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?)

AMn--0.5 Mo steel

B1.25 Cr--0.5 Mo steel

C1 Cr--0.5 Mo steel

DC--0.5 Mo steel

Show Suggested 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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Judy

10 hours 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

6 days 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

11 days 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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