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4140 Threaded Stud Failure
8

4140 Threaded Stud Failure

4140 Threaded Stud Failure

(OP)
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?

RE: 4140 Threaded Stud Failure

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

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

RE: 4140 Threaded Stud Failure

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

RE: 4140 Threaded Stud Failure

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.

RE: 4140 Threaded Stud Failure

(OP)
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!

RE: 4140 Threaded Stud Failure

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

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

RE: 4140 Threaded Stud Failure

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.

RE: 4140 Threaded Stud Failure

Is 4140 susceptible to strain age embrittlement?

RE: 4140 Threaded Stud Failure

2
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.

RE: 4140 Threaded Stud Failure

Metalguy--was the stud plated or unplated?

RE: 4140 Threaded Stud Failure

(OP)
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.

RE: 4140 Threaded Stud Failure

(OP)
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).

RE: 4140 Threaded Stud Failure

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.

RE: 4140 Threaded Stud Failure

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.

RE: 4140 Threaded Stud Failure

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

RE: 4140 Threaded Stud Failure

(OP)
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.

RE: 4140 Threaded Stud Failure

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

RE: 4140 Threaded Stud Failure

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

RE: 4140 Threaded Stud Failure

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

RE: 4140 Threaded Stud Failure

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 m0.5.  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.

RE: 4140 Threaded Stud Failure

I definitely think that the elevated levels of Al & N are contributing to the poor fracture toughness.  N content should never be higher than ~ 0.012% by mass with today's steelmaking practices.  ASTM F 2282 Standard Specification for Quality Assurance Requirements for Carbon and Alloy Steel Wire, Rods, and Bars for Mechanical Fasteners has a requirement for 0.009% N max, but a more typical level is < 0.007% for cold-heading quality steels.  In any case, the N can only be assumed to be fully tied up when the ratio of Al:N is 3:1, so strain aging should be considered as extremely likely.  Dynamic strain aging occurs in the temperature range of 100-300 C (210-570 F).

ASM Handbook Volume 1 has a section on Aluminum Nitride Embrittlement under the more general topic of Embrittlement of Steels.  "The fractures occur at the primary austenite grain boundaries...Platelike aluminum nitride produces small, shiny fracture surface facets that are generally observed."

RE: 4140 Threaded Stud Failure

(OP)
CoryPad,
The toughness has to be very low in order to get 100% cleavage fracture at the relatively low strain rate the mechanic applied.  Twisting one end of the stuck stud with vise-grips is far slower straining rate than an impact tester, etc.  Also, the embrittling elements "usually" cause IG fracture, which I don't have.  That's why this case is so oddball.

RE: 4140 Threaded Stud Failure

(OP)
TVP,
If we look at the Al:N ratio it's 4.5.  Also, strain aging "usually" causes IG fracture.

I'm still baffled!

RE: 4140 Threaded Stud Failure

(OP)
Carburize,
I tried to look at Brians forum but it's blocked--by the computer guru's here at work.  I'll check it when I get home.  But I'm WAY to slow for IPSC!

RE: 4140 Threaded Stud Failure

Metalguy;
I would suggest if you need additional information on this subject, the reference book by Sinha titled “Physical Metallurgy Handbook” has a rather detailed section under tempered martensite embrittlement (TME). The Al-N embrittlement is also discussed separately. I happened to review the TME section last night when I was looking for information on an unrelated topic.

To make a long story short, Sinha states in the TME section that two modes of failure can occur - transgranular or intergranular. The transgranular mode can be caused by precipitation of FeN compounds during tempering along interlath paths of tempered martensite resulting in brittle fracture. This is all I remember, I don’t have this reference book with me today.

RE: 4140 Threaded Stud Failure

Some random thoughts:

1.  We've experienced quasi-cleavage/cleavage (mixed mode)  failures in low alloy lifting lugs under tensile overload conditions at 50°F.  Run a charpy series on the material and get a transition temperature of 75°F.  Take a charpy sample and 3-point bend (tensile test loading rates, not impact) it at LN2 temperatures and get a cleavage failure.  Could you be dealing with a shift in the DBTT during service?

2.  Swall:  If you are looking at embrittlement of leaded steels during moderate temperature exposure, are you talking about the lead melting and causing LME?  We see that in shotgun deslagging related failures.

3.  Strain ageing embrittlement:  You are in the ballpark for mechanism (but that usually affects carbon steels) and for Blue Brittleness (which can affect low alloy steels).  

RE: 4140 Threaded Stud Failure

The ASM Handbook section mentioned that leaded steel embrittlement was observed below the melting point of lead, at temperatures as low as 400F. But, as I mentioned previously, I think it is a stretch that this could be the source of Metalguy's problem, even if the 4140 turns out to be the leaded version.I would have expected the stud to have failed in service, rather than upon disassembly.

RE: 4140 Threaded Stud Failure

I think that something is wrong with this particular stud.  This is based on the fact that we have thousands upon  thousands of B7 studs of every description operating in our polymer process without any signs of any type embrittlement what so ever. The studs operate from ambient to 300°C .  Some have been in service since 1952 either as Crane Alloy or B7.  Some of these studs have been cycled 100's of time to 900° F-1000° F then returned to service.  There is not corrosion as the studs are Dagged prior to use.  I have Magnafluxed, sectioned, and chemically analyzed a very good represenetive sample of  these studs, again finding no problem.  We didn’t analyze for N.   As stated in previous posts I’ve checked several thousands of these studs for distortion using a thread rolling die and check gauges as none of these studs are torqued and are used for seating RTJ and Flexitallic gaskets.  No problems.  Proof testing always revealed the specified yield values.

The only problem I’ve seen with B7 or Allen SHCS is in an area where they was possible or positive exposure to HCL vapors in very low concentrations.  These studs broke with a bang, literally.  The culprit was H2 embrittlement to first degree.  HCL has been the only acid problems.  
When we have been  forced on projects to use B7 with SS flanges in HNO3 service, no problems with H2 as the HNO3 corrodes the fastener too fast.  

We have guest operation on site that did have some embrittlement problems with B7 handling NOX and NO.  We were not privy to the investigation.  

Here are a couple of miscellanous problems that I've encountered over the years.  

Someone called out B7 galvanized and the supplier stripped the zinc with brick wash and was proud of it.

We had an incident with a sister company had a large quanity of B7 studs rust in storage and they gave out an achievment award for someone comming up with a claening procedure using toilet bowel cleaner (HCL).  To compound the problem they comingled the acid treated studs with several thousand others.

RE: 4140 Threaded Stud Failure

Metalguy,

Now I understand why you say this would have to be low toughness steel.  I am sure you would agree that this steel should not have low toughness.  I think you may be placing too much emphasis expecting to see intergranular fracture with the common low toughness phenomena in steels.  

For example, hydrogen assisted cracking usually is intergranular, but it can be transgranular.  Trasgranular propagation is a common mechanism in steels used in sour service for oil/gas production.  In "Hydrogen-induced intergranular fracture of steels", Engineering Fracture Mechanics 68 (2001) 773-788, the following is in the summary:

"There are two kinds of hydrogen-induced cracking found in heat-treated steels.  One is induced by concentrated plastic flow, and the rate of this cracking is controlled by the rate of displacement of the loading points.  This type of cracking is transgranular with respect to the prior austenite grains, and it occurs at high levels of stress or stress intensity."

So, maybe this problem is due to a less common mechanism, but you might need to look more at the issues mentioned here to solve this problem.

Regards,

Cory

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