Forging of SS304L and grain boundary carbides 3

[N K Acharya]

We have ben forging SS 304L grade steel in the temperature range 1150 - 1000 C and water quenching. Further it is Solution annealed and water quenched (soaking at 1060 C - 1 hour per inch thickness). we are getting grain boundary carbides despite the solutionizing treatment. can anybody what could be the reason for this? Sometimes the finish forging temperature goes as low as 900 C.

 

[metengr]

What is the maximum thickness of the forging? You may not be holding the forging long enough at the solution anneal temperature to dissolve all carbides.

 

[EdStainless]

Are you forging cast billets? They would be heavily segregated and you may need to do a homogenization treatment.
What is your C and N level? Is it low enough?

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P.E. Metallurgy, Plymouth Tube

 

[N K Acharya]

The forging is 110 mm thick and 730 mm dia. We gave one hr per inch soaking. We tried going up to 1175 Celsius. Carbon is below 0.03 and Nitrogen is below 0.01% We are using forged round bars.

 

[metengr]

The C and N levels look acceptable. Have you performed a furnace survey to ensure proper temperature control and measurement. I can't tell how you many times I have been on audits where the furnace temperature readings are off because of lack of a proper furnace survey being performed.

The only other option is to check the condition of the in-coming material, for any possible macro-segregation issues.

 

[EdStainless]

Check the starting material, take a sample and sensitize it then examine.
It should not sensitize at that C level, unless there is a lot of segregation.
How are you cooling? You will need something faster than a natural cooling rate, such as fans with water spray.
1150C should be fine for solution anneal.

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P.E. Metallurgy, Plymouth Tube

 

[arunmrao]

110 mm thick forging, will need a more aggressive form of quenching to avoid carbide precipitation.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[metengr]

and I quote from the OP.....

Further it is Solution annealed and water quenched (soaking at 1060 C - 1 hour per inch thickness).

 

[EdStainless]

At 0.03% C it should take 6-8 hours at 1250F for visible GB carbides to form.
If they really are carbides then either the material have some significant segregation which accelerates the process,
Or they are not carbides.....

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

Yes, but he said he was going to 1140 Celcius. Way too high!
I'd think keep the 1140 F degrees because he is so thick, but soak much longer, then try to quench faster. ?? But that's an opinion, not based on the metals he has.

What we don't know is if the problematic crystals are appearing at the surface, or deep within the 4 inch thick mass of his casting.

 

[metengr]

We are talking degrees C and not degrees F. A proper solution anneal is a minimum 1050 deg C for this material. This is all guesswork without evaluating the microstructure.

 

[remetaper]

Excluding a high level of segregations , it's very unusual that this grade with C% lower than 0,03% and N < 0,01% is affected by carbide precipitation with a rapid cooling rate from 1050-1150° C and with the right soaking. I am a little be perplexed about the declared Nitrogen content ( N= 0,01%). In AOD process this content could be N%= 0,06-0,09 % in order to save Argon and Nickel. In any case this is not the reason why of Carbide precipitation even if some techinical papers report that C%< 0,02 is suggested in case of Nickel at top level of range ( ie : Ni%=12). In my opinion, this forging with the declared content of C and N could be directly water fast cooled from forging process provided that final temperature is over 1050-1100°C.
Finally, I agree with the suspicion of Ed Stainless: are really Carbides ?
Or Boron precipitates ?

 

[N K Acharya]

Further during IGC test, the material passed, though there was orange peel appearance. The grain size is 2-3.The furnace TUS and SAT have been done and no worry there. Puzzling though. Thanks to all who have given their views and suggestions.