WO1994009164A1

WO1994009164A1 - Method for the thermal treatment of metal products

Info

Publication number: WO1994009164A1
Application number: PCT/EP1993/002777
Authority: WO
Inventors: Hans-Georg Bittner
Original assignee: Heimsoth Verwaltungen Gmbh & Co. Kg Beteiligungsgesellschaft
Priority date: 1992-10-10
Filing date: 1993-10-11
Publication date: 1994-04-28
Also published as: DE59309308D1; DE4234285A1; ATE175724T1; EP0616646B1; EP0616646A1

Abstract

The invention concerns a method for the thermal treatment of metal products in a roller oven, in particular relatively thin steel articles which distort easily on heating, such as raceways for ball bearings, circular sheet blanks, saw blades and other cutting blades. The products are conveyed through the oven on ceramic rollers and thus heated, and very rapidly and uniformly cooled again so that no distortion occurs.

Description

Process for the heat treatment of metallic goods

The invention relates to methods for the heat treatment of metallic material in a roller furnace, in particular of relatively thin, quickly cooled steel parts, such as ball bearing rings, knives, saws, pliers, cutters and the like, the material after a heating phase in the so-called high temperature range (e.g. For steel, depending on the heat treatment, above 800 - 1250 ° C) is placed directly on the ceramic rollers of the roller furnace.

A device for heat treating metallic material (DE-PS 31 50 576) is known, in which glow racks are arranged on a roller conveyor, which in turn have been fitted with the metallic parts in the individual compartments. In the known furnace, however, it can hardly be achieved that there are metal parts that have a longitudinal dimension in the cm range and a have a small thickness in the mm range, have not distorted after the heat treatment.

In addition, it has hitherto been customary in connection with the heat treatment of metal parts to use furnaces which have a protective gas atmosphere and are partly electrically heated in order to prevent, for example, the carbon content of the metal parts from changing during the heat treatment process or the metal parts oxidizing. In this connection, neither carburization nor decarburization nor oxidation is desired.

The invention is concerned with the problem of enabling a heat treatment of metal parts, in which the metal parts do not warp and only insignificant signs of edge decarburization or edge decarburization and edge oxidation or scale formation occur.

This is achieved by a method of the type mentioned at the outset, which is characterized in that the material is conveyed on the roller conveyor into the cooling zone, in which, by means of nozzle fields arranged both above and below the roller conveyor, for example in the manner of impingement flows, the material is cooled spatially uniformly and very quickly and that the martensite starting temperature is fallen below locally (e.g. in the case of steel), so that even without the use of presses there is very little or almost no distortion in the workpiece. To further intensify the rapid cooling, a boundary layer can also be provided with suction. With this process, even heat transfer coefficients from the range of oil quenching of up to 1000 - 4000 W / m ² K can be achieved. In the method according to the invention, the temperature gradient immediately above the roller conveyor is so low that the metal parts located there cannot warp. Such a temperature distribution can only be achieved with a roller oven.

In connection with the annealing treatment of steel parts, ceramic and carbon-free rollers are used advantageously according to the invention, which have sufficient heat resistance. In addition, it can also be achieved that the phenomena known under the term "pittings" do not occur since no contact of hot metal with hot metal occurs during the annealing process, but rather the steel parts rest on the ceramic rollers. In the case of known roller furnaces for the heat-treating metal industry, metal rollers are predominantly used, which is only possible if a relatively great effort is made for the air or water cooling of the rollers.

In order to keep the oxygen content in the furnace as low as possible, the burners are operated in the high temperature range, for example during heat treatment for hardening steel, the so-called austenitizing range, without excess air. In the method according to the present invention, the burners are set in such a way that a protective gas can indeed be dispensed with. The reason for this is that metal parts of this type are easily scaled on the surface, with a layer thickness in the range of approximately 0.01-0.03 mm. This edge scaling has no influence on the product quality, on the contrary, the scaling prevents the metal parts from carburizing or decarburizing, so that a -4.-

uniform structure is achieved after the heat treatment and only an edge decarburization of approx. 0.01 - 0.03 mm occurs. This slight decarburization or scaling is often permitted because surface treatment is usually carried out in subsequent work steps. At sufficiently high air speeds, it has been found that the material has a scale-free, black surface, so that no additional surface treatment is required.

In the method according to the invention, the metal parts are conveyed through the roller furnace at different speeds. In the unheated heating zones and the heated heating zone, the speed depends on the thickness of the metallic material to be heat-treated, and when a temperature of approximately 850 ° C. is reached, the parts to be heat-treated are transported at a higher speed. As a result, the residence time of the metals in the region of higher temperatures can be limited to a time period which does not permit any noticeable carburization or decarburization processes. In the third zone, for steel, for example, at the austenitizing temperature, the metal parts are finally transported at an even higher speed. After passing through the high temperature zone, the hot material is removed from the oven very quickly. In the roller furnace for carrying out the method according to the invention, no doors are required in the inlet and outlet, instead so-called baffles can also be used there, with one or more baffles being one behind the other at the outlet. If an oven with a light refractory lining, for example made of fiber materials, is used in carrying out the method according to the invention, the oven can be heated up and down in a short time.

Since the temperature field directly above the rollers can be set spatially constant in the method according to the invention, it is possible to treat metal parts with different dimensions at the same time, especially if the thickness of these parts is not significantly different. The temperature can also be made more uniform by using pulse burners or high-speed burners which can be set with a constant gas-air mixture and can also be operated in a clocked manner.

The method according to the invention is also characterized in that the material is conveyed into the cooling zone following the heating phase and the so-called high-temperature region, in order to be cooled here by means of air arranged above and below the roller conveyor, for example with air, so that no or only a little Delay occurs after cooling. The metallic material is hardly decarburized and hardly oxidized. With this method, depending on the pressure selected in the nozzle field, heat transfer coefficients between 600 W / m ² K and 4000 W / m ² K can be set. With this method, values are achieved, such as those obtained from salt bath quenching (600-1000 W / m ² K), oil quenching and water quenching (2000-4000 W / m ² K) from below through the rollers, and the nozzles are precisely positioned in the spaces between the roles. The nozzles are located above the goods from above. _ _

This method is characterized in that there is an exact separation of the atmosphere between the actual heating part, the austenitizing part, for example in the case of steel, and the cooling part. For example, a reducing atmosphere is required in the high-temperature zone and pure, cold air is blown up in the cooling section. The method can be characterized in that the heating part and the cooling part are separated by a lock.

The method is characterized in that the cooling can be operated continuously or pulsating, which means that the cooling only starts when a workpiece in question has reached the cooling point.

The process is characterized in that a nozzle field is used. The nozzle field is designed in two ways. Firstly, to achieve a very high heat transfer coefficient. For the intensity of the cooling, the nozzle field will be operated with different admission pressures, for example from a slight overpressure from 1 bar up to 20 bar overpressure. The nozzle field can be formed on the one hand from nozzles and on the other hand from a plate into which holes and slots are incorporated. The nozzle diameter and the division ratio of the nozzles are selected according to the material requirements of the material to be cooled. In this point it should also be mentioned that in addition to the shape of the nozzle, the nozzle spacing between the workpiece and the nozzle can also be varied in order to achieve intensive cooling. The distances between the nozzles of the two fields should be as small and as large as possible. The rolls should have small diameters. To further intensify the heat transfer, boundary layer suction can be provided at the bottom and / or at the top. The intensity of the suction is determined by the set vacuum.

The task of the cooling mentioned is to exceed the martensite starting temperature of the workpiece at the same time, for example in the case of steel, so that there is no distortion in the workpieces. This is particularly important for workpieces that have a longitudinal dimension in the cm range and a small thickness in the mm range.

The heat released during the cooling process can be returned to the burners in the form of preheated air in the order of 100-400 ° C. This gives you heat recovery and lower energy consumption.

The following nozzle arrangements are conceivable:

square nozzle arrangements, equilateral triangles as nozzle arrangements and isosceles and equilateral triangles as nozzle arrangements.

The nozzle field can be designed so that the outflow of air takes place on one side or also on all four sides or takes place only on two sides or is suctioned off again. The nozzle diameter can range from 1.0 to 4.0 mm. The nozzles can be designed with or without a nozzle neck. The length of the nozzle neck can vary from 0-50 mm. The temperature of the air emerging from the nozzles can be between room temperature and 150 ° C. The Depending on the thickness of the material, the material (the good) can be cooled down to temperatures between 40 and 100 ° C in a few seconds.

A special characteristic is to obtain a very intensive heat transfer. This is the case when the distance from the nozzle to the product is very small. In contrast, it is important for the temperature uniformity that a certain distance is maintained so that the entire impingement flow is formed in such a way that the individual nozzle cones overlap sufficiently. In addition, the nozzle diameter should be chosen to be very small in order to achieve a high heat transfer, which, however, requires greater pressure resistance and results in higher admission pressures. Optimal heat transfer is achieved when the nozzle pitch is equal to the nozzle distance to the product.

The roller speeds in the furnace in the heating section Zone I to III and in the cooling section Zone IV can be different.

The following processes are currently used in the cooling of thin steel sheets:

1. Salt bath.

For environmental reasons, this will no longer be permitted in the future. The salt often has to be washed off of metallic material after the heat treatment. Cost-intensive sewage treatment plants for the washing water and air filters for indoor air cleaning are required. Vacuum systems.

The material is heated in a vacuum and then cooled under a high overpressure with protective gas (intergas). The process takes place in batch or chamber furnaces. This process is very complex in terms of equipment. High distortion occurs here with thin materials.

Heating in protective gas, for example.

After heating in protective gas, cooling takes place in air using straightening presses (cooled), on the one hand to cool quickly, and on the other hand to compensate for the delay that has occurred or has occurred. In special cases, the use of a cooled straightening press alone is not sufficient to convert the metals into the desired structural modification sufficiently quickly. In these cases, cooling takes place in oil or in another liquid medium.

Since there is a large amount of distortion in many flat materials in oil, a combination of cooling in oil and simultaneous straightening is also possible. On the one hand, these processes are very complex. The heat-treated and hardened parts must then be tempered. Here, the oil evaporates or burns. Since this cannot be burned very evenly and without residue, there are numerous environmental problems here. For example, the hydrocarbons cannot be completely oxidized to H ₂ O and C ₂ O.

It is also within the scope of the invention for very small parts to use a grid or net instead of the roller conveyor in the area of the nozzle field. The invention is explained below with reference to the drawing, for example:

Figure 1 shows a diagrammatic longitudinal view of a

Roller furnace for performing the method according to the invention.

Figures 2 to 4 show views of the nozzle fields in different embodiments.

A roller oven is shown schematically in FIG. 1, the roller conveyor being recognizable by the cross-sectional representation of the rollers. The inlet is on the left and the outlet on the right.

The furnace consists of three spatially separated stages I, II and III.

There are no burners in Zone I (VWZI), there is a facility here to absorb the furnace gas and discharge it via the chimney. In addition, air is blown in to set the temperature curve.

In Figure 1, the burners above and below the roller conveyor are shown schematically. Zone II (HZ) shows more burners than Zone II (VWZII), in order to express that Zone II is the high temperature range. The goods must be brought from Zone II to Zone III (rapid cooling zone) as quickly as possible. After the high-temperature zone, zone III, an extraction of the preheated cooling air is provided, which can serve to preheat the combustion air. The heating part can be separated from the cooling part by a lock. The cooling zone V (KZ) is arranged after this zone.

In phase II, the metal parts finally reach the high-temperature range, where, for example, steel is subjected to austenitization during heat treatment for hardening.

In phase III, the material is cooled very quickly using nozzle fields.

FIG. 2 shows a side view of an embodiment of the cooling zone with nozzles arranged above and below the roller.

FIG. 3 shows a view in which the spatial assignment of the nozzles of an upper and a lower nozzle field can be seen. U and 0 denote the individual nozzles of the lower and upper nozzle fields. The best results are achieved with small distances between the nozzles and the product.

FIG. 4 shows a side view of a further embodiment of a nozzle field, in which case air suction devices are to be appointed between the individual upper nozzle fields and the boundary layer is thus suctioned off.

Claims

Process for the heat treatment of metallic material in a roller furnace, in particular of relatively thin steel parts that can be rapidly cooled, such as ball bearing rings, knives, saws, pliers, milling cutters and the like, the material after a heating-up phase in the so-called high temperature range (e.g. in the case of steel depending on the heat treatment above approx. 800 - 1250 ° C) is placed directly on the ceramic rollers of the roller furnace, characterized in that the material is conveyed on the roller conveyor into the cooling zone, in which by means of both above and below the roller conveyor arranged nozzle fields, for example in the manner of impingement flows, the material is cooled spatially uniformly and very quickly and that locally at the same time (for example in the case of steel) the martensite starting temperature is fallen below.

Apparatus according to claim 1, characterized in that air is sucked out between the individual nozzle fields for the purpose of suctioning off the boundary layer.