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Research Papers: Contact Mechanics

Determination of Subsurface Hardness Gradients in Plastically Graded Materials via Surface Indentation

[+] Author and Article Information
Michael A. Klecka, Nagaraj K. Arakere

Department of Mechanical and Aerospace Engineering,  University of Florida, P.O. Box 116250, Gainesville, FL 32611-6250

Ghatu Subhash1

Department of Mechanical and Aerospace Engineering,  University of Florida, P.O. Box 116250, Gainesville, FL 32611-6250subhash@ufl.edu

1

Corresponding author.

J. Tribol 133(3), 031403 (Jul 07, 2011) (5 pages) doi:10.1115/1.4003859 History: Received November 21, 2010; Revised March 07, 2011; Published July 07, 2011; Online July 07, 2011

Graded materials with high surface hardness and ductile cores are popularly used in high performance bearing applications to resist surface wear and fatigue damage. The gradient in hardness with depth is commonly determined using micro-indentation on the cross section of the material which contains the gradation in microstructure or composition. In the current study, a novel method is proposed to predict the hardness gradient profile using solely surface indentations at a range of loads. The method does not require the graded material to be sectioned, and has practical utility in the surface treatment industry. Two case hardened steels, M-50 NiL and Pyrowear® 675, and a through-hardened M50 steel, are used as model materials to illustrate the concepts. For a material with a decreasing gradient in hardness, higher indent loads result in a lower measured hardness due to the influence of the softer subsurface layers. A power-law model is presented which relates the measured surface indentation hardness under increasing load to the subsurface gradient in hardness. It is shown that the response of the material is not influenced greatly by the absolute surface hardness value, but instead sensitive to the sharpness of the gradient in subsurface hardness beneath the indented region. The proposed approach is not specific to case hardened steels and can be used to determine the subsurface hardness gradient for any plastically graded material (PGM).

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Copyright © 2011 by American Society of Mechanical Engineers
Topics: Stress , Steel
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Figures

Grahic Jump Location
Figure 1

Hardness profiles with depth for P675, M-50 NiL, and M-50 materials (trend-lines included for clarity). Unfilled symbols indicate locations of test sections. Inset micrograph reveals indents on the cross section of a specimen.

Grahic Jump Location
Figure 2

Surface hardness versus load for M50 through-hardened and the hardest sections of both graded materials

Grahic Jump Location
Figure 3

Normalized surface hardness as a function of normalized indentation load for all sections and materials tested. The values shown next to each curve label indicates the initial hardness gradient for each section, i.e., change in hardness (kg/mm2 ) over change in depth (mm) on the surface.

Grahic Jump Location
Figure 4

Trend in hardness gradient and power-law exponent for both materials. Arrows indicate the axis to which the data refers to.

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