Rapid Solidification 304 Austenitic SS vs 446 Ferritic SS 3

[JABP123]

I have been working with two types of stainless steels, one austenitic and another ferritic at room temperature, 304 and 446 respectively. They were Rapid Solidified by melt extraction process.
I noticed that they have two completly different thermal expansion values:
304 - around 20x10-6/ºC at 890ºC
446 - around 13x10-6/ºC at 890ºC

I have been trying to find a reason to explain this different values and the only thing I found is that lattice parameters play a role in this thermal expansion coefficients since ferrite(alpha) lattice at 912ºC is 2.895 and austenite is 3.637. Is this the only reason for this variation? If so, why exacly having a bigger lattice means higher thermal expansion coefficient?

Hoping you guys can point me in the right direction.
Kindly,
John

 

[HakiNoir]

Hi,

The atomic packing factor or unit cell density would affect that too. BCC and FCC have different packing factors. Different chemistry within Solid solution (Fe with C and other alloying elements) would affect that too. You couldn't say the bigger the lattice, the higher CTE.
Check one of the general materials txt books e.g. ReedHill or Callister.

 

[JABP123]

Thank you for the reply.
Following your answer, the APF off BCC is 0.68 and FCC is 0.74. Which means that austenite is more dense than ferrite. Why then does it have more thermal expansion at room temperature? By increasing temperature i can understand that when austenite phase changes to alpha iron or delta iron that it expands and creates more thermal expansion, nonetheless i cant understand the values at room temperature.

 

[EdStainless]

The other way to look at this is by looking at CTE for various pure elements.
You will see a pattern that is largely a function of the melting point, high MP = low CTE.
Structure itself does not describe the variations (look at Mg and Re or Fe and Ta).

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Both chemistry and structure relate to variations in thermal expansion characteristics.

https://books.google.com/books?id=_...perties between austenite and ferrite&f=false

 

[MagBen]

3xx SS normally have larger CTE then 4xx even from RT to 200F. To simply put, 3xx is fcc, which is denser, so lower bonding energy, so higher CTE. Note this is a general tendency, there may exist exceptions. The same is true fro MP: higher MP tends to be lower CTE.

 

[JABP123]

Thank you guys for all the information!

@metengr the link you provided talked about fcc austenite having two eletronic states (y0 and y1), each with different densities and with different proportions of atoms that are depedent of temperature. I'm sorry but i couldnt understand the information there.. Is it saying that the volume of bcc only depends on the y1 while the volume of fcc depends on y1 and y0 thats why it has a greater volume in the end?. Could you please clarify me on that? I was quite confused.

 

[metengr]

JABP123;
The link is "Handbook of Residual Stress and Deformation of Steel" by George Totten. There is a reference in the section of interest. You need to locate it and find further information. I do not have this particular reference book.

This may help you get started

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751616/

 

[EdStainless]

Let's go back to the original post, you said that these samples were rapidly solidified.
Do they have crystal texture? Did you anneal them to develop grain structure?
How close are the latice parameters to what we would find in wrought material?

In bcc the distances to nearest neighbors are nearly uniform, in fcc there are two distinctly different spacings.

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P.E. Metallurgy, Plymouth Tube

 

[JABP123]

Yes the samples were rapidly solidified having two different compositions. One 'ME-304' 18Cr8Ni which becomes fully austenitic after rapidly solidification (10^5ºC/s) and 'ME-446' with Cr 23-27% which becomes fully ferritic. This was confirmed after a microstructural analysis where the samples were chemical etched.
This metallic fibers are used to reinforce a refractory slurry and since the thermal expansion is so important in this pratical case I was researching the subject and found that Austenitic (fcc) has higher TE than ferritic (bcc) and couldn't understand why.

The topic of lattice parameters was brought because while I was trying to find an answer I saw that Austenite had a bigger lattice and asked here if this could be related with thermal expansion or not.

 

[EdStainless]

So these weren't quenched fast enough to suppress crystal structure.
I wonder what the CTE would be for them if they were amorphous?

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P.E. Metallurgy, Plymouth Tube

 

[JABP123]

Probably lower CTE but it would probably decrease the ductility of the fiber, needed to bridge and absorb the tensions of the refractory slurry.

 

[MagBen]

might be a higher CTE for amorphous due to lower bond energy. Again a general trend: larger bond energy leads to lower CTE, larger elastic modulus, larger MP. Think about two extreme materials, ceramic vs polymer.
I did ever measure a decreased CTE when a disordered alloy was transformed to an ordered structure (BCC to B2).

 

[JABP123]

@MagBen

Indeed! I have here one fiber (Cr-Al ferritic) that has been cold drawn and since it has an extremely high elastic modulus which results from the cold work, it has a very low CTE.

 

[JABP123]

By the way guys, just one more question. I read somewhere that the microstructure of stainless steel 304 will keep being Austenitic even if I increase temperature because of the composition that stabilizes austenite, the only thing that will change is the formation of carbides and intermetallics.. is this true? What about the 446 ferritic structure?

 

[EdStainless]

Yes, the 304 will stay austenitic clear to the melting point, and there may be secondary phases depending on composition.
If your material is low C and low N then you may need thousands of hours at high temp to form any other phases.

The ferritic will start to form secondary phases at about 300C, with rapid formation at 475C.
Then as you go higher in temp some of these phases wills tart to dissolve and others will form.
A ferritic stainless will have good ductility when hot, but if it has been above 300C for any time the room temp ductility will be very poor.
If you dl this book it has figures that show which phases will form in which temp ranges.

https://www.nickelinstitute.org/en/...ies/11021_HighPerformanceStainlessSteels.aspx

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Attached see phase diagram for 304, when heating it to a temperature above the single-phase austenitic field, i.e. above about 1380C at .05% carbon, primary ferrite will re-form in the alloy. This residual ferrite (in as-cast condition) may or may not a good thing.