Why is Stress Relieving Performed in Heat Treatment Process?

29 March 2018

To some extent, worked alloys have memory. Stress can be absorbed and stored in the metal. Under these circumstances, heat treatment facilities call upon the services of a stress relieving process, a technique that’s designed to reset that solid-form memory. In a nutshell, the grain of the metal is homogenized. Thermal relaxation takes over at this point, then the workpiece is ready for more processing or a post-processing finish operation.

Challenging Stress Relief Actions 

Playing devil’s advocate for a moment, what happens when a heat treatment run skips this crucial material-homogenizing stage? Formed or machined, cold-worked or welded, the alloy piece is saturated with residual mechanical stress. It’s about to be passed back to another processing stage for more machining, with those wildly interacting intragranular still in place. The part deforms, it warps, and it loses dimensional form. Even if those internal forces don’t make themselves known during a secondary heat treatment operation, the workpiece is still packing that trapped energy. Pressed into service by some fly by night heat treatment functionary, the metal part will fail. Prematurely and drastically, it will release those stresses and fail.

Initiating a Heat Treatment Reset 

It takes that kind of reverse logic to really gain an insight into the process. Stress relieving technology sits firmly as a looping branch of the thermal conditioning tree. If the workpiece absorbs stress during the initial machining stage, these forces are thermally relaxed until the part is mechanically equalized once more. Passed through this processing loop and into a heat station that introduces a thermal gradient, more heat-induced stress is encountered. Back into the process sub-branch, that newly added stress is mitigated. The end result, now that the part has left all of the machining stations and heat treatment work behind, is a dimensionally stable, geometrically accurate component. This being the case, the finished metal workpiece is now less susceptible to stress corrosion cracking, post-processing dimensional variations, and grain irregularities.

A number of thermal relaxation temperatures work well here, then there are material normalizing techniques and annealing procedures to further regulate the work. Of the fundamental stress relieving techniques, two types of stress relief dominate. First of all, a 150°C to 200°C method operates as a peak force releasing process. That relatively low-temp system offers significant relief without affecting other treatment work. Using this method, parts lose most internal stress while maintaining their case hardened exterior shell. Alternatively, using a thermal spread of between 600°C to 680°C, the furnace will fully homogenize the alloy grain.

 

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