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Is CA6NM acceptable per MR0175/ISO 15156 at a hardness of max 255 BHN?

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Question:My inquiry concerns CA6NM: Earlier editions of the NACE standard contain a note stating that the hardness correlation in ASTM E 140 doesn’t apply to CA6NM and that for this material the maximum permissible value (in Brinell) is 255 BHN. In NACE MR0175/ISO 15156-3, this statement is no longer used. However, Paragraph 7.3.2 of NACE MR01756/ISO 15156-2 stipulates that users can establish hardness correlations for individual materials. Please see below: Quote For ferritic steels EFC Publication 16 shows graphs for the conversion of hardness readings, from Vickers (HV) to Rockwell (HRC) and from Vickers (HV) to Brinell (HBW), derived from the tables of ASTM E 140 and BS 860. Other conversion tables also exist. Users may establish correlations for individual materials. Unquote Finally the questions: Is CA6NM acceptable per MR0175/ISO 15156 at a hardness of max 255 BHN which has been (empirically) determined to be the equivalent of 23 HRC (but which on the ASTM E 140 scale corresponds to about 25 HRC)?

Answer:

The prescribed hardness limit of 23 HRC for CA6NM in Table A.18 in NACE MR0175/ISO 15156-3 utilizes the Rockwell C scale as the basis for acceptance. Conversions to other hardness scales are no longer included in the standard. Other hardness scales may still be used provided a correlation can be shown between the scale used and the prescribed Rockwell C scale for the particular material being tested. As stated in Paragraph 6.2.1 of NACE MR0175/ISO 15156-3, conversion between hardness scales is material-dependent. The ISO Maintenance Panel cannot make this conversion for you. The user may establish the required conversion tables.

This question is in relation to NACE MR0175/ISO 15156-3 A.6.2 Table A.18

Reference: ISO 15156 Maintenance Panel Inquiry #2004-18 Q1

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