Often when engineers consider mechanical fatigue they envision cyclic, reversing stresses in a spinning shaft or possibly the up and down flex of aircraft wings. Rolling contact fatigue (RCF) or the associated mechanical wear is an important, closely related process to the traditional mechanism of fatigue but with distinct differences. RCF is also often known as spalling.
RCF or wear may be found in bearings of several types and on gear teeth. With RCF there is both a cyclic compressive or bending stress in rolling and varying degrees of sliding-induced shear stress. For example in ball bearings there is compression of each ball into the inner raceway surface of the bearing that is concentrated because of the small contact area between each ball and raceway. All metals are elastic to varying degrees. Compressive stress induces elastic deformation of the ball and raceway interface and some degree of sliding occurs. The sliding causes shear stresses approximately parallel to the contacting surfaces. Typically these shear stresses are highest just below the metal surfaces and are magnified at internal stress concentration features of the metal such as at brittle inclusions. In intermeshing gear teeth sliding is more significant than in the primarily rolling contact of bearings. In gears sliding induces much more shear stress at the metal surface and this is combined with the repetitive bending stresses caused by interacting gear teeth.
RCF in bearings often begins as circumferential cracks just below the metal's surface. Radial cracks then grow from these cracks. As the process continues, and circumferential and radical cracks join, small surface metal plugs fall out. In gears, RCF damage often appears as a series of small cavities or so called pits. These "pits" are not corrosion pits. In spur gears there is often more metal damage on the smaller diameter gear in the couple because it has fewer teeth and thus those teeth are exposed to more repetitions of bending stress than the larger diameter gear's teeth for a given running period.
Traditional wear processes such as abrasion or adhesion differ from RCF primarily because in the latter cyclic stresses are vital. The traditional wear processes usually include little or no cyclic stresses but sliding dominates. Even so RCF processes are often grouped in with wear processes. Traditional fatigue is different from RCF in the mechanism and type of damage that occurs. Standard fatigue occurs due to crystalline slip caused by repeated stresses that form a crack (or cracks) that eventually grows all the way through the metal cross section until there is too little metal remaining to withstand the applied load. Final fracture due to stress overload then occurs. Thus traditional fatigue often causes catastrophic failure of the metal. RCF typically causes only metal loss at the surface. However, if left unattended RCF can lead to traditional fatigue because of the significant damage and stress risers generated.
RCF in steels is most often controlled by case hardening contacting surfaces to provide increased resistance to the stresses and damage that can result. The goal is to make the surface area metal hard to a sufficient depth but not to through-harden the metal being treated and make it susceptible to brittle fracture.
Different types of metallurgical heat treatment processes are available for case hardening. In all cases carbon, nitrogen or a mixture is diffused into the metal to create a concentration profile - greatest at the surface and decreasing in concentration with depth into the metal. All processes involve three steps. The first is absorption and diffusion of the carbon, nitrogen or mix into the metal surface at a high temperature to create a decreasing carbon or nitrogen concentration gradient to a depth of about 0.030-inch. This is followed by a rapid temperature decreasing step (quenching) that transforms most of the crystalline austenite near the surface to the very hard metal constituent - martensite. The metal is then heated to a much lower temperature than used for diffusing so as to temper or slightly soften the near-surface martensite. The specific parameters used in these treatment steps depend directly on the original carbon content of the steel being case hardened.
There is a desirable finished hardness gradient in a properly hardened case to resist RCF. It is one in which the hardness values decrease very gradually from the maximum surface value. Maintaining relatively high hardness values below the surface is important because the maximum shear stresses in service occur not at the surface but just below the surface. Specifications to achieve this should be stated carefully.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
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