Stress-Relief Annealing for CNC Machined Aluminum & Steels
Stress-relief annealing is a sub-critical thermal heat treatment performed after heavy roughing to relax internal residual stresses and prevent long-term dimensional distortion in precision parts.
Engineering Specifications & Parameters
Soak Duration
1 Hour per 25 mm (1 inch) of cross-section thickness
Target Result
Eliminates post-machining warping and dimensional drift
Cooling Method
Slow Furnace Cooling (< 30 C per hour)
Steel Stress-Relief Temp
550 C - 650 C (Below Critical Ac1)
Aluminum Stress-Relief Temp
150 C - 180 C (Preserves T6 Temper)
The Origin of Machining Residual Stress
Heavy roughing cuts introduce intense mechanical shear and thermal gradients that generate internal tension and compression stress layers. When unconstrained, these stresses cause parts to warp over time.
Material Specific Heat Treatment Protocols
6061-T6 / 7075-T6 Aluminum:
Heating to 150 C to 180 C for 2 hours followed by slow furnace cooling.
Preserves T6 precipitation temper while relaxing 60%+ of surface residual stress.
Carbon & Alloy Steels (4140 / D2 / 1018):
Heat to 550 C to 650 C (1020 F to 1200 F) below Lower Critical Temp (Ac1).
Hold 1 hour per 25 mm thickness, furnace cool below 250 C to prevent re-stressing.
Why must high-precision aerospace frames undergo stress relief after roughing?
Removing 80%+ of stock volume redistributes bulk raw material stresses; without stress relief, finishing tolerances will distort when unclamped from the vise.
What is the difference between Full Annealing and Stress-Relief Annealing in steels?
Full annealing heats above the austenitizing temperature to completely soften the metal; stress relief heats below Ac1 to relax stresses without altering hardness.
