Sandvik, UK, produce Nitronic 60 (Fe - 17 Cr, 8 Ni, 8 Mn, 0.1 N2) for MIM application. Someone somewhere must be sintering this powder under vacuum. If we sintered such a high Mn powder ~3*10^-1 bar ~1190 °C, would there be any Mn loss? Mn seems to a have a high vapor pressure.

Is the partial pressure 300mbar? If so what is the constituent of the atmosphere? Manganese has a high vapor pressure and at 3*10^-1 bar you will lose some manganese, the amount being dependant on the sintering time.

In the furnace furniture, we've noticed some gray deposit, perhaps Cr (from high Fe- Cr, Mn free powder).

The gray deposit on the furnace furniture, especially at the colder regions in the furnace, is probably a mixture of Mn and Cr. The vapor pressure of Cr is much lower than that of Mn and so the majority of this deposit is expected to be Mn. Also, at 3*10^-1 bar you will lose a large amount of the 0.1% nitrogen present in the alloy unless the partial pressure is under nitrogen, in which case you could gain nitrogen.