Vacuum heat treatments
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
The absolute vacuum does not exist, we come close to it in interstellar space. Our living environment is a gaseous medium which is established under a certain pressure (1000 mbar for the atmospheric pressure). The heating of metals in contact with air produces an oxidation linked to the presence of oxygen.
To avoid this oxidation, it is necessary either to heat under a neutral atmosphere free of air (nitrogen, argon or hydrogen for example), or to eliminate the oxygen by pumping the air contained in a furnace built to be airtight.
To completely avoid oxidation, the pressure must be reduced to 0.1 mbar; this is achieved by pumping with vacuum pumps.
Vacuum treatments are therefore carried out in special sealed furnaces equipped with pumping systems to maintain a relative vacuum of 10-2 to 10-5 mbar.
However, vacuum has the disadvantage that there is no convection and heating is only by radiation, which is much slower than in an atmosphere, especially at lower temperatures.
For this reason, low-temperature treatments (tempering) and temperature rises can be carried out in vacuum purge furnaces in which a vacuum is first created and neutral gas is introduced to obtain some convection heating.
The recognized quality of vacuum treatments is the absence of surface alteration. The ability to reduce deformation is only related to the less severe cooling conditions that can be implemented, therefore applicable to grades with good hardenability.
Pressure units:
Pascal (Pa) = N (newton) /m2
1 bar = 105 Pa
1 mbar = 102 Pa = 1 hPa
1 mbar = 0,75 Torr ( 1 Torr = 1,333 mbar = 133,33 Pa)
1 mbar = 0,75 mm Hg
Note that at atmospheric pressure, i.e. 1013 mbar: 27 billion billion molecules/cm3 are present, at 10-9 mbar: the number is reduced to 27 million molecules/cm3
The absolute vacuum: 0 molecule is impossible to reach.
The vacuum is obtained by pumping, we distinguish :
Primary or mechanical pumps (piston, screw vane), capable of reaching 10-1 mbar. They can be oil lubricated or dry.
Roots pumps that can reach 10-2 mbar with improved pumping speed from 10 mbar.
Secondary pumps (diffusion pumps) which allow to reach 10-5 mbar
The furnaces are set to trigger different types of pumps depending on the pressure obtained.
Different oven designs:
Horizontal charging furnaces
Vertical load furnaces (shaft or elevator)
The heating chamber: muffle or casing is inside a double-walled steel casing cooled by water to avoid leaks due to expansion. This muffle is for high temperature furnaces (solution heaters) either in graphite or in metallic walls made of molybdenum sheets behind which is a mineral insulator. The muffles for low temperature furnaces (tempering) are made of stainless steel with insulation. The heating resistors made of graphite or metal (molybdenum, tantalum) are judiciously placed to obtain a homogeneous radiation on the high temperature furnaces.
Examples of vacuum furnaces can be seen on the manufacturers' websites listed on the implementation tab.
These furnaces are equipped with a cooling equipment either in the heating chamber or in a separate chamber after transfer of the charge in this one, by injection of a cold gas: nitrogen, nitrogen + hydrogen (5%maxi), argon, helium and gaseous mixtures at pressures going from the atmospheric pressure to 10 bar (pressures of 20 and 40 bar can be proposed) The gas is efficiently turbinated and kept cold by passing over an exchanger inside or outside the furnace.
Cooling by oil quenching is also performed in a separate chamber adjacent to the furnace.
The efficiency of gas cooling is the result of the heat extraction power, which depends on the type of gas used (light gases are a priori more favorable, but mixtures between light and heavy gases are also efficient), the pressure and the design of the furnace itself and its heat exchange system.Gas quenching is often preferred to oil quenching because it saves washing after treatment. The argument of less deformation is usurped and is only due to a less efficient quenching
Solution heat treatment of steels, nickel-based alloys and superalloys (quenching, annealing, hyperquenching...): temperature and vacuum levels according to the alloys. The following are commonly found:
Austenitizing before hardening of tool steels (high alloy steels), before hardening of high hardenability structural steel grades
The solution of superalloys
Solution heat treatment of austenitic stainless steels followed by relatively rapid cooling (known as hyperquenching)
Annealing of titanium alloys
Magnetic annealing of alloys for magnetic applications
Tempering of metal alloys in vacuum purge furnace: steels and nickel-based alloys, hardening of electroless nickel coatings, hardening tempering of copper alloys type UBe2
Annealing of copper alloys (brass)
Brazing of metal alloys: high temperature brazing of various iron or nickel based metal alloys, heterogeneous brazing (ceramic-metal, graphite-metal), brazing of aluminum alloys.
Cementation, carbonitriding of steels in special furnaces dedicated to this application with injection of fuel gas and possible nitriding agent, in sequences alternating with vacuum diffusion sequences. The term used is low pressure carburizing.
Gas nitriding after vacuum purging or under partial pressure of cracked ammonia: after vacuum purging an atmosphere of ammonia and nitrogen for example is established, in the case of the nitriding known as low pressure nitriding the atmosphere is maintained at a pressure of about 200 mbar.
Degassing and decontamination of metal alloys: for this application the lowest pressures are required.
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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