Hardness, as applied to most materials, and in specific metals, is a valuable, revealing and widely used mechanical test that has been in use in different forms for over 250 years. Certainly, as a material property, its importance and value cannot be understated, and the information from a hardness test can complement and frequently be used in tandem with other material verification techniques such as compression or tensile to provide critical performance information.
How Important and Useful is Material and Hardness Testing?
Consider the information provided and its importance in structural, aerospace, quality control, automotive, failure analysis, and several other forms of industry and manufacturing. Determining these material properties offers valuable insight into the strength, flexibility, durability, and capabilities of a wide range of component types from raw materials to finished goods, and prepared specimens.
Hardness testing is an extensively used form of materials test. It is comparatively easy to perform and is typically completely or minimally non-destructive, and also most of the instrumentation is inexpensive when compared to other types of material verification equipment. Furthermore, it can typically be carried out directly on the component without major alteration. While testing hardware and techniques have considerably enhanced as the electronics and computer age has advanced, previous techniques included simple scratch tests.
These tests were based on a bar that increased in hardness end to end. The level at which the material being tested could produce a scratch on the bar was considered to be a determining factor in the specimens hardness. Later hardness testing forms comprised of scratching material surfaces with a diamond and measuring the width of the resultant line and, consequently, indentation of the material using a steel ball under force. With the increased manufacturing requirements brought about by the global industrialization, and then a much more urgent demand during both World Wars, more refined techniques and machines were developed.
Efficient, accurate forms of testing were required in response to structural failures, heavy manufacturing demands, and the need to design adequate material integrity into the growing global infrastructure. Recently, important advances in electronics, hardware, and software have led to much more sophisticated hardness testing equipment that can reliably, quickly, and with extreme precision, offer useful and property critical information.
What Exactly is Indentation Hardness Testing?
The most commonly used and basic definition is the resistance of a material to permanent, plastic deformation. While other forms of hardness testing, such as electromagnetic, rebound, and ultrasonic, are employed in a wide range of applications and measure material hardness via other techniques, indentation hardness testing offers simple, reliable, and a commonly understood test type. It is measured by loading an indenter of specified geometry and properties onto the material for a definite length of time and measuring either the depth of penetration or dimensions of the resulting impression or indentation. The depth of penetration, or indent dimensions become larger as the material being tested is softer.
Common hardness testing varieties include Rockwell (indentation depth or un-recovered indentation), Brinell (area of indentation), and Knoop/Vickers. Rockwell testing is the most frequently used method by virtue of the quick results produced and is usually used on alloys and metals. Knoop and Vickers testing is more ideal for coatings, thin materials, and mounted metallographic components. Brinell testing applications normally include large steel framework, cast iron, and aluminum. Some hardness testing can be performed within seconds with a hand-held device.
The indent made by the hardness test can either be ground out, or can be so small as to not affect the performance or appearance of the component. Since the testing is done to the component itself, each product or a spot check of products can be tested before shipping to the customer.
How are These Common Types of Hardness Tests Performed?
The Rockwell hardness test is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy total (major) load beyond the depth resulting from an earlier applied preliminary (minor) load. Initially, a minor load is applied, and a zero datum position is established. This is followed by applying the major load for a specified period and it is then removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from the zero datum position as a result of the application of the major load. The whole procedure needs as little as a few seconds up to 15 for plastics.
In the Rockwell test, results are rapidly and directly obtained without the need for a secondary, dimensional measurement requirement. The most common indenter type is a diamond cone ground at 120 degrees for testing hardened steels and carbides. Softer materials are usually tested using tungsten carbide balls ranging in diameters from 1/16" up to 1/2". The combination of indenter and test force make up the Rockwell scale. These combinations make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and then the respective scale. A recorded hardness number of HRC 63 indicates a hardness of 63 on the Rockwell C scale. Higher values specify harder materials such as tungsten carbide or hardened steel. These can have HRC values in excess of 70 HRC. Rockwell test forces can be applied by either standard deadweight systems or a closed loop load cell.
Micro or Macro hardness testing, also usually referred to as Knoop or Vickers testing, is also carried out by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the Vickers or Knoop test applies only a single test force. The resultant impression or un-recovered area is then measured with the help of a high powered microscope in combination with filar measuring eyepieces, or more recently, automatically with image analyzing software. The Knoop diamond develops an elongated rhombic-based diamond shaped indent with a ration between long and short diagonals of about 7 to 1.
Knoop tests are primarily done at test forces from 10 g to 1000 g. Knoop tests, mainly called microindentation or microhardness tests, are best used in small test areas or on brittle materials, as minimal material deformation occurs on the short diagonal area. The Vickers diamond produces a square-based pyramidal shape with a depth of indentation of about 1%7th of the diagonal length. The Vickers test has two distinct force ranges, micro (10 g to 1000 g) and macro (1 kg to 100 kg), to cover all testing requirements. The indenter is the same for both ranges, thus Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000).
Vickers tests are mainly known as macro-indentation tests and are used on an extensive range of materials including case hardened, and steel components. Vickers indents are also less sensitive to surface conditions than the Knoop test. In both test types, the measured area is employed in a formula that includes applied force in order to determine a hardness value. Tables or automatic imaging or electronic measurements are a more common and convenient way to produce Knoop and Vickers hardness numbers.
The Brinell test, another common hardness test type, comprises of applying a constant load or force, typically between 500 and 3000 Kgf, for a specified time (from 10 to 30 seconds) with the help of a 5 or 10 mm diameter tungsten carbide ball. The load time period is needed to make sure that plastic flow of the metal has stopped. Smaller diameter balls and lower forces are sometimes used in specific applications. Similar to Knoop and Vickers testing, the Brinell test applies just a single test force. After removal of the load, the resultant recovered round impression is measured in millimeters using an automatic measuring device or a low-power microscope.
Brinell testing is usually used in testing aluminum and copper alloys (at lower forces) and cast irons and steels at the higher force ranges. Greatly hardened steel or other materials are typically not tested by the Brinell method, but the Brinell test is mainly useful in specific material finishes as it is more tolerant of surface conditions due to the heavy applied force and indenter size. Brinell testers are frequently manufactured for accommodating large parts such as large diameter piping and engine castings.
Hardness testing plays a vital role in materials testing, quality control, and acceptance of components. The data is needed in order to verify the heat treatment, structural integrity, and quality of components that will help determine if a material has the properties essential for its proposed use. Establishing a correlation between the desired material property and the hardness result allows this, thus making hardness tests extremely useful in R&D and industrial applications and also in assuring that the materials utilized in the things used every day contribute to a well-engineered, safe and efficient world.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
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