Hardness, as applied to most materials, and in particular metals, is a valuable, revealing, and commonly employed mechanical test that has been in use in various forms for more than 250 years. Certainly, as a material property, its value and importance cannot be understated, the information from a hardness test can complement and often be used in conjunction with other material verification techniques such as tensile or compression to provide critical performance information.
How important and useful is material and hardness testing?
Consider the information provided and its significance in structural, aerospace, automotive, quality control, failure analysis and many other forms of manufacturing and industry. Determining these material properties provides valuable insight to the durability, strength, flexibility, and capabilities of a variety of component types from raw materials to prepared specimens, and finished goods.
Hardness testing is a widely used form of materials test. It's relatively easy to perform, it's typically minimally or completely non-destructive, and most of the instrumentation is inexpensive by comparison to other types of material verification equipment. In addition, it can usually be performed directly on the component without significant alteration. While testing techniques and hardware has significantly improved as the electronics and computer age has advanced, earlier 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 form a scratch on the bar was a determining factor in the specimens hardness. Later hardness testing forms included scratching material surfaces with a diamond and measuring the width of the resultant line and, subsequently, indentation of the material using a steel ball under force. With the increased manufacturing requirements the global industrialization brought on, and then a much more urgent demand during both World Wars, more refined machines and techniques were developed. Accurate, efficient forms of testing were needed in reaction to heavy manufacturing demands, structural failures and the need to design sufficient material integrity into the growing global infrastructure. Recently, significant advances in hardware, electronics, and software has led to much more sophisticated hardness testing equipment that can quickly, reliably, and with extreme precision, provide useful and property critical information.
What exactly is indentation hardness testing?
The most basic and commonly used definition is the resistance of a material to permanent, plastic deformation. While other forms of hardness testing, such as rebound, electromagnetic, and ultrasonic, are used in a variety of applications and measure material hardness through other techniques, indentation hardness testing provides reliable, straightforward, and a commonly understood test type. It is measured by loading an indenter of specified geometry and properties onto the material for a specified length of time and measuring either the depth of penetration or dimensions of the resulting indentation or impression. As the material being tested is softer, the depth of penetration, or indent dimensions become larger. Common hardness testing types include Rockwell (indentation depth or un-recovered indentation), Knoop/Vickers, and Brinell (area of indentation). Rockwell testing is the most commonly used method by virtue of the quick results generated and is typically used on metals and alloys. Knoop and Vickers testing are more suitable for thin materials, coatings, and mounted metallographic components. Brinell testing applications generally include cast iron, large steel framework and aluminum. Some hardness testing can be done 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. Because 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 a previously applied preliminary (minor) load. Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specified period and 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 entire procedure requires as little as a few seconds up to 15 for plastics. In the Rockwell test results are quickly 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 typically 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 signifies a hardness of 63 on the Rockwell C scale. Higher values indicate harder materials such as hardened steel or tungsten carbide. These can have HRC values in excess of 70 HRC. Rockwell test forces can be applied by either closed loop load cell or traditional deadweight systems.
Micro or Macro hardness testing, also commonly referred to as Knoop or Vickers testing, is also performed by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the Knoop or Vickers test applies only a single test force. The resultant impression or un-recovered area is then measured using a high powered microscope in combination with filar measuring eyepieces, or more recently, automatically with image analyzing software. The Knoop diamond produces an elongated rhombic based diamond shaped indent with a ration between long and short diagonals of about 7 to 1. Knoop tests are mainly done at test forces from 10g to 1000g, Knoop tests are mainly known as microhardness or Microindentation tests and 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 (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore 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 a wider variety 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 used in a formula that includes applied force to determine a hardness value. Tables or automatic electronic or imaging measurements are a more common and convenient way to generate Knoop and Vickers hardness numbers.
Another common hardness test type, the Brinell test, consists of applying a constant load or force, usually between 500 and 3000 Kgf, for a specified time (from 10 to 30 seconds) using a 5 or 10 mm diameter tungsten carbide ball. The load time period is required to ensure that plastic flow of the metal has ceased. Lower forces and smaller diameter balls are sometimes used in specific applications. Similar to Knoop and Vickers testing, the Brinell test applies only a single test force. After removal of the load, the resultant recovered round impression is measured in millimeters using a low-power microscope or an automatic measuring device. Brinell testing is typically used in testing aluminum and copper alloys (at lower forces) and steels and cast irons at the higher force ranges. Highly hardened steel or other materials are usually not tested by the Brinell method, but the Brinell test is particularly useful in certain material finishes as it is more tolerant of surface conditions due to the indenter size and heavy applied force. Brinell testers are often manufactured to accommodate large parts such as engine castings and large diameter piping.
Hardness testing plays an important role in materials testing, quality control and acceptance of components. We depend on the data to verify the heat treatment, structural integrity, and quality of components to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications and in insuring that the materials utilized in the things we use everyday contribute to a well engineered, efficient, and safe world.