Tempering Time
Tempering time is the soak duration a hardened ferrous part is held at the tempering temperature after quenching. It is a time–temperature control variable used to reduce brittleness, relieve quench stress, and tune final hardness and toughness. Typical guidance calls for about one hour per 25 mm of section thickness, with thicker parts requiring longer dwells.
On the CNC floor, tempering time is written into the heat-treat sequence for steels such as 1045, 4140, and 4340. The part is quenched to harden it, then reheated below the critical range and held for a soak scaled to section thickness—often one hour per 25 mm. That controlled dwell relieves quench stress and tunes hardness before the part returns to machining. Because quench and temper cycles can disturb dimensions, critical bores and datum faces are left for a final finishing pass after the last temper. That way the material is stable, cutting forces on the finish operation are lower, and the final tolerances are not lost to post-heat-treat movement. In practice, the setup after the oven is a confident finish pass rather than a guess.
- Cracking on the bench: A hardened high-carbon or medium-alloy part can crack if tempering is delayed too long after quench. Industry specs often require tempering within two hours, and snap tempering should cover any unavoidable gap.
- Under-soaked core: A short dwell leaves thick sections surface-tempered but still brittle at the core. Tempering time must be scaled to section thickness; a fixed-shop recipe that under-soaks creates a part with unstable hardness and residual stress.
- One cycle for every alloy: All steels do not temper alike. High-alloy and tool steels need different soak schedules than mild steels, so applying a generic cycle can yield wrong hardness, excess brittleness, or unnecessary softening.
Is a longer temper always better?
No. Tempering is not a 'more time equals better' process. The desired hardness and toughness depend on temperature, time, alloy chemistry, and section thickness, and the same property shift can often be achieved by adjusting temperature and dwell together rather than by extending only the soak.
Why does the soak matter if the part is already hot?
The part must remain at the tempering temperature long enough for the microstructure to transform uniformly through the cross-section. An insufficient dwell leaves residual stress gradients and unstable hardness from surface to core, so the specified hold is needed even after the pyrometer reaches setpoint.
What happens if tempering time is too short on a precision part?
The part retains higher residual brittleness, poorer impact resistance, and a greater chance of post-process distortion. Finish-machined dimensions can drift once the part is released from the fixture or sees service load, so the soak must be adequate before final sizing.