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API-571 Exam - Topic 1 Question 122 Discussion

(What steel alloy is no longer recommended for services susceptible to HTHA?)
C) 1 Cr--0.5 Mo steel
A) Mn--0.5 Mo steel
B) 1.25 Cr--0.5 Mo steel
D) C--0.5 Mo steel

API-571 Exam - Topic 1 Question 122 Discussion

Actual exam question for API's API-571 exam
Question #: 122
Topic #: 1
[All API-571 Questions]

(What steel alloy is no longer recommended for services susceptible to HTHA?)

Show Suggested Answer Hide Answer
Suggested Answer: C

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and API RP 941 (Nelson Curves), 1 Cr--0.5 Mo steel is no longer recommended for services susceptible to High-Temperature Hydrogen Attack (HTHA).

API RP 941 documents industry experience showing that 1 Cr--0.5 Mo steels have suffered HTHA damage below previously assumed safe operating limits, due to insufficient carbide stability. As a result, this material has been removed from the Nelson Curves as an acceptable choice for new construction in hydrogen service.

By contrast:

Mn--0.5 Mo, C--0.5 Mo, and 1.25 Cr--0.5 Mo steels retain higher resistance due to more stable carbide structures.

Referenced Documents (Study Basis):

API RP 571 -- Section on High-Temperature Hydrogen Attack

API RP 941 -- Updated Nelson Curves and Material Recommendations


by Daniela at Apr 06, 2026, 02:50 AM

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Shawna
1 month ago
I’m not sure, but C) 1 Cr--0.5 Mo steel seems risky too.
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Willodean
1 month ago
Agreed, B is the one. HTHA issues are serious.
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Lashaun
1 month ago
I think it's B) 1.25 Cr--0.5 Mo steel. It's been flagged often.
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Malcom
2 months ago
Yeah, B is definitely not recommended anymore.
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Georgene
2 months ago
I thought it was A) Mn--0.5 Mo steel.
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Twana
2 months ago
Wait, are we sure about that?
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Bettina
3 months ago
Totally agree, that's the one!
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Gerri
4 months ago
It's B) 1.25 Cr--0.5 Mo steel.
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Portia
4 months ago
I vaguely recall that 1 Cr--0.5 Mo steel had some issues with HTHA, but I can't remember the details. I wish I had reviewed that section more thoroughly.
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Judy
5 months ago
I’m a bit confused; I thought C--0.5 Mo steel was still acceptable. Did we cover any specific reasons why it might not be recommended anymore?
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Maryrose
5 months ago
I feel like we had a practice question about Mn--0.5 Mo steel being less reliable under certain conditions. Could that be the answer?
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Fallon
5 months ago
I think I remember that 1.25 Cr--0.5 Mo steel was mentioned in our class as being more resistant to HTHA, but I'm not entirely sure.
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