20Cr is generally not recommended for automotive transmission gears, particularly for medium-to-heavy duty applications. While 20Cr is a standard carburizing steel capable of achieving surface hardness of 58–62 HRC after carburizing and quenching, its hardenability is relatively low. The critical diameter in water is only approximately 20–35 mm, which limits its ability to maintain sufficient core strength and toughness in larger gear sections. Automotive transmission gears typically experience high contact stresses and impact loads; under such conditions, 20Cr may exhibit inadequate core support, leading to potential fatigue spalling or fracture. Industry practice favors medium-hardenability carburizing steels such as 20CrMnTi or 20CrMo for transmission gears, as these grades provide better hardenability and the balanced "hard surface, tough core" property required for reliable service. Therefore, 20Cr is better suited for small, lightly loaded components rather than demanding transmission gear applications.
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