WCA July 2012

High B

Base

high B

B

Base

Stress, MPa

Strength, MPa

Strain, % b)

Strain, % a)

❍ ❍ 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.

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. 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. Conclusions

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 .

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Wire & Cable ASIA – September/October 2007 July/August 2012