I was wondering if you have any information on the humidity effect on the vacuum furnace (annealing process). We have experienced an increase in OD discoloration on our products (304L stainless steel) during the summertime and high-humidity days.

The problem of discoloration on stainless steel can often be traced to the build-up of contaminants such as dirt inside the furnace on the (cold) walls of the vessel and in the outer areas of the hot zone. In order to minimize discoloration, it is critical that clean work be processed (the parts must be free of dirt, metal particles, oils, cleaning and/or degreasing fluids, other chemicals, welding flux etc.). All of these will lead to dirty work on humid days.

It is also important not to allow the vessel temperature to drop below ambient (preferably at least 10°F higher). Finally, once the furnace is unloaded, it should never stand open to air more than a few minutes before the door is closed and pumped down to at least half-atmospheric pressure.

If cloth or ceramic work thermocouples are used, these must be baked out prior to use and stored in closed boxes between cycles as these absorb moisture and can be a source of water in the furnace. Stop-off paints to prevent sticking of parts are also subject to moisture absorption in summer weather and should be baked out prior to introduction into the vacuum furnace.

Effects of Pumping Down a Vacuum Chamber
From atmospheric pressure (760 torr) to 1 torr (the initial pumpdown stage from atmospheric pressure), the air in the vessel, with its associated high relative humidity, begins to be removed. As the pressure decreases, water vapor condenses due to the cooling effect of the sudden drop in pressure. A “fog” develops, i.e. a cloud swirls around the interior with a turbulence that is characteristic of gas flow at high pressure and high flow rate. The net result is a loss of pumping efficiency. This is one of the reasons why we should not keep a vacuum furnace open longer than absolutely necessary and that the size of the mechanical pump and blower is important in mitigating this phenomenon. Over time, a (slow) change in the composition of the gas remaining in the vessel takes place. Initially, air is the major component of the gases. Certain oils, grease and water exist, usually on the sides of the vessel. Eventually, almost all the air is pumped out. The grease and water will continue to evaporate, and their partial pressures will constitute a much larger proportion of the total pressure. The main concern during this phase is workload contamination (such as discoloration of stainless steels) due to impurities.