The failure mode you are describing as "flaking" does not sound like a high temperature failure mechanism. If the stainless steel is merely being heated to a high temperature and held there, there will obviously be metallurgical changes that take place. In addition, depending on the temperature, there can be surface oxidation. If there is an uneven heating of the material, there can be thermal fatigue as well. In a thin material this is however unlikely unless you have some kind of a quenching effect. This high temperature exposure does however, to my knowledge, not result in "flaking".
I therefore have to come to the conclusion that the "flaking" is not necessarily a high temperature effect, but possibly corrosion taking place. It is not uncommon to find "pieces" of a sensitized S/Steel comming off that is experiencing intergranular corrosion. This may resemble "flaking".
If you merely go for another 300 series S/Steel that is succeptible to intergranular corrosion, I believe you will have the same failure taking place. To therefore find the correct material, we have to find out what the real exposure is that the component is seeing. Here we need:
1) Temperature that the material reaches. (To see if it is in the sensitization range. - From your description, I believe it would be in the sensitization range, but potentially it could be above this range.)
2) The corrodents that comes into contact with the component both at high temperatures and at low temperatures. (Salt?, Organic acids?, Cleaning fluids? etc.)
I suspect that we may be able to help if we know the above.
Regards
Niekie Jooste
Niekie:
Thanks to everyone for their input thus far. At this time the temp. of the rings is still unknown; however, we will be answering this soon (I would estimate a min. of 1200F. but this IS an estimate only and not very benefical.) The rings are indeed exposed directly to cooked poultry(a good source of amino acids). They roll continuously over a glazed product. The glaze contains: salt, sugar and starches. Would this cause the quenching effect you described? In addition, the entire assembly, shaft and rings, catches an occasional spray of sanatizer (mild chlorine and H2 O). This is highly undesirable as it usually distorts/warps the machined shaft into something resembling a bananna.
Also of interest is the information that the engineers at Rolled Alloys in Michigan supplied. They suggested their product "RA309" and offered that it could withstand 2000F. We are still in the information gathering phase; however, once complete I would like to work up two or more prototype "Charmarker" assemblys and compare the results.
I hope this helps, and we will have an accurate temp. reading taken soon.
Thanks again,
C. Milligan
The more I hear, the more I am convinced that you have an intergranular corrosion problem. The corrosion will not be taking place at the high temperatures, because at those temperatures you have no electrolyte present. The corrosion will be taking place at lower temperatures.
This problem will not necessarily be solved by a material that can handle higher temperatures. - Rather by an alloy that will resist the corrosion effects.
Regards
Niekie
Niekie;
My apologies for the delay on the temperature reading. As suspected, the rings are operating at nearly 1275 deg.F. I hope this is of assistance.
In addition, you suggested that another "300 series" alloy may not address the issue at hand. I am not very farmiliar with alloy selection and would be open to any suggestions. I appreciate all the generous help thus far!
Thanks,
Chris
The sensitizing temperature range for Austenitic stainless steels is 840°F - 1560°F. (450 - 850°C) As you can see, your operating range is slap bang in the middle of this range. Even if you are using an L grade of 304 stainless steel, extended periods at this temperature will result in sensitization. As a result, your S/Steel will have extremely low corrosion resistance and will result in serious intergranular corrosion. Often the material appears to "exfoliate" due to thin slithers of material comming off the surface.
As a first option, I would try either a 321 or 347 grade of S/Steel. These are very simmilar to 304, only they are stabilized to prevent the sensitization from occurring. Grade 321 is stabilized with Titanium while grade 347 is stabilized with Niobium. Usually 321 is cheaper, however I have found that 347 gives slightly better stress corrosion cracking (SCC) resistance. (Although I can not actually tell you why.) SCC may also be a problem for you during times when the material is not very hot. (140 - 570°F - The top temperature is an estimate on my behalf, based on my understanding of your application.)
I must say that your application is actually quite hard, and as a result, the 321 or 347 grades may not remain unsensitized indefinately. It should however give you a huge increase in life.
Let us know of your progress.
Regards
Niekie Jooste
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