Acceptable Hardness of CA6NM under NACE MR0175/ISO 15156

This page addresses an inquiry regarding the acceptable hardness of CA6NM under NACE MR0175/ISO 15156. The inquiry notes that earlier editions of the NACE standard had a note stating that the maximum permissible value for CA6NM is 255 BHN, but this statement is not included in the current version. The inquiry seeks clarification on whether CA6NM is acceptable per MR0175/ISO 15156 at a maximum hardness of 255 BHN, which is equivalent to 23 HRC.

Explanation of Prescribed Hardness Limit

The answer explains that the prescribed hardness limit of 23 HRC for CA6NM in Table A.18 of NACE MR0175/ISO 15156-3 is based on the Rockwell C scale. The standard no longer includes conversions to other hardness scales, but other scales may still be used if a correlation can be shown between them and the prescribed Rockwell C scale for the specific material being tested. The conversion between hardness scales is material-dependent, as stated in Paragraph 6.2.1 of NACE MR0175/ISO 15156-3. The user may establish the required conversion tables.

Reference and Citation

This question pertains to NACE MR0175/ISO 15156-3 A.6.2 Table A.18, and the reference cited is ISO 15156 Maintenance Panel Inquiry #2004-18 Q1.