EP0156378A2 - Method and apparatus for carburizing steel with a gas - Google Patents

Abstract

In the gas carburization of steel, a steel part is exposed in a carbon-enriched atmosphere of a furnace to a diffusion process to form a boundary layer with an increased carbon content, determined as a function of the distance from the surface. The values of importance for the diffusion process are determined intermittently and used as control parameters to affect the diffusion process. The flow of carbon diffusing directly through the surface of the steel part serves as the measured value. The supply of the carburizing gas is controlled as a function of this value.

Description

The invention relates to a method and a device for gas carburizing steel, in which the steel part or the like in a carbon-enriched gas atmosphere of a furnace. is exposed to a diffusion process for forming an edge region with an increased carbon content which is determined depending on the edge distance and in which the measured variables including the temperature which are important for the diffusion process are determined at certain time intervals and used as control variables for influencing the diffusion process.

Such gas carburizing processes, as they precede the case hardening of steel parts, are known (Journal for Economic Manufacturing, Issue 9, September 1968, pages 456 to 464). When steel workpieces are carburized, their surface layer is enriched with carbon. At the end of the process, a certain carbon curve is aimed for, with the target values "carburization depth", "marginal carbon content" and "C curve at the rim". This process can be calculated on the basis of the diffusion laws, because in addition to the temperature and the time, so-called boundary conditions are included as variables with which the carbon supply on the surface is defined.

While in the early days of carburizing technology it was assumed that. that the Rand-C content remains approximately constant during the entire process and that the carburization depth increases with time from the root, the process parameters "carbon potential" and "reaction rate" were introduced at the surface with increasing demands for accuracy. Depending on the respectively measured values, the mathematical relationship and the other process values, the C course in the workpiece can therefore be calculated in advance, whereby the carburizing of the edge area is based on the fact that the C potential of the gas atmosphere in the furnace chamber used for the carburizing is controlled in such a way that the desired target values are achieved. It is also known to use a computing unit to change the C potential during the carburization in order to avoid carbide formation at the edge of the workpiece in the shortest possible time (DE-PS 31 39 622).

However, this type of control of carburizing processes can only be carried out if there are so-called equilibrium atmospheres, because only then is the C potential defined and can it be measured. Atmospheres supersaturated with hydrocarbons are generally avoided because of their limited controllability, although they can save raw materials and energy because they offer even more carbon.

The present invention is therefore based on the object of designing a method of the type mentioned at the outset such that atmospheres supersaturated with hydrocarbons can also be used for carburizing processes in such a way that clear control and determination of the carbon profile at the edge of the workpieces is possible.

To achieve this object, it is proposed according to the invention in a method of the type mentioned at the outset to use the surface of the steel part-diffusing C stream as a measured variable and to control the carburizing gas supply as a function thereof. Such a process can be carried out, although the relationships which are theoretically known for the carbon flow on the surface of the workpieces cannot readily be put into practice because, in contrast to the C potential, this C flow has to be changed in a complicated manner during the carburizing process , if you want to have a certain carbon flow at the end. However, just like in the case of known methods, this can be determined by computers continuously or in certain short periods of time in such a way that the desired regulation is possible if values for the C-stream are made available.

This can be achieved in a simple manner by measuring the C current directly via the change in the electrical resistance of an iron sensor over time. In order not to run the risk that the carbon content of the sensor runs away from the carbon content at the edge of the workpiece to be carburized, it is advantageously ensured that the mean carbon content of the iron sensor is calculated by means of incremental, short-term decarburization phases in a known manner for the edge carbon content of the steel part Value is tracked. Therefore, if the value determined by the C-sensor reaches the target for the marginal carbon content on the workpiece, the supply of carburizing gas must be reduced so that, for example, the harmful formation of carbide is avoided.

The C current cannot only be measured directly via a sensor. It can also be determined indirectly via the difference between the measured, introduced into the furnace and the amount of carbon discharged therefrom or not used. It is expedient if the amount of carbon supplied is determined from the amount of gas supplied in a certain time and its carbon content, the amount of carbon removed from the amount of flare gas removed in the same time and its carbon content and the unused amount of carbon from the soot content remaining in the furnace. It has proven to be sufficient in practice if the amount of carbon removed and the amount of carbon not left in the furnace in the form of soot are combined to form an oven-specific utilization factor and if the C flow is then determined from the amount of carbon supplied, the workpiece surface and the utilization factor becomes. As is known per se, all of these measured variables and process values can be fed to a computing unit which, from this and from those important for the calculation of the course of the marginal carbon content Stored data, such as the geometry of the steel part, core C content and diffusion coefficient are used to calculate the course of the C content in the steel part at the respective point in time and, depending on this, gives signals for controlling the carbon gas supply. This can be done both if the C-current is measured directly via a sensor or if, as described, it is determined from the amount of carbon supplied and removed. One advantage of this is that the values for the carbon transition coefficient and largely also the alloy composition can be dispensed with.

To carry out the method with direct C-flow measurement, a device is advantageously provided which, in a known manner, comprises a carburizing furnace with at least one heatable chamber, with sensors for detecting the chamber temperature, with a carburizing agent feed line in which a controllable control valve is arranged and with a provided in the chamber reaching sensor having at least one electrical measuring resistor exposed to the furnace atmosphere, as described for example in the journal Stahl and Eisen 80 (1960, number 26, pages 1952 to 1954). In such a device, however, according to the invention it is provided that the measuring resistor acts as a C-current sensor and is arranged in a sensor chamber which is connected to a supply line for a decarburization gas, and that in this supply line a shut-off valve which can be controlled as a function of the determined values is arranged. In devices with a sensor which has a comparison resistor around which a comparison gas flows for comparison purposes, this decarburization gas can at the same time be the reference gas which is provided for the compensation comparison resistor. The supply line containing the shut-off valve can branch off in a simple manner from the connecting line to the comparison resistor. This shut-off valve contains its control pulses for opening and closing in a simple manner step by step from the computing unit, which thus tracks the carbon content of the measuring resistor to the carbon content at the edge of the workpiece to be carburized. The time course of the C content of the sensor therefore corresponds to the carburizing process at the edge of the workpiece.

Of course, the carbon content of the sensor can be tracked in the opposite way, even as the carbon content in the atmosphere decreases, analogously to the type just described. The decarburization phases are then replaced by carburization phases.

To carry out the aforementioned second method, in which the amounts of carbon supplied and removed are advantageously determined, a likewise known device with a carburizing furnace with at least one heated chamber, with coolers for detecting the chamber temperature, with a carbonant supply line in which a controllable control valve is arranged and provided with a discharge line leading out of the chamber with a torch. According to the invention, in such a device, the controllable control valve in the carburizing agent supply line is followed by a measuring element which detects the flow rate and a measuring device which detects the outflowing C quantity is also provided in the discharge line. This configuration makes it possible, particularly when certain furnace-specific variables are combined to form a utilization factor, to have the values recorded by the measuring element and the measuring device and the values of the temperature measurement evaluated by the computing unit, which also include values relating to the surface and the initial C content of the workpiece as well as data of the target values to be achieved and the utilization factor are entered.

Of course, it is also possible with the new processes and with the new facilities not only to carry out carburizing processes, but also, for example, processes for carbonitriding. In the case of carburizing processes, it is also possible to switch over to the known C-potential control in the final phase in order to precisely set the 2-C content on the workpieces. The advantage of the new methods and devices is the higher carbon supply in the initial phase, however without the risk of over-charring.

• Fig. 1 die schematische Darstellung einer erfindungsgemäßen Einrichtung zur Stahlaufkohlung, bei der ein C-Strom-Fühler zur Steuerung des Aufkohlungsvorganges eingesetzt ist,
• Fig. 2 die schematische und vergrößerte Darstellung des C-Strom-Fühlers der Fig. 1,
• Fig. 3 eine diagrammartige Darstellung, die den Verlauf des C-Gehaltes des Fühlers der Fig. 2 und dessen Steuerung in Abhängigkeit von der Zeit,
• Fig. 4 Diagramme mit dem Verlauf der Temperatur, des Randkohlenstoffgehaltes, der Aufkohlungstiefe und des C-Stromes jeweils anhand eines auch beschriebenen Beispieles und über dem Zeitverlauf aufgetragen,
• Fig. 5 die diagrammartige Darstellung des Kohlenstoffverlaufes bei Prozeßende in Abhängigkeit des Abstandes vom Rand und
• Fig. 6 die schematische Darstellung einer anderen Einrichtung zur Durchführung des erfindungsgemäßen Verfahrens, bei dem ohne C-Strom-Fühler gearbeitet wird.

The invention is illustrated in the drawing using two exemplary embodiments of devices for carrying out the new methods and is also explained below using an example. Show it:

• 1 is a schematic representation of a device for steel carburizing, in which a C-flow sensor is used to control the carburizing process,
• 2 shows the schematic and enlarged representation of the C-current sensor of FIG. 1,
• 3 is a diagrammatic representation showing the course of the C content of the sensor of FIG. 2 and its control as a function of time,
• 4 diagrams with the course of the temperature, the carbon content, the carburization depth and the C-flow each plotted using an example also described and over the course of time,
• Fig. 5 shows the diagrammatic representation of the carbon curve at the end of the process depending on the distance from the edge and
• Fig. 6 is a schematic representation of another device for performing the method according to the invention, in which one works without a C-current sensor.

In FIG. 1, a carburizing furnace 1 is provided with a circulating device formed by a blower 1 a, in which a workpiece batch 2 for carburizing with the surface A is inserted. A carburizing agent feed line 3, into which the carburizing gas is introduced in the direction of arrow 3a, opens into the furnace. From the carburizing furnace 1, which is provided in a known manner with a heat-resistant insulating wall, leads an exhaust pipe 1b, on which exits in the direction of arrow 4. Exhaust gas is flared. A temperature sensor 6, the measured values of which are fed to a computing unit 7, is also introduced into the furnace 1. A controllable control valve 4 is inserted into the carbonant feed line 3 and can be actuated via an actuator 5, which receives its control impulses from the computer 7.

Also introduced into the furnace 1 is a C-current sensor 8, the sensor head 8a of which is connected to the computer 7, so that the values determined by the C-current sensor 8 can be evaluated by the computer 7. However, this sensor head 8 is also connected to a supply line 9 for a decarburization gas which is supplied in the direction of arrow 9a. In this supply line 9, a shut-off valve 10 is inserted, which can be opened or closed via an actuator 11 controlled by the computer 7. In addition, the desired target values Z and the value for the initial C content CK of the workpiece batch 2 are entered into the computer 7. The computer 7 is connected to a recorder 12 for the carbon profile C-X, which records the carbon profile in the workpiece as a function of the edge distance. This course is determined by the computer 7 in a known manner.

From Fig. 2 it is clear that the C-current sensor 8 consists of a tube divided into two chambers 8b and 8c by the wall 13, which continue into the sensor head 8a. The chamber 8b contains a measuring resistor 14, through which an electric current flows in a manner not shown in detail and which is connected via the connecting line 14a to an evaluation circuit arranged in the computer 7. The chamber 8b is also connected to the supply line 9 for the decarburizing agent, in which the shut-off valve 10 is arranged.

The chamber 8a of the C-flow sensor 8 is connected via a connecting line 16 to the supply line 9 for the decarburizing agent, so that the chamber 8a is constantly supplied with a certain amount of the decarburizing agent which is also used as reference gas in the exemplary embodiment.

The operation of the device of FIGS. 1 and 2 will now be described using an example.

In the carburizing furnace 1, which is designed, for example, as a retort furnace, the batch 2 of steel workpieces with an initial C content of 0.20% C and a total surface area A of 10 m ^{2 is} carburized at 930 ° C. The target variables Z at the end of the process are as follows:

C-course at the edge should be flat.

These target values result in a calculated carbon requirement of 3 ₅ g _{/ m} ² _.

For economic reasons, a short carburization period, i.e. striving for a high carbon supply. The Rand-C content should not exceed 1.00% C during carburization in order to avoid carbide formation. Before moving out, the temperature of Charge 2 should be reduced to the hardening temperature of 860 ° C.

When batch 2 is heated, furnace 1 is flushed with nitrogen.

From 850 ° C, some methanol is added to form CO and H2 as well as natural gas CH4 as carburizing gas, which is done via the carburizing agent supply line 3, and in such a high proportion that a C potential measurement cannot be carried out due to oversaturation. In section I shown in FIG. 4, the C flow is limited in order to avoid excessive soot formation in the furnace 1. In section II, the C flow is reduced because otherwise the marginal C content would rise above one percent.

The atmosphere in the furnace 1 is monitored by the C-current sensor 8 with the measuring resistor 14, which can consist, for example, of an iron wire with a diameter of 0.2 mm, the carbon content of which changes by 0.26% C / h when the C flow is ^{1 g / m2h} , which results from the surface / volume ratio ratio of the sensor 8 results.

3, the carbon content C of the sensor is tracked by the decarburization cycles controlled by the computer 7 to the edge C content of the workpiece batch 2. In Fig. 3 the C content of the sensor is continuously drawn out. This sawtooth-like line is designated 18. The respective decarburization phases 19 are controlled by the computer 7. In this case, the shut-off valve 10 - controlled by the computer - is opened and decarburizing agent flows briefly through the chamber 8b.

The mean value of the course of the C content on the sensor thus corresponds to the dashed line 20, which represents the course of the marginal C content on the workpiece 2, which is determined by the computer.

It can also be seen from FIG. 3 that the time t _E for the decarburization phases is only very short. However, it is sufficient to track the mean value of the C content of the sensor 8 to ^the edge C content 20.

The proportional C current is determined from the value dC. German

The carbonization gas supply in the direction of arrow 3a is interrupted, s: ₁ soon the sum of the carbon supplied corresponds to the predetermined count 35 g / m ² . In the exemplary embodiment shown in FIG. 4, this is achieved after four hours. The mixture is then diffused with the addition of nitrogen until the computer 7 displays the predetermined marginal C content, which after about five hours and 20 minutes according to FIG. 4 is 0.8%, as desired. (see Fig. 4 section III). In the diffusion phase, the temperature is reduced to the hardening temperature, as can be seen from the diagram above with the temperature profile.

The batch report output by the computer 7 (FIG. 4) shows the carburizing process regulated by the C flow. 5 shows the final carbon curve at the end of the process, which corresponds to the predetermined target values. 5 on the The abscissa of the edge distance in the workpiece is plotted. On the ordinate the C content. The marginal carbon content CR is 0.180% C. With a carburizing depth of 1mm, the C content is 0.35% C.

Another possibility of controlling the carburizing process using the C flow is shown in FIG. 6. The same reference numbers are used for the same parts. 1, the controllable control valve 4 in the carburizing agent supply line 3 is followed by a measuring element 21 which flows through. A measuring device 22 which detects the outflowing C quantity is also arranged in the exhaust line 1b. In this device, the amount of carbon supplied is determined from the amount of coal gas supplied in a certain time and its carbon content, which is known, the amount of carbon removed from the amount of flare gas discharged through line 1b in the same time and its likewise determinable carbon content, and from the amount of carbon not consumed , which consists of the soot remaining in the furnace. The resulting difference in carbon has flowed into the workpiece batch 2, the surface A of which is also known. In practice, the procedure is now such that the amount of carbon removed which leads through line 1b and the amount of carbon which is not consumed and remains in the form of soot in furnace 1 are combined to form an oven-specific utilization factor f and that the C stream is fed from the feed Amount of carbon that is detected with the measuring member 21, the known workpiece surface A and the utilization factor f is determined, so that the carbon flow m results in m = fK. K is the amount of carbon supplied in gC / m ² h. These values are entered into the computer 7 in addition to the values for the initial edge carbon content CK of the workpiece and the target values Z. From this, it determines the carbon flow in a similar manner as was also explained with reference to FIG. 4. Depending on this determination, the supply of carburizing agent through the carburizing agent feed line 3 ′ is controlled via the valve 4.

Claims (7)

I. A method for gas carburizing steel, in which the steel part or the like in a carbon-enriched gas atmosphere of a furnace. is subjected to a diffusion process for forming an edge region with an increased carbon content, which is determined depending on the edge distance, and in which the measured variables, including the temperature, which are important for the diffusion process are determined at certain time intervals and used as control variables for influencing the diffusion process, characterized in that directly the the surface of the steel part diffusing C-current serves as a measured variable and that the carbonization gas supply is controlled as a function thereof. 2. The method according to claim 1, characterized in that the C _- stream of an iron sensor (8) is measured directly on the temporal change in electrical resistance. 3. The method according to claim 1 and 2, characterized in that the average carbon content of the iron sensor (8) by gradual, short-term decarburization phases (19) the value determined for the edge carbon content of the steel part (20) is tracked. 4. The method according to claim 1, characterized in that the C-current is determined indirectly via the difference between the measured, introduced into the furnace and the amount of carbon removed or not used and that the amount of carbon supplied from the supplied in a certain time The amount of gas and its carbon content, the amount of carbon removed is determined from the amount of flare gas discharged in the same time and its carbon content and the amount of unused carbon is determined from the soot content remaining in the furnace. 5. The method according to claim 4, characterized in that the amount of carbon removed and the unused, remaining in the form of soot in the furnace remaining carbon amount to an oven-specific utilization factor (f) and that the C-stream from the amount of carbon supplied, the workpiece surface ( A) and the utilization factor is determined. 6. The method according to any one of claims 1 to 5, characterized in that all measured quantities and target quantities are fed to a computing unit (7), from which and from the data stored and important for the calculation of the course of the marginal carbon content, such as geometry of the steel part, diffusion coefficient and the core C content calculates the course (20) of the C content in the steel part present at the respective time and, depending on this, gives signals for controlling the carburizing gas supply. 7. Device for performing the method according to claims 1 to 3 and 6, with a carburizing furnace with at least one heatable chamber, with sensors for detecting the chamber temperature, with a carburizing agent supply line, in which a controllable control valve is arranged, and with one in the chamber Sensor reaching in, which has at least one electrical measuring resistor exposed to the furnace atmosphere, characterized in that the measuring resistor (8) acts as a C current sensor acts and is arranged in a sensor chamber (8b) which is connected to a supply line (9) for a decarburization gas (9a), and that in this supply line (9) a controllable shut-off valve (10) is arranged depending on the determined values. . Device according to Claim 7 with a sensor which, in addition to the measuring resistor, has a comparison resistor for temperature compensation which is flushed with a reference gas, characterized in that the reference gas serves as decarburization gas and that the supply line (9) containing the shut-off valve (10) from the connecting line (16) branches to the comparison resistor (23). . Device according to claims 8 and 9, characterized in that the control pulses for opening and closing the shut-off valve (10) are given step by step by the computing unit (7). . Device for carrying out the method according to claims 1 and 4 to 6 with a carburizing furnace with at least one heated chamber, with sensors for detecting the chamber temperature, with a carburizing agent supply line in which a controllable control valve is arranged and with a discharge line leading out of the chamber with a Torch, characterized in that the controllable control valve (4) in the carburizing agent supply line (3) is followed by a measuring element (21) which measures the flowing quantity, and in the discharge line (1b) also a measuring device (22) measuring the flowing C quantity. is arranged. . Device according to claim 10, characterized in that the values detected by the measuring element (21) and the measuring device (22) and the values of the temperature measurement are evaluated by the computing unit (7), which also includes values over the surface (A) and the beginning -C content (CK) of the workpiece (2), as well as data of the target values (Z) to be achieved and the utilization factor (f) are entered.

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