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applied to the polished surfaces and
optical metallography was conducted.
Pearlite interlamellar spacing was
measured using Field Emission Scanning
Electron Microscopy (FESEM) images
taken at a magnification of 10,000x,
employing a circular intercept method
according to ASTM E-112
[9]
. Error bars
reported for all plots represent the
standard error of the data sets.
Results and Discussion
Hardness values are shown in Figure
1 as a function of cooling rate for
both alloys. It is apparent that the
1080V+Nb alloy exhibits greater
hardness values than the 1080V
alloy. Hardness data is only shown for
cooling rates of 2.5 and 5°C/s for the
1080V+Nb steel since non-pearlitic
microstructural constituents including
martensite and occasionally bainite
were observed via FESEM at higher
cooling rates in the Nb containing alloy.
In addition, more martensitic/bainitic
constituents were observed at these
cooling rates in the Nb alloyed steel
versus the 1080V steel as qualitatively
shown in Figure 2 for a cooling rate of
12.5°C/s. Both samples were etched
using a 4 pct Picral etchant and the
regions that appear light correspond
to pearlite whereas the darker regions
not attacked by the etchant correspond
to martensitic regions. Although no
quantitative analysis was conducted,
it is apparent that more martensite
formed in the 1080V+Nb alloy, which
suggests increased hardenability or
slower pearlite transformation kinetics.
The
obtained
hardness
values
corresponding
to
fully
pearlitic
microstructures are also presented in
Table 2 and greater hardness values
are obtained in the Nb alloyed steel.
The hardness values are also observed
to increase with cooling rate for both
alloys.
As evident from Equation (1), pearlite
colony size and ILS contribute to
strengthening in pearlitic steels. In
addition, microalloying may contribute
to
increased
strength
through
precipitation strengthening. Effects of
niobium additions to eutectoid alloys
have received only limited attention;
Jansto reports ILS refinement through
niobium additions in high carbon
construction steels [7]. FESEM
micrographs are shown in Figure 3 for
the alloys investigated in the present
study for constant cooling rates of
2.5 and 5°C/s. A refinement of ILS
and pearlite colony size for the 5°C/s
cooling rate is qualitatively observed for
the 1080V+Nb alloy. Results from ILS
and pearlite colony size measurements
are presented in Table 2 and plotted in
Figure 4 as a function of cooling rate.
Increased ILS refinement is observed
with cooling rate whereas pearlite
colony size seems less dependent
on cooling rate for the 1080V steel.
ILS refinement is observed for both
cooling rates in the niobium alloyed
steel. A different trend with cooling
800
700
600
500
400
0 4 8 12
S
S
Figure 1 – Vickers hardness as a function of
cooling rate
Cooling Rate, °C/s
HV, 1kgf
S
S
Figure 2 – Low magnification FESEM taken from samples continuously cooled from 1,093°C at 12.5°C/s for
a) 1080V and b) 1080V+Nb steels. Samples etched with 4 pct Picral
a)
b)
S
S
Figure 3 – FESEMmicrographs of 1080V (a and c) and 1080V+Nb (b and d) alloys cooled from 1,093°C at
constant cooling rates of 2.5°C/s (a and b) and 5°C/s (c and d)