What are White Etching Cracks in Roller Bearings?

April 2, 2019

Appearing as pale hairline fissures, white etching marks emerge on bearing surfaces when application stress conditions exert heavy kinetic energies. The true material defect is hidden inside the alloy, so the etching cracks are the tip of the proverbial iceberg here. They’re a sign of a deeper issue. There are no two ways about it, a premature bearing failure lays somewhere in the near future.

What Are White Etching Cracks?

In bearings, they’re a sign of a deeper material issue. Raceway microcrystalline structures develop a fine network of White Etching Cracks (WEC), which take their toll on the rolling elements. Spalling takes place next, with tiny slivers of metal flaking away between the propagating fissures. And that’s just the direct surface effect, which is bad enough. If we’re to resolve the etching effect, the fault-finding procedure has to go deeper. Only then will it be possible to discover the root cause of this fatigue inducing phenomenon.

A Root Cause Study

The effect looks like the tiny cracks that appear on an egg before it cracks open. Alloy structures are clearly a great deal stronger than a fragile eggshell. Still, the causative factors remain the same; some kind of internalized force is pulling the steel apart. Internal forces cause the damage below the surface of the bearing raceways, but a small measure of the stress-energy travels upwards until it forms this white etching pattern. Microplastic deformation, loading stress, work hardening flaws and more, all of these stress multipliers tear at the steel grain from different directions until a structural deformation breaks out into a fully formed subsurface stress fracture.

Microscopic Examinations

Fatigued bearings go under a microscope, where they’re studied by an expert eye. That deeply embedded stress fracture is hidden, but it gives itself away by transmitting the etching cracks to the surface. By the way, the white micro-fissures are that pale colour because they lack heat treated carbide particles. The hairline network of cracks is probably even beginning to spall, which means the isolated islands of formerly unmovable metal will simply flake away and leave the raceways scarred. Such coarse surfaces will soon kick the device out of service as it squeals and vibrates.

The steps are simple enough to follow. A bearing experiences loading stress or some transient energy spike, then that concealed stress zone sends crack energies rippling to the surface, where they appear as micro-fine white cracks. The white etching cracks spread, the metal surface spalls, and the product fails prematurely. All is not lost, though. Knowing the causes, and the effects, the equipment conditions are assessed before a new bearing is pressed into service.

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