Stress Relieve
Stress relieving is a heat-treatment or mechanical process that reduces residual stresses locked into a part from welding, casting, heavy roughing, forming, or uneven cooling. Performed below the critical transformation range, it improves dimensional stability and reduces distortion without materially changing geometry or intended properties, typically applied after rough machining and before finish machining.
On the shop floor, in a CNC cell, stress relief usually sits between roughing and finishing. After heavy stock removal, internal stresses rebalance and the part can move once unclamped. Heating steel below its transformation range—commonly around 1000–1200°F—lets the material yield locally and shed locked-in stress, then slow cooling stabilizes the structure. Aluminum parts often get lower-temperature soaks around 150–200°C to settle before final passes. For welded frames, vibratory stress relief offers a practical alternative when furnace heating is impractical. The sequence matters: rough machine, stress relieve, then semi-finish or finish. Thin walls, pocketed plates, long billets, and asymmetric stock removal all demand this step to hold flatness, bore position, and parallelism. Even a rest period before inspection can help equalize stress after aggressive cutting and clamping loads.
Is stress relieving the same as annealing?
No. Annealing is broader and can alter hardness and microstructure more significantly. Stress relieving is performed below the critical transformation range to reduce residual stress with minimal change to properties.
When should stress relief be done in a CNC process?
Most often after roughing and before finish machining, especially on parts that must hold tight flatness, positional tolerance, or bore geometry. It can also follow welding or assembly before final machining.
What are the main signs a part needed stress relief?
Unexpected movement after unclamping, bore growth or shrinkage after semi-finish, flatness loss after material removal, and repeatability drift across identical parts from the same blank stock.