After quenching, but before tempering, the hardness of 45 steel is greater than HRC55 (up to HRC62) to be qualified.
The highest hardness in practical application is HRC55 (HRC58 for high-frequency quenching).

Quenching hardness:
Quenching is to change the internal structure of steel by rapid cooling, thereby increasing its hardness and strength. For 45 steel, the hardness after quenching can usually reach HRC55-58, and the limit value can even reach HRC62. However, this requires specific conditions to achieve, such as appropriate heating temperature, holding time and quenching cooling rate. It is worth noting that the hardness of 45 steel after quenching may be higher before tempering treatment, so in practical applications, the qualified standard is usually set to a hardness greater than HRC55.
Quenching and tempering hardness:
Unlike quenching, quenching and tempering treatment aims to eliminate the internal stress generated by quenching through tempering, so that the steel can maintain a certain hardness while improving its toughness and plasticity. After quenching and tempering treatment, the hardness range of 45 steel is usually between HRC20-HRC30. This hardness range enables 45 steel to have a certain load-bearing capacity and wear resistance while maintaining good processing performance.

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