In the general gas carburizing process, not only an atmospheric heat treating furnace (called a heat treating furnace hereinafter) but also a transforming furnace has been conventionally required.
Such transforming furnace is to obtain a transformed gas necessary for the atmospheric heat treatment, is charged with a cataylyst within it and is fed with a hydrocarbon gas and air in a retort heated from outside.
The gas obtained from the above mentioned transforming furnace is fed to the above mentioned heat treating furnace and further a carburizing gas is added to the gas to adjust the carbon potential of the atmospheric gas within the heat treating furnace in a carburizing process.
However, with the above mentioned conventional process, there have remained such problems that, as not only the heat treating furnace but also the transforming furnace is required, the heating energy and expensive catalyst are required and further it is expensive to maintain and control the heater and retort.
Therefore, in consideration of the uneconomy accompanying the use of the above mentioned transforming furnace, the applicant of the present case has provided a process for feeding a hydrocarbon gas and oxidative gas directly into a heat treating furnace without using a transforming furnace (Japanese Patent Publication No. 38870/1989).
In this process, a hydrocarbon gas and a small amount of pure oxygen are introduced into a heat treating furnace kept above 730.degree. C. and a nitrogen gas is excluded to carry out a carburizing process.
That is to say, when a hydrocarbon gas and pure oxygen are introduced into a heat treating furnace kept at a predetermined temperature, an atmosphere necessary for carburization will be produced to carry out carburization.
According to this process, as only the gas contributing directly to carburization is fed into the heat treating furnace, the apparent partial pressure of CO in the atmosphere will not be reduced by the gas not contributing directly to the carburization, the carburizing efficiency is high, further no transforming furnace is required, the used amount of the hydrocarbon gas is small and the process is very economical.
However, in the above mentioned process, the amount of the gas fed into the furnace is so smaller than in the case of the process using the carburiz ng gas transformed in the above mentioned transforming furnace that, with the opening and closing of an inlet door, intermediate door and outlet door when an article to be treated is put in and moved, the pressure within the furnace will become negative, atmospheric air (oxygen) will be sucked in through the packing part of the door and the atmosphere within the furnace will be disturbed to cause a danger of an explosion or the like.
Therefore, the applicant of the present application has provided an atmospheric furnace pressure adjusting apparatus wherein, when the pressure within the furnance is negative, a ring burner provided in an atmospheric air introducing path will be ignited to feed the combustion gas into the furnace to eliminate the negative pressure within the furnace (Japanese Utility Model Application Publication No. 16766/1989).
If this apparatus is used, when the pressure within the furnace is negative, oxygen will not be introduced and the furnace will be safe but the N.sub.2 gas not directly contributing to the above mentioned carburization will be introduced to reduce the partial pressure of CO within the furnace.
By the way, the basic gas reaction of the carburization is as follows: ##EQU1## EQU 2CO.fwdarw.[C]+CO.sub.2 ( 2)
That is to say, the gas contributing directly to the carburization is CO, the larger the partial pressure of CO, the more active carburization, a carburized layer of a required hardness and depth can be formed within a short time, further the dispersion of the carburization of a treated article of a complicated form can be reduced and a pore or the like can be effectively carburized.