Bearings are used in rotating equipment to carry axial and radial loads and reduce operating friction. The components of a bearing are subjected to repeated heavy stress loads and must be able to carry those loads while maintaining rotational accuracy and material integrity. To accomplish this goal bearings are made of a material with high hardness that also has good dimensional stability, wear resistance, and rolling fatigue resistance.

These bearings are hardened in 2 different ways. The most common method is Through Hardening, but Case Hardening is done for especially tough applications. Below is a comparison of these 2 methods.

Through Hardening (TH)

Through hardened bearings are exactly what they sound like, the bearing components have the same hardness through the part. The most common bearing steel used for through hardened bearings is AISI 52100 or equivalent. This is a high carbon steel (approx 1.0%) and is hardened by a Martensitic or Bainitic process.

Through hardened bearings made of 52100 steel, have a resulting hardness of approximately 60HRC. The rollers are typically 1-2 HRC harder than the rings to achieve maximum fatigue life.

Through hardened bearings are the standard bearing and are used successfully in most applications in all industries, but there are some especially tough applications where Through Hardened bearings just aren't up to the job.

Case Hardening (CH)

Case hardened bearings have a similar surface hardness of approximately 60HRC but retain a softer, tough core. These bearings are made of low carbon steel (approx 0.2%) and are hardened by diffusing Carbon into the part in an oven.

This combination of a soft, tough core and high hardness surface makes Case Hard bearings very well suited for the toughest applications where bending, torsion and impact loading have to be resisted, and will last longer than the through hardened equivalent.

Case hardened bearings are typically Carburized, but if they are Carbonitrided instead they achieve approximately 64HRC surface hardness and even further gains to wear resistance, fatigue resistance and bearing life.

Through Hardened vs Case Hardened

The increased process time required by the case hardening processes does increase the cost compared to a typical through hard part. In these tough applications the small increase in cost is far outweighed by the many advantages that the case hardened bearing has.

Impact Loading & Toughness:

Case hardened bearings typically have high retained austenite where through hardened bearings have very little. Retained austenite is soft but very tough and has high ductility. These characteristics offer great improvements in fatigue life and impact strength over a through hardened bearing, which is comparatively brittle. Case hardened bearings will last longer in applications with impact loading.

Debris Resistance

The tough core and high retained austenite in case hardened bearings offers a great increase in life in contaminated environments. The debris dent on this case hard part is shallower and narrower (c) because of the toughness but also the radius of hills (r) on either side of this dent are larger minimizing additional damage.

Low retained austenite: sharp edge corners High retained austenite: smooth edge corners

Crack Resistance

Through hardened bearings have a tendency to fail by cracking. Through Hardened bearings have a neutral to tensile residual stress on the surface. When you add to this the hoop stress because of shaft fit and design loads it can approach the total force required to open a crack in the bearing. In Wind applications there are a lot of unknown and variable loads that also get applied to the bearings, and these could very easily push past the strength of the material and can initiate this failure.

The benefit of Case hardened bearings here is they have a compressive residual stress at the surface that introduces more service factor in to this equation and can eliminate crack initiation. If a crack does begin to form however, the compressive residual stress will serve to keep the crack closed in a Case Hard part.

As mentioned above there are 2 different Case Hardening methods applied to bearings; Case Carburizing and Case Carbonitriding. NSK’s tough steel family of materials combines the case carbonitriding heat treatment with special steel alloy formulations to maximize the life of bearings in tough conditions. Read more about Super Tough bearings here.

Conclusions for Wind Applications

In ordinary applications where, rolling fatigue is the only concern through hardened and case hardened bearings can be used interchangeably. In general, only 3-5% of bearing failures are due to material fatigue from load cycles, and 50% are initiated by contamination. Add to that the highly fluctuating loads, speeds, and environmental conditions found in Wind Turbine applications and you will find that case hardened bearings offer great improvement in bearing life.

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-Cory Mittleider

Through Hardened vs Case Hardened Bearings