Failure analysis of a thin-walled tube

[IngeniousQuest]

Could someone assist me in identifying the failure mode of the following thin-walled tube
Microscopic images of locations A, B, and C (x1000) of the fracture surface as attached. I'm attempting to interpret the failure mode.

1. Thin-walled tube : [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1713285821/tips/1_tbhwer.tiff[/url]
2. A : [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1713285894/tips/A_btoelf.tiff[/url]
3. B : [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1713285909/tips/B_n5mhoh.tiff[/url]
4. C : [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1713285926/tips/C_yctap8.tiff[/url]

 

[TugboatEng]

The failure occurred in the valley of a thread. I doubt there is a weld there.

 

[IngeniousQuest]

Hi mrfailure, I did some EDX on the polished samples at 20kV and there were no any Mo peaks.
The Mn and S are coming from the inclusions (MnS?) in the ferritic-pearlitic microstructure of the steel.

No welds are present, and the intergranular crack shown in the microscopy image above is located in the threaded region of the fractured tube. It isn't spreading from the fracture surface, but from the surface of a machined thread.

 

[MagBen]

have you ever "seen" MnS, or just speculate based on high Mn and S contents from EDX? if the former, are they taken a form of elongated inclusions? MnS can act as macro-voids or micro-cracks. MnS inclusions can also promote HIC. If all these are true, it seems to me cracks initiate around the inclusions, in response to stress concentrations on OD, and cracks propagate intergrannularly. H prefers to diffuse and accumulate at grain boundaries. H from Hydraulic oil?

 

[EdStainless]

Threads cut too deep/sharp/long and bending forces being applied at the threaded connection.

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P.E. Metallurgy, consulting work welcomed

 

[IngeniousQuest]

Hi MagBen
On the polished and etched sample at 2k magnification, the microstructure consists of inclusions with spherical and elongated morphologies. I did EDX point analysis on these inclusions. Some of the inclusions only had peaks corresponding to Mn and S. Hence, the conclusion that these inclusions are MnS.

 

[mfgenggear]

A lot of speculation, make a fish bone diagram.
And list all the advice of causes. These guys are experts. Send samples out to a met lab. A) to establish exact materials, spectral analysis, B ) met lab of failure area. Take a small sample of the parent material do a sharpy test, and a tensile test. Take actual harness test.
Have a test done fore exact heat treat condition.
Is it annealed, work harden or heat treated. Grain structure will give a clue.
Now who manufactured this part, where did the parent materia or who manufatured it. Is there historical of all the Manufacturing and heat treat records.

 

[mfgenggear]

I would also suggest
NDT of any parts in stock, zyglo and inspect for indications, cracks, intergranul attack,
Test the failed part as well. Onaddition what the assembly procedure, was it torqued or installed correctly. Was there any welding or post heat treat. Most important. Manufacturing records.
Where any of the parts yielded. Or work harden Un-intentionally