Extremely corrosive operating environments require a thorough analysis of bearing ring materials to specify the most suitable option. While a majority of situations might ultimately call for the most common and reliable stainless steel, AISI 440C, specific applications demand a more resistant alloy, especially those that regularly expose bearings to high levels of contaminants, water vapor, or acidic solutions. The choice of an alternative stainless steel ring material that is durable enough to withstand such abuse is increasingly likely to be Corrosion Resistant Nitrogen-Enriched Stainless Steel (CREN).
CREN stainless steel, as defined by the AMS 5898 specification, is martensitic steel that possesses high tempering resistance, excellent fatigue resistance, and high hardness levels ranging from 60 to 64 HRc. The increased concentration of nitrogen produces a higher resistance to corrosion and wear than 440C stainless steel. As a result, CREN is finding its way into a variety of aerospace, food and beverage, and medical device applications that expose bearings to highly corrosive substances.
The superior corrosion resistance of CREN proved to be a beneficial solution for one of NHBB’s customers in the medical device manufacturing industry. The customer requested NHBB’s help with resolving a problem related to a pulmonary application. The customer’s original specification called for a radial bearing with 440C stainless steel rings, silicon nitride ceramic balls, a polyamide-imide retainer, and two shields. However, the 440C stainless steel bearing it had specified through another supplier was rapidly corroding in the application’s saline solution. NHBB’s preliminary recommendation was to consider using CREN for the bearings rings instead of 440C because of CREN’s superior corrosion resistance.
Customer testing of the CREN material involved submerging a CREN stainless steel bearing in a proprietary saline solution for several weeks. This testing resulted in favorable preliminary results. To better understand CREN’s characteristics and validate the specification of the CREN material, NHBB consulted the alloy supplier and reviewed the raw material and processing specifications. Test engineers then initiated a series of experiments in the product development and testing lab designed to evaluate the corrosion resistance of different batches of CREN material that had undergone alternative processing sequences. The positive results of these experiments enabled NHBB to adjust its production process in a way that best achieved the required outcome.
Working closely with the customer to understand their application challenges and then engaging in a development and testing cycle to specify the right solution is a routine practice at NHBB. In this case, it led to an approved specification for CREN stainless steel ball bearings. Other situations might dictate the use of different materials.
|Material||Specification||Attributes||Ring Hardness (HRc) at Room Temp.|
|AISI 440C||AMS 5618||58–62||Corrosion resistance|
|BG42®*||AMS 5749||61–65||High temperature tolerance, corrosion resistance|
|CREN||AMS 5898||60–64||Extreme corrosion resistance, long life|
BG42® is a registered trademark of Latrobe Specialty Steel Company.