Vacuum level as indicated by vacuum gauge readings is not always a true indication of the actual conditions within a vacuum furnace. It is possible to have two identical furnaces operating at the same pressure but producing entirely different results on heat treating.
The reason for this can be explained by the relative leak tightness or leak rate of each furnace. Most furnaces are equipped with pumping systems sufficient to overcome reasonably significant leaks. On the furnace with the higher leak rate, air would be leaking continuously into the furnace resulting in a higher residual oxygen content than in the leak tight furnace. The higher oxygen content would adversely affect the heat treating results. Therefore, leak rate checks as prescribed by the furnace manufacturer should be performed at regular intervals to prevent this problem.
To ensure that the processing environment is not being contaminated by impurities leaking into the furnace, the relative leak tightness of the furnace needs to be regularly tested. Every leak rate test should be preceded by a high-temperature dry run (bake out) of the furnace. In the dry run, the empty furnace is usually heated to a temperature at least 55ºC (100ºF) above the its normal operating temperature, then allowed to cool under vacuum. For most general heat treating furnaces, a dry run temperature of at least 1200ºC (2200ºF) should be used. The furnace should be held at this temperature for at least 30 minutes but not longer than 60 minutes. The purpose of the dry run is to bake out any residual moisture or other impurities accumulated from lower temperature processing.
The leak rate test is performed immediately after the dry run cycle without venting the furnace to atmosphere. First, the pumping system is started in the manual mode and the furnace is pumped down to 1 x 10-4 torr or better. The pumping system is then isolated from the furnace chamber. The vacuum level is recorded after 30 minutes and again after 60 minutes. The leak rate can then be calculated in microns per hour and compared to the acceptance criteria in governing specifications.
Newer furnaces should not have leak rates exceeding 10 microns per hour at a pressure of 70 microns or less. For older furnaces, leak rates of 20 to 25 microns per hour are not unusual. These leak rates ensure that the volume of impurities that may be leaking into the furnace is sufficiently low so as not to cause any significant detrimental effects to the materials being processed.
A furnace exhibiting a leak rate greater than these limits should not be used for production until repair of the leak. In this case, the normal procedure is to backfill the furnace with nitrogen, but do not open the chamber to atmosphere. All thermocouple fittings and other vacuum feed-throughs should be tightened. The furnace can then be re-tested for leak rate as before. Failure of the second leak rate test is an indication that the furnace requires more extensive maintenance, possibly including helium leak checking.
A comprehensive preventive maintenance program is essential to minimizing downtime due to vacuum leaks. Proper care of pumps, O-rings and flange sealing surfaces and regular inspection of vacuum feed-throughs can help prevent leaks. Continuous monitoring of vacuum levels during processing can also help to identify potential problems before they develop into major repairs. VAC AERO technicians are expert in all phases of furnace maintenance and repair. Contact us to discuss your service requirements.
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