PSI - Issue 3

Jesús Toribio et al. / Procedia Structural Integrity 3 (2017) 3–10 Author name / Structural Integrity Procedia 00 (2017) 000–000

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5. Relationship between microstructure and strength One important issue in cold-drawn pearlitic steel wires is the relationship between microstructure and strength (Toribio, 2004), i.e., the effect of microstructural evolution on the (macroscopic) mechanical properties (Toribio et al., 2014), since the final aim of the manufacture process by cold drawing is the improvement of mechanical properties (increase of strength) by a strain hardening mechanism. Fig. 6 plots the stress-strain curves for the progressively cold-drawn pearlitic steels form A0 (hot rolled steels, not cold drawn at all, 0 drawing steps) to the commercial prestressing steel wire A6 (heavily cold drawn pearlitic steel that has undergone 6 drawing steps). It is seen that both the yield strength  Y and the ultimate tensile strength (UTS  max ) increase with the drawing degree.

2.0

1.5

1.0

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 A6 A5 A4 A3 A2 A1 A0  (GPa)  0.5

Fig. 6. Stress-strain curves of the progressively drawn pearlitic A0 to A6 (from 0 to 6 cold-drawing steps).

With regard to the materials-science relationship between microstructure and strength, the Hall-Petch equation has been proposed by Choi and Park (1996) and Nam et al. (2002) to correlate the pearlite interlamellar spacing and the material strength. It is based on the classical research works by Hall (1951) and Petch (1953). Although the Hall Petch equation seems to be effective to describe the relationship between microstructure and strength in randomly oriented pearlitic microstructures, it does not properly work for drawn pearlite, i.e., for the case of oriented pearlitic microstructures (Toribio, 2004). However, for cold-drawn pearlite oriented in a direction quasi-parallel to the wire axis or cold-drawing direction, an Embury-Fisher equation (Embury and Fisher, 1966) has been proposed by Toribio et al. (2014) to describe the relationship between microstructure and strength in cold-drawn pearlitic steels (Fig. 7).

2.0

1.5

1.0

 R (GPa)

 

0.5

max

Y

0.0

1.0 1.1 1.2 1.3 1.4 1.5

exp   P /4)

Fig. 7. Embury-Fisher relationship betwwen strength (  Y and  max ) and cumulative plastic strain 

p after drawing.