Tensile Strength of Steel and Other Metals
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The term tensile strength refers to the amount of tensile (stretching) stress a material can withstand before breaking or failing. The ultimate tensile strength of a material is calculated by dividing the area of the material tested (the cross section) by the stress placed on the material, generally expressed in terms of pounds or tons per square inch of material. Tensile strength is an important measure of a material’s ability to perform in an application, and the measurement is widely used when describing the properties of metals and alloys.
The tensile strength of an alloy is most commonly measured by placing a test piece in the jaws of a tensile machine. The tensile machine applies stretching stress by gradually separating the jaws. The amount of stretching needed to break the test piece is then measured and recorded. The yield strength of metals may also be measured. Yield strength refers to the amount of stress a material can withstand without permanent deformation.
We choose metals for their many applications based on a number of properties. One of these properties is tensile strength. Metals need to be very strong in some instances, relatively soft and ductile in others. In some cases, they have to be strong and tough. Corrosion resistance, heat resistance, weldability and machinability are other properties that come into play in the selection of a metal or alloy for a specific application.
We’ll deal here with the properties most associated with engineering metals and alloys, namely their yield strength (Y.S.) tensile strength (U.T.S.), elongation (EL%) and reduction of area (R.O.A.%).
When a tensile stress is applied to a test specimen of a metal or alloy bar it will deform, or stretch. Up to the application of a certain stress force the metal will revert to its original length. If, for example, we’re putting a tensile stress on a steel or aluminum specimen, the bar will return to its original length until a stress sufficient to cause permanent deformation is applied. When this stress point is reached, the bar’s cross-section will decrease and with further increasing stress the bar will rupture.
The stress required to cause permanent deformation is known as the metal’s yield strength, and up to this point the metal is undergoing elastic deformation. Application of further stress causes plastic or permanent deformation, until the point where the metal can no longer withstand the stress being applied to it, and it ruptures. The stress value at which rupture occurs is the metal’s ultimate tensile strength.
Once the yield strength has been exceeded, the metal will stretch and will continue to do so until the rupture point. The extent to which the bar stretches before rupture is a measure of the metal’s ductility which is expressed as the percentage elongation. Similarly, the reduction of area of the test specimen may be defined as the difference, expressed as a percentage of original area, between original cross-sectional area and that after straining the test specimen to its rupture point.
It should be noted that the above-noted definitions and data apply to those materials known as ductile materials, or those materials capable of withstanding a good deal of deformation prior to rupture. Brittle materials, or those materials that are brittle by nature or are designed purely for high strength and hardness, will show effectively no plastic deformation prior to rupture and their elongation and reduction of area values will be near to zero.
A metal’s yield strength and ultimate tensile strength values are expressed in tons per square inch, pounds per square inch or thousand pounds (KSI) per square inch. For example, a tensile strength of a steel that can withstand 40,000 pounds of force per square inch may be expressed as 40,000 PSI or 40 KSI (with K being the denominator for thousands of pounds). The tensile strength of steel may also be shown in MPa, or megapascal.
The properties of engineering metals and alloys may, in most instances, be optimized by heat treatments such as quenching and tempering or annealing. The temperatures employed during such thermal treatments will determine the properties obtained in the finished product. Toughness, as measured by the Izod impact test, is greatly enhanced by tempering and annealing treatments.
The IZOD impact test is an ASTM (American Society for Testing and Materials) standard method of determining the impact resistance of materials. The test is similar to the Charpy impact test but uses a different surface standard than the Charpy V-notch test.
All testing methods for engineering metals and alloys are covered by ASTM material specification standards. Each material specification for a metal alloy includes the ultimate tensile strength of steel, plus its yield, elongation and reduction of area values.