EP2345747A1

EP2345747A1 - Method and device to reduce risk of and to suppress fumes in oil quenching.

Info

Publication number: EP2345747A1
Application number: EP10000364A
Authority: EP
Inventors: Tero Ristolainen; Reijo Luukkanen
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2010-01-15
Filing date: 2010-01-15
Publication date: 2011-07-20

Abstract

The invention relates to a method for cooling a metal part in a bath of a quenching medium wherein said metal part is submerged into said bath, wherein an inert gas is blown to the surface of said bath. (Fig. 2)

Description

The invention relates to a method for cooling a metal part in a bath of a quenching medium wherein said metal part is submerged into said bath. Further the invention relates to a device for cooling a metal part comprising a tank with a bath of a quenching medium.
It is well known to subject hot metal parts to a quenching operation in order to obtain certain hardness and mechanical property requirements. By quenching heat is rapidly removed from the hot metal part to achieve a desired internal microstructure as well as to ensure uniform mechanical properties and to minimize residual stresses.
The quenching is often practiced using oil as quenching medium. The hot metal part is submerged into an oil bath and rapidly cooled down. When submerging the metal part into the oil bath there is a considerable risk that oil will flame up. Oil boiling and the risk for fire might also occur if the viscosity of the oil increases to a point where it overheats or if there is a loss of thermal stability in the oil and low flash point fractions are produced.
Therefore, it is an object of the invention to reduce the risk of fire and to suppress fume production.
This object is achieved by a method for cooling a metal part in a bath of a quenching medium wherein said metal part is submerged into said bath, and which is characterized in that an inert gas is blown to the surface of said bath.
The inventive device for cooling a metal part comprising a tank with a bath of a quenching medium, is characterized in that an inert gas supply and an inert gas supply line is provided wherein said inert gas supply line comprises at least one opening for supplying inert gas to said tank.
According to the invention the bath of the quenching medium is blanketed with an inert gas. The inert gas is blown to the surface of the bath such that an inert gas layer is built up on or above the surface of the quenching medium.
The quenching medium is a liquid. According to a preferred embodiment the quenching medium is an oil or comprises an oil, for example a hydrocarbon based oil or a mineral oil, or a non-aqueous fluid, especially a synthetic non-aqueous fluid.
Preferably, nitrogen is used as inert gas. Nitrogen does not react with the metal part which is cooled and nitrogen does also not effect the quenching medium. For example, when using oil as the quenching medium the oil composition will not be effected by the nitrogen. In the state of the art technology, when the temperature of the quenching oil raises due to the hot metal submerged into the oil the quenching oil is degraded by the oxygen present in the air. This degradation does not occur when the bath is covered by a layer of nitrogen gas according to the invention.
In addition, the inert gas will supersede the air normally covering the bath of quenching medium. By substituting inert gas for air the oxygen concentration directly over the bath of quenching medium will be reduced. This will reduce the risk of fire hazards and decrease fumes at the operation site, for example in a production hall.
According to a preferred embodiment the inert gas is blown to the surface of the quenching bath in an essentially horizontal direction. By providing the inert gas in a horizontal direction the atmosphere, e.g. the air, above the quenching bath will be blown away by the inert gas. The time for exchange of the atmosphere above the bath, from air to inert gas, is considerably reduced.
The inert gas can be continuously provided to the bath surface. Preferably the same flow of inert gas is applied to the quenching bath all the times. In that case the inert gas flow is preferably set to a constant value such that there is always a satisfactory inert atmosphere above the quenching bath in order to prevent ignition of the quenching medium and/or to avoid degradation of the quenching medium by a too high concentration of oxygen.
It has been found that the most crucial moments with respect to ignition of the quenching medium are when the hot metal is lowered into the quenching bath. At that time boiling of the quenching medium is most probably to occur and the risk of fire is highest. Therefore, according to a preferred embodiment of the invention the flow of inert gas is changed with time. The inert gas flow is changed from a lower flow rate to a higher flow rate and/or vice versa. Of course, the inert gas flow can be changed several times and can vary between several flow rates. The inert gas flow is preferably controlled depending on the time when the metal object is submerged into the quenching bath. Other parameters which can be used to control the inert gas flow are the oxygen concentration above the quenching bath and/or on another parameter of the quenching process.
The inert gas flow can be changed between zero flow and maximum flow. Preferably, the inert gas is controlled such that there is always a minimum inert gas flow which is temporarily increased. The minimum inert gas flow is preferably set to such a value that the oxygen concentration above the quenching bath is low enough to avoid negative interactions of the oxygen with the quenching medium. The flow rate of the minimum inert gas flow depends on the surface area of the quenching bath and/or on the open volume above quenching bath.
In addition to that minimum inert gas flow the gas flow is increased depending on pre-defined parameters, such as oxygen concentration above the quenching bath, temperature of the quenching bath, the temperature, size and / or heat capacity of the metal object to be cooled, or the time the metal object is lowered into the quenching bath. The change of the inert gas flow can be stepwise or continuously.
As already described the most critical moment is when the hot metal object is lowered into the quenching medium. Therefore, it has been found advantageous to increase the inert gas flow for a defined time period before the hot metal object is lowered into the bath. It is further advantageous to increase the inert gas flow during the time when the metal object is brought into contact with the quenching medium and/or for a defined period of time after the metal object has been submerged into the bath.
Preferably the inert gas flow is increased for a few minutes, preferably for 1 to 10 minutes before the hot metal is submerged into the quenching medium. Thus, the inert gas layer is increased and the oxygen concentration at the surface of the quenching bath decreased and consequently the risk of fire is prevented or at least considerably reduced.
As mentioned above, it is further preferred to have an increased inert gas flow for a certain period of time after the metal object has been lowered into the quenching oil. This period of time is preferably between 1 and 10 minutes.
Right before the metal touches the quenching medium the inert gas flow is increased to at least 500 Nm³/h, more preferred to at least 1000 Nm³/h, even more preferred to at least 1200 nm³/h. When the metal has cooled down the inert gas flow is again reduced to a flow rate below 300 Nm³/h.

Example:

Hot metal parts shall be cooled in a bath of quenching oil. For safety reasons, during idling the oil bath is covered with a lid which is opened or removed before quenching. The hot metal parts are lifted with a crane and then lowered into the tank with the oil bath.
The whole volumen of the tank with the oil bath is around 20 m³. The open space between the lid and the oil depends on the level of the oil and is normally between 5 and 10 m³. The inert gas flow, preferably nitrogen gas, is set to a minimum flow rate of 5 to 10 Nm³/h which is always always blown onto or along the surface of the bath of quenching oil. This minimum inert gas flow guarantees that the atmosphere above the oil bath remains inert and that the quality of the quenching oil is good. A few minutes, preferably 5 to 10 minutes, before the metal part which shall be cooled is lowered into the oil bath the inert gas flow rate is increased. The increase of flow rate could be for example to a level of about 300 m³/h to decrease the oxygen content in a layer above the oil bath. The increased flow rate could be held for a certain period of time, for example 10 minutes, or until the oxygen content at a certain distance above the oil bath goes below a pre-defined value. One to three minutes before the metal part is lowered into the oil bath the inert gas flow is increased to roughly 1200 m³/h. This high inert gas flow rate will last for 3 to 4 minutes. Then the inert gas flow is lowered back to 300m³/h for a couple of minutes, for example 5 to 15 minutes. Finally, the inert gas flow is reduced to the minimum flow rate of 5 to 10 m³/h.
According to another preferred embodiment the inert gas flow is automatically controlled. The inert gas flow is set to a minimum gas flow which is, according to a pre-defined schedule, increased at certain times, at certain conditions and/or when a particular variable, for example a monitored variable, meets a pre-defined value.
For example, the degradation or oxidation of an oil used as quenching medium can be detected by infrared spectroscopy. It could also be measured by several other methods, including precipitation number, total acid number, sludge content and viscosity.
It has been found that the invention also works with a zero minimum inert gas flow. The inert gas flow is started before the metal to be cooled is lowered into the bath. The flow rate of inert gas and / or the time period between starting of the inert gas flow and submerging the metal part into the quenching bath is in that case increased compared to a process wherein a non-zero minimum inert gas flow is used.
The inventive method is in particular useful for quenching objects made of iron, steel, copper, aluminium or other metals.
The inventive device comprises a tank with a bath of a quenching medium, an inert gas supply and an inert gas supply line wherein the inert gas supply line comprises at least one opening for supplying inert gas to the bath of a quenching medium.
According to a preferred embodiment the inventive device comprises an inert gas supply line which comprises a ring line with several openings for providing inert gas. The ring line is preferably arranged along the circumference of the tank of quenching medium. For example, if the tank is of cylindrical design the ring line will be a circular line with several gas outlet openings which are preferably directed in a radial direction to the center of the ring line such that any gas provided through the gas outlet openings flows in a more or less horizontal direction. The gas outlet openings are preferably in the form of bores or slots. It is also possible to provide only one slot which extends along the whole inner circumference of the ring line.
The invention as well as further preferred details and advantages of the invention will become apparent from the attached schematic drawing wherein

fig. 1: schematically shows the inert gas supply for a quenching bath,
fig. 2 and 3: show the quenching bath in more detail and
fig. 4: shows an example of the variation of the inert gas flow with time.

Figure 1 schematically shows an inventive system for supplying inert gas to a quenching bath 1. A tank 1 is filled with oil which is used as a quenching medium to rapidly cool down hot metal objects. The hot metal objects are submerged into the quenching bath 1 whereby they are shock-cooled by direct heat exchange with the quenching oil.
According to the invention an inert gas is supplied to the surface of the quenching bath 1. Nitrogen is withdrawn from a nitrogen tank 2 via a nitrogen supply line 3. Nitrogen supply line 3 splits up into two parallel lines 4a, 4b. Each line 4a, 4b comprises a flow control unit 5a, 5b and a shut-off valve 6a, 6b arranged in series with the flow control unit 5a, 5b, respectively. Each flow control unit 5a, 5b comprises two flow meters 7a, 8a, 7b, 8b also arranged in parallel.
Downstream the flow control units 5a, 5b and the shut-off valves 6a, 6b the parallel lines 4a, 4b are again combined to a common nitrogen supply line 3. At the quenching bath 1 nitrogen supply line 3 ends in a kind of sprinkler system which is shown in more detail in figure 2.
The nitrogen flow to the quenching bath 1 is controlled by means of a control unit 9. A measuring device 10 determines the oxygen content in the atmosphere above the quenching bath 1. The measured value for the oxygen content is sent as an input parameter to the control unit 9. Control unit 9 further comprises a timer control such that shut-off valves 6a, 6b can be controlled depending on the time and on the measured parameter.
In the embodiment shown in figure 1 the nitrogen gas flow can be controlled by opening or closing one or both of the shut-off valves 6a, 6b. It is further possible not only to control the shut-off valves 6a, 6b but also the flow control units 5a, 5b or even each of the flow meters 7a, 8a, 7b, 8b. This will allow for a more continuous variation of the nitrogen gas flow to the quenching bath 1 rather than a stepwise change of the nitrogen gas flow.
Figure 4 shows the correlation between the nitrogen gas flow for blanketing the bath of quenching oil 1 and time. Independent of time there is always a constant flow F1 of nitrogen gas in order to create and maintain a nitrogen gas layer above the quenching oil 1. This is achieved by leaving shut-off valve 6a open and closing shut-off valve 6b. Thus, a gas flow F1 is continuously supplied to the sprinkler system.
At time t2 a hot metal object will be shock-cooled in the quenching bath 1. At the moment when the hot metal object is lowered into the quenching bath 1 there is a considerable risk that the quenching oil will boil, overheat, flash and flame up. Therefore, the nitrogen gas flow to the surface of the quenching bath 1 is already increased for a certain period of time before the metal object comes into contact with the quenching oil. At time t1 shut-off valve 6b is also opened and thus the nitrogen gas flow is increased to F2. Thereby, a nitrogen gas layer of sufficient thickness will be produced above the quenching bath 1. Any oxygen close to the surface of the quenching bath 1 is replaced by nitrogen and thus the risk of flames or oil ignition is avoided.
In the system shown in figure 4 nitrogen gas flow F2 is more than double of nitrogen gas flow F1. The magnitude of the gas flows F1 and F2 depends on the design and settings of flow control units 5a and 5b. The magnitudes of nitrogen gas flows F1 and F2 will be set according to the requirements of the process. Of course, any other ratio of F1 to F2 is also possible. For example, often it is advantageous to use identical flow meters 7a, 8a, 7b, 8b. In that case by opening shut-off valve 6b the nitrogen gas flow will be doubled, that is F2 = 2 * F1.
After the hot metal object has been submerged into the quenching bath 1 the nitrogen gas flow is maintained at flow rate F2 until time t3. At time t3 the metal object has been sufficiently cooled down and oil splashs will no more occur. Therefore, there is no more risk of fire and the main task of the inventive nitrogen gas flow is to protect the quenching oil from oxygen in order to avoid degrading of the oil. So the nitrogen gas flow can be lowered to F1 again.
Figure 4 shows a stepwise control of the nitrogen gas flow to the quenching bath 1. As described above, it is not only possible to control shut-off valves 6a, 6b depending on the time of contacting the hot metal object with the oil bath 1. According to another preferred embodiment the nitrogen gas flow is not stepwise changed but continuously. Flow meters 7a, 8a, 7b, 8b could be controlled by magnetic valves which are controlled by control unit 9. Further control parameters beside the time control already described could be the oxygen content above the oil bath 1 or the quality of the quenching oil.
Figures 2 and 3 show the quenching bath 1 in more detail. Figure 2 shows a vertical cross section of the quenching bath 1, figure 3 shows a top view of the quenching bath 1.
The quenching bath 1 comprises an oil drum 14 which is filled with a quenching oil 11 up to a height 12. Inside the oil drum 14 there is a stainless steel annular tube or ring torus 13 which is connected to nitrogen supply line 3. The annular tube 13 is arranged near the top of the oil drum 14 at a distance between 5 and 300 mm from the top of the oil drum 14. The annular tube 13 abuts with its outer circumference on the inner wall of the oil drum 14.
At its inner surface - opposite to the inner wall of the oil drum 14 - the annular tube 13 comprises a number of openings 15 in the form of horizontal slots. Gaseous nitrogen supplied via nitrogen-supply line 3 to the annular tube 13 will flow out of annular tube 13 through the slots 15. The nitrogen gas flows out in an essentially horizontal direction directed to the center of the oil drum 14 (see figure 3). Thus the nitrogen gas will create an inert gas blanket over the oil bath 11.

Claims

Method for cooling a metal part in a bath (1, 14) of a quenching medium (11) wherein said metal part is submerged into said bath (1, 14), characterized in that an inert gas (3, is blown to the surface of said bath (1, 14).
Method according to claim 1, characterized in that nitrogen is blown to the surface of said bath (1, 14).
Method according to any of claims 1 or 2, characterized in that said inert gas is horizontally blown to said surface of said bath (1, 14).
Method according to any of claims 1 to 3, characterized in that said inert gas is continuously blown to said surface of said bath (1, 14).
Method according to any of claims 1 to 3, characterized in that said inert gas is intermittently blown to said surface of said bath (1, 14).
Method according to any of claims 1 to 5, characterized in that the flow of said inert gas is increased before said metal part is submerged into said bath (1, 14).
Device for cooling a metal part comprising a tank (1, 14) with a bath of a quenching medium (11), characterized in that an inert gas supply (2) and an inert gas supply line (3) is provided wherein said inert gas supply line (3) comprises at least one opening for supplying inert gas to said tank (1, 14).
Device according to claim 7 characterized in that said inert gas supply line (3) comprises a ring line (13) comprising several openings (15).