Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 515 of 707

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 dur- ing this period will give low, false readings. Running motors; fans; people walk- ing 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 loca- tion to minimize any vibration effects. The operator should be aware of this sen- sitivity 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 encoun- tered 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 test- ing 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 work-hardened material may be removed by repeated light etching and light pol- ishing steps. If the sample has been accidentally heat treated a new sample must be prepared. With a machine properly calibrated against a stage micrometer it has been determined that measurements should not vary more than 5%. The range of hard- ness 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 vari- ance can be expected, and it is often recommended that a practical range be spec- ified, rather than a single figure. Measurement Problems Other Common Errors Reporting errors: Omitting force (load), not expressing hardness numbers prop- erly, and not including the other data as required by the pertinent specifications. Converting from one hardness scale to another: Most specifications advise against 514

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