Melting
Melting
Steel cleanness is critical to the performance of most bearing applications, and literature has shown a direct relationship between how many inclusions are present in the steel and bearing life.
There are many steelmaking processes, but most bearings are manufactured with steel from either a continuous strand-cast or ingot-cast process. Strand-cast steelmaking is a more efficient process because you make a continuous bloom of steel and simply cut it to length. As a result, there is less waste.
There are two types of continuous casting methods: a curved caster and a newer vertical-casting process. My experience has been that the vertical casters produce very clean material compared to the more traditional curved caster. There are very few vertical casters that make bearing-quality steel because this technology is relatively new.
Ingot casting involves bottom pouring liquid steel into molds and waiting for them to solidify. Each ingot mush then be trimmed or cropped to remove porosity and inclusion material. While this generally produces very clean steel, when well controlled, it is not as efficient as continuous casting.
Visiting many steel mills around the globe, it is my experience that the true secret to making the cleanest steel is not as much the equipment but the attention to detail and the process-control discipline by knowledgeable people. While the basic measure of cleanness is the oxygen content in the steel, this only gets you in the stadium. To get the best seat, or very clean steel, you must measure inclusion results in finer detail with ultrasonic testing, extreme value analysis or a similar technique. You must then connect the results with process controls.
Beyond the standard bearing performance needs, ultra-clean steels will be made using electro-slag remelting (ESR), vacuum remelting (VAR) or vacuum induction melting plus vacuum arc remelting (VIM-VAR). These are techniques that melt under vacuum to prevent oxidation of the steel (harmful inclusions are usually oxides) or slowly melt an ingot a second time to remove inclusions. A price premium accompanies this extra processing.
Click on an image to view it in full size.