This paper presents the minimum specimen thickness limit (
Tmin) of copper alloys (brass, 2 kind of phosphor bronze and nickel silver). Standardized hardness blocks were tested by HRB·HRBQ, HR 30 T·HR 30 TQ scales, errors of the similarity in hardness were found to be lower than 1% (2nd report). To test thinner specimen, scales are extended to HR 15 T·HR 15 TQ. Hardened steel spot anvil (SSA) and diamond spot anvil (DSA) are used, both spot diameters are 4 mm. Criterions of the
Tmin, are : (1) Difference of hardness measured with SSA, DSA is lower than ±0.1 hardness unit. (2) The bulge or mark appears on underside is barely visible. (3) The elastic deformation of SSA is reasonable that is 0.2-0.9 hardness unit depending on the specimen thickness. This test uses specimens which are extracted from usual factory products. They were hardened by cold rolling and hardness were changed by working ratio, heat treatment, tension annealing, etc. Therefore the hardness of these copper alloys generally vary throughout thickness of specimens and the thickness is decreased by etching (HNO
3) and polishing, the hardness changes. The values in the test results shoud be taken as average ones. The hardness test data are fitted with second or third order polynomials by least square method. The above 5 materials are divided into 2 groups. (1) brass, phosphor bronze (Bs, PB) and (2) Nickel silver (NS) in the relation between
Tmin and hardness. The hardness for the same
Tmin differs significantly. HRB 3.7-7.3, HR 30
T 1.8-3.1, HR 15 T 1.8-3.0, in middle and high hardness range (H, EH, SH). But the difference between polynomials within the same group is lower than 1.0 unit (except phosphor bronze ≤ 1.4).
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