GB2092183A

GB2092183A - Method of controlling furnace atmospheres

Info

Publication number: GB2092183A
Application number: GB8200827A
Authority: GB
Original assignee: Holcroft and Co
Current assignee: Holcroft and Co
Priority date: 1981-01-14
Filing date: 1982-01-12
Publication date: 1982-08-11
Also published as: ES508674A0; ES8300867A1; JPS57140821A; DE3149212A1; FR2500848A1

Abstract

Carrier gas composed essentially of a mixture of a suitable hydrocarbon such as methanol and nitrogen is supplied to a furnace for adjusting the carbon content of steel or cast iron, and the carbon potential of the furnace atmosphere is controlled by varying the amounts of additive gases. In accordance with the present invention, the ratio of flow of hydrocarbon gas to flow of nitrogen is varied to maintain the CO percentage of the furnace atmosphere at a predetermined value. This renders more effective the independent control by convertional means of the flow of additive gases to maintain the carbon potential of the furnace atmosphere at a predetermined value.

Description

SPECIFICATION Method of controlling furnace atmospheres In atmosphere type furnaces for controlling the carbon content of steel or cast irnn it has been a common practice for the furnace atmosphere to consist essentially of a substantially neutral carrier gas with smaller amounts of carburizing or decarburizing gases, commonly known as additive gases, added to shift the carbon potential of the active parts of the furnace atmosphere as required for the particular process involved. These processes include gas carburization, carbonitriding, carbon restoration, clean or neutral hardening and annealing. Thus as different locations in a continuous furnace or at different times within the cycle of a batch furnace, it may be advantageous to control the atmosphere, or the atmosphere within a particular zone, at different controlled carbon potentials.In any case the steel or cast iron is heated to a controlled high temperature in a furnace containing an atmosphere that will provide carbon to the steel surface, or maintain its carbon content.

In the past a widely used source of carrier gas for furnace atmosphere was by thermal decomposition of a hydrocarbon such as natural gas or propane, with a limited amount of air supplied over a nickel catalyst. The resulting gas was commonly known as AGA type 302 endothermic generator gas and when natural gas was used, had a basic analysis of 20% CO, 40% H2, and 40% N2.

It is currently becoming more prevalent to use a mixture of methanol (CH30H) and N2 as the carrier gas. In the presence of heat, the methanol dissociates into one part CO and two parts H2. By adding two parts gaseous N2 a furnace atmosphere with the same basic analysis as the AGA type 302 endothermic gas, namely 20% CO, 40% H2, and 40% N2 can be produced.

In either case it has of course been recognized that the carbon potential of the furnace atmosphere must be controlled in order to provide the desired amount of carbon at the surface of the metal being treated. The carbon potential determines the ability of the atmosphere to supply, maintain or extract carbon from the surface of the steel or cast iron at the temperature to which it has been heated. This control is normally accomplished by measuring one or more constituents of the furnace atmosphere such as, but not limited to, C 2, 2 or H20 and making suitable adjustments in the feed rate of the additive gas or gases to provide the required carbon potential.This method is based on the assumption that the constituents in the furnace atmosphere other than the constituent or constituents being measured, remain constant and therefore that the measured constituent or constituents represent a direct relationship to the carbon potential of the atmosphere. This assumption is found to be incorrect because of the well known interaction between the various constituents of the furnace atmosphere.

It was customary in the past when using AGA type 302 endothermic generator gas as the carrier gas to control the analysis of the finished'gas leaving the generator by monitoring the CO2 or H20 (dewpoint) of the finished gas and to adjust the mixture ratio of the air and hydrocarbon used to produce the finished carrier gas. Similarly when the carrier gas is supplied as a mixture of methanol and N2, the practice has been to measure or monitor the feed rate of both constituents and to adjust the ratio of methanol to N2 accordingly.

The atmosphere within a heat treating furnace, or at a specific zone within such furnace, is maintained by a flow of gases thereto, and where these gases are methanol or a similar hydrocarbon, N2, and a suitable additive gas whose function is to control the carbon potential.

it is recognized that there is a definite interrelationship between the flow of the several gases and the CO content and the carbon potential of the furnace atmosphere. Thus it is possible to anticipate a change in CO content which will result from a change in the flow rate of the additive gas or gases, and further anticipate a change in the carbon potential as it is monitored in the furnace which will result from a change in the relative flow rates of methanol or similar hydrocarbon and N2. Therefore, by separately monitoring the CO content and the carbon potential of the furnace atmosphere, it may be said that there is an interrelated control of the related rates of flow of the methanol or similar hydrocarbon and the N2, primarily responsive to the monitored CO content, but also with reference to the monitored carbon potential.Similarly in an integrated control system in which the flow rates of the methanol dr similar hydrocarbon, the N2, and the additive gas are all separately controlled, the control of the additive gas is primarily responsive to the monitored carbon potential but also with reference to the CO content of the atmosphere.

The present invention relates to a method of controlling the ratio of methanol or similar hydrocarbon N2 flow ratio when this mixture is used as the carrier gas in a furnace for controlling the carbon content of steel or cast iron. The invention consists in monitoring the CO content of the atmosphere at an appropriate location or locations within the furnace and adjusting the relative flow or flows of methanol or similar hydrocarbon and/or nitrogen to maintain a desired CO content. At the same time, the amount or flow of additive gas or gases supplied to the furnace to alter the carbon potential of the furnace atmosphere is controlled by separate means responsive primarily to monitored carbon potential.

It is emphasized that this control of the proportion flow of methanol or similar hydrocarbon and nitrogen is primarily for the purpose of maintaining the CO content of the furnace at a predetermined value.

However, by establishing a condition in which the CO content of the furnace atmosphere is at a predetermined value, the constituent or constituents measured to control the flow of additive gas or gases by conventional practices becomes more representative of the furnace atmosphere's carbon potential.

These include measurements of one or more constituents of the furnace atmosphere such as but not limited to CO2, 02, or H20 (dewpoint) and to adjust the feed rate of the required additive gas to increase or decrease the carbon potential.

Instead of determining the carbon potential of the existing atmosphere in the furnace as described above, other means of determining the carbon potential. may be substituted, such as for example (but not limited to) the well known hot wire method of determination of the carbon potential, or by measuring flame temperature.

In any case it has been found that a much better control of the carbon potential of the furnace atmosphere is made possible when the CO content of the atmosphere is maintained at a predetermined value. Thus, the CO content of the atmosphere may be maintained for example at 20%, and if it drops below this value, the proportion of methanol or similar hydrocarbon to N2 is increased.

It is recognized that adjustments to the relative flow rates of the methanol or similar hydrocarbon and N2 primarilyforthe purpose of maintaining the CO content at a predetermined value will affect the carbon potential of the furnace atmosphere.

Similarly adjustments to the flow of additive gas primarily for the purpose of controlling the carbon potential of the furnace atmosphere will affect the CO content of the atmosphere. The interrelationship between the rates of flow of the three gaseous components of the furnace atmosphere is predictable, and the adjustment of the valves controlling these flows can take this into account to anticipate the secondary effects thereof. The mechanism for controlling the valves to regulate the flow of the several gaseous components can but does not necessarily include programmable microprocessors, so that this may be readily accomplished.

The present invention is applicable to continuous or batch type processing, and is also applicable to controlling carbon potential at different values during different phases of the processing or in different compartment of a furnace through which the metal parts are advanced, either continuously or intermittently.

The rate of addition of the mixture of methanol or similar hydrocarbon and nitrogen, as well as the additive gases, must of course take into account the necessity of purging the furnace of undesired impurities or atmosphere constituents introduced as a result of the necessary opening of vestibule doors when metal work pieces are fed to or removed from the furnace, as well as the need to maintain a positive pressure within the furnace to prevent infiltration of contaminants such as oxygen. The overall flow rate can of course be varied either upward or downward in accordance with processing and operational requirements.

Thus, the invention is applicable to situations where the overall flow rate is reduced during specific periods in the processing cycle.

In the foregoing reference has been made to the carrier gas portion of the furnace atmosphere produced by a mixture of methanol or similar hydrocarbon and nitrogen. The preferred hydrocarbon is methanol, since it dissociates into desirable proportions of CO and H2. However many other hydrocarbons may be employed, selected to produce by dissociation proportions of CO and H2.

Reference herein to monitoring will be understood to encompass continuous or intermittent measurements.

Claims

1. The method of controlling the atmosphere of a furnace for controlling the carbon content of steel or cast iron, which comprises introducing a major amount of substantially neutral carrier gas in which the constituents consist essentially of a hydrocarbon and nitrogen, and a minor amount of an additive gas to control the carbon potential of the furnace atmosphere, which is characterized by the steps of maintaining the CO content of the atmosphere substantially at a predetermined value by controlling the hydrocarbon/nitrogen ratio, and controlling the carbon potential of the atmosphere by controlling the addition of the additive gas to the atmosphere.

2. The method as defined in claim 1 in which the hydrocarbon is methanol.

3. The method as defined in claim 1 in which the control of the carrier gas is by adjustment of separate valve means which determine the flow rate of the constituents of carrier gas to the furnace and in which the control of the addition of the additive gas is by adjustment of valve means controlling the flow rate of the additive gas, which comprises separately monitoring the CO content and the carbon potential of the atmosphere, regulating the valve means which determine the flow rate of the carrier gas constituents primarily in response to the monitored CO content of the atmosphere, and regulating the valve means which determine the flow rate of the additive gas primarily in response to the monitored carbon potential of the atmosphere.

4. The method as defined in claim.3 in which the hydrocarbon is methanol.

5. The method as defined in claim.3, which comprises modifying the regulation of the separate valve means controlling the flow of hydrocarbon and N2 to take into account the changes in CO content resulting from changes in the flow rate of additive gas.

6. The method as defined in claim 3, which comprises modifying the regulation of the valve means controlling the flow of additive gas to take into account the changes in carbon potential resulting from changes in the flow rates of the hydrocarbon and N2.

7. The method as defined in claim 3, which comprises modifying the regulation of the separate valve means controlling the flow of hydrocarbon and N2 to take into account the changes in CO content resulting from changes in the flow rate of additive gas and modifying the regulation of the valve means controlling the flow of additive gas to take into account the changes in carbon potential resulting from changes in the flow rates of the hydrocarbon and N2.

8. The method as defined in claim 7, in which the hydrocarbon is methanol.

9. The method as defined in claim 6, which comprises controlling all valve means to provide an overall flow to the furnace to take into account purging, varying treatment cycles and thqWlike.

10. A method of controlling the atmosphere of a furnace for controlling the carbon content of steel or cast iron, substantially as hereinbefore described.

11. Steel or cast iron whenever made by a process in which the atmosphere of the furnace wherein the process is conducted is controlled by a method as claimed in any preceding claim.