Materials Engineering, Evaluation, and Inspection
Spectrum uses hardness tests to determine the strength of various materials. Hardness is generally defined as a measure of the materialís resistance to scratching, deformation, or cutting. A hardness test gauges this parameter by assessing the depth of an indent or incision made by a precisely shaped indentor applied to the material with an exact amount of force over a specified period of time. Appropriate candidates for hardness tests include structural steel, castings, forgings, welds, weld overlays, piping, machined parts and pressure vessels.
Depending on the requirements, Spectrum will typically use one of four test methods: Rockwell, Brinell, Vickers or Knoop.
Hardness Rockwell Testing requires different indenters and loads for different strength or hardness ranges. In addition to gauging hardness, Rockwell numbers can be converted to approximate tensile strength for a number of alloys.
Test material is indented using tungsten carbide tips, except for the Rockwell C scale, which specifies a diamond tip. The tip is forced into the metal under a specified load, usually between 15 and 150 kilograms, and a corresponding value from the Hardness Rockwell scale is determined by the depth of penetration. Test samples can have rounded or flat surfaces and be as small as 5 millimetres square. Hardness Rockwell testing is only conducted on metals and is often used to help ascertain the grade of a metal.
Hardness Brinell testing utilizes a much larger indenter with higher loads, and subsequently requires thicker samples than standard Rockwell hardness testing. The minimum test material thickness is typically ľ inches. Brinell test values are derived by measuring the diameter of a circular impression made by a tungsten carbide ball indenter, normally under a load of 500 kilograms or 3,000 kilograms applied for 10 to 15 seconds on iron or steel, and at least 30 seconds for other metals. Penetration depth is measured by a microscope equipped with an optical light source.
Spectrum uses the Vickers test, also known as a microhardness test, to determine the ability of test material to withstand pressure from a standard source. Vickers test calculations are independent of indenter size. As with the Brinell test, values are determined by measuring the depth of an indentation made by an indenter. Tungsten carbide or diamond tips are normally used, along with an optical light source and a microscope.
The Knoop method is another microhardness test procedure, and is used mainly to measure the hardness of smaller, thinner test samples, and when examining case depth. Because the test samples are smaller, they must be very carefully prepared. As with the Vickers method, the Knoop test utilizes an optical measurement system. A rhombohedral-shaped diamond indenter is used for testing in the Knoop scale, and it is more elongated when compared to the pyramid indenter used for a Vickers test. The Knoop method is often used when indentations are close together or close to the edge of a sample. The greater width and shallower depth of a Knoop indentation provides better measurement resolution and makes it more appropriate for testing very thin materials.