EoW May 2012

Technical article

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 .

Temperature, °C

Time, s

▲ ▲ Figure 3 : Transformation start (squares) and finish (triangles) temperatures for different constant cooling speeds. Filled symbols: base alloy and open symbols: B steel

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

steel. In addition, the B steel exhibited a larger pearlite transformation region. Stress-strain curves and tensile properties of the hot rolled rods are given in Figure 4 and Table 2 . The Base and B steels exhibit very similar stress-strain behaviours albeit that the B steel exhibits a yield point elongation (YPE) whereas the Base steel exhibits continuous (ie smooth, “round-house”) yielding. The occurrence of YPE might be somewhat unexpected as the alloy was designed to have nitrogen tied up to boron and the YPE should hence not result from “free” nitrogen strain aging. The behaviour hence presumably relates to carbon strain aging. It should be recognised that the rods were straightened at room temperature following hot rolling, and non uniform strain during straightening may have led to removal of YPE in some cases. Similar tensile strengths and elongations were obtained in the Base and B steel. The High B steel exhibited lower strength values; smooth yielding is observed at lower strengths compared to the other steels and an ultimate tensile strength value lower by about 25 MPa was obtained. This strength difference cannot be ascribed to carbon as samples with the same carbon content were selected for testing. A higher tensile elongation was exhibited by the High B steel. It is interesting to note that reduced tensile strength with boron alloying is in agreement with earlier work on low1

▼ ▼ Table 2 : Tensile properties of the hot-rolled rods

UTS, MPa

UE, %

TE, %

Base

952 951 926

9.4 8.2

13.7 13.9 16.6

B

High B

11.2

▼ ▼ Figure 4 : Stress-strain curves of the hot-rolled rods

High B

Base

High B

Base

Engineering stress, MPa

Engineering strain, %

66

EuroWire – May 2012