Abstract: In today’s rolling bearing industry, the demand for more and more reliable and effective products is continuous. For standard operating conditions the rating lives of bearings, with ideal internal geometries and correct mounting, are ultimately established by the quality of the steels used in the manufacturing process. The present paper presents an overview of the importance of the proper selection of the bearing steel for manufacturing high quality rolling bearings, according to the highest international standards. In addition, the complex interrelation between steel cleanliness and adequate heat and thermo-chemical treatments applied to bearing components is pointed out.
Key words: Bearing steel, cleanliness, fatigue life, heat treatment, metallography, non-metallic inclusions
Rolling bearings are one of the most frequent machine elements and play a key role in the reliability of the majority of industrial equipment. In this respect, the selection of the proper bearing material and its processing are of the utmost importance.
Over the years, the specific requirements of rolling bearings, i.e. rolling contact fatigue strength, resistance to contact deformation and wear, toughness, machinability and hardenability, conducted to a special grade steel commonly known as bearing steel .
There are two major rolling bearing steel variants :
– the through-hardened steels (table 1), for which, after the final heat treatment, the hardness attains the high value of 58-64 HRC in the entire volume if the bearing components;
– the case-hardened steels (table 2),for which a carburizing treatment increases the carbon content up to 0.65-1.10 % only in a superficial layer of 0.7-3.5 mm thickness, where a proper hardening and tempering treatment ensures the needed high hardness of 58-64 HRC,while maintains the rest of the material at a much lower hardness 0f 32-48 HRC that provides a better operating toughness.
The appropriate steel selection for the manufacture of rolling bearings is based on the particular bearing industrial application, geometry, size, production technologies and associated costs .
One major factor that determines the rolling bearing fatigue life is the homogeneity and purity or cleanliness of the steel. A given steel can be considered clean if its content of impurities, such as phosphorous, sulphur, silicon and aluminum, is very low. These impurities in combination with oxygen form non-metallic inclusions .
If two groups of identical rolling bearings were manufactured using the same type of materials, but from different lots, which means possible different contents of non-metallic inclusions, a considerable difference in the distribution of the actual fatigue lives might be expected . The existing non-metallic inclusions act as stress risers able to initiate micro-cracks during the repeated contact stresses introduced by the rolling load in the shallow layer of the raceways. Eventually these micro-cracks can break through the raceway surface and in time spalling may occur and lead to total bearing failure (figure 1) .
To obtain an appropriate cleanliness grade it is necessary to control and improve a wide range of operating practices during the steel manufacturing process, like deoxidant and allow additions, secondary metallurgical treatments and casting activities .