Yield Strength And Heat Treatment

♣YIELD STRENGTH AND HEAT TREATMENT

Yield strength is an important indictor for the most engineering design, which is influenced by many factors such as raw material quality, chemical composition, forming process, heat treatment process, etc. This article presents an example indicating the effect of heat treatment on yield strength of AISI 4140 alloy steel.

1. YIELD STRENGTH

Yield strength is the amount of stress at which plastic deformation becomes noticeable and significant. Fig.1 is an engineering stress-strain diagram in tensile test. Because there is no definite point on the curve where elastic strain ends and plastic strain begins, the yield strength is chosen to be that strength when a definite amount of plastic strain has occurred. For the general engineering structural design, the yield strength is chosen when 0,2 percent plastic strain has taken place. The 0.2% yield strength or the 0.2% offset yield strength is calculated at 0.2% offset from the original cross-sectional area of the sample (s=P/A).

During yielding stage, the material deforms without an increase in applied load, but during the strain hardening stage, the material undergoes changes in its atomic and crystalline structure, resulting in increased resistance of material to further deformation.

Yield strength is a very important value for use in engineering structural design. If we are designing a component that must support a force during use, we must be sure that the component does not plastically deform. We must therefore select a material that has high yield strength, or we must make the component large enough so that the applied force produces a stress that is below the yield strength. In contrast, the tensile strength is relatively unimportant for ductile materials selection and application since too much plastic deformation takes place before it is reached. However, the tensile strength can give some indication of the materials, such as hardness and material defects.

Fig.1 Stress – strain diagram