As engineers we are obviously conscious of the vital need to define and then design for the largest applied stresses that a product is liable to encounter in service. The product’s long-term reliability and safety for the user demands this acknowledgement. However, many of us may overlook residual stresses that can be created inside metallic materials.

If these residual stresses are of sufficient magnitude and if they are tensile in nature they will accelerate certain failure mechanisms. These forms of failure include “pure” mechanical fatigue (that is without a significant corrosion effect), corrosion fatigue, hydrogen embrittlement (HE) and stress-corrosion cracking (SCC). Tensile stresses can be created by a variety of manufacturing processes such as severe cold working, forging, casting, machining, grinding and electroplating. They may also be generated in service due to thermal growth and contraction.

By contrast, compressive residual surface stresses as created by shot peening, cold rolling or by case hardening processes such as carburizing or nitriding are beneficial. They protect the metal against failures that typically initiate at the metal surface, e.g., any variety of fatigue or SCC. HE originates further below the metal surface and its control generally does not benefit from surface compressive residual stresses.

Correct stress relief annealing heat treatments can be used to eliminate or greatly decrease both types of residual stresses. The temperature and duration of an effective stress relieve heat treatment depend on the specific alloy and dimensions of the part being treated. These treatments may then be followed with beneficial shot peening, cold rolling or a case hardening process if desired. Residual stresses, tensile and compressive, are algebraically additive to each other and to any applied stresses that may be acting. The key is – don’t overlook residual stresses – whether undesirable or intended.

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