The Testing of Metals

Metal materials are used in a myriad of applications in today’s world. There are a wide variety of physical and chemical properties of metals that can be tailored to specific engineering applications. There are many reasons to conduct testing of metals both in the research stage when metal materials are being developed and in the application stage when they are being produced and used in specific applications.

There are many types of tests that are available for metals. The following are some of the more common types of tests and the reasons for their importance:

Chemical analysis: The composition of metals is very important as it can dictate a number of properties once they are placed in service. These include corrosion resistance, creep resistance, strength, reaction to heat treatment, electrical properties, hardness/wear resistance, etc. Chemical analysis can be performed in a number of ways including Optical Emission Spectroscopy (OES), wet chemical analysis (generally refers to chemistry performed on samples in the liquid phase), Leco combustion method (carbon, sulfur, nitrogen, oxygen in steels), X-ray Fluorescence (XRF), etc.

Hardness Measurement: The hardness of a metal is often considered the most important measured property as it is used in design and quality control and can be measured relatively easily (and inexpensively). The hardness can give an indication of strength and success of a heat treatment process (especially with steels). Standard hardness measurement techniques include Rockwell (standard and superficial), micohardness (Vickers and Knoop), Brinell and file. The hardness measurement method used is usually dictated by the sample geometry and general hardness level. For large samples, the Rockwell and Brinell methods can be readily used. For very small samples a microhardness method may be required as the indentation used to measure hardness is extremely small (measured using a microscope). Microhardness is used to document the hardness profile in a metallurgical case. File hardness is very easy to use and is often used in the field to get general hardness measurements. Files of differing hardness levels are used to determine the hardness range of a sample.

Tensile Testing: The strength of a metal material is often the most important characteristic when the initial engineering is performed and the material is selected for a specific application. The properties of greatest interest when performing tensile testing are yield strength, ultimate tensile strength and elongation. Tensile tests are performed using samples that have been prepared to a specific test geometry. The tensile tester pulls the sample to failure and creates a stress/strain curve which allows for the determination of the yield strength, ultimate tensile strength and amount of elongation. The yield and ultimate tensile strengths are reported in pounds per square inch (or some other scale with a force per unit of area) and elongation is reported in percent.

Metallography: The internal structure of a metal material (microstructure) can be evaluated after samples have been carefully prepared and examined at high magnification using a metallograph. The samples typically are sectioned and placed in a metallographic mount that allows for easy laboratory handling while the sample surface of interest is polished to a mirror-like finish and etched to reveal the internal features. A trained metallurgist can examine the etched sample with a metallograph (special microscope) to examine and measure features such as grain size, inclusion content, phases present (ferrite, pearlite, martensite, bainite, austenite in steels). The phases and grain size usually have a bearing on the material strength, hardness, fatigue resistance, corrosion resistance, toughness, etc. so it is very important to examine the internal structure of the metal. rockwell hardness tester for sale

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