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Research Papers

Influence of Residual Stress and Temperature on the Cyclic Hardening Response of M50 High-Strength Bearing Steel Subjected to Rolling Contact Fatigue

[+] Author and Article Information
Abir Bhattacharyya, Nagaraj Arakere

Department of Mechanical
and Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Ghatu Subhash

Department of Mechanical
and Aerospace Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: subhash@ufl.edu

Bryan D. Allison, Bryan McCoy

SKF Aeroengine North America,
Falconer, NY 14701

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received July 28, 2015; final manuscript received December 3, 2015; published online January 21, 2016. Assoc. Editor: Hareesh Tippur.

J. Eng. Mater. Technol 138(2), 021003 (Jan 21, 2016) (14 pages) Paper No: MATS-15-1173; doi: 10.1115/1.4032321 History: Received July 28, 2015; Revised December 03, 2015

Microstructural and mechanical characterization investigations on three variants of a through-hardened M50 bearing steel are presented to compare and contrast their performances under rolling contact fatigue (RCF) loading. Baseline (BL) variant of M50 steel bearing balls is subjected to: (i) a surface nitriding treatment and (ii) a surface mechanical processing treatment, to obtain distinct microstructures and mechanical properties. These balls are subjected to RCF loading for several hundred million cycles at two different test temperatures, and the subsequent changes in subsurface hardness and compressive stress–strain response are measured. It was found that the RCF-affected subsurface regions grow larger in size at higher temperature. Micro-indentation hardness measurements within the RCF-affected regions revealed an increase in hardness in all the three variants. The size of the RCF-affected region and intensity of hardening were the largest in the BL material and smallest in the mechanically processed (MP) material. Based on Goodman's diagram, it is shown that the compressive residual stress reduces the effective fully reversed alternating stress amplitude and thereby retards the initiation and evolution of subsurface plasticity within the material during RCF loading. It is quantitatively shown that high material hardness and compressive residual stress are greatly beneficial for enhancing the RCF life of bearings.

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Figures

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Fig. 1

Optical micrographs showing carbide morphologies at the core regions of the three M50 steel variants: (a) BL, (b) NT, and (c) MP. (d) Magnified image showing large primary carbides and smaller spherical carbides in BL. Note that carbides in MP are larger than those in BL and NT.

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Fig. 2

Scanning electron micrograph showing the carbide morphologies in BL and the corresponding EDS spectra. (a) Image showing coarse, irregular carbides and (b) corresponding EDS spectra showing the elements present in the carbides. (c) Image showing small spherical carbides and (d) corresponding EDS spectra showing the elements present in the small carbides. Note that the coarse carbides contain Cr, Mo, and V, whereas Fe, Mo, and small amount of V are present in small carbides.

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Fig. 3

(a) Virgin material hardness distribution of M50 steel variants BL, NT, and MP and (b) residual stress distribution over depth

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Fig. 4

(a) Single-ball RCF test rig and (b) magnified image of the contact region between ball and cylinders

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Fig. 5

(a) Deformed track formed due to RCF test and the RCF-affected regions on equatorial and meridial planes and (b) another view of RCF-affected regions on equatorial and meridial plane

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Fig. 6

Schematics of the indentation grid for hardness mapping inside the RCF-affected regions in (a) equatorial section and (b) meridial section

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Fig. 7

Optical micrographs revealing the RCF-affected regions observed after 415 × 106 cycles of RCF loading at low temperature (79 °C) in (a) NT and (b) MP steel balls

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Fig. 8

Optical micrographs revealing the RCF-affected regions observed after 415 × 106 cycles of RCF loading at 135 °C in (a) BL, (b) NT, and (c) MP

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Fig. 9

Magnified optical micrographs of RCF-affected regions of three variants showing preferentially oriented needle-shaped features. The features were observed after 415 × 106 cycles of RCF at 135 °C. No such preferential orientation was observed at lower temperature RCF loading: (a) BL (135 °C), (b) NT (135 °C), and (c) MP (135 °C).

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Fig. 10

Hardness as a function of depth within the RCF-affected regions of the meridial section of the three variants after 415 × 106 cycles at (a) 79 °C and (b) 135 °C

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Fig. 11

(a) RCF-affected region in the equatorial section of BL steel observed after 415 × 106 stress cycles at 135 °C and (b) hardness distribution within this region. The dashed horizontal line shows the virgin material hardness. Considerable hardening and some softening are observed at various depths within the RCF-affected region. The label indicates depth from the surface in micrometer, where the hardness measurements were made.

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Fig. 12

The compressive stress–strain responses of (a) virgin material variants. The stress–strain responses of RCF-affected materials obtained from the equatorial and meridial sections of (b) BL, (c) NT, and (d) MP balls. RCF tests were conducted at 135 °C for 415 × 106 cycles. The yield strength of RCF-affected materials is greater than the virgin materials.

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Fig. 13

Variation of the principal stress components and von Mises stress as a function of depth along the centerline of contact between the ball and the cylinder in our single-ball RCF test at 2.9 GPa Hertzian contact stress level. Note that the maximum von Mises stress occurs at the depth of 0.64b from the surface.

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Fig. 14

(a) Typical von Mises stress cycles at a material point within the RCF-affected zone in the absence of residual stress. Note that the material points are in released compression with a mean stress and (b) von Mises stress cycles in the presence of a compressive residual stress. Note that there is no change in alternating stress despite a change in mean stress. (c) Schematic showing the effect of mean stress on effective alternating stress at a material point according to the modified Goodman's approach and Haigh's approach. An intermediate approach has been selected in the current study.

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