Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 532 of 843

SOURCES OF ERRORS Vibration During the indentation process, while the indenter is in contact with the test part, vibration may be a major factor, often underestimated. The slightest vibration during this period will give low, false readings. Running motors; fans; people walking near the machine; auto, truck, or train traffic nearby; even thunderstorms in the area can cause significant vibration, which affects the microhardness readings. It is important to place the microhardness testing machine in a fashion and location to minimize any vibration effects. The operator should be aware of this sensitivity to vibration and keep his or her motions smooth and maintain minimum movement during the indentation process. At lower test loads vibration is even more critical. ASTM E 384 limits vibration to less than 0.005 g (2 in/sec2). Slight, sporadic vibration can be a major source of the wide ranges often encountered in microhardness readings. Equipment Problems Mechanical: Indentation process must be smooth, with a controlled velocity (slow) of the indenter. Full force should be applied for usually 10 or 15 seconds. Longer load-retention times can be more prone to the effects of vibration. ASTM E 384 contains information on verification and calibration of microhardness testing machines. Optics: 400 magnification should be the minimum; higher magnification would be better. Electronically projected images are more easily measured. Results obtained with lower magnification have varied widely and not agreed with results at 400 and higher. Indenters: Should be examined at about 100× periodically for defects. Any chipped indenter must be replaced. Sample Preparation Problems Surface not perpendicular to indenter: This results in unsymmetrical indents, which can be corrected by repolishing the mount or by the use of a stage-leveling device. Excessive surface scratches: These can interfere with locating the exact start of the indentation. Repolish the sample. Sample overheated or surface work-hardened during preparation: A thin layer of workhardened material may be removed by repeated light etching and light polishing steps. If the sample has been accidentally heat treated a new sample must be prepared. Measurement Problems With a machine properly calibrated against a stage micrometer it has been determined that measurements should not vary more than 5%. The range of hardness numbers, however, increases substantially when testing harder materials (with the widest ranges found with the Vickers indenter), and measurement errors will exceed 5%. With microhardness numbers over about 800 HK100, considerable variance can be expected, and it is often recommended that a practical range be specified, rather than a single figure. Other Common Errors Reporting errors: Omitting force (load), not expressing hardness numbers properly, and not including the other data as required by the pertinent specifications. Converting from one hardness scale to another: Most specifications advise against 523

Articles in this issue

view archives of Metal Finishing Guide Book - 2013