EP1767660B1 - Method of operating a single chamber vacuum furnace for hydrogen quenching - Google Patents

Abstract

A method for operating a single-chamber vacuum tempering furnace with hydrogen (H 2) cooling involves (a) filling the furnace cavity (FC) with workpieces (WP's), then closing and evacuating the FC, (b) heating the WP's and keeping a specific temperature, (c) filling the FC with pressurized H 2, ceasing heating and circulating the H 2to cool the WP's, (d) discharging the H 2and evacuating the FC, (e) filling the FC with inert gas up to atmospheric pressure and (f) opening the furnace and removing the WP's. An independent claim is included for apparatus for carrying out the process, comprising a tempering furnace surrounded by a safety zone (20), in which travel or transfer of the immediate environment of the furnace (11), excluding the openable front side (18), is mechanically or electrically inhibited.

Description

The present invention relates to a method for operating a heat treatment plant with a single-chamber vacuum furnace having the features of the preamble of claim 1 and a single-chamber vacuum furnace.

Conventional single chamber vacuum ovens with gas quenching have heretofore been subjected to various heat treatment processes. These processes include annealing, brazing, sintering, degassing, hardening of tool steels, high-speed steels, hot and cold work steels, as well as deep-freezing and tempering.

In future, processes for hardening low-alloy steels and vacuum gas carburization will be added to these heat treatment processes. The need to expand the scope of application results from the heavy cost pressure in heat treatment plants. The largest influencing factor on the production costs is the number of operating hours of the kiln plants per year. A heat treatment plant may use special ovens for the latter two processes, if long-term contracts with customers are available. For the rest of the business, which has only short-term commitments, great flexibility is required. This requires that the latter two methods should be carried out as far as possible with the same ovens as well as the standard methods mentioned above.

Low-alloyed steels have hitherto been hardened mainly in protective gas furnaces with oil quenching (so-called sealed quenching). The vacuum gas carburization is carried out in special multi-chamber plants using quenching oil baths or high pressure quenching stations with nitrogen or helium.

Because quenching in gas causes less distortion on the material to be hardened and then no cleaning is required, gas quenching is preferred. The currently used multi-chamber furnaces are very expensive and are especially suitable for the production of large series of automobile parts or the like. Parts have been developed. They lack the flexibility to be adaptable to changing tasks. In addition, with single-chamber furnaces, the control and monitoring of the process is much easier to carry out because the workpieces do not have to be moved during the process and therefore sensors can be placed directly on or in the workpiece, which can detect its actual temperature.

$$\mathrm{Lambda}=0,65\mathrm{für\; Bolzen}\mathrm{40}\mathrm{mm\; ⌀}\times \mathrm{80}\mathrm{mm\; lang}$$

$$\mathrm{Lambda}=1,50\mathrm{für\; Bolzen}\mathrm{80}\mathrm{mm\; ⌀}\times \mathrm{160}\mathrm{mm\; lang}$$

$$\mathrm{Lambda}=2,35\mathrm{für\; Bolzen}120\mathrm{mm\; ⌀}\times 240\mathrm{mm\; lang}$$

Standard single-chamber vacuum furnaces currently work with 10 bar of nitrogen for quenching and achieve the following lambda values in the material core of bolts made of structural steel: $$\mathrm{lambda}=0.35\mathrm{for\; <figure-callout\; id="20"\; label="bolts"\; filenames="imgf0002.png"\; state="\{\{state\}\}">bolts</figure-callout>}20\mathrm{mm\; ⌀}\times 40\mathrm{mm\; long}$$

$$\mathrm{lambda}=0.65\mathrm{for\; bolts}\mathrm{40}\mathrm{mm\; ⌀}\times \mathrm{80}\mathrm{mm\; long}$$

$$\mathrm{lambda}=1.50\mathrm{for\; bolts}\mathrm{80}\mathrm{mm\; ⌀}\times \mathrm{160}\mathrm{mm\; long}$$

$$\mathrm{lambda}=2.35\mathrm{for\; bolts}120\mathrm{mm\; ⌀}\times 240\mathrm{mm\; long}$$

The lambda value is the cooling time from 800 ° C to 500 ° C measured in seconds divided by 100. These values for the cooling rate are significantly slower than those achievable with quenching in an oil bath.

The current state of the art is described in the journal article of R. Hoffmann, H. Steinmann and D. Uschkoreit: Possibilities and Limitations of Gas Cooling - HTM 47-1992, p. 2 ff , For hardening low-alloyed steels and for vacuum gas carburizing, the quenching speed must be significantly increased, namely to values that have hitherto been considered unachievable in the single-chamber furnace.

DE-A 4121277 relates to a method and a device for monitoring the operational safety and for controlling the process sequence in a vacuum heat treatment furnace.

In order to extend the range of use for single chamber vacuum ovens in the manner described above, it is therefore an object of the present invention to achieve quenching rates that correspond to quenching in an oil bath.

This object is achieved by a method having the features of claim 1.

Preferred features of the present invention are set forth in the dependent claims.

Figur 1:

den prinzipiellen zeitlichen Verlauf von Temperatur und Druck sowie der jeweils zugeführten Gasart beim Gasaufkohlen von Werkstücken;

Figur 2:

die räumliche Anordnung der wesentlichen Anlagenteile in einem Betrieb in einer schematischen Darstellung; sowie

Figur 3:

einen für das Verfahren gemäß Figur 1 geeigneten Härtereiofen in einem Querschnitt von der Seite.

The invention will be described with reference to an embodiment and the drawing. Show it:

FIG. 1:

the basic time course of temperature and pressure and the respectively supplied gas during gas carburizing workpieces;

FIG. 2:

the spatial arrangement of the essential system parts in one operation in a schematic representation; such as

FIG. 3:

one for the method according to FIG. 1 suitable hardening furnace in a cross section from the side.

In the FIG. 1 in the curve 1, the time course of the temperature inside a hardening furnace in the implementation of a method according to the invention is recorded. Curve 2 shows the course of the pressure within the curing oven over the duration of the process. Here, the time scale is arranged on the horizontal X-axis, which represents a total of five hours from the beginning of the process to the end. The temperature scale covers a temperature range from 0 ° C to 1200 ° C. The pressure scale is located on the right side of the diagram. This indicates the pressure in absolute bar. It goes from 0 bar to 10 bar, where 0 bar is vacuum.

Below the diagram is illustrated which gases are fed into the device at which times and when vacuum is applied. This presentation will be described later in detail.

First, the temperature profile over the process time of 5 hours is described. The curve 1 first begins at room temperature. This is the section 1a on the temperature curve 1. Thereafter, the heater is turned on and brings along the section 1b the furnace to a temperature of about 1050 °. The required temperature range for the various carburizing applications for which the furnace is to be suitable is from 800 ° C to 1100 ° C.

In the specified in the present embodiment target temperature of 1050 ° C, the furnace temperature is kept constant in section 1c. Section 1c is about an hour long. In section 1d, the furnace is rapidly cooled, ie from 1050 ° to room temperature within about 20 minutes. There, the temperature is then kept constant until the end of the process, ie until unloading of the workpieces. This section is labeled 1e.

The pressure curve, which is illustrated in the curve 2, initially starts at 1 bar, ie at ambient pressure. This corresponds to the air when loading the hardening furnace in the interior of the oven is present. In section 2a of the curve, the furnace chamber is first evacuated for a period of about 20 minutes. The air in the oven chamber is removed before switching on the heater, so that no oxidation can take place. Instead, with the switching on of the heater, ie in the transition from 1a to 1b on the temperature curve, the furnace chamber is flooded with about 2 bar of nitrogen as inert gas. For a period of about 2 hours, the pressure is maintained. This corresponds to section 2b of the pressure curve. The nitrogen filling of the furnace is maintained approximately up to a temperature of 700 ° C. Up to this temperature range, the workpieces in the furnace will be heated by convection heating. Thereafter, the furnace interior is evacuated by applying vacuum. The associated pressure drop from 2 bar to 0 bar is marked with the section 2c. The further heating of the workpieces from 700 ° to the final temperature of 1050 ° takes place via radiant heating.

After the vacuum is reached in the furnace chamber, which is indicated by 2d, a carbon-containing gas is repeatedly introduced at a pressure of about 30 mbar into the furnace chamber for a short time. This gas, for example acetylene, causes the surface to be exposed to carbon via the thermal decomposition on the surface of the workpieces during the time periods 2e. This carbon diffuses from the surface into the workpiece. In order to obtain the carbon concentration more uniformly over the thickness of the carburized layer, so-called diffusion phases 2f are provided between the carburization phases 2e, in which the gas is removed from the furnace chamber by applying a vacuum. Carbon taken up by then from the workpiece surface can then diffuse into the workpiece without newly added carbon. Depending on the desired carbon distribution, phases 2e and 2f can be repeated. In this embodiment, there are a total of four carburization phases 2e. This embodiment contains method steps as they are for thin-walled Workpieces would be suitable in which a relatively low carburization is sought.

After completion of the last diffusion phase 2f, the furnace interior evacuated until then is flooded with hydrogen gas, up to an absolute pressure of 10 bar. At the same time, the heating is switched off, which has until then kept the temperature in the curve section 1c constant. The temperature drops rapidly to ambient temperature through hydrogen cooling along curve 1d. The phase of hydrogen cooling is designated 2g. A circulation of the hydrogen gas with a powerful fan in the furnace interior supports the heat dissipation. For uniform cooling, which minimizes the distortion of the workpieces during cooling, the hydrogen flow within the furnace chamber is deflected several times, so that the workpieces are charged from several sides with the cooling gas. If the cooling is completed close to room temperature, the hydrogen gas is removed in section 2h from the furnace chamber, until reaching the vacuum. Thereafter, to discharge the furnace interior, the interior is flooded from 0 bar to ambient pressure with nitrogen, which is illustrated by the curve portion 2i. If the oven is then opened, air enters the interior. The pressure adjusts to the atmospheric pressure. This section of the pressure curve is labeled 2k.

In the case of solid workpieces, provision may also be made for quenching to initially take place above the martensite start line and for the temperature to be maintained there until the edge temperature and the core temperature of the workpiece have become equal. Thereafter, the further quenching can take place up to the range of room temperature.

The method described offers the possibility in a single-chamber vacuum furnace of achieving cooling rates which otherwise could only be achieved during oil quenching or water quenching. The cooling rate depends from the slope of the curve 1 in section 1d. Although it is known to cool workpieces in the hardening with hydrogen, this method is not used in practice in single-chamber vacuum ovens, because the security problems are considered not economically solvable.

Here, a new solution to the security problems is found with the method described. The danger of explosion when filling the interior of the furnace with hydrogen arises as a result of ignition sources in the furnace, namely the workpieces kept at more than 1000 ° C., and, on the other hand, that hydrogen is present as oxidizable gas. To exclude an explosion hazard, therefore, all oxygen must be kept away from the interior of the furnace. In the method according to the invention, this is achieved by first evacuating in section 2a the air present after loading the furnace almost completely. Thereafter, a nitrogen atmosphere is first built (2b) which is then pumped out again later in the process. Any existing residual oxygen from the atmosphere or by degassing of the workpieces is then rinsed out with the nitrogen. Finally, the carbonizing gas is placed in the oven, which does not react with hydrogen. This gas is pumped out repeatedly, which is equivalent to a further flushing of the furnace interior.

If the hydrogen is then introduced into the interior in section 2g, then there is no more oxygen there. An explosion hazard is thereby completely excluded. During the cooling phase, no oxygen comes into the furnace interior. The hydrogen is then vented to the flue through bleed valves, and upon reaching atmospheric pressure, the residual hydrogen is pumped out of the furnace interior by vacuum pumps (section 2h). Any existing residual hydrogen is diluted to such an extent by flooding with nitrogen in Section 2i that in each case no ignitable mixture can arise. At the time it is missing too an ignition source in the furnace interior, because the entire contents of the furnace is cooled to near room temperature. The drive motors of the fans and the heater are de-energized. The opening of the oven in section 2k for unloading the inventory therein is then completely uncritical. The air entering at the opening will have neither an ignition source nor sufficient hydrogen concentration to form explosive conditions.

The hydrogen pumped out in section 2h is vented to the atmosphere through gas-tight pipes and vacuum pumps via a chimney, outside the plant building. After pumping off the hydrogen, the chimney 17 is completely purged with nitrogen to ensure that there is no hydrogen left in it, which could form an ignitable mixture.

The factory building is in the FIG. 2 illustrated in more detail.

The FIG. 2 shows a schematic representation of an operating building 10 for carrying out the method described above. The operating building is designed as a hall in which a hardening furnace 11 is set up in a conventional manner. Outside the building, a storage tank 12 is provided for hydrogen. Next, a storage tank 13 is arranged for gaseous nitrogen in addition to another storage tank 14 for liquid nitrogen. The two storage tanks 12 and 13 for the gaseous supply are connected via connecting lines 15, 16 with the hardening furnace 11.

The hardening furnace 11 is further provided with a chimney 17 which leads out of the building into the atmosphere. The chimney 17 is made higher than the ridge line of the building 10th

The hardening furnace 11 has on its left front side a closure lid 18 which is to open for loading and unloading of the curing oven 11. Behind the plane of the cap Figure 18 illustrates a hatched area 20 in which special provisions are provided against mechanical damage to the external attachments and piping. This area 20 is mechanically secured in such a way that driving this area in the vicinity of the hardening furnace 10 with machines such as forklifts and the like is not possible. Also, driving over the area 20 with a gantry crane is precluded by appropriate mechanical devices or electrical precautions that affect the control of the crane. For this purpose, barriers, crash barriers or even a cage can be provided. These safety measures prevent the hydrogen-carrying lines 15, 17, the associated valve means and pumps and the hardening furnace itself from being damaged in such a way that hydrogen can escape within the operating building 10.

The closure lid 18 of the curing oven 11 is further provided with a circumferential seal that hermetically seals in operation safely by overpressure of a protective gas. In this way it is avoided that when changing from negative pressure to overpressure, as occurs in operation and in the curve 2 of FIG. 1 Illustrated is leaks.

Due to the described safety precautions, it is not necessary to carry out area 20 explosion-proof. This leads to a reduction of plant and operating costs compared to the previously known concepts for hydrogen cooling in the hardening shop.

The FIG. 3 finally shows the curing oven 11 in an enlarged view. The oven is of the single chamber vacuum oven type with a fan whose axis of rotation is identical to the central axis of the oven. The oven door 18 is particularly equipped to leakages when switching between negative pressure and pressure. This is in the published patent application WO 2004/096427 A1 described in more detail, which goes back to the same applicant. It should be mentioned that also vertical Einkammeröfen be built, and that the cooling fan and the heat exchanger can also be installed in an external housing, which is then connected to the furnace housing.

$$\mathrm{Lambda}=0,26\mathrm{für\; Bolzen}\mathrm{40}\mathrm{mm\; ⌀}\times \mathrm{80}\mathrm{mm\; lang}$$

$$\mathrm{Lambda}=0,72\mathrm{für\; Bolzen}\mathrm{80}\mathrm{mm\; ⌀}\times \mathrm{160}\mathrm{mm\; lang}$$

$$\mathrm{Lambda}=1,30\mathrm{für\; Bolzen}\mathrm{120}\mathrm{mm\; ⌀}\times \mathrm{240}\mathrm{mm\; lang}$$

Initial tests with the described plant and method have shown that the following lambda values can be achieved in the material core of structural steel bolts: $$\mathrm{lambda}=0.10\mathrm{for\; <figure-callout\; id="20"\; label="bolts"\; filenames="imgf0002.png"\; state="\{\{state\}\}">bolts</figure-callout>}20\mathrm{mm\; ⌀}\times 40\mathrm{mm\; long}$$

$$\mathrm{lambda}=0.26\mathrm{for\; bolts}\mathrm{40}\mathrm{mm\; ⌀}\times \mathrm{80}\mathrm{mm\; long}$$

$$\mathrm{lambda}=0.72\mathrm{for\; bolts}\mathrm{80}\mathrm{mm\; ⌀}\times \mathrm{160}\mathrm{mm\; long}$$

$$\mathrm{lambda}=1.30\mathrm{for\; bolts}\mathrm{120}\mathrm{mm\; ⌀}\times \mathrm{240}\mathrm{mm\; long}$$

These are values that correspond to quenching in oil.

For a further improvement of the achievable cooling rates, provision may be made for a cold water supply of a few cubic meters of cooling water at a low temperature of about 3 ° C.-5 ° C., which are introduced into the heat exchanger for the first 30-60 seconds of the cooling process. As a result, the particularly critical time phase of the first quenching of the holding temperature 1c is favorably influenced.

The quality of the workpieces is also determined by the delay that arises during the hardening process. In order to reduce the residual distortion problems in ovens with gas quenching, the gas flow with frequent reversal of direction in vacuum ovens was successfully introduced several years ago. Now, the present invention provides a new, complementary approach to this problem.

In the method according to the invention and with the device, it is possible to achieve specific lambda values in a targeted manner. The full gas pressure of hydrogen is introduced into the furnace for quenching. Depending on the oven, this pressure can be 10bar amount, but also 20bar or 40bar. The cooling rate to be set to achieve a given lambda value is controlled by the gas velocity and ultimately the rate of circulation within the furnace. The circulating fan is regulated in its speed, with a control range of 10% of the maximum speed is provided to the full maximum speed.

The technical effect is that there are three factors influencing the rate of cooling, namely the type of gas, the gas pressure and the gas flow rate. So far, the experts believe that these three components are of equal importance. This may apply to the narrative hardness. In the delay of the workpieces, however, differences have been found. Thus, the type of cooling gas used affects all surfaces of the workpieces exposed to the cooling gas. The same applies to the gas pressure, which is the same everywhere in the treatment room of the furnace. However, the flow velocity of the cooling gas will vary on the workpiece surfaces, depending on how they are reached by the gas flow.

The use of hydrogen gas at very high pressure determines the first two points. If now lower lambda values are sought than they would be possible at full fan power, the performance of the circulating fan is reduced and the parameter "flow velocity of the cooling gas" adapted to the needs. The slower gas recirculation will result in less distortion of the machined workpiece than if the fan were operated at full power and one of the other parameters was changed.

As an alternative to the regulation of the blower power, a swirl throttle or a similar means for influencing the flow velocity can also be used.

Claims (9)

1. Method for operating a single-chamber vacuum furnace with hydrogen cooling, having the following method steps:

   a) filling the furnace interior with workpieces;

   b) closing the furnace interior;

   c) evacuating the furnace interior;

   d) heating the workpieces and maintaining a set temperature;

   e) filling the furnace interior with hydrogen at high pressure, switching off the heating and circulating the hydrogen to cool the workpieces;

   f) releasing the hydrogen and evacuating the furnace interior;

   g) filling the furnace interior with an inert shielding gas until it reaches approximately atmospheric pressure;

   h) opening the furnace and unloading the workpieces.
2. Method according to Claim 1, characterised in that after step c), the following is provided:

   c1) filling the furnace interior with a shielding gas;

   c2) convectively heating the workpieces by circulating the shielding gas;

   c3) evacuating the furnace interior.
3. Method according to one of the preceding claims, characterised in that in step c2) and/or in step g), the shielding gas is nitrogen.
4. Method according to one of the preceding claims, characterised in that in step f) the hydrogen is fed out of the plant building (10) by means of a chimney (17).
5. Method according to Claim 3, characterised in that the chimney (17) is higher than the roof height of the plant building (10).
6. Method according to one of the preceding claims, characterised in that during and/or after method step d), a gas containing carbon is introduced into the furnace interior at least once.
7. Method according to Claim 6, characterised in that the gas is pumped out again before method step e).
8. Method according to one of the preceding claims 6 or 7, characterised in that the gas is acetylene, to which optionally a carrier gas is added.
9. Method according to one of the preceding claims, characterised in that at least in step e) the rotational speed of the fan for circulating the hydrogen can be regulated.

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