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According to NACE, what is the correct chemistry of inconel 625 (UNS N06625)?

Question:

For the interpretation question of NACE spec MR0175-3, table D.3 Inco 625 alloy chemistry. I believe it is a “typo” but need your concurrence or interpretation if it is not a “typo”. Table D.3 as attached below lists Inco 625 chemistry as Nb max weight % 3.15 to 4.15. Typical Inco 625 would have Tantalum in addition to Niobium and typical mill chemistry would specify Nb + Ta as 3.15 % to 4.15 % instead of Nb only. Industry standard spec ASTM B446 chemistry for Inco 625 bars specifies Nb + Ta as 3.15 to 4.15% instead of Nb only. So the question to you (or to NACE) is as follows: 1. Is it a “typo” in the NACE spec and it should be interpreted as Nb + Ta as 3.15% to 4.15% instead of Nb only as specified in the MR0175 spec? 2. If it is not “typo”, is there any technical reason why Tantalum would not be accepted as part of standard Inco 625 chemistry?

Answer:

1) No. 15156-3 Annex D is an INFORMATIVE annex. ISO 15156 specifies material requirements for specific material groups or UNS numbers. The chemistries of these UNS numbers are established by the international standards organizations that maintain the UNS. Annex D lists chemical composition data using the values and elements defined by UNS. It should be noted that the compositions listed in Annex D have no effect on the NORMATIVE composition limits in Table A.12.

2) The scope of the MP is limited to interpretation to wording in ISO 15156. The technical reason for tantalum not being included in Annex D for UNS N06625 is, as stated above, the standard adopted the UNS chemistries.

This question is in relation to NACE MR0175/ISO 15156-3 Table D.3

Reference: ISO 15156 Maintenance Panel Inquiry #2013-01

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