 0.53-0.54%. Above this carbon level, boron has a detrimental effect on
hardenability.
The above type of experiment was also carried out in base steels containing
0.8% Mn and the results are summarized in Figure 3.18. In this case, the
efficiency of boron as a hardenability agent is expressed by means of a
multiplying factor B F, calculated on the basis of:

BF=

hardenability of (base steel + boron)

hardenability of base steel

Thus the effect of boron on hardenability is again reduced with increase in carbon
content but, in this particular base composition (0.8% Mn), reaches a value of
zero (BF = 1.0) at a carbon content of about 0.85%.
From the above data, it is evident that the boron effect varies not only with
carbon content but also with the alloy content of the base steel. Llewellyn and
Cook have proposed that the critical carbon content may well correspond to the
eutectoid level for the compositions concerned and that bainite might be promoted
at the expense of martensite in boron-treated, hyper-eutectoid steels.

50"8 f Hmax -i 25 HV 2///~

38.1

25.4 ree~.._........~~.

~, 12.7 l--B-f 53

E E 0 / I! = = I = I J l I l I I = "'

| 50.8 r I"- Hmax -100 HV

25.4 B

.r >" 12.7 B-fr^53

E o 0t. ,'F'- I I I i I I

50.8 I'- Hmax -50 HV

38.1

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Carbon (%)

Figure 3.17 Effect of boron on hardenability at various carbon levels (After Llewellyn
and Cook 6)