Question: I am looking for a definitive interpretation of NACE MR0175 concerning nickel-based alloys. My company produces alloy 20 (UNS N08020). The standard ASTM/AMS/UNS composition is listed in Table D3 on pg. 65 showing a Mo range of 2.0-3.0. Section A.4 of MR0175 contains Table A.12–Materials types of solid solution nickel-based alloys. This table does not list any specific alloys, but it does list the minimum Mo (which I assume is for compliance with the standard) at 2.5%. Does that mean that in order to certify our 20Cb3 to MR0175 the heat must have at least 2.5% Mo, even though the acceptable Mo range is 2.0-3.0? If so, what is the impetus behind requiring a higher Mo range than is standard for the alloy?
You are correct, a minimum of 2.5% Mo is required for N08020 to be compliant with Table A.12, material types 4a and 4c. Heats of N08020 not meeting the 2.5% Mo minimum may be acceptable as “Highly alloyed austenitic stainless steels” under the limits provided in Tables A.8, A.9, and/or A.11.
The 2.5% Mo restriction for N08020 is similar to the PREN restrictions for Duplex Stainless steels. The full chemistry ranges of the UNS numbers listed in the Annex D tables are for reference only.
This question is in relation to NACE MR0175/ISO 15156-3 A.4, Table A.12
Reference: ISO 15156 Maintenance Panel Inquiry #2011-07
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