For this reason, some type of surface protection during austenitizing is required. Vacuum, controlled-atmosphere, or neutral salt bath furnaces all offer surface protection. If neutral atmosphere furnaces are not available, parts are sometimes wrapped in stainless foil to minimize oxygen exposure. Salt furnaces usually offer the quickest and most uniform heating, but leave a residue which must be cleaned from the tool surface. Salt bath heat treating has traditionally used for high speed steel cutting tools, and often cannot accomodate large tools or high-volume hardening.
Vacuum furnaces offer the best surface protection, but usually require longer process cycles.
Quench rate may be limited due to the ability to remove heat from a hot part fast enough to obtain maximum hardness. Vacuum heat treating may result in slightly lower hardness than salt bath. Wrapping parts in foil may also slow down the quench rate because of the slight insulating effect of the foil layer. In addition, the type of foil must be chosen to withstand the austenitizing temperature used. Recommended heat treatments for specific tool steels are usually thoroughly described in datasheets from the suppliers.