Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Patent
1996-04-15
1998-11-03
Walberg, Teresa J.
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
148233, 432 72, 432175, 432176, 432179, C23C 820
Patent
active
058302840
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and a device for the heat treatment of workpieces, wherein the workpieces are heated with radiant heat which is generated through the combustion of gaseous fuel, in particular natural gas, and wherein at least some of the workpieces are subjected to a carburisation atmosphere.
BACKGROUND OF THE INVENTION
Such carburisation methods and devices known from practice have a relatively high energy requirement. Thus, those skilled in the art are continually striving to improve the cost-effectiveness of carburisation processes. At the same time, the environmental protection requirements have also to be considered. Great importance is attached to the reduction of pollutant emissions. However, such emission reduction measures are cost-intensive and thus run counter to the efforts to improve cost-effectiveness.
SUMMARY OF THE INVENTION
The object of the present invention was to optimise the carburisation process under these aspects, i.e. to reduce the pollutant emissions and at the same time to improve process control at least to such an extent that there is no appreciable negative effect on cost-effectiveness.
This object is achieved by the method according to the present invention characterised in that carbon dioxide is separated out of the exhaust gas which forms during the combustion of the gaseous fuel, that the carbon dioxide is mixed with hydrocarbon-containing gas, in particular with natural gas, and that the gas mixture is heated to produce the carburisation atmosphere.
The impact on the environment is quite considerably reduced through the removal of the carbon dioxide from the exhaust gases of the radiant heating system. The share of carbon dioxide in the exhaust gas is approx. 11%. Most of said carbon dioxide can be removed. With the appropriate process control the remaining exhaust gas only contains less than 1% carbon dioxide after the treatment.
The carbon dioxide is fed to the carburisation process as a supplier of oxygen and carbon with the effect that the carburisation time is considerably reduced, namely by 20 to 40%. The carburisation time depends on the temperature, the diffusion coefficient and the mass transfer coefficient. At a given temperature the two latter coefficients govern the speed of the carburisation process, in the case of small or medium-sized carburisation depths (0.2 to approx. 0.8 or 1.0 mm) said coefficients having equal ranking. The present invention has a particularly favourable effect in this range. It leads to an increase in the mass transfer coefficient by a factor of roughly 2.5.
A particular advantage of the present invention consists in the fact that the entire process can be operated continuously, it being possible, if necessary, to place the carbon dioxide removed in an intermediate store.
The carburisation atmosphere is generated by an endothermic reaction. The heat necessary herefor is preferably removed from the radiant heat which is used for heating the workpieces. This can, for example be achieved by introducing the mixture of carbon dioxide and hydrocarbon-containing gas directly into the furnace chamber. However, with this procedure there is the danger of inadmissible soot formation occurring. Therefore, it is more advantageous to pass the gas mixture over a catalyst which ensures that the endothermic reaction can take place without the formation of soot. The catalyst also ensures optimal intermixing of the components. The endogas generator can be arranged outside the furnace chamber. However, then a separate heating system is generally necessary. Therefore it can be more advantageous to arrange the generator in the furnace chamber, preferably in the furnace roof area, i.e. where a high temperature prevails and also where the fans are located.
A further embodiment of the present invention proposes that the carbon dioxide is removed from the exhaust gas formed during combustion of the gaseous fuel by changes in pressure. This method makes use of the pressure-dependent attachment properties o
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patent: 3207493 (1965-09-01), Swain
patent: 3712597 (1973-01-01), Waitkus et al.
patent: 3870474 (1975-03-01), Houston
patent: 3957418 (1976-05-01), Sata
patent: 4217091 (1980-08-01), Whike et al.
patent: 4219324 (1980-08-01), Zahniser
LOI Thermprocess GmbH
Lu Jiping
Walberg Teresa J.
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