Displace the air with an inert gas.

Flood it with nitrogen.

You're 78% of the way there already.

Why off topic?

You could be having problems with the gasses that are being released from the parts contaminating the belt and the muffle. This means you might need a more advanced belt cleaner like an ultrasonic cleaner.

You can also use a N2/H2 combined gas instead of pure nitrogen or pure hydrogen.

If you don't have a flame curtain already attached, then one should be installed to ensure proper coverage of the open end



Reference websites:

http://heattreatment.linde.com/International/Web/LG/HT/like35lght.nsf/repositorybyalias/wp_sntrg_8/$file/8.pdf



http://Beltfurnaces.com



http://www.airproducts.com/industries/Metals/ask-the-experts-metals/sintering-powder-metals-and-powder-metal/sintered-parts-frosty-matte-finish.aspx



http://www.abbottfurnace.com/