FAQ's

NACE MR0175 and the heat treatment of Inconel X750

 

Based on NACE paper 03133, the cause of failure of UNS N07750 was identified as sensitization of the material when it was exposed to a temperature of around 2000⁰F and cooled slowly. The paper points to the lack of defined quality control procedures, specifically the definition of Solution Heat Treatment. This no longer seems to be the case as Table A.27 of NACE MR0175-3 lists very specific heat treating process requirements for UNS S17400.
If the heat treatment of UNS N07750 were properly controlled per a well-defined standard such as AMS 5667, would this material be acceptable for qualification by lab testing or field experience?

Or are there additional causes to doubt this materials ability to resist hydrogen embrittlement or SSC?

ANSWER:

Question 1: If the heat treatment of UNS N07750 were properly controlled per a well-defined standard such as AMS 5667, would this material be acceptable for qualification by lab testing or field experience?
Please review , Clause 6.1 of ISO 15156-3. You may test in accordance with Annex B of ISO 15156-3 and, with successful tests, the alloy would be acceptable for use with the agreement of the end-user for the application. The documentation requirements for these tests shall be in accordance with ISO 15156-1 Clause 9. You may also use field experience noting the requirements of ISO 15156-1 Clause 8.2 with the documentation requirements of Clause 9.
NACE Corrosion 2003 Conference Paper 03133 demonstrated a sensitization of X750 alloy at grain boundaries as the root cause of embrittlement. Question 2: are there additional causes to doubt this materials ability to resist hydrogen embrittlement or SSC?

There are known cases of embrittlement in this alloy due to hydrogen charging. We do not know whether some or all the reported instances were due to sensitization.

Reference: MP INQUIRY #2016-16 (Ref NACE MR0175 part 3 A.8.2, Table A.27)

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