1) Can stress relieving minimize part distortion?
Yes! A cursory read of the machinist’s handbook shows stress relieving recommended as a best practice. We commonly see requests for stress relieving on weldments, fabrications, and machined components, but increasingly on additive-manufactured (AM) and printed parts as this technology advances. Stress relieving removes machining stresses, cold working, or with AM parts, the unbalanced heat-affected areas that can contribute to distortion.
2) Is stress relieving allowed when working to MIL or AMS heat treat specifications?
In some cases, it’s mandatory. We work to many MIL, AMS, and BAC specifications on a daily basis. AMS2759, AMS2801, and BAC-5613 represent a large portion of our stress relieving business. Did you know precipitation hardened (PH) grade materials, in the aged condition, can be stress relieved? Per Table 1 of AMS2759/11, stress relieving must be done 100°F below the aging temperature. Using this method has become popular and necessary to stabilize parts in use or during machining.
3) Are material properties affected during stress relieving?
Trick question! You must know the scope of work, processes, and needs of your customer. Stress relieving consists of heating the steel to a temperature below the critical range to relieve stress from cold working, shearing, mill/cutting, etc. It’s not intended to alter the microstructure or mechanical properties. You wouldn’t want to stress relieve a 17-4 stainless steel part at 1,250°F if it is going to be aged (H-900) at the final process. However, if you knew it was going to be solution-treated and then aged, you’d be fine. If desired, the material properties of some alloys can be altered by stress relieving, then restored with an annealing or hardening process. Again, consult with your heat treater.
4) What are the negative impacts of stress relieving?
Unintended results or falling short of the goal. Heat treating complex geometries or configurations may require post heat-treat straightening or flattening. These post processes must be built-in ahead of time. Predicting results to leave stock on, for instance, can prove challenging. In many cases, a trial run in a smaller furnace will yield the knowledge needed to ensure stress relieving’s usefulness. If stress relieving doesn’t work, then we will work on plan B. There is always a plan B.
5) Can finished machined parts be stress relieved?
Absolutely, vacuum stress relieving is most advantageous! Vacuum stress relieving produces bright, clean finishes in near-net shape. In some cases, stress relieving can correct (straighten or flatten) a warped part post-machining or grinding. Influencing the materials at temperature by exercising techniques such as static weight to flatten or using coefficients of thermal expansion (CTE) of dissimilar metals may improve such conditions. It’s always best to describe the conditions you’re seeing during machining or after a thermal process to see if stress relieving can benefit your production flow. You might walk away saying you wish you'd have done it sooner.
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