Gas problems with CA-15 Stainless Steel 2

[berkrefik]

I am having gas porosity problems with CA-15 type stainless steel castings and can't find a way to prevent. I have this problem both in resin bonded and clay bonded sand moulds. During solidification i observe expansion on the feeders and after breaking the feeders porosities appear on the cracking surface. Also during machining small gas pores appear beneath the surface.
Chemical analysis of my castings are;
0.15% C
1.0% Mn
1.5% Si
13.0 % Cr
0.5% Ni
0.5% Mo

The thing I cannot understand is; having this problem in some of the moulds cast in the same tap, not all of them. I've tried deox. with Al but couldn't get a success. I have neutrally lined induction furnace and magnesite lined ladles. My pouring temp. is about 1600 C. I would be very grateful if you share your experiences.
Thanks

 

[blacksmith37]

Since it occurs in various mold types and with Al deox , my guess would be Hydrogen. Other than holding the ladle in a vaccum, I don't know how to remove H2.

 

[metengr]

You might be interested in reviewing this paper

http://www.rastgar.com/rec/papers/Porosity in Castings.pdf

 

[arunmrao]

Sorry,if it is too late. To begin with what is the charge material being used,also in terms of foundry returns what percentage is being added?

CA15 does exhibit gassing tendency but can be easily controlled.. Trick is to pour at as low a temperature and quickly. I have no experience with neutral linings,but have used basic linings for the purpose.

If aluminium addition is inadequate safely add some CaSi and ferrosilicon zirconium.too.( Your test bar will never fail). You can also add a few grams of mischmetal too,if desired,(though not always recommended ,unless you have an epidemic).

With these precautions,prepour a sample cup and observe for any gas rising(cauliflower shape). A sunken surface indicates that the metal quality is good.

Regarding molding materials and their effect,you will have to carefully evaluate against the requirements and facilities available. Hope this helps.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[EdStainless]

The previous comments are spot on, lower pour temp and CalSil should be your first shot.
If it is hydrogen then the source is moisture. Charge metal, molds, cores, ladles, anyplace could be the source.

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Plymouth Tube

 

[berkrefik]

I've supplied ferrosilicozircon. Ln the article metengr posted it says zircon should be added %0,2 after deoxidation with aluminum to remove the nitrogen in the melt. I usually add aluminum during tapping, if i add ferrosilicozircon after tapping, would it be effective or what do you suggest?