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Wire & Cable ASIA – September/October 2007
54
July/August 2012
A decrease in tensile strength with boron alloying is again
apparent along with a slight increase in uniform and total
elongation. The number of twists to failure is however
not altered by the alloying whereas a slight decrease in
number of reverse bends is observed with increased boron
levels.
In order to assess aging response of the 1mm drawn wire,
isothermal aging was conducted at 150ºC for one hour and
the results are given in
Table 5
.
A tensile strength increase by about 170MPa is obtained
whereas tensile elongations are reduced to 0.4% uniform
and 1.5% total elongation. Similar elongations were
obtained in all alloys.
Similar twists to failure were again observed in all alloys
albeit at lower levels as for the unaged material.
The trend of reduced reverse bends with increased boron
levels is again observed in the aged condition and about
one bend less is obtained in the aged condition versus the
unaged condition for all steels.
This suggests that the boron alloying does not affect
ductility significantly at the levels of nitrogen investigated.
It should be noted that the nitrogen levels of the present
heats of approximately 40ppm are on the lower end of
industrially produced material.
Conclusions
The effect of boron alloying of 0.80C steels to tie up “free”
interstitial nitrogen was investigated.
Heats with B:N ratios of 1.4 and 2.4 in addition to a base
alloy without boron were laboratory prepared, hot-rolled,
drawn, patented and further drawn to a final diameter of
1mm.
Microstructural characterisation was conducted and
tensile properties were assessed.
Base
High B
B
Base
high B
Strength, MPa
Strain, %
a)
Strain, %
b)
Stress, MPa
❍
❍
Figure 5
: Stress-strain curves of wire a) drawn to 2.5mm and b) patented at 2.5mm
and high7 carbon steels and is also in agreement with
increased hardenability observed in the dilatometry study.
Increased pearlite transformation kinetics may lead to
increased lamellar spacing and/or coarser pearlite. One
might also argue that the reduced strength level may be
related to reduced solid solution strengthening. It should
however be recognised that the B alloy does not exhibit
strength reduction compared to the Base.
It has been suggested previously that the strength
reduction relates to an alloying effect on the austenite to
ferrite1 or pearlite
11
transformation.
Mechanical properties following wire drawing to 2.5mm
diameter are given in
Figure 5a
and
Table 3
.
In the drawn condition, the B steel exhibits the lowest
tensile strength and elongation, the High B steel exhibits
the highest tensile strength and higher elongation
compared to the B steel.
The Base steel exhibits similar uniform and total elongation
compared to the High B steel albeit at a lower tensile
strength. It should be recognised that failures occurred at
the tensile grips which likely influenced the total elongation
values.
Tensile properties obtained after patenting at 2.5mm
diameter are given in
Figure 5b
and
Table 3
.
Similar tensile strengths are obtained in the Base and B
steel whereas the High B steel exhibits an ultimate tensile
strength lower by about 50 MPa.
This lower strength value may again be related to
increased austenite decomposition kinetics. Slightly higher
total elongation is obtained for both boron containing
steels.
The patented wires were subsequently drawn to 1mm
diameter in consecutive passes and resultant tensile
properties in addition to number of twists to failure (Nt) and
number of reverse bends (Nb) are given in
Table 4
.