FAQ's

Is there a Fahrenheit to Centigrade discrepancy in NACE MR0175?

The age hardening temperature midpoint in Centigrade for UNS S17400 and UNS S15500 in Tables A.27, A.28, A.29 and A.30 is 620 C; should be 621 C.
NACE MR0175-3 2015
In Centigrade, 634 °C max.
For comparison, NACE MR0103-2015:
In Centigrade, 635 °C max.
I believe the intent is to adhere to the Fahrenheit range of 1125 – 1175 F, or 1150 +/- 25 F.
By direct conversion, 1125 – 1175 F = 607.2 – 635 C, which is 621 +/- 14 C. Not 620 +/- 14 C. NACE MR0103 has it right, NACE MR0175-3 is off by 1 degree.
Note that in Table A.28 for UNS S45000, it has it correct as 621 C.
Why does this one degree matter? When a heat treater reports age hardening temperature in Centigrade as 635 C, in Centigrade it meets MR0103, but not MR0175 and therefore it is not acceptable for NACE MR0175 service. When converted to Fahrenheit it meets both.
Additionally, in Table A.29 for USN S15700, c) has 560 C = 1150 F. But 560 C = 1040 F. 565 C = 1150 F. That too should be corrected to 565 C to match ASTM A564 Table 4, and I note that there is no tolerance, or min./max. stated in this table, but maybe that is OK.
What steps need to be taken to initiate this change?
Background: There are inconsistencies and errors in the specified age hardening temperatures and ranges for precipitation hardening stainless steels.
1. For UNS 17400 and UNS 15500, NACE MR0175-3 Tables A.27 (S17400 only), A.28 and A.30 specify age hardening temperatures as (620 + 14) °C [(1150 +- 25) °F]
Note that 620 °C = 1148 °F; 621 °C = 1150 °F
For comparison, NACE MR0103 (clause 13.9.2.3 & 4) specifies age hardening for these same materials as 621 °C + 14 °C (1 150 °F ± 25 °F)
2. For UNS S45000, NACE MR0175-3 Table A.27 specifies (620 + 8) °C [(1150 + 15) °F].
For UNS S45000, NACE MR0175-3 Table A.28 specifies (621 + 8) °C [(1150 + 14) °F].
For UNS S15700, NACE MR0175-3 Table A.29 specifies 620 °C (1150 °F) (no range specified).
For UNS S45000, NACE MR0175-3 Table A.30 specifies (620 + 8) °C [(1150 + 15) °F].
For comparison, NACE MR0103 (clause 13.9.2.5) specifies age hardening for UNS S45000 as 621 °C (1 150 °F) (no range specified)
For comparison, NACE MR0103 (clause 18.6.2) specifies age hardening for UNS S15700 as 621 °C (1 150 °F) (no range specified)
Q1: Is it correctly understood that the occurrences in MR0175-3 of 620 °C in Table A.27, A.28, A.29 and A.30 are in error and those should be changed to 621 °C?
A1:

For BOTH UNS S17400 and UNS S45000, NACE MR0175-2002 has 620°C AND NACE MR0175-2003 has 621°C. In NACE MR0175/ISO 15156-2003, UNS S17400 has 620°C and UNS S45000 has 620°C in Tables A.27 and A.30 but 621°C in Table A.28. The 1150°F in the heat treatment of both these materials is a constant and the technical basis. This is a conversion issue. The conversion equation is T°C = (T°F -32) x 5/9. Using this equation, 1150°F becomes 621.1°C. 621°C is the correct conversion for these materials in Tables A.27, A28 and A30.
The conversion issue for UNS S15700 and UNS S15500 respectively in Tables A.29 and A.30 is identical and the correct conversion from 1150°F is 621°C.

Reference: MP Inquiry 2017-09

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