ASTM E92-16 pdf download.Standard Test Method for Vickers Hardness of Metallic Materials.
5. Test Specimen
5.1 The Vickers hardness test is adaptable to a wide variety oftest specimens ranging from large bars and rolled sections to minute pieces in metallographic mounts. In general the backs of the specimens shall be so finished or the specimens shall be so clamped that there is no possibility of their rocking or shifting under the test force. The specimens shall also conform to the requirements given in the following 5.1.1, 5.1.2, and 5.1.3.
5.1.1 Thickness—The thickness of the test specimen shall be such that no bulge or marking showing the effect of the force appears on the side of the specimen opposite the impression. In any event the thickness ofthe specimen shall be at least one and one halftimes the length ofthe diagonal. When laminated material is tested, the thickness of the individual component being tested shall be used for the thickness- diagonal length relationship.
5.1.2 Finish—The surface of the specimen should be so prepared that the ends of the diagonals are clearly defined and can be read with precision of 60.0005 mm or 60.5 % of the length of the diagonals, whichever is larger. Care should be taken in specimen preparation to avoid tempering during grinding, or work-hardening the surface during polishing.
5.1.3 Alignment—The specimen should be so prepared or mounted that the surface is normal to the axis of the indenter within 61° of angle. This can readily be accomplished by surface grinding (or otherwise machining) the opposite side of the specimen to parallelism with the side to be tested. 5.1.4 Radius ofCurvature—Until further investigative work is accomplished to determine the effect of the radius of curvature on readings, due caution should be used in interpret- ing or accepting the results of tests made on cylindrical surfaces.
6. Verification of Apparatus
6.1 The hardness testing machine shall be verified as speci- fied in Part B.
6.1.1 Two acceptable methods ofverifying Vickers hardness testing machines are given in Part B.
7. Procedure
7.1 Magnitude of Test Force—Test forces of 1 kgf to 120 kgf may be used, depending on the requirements of the test. Although tests on homogeneous materials indicate that the Vickers hardness number is nearly independent of the test force, this condition will not be present in cases where there is a hardness gradient from the specimen surface to the interior of the specimen. The magnitude of the test force should therefore be stated in the test report (Section 11).
7.2 Application of Test Force—Apply the test force and release smoothly without shock or vibration. The time of application of the full test force shall be 10 to 15 s, unless otherwise specified. 7.3 Spacing ofIndentations—The center of the impression shall not be closer to any edge of the test specimen or to another impression than a distance equal to two and one half times the length ofdiagonal ofthe impression. When laminated material is tested, a bond surface shall be considered as an edge for spacing of indentation calculations. 8. Measurement of Impression
8.1 Both diagonals of the impression shall be measured and their mean value used as a basis for calculation of the Vickers hardness number. It is recommended that the measurement be made with the impression centered as nearly as possible in the field of the microscope. 8.2 In the case of anisotropic materials, for example mate- rials that have been heavily cold worked, there may be a difference between the lengths of the two diagonals of the impression. In such cases, the test specimen should be reori- ented so that the diagonals of a new impression are approxi- mately of equal length. 9. Accuracy 9.1 The accuracy of the Vickers hardness method is a function of the accuracies of the test force, indenter, and measuring device. The condition of the test and support surfaces and support of the test piece during application of the test force also affect accuracy. Under optimum conditions of these factors the accuracy that can be expected is the equivalent of 4 % of the Vickers hardness number of the standardized reference hardness test blocks (see section 18.2). Under less than ideal conditions the reduction in accuracy, when required, can be established empirically by employing statistical methods.ASTM E92 pdf download.