Catastrophic Failure of Stainless Cooking Pot 1

[AdamPrince2]

I saw a cooking pot that was made of stainless steel. It was used to cook potato soup. After cooking a restaurant owner decided to soak the pot in hot soapy water over night in the sink. The next day he found the pot literally cracked up like a banana peel! I have attached a photo. What failure mechanism would cause such a spectacular failure of a cooking pot?

 

[AdamPrince2]

The place was located in Oldham, County Texas.

 

[AdamPrince2]

The restaurant name is:
Twisted Tulip
1208 W Main St, Vega, TX 79092
Phone: (806) 340-4245

 

[Dhurjati Sen]

AdamPrince2

A nice brain teaser, thanks for posting.

The discussion has gone around forming operations and SCC. But seriously, where was the tensile stress that caused SCC? Residual stresses at the time of forming has caused CSCC...wow!

Did anybody ask how old was the cooking pot?

If possible, take a sample and carry out metallography. Then the world would know the answer.

 

[mrfailure]

It does not take much stress. I have the example of an oven used in a pharmaceutical process made from flat 304 sheet where the cleaning agent used contained chlorides and was not thoroughly rinsed. That cracked cracked completely throughout.

 

Residual stresses at the time of forming has caused CSCC...wow!

Yes, and the magnitude in this case is more than sufficient to provide one of the three prerequisite conditions for SCC.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

p.s., at the right (i.e. wrong) temperature and with a susceptible austenitic stainless steel, chloride SCC has been observed with Cl- concentration in the ppm range.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[ornerynorsk]

Mrfailure, at second glance, I believe you are correct. The sheen on the outside of the pot does look much more like stainless than aluminum. The rolled lip could be on both the inner pot or the liner, but the liner is generally not intended to be removed, at least not in normal use. Interesting failure to say the least!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Dhurjati Sen]

For this particular cooking pot, two things are assumed:

1. It is an austenitic SS (because of its thickness and the way it is formed)

2. The quality of the SS, specially with respect to Carbon content, is not maintained.

Don't you think sensitization is more likely the cause of failure than SCC?

@ironic metallurgist: thanks for the input on residual stress.

 

[NBrink]

I would assume such a pot would be cold drawn from annealled sheet. Why would it sensitize?


My bet is on SCC. Plenty of residual stresses, plenty of chloride, plenty of oxygen.


Makes me look at my stainless pressure cooker a little warily to be sure..

Nathan Brink

 

[mrfailure]

SCC in austenitic steels is not dependent on carbon content and happens with the right exposure in low-carbon versions of stainless as well.

 

mrfailure,
Many folks confuse sensitization with stress corrosion cracking.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

They confuse the two because in many environments sensitized material is more susceptible to SCC.
Actually in a part that is heavily formed higher C can be good since it is an austenite stabilizer.


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P.E. Metallurgy, Plymouth Tube

 

[metengr]

For metallurgists stress corrosion cracking (SCC) is such a broad term to define cracking related to three conditions (environment, susceptible material and tensile stress), which must be present simultaneously. You have subsets of SCC, like intergranular stress corrosion cracking (IGSCC) because of sensitization, you also have transgranular stress corrosion cracking (TGSCC) associated with chloride exposure, and hydrogen related and even sulfur related, which can result in SCC (transgranular or intergranular) or SSCC, respectively.

 

My theory about sensitization is that it is the only thing many non-metallurgical engineers retain from their 2nd year material science course, and so it perennially arises when discussing anything to do with stainless steel.
I recall arguing with design and manufacturing engineers who were sure that austenitic SS weld overlay could not be PWHT before machining or it would be certain to crack in cold, fresh water. Sensitization was always given as the reason for making manufacturing of these components more onerous and costly.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The bigger name is EAC (environmentally assisted cracking).
The curious thing is that there are environment that will cause cracking in all known alloys (even gold will crack in HF).

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Would we really expect a component with residual stress from deep drawing to have cracks ONLY in the axial and none in the radial direction? And no cracks on the rolled lip either, except through the lip?

Would we also expect the cracking to happen when the object was sitting around in a presumably cold soap solution?

Agree that the conditions for chloride SCC are all there- heat, stress, oxygen, chloride and stagnant conditions. But those conditions are encountered in all cookware- perhaps the exposure duration of this component, being used in a restaurant, was long enough for pits to start- from what I've read, pit initiation is necessary before Cl SCC can start.

Better photographs would be needed for the people (unlike me!) competent in failure analysis to do more than just guess about it. There's a burr on one edge of one of the cracks that makes it look rather like a sawcut- that and the regularity of the distance between the cracks looks a little suspicious. I'm sure the owner could tell the difference between a sawcut and a crack, but it's impossible to tell from this photo.

 

[EdStainless]

Yes we would expect cracking that that direction, because the stresses are much higher in the hoop than in longitudinal.
Mu hunch is that the rim is usually dry, in use any liquid there would evaporate. But this was being soaked and there could have been fluid there. Combined with the hot water in a restaurant being 185F I could see this finally happening.
I have never seen a pot crack while cooking, but while soaking and cleaning it isn't unheard of.
Yes you do need active corrosion for SCC to occur, but the pits can be microscopic so that there is no apparent corrosion.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Typical SCC often occurred on deep-drawn brass and 3xx SS! critical temperature > 70 degree C.

 

[Metalero]

It’s SCC failure where the strees comes mainly as residual from the fabrication (and use, of course) due to a deep drawing deffect like wrinkles or microscopical scratches. Thermal cycles in cooking, chlorides in the food and soap contrbuted to the mechanism.

 

[cavbilly]

It appears to be case closed, based on the reports of high residual stress from deep drawing, and propagated cracks appearing to run parallel to axis, not hoop.
SCC will most definitely take place in 316SS when there is high heat stress (as in cooking on a high temperature burner, with little or no water left in pot, or steam with exposure to chloride containing water as a quencher/coolant. I have seen boiler sample cooler fail within a period of 2-3 days when subjected 316L stainless steel tubing coils to raw hard water containing 200-300 ppm chloride.

Generally, this was a failure waiting for a place to happen.



I guess time ran out, or actually the soup was going to run out?
Is this why most pressure cookers are made from Aluminum alloy?