Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 513 of 707

WHAT LOAD (FORCE) TO USE It is important to recognize that microhardness numbers can vary with the force used. This is more pronounced at the lower forces. For the harder plated met- als, such as hard copper, nickel, or chromium, 100-gram force (0.981 N) should be used. Higher loads of 200-gram force generally show less variance of readings between operators and laboratories but can produce cracking, not always appar- ent, with the harder, more brittle deposits. At loads of 50-gram force or less, high- er values are obtained with considerably more variance, especially with harder materials. Many specifications recommend against using 50-gram loads with these harder materials. Lower forces of 25-gram force or less should be used only for the softer metals such as gold or lead. HOW MICROHARDNESS IS EXPRESSED The correct way to express microhardness is to place the microhardness number first, followed by the term for the scale (indenter) as either HK, for hardness Knoop, or HV, for hardness Vickers, with the force used as a subscript. ASTM B 578 specifies this in grams force. The international ISO 4516 specification expresses the force in kilograms force. Both specifications agree that the system of designation of hardness shall be such that the numerical value of the hardness number remains unaffected by the introduction of the SI unit of force, the newton, in place of the old unit. (The force, in newtons, divided by 0.00981 is equal to gram force.) Thus, a microhardness of 550 using a 100-gram force (0.981 N) and a Knoop indenter should be expressed as 550 HK100 . RECOMMENDED PROCEDURE Preparation of the Sample Adequate plate thickness is necessary to achieve good results. Minimum plate thicknesses of 0.002 in. (about 50 mm) are preferred. Using the Knoop indenter an absolute minimum of 0.001 in. (25 mm) is sometimes used, although con- sidered too thin by some. When production parts have insufficient plate thick- ness, it will be necessary to continue the plating of a sample part under constant conditions until minimum thickness is obtained. The use of test panels plated under the same conditions as the work is a common practice. Except in rare instances, hardness is measured on a cross-section of the plated part. Cut the sample using adequate cooling to avoid any accidental heat treating of the specimen. Overplate the specimen with another metal to protect the edge and provide a visible contrast; e.g., use copper to overplate nickel or vice versa. The hard- ness of the overplate should be in the same range of hardness as the plate to be test- ed. Usually, an overplate thickness of about 0.001 in. (25 mm) is satisfactory. Mount the specimen in a plastic medium suitable for metallographic work. If the hardness of the specimen may be affected by the time at temperature involved in the mounting operation use low-temperature-setting plastics. Grind and Polish the Mount Avoid heat and "buffing" or work hardening the surface. Normal metallographic techniques require using successively finer grinding and polishing media, rins- ing carefully between steps. The mount is rotated 90O at each step to help keep the surface of the mount flat and parallel with the bottom of the mount. The final finish should show few, if any, scratches when viewed at 400 magnification. The final polishing step is often a fine-grit diamond paste. 512

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