Page 168 - Materials Science and Engineering An Introduction

C h a p t e r 5 Diffusion

The first photograph on this page is of a steel gear that has been case
hardened—that is, its outer surface layer was selectively hardened by
a high-temperature heat treatment during which carbon from the sur-
rounding atmosphere diffused into the surface. The “case” appears as the
dark outer rim of that segment of the gear that has been sectioned. This
Courtesy of Surface Division Midland-Ross in Section 10.7), which in turn leads to an improvement of wear resist-
increase in the carbon content raises the surface hardness (as explained

ance of the gear. In addition, residual compressive stresses are introduced
within the case region; these give rise to an enhancement of the gear’s
resistance to failure by fatigue while in service (Chapter 8).
Case-hardened steel gears are used in automobile transmissions,
similar to the one shown in the photograph directly below the gear.

Courtesy Ford Motor Company

© iStockphoto
© BRIAN KERSEY/UPI/Landov LLC

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