News. Practical experiments of manual vs. automated hardness testing

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1 Hardness Testing News 1 Welcome Welcome to the second edition of this newsletter, focusing entirely on hardness testing. The newsletter will be published 3-4 times per year and will include a combination of application studies, real life case examples and Struers hardness testing product news. We hope you will find the newsletter interesting and that you will forward it to your colleagues working with hardness testing. Content In this issue: Practical experiments of manual hardness testing Hardness testing on rail segments experiences from a customer New DuraVision models and accessories Application Practical experiments of manual vs. automated hardness testing A number of factors influence the accuracy and repeatability of the hardness result. If these factors are not controlled or eliminated, it can result in inaccurate or variable test results. One of the factors influencing the test results is the influence of the operator during optical indent evaluation, as used in Vickers, Knoop and Brinell testing. In the following will be described a few practical experiments illustrating the potential consequence of the operator influence. a) In the first experiment, 14 different operators, all with years of experience, were asked to manually measure the same Vickers hardness indent (~115 HV0.05) on a PC with hardness testing software installed. Test conditions were reasonable and according to standard and provided operators a decently sized indent to measure. Findings were that hardness values ranging from 108 to 118 HV0.05 were obtained (with a standard deviation of 2.8), i.e. a variation of almost 10%.

2 2 As the hardness evaluation was carried out on the same indent and under same conditions, the observed differences can therefore be related to the human influence during the evaluation process. This is quite interesting as all operators had many years of experience within hardness testing. Both Knoop and Vickers hardness numbers increase in proportion to the inverse of the square of the indentation diagonal length. An absolute measurement variation or error of a few µm can therefore have great impact on the resulting hardness number, especially when working with small indents. So although an experienced operator can reduce the variation in manual hardness indent measurement carried out by him, a potential problem occurs when multiple operators are involved. So if that s the case, and unless thorough training is carried out, hardness testing equipment with an automatic indent evaluation is therefore strongly recommended.

3 3 b) In the next experiment are demonstrated variations of hardness evaluations made on a calibrated hardness test block. This time 36 different hardness indents is made on the test block and each indent is measured manually and automatically by 12 different (experienced) operators. Not surprisingly, and similar to the first experiment, it is observed that manually evaluated hardness values show a much greater variation than automatic evaluated values (11% variation vs. 2%). But it is also observed that manual evaluations have resulted in a slightly higher average hardness value (713 vs. 698 HV0.2). Although reasons for this can be many, human operators apparently have a tendency to measure the diagonals shorter than they are supposed to be (thus resulting in higher hardness values) something also observed in other experiments. Part of this error could be due to the thickness of the lines used for indicating the indent tips, i.e. confusion of whether the lines should be placed just across the indent tips, just inside the indent tips or just outside. Usually the lines should be placed just outside the indent tips, although the correct placing naturally depend on the actual equipment used and how the calibration has been carried out. Other reasons could be uneven the focusing of the indent or manual tampering with the illumination resulting in the indent to appear smaller. Under all circumstances hardness testing equipment with an automatic indent evaluation, as well as automatic focus and illumination control, is therefore strongly recommended.

4 4 Case Example Hardness testing on rail wheels Below example illustrates how a large steel manufacturer uses a Struers DuraVision-x50 for fully automatic Brinell and Rockwell testing on railway wheel cross sections. Purpose of tests: Due to the high pressure and wear of railway wheels, the customer must check that the material hardness lies within certain hardness intervals to ensure adequate wear resistance and ductility of the material. Sample

5 5 Sample characteristics: Samples are cut into pieces of app. 130 x 100 mm, using the Struers Exotom-150 cutting machine. Due to the good surface quality hereby obtained, as well as the automatic illumination adjustment of the DuraVision-x50, hardness tests can be performed directly on the cut surface. The sample can therefore be placed directly on the XY-stage of the hardness tester, as shown below. Sample seen from side

6 6 Test point pattern: The location of the test points are specified by American Association of Railroads (AAR) for testing of rail wheels (see below). Tests are made with Brinell HBW 10/3000 and Rockwell C at specified locations. At initial setup test points are programmed and positioned under the overview camera of the hardness tester, as shown below. When tests are performed, the test point pattern, incl. all related settings, are automatically stored as a template for easy setup next time tests are to be performed: Location of test points on the rail wheel cross section. Left side Right side

7 7 Test run and evaluation is carried out fully automatically. If the DuraVision-x50 is equipped with a motorized turret (as in this case), the Brinell and Rockwell series can be executed simultaneously. Example of HBW 10/3000 indent Outcome: The entire test run, including setup, takes minutes to complete (3x rows of HRC and 3x rows of HBW 10/3000), in total ~40 measurements. Operator influence is only required during initialization of the test. When finished, test results can be exported to Excel or reported directly in the reporting software. Due to the template management capabilities of the ecos Workflow software, the test point pattern is automatically saved and can easily be loaded and positioned for the next sample, saving significant setup time and minimizing risk of errors. All in all the automation of the hardness test procedure, as indicated above, has saved the customer significant time compared to previously, as well as optimized the repeatability and reproducibility of test results.

8 8 Product News New DuraVision models! Struers have introduced new DuraVision models (DuraVision-40/-400/-450) covering a (main) test load range of kgf. They complement the existing models with test load ranges kgf and kgf respectively. Together with the new models we are also introducing various new accessories, including a QR-code scanning function, an improved data export editor, nose cone inserts for small samples, a foot switch and a remote access interface. Read more about the DuraVision and the new accessories here NEXT In the next couple of issues we will cover: Influence of the mounting resin on the test result Influence of the surface preparation on the test result Automatic case hardness depth measurement Hardness testing on ceramics Hardenability evaluation via Jominy / end quench testing