Accelerated Cementite Spheroidization in a Hypereutectoid Bearing Steel Through Controlled Divorced Eutectoid Transformation

Cementite spheroidization heat treatments of hypereutectoid steels are of major industrial relevance as a prerequisite for the production of rolling bearing components such as bearing balls, rollers, and races. Owing to their long process durations and high energy demand, optimizing these heat treatments offers significant potential for reducing energy consumption and associated CO 2 emissions. In this work, two fundamentally different cementite spheroidization mechanisms—sub- Ac1 A c1 spheroidization and spheroidization via the divorced eutectoid transformation—are systematically investigated with respect to their microstructural evolution, mechanical properties, and potential for process optimization. This study combines dilatometry, hardness testing, scanning electron microscopy with AI-assisted particle analysis, electron backscatter diffraction, and transmission electron microscopy to establish quantitative correlations between processing parameters, microstructure, and properties. Spheroidization via the divorced eutectoid transformation proves to be superior to sub- Ac1 A c1 heat treatments in terms of time efficiency and achievable spheroidization degree. A broad parameter study covering austenitization temperatures and cooling rates reveals that temperatures slightly above Ac1 A c1 combined with cooling rates of 125\,∘C\,h⁻¹ 125 ∘ C\,h - 1 or lower yield the highest spheroidization degrees and the lowest hardness values.