GX120MnCr18-2 manufacturing porosity defect

[Klamer]

I have been having trouble casting GX120MnCr18-2 with Mn content at 17.5% due to porosity defects-blowholes (probably gas porosity due to hydrogen solubility). The mold humidity as well as the sand's too were tested and are in given range. The same molds were casted with Mn content of 12$ and had no problem in the final product. This is where it gets tricky, when added Mn to 18% the castings had blowholes (pumped up instead of dragged as seen on the surface of the feeder). Casting temperature was methodically tested and never exceeded 1500 degrees C. Based on current research the more you increase temperature above casting temperature the more hydrogen dissolves and diffuses in the melt and when solidifies it makes those porosity defects. But when you increase Mn content hydrogen solubility should decrease.
As I said earlier 12% castings had no problem at all. Any ideas??

 

[EdStainless]

What is your deoxidation practice?
What are you adding, is it in the ladle?

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

 

[Klamer]

It is a mixture of flourite + lime and metallic aluminum. Melting is taking place in an induction furnace. LF is not available, the method is from melting +deoxidation +ferroalloys to casting. I have been adding FeMn with 76%wt Mn and 1-2%C with some Si addition if necessary. All ferroalloys have been tested for extra humidity. Again the same ferroalloys were used to Mn-12% as well as the same materials for the molds(sand, catalyst, resign etc) and did not confront any problems.

 

[EdStainless]

Have you tested samples of the metal for Nitrogen content?

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

 

[Klamer]

I haven't, I will be taking samples to test outside the plant though. But I tested for Nitrogen in practice last night, increased Al% to 0.4% with extra CaO to form AlN.
It is a common practice among foundrymen when casting high Mn steels in an EAF to dilute it with approximately 30% low alloy steel to prevent gas porosity-blowholes-blisters. However I was melting in induction furnace so I figured it wouldn't have to be this much, so along with the increase in Al, I diluted with 5% low alloy steel.
In the end castings had satisfactory results. I will be checking for Nitrogen this week too so I can extract a valid conclusion.
Thanks for the recommendation.

 

[arunmrao]

Manganese steels do not need any deoxidizers. The high Manganese and Carbon content prevent any gas related defects. Do not exceed your tapping temperature beyond 1480C. If you have left the metal long in the furnace or have wet ladles, gas can be picked up.

If need be add a mixture of Al And CaSi to the melt while tapping.

Finally do not exceed in your charge foundry returns beyond 40%.

Hope it helps.

 

[EdStainless]

While oxygen and hydrogen are not a big concern Nitrogen is.
The high Mn alloys have huge N2 solubility, that goes down when they cool.
Even as solid alloys you can often see 0.5 wt% or higher in these alloys.

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