Primary Causes of Residual Stresses in Metals and How to Mitigate Them

25 March 2022

Numerous materials are being processed every day so that various machines and products will be generated. One of the materials that are often used by manufacturers is metal.

What is great about metals is they can be processed easily. And as more elements are integrated into metals, they can easily resist heat, moisture, corrosion, and other damaging elements. However, there are instances where the metal workpieces will boast areas that are restricted from expanding, contracting, and releasing elastic strains. These restrictions occur whenever residual stresses are present.

Residual stresses are stresses that may remain in a metal workpiece despite the lack of external loading or thermal gradients. A metal with residual stresses is known to be stressed way past its elastic limit. The presence of residual stresses in metals often leads to warping, distortion, fracture, and fatigue.

Primary Causes of Residual Stresses

To date, there are three primary causes of residual stresses.

1. Thermal Differences: One of the primary causes of residual stresses on metal workpieces is thermal variations. Metal workpieces are typically cooled after they undergo heat treatment processes. However, the cooling rate of their surfaces is often faster than the one on their interiors, constraining the interiors from cooling equally. The localised thermal contractions that develop due to cooling constraints lead to residual tensile stresses on both parts of the workpieces.
2. Phase Alterations: Another cause of residual stresses on metal workpieces is phase alterations. Some processes conducted on metal workpieces may change their phase to ensure that they can be workable. But as some metal workpieces undergo a phase alteration or transformation, a volume difference between their newly generated phase and the surrounding material may appear. This specific difference can contract or expand the materials that lead to residual stresses.
3. Mechanical Processes: One more cause of residual stresses is mechanical processes. Bending, drawing, rolling, and extruding processes are often applied to metal workpieces to attain their needed specifications. As the metal workpieces undergo these processes, some of their parts become elastic, while others become plastic. As the load is removed, the metal workpieces would attempt to recover the elastic part. However, their full recovery is prevented due to plastically deformed parts.

Mitigating Residual Stresses in Metals

Manufacturers can mitigate residual stresses in metal workpieces by controlling the type and magnitude of residual stresses. Controlling them can be done through stress relief heat treatment, mechanical treatment, regulating heat treatment processes, and alloy selection. Residual stresses can also be mitigated by utilising reduced cooling rates, choosing alloys with slow cooling rates, and maximising post-weld heat treatments. Once these things are done, cracks and deformation on metals can be avoided.

To know more about residual stresses in metals, you can call us at Alpha Detroit Heat Treatment.

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