MATERIALS TEST & ANALYSIS SPECIAL REPORT: Making Hardness Tests a Moving Experience
In response to the need to test products that are too large for conventional Rockwell, Brinell or Vickers hardness testing methods, quality and manufacturing professionals are making more use of portable hardness testers. For production testing, the ability to test complex shapes and access difficult test areas allows portable testers to complement stationary testers.
Two common methods used by portables employ the Ultrasonic Contact Impedance (UCI) and rebound. Instruments based on the UCI principle measure a frequency shift of an oscillating rod with a diamond tip to determine the area of an indentation. Instruments using the rebound principle measure voltage to indirectly measure hardness through the loss of energy of a so-called impact body. Each method has its individual advantages and appropriate applications.
The UCI Method
The UCI method uses the same pyramid-shaped diamond as a conventional Vickers hardness tester. Unlike Vickers testing, no optical evaluation of the indentation is required, enabling fast and portable testing when using the UCI-based instruments.
The UCI method uses a rod that is excited into oscillation at an ultrasonic frequency by a pair of attached piezoelectric ceramics while a second pair of piezoelectric ceramics constantly monitors the frequency. The rod has a Vickers diamond indentor attached to one end and a spring at the other that applies a test load ranging from 0.1 to 10 kilogram of force. As the operator forces the diamond into the material, the frequency of the rod oscillation changes in response to the contact area between the diamond and the material under test. The instrument detects the shift in frequency, converts it to a hardness value and immediately updates the display with a hardness value. The following equation describes this basic relation in comparison to the definition of the Vickers hardness value.
Where: f = frequency shift; A = area of indentation; Eelast = Young's modulus; HV = Vickers hardness value; and F = Force applied in the Vickers hardness test.
The frequency shift is not only proportional to the indentation size of a Vickers indenter but is also dependent on the Young's modulus of elasticity for the material. Factory calibration is performed on unalloyed and low alloy steel test blocks and when testing materials having a different Young's modulus, a special calibration is required. All that is required to correctly calibrate the instrument are samples of the special material whose hardness has been established using the specified conventional Rockwell, Brinell or Vickers tester. The calibration procedure to compensate for any difference in the Young's modulus involves simple adjustment of the uncalibrated UCI instruments to the value established with the conventional tester.
The Rebound Method
he rebound method is based on measuring voltages to indicate the loss of energy of a so-called impact body after it strikes the test piece. In an instrument using the rebound principle, a spring propels an impact body through a guide tube toward the test piece. As the impact body travels unimpeded toward the test piece, a magnet contained within generates a voltage in a coil system that encircles the guide tube. A tungsten carbide-ball indentor, located on the end of the impact body, strikes the material, causing the impact body to rebound from the surface at a slower velocity. The softer the material, the bigger the indentation, causing a larger loss of energy and a slower rebound speed, which in turn produces a proportionally lower voltage as the magnet returns through the coil.
The hardness value (HL) is calculated from the ratio of the impact and rebound speed according to:
However, because few people indicate the HL value in specifications or test reports, portable instruments typically contain internal conversion tables to convert the HL values into the required hardness scales such as Vickers, Rockwell or Brinell.
Choosing the appropriate portable method depends on the task. Selection is based on controlling the indentation size and overcoming certain mass limitations of the method.
Generally because of the relatively small indentation created by the UCI method, it is best suited for testing fine-grained materials having a variety of shapes and sizes. Rebound testers produce larger indentations than UCI instruments for more consistent results when testing large coarse-grained materials typical of forged and cast components.
Consistency of test results requires the indentation size be large in comparison to the material's microstructure. Therefore, rebound instruments, with their larger indentation, should be given first consideration over UCI instruments when testing coarse-grained materials. However, both methods can be influenced by the mass of the part to be tested. With the UCI method, parts that weigh more than 0.3 kilogram and with thickness greater than 6 millimeter have sufficient mass to prevent them from going into a state of self-oscillation. Requirements for the rebound method are that parts must weigh at least 5 kilograms and have a minimum thickness of 20 millimeters to prevent them from yielding or flexing under the large force created during the time of impact. The appropriate use of a test support or couplant will allow either method to test thinner and lighter parts.
In certain applications, such as testing the small width of a heat affect zone (HAZ) to determine whether a welding process was correctly performed requires the smaller indentation of the UCI method. In this situation, the larger indentation of the rebound method could be affected by the weld or base material and mask the presence of a high hardness peak in the HAZ which indicates high martensite content that can often lead to cracking.
What is Hardness?
Determining metal hardness has always been a subject of much discussion among technical people that has resulted in a wide range of definitions. Hardness properties include such varied attributes as resistance to abrasives, resistance to plastic deformation, high modulus of elasticity, high yield point, high strength, absence of elastic damping, brittleness or lack of ductility.
To a technician, hardness is a material's resistance to penetration. In general, an indenter is pressed into the surface of the material to be tested under a specific load for a definite time interval and a measurement is made of the size or depth of the indentation.
Hardness is not a fundamental property of a material, but a response to a particular test method. Basically hardness values are arbitrary, and no absolute standards for hardness exist. Hardness has no quantitative value, except in terms of a given load applied in a specific, reproducible manner and with a specified indenter shape.
Static indentation tests in which a ball, cone or pyramid penetrates into the surface of the test material are widespread. The relationship of load to the area or depth of indentation is the measure of hardness, such as in common benchtop Brinell, Rockwell, Vickers or Knoop hardness testers.
The different methods and differently shaped indenters used by Brinell and Rockwell produce dissimilar responses of the material under test. Conversion tables that list values are only approximations-no mathematical equation exists that can transfer measurements from one scale to another. The so-called conversion tables are determined empirically by experimental evaluation of a specific material's hardness with the different test methods. To compare the hardness of two different samples, both must be measured using the same hardness scale, or a scale must be developed to convert from one measurement to the other.
Source: Portable Hardness Testing Application Guide, by Dr. Stefan Frank of Krautkramer.