Answer
Austempering is a heat-treating process that results in a Bainite microstructure, whereas Martempering produces Tempered Martensite. For spiral retaining rings, which are thin and prone to distortion, Austempering offers several advantages: (1) Reduced distortion because the transformation occurs at a constant temperature above the Martensite start ($M_s$) point. (2) Increased toughness and 'ductile-to-brittle' transition resistance at a given hardness (typically HRC 45-52). (3) Superior fatigue life. Because retaining rings must be expanded/contracted during installation, the higher ductility of the Bainitic structure prevents cracking during the assembly process. Martempering, while effective for larger cross-sections, often leaves residual stresses that can lead to 'quench cracking' in the thin, multi-turn geometry of spiral rings.
