Gases used in atmosphere treatment
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.
ARGON
(Ar) Neutral gas delivered in liquid form in tanks or cylinders.
Argon is produced by fractional distillation of air. It is used as partial pressure gas during vacuum treatments and as inerting and cooling gas. Not very suitable for cooling, this inert gas is also used for the treatment of very oxidizable metals, such as titanium and its alloys, certain stainless or special steels. Titanium alloys in particular, which cannot be treated in atmospheres containing hydrogen or nitrogen, are treated in a pure and dry argon atmosphere. Beware, this gas being very heavy, the risks of flow in a workshop as well as the purging of furnaces must be rigorously controlled.
NITROGEN
(N, N2 molecule nitrogen) Neutral gas delivered in liquid form in tanks or cylinders.
Molecular nitrogen is a neutral gas that is inert to many metals, especially mild, low carbon steels. Pure and dry nitrogen is an excellent protective atmosphere against oxidation. It is delivered in liquid form stored in cylinders or tanks with a pressure of about 8 bar for common applications or 30 bar for high pressure quenching applications. It is sent into the circuits after passing through an atmospheric heater (plate heat exchanger) where it is expanded to atmospheric pressure before being sent into the furnace. One must be careful with the presence of water vapor which can transform this neutral medium into an oxidizing or decarburizing atmosphere for high carbon or alloy steels.
It is used as an active gas for thermochemical treatments and coatings via thermal dissociation (case of ammonia dissociation), or plasma activation (case of plasma assisted treatments, nitriding, PVD deposits).
HYDROGEN
(H, dihydrogen H2) Reducing gas delivered liquid in cylinders.
Dry hydrogen is a very reducing protective gas from 300°C. In particular, it reduces most metal oxides to metals. Wet hydrogen can be decarburizing depending on the furnace temperature, the holding time, the amount of water vapor in the furnace and the carbon content in the steel. The decarburizing effect of hydrogen is negligible below 700°C. It is most often used in addition to nitrogen to avoid oxidation and discoloration that could be linked to low oxygen levels in the atmosphere or during gas quenching to improve the cooling rate. Its content is limited to 5% to avoid the risk of explosion. It is exceptionally used in its pure state on dedicated installations for annealing metallurgical wire coils for example (its lightness allows it to penetrate between the turns of the coil) or as a magnetic annealing atmosphere for its decarburizing and depassivating effects.
The atomic hydrogen intervenes fleetingly in various gaseous reactions, in particular during the decomposition reactions of hydrocarbons or ammonia, before recombining into dihydrogen (carburizing atmospheres of endothermic type contain from 66,6 to 30% of dihydrogen).
PROPANE
C3H8 carburizing enrichment gas
A saturated hydrocarbon or alkane gas which, by cracking under the effect of temperature, produces an atmosphere rich in carbon, thus allowing this element to be added to the surfaces of metal parts. This gas is used either alone (plasma treatment processes, low pressure carburizing), or diluted with nitrogen in carburizing, or as a complement to a carrier gas (endothermic atmosphere or cracked methanol nitrogen, etc.). It then has the effect of increasing the carbon content of the mixture by increasing the CO and decreasing the CO2
This gas is available in bottles or cylinders, delivered in bulk in liquid state (LPG).
It is often recommended to take the quality called depropylenated.
METHANE
CH4 enrichment gas for carburizing
Gas of the saturated hydrocarbon or alkane type which, by cracking under the effect of temperature, will produce an atmosphere rich in carbon, thus allowing a contribution of this element on the surface of metal parts. This gas is most often used in addition to a carrier gas (endothermic atmosphere or methanol nitrogen, etc.). It increases the carbon content of the mixture by increasing CO and decreasing CO2. It is also used as an additive to plasma atmospheres in ionic nitrocarburizing. It is also used mixed with air for case hardening ("in situ" atmosphere).
It takes three times as much methane flow to achieve the same result as with propane.
This gas is the almost unique component of the town gas or natural gas which is distributed by the GRDF network.
AMMONIAC
NH3 enrichment gas in carbonitriding and treatment gas in nitriding and nitrocarburizing.
Gasreleasing atomic nitrogen when it arrives on the surface. In carbonitriding, the ammonia rate must be well controlled to avoid defects in the carbonitrided layer (austenite, white layer, porosities). In nitriding, the measurement of the dissociation rate allows to measure the part of ammonia still whole likely to enter in reaction with the surface to be enriched.
Storage of liquid ammonia in cylinders distributing ammonia in gaseous phase by expansion with or without vaporization of the liquid phase.
At 17°C, 1 Kg of ammonia provides 1.3 Nm3 of NH3 gas and 2.8 Nm3 of cracked gas (at atmospheric pressure).
It should be stored at a temperature below 50°C.
It has physiological effects in concentrations above 50 p.p.m. in air and can cause irritation of the eyes and bronchi.
The gas can be withdrawn :
In gaseous phase (without dip tube): depending on the flow rate drawn, there is a cooling of the cylinder with a risk of flow variation by icing (>500l/h). Warm up the storage or oversize the stored volume.
In liquid phase with dip tube: the gas is drawn from the liquid phase and heated to a gaseous state in an evaporator (electric)
ENDOTHERMIC GAS MIXTURES
The so-called endothermic atmospheres are made up of a mixture of nitrogen, hydrogen and carbon monoxide CO obtained by cracking in a reactor or endothermic generator a mixture of a gaseous hydrocarbon (methane, propane or butane) and air passing over a catalyst heated to about 1000°C. The system is in default of air to obtain a CO rate of about 20%. Nitrogen is present by the volume of air introduced (about 40%) and hydrogen from the reaction with the oxygen of the air introduced (about 40%).
In common parlance, the gas produced is called endothermic gas because it comes from an endothermic reaction which must involve a reaction at high temperature consuming heat as opposed to exothermic reactions which are maintained by the release of heat from the reactions.
EXOTHERMIC GAS MIXTURES
Float or rotameter flow meter
The "lean" exothermic gas is obtained by cracking in a generator, a mixture of gas between a hydrocarbon such as methane or propane and air after passing in overpressure in a burner. The atmosphere thus created contains at the exit of the generator (after having been cooled by passage in a condenser or soaked by a refrigeration unit) nitrogen (from air) (85%), carbon dioxide (CO2) (10, 12%), carbon monoxide (CO) (2.5%) and dihydrogen (2.5%). In stoichiometric condition (air - gas ratio =1 with methane) all the carbon and oxygen are burnt into CO2. If this ratio deviates slightly from 1 to be in low air deficit, the atmosphere is said to be poor because of low CO content. When the mixture is in stronger defect of air, the combustion is incomplete, it remains then of the gas generator of soot, CO and H2, the atmosphere is known as rich because with stronger content of CO.
These atmospheres may not be inert to the steel depending on the carbon content of the steel.
ETHYLENE
C2H4 Fuel gas used in low pressure carburizing.
ACETYLENE
C2H2 Fuel gas used in low pressure carburizing.
METHANOL
Methanol or methyl alcohol (very toxic liquid when ingested), although not a gas, is a liquid that cracks at high temperature into CO and H2, directly affected to the production of fuel atmospheres.
It is introduced into the furnace by an injection tube via a pipe coupling a pumping and pressure effect.One liter of liquid produces 1.66 m3 of gas (0.553 m3 CO, 1.106 m3 H2).
The proportion of the nitrogen-methanol mixture determines the composition of the gas mixture
The control of gas flow rates is obtained by float or rotameter flowmeters. It is then necessary to check that the float has a stable position, without bubbles in the circuit, or by mass flowmeters.
Implementation
Main equipment (furnace, reactor, line, machine...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
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