The hardness of 42CrMo is directly controlled by its heat treatment, determining whether it is suitable for machining or ready for high-stress service. The spectrum ranges from soft for shaping to hard for final use.
In the Annealed (+A) condition, the steel is slowly cooled to produce its softest state, with a typical hardness of 180-220 HB. This creates a microstructure ideal for easy chip removal, allowing for heavy machining, drilling, and tapping with excellent tool life. It is the standard starting condition for components that will be shaped before final hardening.
The Quenched and Tempered (+QT) condition is the state of final use. Here, the steel is oil-quenched to form a hard martensitic structure and then tempered. The hardness in this state is not a single value but a range dictated by the tempering temperature, which balances strength and toughness. For most high-strength applications, a tempering range of 550-600°C is used, resulting in a hardness of 270-320 HB and a tensile strength of 900-1100 MPa. While machining is possible in this state ("hard machining"), it requires specialized tooling and is typically limited to light finishing or grinding operations.
| Condition | Typical Hardness (HB) | Primary Microstructure | Purpose & Implication |
|---|---|---|---|
| Annealed (+A) | 180 - 220 | Ferrite + Spheroidized Carbides | Optimal machinability. Used for all major shaping operations. The part is not for service in this state. |
| Quenched & Tempered (+QT) | 270 - 320 | Tempered Martensite | Final high-strength state. Ready for service after final grinding/finishing. Poor machinability, excellent wear resistance. |