Patent Application
20120247627
1. Field of the Invention
This invention relates to a method for quenching heat treated metallic work pieces and to an apparatus for carrying out the method.
2. Description of the Related Art
In some of the known heat treatment systems, a high pressure gas quench subsystem is used to rapidly cool the metal work pieces from the heat treatment temperature. As shown in
In the case where the final quench pressure is high, e.g., on the order of about 20-30 bar, for example, many large accumulator tanks would be required, each storing gas at a pressure much higher than the final quenching pressure. Such tanks are expensive and take up a lot of space in the processing facility. The rapid filling of the furnace requires a large pipe and valve size to allow the furnace to reach the final quench pressure in a short time. In order to pressurize the large accumulator tanks to the required high pressures, a compressor system or very high pressure gas delivery system is sometimes employed. Both of those systems require additional energy to fill the tanks That energy ultimately is wasted because it does not convert into useful energy in the furnace quenching process.
The main problems the invention is meant to address are summarized as follows.
1) Physical space used by high pressure backfill tank(s).
2) The compressor systems that charge these tanks to high pressures (up to 30 bar or more) have periodic maintenance issues with wear parts and also add unwanted energy into the process of furnace quenching.
3) If a compressor system is not used, the end user of the furnace equipment would have to change the bulk gas storage system in the facility and the high pressure gas delivery line from what would be typically a 10 bar or an 18 bar gas delivery system to at least a 30 bar gas delivery system.
4) Typically gas is kept in a liquid state in bulk storage systems. It takes energy to change the gas into a liquid form, energy that the end user already paid for when they bought the liquid gas. If the liquid gas is used downstream of the bulk storage system, it commonly goes through a vaporizer to turn it back into a gaseous state before delivery. The conversion of liquid gas to the gaseous state gives up stored energy by cooling the vaporizer. This energy is wasted and is not useful in the furnace quenching process.
This invention provides a process and associated apparatus to deliver a liquid, a liquefied quenching gas or vapor directly into a furnace chamber such that the liquid, liquefied gas, or vapor converts to a fully gaseous state thereby rapidly increasing the pressure inside the chamber.
The process and apparatus according to this invention eliminate the need for large high pressure gas storage tanks The conversion of liquefied gas to the gaseous state inside the furnace chamber utilizes the energy stored in the liquefied gas and eliminates the need for compressors or other high pressure gas delivery systems.
In accordance with a first aspect of the present invention there is provided a method for rapidly cooling a load of heat treated metal parts from an elevated temperature. The method includes the steps of injecting a pressurized liquid quenchant into a pressure vessel containing a load of heat treated metal parts such that a vapor of the liquid quenchant forms rapidly and cools the metal parts and continuing to inject the pressurized liquid quenchant for a time sufficient to establish a desired peak vapor pressure in the pressure vessel. Preferably the liquid quenchant is readily vaporizable at temperatures and pressures utilized for the heat treatment of metal work pieces.
In a preferred embodiment of the process the pressurized liquid quenchant is injected for a time sufficient to establish a vapor pressure in the pressure vessel of about 5 to 100 bar.
In another preferred embodiment the quenchant vapor is circulated in the pressure vessel at high velocity while the liquid quenchant is injected into the pressure vessel such that the quenchant vapor penetrates through the load of metal parts.
In another preferred embodiment the injecting step includes the step of spraying the liquid quenchant in a preselected direction in the pressure vessel.
In further preferred process, the injecting step includes providing the liquid quenchant at an initial pressure prior to the start of the injecting step that is higher than the desired peak vapor pressure in the pressure vessel. Preferably the initial pressure of the liquid quenchant is higher than the quenchant vapor pressure in the pressure vessel by at least about 3 bar.
Preferably, the method comprises the step of continuously raising the pressure of the liquid quenchant during the injecting step such that the liquid quenchant pressure is always higher than the instantaneous quenchant vapor pressure in the pressure vessel.
Preferably the process includes the step of continuously raising the pressure of the liquid quenchant during the injecting step such that the liquid quenchant pressure is about 3 to 5 bar higher than the instantaneous vapor pressure in the pressure vessel.
Preferably the injecting step is stopped once the desired peak vapor pressure in the pressure vessel is reached.
In another preferred embodiment the steps of maintaining the peak quenchant vapor pressure in the pressure vessel and continuing to circulate the quenchant vapor are carried out for a time sufficient to lower the temperature of the metal parts to a temperature lower than the elevated temperature of the metal parts.
Preferably the process includes the step of continuing the injecting step for a period of time after the peak vapor pressure in the pressure vessel is reached.
Preferably the peak vapor pressure in the pressure vessel is maintained at the desired level by exhausting a portion of the quenchant vapor from the pressure vessel.
Preferably the peak vapor pressure in the pressure vessel is maintained at the desired level by injecting additional liquid quenchant into the pressure vessel.
A further preferred embodiment includes the step of reducing the quenchant vapor pressure in the pressure vessel to a lower pressure when the load of metal parts reaches the first lower temperature.
Preferably the method includes the step of holding the quenchant vapor pressure in the pressure vessel at the lower pressure until the load of metal parts reaches a selected final temperature.
In a still further preferred embodiment the circulating step includes circulating the quenchant vapor through a heat exchanger and circulating a heat absorbing fluid in the heat exchanger to absorb heat from the quenchant vapor.
In a still further embodiment the injecting step is carried out with a flow rate that is effective to raise the vapor pressure in the pressure vessel to the desired peak vapor pressure within about 2 to about 60 seconds from the start of the injecting step.
In one embodiment, the process according to the invention uses a liquefied gas as the quenchant. In a particularly preferred embodiment the liquid quenchant is selected from the group consisting of liquefied nitrogen, liquefied helium, liquefied argon, liquefied air, a liquefied hydrocarbon gas, liquefied carbon dioxide, and a combination thereof. In another embodiment, a liquid quenchant such as water or an aqueous quenchant solution can be used to provide a high pressure steam quench. In a further embodiment, the process according to this invention is carried out with oil as the liquid quenchant.
In accordance with a second aspect of this invention, there is provided an apparatus for rapidly cooling a work load of heat treated metal parts. An apparatus according to the invention includes a pressure vessel having an internal chamber for holding a work load of heat treated metal parts. The apparatus also includes a liquid quenchant supply vessel adapted to contain a liquid quenchant at a first pressure and a quenchant conducting means for conducting the liquid quenchant from the supply vessel to the internal chamber of the pressure vessel. The apparatus further includes a pressure control means operatively connected to the pressure vessel and the quenchant conducting means for maintaining the liquid quenchant conducted to the pressure vessel at an elevated pressure differential sufficient to establish a desired peak vapor pressure in the internal chamber of the pressure vessel.
Preferably the pressure control means is adapted for controlling the flow rate of the liquid quenchant from the supply vessel to the internal chamber of the pressure vessel.
Preferably the quenchant conducting means comprises means for increasing the pressure of the liquid quenchant conducted to the pressure vessel which may be embodied as a liquid pump or a source of pressurized gas.
In another preferred embodiment the quenchant conducting means includes a storage tank adapted for concurrently holding liquid and vapor phases and means for increasing the vapor pressure inside the storage tank.
In a still further preferred embodiment the means for spraying the liquid quenchant comprises at least one spray nozzle mounted in the pressure vessel and connected to the means for conducting the liquid quenchant.
The foregoing summary of the invention as well as the following detailed description of the invention will be better understood when read in conjunction with the drawings, wherein:
Referring now to the drawing and in particular to
The heat treating furnace 12 is constructed for holding a load of metal work-pieces 16 that are heat treated in the furnace. The load will typically be in the form of stacked baskets or containers of the metal work pieces. The heat treating furnace 12 includes a pressure vessel or quenching chamber that is capable of holding a quenching gas, such as nitrogen, at pressures of at least about 5 bar up to about 100 bar. The pressure vessel or quenching chamber preferably includes a recirculation fan 13 which operates to circulate the quenching gas in the furnace chamber. A heat exchanger (not shown) is also included for extracting heat from the quenching gas as it is recirculated through the heat exchanger. The heat exchanger is preferably located internally to the pressure vessel, but may be located externally in accordance with arrangements generally known to persons skilled in the art. Likewise, the recirculation fan may be located externally to the pressure vessel in accordance with arrangements generally known to persons skilled in the art. One or more spray nozzles 15a, 15b, 15c, may be connected from a cryogenic manifold 14. A second cryogenic pipe 33 is connected between the LN2 storage tank 18 and the cryogenic manifold for supplying LN2 gas to the spray nozzles 15a, 15b, and 15c. The LN2 storage tank is preferably located in close proximity to the heat treating furnace, specifically to the quenching chamber of the furnace. In this way, second cryogenic pipe 33 is kept as short as possible. The second cryogenic pipe 33 preferably has an inside diameter that is dimensioned to allow the LN2 to flow into the manifold 14 at a rate of about 1 to 15 l/s. Such a flow rate may allow the heat treating furnace 12 or quenching chamber to be pressurized to the desired quenching gas pressure within as little as 2-5 seconds. More typically, it is expected that the desired quenching gas pressure will be attained in about 10 to about 50 or 60 seconds. The spray nozzles are preferably constructed to provide a wide angle spray as shown in
A pipe or tube 47 extends from the interior of the pressure vessel or quenching chamber 12 to provide an overpressure exhaust port. A solenoid-operated valve 48 is connected in the pipe or tube 47 to control the flow of quenching gas from the interior of the pressure vessel or quenching chamber through the exhaust port and out to the atmosphere when the gas pressure inside the pressure vessel reaches a predefined peak value.
A high pressure source of pressurizing gas 22, preferably nitrogen, is connected to the storage tank 18 through high pressure gas tubing or pipe 35. The pressurizing gas source is preferably realized with a high pressure gas cylinder. A pressure regulator 26 may be connected in the high pressure tubing 35 in proximity to the high pressure gas source 22. A solenoid-operated control valve 46 is connected in the high pressure gas tubing 35 in proximity to the storage tank 18 for controlling the flow of gas from the source 22 to the storage tank 18. A pressure switch 24 is provided at the heat treating furnace 12 and is adapted to sense the gas pressure inside the pressure vessel or quenching chamber. The pressure switch 24 is connected to the control valve 46 for controlling the high pressure gas flow to the storage tank 18 from the gas source 22. In an alternative embodiment, a cryogenic fluid pump (not shown) can be connected in the LN2 supply line 31 to pump the LN2 up to a desired pressure in the storage tank 18.
The filling of the storage tank 18 is achieved by establishing a positive pressure differential in the LN2 supply tank 20 relative to the storage tank 18. The volume of the storage tank 18 is selected such that the amount of LN2 stored will be sufficient to bring the high pressure gas quench system of the heat treat furnace 12 to the desired gas pressure for quenching after evaporation of the liquefied nitrogen. For example, a high pressure gas quench system having a volume of 2 m3 can be used for a quenching cycle that requires a gas pressure of 30 bar. This means that 60 m3 of nitrogen gas are needed to reach this pressure, which requires at least 90 liters of LN2 to be filled into the LN2 storage tank 18.
When the storage tank 18 is filled with a sufficient amount of LN2, it is closed-off completely by the valve 44 in the first cryogenic pipe 31 and valve 43 in the second cryogenic pipe 33. The pressure inside the storage tank is allowed to build up to a value sufficient to cause the liquefied nitrogen to flow from the storage tank 18 into the manifold 14 and spray nozzles 15a-15c in the heat treat furnace 12 at a flow rate sufficient to provide an amount (volume) of LN2 that will cause the desired quench gas pressure to occur after evaporation of the LN2 inside the furnace.
To achieve rapid evaporation of the LN2 inside the heat treating furnace or quenching chamber, it is advantageous to spray the LN2 flow with a widely diverging spray pattern. Although the embodiment shown in
Preferably, a constant pressure differential is maintained across the spray nozzles to provide a constant flow of LN2. As an example of a suitable operating characteristic, the desired flow can be achieved by using a starting pressure of about 5 bar in the storage tank 18 and increasing the pressure in the storage tank during outflow of the LN2 so that the storage tank pressure is always higher than the instantaneous gas pressure in the pressure vessel by at least about 3 bar. Thus, a final pressure of about 30 bar, for example, in the heat treating furnace 12 can be achieved by causing the pressure in the LN2 storage tank to be about 33 bar, for example, during the cycle of supplying the liquefied nitrogen to the heat treating furnace. Alternatively, the pressure in the storage tank can be raised by starting at a pressure of 5 bar and continuously raising it to about 33 or 35 bar during the filling operation. The high pressure needed in the LN2 storage tank is easily established by connecting it to the source 22 of nitrogen gas under very high pressure to the LN2 storage tank.
The process according to the present invention is preferably realized through use of the apparatus described above. However, it is contemplated that other systems can be designed for carrying out the process. The quenching process according to the present invention is preferably utilized in an industrial metal heat treating process. Such a process typically includes the steps of heating a load of metal work pieces in a heat treating furnace to a desired temperature and then holding the metal work pieces at this temperature for a period of time sufficient to effect a desired metallurgical change in the metal work pieces. The heat treating furnace may be a vacuum furnace or an atmosphere furnace. The desired change in the metal work pieces is often effected or locked in by cooling the metal work pieces at a rapid rate.
In the method according to the present invention the heated metal parts are cooled by application of a cooling gas, preferably nitrogen, at high pressure. The cooling gas is preferably injected into the furnace or quenching chamber by conducting LN2 from a local storage tank into the heat treating furnace chamber or into a standalone quenching chamber as the case may be. Feeding the LN2 into a furnace quench chamber at a high flow rate against a gas pressure that has built up to about 25 bar or more requires a pressure in the LN2 storage tank of at least about 30 bar or more. However, at such a pressure the boiling point of the LN2 rises to about −151° C., which is 45° C. higher than when the pressure in the storage tank is at 1 bar. The spraying of LN2 at a temperature of −151° C. into the high pressure quench chamber results in a reduction of the cooling capability of the quenching medium by about 22% as compared to spraying the LN2 at a temperature of −196° C. Therefore, more effective cooling with LN2 spray quenching can be provided when the LN2 is super-cooled. Super cooling of the LN2 can be accomplished by using the following steps.
Prior to the injection of LN2 into the heat treating furnace or quenching chamber, the LN2 is preferably held in the storage tank 18 at a relatively low pressure, for example at about 1 bar. As the process proceeds and LN2 flows toward the heat treating furnace or quenching chamber, the pressure in the storage tank 18 is increased to a pressure that is greater than the final pressure required for the specific gas quench cycle. Alternatively, the pressure in the LN2 storage tank can be set directly to a pressure of at least about 3 bar at the start of the quenching cycle and then, while the LN2 flows toward the furnace or quench chamber, the pressure in the LN2 storage tank is continuously increased at such a rate that the pressure is at any point of time during the quenching cycle at least 3 bar higher than the pressure in the furnace or quench chamber at the same time. The pressure in the storage tank is preferably increased or maintained, as the case may be, by injecting nitrogen gas at elevated pressure into the storage tank. The gas injection is preferably carried out by allowing nitrogen gas from the high pressure gas source 22 to flow into the storage tank 18 thereby providing a blanket of gas whose pressure is determined by the pressure regulator 26.
It is understood, that in carrying out the process of this invention, the LN2 will initially evaporate as it is conducted from the storage tank to the furnace or quenching chamber because the supply pipe from the storage tank to the furnace chamber will not initially be at cryogenic temperature. As the supply pipe cools down to cryogenic temperature, the nitrogen will enter the chamber as a combination of cold nitrogen gas and liquefied nitrogen. When the supply pipe has cooled to substantially cryogenic temperature, the LN2 will be conducted into the spray manifold in the furnace chamber and exit from the spray nozzles to be sprayed over the batches of metal work pieces. The conduction of the cooling gas in liquid form will provide a greater mass of the cooling gas into the furnace chamber thereby causing the gas pressure in the furnace chamber to rise rapidly. More specifically, it is expected that peak gas pressure for cooling in the furnace chamber can be achieved in 30 seconds or less from the start of the liquefied gas injection process.
During the injection of the cooling liquid into the furnace chamber, the vaporized nitrogen gas is preferably continuously circulated inside the chamber by means of the recirculation fan 13. The continuous circulation of the LN2 mist and the cold nitrogen gas causes the gas/mist mixture to penetrate into the lower layers of the work piece load so that the lower layers of the stacked baskets or containers are cooled at the same or a similar rate as the uppermost baskets of work pieces. As the nitrogen gas/mist mixture absorbs heat from the metal work pieces, it transforms to all gas and rapidly expands inside the pressure vessel. The rapid expansion of the gas causes the pressure to rapidly rise also.
Once the gas pressure inside the furnace chamber reaches the desired peak value, the injection of the LN2 can be stopped. The recirculation fan preferably continues to run so that the quenching gas is recirculated through the heat exchanger to remove additional heat from the load in the furnace chamber. The gas recirculation at the elevated pressure continues until the work pieces reach a preselected temperature in accordance with the known gas quenching processes.
Depending on the geometry of the load of metal parts, it may be advantageous to spray the liquid quenchant in a particular direction to maximize penetration of the gas/mist mixture into the work load. When such directional spraying is used, it may also be preferable to circulate the gas/mist mixture in a direction selected to further enhance contact of the cooling gas and mist with the metal parts. Therefore, in some embodiments the direction of circulation is selected to be parallel to the spraying direction. In another embodiment, the circulation of the gas and mist is circulated in a direction that is at an angle to the spraying direction, for example, at an angle of 90 degrees or 180 degrees relative to the spraying direction.
Referring now to
Depending on the overall load size, the section size of the parts in the load, and especially the type of steel or metal of the parts, the quenching speed of the second stage in the process of this invention (i.e., circulation of gas at high pressure) might not be sufficient. In such situation, it is possible to further supply the liquid quenchant into the furnace during (and vent off the vapor produced once it supersedes the chosen final peak pressure) for an additional time period during the first stage, until subsequently the transition to the second stage (pure high pressure gas quench) is made (stopping the flow of liquid). Such a process is exemplified in the following description of the example illustrated in
Referring now to
During further cooling in the third stage of the process according to this invention, i.e., pure gas quenching, the gas temperature decreases which causes the gas to contract, thereby reducing the pressure in the quenching chamber. In order to maintain the pressure during a given cooling stage constant, the pressure control system is preferably adapted to intermittently open the valve for the liquid quenchant and allow more liquid to enter the furnace. The evaporation of the additional liquid increases the pressure in the quenching chamber back to the desired level.
It will be appreciated by those skilled in the art that the apparatus according to the invention can be realized by configurations other than that described above and shown in
The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features or steps shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/468,267, filed Mar. 28, 2011, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61468267 | Mar 2011 | US |
