Bearing Steel Cleanliness
Nonmetallic inclusions typically have a negative effect on rolling contact fatigue life. The presence of globular oxides, sulfide, alumina and silicate type inclusions, including carbide banding, can result in a reduction in bearing life. Typically, nonmetal inclusions in steel act as stress risers that cause micro-cracks to propagate to the surface. When the micro-cracks break through the raceway over time, they cause spalling, leading to premature bearing failure.
The good news is, cleanliness levels in bearing steels have increased dramatically since the 1960’s and 1970’s when an attempt to decrease the oxygen content was accomplished through the introduction of the acid hearth furnace. Before that time, there was a large content of oxygen, around 35 ppm. Currently, the oxygen content in bearing steels is around 6 ppm, which has improved bearing life by almost ten times. Advancements in vacuum induction melting and vacuum arc remelting have also significantly improved bearing cleanliness properties, however, on the global level, the older basic electric arc furnace is still in use.
Although there has been a marked improvement in steel cleanliness, differences in bearing performance can still be observed due to varied processing techniques. Testing has shown that two sets of bearing rings, manufactured from the same material but different heat lots can have dramatically different levels of inclusions causing a marked difference in bearing life. The same difference in life and steel can hold true for bearing manufacturers with multiple manufacturing facilities. An identical part number manufactured in two different facilities from the same company can have different steel cleanliness and life. It is a function of the controls put in place to avoid such irregularities.
One way steel cleanliness can be ensured is through the use of a digital metallograph, an instrument with the capability of 1000x magnification. Metallography, used to examine microstructure, grain size, retained austenite and for inclusion content of steel per ASTM E-45, is one of the tools used in our Source Qualification Inspection (SQI) program. The unbiased program identifies the design intentions, manufacturing capabilities and quality of workmanship between bearing manufacturers and even manufacturer's facilities. The SQI allows for risk reduction when qualifying new sources of supply and increases bearing knowledge and design within your own application. Contact us today to learn more about SQI and how it can help your application.