Carbides in Austenite phase 1

[Mett87]

Hi I have a samples of F55 duplex stainless steel that appears to have a high percentage of carbide/intermetallic phase in the austenite grains, could this be down to a poor solution treat after forging?

 

[jwhit]

That is most likely the cause. You may want to bump your SA Temp. up a bit and make absolutely certain that the quench rate is fast enough to get the part cooled to <800f within a minute from the time the furnace door is fully open. Also, allow enough soak time in the furnace to be certain the part is heated all the way through.

 

[TVP]

I agree with everything jwhit wrote.

 

[EdStainless]

Have you done impact testing?
My hunch is that at higher magnification you will find a lot of secondary phases along the ferrite/austenite boundaries as well.
You really need to cool fast, under 1000F in less than 30 sec, and under 600F in less than 4 min. are the values that I use.
It is going to take a good soak to get rid of this stuff.

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Plymouth Tube

 

[Mett87]

Thanks Guys,

I have not done impacts yet, I have not actually produced the forgings so I do not know the exact parameters used for ST. I am just carrying out some further testing and noticed them when I have prepared the sample for ferrite count. No deleterious 3rd phases appear at the boundaries, etchant used was 40% aqueous NAOH electrolytically.

 

[metengr]

Good follow-up. If this is the case, the solution treatment temperature was probably below minimum specification for this duplex or insufficient soak time. If cooling rate was an issue, you would see decorated grain boundaries.

 

[Mett87]

Would I not also expect to see carbides in the ferrite graines also,? this was what originally puzzled me.

 

[metengr]

Not necessarily. Depending on carbide-type the formation and dissolution time/temperature will vary in comparison to intermetallic phases.

 

[EdStainless]

More carbon in the austenite (it is a austenite stabilizer).
I agree that it looks like a solution anneal temp/time issue.
My hunch is that they finished forge a bit low, formed carbides, and then didn't fully re-solution them.

From high temp to lower the order of preferential formation is: sigma, chi, nitride, carbide.
If it sat for a few min in the range of 600C-700C you would preferentially form carbides

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Plymouth Tube

 

[Mett87]

Thanks Ed,

Also with regard to carrying out the austenite spacing - what is the calc used to actually work out the austenite spacing? ASTM E112 states at a magnification suitable to count 50 fields, also it would depend how many lines are superimposed on the image.