US3413161A - Process for the generation and utilization of furnace atmospheres for the heat treatment of metals, especially of steel - Google Patents

Description

NOV. 26, 1968 w, GOEHRING 3,413,161

PROCESS FOR THE GENERATION AND UTILIZATION OF FURNACE ATMOSPHERES FOR THE HEAT TREATMENT OF METALS, ESPECIALLY OF STEEL Filed Sept. 19, 1966 INVENTOR WERNER GOEHRING Attorney United States Patent 6 Claims. (Cl: 148-16.5)

ABSTRACT OF THE DISCLOSURE Hydrocarbon fuel is mixed with a quantity of air less than that required for complete combustion. This mixture is burned, while steps are taken to prevent the escape of generated heat from the burning mixture. This generated heat is accumulated until chemical equilibrium of the products of combustion is reached at a temperature above the processing temperature of the steel to be treated. The steel is then exposed to the products of combustion for earburizing, bright annealing and oxidation-free cooling.

Cross-reference to related application This is a continuation-in-part application to copending parent application Ser. No. 396,980 which matured into U.S. Patent No. 3,290,030, on Dec. 6, 1966.

Field of the invention The present invention relates to a process for generating and using furnace atmospheres for the heat treatment of steel.

Summary of the invention The objects of the invention are to provide a process:

For producing high-quality furnace atmospheres from fuels consisting essentially of hydrocarbons;

For producing atmospheres that affect the carbon content of the steel completely according to the formation and decomposition of CO on the metal surface;

For producing atmospheres that are in equilibrium and which can therefore be judged for their carburizing effect by the measurement of only one gas component;

For producing a furnace atmosphere without the necessity of removing CO and H 0, said atmosphere therefore being available immediately after combustion for contact with the metal to be treated and therefore also being usable for regeneration heat transfer.

For producing atmospheres that can be used even at low heat treating temperatures and that prevent oxidation, even during cooling of work pieces;

For producing furnace atmospheres without the necessity of specially heating the combustion chamber for the fuel-air-mixture;

For producing high-quality furnace atmospheres from combustion gases of hydrocarbon fuels without any preliminary treatment, which atmospheres can be used for bright annealing of steel and subsequent cooling Without any oxidation of the work pieces.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description.

The only figure of the drawing is an equilibrium chart.

In outline, the steps of the process of the invention comprise mixing with a hydrocarbon fuel a quantity of air less than that needed for its complete combustion, burning the air-fuel mixture, preventing the escape of 3,413,16 1 Patented Nov. 26, 1968 heat from said burning mixture and from the resulting gaseous products of combustion until said products reach chemical equilibrium at a temperature above the processing temperature of the metal to be treated, collecting said products and exposing said metal to said products. As will be seen in the following paragraphs, other steps are added to these, depending on the particular advantages desired.

Description 0 the preferred embodiments The various types of furnaces conventional in the are suitable for the performance of the process of this invention. The drawings of the furnaces of the copending parent case and their descriptions are hereby incorporated by reference only for the purpose of showing two furnaces in which the process can be performed. The process is, of course, not limited to its performance in these two furnaces.

Hydrocarbons, the molecules of which consist only of carbon and hydrogen, or fuels essentially containing these hydrocarbons are burned in a combustion chamber which is connected with the processing chamber of the metal work pieces. The combustion chamber is protected against heat losses and is of such small volume that the gaseous product of combustion, hereinafter referred to as combustion gases, reach a high combustion temperature and assume a state of chemical equilibrium above the temperature of the material to be treated. Thus, these gases influence the carbon content of the steel exclusively by the combustion gas component CO and/ or prevent any oxidation during annealing and subsequent cooling of the steel. Chemical equilibrium means that the relative amounts of the various products of combustion at a given temperature are neither increasing nor decreasing.

As a rule, in comparison to other flowable tools as for instance coke oven gas, the hydrocarbons contain little hydrogen in proportion to carbon. They therefore bring the advantage that during the combustion, combustion gases low in Water vapor are generated. Although the combustion may take place under conditions of insufficient air for generation of the required content of combustion-gas components which have a reducing effect, namely H and CO, hydrocarbons still permit a relatively high feeding of air, since they have no combined oxygen. This makes the theoretical combustion temperature in many cases higher than the conventional processing temperatures of the work pieces. In the case of combustion of the hydrocarbon-air mixture in a small combustion chamber, the volume of which is designed no larger than required for the movement of the combustion reaction to chemical equilibriurru a high combustion temperature occurs. Combustion is effected preferably in the presence of reaction accelerating, catalytic filling materials, such as nickel oxide or platinum black. The high temperature attained is due to heat reflection from the adjacent cham ber surfaces onto the burning gases. Due to the small surface and the thermal protection of this combustion chamber as well as possibly through the structural unification with the chamber of the work pieces, the heat losses can be kept so low that the combustion gases almost reach the theoretical combustion temperature. Then, due to the high reaction velocity at a high combustion temperature, they assume the state of equilibrium.

In the case of employing these combustion gases for carburizing work pieces, in accordance with Boudouards reaction 2CO CO +C, the transfer of carbon takes place exclusively through the dissociation of the carbon monoxide, more stable at higher temperatures, at the cooler surface of the work pieces. Because of the attained state of equilibrium the composition of the combustion gases permits an unambiguous conclusion to be made concerning the carburizing effect. This conclusion can be made by the measurement of one single combustion-gas component. This is usually the water vapor and is determined by means of measurement of the dew point.

In the case of the bright annealing of steel, cooling work pieces are not oxidized by combustion gases that are generated in the manner of the process, even though they have been generated with a relatively high feeding of air. The gases contain more Water vapor in proportion to hydrogen than is permissible for oxidation-free cooling according to teachings of the prior art. Obviously, this oxidation is avoided through the high content of carbon monoxide that is present due to the composition of the fuel. Here, it is likely that water vapor reacts locally with carbon monoxide under the catalytic influence of the Work piece surfaces to reach the state of equilibrium that corresponds to the momentary temperature of the metal material; at this state of equilibrium the gases then have a reducing effect in accordance with the known values.

In a further development of the process according to the invention, air or a hydrocarbon-air mixture is introduced into the combustion chamber via a preheater. This supplies the combustion components with an amount of heat which, together with the amount of heat released during combustion, provides for the required high reaction temperature. For this purpose the heating installation of the processing chamber as well as the heat of the cooling work pieces of continuously fed furnaces can be used. Regeneration heat transfer using previously charged gases of the atmosphere is also applicable.

It has been found that, in the case of a hydrocarbon-air mixture which flows faster than its combustion velocity, no precornbustion and, above all, no cracking connected with undesirable separation of carbon occurs within the preheater. This fact is based upon the circumstance that the carbon of the hydrocarbon, which disassociates upon strong heating, adds in statu nascendi to the oxygen present in the mixture. Thus, only gaseous products of cracking are brought about; these burn free of soot in the combustion chamber.

The purpose of the preheating of the combustion components is, on the one hand, to reach, in the case of some hydrocarbons, the required high theoretical combustion temperature, especially in the case of the small 4 on the H/C ratio of the fuel. The figure shows the fa miliar iron/iron oxide equilibrium curve in a mixture of CO CO, H 0, and H at different temperatures. The combustion gases from fuels, as entered on the chart, form curves because their composition changes as a function of temperature according to the reaction where k the lack of air (ratio of the air fed to the air needed for complete combustion);

a=the desired CO /CO ratio;

b=the desired H O/H ratio;

x=the H/ C ratio of the fuel.

If bright annealing and oxidation-free cooling of steel is desired, the combustion gas curve must be located some certain distance from the iron/iron oxide equilibrium curve in the reducing region of the chart. For instance, if the choices a=1.1 and b=0.025 are made for the low temperature region, the required amount of air follows the equation:

For the choices a=0.25 and b:0.55 made for the high temperature region, the required amount of air is:

In the case of fuels having an H/ C ratio greater than 1.8, the composition curve of the combustion gas comes closest to the iron/ iron oxide curve in the region of lower temperatures. Therefore, here the Equation 1 must be used.

In the case of fuels with H/C smaller than 1.8 (for example, benzene), the closest approach is in the region of higher temperatures. This dictates use of Equation 2.

TABLE I.TABLE OF EXAMPLES Lack of air Required preheating in (air fed in Composition of the combustion gases order to reach the proportion Theoretical (at 1,l00 C. in a state of equilibrium), percent theoretical combustion Type of heat Fuel to amount combustion temperature of 1,100 0.

treatment (hydrocarbons) of air in the temperature,

case of 0. Air Mixture complete CO2 00 E20 Hz N2 preheating, preheating, combustion) 0. 0.

Bright annealing and Benzene vapor 0.55 1, 620 5. 0 19. 5 4.15 8.1 63. 25 None None oxidation-free a s. cooling of steel. Light fuel oil 0. 52 1, 300 4. 3 16. 2 6. 9 13.0 59. 6 None None Propane C3H5 0. 46 1,230 3.0 16. 0 7.0 18. 3 55. 7 None None Methane CH4 0.4 975 2. 1 14.5 7. 7 25. 5 50. 2 220 160 Cementatiou or de Benzene vapor O. 4 1, 180 29. 5 Trace 14. 75 55. 75 None None carburization-free CGHG. Trace annealing of steel. Propane O Hg 0.3 600 Trace 23. 7 Trace 31.6 44. 7 890 590 permissible feeding of air for the production of combus- 0 Table I glves examples of fuels for use in the process tion gases with a carburizing effect. For the production of combustion gases that only prevent oxidation, in the case of which a larger feeding of air is permissible, the processing chamber can be directly heated by means of the hot combustion gases. In the case of the employment of a heating installation, it is expedient to use it only for the preheating of the processed material, since then it is subject to a smaller thermal stress.

By means of the process according to the invention it is thus feasible to generate furnace atmospheres for bright-annealing and bright-cooling purposes for as well as for cementation-free annealing, and also for carburization of steel in a simple furnace installation.

The amount of air to be used during combustion depends on the desired CO /CO and HgO/H ratios and of the present invention. Other fuels can be analyzed for application using the above equations.

The process according to the invention can be employed at all furnace installations suitable for protective-gas operation with burned gases.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

The various types of furnaces conventional in the art can be used for the performance of the process of this invention. The drawings of the furnaces of the copending parent case and their descriptions have been incorporated herein only for the purpose of illustrating two furnaces in which the process of the invention can be performed. The invention is however not limited to performance in these furnaces.

I claim:

1. A process for the production and utilization of furnace atmospheres for carburizing, bright annealing and oxidation-free cooling of steel, comprising the steps of: mixing with a hydrocarbon fuel a quantity of air less than that required for complete combustion, burning the airfuel mixture, preventing the escape of heat from said burning mixture and from the resulting gaseous products of combustion, raising the temperature of said gaseous products of combustion to a temperature above the processing temperature of the steel to be treated, bringing said gaseous products to chemical equilibrium at said temperature above the processing temperature, both the steps of raising and bringing being accomplished only by burning said air-fuel mixture, without supplemental external heating of said gaseous products, collecting said products and exposing the steel to said products.

2. A process as in claim 1 including the step of preheating said air prior to said step of burning.

3. A process as in claim 1 including the step of pre heating said air and a part of said mixture prior to said step of burning.

4. A process as claimed in claim 2, said step of preheating includin the step of transferring heat from said products.

5. A process as claimed in claim 3, said step of preheating including the step of transferring heat from said products.

6. A process as claimed in claim 1, said step of burning including the step of contacting said air-fuel mixture with a reaction accelerating material.

References Cited UNITED STATES PATENTS 2,763,476 9/1956 Ness et a1. 148-465 X 2,763,582 9/1956 Rusciano 14816.5

2,799,490 7/1957 Rusciano 14816.5 X

2,886,303 5/1959 Rusciano 14816.5 X

CHARLES N. LOVELL, Primary Examiner.

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