Steel yield strength is the amount of stress a piece of steel must undergo in order to permanently and measurably deform. The yield strength is most often defined as the point at which a measurable deviation of 0.2 has occurred in the steel. Steel yield strength can be increased using heat treatments or by mixing other materials with the steel to create a steel alloy. Engineers must know the yield strength of the materials they are using in order to ensure that the structures they build will withstand the stresses put on them.
Yield strength should not be confused with tensile strength. Tensile strength is the point at which stresses on a metal will cause it to break or snap. By contrast, yield strength is the point at which a metal will undergo plastic deformation. A material is said to have deformed plastically when it bends under stress and cannot return to its original shape after the stress is removed. If the material is able to return to its original shape, this is known as elastic deformation. Elastic deformation can go on for some time before a metal deforms plastically.
The yield strength of a material is determined in a laboratory using a tension test. First, hundreds of samples of the material are subjected to stress. The point at which plastic deformation occurs is recorded for each sample. The measurements are then averaged to determine the yield strength of the material as measured in pounds per square inch (psi). If more stress is applied to the material after it has reached its yield strength point, the material will soon reach its fracture point and break.
The technical definition of steel is iron that has had carbon added to it to form an alloy. Steel yield strength and other properties of the steel will vary according to the percentage of carbon that has been added. Adding other materials such as copper, lead, and manganese may also have an effect on steel yield strength. Manufacturers of metal products release statistics on yield strength and other characteristics of specific products.
Heat treatments can also affect steel yield strength. The purpose of a heat treatment is to either strengthen or soften the metal. Heating and cooling the metal to specified temperatures at controlled speeds changes the crystalline structure of the metal. Crystalline structure is one of the major factors that determines metal strength.