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Vacuum Furnace Quenching Systems: External versus Internal
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For heat treating purposes, "quenching" can be defined as the rapid cooling of a metal to impart some desired property such as hardness.
Different metals and alloys require different quenching rates to achieve their optimum properties. Regardless of equipment design, gas quenching in vacuum furnaces involve the same basic principles. The gas quenching process normally consists of the following sequence of events. First, the power to the heating elements is shut off. Next, the furnace chamber and quench loop are backfilled with a non-reactive gas, commonly nitrogen or argon.
The quench blower then activates, forcing the gas through quench nozzles located circumferentially in a manifold that is part of the hot zone and into the hot load. As the gas passes over the load, it picks up heat. The hot gas then exits the main chamber and travels through the quench piping to a water-cooled heat exchanger, where it is cooled. After exiting the heat exchanger, the cooled gas is drawn back through the blower to be recirculated through the chamber in a continuous cooling loop. READ MORE |
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Cleaning Practices Prior to Vacuum Heat Treating
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There are several factors to be considered in preparing workpieces for vacuum heat treating or brazing. Cleanliness of the workpieces and baskets or fixtures is very important.
They must be free of oil, dirt, machining lubricants or other contaminants prior to being loaded into the furnace. Some lubricants contain sulphur compounds which can adversely affect the alloys being heat treated. Inadequate cleaning can also cause staining and discoloring of the end product or result in poor braze alloy flow. Contaminants with high vapour pressures will evaporate during heating causing loss of vacuum. The vapours may eventually condense on colder surfaces in the furnace only to re-vaporize to cause contamination problems in subsequent runs.
It is good practice to check all workpieces for cleanliness prior to loading for vacuum processing. Tubular assemblies or parts with deep holes or recessed passages should be inspected for entrapped lubricants, machining chips or residual casting core debris. Particular attention should be paid to castings. In some cases, they are cleaned in molten salt baths to remove core residues. READ MORE
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