Thermal Fit 1

[mcquillan]

I am building a device out of 309 stainless steel to operate in the 700-1000C temperature range, it will also be submerged in liquid aluminum. The stresses on the device will be moderate so I am thinking of doing thermal fits to hold everything together. My question is: Will the fits loosen to a considerable degree over time due to stress relieving? And is this an appropriate method for joining parts in high temperature applications? I estimate a thermal strain of 0.0190 micro-m/mK to make the fit.

Thank You.

 

[desertfox]

Hi mcquillan

Can you give more information like the size of the two parts your joining and are they of the same material.
I have formula's for thermal fits and calculating the stresses in the parts. If your worried about stress relieving of the parts is this detrimental to the function of the job? the parts could be stress relieved before being assembled therefore if the design of the thermal fit is correct there should be no problem.

regards desertfox

 

[CalvinKelly]

Unfortunately, molten aluminum is the Universal Solvent for most metals. 309, and about anything else I can think of, will very quickly dissolve. It is hard even to find a ceramic that aluminum will not reduce.
Graphite is OK, used to line the pots where aluminum ore is electrically reduced to the metal.
Although it is academic here, thermal fits won't work at high temperature, they very quickly relax.
What other than aluminum will be present? That is, will there be any oxygen or oxidizing species present (CO2, H2O)?
If there is no partial pressure of oxygen at all then one might search a little further to find if some refractory metal might work.

James Kelly

http://www.rolledalloys.com

 

[mcquillan]

Thank you for the advice. The parts are shafts that will fit into holes in disks. The shaft diameters are 0.375". I had planned to experiment to determine the hole sizes, I was going to try 0.368" and 0.3701" diameter holes. Both parts are SS 309, I believe the thermal expansion coefficient to be about 18 micro-m/mK. The operational temperatures will not be much more than 700C. My concern with stress relieving is the rapid loosening of the fit and having the componenents falling apart. This now seems like a possible reality, but to what degree do the fits relax? I will not be having significant forces but should I weld instead? This of course presents unpleasant complications.

How fast does molten aluminum dissolve metals? I do not expect more than a few hours of exposure weekly.

 

[aybee]

desertfox, could you send me the formulas for the thermal fits and stresses?

eckybo2001@yahoo.com

 

[desertfox]

Hi aybee

Thread 404-42504 as all the formula for thermal fits.

Regards Desertfox

 

[desertfox]

Hi mcquillan

Having read Calvinkelly's post, I immediately thought of the materials used for die's in aluminium diecasting and I am not sure whether your 309 material would be classified as a die material or not, however hot die steels according to the book I have here are classed as H10,H10a,H11,12 and 13, 19 and 21 which are Chrome Vanadium Steels and are suitable for die casting dies. Reading further into the book
the steel dies are cooled during operation with water and as
mentioned by Calvin the surfaces in contact with the molten metal are coated with french chalk, ceramic powder etc.
Would it be possible within your design to facilitate cooling and coating of the shafts and discs?
As a final thought both Inconel and Nimonic are both high temperature materials is it possible you could use those?
Apart from the fact I am outside my field here, have you
considered contacting some Die casting Companies who may well have expertise on similar problems that you are facing.


regards Desertfox

 

[kenvlach]

Hi,
CalvinKelly (James) gave a good idea of the problems you are facing.
I have a bit of experience melting Al alloys, plus some reference info.
First, steel is not compatible with liquid Al, and Ni-containing steel is worse. Metals Handbook, 9th edn., Vol. 13, Corrosion, p. 59. This is consistent with large, exothermic heats of mixing between Fe & Ni with Al.
Further, liquid Al was reported to cause embrittlement of steel at 690^oC.—ibid. pp. 179-180.
Aluminum, vol. III, p. 16-17, ASM (1967), mentions that cast iron is attacked at a slower rate than steel, and is hence used for powered mixers (where wash coatings cannot adhere). Steel tools must be adequately coated by ceramic coatings. When I was preparing experimental Al melts, we coated metal molds and tools with easy-to-apply ZrO₂ or Y₂O₃ coatings from ZYP:

http://www.zypcoatings.com/

This webpage shows a number of coatings resistant to liquid Al.
However, if your device is a high speed stirrer or similar, you will need to apply a ceramic coating via flame or plasma for durability (adhesion).

To summarize, you are better off with a non-Ni alloy, a ceramic coating thermodynamically resistant to Al should be applied, and if there is high shear rate between your device and the liquid Al, the ceramic should be high-T spray applied.

 

[mcquillan]

Thank you all very much. I contacted ZYP for information, they haven't gotten back to me yet. Is anybody familiar with embrittlement? I have read and heard a lot about it but I have never found out exactly how rapid and severe it is. How fast does it happen and what exactly happens physically, does the metal just disintegrate? My setup is somewhat experimental so I am not looking for long term dependability, of course I do need it to last for at least 40 hours of service.

 

[aybee]

thanks Desertfox..all I gotta do now is find it...how do I do that?..I have been around all the pages looking for clues , so far no good

 

[TVP]

aybee,

Click on the following link:

thread404-42504

 

[desertfox]

Hi TVP


Thanks for providing the link for abyee you beat me to it.

regards desertfox

 

[aybee]

thanks guys

 

[aybee]

how was it done? I am grateful for the link but would like to know how you got it?

 

[CalvinKelly]

With respect to embrittlement, many Ni-Cr-Fe alloys will form a brittle intermetallic, sigma phase, after many hundreds or a few thousand hours exposure to roughly 600--850C. Won't be a problem with 40 hour life. As other posts said, you will need a good ceramic coating to keep 309 from dissolving. I don't know how fast, would estimate hours. Could be minutes. And interference fits do not hold at these temperatures.

James Kelly

http://www.rolledalloys.com

 

[JimMetalsCeramics]

Is the application being used in open air?

In this temperature range, I would not use stainless.

There is a separate problem. Many metals will dissolve in aluminum at these temperatures.