4140 Threaded Stud Failure 8

[Metalguy]

Had a somewhat strange failure recently that I had to do the analysis on. The stud is 1/2" dia A-193-B7. Operated for years at ~475 deg F. During removal from the valve (room temp)it snapped off. 100% brittle cleavage fracture. Did the usual checks-small grain size, normal chem, Rc28, only trace amounts of Sb, Sn. Had ~.12% Al, but N was high at 200 PPM. Microstructure looked fine-tempered martensite. Had the lab. make a small tensile bar from the stud-met all requirements!

Thought about temperature/temper embrittlement, but most of my tech. sources indicate IG fracture, not TG. Thread was cut, not rolled, and the root was pretty rough.

Any ideas?

 

[unclesyd]

What were the environmental factors the stud saw other than temperature?

Though the temperature is a little low I would analyze for Boron.

 

[unclesyd]

PS:
Where there any other cracking in the stud be either MT or microscopically?

 

[metengr]

metalguy;
In addition to uncelsyd's advice, and if nothing else pans out, the key could be nitrogen. I have read where in low alloy steels, nitrogen likes to segregate to dislocations and tie them up. If this is the case, it could well be that the notch toughness would be severely reduced because you have no ability to activate slip systems.

 

[Metalguy]

Thanks, guys.

Unclesyd-no other cracking was found. The studs were exposed to room air, but minor corrosion locked them into the valve. A mech. removed them with Kroil and vise grips. Maint. says they have broken them before but didn't bother to tell eng.

I'll check the B.

Metengr-I was thinking the same thing, but I thought the Al was supposed to form a harmless nitride compound with the N. I wanted to persue this investigation with a TEM, but a beancounter shot that down.

Stars to both of you!

 

[metengr]

metalguy;
Here is a paper of interest regarding effects of nitrogen in ferrite. By the way, glad to have you back in the forum.

http://www.nitrovan.com/Infacon-MK-8-01.pdf

 

[arunmrao]

The residual Al is high. If this is present along with Nitrogen, Aluminium Nitride inclusions are formed and they are continuous and stringer type. This can result in rock candy fracture. Please observe in unetched condition for the inclusions.

 

[Carburize]

Is 4140 susceptible to strain age embrittlement?

 

[CoryPad]

What were the phosphorus and sulfur concentrations? Can you perform a fracture toughness test. A cylindrical tension specimen can have a circumferential groove cut into it and you can calculate fracture toughness using the force to fracture plus geometry. I have done this before - let me know if you want the equations.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Metalguy--was the stud plated or unplated?

 

[Metalguy]

Metengr,
Good paper! It's good to be back here and at work. I've been working since the first week in Jan. Just drove my old 1 ton Chevy K30 truck from my sisters in Ft. Lauderdale to Phoenix--carrying the rifles and pistols I left with her 6+ years ago when I went to Italy. Left FL Tues. at 0930, arrived home Thurs. at 2030--slept right in the truck when I got sleepy.

Arunmrao,
The Al was actually .09%, not .12% as I wrote (at home from memory). I thought that small amounts of Al were beneficial. At what % do the problems begin?

S was .015, P was .014, and Si was .24

Carburize,
It is susceptible, and that was my first suspicion. But the reports I've read say you should expect IG fracture, not TG. Also, the Mo doesn't really help with long-term exposure to high temps. like it does w/short term.

 

[Metalguy]

CoryPad,
There isn't much stud left, but I doubt the fracture toughness could be anything but very low. I think the thread/stress conc. is the culprit, but correctly HT'd 4140 should have a trans. temp. way below RT, I'd think.

Swall,
No plating involved. I don't think H is involved (no IG fracture).

 

[CoryPad]

Metalguy,

Your first sentence confused me. Do you think the fracture toughness is low or high? Based on what you have dismissed as the causes, it seems like low fracture toughness (possibly due to chemical segregation, poisoning) may be the cause.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Actually, I was thinking more along the lines of liquid metal embrittlement, considering the somewhat elevated temperature. But, given the absence of plating and lack of I.G fracture, we can rule that out. Another stretch--this wasn't leaded 4140, was it? Leaded steels can also undergo embrittlement at moderately elevated temperatures, according to the ASM handbook.

 

[swall]

Another question--was the fracture a tension fracture or torsional fracture? Given your description of the removal process, I would expect a torsional fracture.

 

[Metalguy]

CoryPad,
Sorry for the poor wording, but I am sure the toughness is very low.

Swall,
Good point about the Pb-I missed that one. I'll find out. The fracture occurred during a high torsion load.

 

[Carburize]

Metalguy - way off subject but do you visit the Brian Enos forum as well?

 

[Metalguy]

Carburize,
Never heard of that. What's it about?

 

[Carburize]

Metalguy - it's about what you collected from storage at your sister's.

 

[CoryPad]

Metalguy,

Why do you think the fracture toughness is low? ASTM A 193, B7 with fine grain size, low impurities, etc. should have good fracture toughness of around 40 - 50 MPa m^0.5. If it is lower than this, then I would suspect some embrittling agent like Hydrogen, Sulfur, etc.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.