What are the hardness requirements for cobalt alloys as per NACE MR0175?
The main obstacle we foresee is that in Section A.10 Table A.39, Environmental limits for cobalt-based alloys used as springs, we see that the requirements for UNS R30003 are:
-Shall be cold worked
-[Shall be] age-hardened
-[Shall be] maximum 60 HRC
-This requirement means, in our interpretation, that after the cold working, and age hardening, we must be able to prove that the HRC hardness of the end product falls at or below 60HRC.
-This presents a problem – due to the geometric constraints of spring design, the area presented for hardness examination can be both very narrow and thin. This drives testing to either Knoop or micro-Vickers hardness. This means we are relying on a conversion, like ASTM E140 Table 3, to approximate hardness. ASTM very clearly states that converted values are approximate, and may not be accurate. -ASTM E140 also lacks a conversion table that covers high nickel alloys in the range up to 60 HRC.
Q1: Regarding 15156-3 Table A.39 for UNS R30003, we believe that the limitations for the alloy as springs are (1) shall be cold worked, (2) shall be in the age-hardened condition and (3) shall have a maximum hardness of 60 HRC.
A1: Your interpretation is correct.
In what way is conformance to NACE MR0175:3 Table A.39 able to be assessed, for springs of small cross section which cannot be tested with the standard 150kgf HRC hardness test?
Q2: How do we prove compliance to the 60 HRC limit when (1) the geometry prevents use of HRC and (2) suitable hardness measurements such as Vickers (DPH) or Knoop can be used but there are no published conversion Tables such as ASTM E140 that apply to this alloy/hardness range?
A2: This is outside the scope of NACE MR0175/ISO 15156 but there is no prohibition from you developing hardness conversions based on test data.
Additionally, can the material also be considered compliant without age-hardening?
Are there corrosion issues related to non-age hardened cobalt alloys?
Q3: Additionally, can UNS R30003 also be acceptable in the cold worked condition without age-hardening? Additionally, are there any corrosion issues with the nonage hardened cobalt base alloys?
A3: There was a recent successful ballot to NACE MR0175/ISO 15156-3 Table A.40 that once published will permit the use of the UNS R30003 in both the cold worked and the cold worked + aging conditions. However, Table A.39 does not include the as cold worked condition without subsequent age hardening. The inclusion of this ascold worked condition to Table A.39 would require a successful ballot to NACE MR0175/ISO 15156.
This question is in relation to NACE MR0175/ISO 15156-3 Table A.39
Reference MP inquiry #2017-07
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