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A theoretical or ideal vacuum is an empty space that does not contain either vapours, particles, gases or other matter, and as a consequence has no absolute pressure. Because this condition does not exist, even in outer space, an ideal vacuum cannot be achieved.
Normally when the term vacuum is used it refers to an absolute pressure below that of a normal atmosphere. Normal atmospheric pressure is 14.7 lb/sq in, commonly termed 1 Bar. Nowadays vacuum gauges measure pressures in millibars (mbar) where 1000 mbar = 1 Bar. For use in vacuum heat treatment operating pressures are classified as:
With the development of vacuum technology it has become possible, by means of an array of roughing pumps, rotary pumps and diffusion pumps, to progressively evacuate a furnace chamber to high vacuum conditions, reducing the available oxygen to miniscule levels. The resulting environment is unreactive, even to alloys of titanium which are especially prone to oxidation. For all grades of steel, including those requiring high temperature austenitisation, such as high speed steels at 1320°C and all nickel alloys, vacuum heat treatment is the optimum method.
For those alloys which require quenching for hardening, such as steels, or quenching during solution treatment, such as some nickel alloys and stainless steels, integral quench systems have been developed based upon oil or inert gas. Various quenching rates can be obtained by delivering the inert gas to the furnace chamber at a pressure of up to 20 bar. Provision is made in some furnaces for alternating the direction of flow of the quenching gas from top to bottom of the furnace load and the reverse. Thus, steels of relatively low hardenability, such as low alloy engineering steels can be fully hardened. Since the work pieces remain stationary in the furnace chamber throughout heating and quenching there is no risk of component damage due to work movement at high temperatures.
Multi zone heating is provided by electrically heated elements surrounding the furnace chamber. The elements are made of graphite or high nickel alloys and the furnace chamber is surrounded by heat shields made from molybdenum and backed by stainless steels and insulating media such as ceramics. Temperature uniformity throughout the furnace chamber can be controlled to very tight limits, +/- 2°C at temperatures of 1300 – 1350°C.
Vacuum heat treatment is the cleanest and most environmentally friendly of all hardening methods and, as the size of furnaces has increased and computerised process controls are now standard, treatment economics are increasingly attractive. Tempering following hardening can be carried out in vacuum furnaces evacuated to low pressures, using roughing and rotary pumps only, since the risk of oxidation is less due to the lower temperatures employed.
A theoretical or ideal vacuum is an empty space that does not contain either vapours, particles, gases or other matter, and as a consequence has no absolute pressure. Because this condition does not exist, even in outer space, an ideal vacuum cannot be achieved.
Normally when the term vacuum is used it refers to an absolute pressure below that of a normal atmosphere. Normal atmospheric pressure is 14.7 lb/sq in, commonly termed 1 Bar. Nowadays vacuum gauges measure pressures in millibars (mbar) where 1000 mbar = 1 Bar. For use in vacuum heat treatment operating pressures are classified as:
With the development of vacuum technology it has become possible, by means of an array of roughing pumps, rotary pumps and diffusion pumps, to progressively evacuate a furnace chamber to high vacuum conditions, reducing the available oxygen to miniscule levels. The resulting environment is unreactive, even to alloys of titanium which are especially prone to oxidation. For all grades of steel, including those requiring high temperature austenitisation, such as high speed steels at 1320°C and all nickel alloys, vacuum heat treatment is the optimum method.
For those alloys which require quenching for hardening, such as steels, or quenching during solution treatment, such as some nickel alloys and stainless steels, integral quench systems have been developed based upon oil or inert gas. Various quenching rates can be obtained by delivering the inert gas to the furnace chamber at a pressure of up to 20 bar. Provision is made in some furnaces for alternating the direction of flow of the quenching gas from top to bottom of the furnace load and the reverse. Thus, steels of relatively low hardenability, such as low alloy engineering steels can be fully hardened. Since the work pieces remain stationary in the furnace chamber throughout heating and quenching there is no risk of component damage due to work movement at high temperatures.
Multi zone heating is provided by electrically heated elements surrounding the furnace chamber. The elements are made of graphite or high nickel alloys and the furnace chamber is surrounded by heat shields made from molybdenum and backed by stainless steels and insulating media such as ceramics. Temperature uniformity throughout the furnace chamber can be controlled to very tight limits, +/- 2°C at temperatures of 1300 – 1350°C.
Vacuum heat treatment is the cleanest and most environmentally friendly of all hardening methods and, as the size of furnaces has increased and computerised process controls are now standard, treatment economics are increasingly attractive. Tempering following hardening can be carried out in vacuum furnaces evacuated to low pressures, using roughing and rotary pumps only, since the risk of oxidation is less due to the lower temperatures employed.
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