The development of hardness scales in Europe

The development of standard references for a conventional quantity like hardness needs a large amount of cooperation between the certifying authorities to coordinate the work involved. The first problem consists in the very definitions of the hardness scales, which do not al­ low the influence quantities (mainly due to the cinematics of indentation and the stif­ fness of the machine itself, which are insufficiently or not defined in standard docu­ ments) to be completely controlled. In Europe the Bureau Communautaire de Reference (BCR) of the EEC has organized a number of international comparisons to enable the national certifying authorities to reali­ ze the differences in the results and find the way to make these more consistent. Since the first unofficial comparison to the present results, dispersions have changed, for in­ stance for the HRC scale, from more than 1.5 HRC to few tenths of HRC. HRC is one of the more deeply studied hardness scale, and the results obtained in the three international comparisons held in 1980, 1985 and · 1989 gave an indication of the very good long term stability of Hardness Standard Machines; consequently, it has been possible to define a "European HRC Scale" on the basis of measurements with the French, Italian and German Hardness Standard Machines. The international comparison activities concerning hardness scales have made it possi­ ble to obtain experimentally the typical level of uncertainty of Hardness Standard Blocks and thus avoid much misunderstanding at the industrial level.