Hub bearing units (HBUs), with ball rolling elements, have been known for a long time and used in high volume automotive wheel bearing applications since the 1970s. Truck hub units (THUs), with rollers, are a variant of HBUs. Because HBU rings, and special quality balls, have to be tolerant to rolling contact and structural loads, the design and related steel selection and heat treatment are not trouble-free. The flanged ring hot forging operation needs to impart the optimum material flow for both the angular contact ball or roller raceway surfaces and the flange bending loads. The manufacturing chain for the flanged rings is generally forging, softening, shot blasting, soft turning, selective induction surface hardening, raceway finishing and flange drilling and taping. The flange structural core strength properties are therefore a function of the steel's metallurgical quality, forging shape and the steel's soft microstructural condition. Cost is a major factor in high-volume automotive and truck bearings. In the early days of production, in Europe, 52100 steel was the only commercially available metallurgically clean bearing steel. With the introduction of HBU production in the United States and Japan the use of simpler carbon steels came about. The well-known 1070 steel was modified to become an ASTM A295-14 rolling bearing quality under the designation 1070M. For cost and strength reasons lower carbon rolling bearing quality, structural steels have been applied under designations such as 1053, C56E2, and 56Mn4. The steel technology cost saving is achieved by their suitability for economical continuous casting methods and the elimination of the postforging softening operation after hot forging. The core strength of 52100 and 1070M steels may be improved by quench and tempering operations. A cost-effective alternative to quench and tempering of carbon steels is micro-alloying with vanadium and nitrogen to improve the as-forged properties.
