High Strength Fittings in Sour Service are Prone to SSC

Hello fellow Stressers!!

My question is not related to stress analysis. It's somewhat related to Piping Material Engineering. I have received a comment regarding a reducing tee in which the material is "A234 Gr WPB, SMLS", in which I've prepared a Material Requisition. The comment is shown below:

"Check connected pipes to be API 5L Gr B or Gr X60 accordingly A234 WPBW or A860 fitting to be selected as high strength fittings in sour service are prone to SSC."

My question are the following:

1) Does A234 WPBW (Gr. B) fitting material is better than high strength fittings (Gr. X60) in terms of sour service and is not prone to SSC?

2) Is it necessary that pipes & fittings should have same grade (API 5LB pipe to A234 WPB fitting) & (API 5L X60 pipe to A860 WHPY 60 fitting) or they can be mixed? I'm wondering why we need to check the material of connected pipe, maybe this is the reason.

3) What is SSC and why the high strenght fittings (A860 WHPY 60) is prone to this and not the low or medium strenght fittings (A234 WPB)?

When I search google, A860 is high strength fittings, but in the comment it seems like it can be selected for sour service which is conflicting with the statement that high strength fittings is prone to SSC in Sour Service.

Please correct any wrong statement I've made, as I am starting to enter the world of Material Engineering & trying to read some NACE stuff.

Cheers!!!

My understanding is the originator of the comment is rather concerned on pipes material in your system, refers to possible combinations API 5L Gr B- A234 WPB vs higher grades API Gr X60- A860 WHPY 60; in the end of the sentence gives a reason (which would be better said vs the concern) as "fitting to be selected as high strength fittings in sour service are prone to SSC".

In practice SSC is frequently associated with welds- this may be the root of concern on mixing materials, you risk to develop SSC in the connected pipes, even the tee material selection is correct.

Sulfide Stress Cracking is a form of cracking mechanism in contact with sour/ wet H2S. The qualification of material is by NACE MR0175 for Oil&Gas applications. I don't know your case needs NACE qualification.

In fact today NACE describes the mechanism of SSC as either type I SSC (SOHIC) or type II, typical hydrogen embrittlement and the details are quite complicated. Into these complex details/mechanism theories one can find an explanation of the practical fact that SSC is prevented (but there are limits of prevention, you can reduce the risk as acceptable vs H2S amount) by minimizing the hardness and strength of the alloys used in wet H2S systems; this is accomplished through material selection, weld procedures and PWHT.

Thanks Mariog. In this case, it's really not all about strength that we need to look at to decide if material is acceptable or not for a particular case (especially the wet sour service). I think that's the reason why I have read in one spec "The flange & pipe should have equal yield strength", & the spec have given some combination of flange-pipe material that is acceptable.

Any other opinion is highly appreciated.

Cheers!!!

For carbon steel, strength is like a warning alert in corrosion case; mild carbon steel- may be good for service; hard/ high strength carbon steel- likely to corrode, especially when welded.
But for sure this is not the end of knowledge and a deep analysis is needed for a proper selection. There are many cases when carbon steel is not adequate- you need SS or Duplex.

I can't say anything better than what Mario just said, but as a point of clarification, high strength carbon steel is commonly of higher hardness, and being of higher hardness means higher susceptibility to *cracking* in particular.

H2S + H2O = H+ + HS- + H3O + H2S + H2O and all fluctuate.

How that manifests specifically into cracking, I'm not entirely sure.

I speculate that the entirety has the capability of stripping surface atoms off (corrosion), increasing roughness. The H+ has the capacity to intrude "far" and the metallic electrons look particularly appealing to them. H3O is also thinking those metallic electrons look scrumptious and collects into those rough patches.

When you see a temperature spike, you'll either make more room for more H3O and H or you'll see a crack, depending on whether it gets hot or cold.

Thanks Michael & Mariog for sharing your knowledge. This is a good subject to explore more deeply. Currently, I'm assigned at construction site & many questions arise in different aspects of design, material selection, etc.

Any other opinion is highly appreciated.

Cheers!!!