What Is Flow Stress?

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  • Written By: Andrew Kirmayer
  • Edited By: Shereen Skola
  • Last Modified Date: 25 September 2019
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Allowing for a measure of the stress that causes material to flow in one direction, flow stress is used to characterize metal forming machines. The rolling of metal sheets requires certain levels of power and torque depending on the type of material being processed. Other methods of metalforming such as forging, extrusion, and smelting have their own impacts on the level of stress as well. A flow stress curve represents how elastic a particular material is and how elongated it can get before it reaches its maximum tensile strength. At what point the material fails is important to know when processing metals.

Temperature has an effect on steel flow stress, which typically decreases when a material becomes softer as it becomes hotter. Stress also increases in a metal that changes phases when heated, as well as if a material re-crystallizes during a temperature change. Re-crystallization and temperature changes also increase the strain on the material, which directly correlates to rises in flow stress. So do grain size as well as other changes in the nature of the grains as the temperature and metal state alter during the metal forming process.


The chemical composition of the steel, its crystal structure, as well as the purity of its makeup are factors that affect its flow stress. Heat treating of cold-formed steel drastically increases the stress state, as does changes in the metal’s hardness and softness during processing. A sample of the material is subject to a tensile test, and data on its tensile strength are used to determine the flow stress of the substance. In this case, the state of the sample influences what metal forming specialists believe are the characteristics of the overall steel sheet, and at what point it would fracture.

A particular stress flow chart is used to graph the relationship between a parameter and the flow stress. The chart helps understand these relationships under specific circumstances, and to determine which techniques work for the type of material used. Based on data from various experiments which researchers have carried out, the flow stress properties of different grades of steel are known. Mathematical equations are also utilized to estimate the stress level based on a variety of factors. A particular material’s limits are important for manufacturers to know when selecting the right metal forming equipment, so it produces high-quality steel products in a cost-effective manner.


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