Microstructure and Properties of Pearlitic Steel during Cold Wire Drawing: A Residual Stress Perspective*

Abstract

The residual stress evolution during cold drawing of pearlitic steel wire was followed over a broad strain range by diffraction techniques. The present work discusses possible links between microstructural changes induced by severe deformation and their “footprints” left in the residual stresses on different – macro, micro and nano – scale levels. Energy dispersive synchrotron diffraction revealed a significant divergence in the grain microstress evolution among differently oriented ferrite grains in the high deformation regime beyond an accumulated true strain of about ∊t ≥ 2.3. A possible physical reason for the observed divergence is discussed in terms of a distinct microstructure development in this stage of the cold wire drawing.

Evolution of Residual Micro Phase and Orientation Dependent Stresses during Cold Wire Drawing

“…Moreover, one can clearly observe a trend of the ferrite grain microstress saturation in the range of ε t = 0.6 -2.3. This finding fits with the present (growing tensile) total phase stress evolution and the assumed predominant role of macrostress in this regime [2,3]. Analogously to the reduction of axial internal stresses during loading of pearlitic steel [5], a competition between elastic and plastic anisotropy in the ferrite leads to the monotonous residual grain microstress evolutions.…”

“…This can be attributed to inhomogeneous elastic and plastic deformation of cementite lamellae and ferrite matrix, respectively [2]. Recently, a microstructural analysis pointed to a dependence of the evolutions of the apparent interlamellar spacing and cementite lamellas thickness on the alignment of the lamellae with respect to the drawing direction [3]. It has been suggested that the cementite lamellae of pearlite undergo predominantly elastic deformation during the first passes.…”

“…This speculation would support the assumption of a sharp onset of cementite decomposition to influence considerably the micromechanical response of pearlitic steel during severe cold deformation. It should be noted, however, that significant microstructural changes like the end of the lamellae alignment in drawing direction, intense thinning and fracture of cementite lamellae related to the dissolution process [3,[8][9][10] Residual phase microstress and macrostress evolution. The average residual ferrite phase microstresses were determined from the residual stress values recorded for the particular {hkl} lattice plane families (taking into account their multiplicity factor related to a number of equivalent scattering planes).…”

“…can be excluded as a constitutive factor behind the rapidly changing behavior of the (surface) total phase stress evolution during cold drawing at high strain levels. Consequently, it is speculated that the physical reason for the remarkable "oscillatory" behavior at large strains could consist in the distinct microstructure development over the interval of the accumulated true strains ε t ≈ 1.8 -2.3 and further beyond this level [2,3,[8][9][10]. Therefore, this microstructure change and its footprint in the stress state is discussed hereinafter based on different length scales on which the internal and residual stresses arise and accumulate respectively during severe cold drawing.…”

“…Moreover, complementary neutron diffraction investigations [2] revealed a similar trend and showed comparable results of the ferrite phase microstress evolution in the first stage of cold drawing (see later discussion). A total residual ferrite phase stress pattern from the {211} reflection (recorded by laboratory XRD) [2,3] is employed as reference stress evolution for an investigation of the "footprints" left by (1) microstructural changes on different length scales and (2) mechanical responses of the examined wires to externally applied strain/stress fields. The {211} total phase stress pattern (i.e.…”