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An interlaboratory test program was conducted by ASTM Committee E-4 on Metallography, according to ASTM Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods (E 691), to develop information regarding the precision, bias, repeatability, and reproducibility of measurements of Knoop and Vickers microindentation impressions. Both types of indents were made using loads of 25, 50, 100, 200, 500, and 1000 gf (five of each type at each load) using three ferrous and four nonferrous specimens of varying hardness. The indents were measured by 24 laboratories. Analysis of the test results according to E 691 have been used to prepare a Precision and Bias section for ASTM Test Method for Microhardness of Materials (E 384).
Fourteen laboratories measured the indents in the three ferrous specimens and nine labs had similar Vickers hardness measurements. Of the remaining five laboratories, two were consistently lower while three were consistently higher in measured Vickers hardness. For the Knoop indents in the ferrous specimens, the results were similar except that one lab that got consistently lower Vickers hardnesses had acceptable Knoop hardnesses.
Twelve laboratories measured the indents in the four nonferrous specimens, and the hardness data were in better agreement than for the ferrous specimens due to the much larger indents in the nonferrous specimens. For the Vickers data, one laboratory was consistently lower in hardness while two laboratories were consistently higher in hardness. For the Knoop data, three laboratories were consistently lower in hardness while one laboratory was consistently higher in hardness.
Three laboratories measured both ferrous and nonferrous Vickers and Knoop indents, although one of these laboratories (N) measured only one of the nonferrous specimens. Test results for laboratories N and O were acceptable while those for laboratory M were consistently lower in hardness for all specimens and for both Knoop and Vickers indents. This result suggests a consistent bias either in the calibration or the manner in which the indents were sized.
The repeatability and reproducibility intervals increased with increasing specimen hardness and decreasing test load, that is, with decreasing indent size. The within-laboratory and between-laboratory precision values improved as the specimen hardness decreased and the test load increased, that is, as the indent size increased.
microhardness, microindentation hardness, Knoop hardness, Vickers hardness, load, precision, bias, repeatability, reproducibility
Vander Voort, GF
Supervisor, Metal Physics Research, Carpenter Technology Corp., Reading, PA