Taking Two Directions in Hardness Testing Development
Hardness testing is being strongly affected by manufacturing's demands for increased hardness accuracy, reduced labor and rapid feedback. Two approaches have emerged for achieving these goals: the automation of each aspect of existing test methods and the application of depth of indentation measurement techniques. Each approach has advantages for a range of applications. Automating existing methods applies modern gage technology, video technology and computers to a wide range of applications. The depth of indentation measurement methodology forges beyond the traditional hardness standards to provide even greater advantages for a more narrow range of specific and demanding applications.
Automating existing standard test methodsRockwell tester manufacturers have made great strides in automatically controlling many of the potential variables in Rockwell testing in higher-end testers. Loads often are now controlled by load cells and low-friction load techniques. The friction of a dead weight or spring-powered load is almost free from variation, unlike the traditional deadweight style using knife edges. "Closed loop" refers to the load-cell units and has become a fashionable marketing term because it implies tester infallibility. Both the load cell and low-friction types operate exceptionally well on test blocks, but in actual operation are subject to error from deflection.
Hardness testers measure at a millionth-of-an-inch resolution with large forces-picture an exceptionally accurate micrometer being closed with 330 pounds of force. The smallest "give" in the unit's frame, the sample, or contamination on the sample or anvil can give bad results. With a closed-loop system, the operator is not aware of deflection any more than he is using an old knife-edge unit. Fortunately, there is an answer for this problem too, through deflection compensation. This eliminates the effects of deflection resulting from contaminating materials on the sample and anvil, or from the sample itself deforming or compressing under load-as in a cylinder. This feature makes hardness testing more repeatable in actual operating conditions, not just during calibration routines.
Brinell testing has seen tremendous improvements automating each aspect of test method ASTM E-10. The Brinell method traditionally has suffered from several problems: a lengthy two-or-three step process-indentation then measurement-often preceded by grinding. There has been a gross lack of accuracy and repeatability-mostly due to operator influence in the test measurement.
The biggest advance in Brinell methodology has been the adoption of automatic measurement systems using vision systems. These systems operate faster and more accurately, and are more repeatable than the traditional eyeball-on-scope method.
One challenging area for these automatic measurement systems has been the edge detection on surfaces where a grinder has been used. The rough edge was less of a problem in the traditional make-believe measurement system where operators saw what they wanted to see-or quite often what they needed to see in order to pass the part. Generally, the quality of the grinding, the illumination of the specimen, and the edge-detection algorithms have all been forced to improve to provide for real accuracy in automatic measurement. This automatic accuracy now far exceeds manual measurement techniques.
The Brinell testing or "hole punching" part is less challenging for the actual testing machine, unlike Rockwell where there is a preload and measurement at millionth-of-an-inch resolutions. In Brinell, the tester simply needs to make an impression at a given load. Proving rings with transducers and built-in load cells have provided the needed assurance of load accuracy to improve the testing. The other major feature that has been developed is the use of a stroke in the Brinell machine-either in the head or in elevating the specimen-to eliminate the time operators spend adjusting the elevating screw.
Microhardness testing, like Brinell, uses a two-step, test-then-measure process similar to Brinell, although the results have had more accuracy and repeatability than Brinell due to the use of polished surfaces and skilled lab personnel. In microhardness testing, the high-labor cost has been more the driving force for increased automation, although better accuracy has been a surprising result as well-especially in the highest volume applications. Everything an operator might do, such as selecting loads, testing positioning, focusing, impression reading and data recording has been subject to automation.
Newer systems provide an electronic overview of the test sample for better positioning, faster and more accurate focus detection, better image analysis algorithms on etched samples or other darker surfaces, more accurate positioning tables with optical encoders, and better load and wear capabilities.
The operator of advanced systems today only loads samples on the tester, tells the tester where to start and what predetermined batch of specifications to use. All of these operations are likely to be the subject of future improvements.
Depth of indentation measurementFor Brinell and microhardness testing, there is a second method of testing that is making inroads in the market. For Brinell testing, this method is discussed in ASTM E-103 standard for rapid indentation Brinell hardness testing. This standard addresses the practice of making an indentation with a shorter dwell time than the 10 to 15 seconds recommended in the ASTM E-10 test cycle. Just as important in this minimalist three-page standard is a buried reference to depth-of-indentation measurement for Brinell-type testing.
Depth testing is usually associated with the Rockwell method. Why would someone want to make a Brinell test in the manner of a Rockwell test? Anyone who has done Rockwell, Brinell and Vickers/Knoop testing can appreciate the speed, accuracy and freedom from operator involvement of the Rockwell test. A Rockwell test is done with the measurement process performed integrally to the indentation's formation. As previously mentioned, it now relies on dependable electromechanical systems for making a depth measurement to a very high resolution between the preload and full-load penetration depth.
There are some differences between the depth measurement method and the width measurement tests. This occurs in the metal's elastic recovery after the load, especially in anisotropic materials, although it is always possible to go back and verify a depth measurement with a Brinell scope. Also, the potential for downward deflection of the sample between the full load and preload positions is magnified in a 3,000 kg load compared to a 150 kg load. However, the technical developments in compensating for deflection minimizes this last concern. That leaves only the elastic recovery problem, which can be handled, in an electronic offset.
A number of manufacturers have produced depth Brinell units. Typically they are used in high-volume applications where there is a single type of part being tested in high volumes and where high accuracy is needed. Often, material handling and automatic milling-rather than grinding-is utilized to further reduce labor costs and variables like surface roughness.
In microhardness testing, the same Rockwell-like depth measurement testing has been done, although there is no standard written for it. It initially occurred as an extension of the superficial Rockwell method into the micro- and near-micro load range (< under 2 kg) as the Rockwell method capability increased. Another developmental factor relates to the most frequent, high-volume application for microhardness testing-case depth analysis of surface-hardened samples-which has traditionally been reported in a Rockwell value for the total case depth. This case depth testing has been done in Knoop and Vickers scales for years, but the traditional relationship to Rockwell lead to using a Rockwell test for the traverse testing when it became feasible to return to a Rockwell-type tester and maintain accuracy.
This depth measurement method presents several advantages over microhardness testing in addition to the obvious labor reduction and increase in accuracy from hardworking, eye-weary technicians. Because a Rockwell tester uses a preload, the surface finish of a tester does not need to be as finely finished, requiring only 400-grit final polish.
There are several limitations of the depth measurement method at micro-load levels. The sensitivity of the tester must be extremely accurate and deflection can be a factor-not due to high loads, but due to extremely sensitive measurements. Unlike Brinell testing, there are not as many elastic recovery and flow issues because the test method is so commonly used for case depth analysis with its Rockwell roots and due to the high hardness of the samples being tested. But this testing is usually limited to these large-volume, case-depth-traverse applications. It does not yet have the capability of the lighter loads or of a wide selection of loads.
Automated hardness testing's futureThe depth method has become more common as the capability of technology applicable to the Rockwell method has improved. But so far, it has only found real applications in high-volume, high-payback applications due to lack of flexibility in its capability. Another consideration is that the standardization from approved test methods has not been established. Even ASTM E-103 cautions that the test is not to be regarded as a real ASTM E-10 Brinell test. Part of the problem lies in the different methodology each manufacturer uses in their machines, and the fact that they all are trying to compare back to and replicate results from other hardness test methods like Brinell and microhardness.
One can assume that there will be further development in both standard methods of hardness testing and in applications that are appropriate for the depth method-larger volume applications with little test sample variation. One note for the future is the "sub-micro" range of hardness testing, consisting of hardness testing below a single gram. These testers are frequently using a three-sided diamond for repeatability in the indenter tip formation together with depth measurement techniques-because the limitations of the width measurement technology become more pronounced. This method of hardness testing will undoubtedly take on more importance.