Induction melting of AISI 321 mod

[MaxKortau]

Hi everyone,

we are about to purchase an experimental 3t 1000Hz/2500KW coreless induction furnace to melt the AISI 321 scrap we've stored for years on our scrap yard (we run production shop for machining and assembling of special components for nuclear power reactors) and are about to start our own mini-mill, in order to produce high-quality steel ingots for further forging and processing for our needs.

The common grade we use is AISI 321 mod, with P max. 0.035, S max. 0.035, Cu max. 0.3 and Co max. 0.2 as well as 0.05 for some special requirements.

However, we do have lack of experience on melting practice of this grade. Any guideline or advice based on practical experience is highly appreciated.

Should we just use the scrap/rest of processed material as basis for furnace charging? Any slag (CaO - Al2O3 - SiO2) treatment needed? Any ferroalloy (FeSi, FeCr) to be charged? Any desoxidation?

Thks in advance and best regards

 

[EdStainless]

What size/shape is the scrap?
Is it very clean and dry?
The big issue with 321 from all scrap is first of all C pickup from dirt.
And secondly getting the Ti correct.
Can you do the chemical analysis in real time with enough precision and accuracy?

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P.E. Metallurgy, consulting work welcomed

 

[MaxKortau]

Hi Ed, thanks for your comment.

I. Scrap size/shapes

Currently we do have:
a) metal chips, after machining, stored in hermetical bags
b) cut rests of rolled plates: 25-45 mm thickness // 300-400 mm brightness // 400-500 mm length range, stored under open air
c) cut rests of rolled/forged round bars: 100-350 mm diameter // 400-600 mm length range, stored under open air
d) rest of diverse testing samples/specimen cut from rolled/forged raw material delivered: stored in our laboratory store

So, if clean and dry charging material needed - we'll fix it.

II. C pickup from dirt - any practice/method of decarburization during the induction melting process? As per current stage, we haven't planed any refining equipment (AOD/VOD etc)

III. Chemical analysis on heat sample: yes, we can take sample from a melt, using spoon sampling or immersion sampling probes (the latter one for the determination of O and H content)

 

[EdStainless]

We used to wash chips and then oven dry them at about 125C.
You must remove all oil residue.
There is no decarb method.
But if you pick up C you will need to be adding more Ti also.
And dry is super important, on chips and solids.
You will need to start the heat with solids, large preferred.
You don't want chips sitting and taking a long time to get hot.
You could always use some Ar purge to reduce oxidation.
We used to just bleed some in just below the top of the crucible and let if settle in the bottom.
You will want to limit the amount of chips used in a heat, maybe no more than 15% of the total.
Do you have any high purity metal (Fe, Ni, Cr, Ti) around in case you need to adjust the chemistry?
For the Ni and Cr you can buy pure electrolytic material.
For the Ti I would buy solids of scrap Ti Gr1 or Gr2 to use.
Pure Fe is difficult, there is some ELI/high formability/extra deep draw material that is very clean 1002.
We used to buy scrap punchings from appliance manufacturers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MaxKortau]

Thanks, Ed.

No slag treatment applicable/needed?

High purity metals - we can get some, but how about using ferroalloys?

BG, Max

 

[EdStainless]

No because the ferroalloys will have too much C in them.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[bobjustbob]

I have not melted 321, but have made a lot of CFXX grades using stainless plate scrap. I doubt you will have a difficult time with your carbon, assuming you are not using a lot of 321H. I expect that titanium is going to be the tough part to keep in solution, especially when melting the chips. You will want to start your melt with the heavy stuff. Add the finer material once you have a bath for it to dissolve into.

Ni is typically added as 99.9+ nickel pellets. You can get some low carbon ferrorchrome with <.025% carbon. With the amounts of Mn and Si you should be able to get by with adding those as ferro-alloys as well. When melting stainless Ni and iron were the only "pure" elements that we typically used. You should be good with ferroti as well. Again with the trim amounts you will be adding.

I would probably preheat your material, as it will probably have a bunch of cutting fluids on it. Do you have the spectrometer and combustion analyzer to check the chemistry? You need to balance the Ti with the C and N in the melt correct?

Bob

 

[MaxKortau]

Hi Bob, thks.

We do have OES and combustion - doing chemical analysis shouldn't be a big issue here.

What about slag treatment in induction melting of stainless steel - is it general applicable or Ar purging would be enough?

 

[MagBen]

If you add Al to kill oxygen/prevent Ti, you may need a slag treatment. However, due to Ca easy evaporation nature, composition control becomes a challenge.

Ti has a very high affinity with N, you probably need to add some extra Ti, target is to increase Ti/C+N ratio to 6-8 with respect to a final ratio of 5.

Depending on how "dirty" your scrap material is, you may still end up with a refining process (AOD) for this Ti-added grade, especially if you want to control carbon to a low level that could increase oxygen.