EoW May 2012

Technical article

High B

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

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. was conducted and tensile properties were assessed. Microstructural characterisation

UTS, MPa

UE, %

TE, %

Drawn to 2.5mm Base

1644 1592 1677 1324 1317 1277

1.2 1.0 1.2 7.3 6.7 6.7

1.5 1.1 1.5 8.6 8.9 9.1

B

High B

Patented at 2.5mm Base

B

High B

▲ ▲ Table 3 : Tensile properties Ultimate Tensile Strength (UTS), Uniform Elongation (UE), and Total Elongation (TE) of the wires drawn to 2.5mm and patented at 2.5mm

UTS, MPa

UE, % TE, %

Nt 41 42 41

Nb

Base

2106 2096 2087

1.1 1.3 1.4

2.1 2.4 2.5

12 11

B

High B

9

▲ ▲ Table 4 : Tensile properties Ultimate Tensile Strength (UTS), Uniform Elongation (UE), and Total Elongation (TE) of the wires drawn to 1mm after patenting

UTS, MPa

UE, % TE, %

Nt 35 36 36

Nb

Base

2263 2283 2257

0.4 0.4 0.4

1.5 1.5 1.5

11 10

B

High B

8

▲ ▲ Table 5 : Tensile properties Ultimate Tensile Strength (UTS), Uniform Elongation (UE), and Total Elongation (TE) assessed following aging at 150 ºC for one hour of the wires drawn to 1mm after patenting

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 . 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.

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

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EuroWire – May 2012