The leak rate for a vacuum furnace can vary depending on the application and volume of the vacuum chamber. Generally, a leak rate of 10 microns/hour is acceptable for most vacuum furnaces. However, for critical applications such as processing of superalloys or reactive metals, a leak rate of less than 5 microns/hour is mandatory. It is important to have a leak-tight vacuum chamber, doors, feed-through, and penetrations to maintain the desired vacuum level.
Leak rate testing is performed to ensure the integrity of the vacuum system. This test is typically done immediately after the dry run cycle without venting the furnace to the atmosphere. 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, and 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 specified in governing specifications.
For newer furnaces, leak rates should not exceed 10 microns per hour at a pressure of 70 microns or less. Older furnaces may have leak rates of 20 to 25 microns per hour, which are still considered acceptable. These leak rates ensure that the volume of impurities leaking into the furnace is sufficiently low to avoid significant detrimental effects on the materials being processed. If a furnace exhibits a leak rate greater than these limits, it should not be used for production until the leak is repaired. In such cases, the furnace may need to be backfilled with nitrogen and all vacuum feed-throughs tightened before re-testing for leak rate.
It is important to regularly perform leak rate checks as prescribed by the furnace manufacturer to prevent any issues caused by leaks. A higher leak rate can result in higher residual oxygen content in the furnace, which can adversely affect the heat-treating results. Therefore, proper attention should be given to the entire vacuum system, and leak detection of all joints, welds, seals, valves, pumps, and the vessel itself is critical.
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