Gas quenching installation and the corresponding quenching method

Abstract

The invention relates to a gas quenching installation comprising a quenching cell (12), a buffer tank (10) which is intended to contain the quenching gas (6), gas connection means (32) which link the buffer tank (10) with the quenching cell (12) and gas supply means. Said gas supply means consist of an evaporator (8), the outlet (20) of which is connected to an inlet (22) in the buffer tank (10), and a high pressure storage tank (4) for the liquid-state quenching gas (6) which is connected to an inlet (16) in the evaporator (8), the connection means between the evaporator and the buffer tank having no gas compression means. The invention applies to installations used for the nitrogen quenching of steel parts.

Description

The present invention relates to a gas quenching plant, of the type comprising a quenching cell intended to receive objects to be quenched and suitable for withstanding a specified quenching pressure of the quench gas and, connected to this cell, pressurized quench gas delivery means.

It applies especially to gas quenching plants for steel components.

Gas quenching plants are known. Such a plant comprises a quenching cell, in which the objects to be quenched are placed, and, connected to this cell, pressurized quench gas delivery means. Such quenching plants generally include a buffer tank for intermediate storage of the gas, located between the gas supply and the quenching cell.

In the case of a quenching plant operating at high pressure (for example 20 bar and higher), the gas delivery means comprise a compressor permanently connected to the quench gas supply.

The use of a compressor in such a plant has in particular the following drawbacks: the investment cost of the compressor; the down-time of the quenching plant for the time needed to carry out maintenance on the compressor; the power consumption of the compressor; and the noise generated by the compressor.

The object of the invention is to alleviate these drawbacks by means of an inexpensive quenching plant.

For this purpose, the subject of the invention is a plant of the aforementioned type, characterized in that the gas delivery means comprise:

• • an evaporator, the outlet of which is connected to an inlet of the buffer tank via second connecting means; and
  • a high-pressure storage tank for storing the quench gas in the liquid state, under a storage pressure (P_storage), the said high-pressure storage tank being connected to an inlet of the evaporator (8) via third connecting means;
  • the said second connecting means having no gas compression means between the evaporator and the buffer tank. Depending on the particular embodiments of the plant, the latter have one or more of the following features:
  • the first shut-off means divide the said first connecting means into a buffer part associated with the buffer tank and into a cell part associated with the quenching cell and in that the volume (V_buffer) of the buffer tank and the volume (V_ct) of the buffer part of the connecting means form an effective buffer volume (V_buffer,eff), whereas the volume (V_cell) of the quenching cell and the volume (V_cc) of the cell part of the connecting means form an effective cell volume (V_cell,eff), the effective buffer volume (V_buffer,eff) being greater than the effective cell volume (V_cell,eff);
  • the effective buffer volume (V_buffer,eff) is greater than 1.2 times the effective cell volume (V_cell,eff), and especially between 1.4 times and 5 times this volume;
  • the buffer pressure (P_buffer) is higher than the quenching pressure (P_quench) and the effective buffer volume and the effective cell volume are related by the relationship: $V_{\mathrm{buffer},\mathrm{eff}}\ge V_{\mathrm{cell},\mathrm{eff}}\frac{P_{\mathrm{quench}}}{P_{\mathrm{buffer}}-P_{\mathrm{quench}}};$
  • the effective buffer volume and the effective cell volume are related by the equation: $V_{\mathrm{buffer},\mathrm{eff}}=V_{\mathrm{cell},\mathrm{eff}}\frac{P_{\mathrm{quench}}}{P_{\mathrm{buffer}}-P_{\mathrm{quench}}};$
  • the buffer pressure (P_buffer) is greater than or equal to 1.2 times the quenching pressure (P_quench) and is preferably between 1.2 and 1.8 times this pressure;
  • the buffer pressure (P_buffer) is between 15 and 35 bar;
  • the quenching pressure (P_quench) is between 5 and 25 bar and preferably between 10 and 20 bar;
  • the storage pressure (P_storage) is higher than the buffer pressure (P_buffer) and especially higher than 8 bar, preferably between 10 and 40 bar and even more preferably between 15 and 40 bar;
  • the plant furthermore includes an auxiliary quench gas supply connected to the buffer tank via auxiliary connecting means that are capable of supplying the buffer tank with auxiliary quench gas;
  • the auxiliary quench gas supply comprises at least one of the gases from the group formed by hydrogen and helium;
  • the said quench gas is nitrogen;
  • the said quench gas is CO₂ or comprises CO₂.

The subject of the invention is furthermore a process employing a plant as defined above, characterized by the following successive steps:

• • the buffer tank is filled with quench gas at the buffer pressure;
  • the object to be quenched is placed in the quenching cell;
  • the first controllable shut-off means are opened, establishing a connection between the buffer tank and the quenching cell until the quenching pressure is established in the quenching cell;
  • the object to be quenched is cooled to a specified temperature; and
  • the quenched object is removed from the quenching cell.

The invention will be more clearly understood on reading the description that follows, given solely by way of example and with reference to the appended drawing, the single FIGURE of which shows schematically a plant according to the invention.

The single FIGURE shows a gas quenching plant according to the invention, denoted by the general reference 2.

The plant is intended for quenching an object 3 by heat treatment. The object is, for example, a steel component.

The quenching plant 2 comprises a high-pressure storage tank 4 for storing a quench gas 6 in the liquid state, an evaporator 8 for evaporating this gas, a buffer tank 10 and a quenching cell 12.

The storage tank 4 is a “high-pressure” storage tank. The quench gas 6 is stored therein at a storage pressure P_storage above 8 bar, in this case between 15 and 40 bar. The quench gas 6 is, for example, nitrogen.

The storage tank 4 is provided with an outlet 14 that is connected to an inlet 16 of the evaporator 8 via a first connecting line 18. The evaporator 8 is preferably a “high-pressure” evaporator suitable for generating a gas at a high pressure from the storage tank 4. This pressure lies slightly below the storage pressure.

The evaporator 8 has an outlet 20 connected to an inlet 22 of the buffer tank 10 via a second connecting line 24, the said line being provided with a first shut-off valve 26. The first shut-off valve 26 is placed at the inlet of the buffer tank 10.

According to the invention, these connecting means have no gas compression means between the evaporator and the buffer tank.

The buffer tank 10 is suitable for withstanding a gas pressure, called the buffer pressure P_buffer, during the operation of the plant. This pressure is substantially identical to the pressure of the gas leaving the evaporator 8.

An outlet 28 of the buffer tank 10 is connected to an inlet 30 of the quenching cell 12 via a third connecting line 32, the said third connecting line being provided with a second shut-off valve 34.

The quenching cell 12 is suitable for withstanding a gas pressure, called the quenching pressure P_quench, during the quenching operation.

The actual buffer tank 10 has a volume V_buffer. The volume of the third line 32 is formed by a first part V_ct associated with the buffer tank 10, extending between the buffer tank 10 and the second shut-off valve 34, and a second part V_cc associated with the quenching cell 12, extending between the second valve 34 and this cell 12. The quenching cell has a volume V_cell.

The two volumes V_buffer and V_ct taken together define an effective buffer volume V_buffer,eff, whereas the two volumes V_cell and V_cc taken together define an effective cell volume V_cell,eff.

According to one advantageous embodiment of the invention, the effective buffer volume V_buffer,eff is greater than the effective cell volume V_cell,eff. Preferably, the effective buffer volume V_buffer,eff is greater than 1.2 times the effective cell volume V_cell,eff and is especially between 1.4 times and 5 times this volume.

The effective buffer volume V_buffer,eff advantageously satisfies the condition $V_{\mathrm{buffer},\mathrm{eff}}\ge V_{\mathrm{cell},\mathrm{eff}}\frac{P_{\mathrm{quench}}}{P_{\mathrm{buffer}}-P_{\mathrm{quench}}}.$ In one particular embodiment, the effective buffer volume and the effective cell volume are related by the equation $V_{\mathrm{buffer},\mathrm{eff}}=V_{\mathrm{cell},\mathrm{eff}}\frac{P_{\mathrm{quench}}}{P_{\mathrm{buffer}}-P_{\mathrm{quench}}},$ thereby making it possible to minimize the overall size of the tank 10.

The plant also includes means for creating a vacuum in the quenching cell 12. These vacuum means consist of a vacuum pump 36 connected to the quenching cell 12 via a fourth line 38.

The plant according to the invention operates in the following manner.

Initially, the buffer tank 10 contains quench gas at a residual pressure and the quenching cell 12 contains a residual atmosphere, generally at a pressure below atmospheric pressure. In the quenching cell, there is no object to be quenched.

The second shut-off valve 34 is closed and the first shut-off valve 26 is opened. The evaporator 8 is thus put into operation and produces quench gas, which is at approximately the buffer pressure P_buffer and a temperature T_buffer. This pressure P_buffer is in this case between 25 and 35 bar.

When the pressure in the buffer tank 10 reaches the pressure P_buffer, the evaporator 8 is stopped by closing the first valve 26. Typically, it takes between 5 and 20 minutes to fill the buffer tank 10.

During this time, the object 3 to be quenched undergoes a heat treatment (not shown).

The object 3 to be hot quenched in then placed in the quenching cell 12 and the latter is closed. The pressure within the quenching cell 12 is often the ambient pressure, i.e. about 1000 hPa (however, the cell could also be under vacuum initially), and the temperature is the ambient temperature.

Next, the second shut-off valve 34 is opened. The quench gas 6 contained in the buffer tank 10 flows rapidly via the third line 32 into the quenching cell 12 until the desired quenching pressure is reached. This quenching pressure P_quench is between 5 and 25 bar, preferably between 10 and 20 bar. Moreover, it is advantageous that the buffer pressure P_buffer be chosen between 1.2 and 1.8 times the quenching pressure so as to be able to use a buffer tank that is not very bulky.

When the object 3 to be quenched has reached the desired temperature, the quenching cell 12 is depressurized and the quenched object 3 is removed from the cell. Finally, the second shut-off valve 34 is closed and the first shut-off valve 26 is opened.

The treatment cycle may recommence.

The plant according to the invention has the following advantages.

Given that the effective buffer volume V_buffer,eff is greater than the effective cell volume V_cell,eff, the pressure of the gas in the buffer tank P_buffer is relatively low for a given quenching pressure P_quench. Consequently, the thickness of the wall of the buffer tank 10 may be relatively small.

Furthermore, this plant has no compressor placed between the evaporator 8 and the buffer tank 10, which avoids the consumption of electrical power during generation of a gas at a pressure P_buffer. The plant also has a short down-time thanks to the short maintenance time needed.

In addition, the plant is compact and generates little noise.

As a variant, the buffer tank 10 may be placed adjacent to the quenching cell 12 in such a way that the third line 32 may be omitted. In this case, the volumes V_ct and V_cc are equal to zero and the effective volumes V_buffer,eff and V_cell,eff are identical to the respective volumes of the buffer tank 10 and of the quenching cell 12.

Although the invention has more particularly been illustrated by a structure in which the buffer tank is supplied only from the high-pressure liquid storage tank/evaporator assembly, it is also possible and extremely advantageous according to the invention to provide the possibility of supplying the buffer tank also with an auxiliary quench gas via an auxiliary quench gas supply connected to the buffer tank via auxiliary connecting means, such an auxiliary quench gas advantageously comprising at least one of the gases from the group formed by hydrogen and helium, for example, so as to thus have available nitrogen/helium, nitrogen/hydrogen, CO₂/helium etc. quenching mixtures. Likewise, as will be clearly apparent to those skilled in the art, the terminology “high-pressure storage tank for storing the quench gas in the liquid state” has been used throughout the foregoing to mean that there is thus a high-pressure reserve of the quench gas in liquid form. This reserve will advantageously be quite simply in the form of a high-pressure storage tank, but it should be understood that it is also possible (for example for practical reasons connected with the local site), and without in any way departing from the scope of the present invention, to envision this “high-pressure storage tank” in the form combined with a low-pressure storage tank for storing the gas in liquid form, which feeds a system for pressurizing the quench gas in the liquid state. This being so, of course, with the proviso that the condition according to the invention, whereby the second means of connection between the evaporator and the buffer tank do not contain any means of compressing the gas, is indeed satisfied.

Claims

1-14. (canceled)

15. An apparatus which may be used for gas quenching comprising: a) a quenching cell to receive the objects to be quenched, wherein said cell can withstand pressurization with a quench gas up to a quenching pressure (Pquench); and b) a pressurized quench gas delivery means connected to said cell, wherein said gas delivery means comprises: 1) a buffer tank for containing said quench gas at a specified buffer pressure (Pbuffer); 2) a first gas connection means that connects said buffer tank to said cell; 3) a first controllable shut-off means for said first gas connection means; and 4) a feeding means for feeding said buffer tank with said quench gas at a pressure greater than about atmospheric pressure, wherein said feed means comprises: i) an evaporator; ii) a second connecting means connected to both said evaporator and said buffer tank, wherein said second connecting means comprises no gas compression means; iii) a high pressure storage tank for storing said quench gas in the liquid state at a storage pressure (Pstorage); and iv) a third connecting means connected to both said evaporator and said storage tank.

16. The apparatus of claim 15, wherein: a) said first shut off means divides said first connecting means into a buffer part associated with said buffer tank and into a cell part associated with said cell; b) the volume of said buffer tank (Vbuffer), and the volume of said buffer part (Vct) form an effective buffer volume (Vbuffer,eff); c) the volume of said cell (Vcell), and the volume of said cell part (Vcc) form an effective cell volume (Vcell,eff); and d) said effective buffer volume (Vbuffer,eff) is greater than said effective cell volume (Vcell,eff).

17. The apparatus of claim 16, wherein said effective buffer volume (Vbuffer,eff) is at least about 1.2 times greater than said effective cell volume (Vcell,eff).

18. The apparatus of claim 17, wherein said effective buffer volume (Vbuffer,eff) is between about 1.4 times and about 5 times greater than said effective cell volume (Vcell,eff).

19. The apparatus of claim 16, wherein: a) said buffer pressure (Pbuffer) is greater than said quenching pressure (Pquench); and b)⁢ ⁢Vbuffer, ⁢eff≥Vcell, ⁢eff⁢PquenchPbuffer-Pquench.

20. The apparatus of claim 19, wherein:

21. The apparatus of claim 15, wherein said buffer pressure (Pbuffer) is greater than or equal to about 1.2 times the quenching pressure (Pquench).

22. The apparatus of claim 21, wherein said buffer pressure (Pbuffer) is less than about 1.8 times said quench pressure (Pquench).

23. The apparatus of claim 15, wherein said buffer pressure (Pbuffer) is between about 15 and about 35 bar.

24. The apparatus of claim 15, wherein said quenching pressure (Pquench) is between about 5 bar and about 25 bar.

25. The apparatus of claim 24, wherein said quenching pressure (Pquench) is between about 10 bar and about 20 bar.

26. The apparatus of claim 24, wherein said storage pressure (Pstorage) is greater than said buffer pressure (Pbuffer).

27. The apparatus of claim 26, wherein said storage pressure (Pstorage) is greater than about 8 bar.

28. The apparatus of claim 27, wherein said storage pressure (Pstorage) is between about 10 bar and about 40 bar.

29. The apparatus of claim 28, wherein said storage pressure (Pstorage) is between about 15 bar and about 40 bar.

30. The apparatus of claim 15, further comprising an auxiliary quench gas supply connected to said buffer tank via an auxiliary connecting means capable of supplying said buffer tank with an auxiliary quench gas.

31. The apparatus of claim 30, wherein said auxiliary quench gas supply comprises at least one member selected from the group consisting of hydrogen and helium.

32. The apparatus of claim 15, wherein said quench gas comprises nitrogen.

33. The apparatus of claim 15, wherein said quench gas comprises carbon dioxide.

34. A method of operating the apparatus of claim 15, comprising: a) filling said buffer tank with said quench gas at said buffer pressure (Pbuffer); b) placing said object in said cell; c) opening said shut-off means until said quenching pressure (Pquench) is reached in said cell; d) cooling said object to a specified temperature; and e) removing said object from said cell.

35. A method of gas quenching comprising: a) placing objects in a quenching apparatus, said apparatus comprising: 1) a quenching cell to receive said objects to be quenched, wherein said cell can withstand pressurization with a quench gas up to a quenching pressure (Pquench); and 2) a pressurized quench gas delivery means connected to said cell, wherein said gas delivery means comprises: i) a buffer tank for containing said quench gas at a specified buffer pressure (Pbuffer); ii) a first gas connection means that connects said buffer tank to said cell; iii) a first controllable shut-off means for said first gas connection means; and iv) a feeding means for feeding said buffer tank with said quench gas at a pressure greater than about atmospheric pressure, wherein said feed means comprises: aa) an evaporator; bb) a second connecting means connected to both said evaporator and said buffer tank, wherein said second connecting means comprises no gas compression means; cc) a high pressure storage tank for storing said quench gas in the liquid state at a storage pressure (Pstorage); and dd) a third connecting means connected to both said evaporator and said storage tank; and b) filling said buffer tank with said quench gas at said buffer pressure (Pbuffer); c) placing said object in said cell; d) opening said shut-off means until said quenching pressure (Pquench) is reached in said cell; e) cooling said object to a specified temperature; and f) removing said object from said cell.