The quenching medium controls the cooling rate, which is critical to avoid cracking and achieve the desired microstructure.
Oil (Fast Oil): The standard and recommended quenchant for 42CrMo. It provides a cooling rate fast enough to achieve full martensite transformation in sections up to ~100mm due to the steel's high hardenability, but slow enough in the martensite transformation range to minimize thermal stress and distortion/cracking risk.
Water or Brine: Provides much faster cooling. Rarely used for 42CrMo because it drastically increases the risk of quench cracking and severe distortion due to high thermal and transformation stresses. It might be used for very large, simple sections where ultimate hardness is critical and the risk is managed.
Polymer Solutions (e.g., PAG): Can be tailored to cool at rates between water and oil. They are sometimes used as a safer, cleaner alternative to oil. The concentration is adjusted to achieve a cooling rate similar to fast oil for 42CrMo.
The selection is based on section size, part geometry (risk of stress concentration), and the required core properties.