EP0156378B1 - 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 for gas carburizing steel according to the preamble of patent claim 1.

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 carburizing steel workpieces, 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 margin". This process can be calculated on the basis of the diffusion laws (Zeitschrift Forsch. Ing. Wes. 13, 1942, p. 177), because in addition to temperature and time, so-called boundary conditions are included as variables with which the carbon supply on the surface is defined.

While it was assumed in the beginning of carburizing technology that the Rand-C content remained approximately constant throughout the entire process and that the carburizing depth increased with the root of time, the process parameters "carbon potential" and "reaction rate" became more and more exacting. introduced on the surface. Depending on the measured values, the mathematical relationship and the other process values, the C profile in the workpiece can therefore be calculated in advance, whereby the carburization of the edge area is based on the fact that the C potential of the gas atmosphere in the furnace chamber used for carburizing is regulated in such a way that 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-3 139 622).

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 due to their limited regulating capacity, although they can save raw materials and energy because they offer an even larger amount of carbon.

From DE-1 222 762 B 1 it is known to use the change in the electrical resistance of an iron sensor as a measure of the carbon absorption of the steel part and to control the carbonization gas supply as a function thereof. Depending on the measurement, the carbon emission capacity of the carburizing gas is regulated, which is relatively complex. This process offers no possibility of working with supersaturated hydrocarbon atmospheres without the risk of soot or carbide formation.

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

To solve this problem, the characterizing features of claim 1 are provided. Such a process can be carried out, although the relationships theoretically known for the carbon flow on the surface of the workpieces cannot be put into practice easily, because this C flow, in contrast to the C potential, 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 current are made available, which can be done in a simple manner by changing the time electrical resistance of an iron sensor is possible.

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 by step-by-step short-term decarburization phases in a known manner for the edge carbon content of the steel part calculated value is updated. 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, harmful carbide formation is avoided.

To implement the new method, a device is provided which, in a known manner, provides a carburizing furnace with at least one heatable chamber, with a sensor for detecting the chamber temperature, with a carburizing agent feed line in which a controllable control valve is arranged and with a sensor reaching into the chamber. which has at least one electrical measuring resistor exposed to the furnace atmosphere, as described, for example, in the journal Stahl und Eisen 80 (1960, number 26, pages 1952 to 1954). In such a device, however, it is provided according to the invention 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. This decarburization gas can be used in facilities with egg nem sensor, which has a comparison resistor around which a comparison gas flows for temperature compensation, at the same time be the comparison 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.

Of course, it is also possible with the new processes not only to carry out carburizing processes, but also, for example, processes for carbonitriding. In carburizing processes, the known C potential control can also be used in the final phase to precisely adjust the edge C content on the workpieces. The advantage of the new process is the higher carbon supply in the initial phase, but without the risk of over-carbonization.

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.

• 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.

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.

In FIG. 1, a carburizing furnace 1 is provided with a circulating device formed by a blower 1 a, into 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 line 1b, on which the exhaust gas emerging in the direction of arrow 4 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 8a 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. The desired target variables Z and the value for the initial C content CK of the workpiece batch 2 are also 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 feed line 9 for the decarburizing agent, so that the chamber 8a is constantly acted upon 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.

• Aufkohlungstiefe AT = 1 mm bei 0,35 %C;
• Randkohlenstoffgehalt CR = 0,80 %C;

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:

• Carburizing depth AT = 1 mm at 0.35% C;
• Marginal carbon content CR = 0.80% C;

C-course at the edge should be flat.

These target values result in a calculated carbon requirement of 35 g / m ² .

For economic reasons, a short carburizing period, i.e. H. 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 extending, 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 / m ² h, which results from the surface / volume ratio of the sensor 8.

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 supply the mean value of the C content of the sensor 8 according to the edge C content 20.

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

The carbonization gas supply in the direction of arrow 3a is terminated as soon as the sum of the carbon supplied corresponds to the predetermined target value of 35 g / m ² . In the exemplary embodiment according to 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 edge C content, which is approximately 0.8%, as desired, after about five hours and 20 minutes according to FIG. 4. (See section 111 in Fig. 4). 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. The edge distance in the workpiece is plotted on the abscissa in FIG. 5. On the ordinate the C content. The marginal carbon content CR is 0.180% C. With a carburizing depth of 1 mm, the C content is 0.35% C.

Claims (2)

1. A process for gas carburization of steel, in which the steel part is exposed within a carbon- enriched gas atmosphere of a furnace or the like to a diffusion process to form a boundary layer with an increased carbon content determined as a function vals the values important for said diffusion process, including temperature, are determined and used as control parameters to affect said diffusion process, and in which the change in the electrical resistance of an iron sensor (8) serves as the dimension for the carbon content of said steel part and the carburizing gas flow is controlled as a function thereof, characterized in that the gas atmosphere is supersaturated with carbon and the mean carbon content of said iron sensor (8) is followed up by short, intermittent decarburization phases (19) to the value (20) determined for the boundary carbon content (CR) of said steel part.

2. A process according to Claim 1, wherein all measured values and target values are supplied to a computer unit (2) which therefrom and from the stored data important for computation of the variation in the surface carbon content, such as the geometry of said steel part, the coefficient of diffusion and C content of the core, computes the instantaneous variation (20) of the C content in the steel part and issues as a function of the latter signals to control the supply of the carburizing gas.

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