1. Typical Hardness Range
Carbon steel (such as 45# steel): The hardness after heat treatment is typically HRC25~35 (corresponding to HBW210~250). When quenched but not tempered, it can reach HRC55~62, but the hardness decreases significantly after tempering.
Alloy steel (such as 40Cr): The hardness is slightly higher, approximately HRC28~38, because alloying elements inhibit tempering softening.
2. Factors Affecting Hardness
Tempering temperature: For every 100℃ increase in tempering temperature, the hardness decreases by approximately HRC3~5 (e.g., the hardness of 45# steel after tempering at 600℃ decreases to HRC28).
Wall thickness: The hardness of the core of thick-walled tubes is 5~10 HRC lower than the surface due to insufficient hardenability caused by differences in cooling rate.
Alloying elements: Molybdenum (Mo) or chromium (Cr) can delay tempering softening and improve hardness stability.
3. Special Process Adjustments
Surface hardening: Heat treatment + high-frequency quenching can achieve a surface hardness of HRC58~62, while the core maintains HRC30~35.
Low-temperature tempering: Tempering at 150~250℃ can retain higher hardness (HRC40~45), but the toughness is reduced.
