Minimum thickness requirements for Rockwell hardness testing

A study was performed in an attempt to resolve the conflicting thickness requirements for Rockwell hardness testing specified in ASTM Standard Test Methods E 18-93 and in ISO Standard 6508. Commercial standardized hardness test blocks were machined to ''steps'' of various thicknesses using a wire EDM (electrical discharge machining) process. Rockwell hardness tests were performed on the original standardized surface of these ''stepped'' blocks in an attempt to determine the minimum thicknesses required for reliable Rockwell hardness testing. The extremes of test force and hardness were investigated for both the diamond brale and 1/16 in. (1.588 mm) ball indenters. Results of the study indicate that requiring a single multiple (10 or 15) times a measure of indentation depth (total depth or permanent increase in depth from the preliminary test force to the total test force) is not the best way of specifying minimum test piece thickness. In general, higher multiples are needed for harder materials (when the indentation depth is small) and when the 1/16 in. (1.588 mm) ball indenter is used, Use of individual, empirically determined figures and/or tables to specify minimum thickness appears to be necessary. This report was submitted to the members of ASTM Subcommittee E28.06 on Indentation Hardness Testing at their May 1995 meetings in Denver, CO. Once the subcommittee has made a decision as to the best approach to use in ASTM Methods E 18, the report will be circulated to members of the European Standards and ISO hardness testing bodies in an attempt to standardize Rockwell hardness test piece minimum-thickness requirements worldwide.