Relationship between anisotropic microstructure and strength in cold drawn pearlitic steel wires with oriented pearlite: On the validity of the Hall-Petch equation

This paper studies the relationship between anisotropic microstructure and strength in heavily cold drawn pearlitic steels supplied in the form of prestressing steel wires for prestressed concrete, analyzing the influence of the microstructural changes undergone by eutectoid pearlitic steels during continuous cold drawing on their yield strength. To this end, samples from a real manufacturing process (wire drawing) were studied, and consideration was given to the microstructure evolution as the drawing progresses up to the final commercial product that is heavily drawn and has undergone severe straining. It is seen that the pearlite interlamellar spacing (decreasing with cold drawing) influences clearly the improvement of yield strength (increasing with cold drawing, which is the final aim of manufacturing), although a relationship of the Hall-Petch type cannot be fitted in this case, probably because cold drawing produces not only an increasing closeness of the pearlitic packing, but also a microstructural orientation in the material. To overcome this difficulty, the strength is related to the cumulative plastic strain after cold drawing by means of the Embury-Fisher equation, and also by a modified Hall-Petch expression, where to calculate the distance between barriers against dislocational movement one must consider, apart from the average interlamellar spacing, the average orientation angle.