The selection of the proper material for a bearing application can be confusing for the bearing end user. This is a general overview of material options for bearings.
First, there are some considerations outside of material properties that one should think about during the bearing selection process:
Material Availability: Beyond the basic bearing materials, many of the high-performance materials can have limited availability and, thus the bearings can have long lead-times. When selecting a bearing type and material, the user should consult with the bearing manufacturers to determine which materials are available for the style of bearing being considered. I.e., if you are looking at a high precision ball bearing, there may be many material options because the base ring materials and ball materials are generally more readily available. If, instead, you are looking at a spherical roller bearing, there may be limited options based on the cost of manufacturing the rollers with a special material.
Additionally, different bearing manufacturers specialize in different materials. Some manufacturers prefer to stick with standard materials and manufacture in high volume. Most ball bearing manufacturers offer at least some of their product line in both 52100 (non stainless) and 440C (stainless). Other manufacturers are more specialized and will offer a variety of materials to suit a wide range of applications.
Cost: The cost difference between bearings with standard materials and special materials is very often much higher than just the difference of the cost of the raw materials. The standard material bearings are generally run in much higher quantity, so the manufacturer is able to amortize the manufacturing cost over much larger quantities. If choosing one of the specialty materials, these bearings would generally be manufactured in much lower quantities, so the cost could be many times higher than the standard material bearing. Consulting with the potential bearing manufacturer or an industry expert will help guide you through the selection process.
Overview of materials:
Standard Non-Stainless:
52100: The baseline bearing material used in for most precision bearings. 52100 is a high carbon chromium alloy steel. It has excellent properties for rolling contact fatigue applications. The material is typically through-hardened, but can be zone-hardened via induction hardening for applications requiring greater ductility. Air melt version is AMS 6440 and VAR is AMS 6444.
8620: AMS 6274. This is a carburizing grade steel used for applications that require ductility or zone hardening.
Standard Stainless Grade:
440C: Air melt is AMS 5630 and AMS 5880. VAR is AMS 5618. This is a high chromium, high carbon grade steel that is widely used in bearing applications. It is the most common "stainless" bearing steel. It has high hardenability and moderate corrosion resistance (other stainless steel grades have higher corrosion resistance). Additionally, 440C has lower rolling contact fatigue resistance compared to 52100 and other bearing material options. In general, this material has roughly 50% of the fatigue life compared to 52100. Designers should consider the drop in fatigue life when determining if a given bearing is adequate for their application.
Specialty Grades:
Non-Stainless:
M50: VIM-VAR is AMS 6491. M50 has excellent rolling contact fatigue resistance. Also, it can be heat-treated to operate in temperatures up to 900 F. It is used in highly critical aerospace applications such as aircraft engines and helicopter rotor systems. It can be used in applications where long life and reliability are critical and also in high temperature applications.
Stainless:
CREN (High Nitrogen Stainless): High nitrogen martensitic stainless steels offer improved fatigue and corrosion resistance over 440C. They offer excellent rolling contact fatigue resistance (similar to M50) with significantly better corrosion resistance when compared to 440C. Available grades are XD15NW (AMS5925) and X30 (AMS5898). Additionally, these steels have a supplemental heat treatment process that can handle operating temperatures up to 900 F (note: The high operating temperature heat treatment process sacrifices some of the corrosion resistance. Therefore, this option should only be used for application requiring operating temperatures over 400 F.)
