214 Steels: Metallurgy and Applications- The effect of boron on hardenability is relatively constant provided a minimum
level of soluble boron is present in the steel. - The potency of boron is related to the carbon content of the steel, being very
effective at low carbon contents but decreasing to zero at the eutectoid carbon
level.
Because of its high affinity for oxygen and nitrogen, boron is added to steel
in conjunction with even stronger oxide- and nitride-forming elements in order
to produce metallurgically active, soluble boron. In electric arc steelmaking,
this involves additions of about 0.03% A1 and 0.03% Ti, either separately or
in the form of proprietary compounds containing the required levels of boron,
aluminium and titanium. Without these additions, boron would react with oxygen
and nitrogen in the steel and form insoluble boron compounds which have no
effect on hardenability.
Gladman 8 has shown that the location of boron in steels is very dependent on
the heat treatments that are applied. In low-alloy steels, boron was distributed
uniformly throughout the microstructure of a 25.4 mm bar after water quenching
from the austenitic range. However, in air-cooled samples, the grain bound-
aries were enriched in boron compared with the body of the grains. Ueno and
Inoue 9 also investigated the presence of boron in a 0.1% C 3.0% Mn steel and
showed that boron first segregated to and then precipitated at the grain bound-
aries, according to a typical 'C' curve pattern. They also showed that an increase
in boron content decreased the incubation periods for segregation and precipita-
tion, and in a steel containing 0.002% B, solution treated at 1350~ quenching in
iced brine was required in order to prevent the segregation of boron to the grain
boundaries. It would appear therefore that under normal heat treatment conditions,
involving oil quenching from temperatures of 820-920~ boron segregates to
the austenite grain boundaries and suppresses the formation of high-temperature
transformation products.
LleweUyn and Cook 6 carded out a detailed investigation of the metallurgy
of boron-treated engineering steels containing a wide range of carbon contents.
The effect of boron content on hardenability was studied in a base composition
of 0.2% C 0.5% Ni 0.5% Cr 0.2% Mo (SAE 8620) and the following Jominy
hardenability criteria were examined:
- Jominy distance to [hardness at J 1.25 mm-25 HV].
- Jominy distance to 350 HV, i.e. near the inflexion in the hardenability curve.
- Hardness at J 9.8 mm, equivalent to the cooling rate at the centre of an oil-
quenched, 28 mm bar.
As illustrated in Figure 3.15, each of these criteria reaches a maximum at a
soluble boron content of about 0.0007%. Further additions produce a reduction
in hardenability but a reasonably steady-state condition is achieved at boron
contents in excess of 0.0015%. A similar pattern of results was also observed by
Kapadia et al. l~ and, in commercial practice, it is usual to aim for soluble boron
contents of 0.002-0.003%, accepting a slight loss in hardenability in favour of
a consistent hardenability effect.