FALL 2024

130 THE DISTRIBUTOR’S

130 THE DISTRIBUTOR’S LINK ROB LaPOINTE HARDNESS TESTING SMALL FASTENERS: USING MACROHARDNESS & MICROHARDNESS TECHNIQUES from page 90 FIGURE 4 MICROHARDNESS TEST INSTRUMENT WITH MICROSCOPE OPTICS AND THERMOSETTING PLASTIC MOUNT Sanding, griding or cutting a small distance into surface is sufficient to avoid a potential effect from decarburization. Testing the fastener in the arbitration location is the best practice to avoid surface effects such as decarburization. Sample preparation for microhardness testing is significantly more involved than for macrohardness testing. Microhardness indentations are just a few tens of micrometers (1 micrometer = 0.000039 inch) across and are so small that they typically cannot be seen without magnification. Thus, the term microhardness is used to denote measurement with the aid of a microscope (Figure 4). Depending on the hardness of the material and the load applied during testing, the observation magnification is usually 100-500 times that of normal vision. Both Vickers and Knoop microhardness results are calculated based on the width of the indentation. Since the microhardness indentation is so small, the result is particularly susceptible to the smoothness of the surface being tested. Samples must be ground and polished to a mirror-like finish to achieve accurate results (Figure 5). Surface scratches seen through the microscope may cause the indenter to not travel as FIGURE 5 MIRROR POLISH OF MICROHARDNESS SAMPLES deeply into the material during the loading phase of the test and therefore the indentation will not be as wide as it would be on a scratch-free surface. The resulting hardness value will be higher than it should be due to the decreased penetration into the material. Microhardness samples are typically mounted in a thermosetting or catalyst-setting plastic to aid the surface preparation process (Figures 4 and 5). The mounted samples are placed in grinding and polishing machines which facilitate the surface preparation. Instrument Preparation The hardness testing instrument must be in good working order and free of debris on top of and under the testing anvil or stage and on the indenter (Figure 6). Dust, oil or debris on the anvil, stage or sample may cause the instrument to indicate a softer hardness value as those materials can compress under the test load. Rockwell testers are particularly susceptible to the compression of debris under the test sample or anvil. When the sample is loaded, the debris will compress, causing the indenter to travel further. CONTINUED ON PAGE 150

MWFA FSTNR WEEK ’24 TABLE TOP SHOW AND FASTENER BASH - AUGUST 20, 2024 CONTINUED ON PAGE 145