The hardness of 17-4PH directly depends on the aging temperature—the lower the aging temperature, the higher the hardness. The typical range is 30–45 HRC.
Hardness at Different Aging Temperatures (Typical Process: 4 hours hold + air cooling):
480°C aging: 39–45 HRC, higher hardness, commonly used for maximum strength.
550°C aging: 36–42 HRC, balanced strength and toughness.
590°C aging: 33–38 HRC, better toughness.
620°C aging: 30–32 HRC, good toughness, suitable for low-temperature or impact-resistant applications.
In the solution-treated state (unaged), hardness is only 28–32 HRB, much lower than after aging, making it easier to machine.
Workpiece thickness and cooling rate can slightly affect hardness. Values above are typical industry references.
If welded without post-weld aging, hardness decreases significantly and requires aging to restore target hardness.