Experimental observation and numerical modeling of formation of local plastic zones in hardened surface layers due to contact overloading

Abstract

a b s t r a c tThe problem of formation of plastic zones in case-hardened metallic bodies due to contact overloading is studied both experimentally and numerically. Metallic materials exposed to surface hardening demonstrate spatial variation of the material hardness and yield strength with a decreasing profile with depth and belong to the class of so-called plastically graded materials. The presented experimental program employs micro-Vickers hardness tests to map the variation in material hardness and corresponding yield strength for both virgin and loaded case-hardened specimens made of a chromium tool steel. It is shown that, depending on the profile of the yield strength in the near-surface zones and contact parameters, a plastic deformation can originate underneath the hardened layer. The distribution of the effective plastic strain extracted from the micro-hardness increment measurements are found in good agreement with the results of finite element simulations of a plastically graded material subjected to similar loading conditions. Numerical analysis reveals significant perturbations in the stress field distribution within the hardened layer due to formation of a closed-shaped plastic zone in the gradient layers, including development of a tensile stress on the boundary between the elastic and plastic zones as well as an overall increase in the effective stress intensity. It is shown that the hardened layer behaves similar to an elastic beam on a compliant foundation. These stress field perturbations in the hardened layers with low deformation capacity can greatly affect the durability and serviceability of surface treated mechanical parts.

Effects of local overloads on cold rolling mill work rolls

“…Previous experiments and numerical modelling of local contact loading of cylinders with a plastically graded surface layer [15] have shown that plastic deformation initiates with formation of a closed-shape plastic region at some depth without reaching the surface. Maximum plastic strains form at the depths of 1.5 − 2 of half the contact length , which exceed the depth at which the maximum effective stress develops in purely elastic contact conditions (0.785 , according to the Hertz solution).…”

“…We consider a twodimensional problem of a case-hardened half-plane, representing the roll, with a plastically graded surface layer in contact with an elastic circular disc, while the total contact load and the radius of the disc are chosen such that the corresponding loading parameters (mean load pressure p and contact length 2b) are in accordance with the estimates of the illustrative example, described in the previous section ( Table 1). Validation of the model can be found in [15]. The straight boundary eliminates the influence of the contact surface curvature of the plastically graded body on the distribution of stress/strain fields in the near-contact zones.…”

“…Based on accurate experimental measures of micro-hardness distribution in a surface-quenched tool steel [15] (with 0.9% C, 2% Cr) , the yield strength distribution ( ) with depth (along the Y axis) is assumed to be most accurately described by the following Weibull-type exponential function:…”

“…Behavior of the half-plane material beyond the elastic limit is approximated by adopting linear strain hardening. The average value of the hardening modulus at strains of 0-10% is accepted at 3 GPa, based on systematic experimental mapping of micro-hardness increments due to plastic deformation presented in [15].…”