I am interested in sintering 316L or 17-4PH MIM materials. Should I sinter these steels in a graphite or metal hot zone? What are the differences in a graphite versus a metal hot-zone furnace?

This is an important question. Common vacuum furnace hot-zone materials include:
  • All metallic (radiation shields)
  • Combination radiation shields and other (ceramic) insulating material
  • Multiple-layer (sandwich) insulation
  • All graphite (board, fiber, carbon-carbon composite)
Most all-metallic designs consist of a combination of materials. For example, three molybdenum shields backed by two stainless steel shields would be typical for 2400°F (1150°C) operation. Radiation shields are (relatively) expensive to purchase and maintain and often require greater pumping capacity to remove any moisture trapped between the shields. Compared with other types of insulation, their heat losses are high and become higher with loss of emissivity (reflectivity) due to the gradual contamination of the shields.

Graphite-fiber insulation designs cost somewhat more than sandwich insulation. However, as their heat losses are lower, a smaller thickness is sufficient. In these designs, the adsorption of gases and water vapor is considerably reduced. Furthermore, the heating costs are lower, and the lifetime of this type of insulation is much longer. The maximum operating temperature is around 3630°F (2000°C). The speed at which they reach their ultimate vacuum and their life depends strongly on the purity of the graphite. In some applications, such as brazing, a sacrificial layer is used to protect the insulation beneath.

With regards to sintering MIM materials, either insulation type can be used. I know of some companies that prefer metallic designs due to a concern over carbon transfer into the parts when running hydrogen partial pressure in a graphite chamber. I encourage readers to contribute their thoughts on the matter.