John Boettcher, Field Service Engineer
Hot zone life can vary drastically depending on the vacuum furnace application. Some processes are harder on the insulation than others. Furnaces with higher bar quenching capabilities are also more stressful to the insulation. Some simple things can be done to help the hot zone life in almost all situations.
Keep the furnace leak-free. Check your leak-up rates as often as possible. Some customers add a short leak-up segment before or after their cycle as a verification that the furnace is still vacuum tight. Air/Oxygen leaking into the furnace will not only discolor most metals but also deteriorate the graphite insulation. If your furnace is leaking up over 30 micron/hr, it may be time to do a helium leak-test on the furnace.
Do burn-out/bake-out cycles on a regular basis. This should be a recipe that goes at least 100f over your typical process temperature. Some customers heat up under partial pressure and some just do straight vacuum. It is most typical to vacuum cool to around 100f and directly after, do a leak-up check. Burn-out cycles are good for getting the process contaminants out of the insulation and generally improve pump-down times. Make sure the furnace does not haave a large vacuum leak before dong a burn-out.
Keep the furnace clean! Sometimes customers will use Inconel or SS tie wires to hold the parts that are being heat treated to prevent them from moving during the quench. Many times these wires will break off and fall onto the insulation or heating elements. These metals can easily burn into the heating elements and insulation during heating causing premature damage to the graphite. Also general debris from the product should be cleaned off on the insulation.
Fix damaged areas immediately! This is the most critical on high pressure quench furnaces. A small nick in the insulation or a broken heating element support can quickly turn into a huge repair if not taken care of. Keeping some CFC board and moly retaining pins in stock can be very handy for doing insulation repairs.
Clean/dry your process parts if possible. Excess oil or cleaning fluids can cause poor pump downs and add to graphite insulation deterioration. If you have a cleaning process, make sure your parts are dry before placing them into the furnace.
Keep the bars down. If your process parts can meet the hardness or other specifications at a lower quench pressure, then take advantage of that. The higher the quench pressure, the harder it is on the insulation and elements. Try step-down quenches, high pressure for the first few minutes then drop the pressure down for the remaining cool. It will not only help on insulation life but also reduce your energy usage.
Load gently. Many hot zone repairs are a result of carelessness when loading and unloading the furnace. Taking an extra minute or two by being careful loading/unloading will save hours of down time doing repairs.
Check the hot zone before and after each load. A good visual inspection can prevent long term issues. Getting inside and checking all the heating element hardware for tightness/breakage along with checking for loose insulation retaining pins can save you time later. Don’t forget the door insulation and convection fan hardware.
Make sure flaps/bungs are working properly and are restriction free. If there are restrictions beyond the design, then added quench pressure will be forced to undesired areas which can get behind the insulation breaking it or blowing it off the retaining pins. Also make sure that the door insulation is mating up to the hot zone shell insulation correctly to prevent quench gases from getting in behind it.
Schedule a yearly preventative maintenance visit from the furnace manufacturer. Sometimes a well experienced field service engineer can see things that aren’t so obvious to your regular maintenance staff and can provide first hand tips and cost saving suggestions.
