Development of controlled atmospheres

Controlled atmospheres are used nowadays at an ever growing Iarger extent. By means of it. there is not only attained  a rationalization of production, but frequently a substantial  improvement of quality. In some cases producers passed over at application of controlled atmopheres to entirely  new production methods.

It is necessary to enlighten the application and production  of various controlled atmospheres. At present, there are offered at the market generators of controlled atmospheres, the application of which is for a certain heat-treatment process entirely univocal. In some cases, however, application  of the generators is not so univocal, we are therefore recommending consultation with a specialist.

For the producer of equipment for controlled atmospheres  it follows hence, to take an intensive interest into all que- stions regarding technics of thermal treatment. Thereto does not only belong metallurgical knowledges and experiences, but also close cooperation with the build-up of the fur nace. Best results ar attained in cases, when the individual  parts for thermal treatment, as for inst. the furnace, the generator, the transportation of equipment, the washing machine and the Iike are jointly designed and mutually technically brought into harmony. According to present requirements for equipment for controlled atmospheres, there turns up the possibility of reducing production costs by means of partial series production. Th,at re&ults in the requi- rement for typifying individual generators, on the other side it is, however necessary for this narrowed assortment of generators to meet all requirements of the customers. It is necessary, to divide the whole production process of controlled atmospheres into individual partial processes, and to construct for them in unit assembly principle production units.

Thermal treatment in controlled atmospheres

With the aid of controlled atmospheres we can prevent exchange of components between the material surface and the gaseous environment or to regulate such exchange.  For the exchange of components, there are coming into consideration such steel components, which at thermal-treatment temperatures are turning into gaseous state or into gaseous compounds. Those are primarily oxygen, car bon, nitrogen. On the other hand components of hydrogen, sulphur, chromium and the like may also take part in the referred to exchange of components, playing technically a more inferior role. 

For the beforehand referred to components (oxygen, car bon, nitrogen), there are stated in Table 1. the gaseous compounds, which are taking part in the exchange of components. Carbon and nitrogen are listed at the Table together,  because the simultaneous exchange of both components is technically significant. Articulation of individual columns  according to directions of exchange of components is shown in the diagram, which is describing us the processes  of the entire thermal treatment with controlled atmospheres. ln spite of this very lucid arrangement of individual processes, we cannot read out of this diagram any direct hints for dividing individual controlled atmospheres.

CO/CO₂

>

<

no element transport 

scale reduction

bright annealing

CH₄/H₂

 >

<

no element transport

decarburization

nodecarburization annealing

 >

<

no element transport

nitrogen depreciatin

CH₄/H₂

NH₃/H₂

 >

<

no element transport

decarburization nitrigen depreciation

Kinds of controlled atmosphere

      Because partial pressures of components, which are in the steel as well as in gaseous mixtures of different composi- tion are known, it is possible to count the behaviour of individual atmospheres. Molecular nitrogen does not take part at normal circumstances in reactions, and it is therefore unnecessary to take it henceforth into consideration. We can derive the following qualitative conditions for the compositions of artificial atmospheres.

a) limits between oxidation and reduction (change of oxy- gen) at plain carbon- and low-carbon steels is accor ding to temperature for the proportion CO₂/CO fromO,3 to 3,0 and for the proportion H₂O/H from 0,02 to 1,0 in a controlled atmosphere. If both these gaseous mixtures are within the controlled atmosphere, composition of these four components are to be adjusted according to the temperature and the !eaction of the water gas.. The carbon potential of such gaseous mixture Iies below 0, 1 % C (compare Fig. 1 and 2).

b) The Iimit for Iight carburizing up to material saturation with carbon is attained only at very low proportions of,  CO₂/CO and H₂O/H₂. Already negligible CH₄ contents 1 are Ieading to steel saturation with carbon (compare Fig. : 1 and 3)

c) At processes at which it should not come to any exchange of components, it is possible, to use the gaseous mixtures ad a) and ad b), if their composition complies at the given temperature exactly with the equilibrium. 

d) When the steel contains greater contents of components, which have a very low partial oxygen pressure (chromium, aluminium, silicon), then the controlled atmosphere must be relieved of the CO₂ - and H₂O -as well as the CO components, if the steel will have to be oxygenfree annealed, because even the partial oxygen pressure in pure CO is higher and might oxidize the referred to components.

e) The partial pressure of normal molecular oxygen is too small to cause steel nitridation. On the other hand, the mixtures NH₄H₂ are displaying an adequate partial nitrogen pressure.

     ThiS division provides us with four basic types of controlled atmospheres with the range of application. The mixture of gases quoted under e) is for the production of artificial atmospheres Iess interesting, because it is possible to prepare it without any special equipment. Equilibriums between the controlled atmosphere and steel are depending only upon the proportion of actual components in the mixture, and not at their absolute hight. For practical application there occur even nevertheless definite optimal conditions.

        For production of controlled atmospheres. there are coming into consideration hydrocarbons or gases comprising hydrocarbon. In order to acquire fissionable gas, procedure starts usually from carbon-free ammonia.

Analysises of controlled atmospheres are performed essentially from the determination of the proportion of carbon to hydrogen in the starting gas. In Table 2. there are quoted the most important starting materials. At the generator gas proportion of carbon is outweighing so strongly , that at production of controlled atmospheres there are occuring unfa- vourable proportions.

At the selection of the starting material. it is further necesi saryto take into consideration, that at production of controlled atmospheres necessary alternations are taking place without disturbing products (for inst. forming of soot), and it is also necessary to evade undesirable components (for inst. sulphur compounds).

At comparision of expences it is necessary to take into consideration the quantity of the controlled atmosphere produced of the quantity unit of the initial substance.  

      The basic process at the production of control'ed atmospheres is the reaction of hydrocarbon materials with the air at temperatures over 1000 °C. The resulting comosition of gaseous mixture is read of the combustion diagrammes. As an example there is quoted the combustion diagram of pure propane - see Fig. 1. 

Figure 1. Production of controlled atmoshpere from propane 

1 - free C ( soot )

2 - atmosphere for bright annealing

3 - inert atmosphere

4 - fre oxigen

5 - carburizing atmosphere

6 - endotherm. atmosphere

7 - exotherm. atmosphere

Combustion diagrammes of the other initial gases have a similar procedure. The working range of the referred-to diagram is demarcated by two ver ticallines: to the Ieft there is taking place a soot deposition, to the right occurence of free oxygen. In the centre, there is a zone for bright annealing, close to the boundary of soot forming, there is narrow zone for carburizing, and at the boundary of free oxygen, there is an inert-gas zone. Between the carburizing gas and the gas for bright annealing, there is the change from the "endothermiců" to the "exothermic" atmosphere. In the first case, the reaction chamber must be heated from the outside. In the range of the inertgas zone, the curve for CO₂ is illustrated dashed, because this component must be subsequently removed. For determination of gas behaviour for bright annealing of steel, there is very useful the diagram of Fig. 2, which is related again to propane as initial gas.

Figure 2. Composition of a controleld atmosphere for bright propane annealing

1 - not dried

2 - dried

It shows vividly, where the controlled atmospheres of different composition are situated against the oxidation curve. Drying of gas is then necessary , if cooling shall take place at maintenance of the bright surface, where by the degree of drying is dependent upon the speed of cooling. 

Carbon gases are adjusted in accordance with known dia- grammes of the dew point Fig. 3. 

Figure 3. Relation between the carbon potential, the dew point and the temperature of the furnace atmosphere containing 31% H₂ a 23% CO

The proportion gas-air at the production of carburizing gases is entirely delimitated, which emanates from the narrow range of the carburizing zone in Fig. 1. The content of H₂ and CO must therefore not be altered. At the inert atmosphere, there it is unnecessary to apply equilibrium diagrammes, because at that atmosphere, there is not taking place an exchange of elements Ij>etween material and environment as a result of equilibrium: but due to the absence of reaction partners. Handling with these at- mospheres is proportionally simple. For the negligible content of inflammable components they are non-explosive and are advantageously used anywherey, where the processes ot thermal treatment are taking place for a long pe- riod in temperatures below the Iimit of inflammability (500 to 700 °C). At fission-gas application for bright snnealing it must be dried 50 as to prevent oxidation of iron and its alloying ele ments.

Conception diagram of EXO generators

1 - Gas-air mixing unit

2 - Combustion chamber

3 - Cooller

4 - Adsorption dryer

Low-carbon steel -bright surface

Medium-carbon steels up to a content of 0,25 % C, bright surface,

cast iron -application for decarbonization surface clean steel, plates with content of Si -release of stress -surface clean

copper -H₂ content in an atmosphere of up to 2 vol. % - surface bright

alloyes Cu-Ni -surface bright bronze Cu-Si surface bright

nickel and its alloys -surface clean gold surface bright silver -surface bright  

medium-and low carbon steels -surface clean

steels comprising alloy elements with higher Cr content - surface clean  

low-and medium carbon steel -hard solder Cu -surface clean  

solder Ag with flux -surface clean

non-rusting steel -hard Cu solder with flux -surface clean cast iron -solder Ag with fllux -surface clean brass -solder Ag with flux -surface clean  

low-carbon steel -surface bright Cu-Fe alloyes -surface bright Cu-Pb alloyes -surface clean

high-carbon steel -surface bright Cu-Sn graphite -surface clean Fe-graphite -surface clean Copper -surface bright

Cu-Fe alloyes --surface clean and bright nickel -surface bright silver -surface bright

bronze -surface clean  

Conception diagram of EXO- MONO generators 

1 - Gas-air mixing unit

2 - Combustion chamber

3 - Cooller

4 - Absorption column

5 - Adsorption dryver

6 - Compressor unit

medium-carbon steels

high-carbon steels - surface bright special steels  

medium-cArbon steel without decarburization -surface bright

High-carbon steels. heating no longer than 2 hrs. -surface bright

alloyed steels -without decarburization

medium-carbon steels. heating no longer than 2 hrs. -sur- face clean

high-carbon steels. heating na longer than 2 hrs. -surface clean

high-speed steels without decarburization. heating no lon- ger than 2 hrs. -surface clean

with 3% Si orient. transformer sheet. stress relieve -surfa- ce clean

dynamosheet. stress relieve -surface clean bronze -surface clean

Cu-Ni alloyes -surface bright 1)

Cu-Si alloyes -surface bright ~. Cu-Be alloyes -surface clean Cu-AI alloyes -surface clean  

high-carbon steel. without decarburization -surface clean up to bright

carburized steel. without decarburization -surface clean up to bright

alloyed steels:

medium carbon steels,

without decerburization -  

surface bright up to clean high-carbon steels

high-speed steels. without decarburization -surface clean 

medium-and high-.carbon steels. without decarburization - heating no longer than 2 hours -surface clean

low-carbon steel -as bearing atmosphere enriched by sui" table hydrocarbon -surface clean  

medium- and high-carbon steel. Cu brazing solder. without decarburization -surface bright alloyed steel:

medium carbon steels

Cu solder, -without decarburization, surface bright.

high-carbon steels clean brass

copper Ag solder with flux - surface clean  

iron -surface clean up to bright

medium -and high-carbon steels without decarburization -surface clean up to bright

iron -copper -graphite -surface clean iron -graphite -surface clean bronze -surface clean nickel -surface clean  

Conception diagram of ENDO generators

1 - luquid gas storage bottles 

2 - vaporiser

3 - air - gas mixing unit

4 - catalytic reactor

5 - cooler

They comprise a high % of H₂ ,CO and N₂ components and a low vol. of CO₂ and H₂O %.  

low-carbon steel -as bearing atmosphere enriched by a suitable hydrocarborr -surface clean  

nitro-carburizing. low-carbon steel -as bearing atmosphe- re enriched by suitable hydrocarbon and ammonia -surfa- ce clean.  

t

medium- and high- carbon steels. Cu solderwithout decar- burization -surface bright alloyed steels:

medium-carbon steels

by means of a Cu solder

without decarburization - high-carbon steels surface bright

non-rusting steels. Cu solder -surface clean up to bright Imedium- and high-carbon steel. Ag solderwith flux -surfa-

ce clean  

low- and medium-carbon steel -surface bright

medium and high-carbon steels, short-time heating -sur- face clean up to bright

high-spe~d steels, short-time heating, surface clean alloyes Ni-Cr -surface clean up to bright

alloyes Ag-Zn -surface clean up to bright

transformer sheets, short-time heating. stress relieve. re- duction of magnetic losses -surface clean  

carburized steel

high-carbon steel

}...dIwlthout decarburlzatlon -

alloye stee s:

rfb.

h.-su ace rlg t

medlum-carbon steels high-carbon steels

carburized steel

high-carbon steel

alloyed steels: without decarburization - medium carbon steel surface clean up to bright high-carbon stee1 high-speed steels    

Conception diagram of the splitting device

1 - luquid gas storage bottles 

2 - vaporiser

3 - catalytic reactor

4 - cooler

high alloyed steel - surface bright  

sheets for electrotechnics -decarburizating annealing with water vapour addition - surface clean brass -surface clean up to bright  

Normalization:  

carbon steel -surface bright 

steel with a high chrome content -surface bright 

steel with a high chrome content -surface bright  

steel with a low carbon content -surface clean  

steel with a low carbon content

steel with a medium carbon content ' -surface steel with a high carbon content clean special steel ~  

To the basic types of generators of controlled atmospheres, there can be supplied a series of accessories. Their applica- tion emanates from functional, technological and safety re- quirements. As main parts, there can be quoted:

• box of the H₂ analyzer with a range of 0-10 or 0-20 vol. % H₂ for recording event. automatic control of H₂ content at the EXO and EXO MONO generator,

• box of the CO₂ analyzer with a range of 0-1 vol. % CO₂ for automatic control (and recording) of CO₂ content at the ENDO generators and for automatic control of the C - potential in the operating space of the furnace,

• actuators for automatic addition of gaseousor Iiquid mediums at the requirement of automatic control of composition of controlled atmospheres,

• box for dosing Iiquids for transportation of Iiquid hydrocarbons into the operating spaces of the furnaces (f.or diffusion processes),

• compressor unit for creation of stocks of controlled atmospheres,

• storage tanks of controlled atmospheres for a maximum overpressure of 1,5 MPa and a maximum volume of 25 m³,

• ammonia and nitrogen systems, ammonia vaporizer, measuring  and control stands,

• bottle boxes (metal pressure vessels) for external execution.