HARDNESS MEASUREMENT OF ROLLING BEARING BALLS SUBJECTED TO MULTI-CYCLE CRYOGENIC TREATMENT

Rolling bearings are essential components in a wide range of mechanical systems, where they support loads and enable relative motion with minimal friction. Their operational reliability and service life strongly depend on the mechanical properties of the rolling elements. Among various surface and heat treatment techniques, Deep Cryogenic Treatment (DCT) is a promising method for enhancing the performance of bearing steels. DCT is a process in which materials are cooled to cryogenic temperatures (typically below −150°C), maintained at that temperature for a defined period, and then gradually reheated under controlled conditions. This process leads to several beneficial microstructural changes, such as the transformation of retained austenite into martensite, precipitation of fine carbides, and redistribution of internal stresses. These effects can contribute to improved hardness, dimensional stability, and fatigue life. For this research, bearing balls were selected because of their distinctive cooling patterns. Unlike bearing rings, which generally cool evenly, bearing balls experience notable temperature differences between their outer surface and inner core during cooling. These thermal gradients can influence residual stress distribution and microstructural evolution, making balls an excellent subject for studying the effects of DCT. This study investigates the influence of multicycle Deep Cryogenic Treatment on the hardness of bearing balls made of 100Cr6 steel, intended for use in standard deep groove ball bearings (types 6306, 6308, and 6310). The test specimens were commercially available balls, previously subjected to conventional quenching (Q) and tempering (T). The aim is to determine whether DCT, when applied as an additional treatment after Q-T, leads to significant changes in hardness and whether the treated rolling elements maintain hardness values within the range required for reliable bearing operation.