Section 7.3.2 of Part 2 of NACE MR0175/ ISO 15156 states that when hardness of parent metals are taken, after welding, using the Rockwell C method, as long as no individual reading is greater than 2HRC above the specified value, you are allowed to average several readings in close proximity. For example, if you punch a 23HRC (where 22 HRC max is the requirement), you can take 3 or 4 reading (e.g. 21, 21,22, 22) and if the average does not exceed 22, the test is valid and the qualification or procedure is successful. When using the HV5, HV10 or HRC 15N scales, the document is silent on averaging.
If you look on the ASTM E140 conversion chart, 22 HRC is approximately 250 HV, and 20 HRC is approximately 234HV. In this hardness range there is basically a 16HV difference that equates to 2 HRC—or 8 HV points for every 1HRC point. It does not seem to make any technical sense to reject a weld procedure for being off say, 1 2 or 3 HRV points, which would amount to less than 0.5 HRC points. It seems to that averaging a few points (for example, if you punched a 252HV in one area) would be the logical and prudent thing to do.
The basic question would be is there anything preventing someone from averaging HRV values right now? If there is, does the group feel that a formal ballot would be warranted?
The referee hardness tests in for proving out isolated hard readings is not currently addressed in NACE MR0175/ISO 15156. A successful ballot would be required to add this into the standard.
This question is in relation to NACE MR0175/ISO 15156-2 7.3.2 and 7.3.3
Reference: ISO 15156 Maintenance Panel Inquiry #2016-10
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