Patent Grant
6902635
This invention relates to thermal processing of workpieces and in particular to a multi-cell thermal processing unit comprising a plurality of thermochemical processing cells, wherein each cell is operated at a substantially fixed predetermined atmosphere and temperature.
Heat treating of metal is a commonly used technique to improve material characteristics of a workpiece for specific applications. For example, surface hardening involving a change in the composition of the outer layer of an iron-base alloy through application of an appropriate thermal treatment. Typical processes are carburizing carbo-nitriding and nitriding. Application of such processes enhances wear resistance, corrosion resistance, and fatigue strength of such treated workpieces. Other heat treatment processes involve annealing and aging.
However, in order to reproducibly obtain predetermined results using these surface hardening processes control of operating parameters such as composition of the atmosphere, temperature, and pressure during the hardening process is required. This is particularly necessary for nitriding processes. From the control point of view nitriding is a very complex process influenced by thermodynamic relations at the gas/metal interface during breakup of the atmosphere's components. The exact nature of the reactions taking place, i.e. mass transport of the gaseous phase, adsorption, diffusion and nitride phase formation is determined by the kinetics of this process. In order to control this process accurate provision of the atmosphere's components as well as temperature and pressure are essential.
Normally, a heat treating process of a workpiece comprises a number of processing steps such as preheating, carburizing or nitriding, and cooling or quenching. Numerous prior art systems have been disclosed teaching cascading of various chambers for preheating, thermal treating and cooling in order to avoid, for example, cooling of the nitriding furnace for loading and unloading of a batch of workpieces. Such systems are disclosed, for example, in U.S. Pat. No. 3,598,381 issued to Schwalm et al. in Aug. 10, 1971, U.S. Pat. No. 3,662,996 issued to Schwalm et al. in May 16, 1972, U.S. Pat. No. 4,653,732 issued to Wunning et al. in Mar. 31, 1987, U.S. Pat. No. 4,763,880 issued to Smith et al. in Aug. 16, 1988, and U.S. Pat. No. 5,052,923 issued to Peter et al. in Oct. 1, 1991, which are incorporated hereby for reference.
However, these systems are very inefficient for modern applications. Nowadays, use of thermal processing of metal workpieces in order to improve their material characteristics is numerous. This results in an increasing demand of a plurality of differently treated workpieces meeting different material characteristic requirements. The above mentioned heat treating systems only allow treatment of workpieces using a same process. Furthermore, change of thermochemical processing parameters such as atmosphere composition or temperature for different workpieces requires change of the operating parameters of the heat treating cell of the system. Therefore, a complex heat treating cell being able to provide numerous different heat treating parameters is required. Additionally, change of the heat treating parameters requires a substantial amount of time for adjusting the heat treating cell, which is not acceptable in modern manufacturing processes. Another disadvantage of these prior art systems is the inefficient use of the various system components through the cascading of these components. For example, the thermochemical processing step requires substantially more time than the cooling or quenching step. Thus, during the thermochemical processing step the cooling or quenching cell is sitting idle.
However, it would be advantageous for modern manufacturing applications to divide the thermal process into steps performed under substantially fixed conditions or performed within a narrow range of conditions based on the different processing steps required for the different heat treating of workpieces. Manufacturing and operating costs would be substantially reduced if each of the processing modules is operated at substantially fixed parameters such as atmosphere composition and temperature.
It is, therefore, an object of the invention to provide a method for thermal processing workpieces by dividing the thermal process into steps performed under substantially fixed conditions or performed within a narrow range of conditions based on the different processing steps required for the different heat treating of workpieces.
It is further an object of the invention to provide a multi-cell thermal processing unit wherein each of the processing cells is operated at substantially fixed operating parameters.
The multi-cell thermal processing units according to the invention are highly advantageous for modem thermochemical processing applications. For example, keeping the operating conditions in each of the thermochemical processing cells constant or varying these conditions only within a range smaller than the range required for a complete thermochemical processing process provides considerable time as well as energy savings. Furthermore, operating a thermochemical processing cell under substantially constant conditions considerably facilitates control functions for providing predetermined conditions. This allows a substantially more accurate control of the heat thermochemical processing conditions which is especially advantageous for reproducibly thermochemical processing workpieces using nitriding processes such as the NITREG® process.
In accordance with the present invention there is provided a multi cell thermal processing unit comprising:
an air tight expandable common chamber module for containing an atmosphere other than ambient air, the chamber module comprising N ports;
a loading cell linked to the first port of the common chamber module via a gas tight door for providing to and receiving from the common chamber module a first and a second workpiece;
a first thermochemical processing cell linked to the second port of the common chamber module via a heat insulating door, the first thermochemical processing cell for providing substantially fixed first thermochemical processing conditions for thermochemical processing the first workpiece;
a second thermochemical processing cell linked to the third port of the common chamber module via a heat insulating door, the second thermochemical processing cell for providing substantially fixed second thermochemical processing conditions for thermochemical processing the second workpiece;
a transport mechanism disposed within the common chamber module for handling and transporting the first and the second workpiece within the thermal processing unit; and,
N-3 sealing covers for airtightly sealing the remaining N-3 ports, the covers being removable for mating the common chamber module to a processing cell or another common chamber module.
In accordance with the present invention there is further provided a multi cell thermal processing unit comprising:
an air tight common chamber for containing an atmosphere other than ambient air;
a loading cell linked to the common chamber via a gas tight door for providing to and receiving from the common chamber a workpiece;
a first thermochemical processing cell linked to the common chamber via a heat insulating door, the first thermochemical processing cell for providing substantially fixed first thermochemical processing conditions for nitriding the workpiece;
a second thermochemical processing cell linked to the common chamber via a heat insulating door, the second thermochemical processing cell for providing substantially fixed second thermochemical processing conditions for second nitriding treatment of the workpiece;
a cooling cell linked to the common chamber for controllably cooling the workpiece; and,
a transport mechanism disposed within the common chamber for handling and transporting the first and the second workpiece within the thermal processing unit.
In accordance with the present invention there is yet further provided a multi cell thermal processing unit comprising:
an air tight common chamber for containing an atmosphere other than ambient air;
a loading cell linked to the common chamber via a gas tight door for providing to and receiving from the common chamber a first and a second workpiece;
a preheating cell linked to the common chamber via a heat insulating door, the preheating cell for providing a substantially fixed temperature for activating the workpiece;
a first thermochemical processing cell linked to the common chamber via a heat insulating door, the first thermochemical processing cell for providing substantially fixed first thermochemical processing conditions for thermochemical processing the first workpiece;
a second thermochemical processing cell linked to the common chamber via a heat insulating door, the second thermochemical processing cell for providing substantially fixed second thermochemical processing conditions for thermochemical processing the second workpiece; and,
a transport mechanism disposed within the common chamber for handling and transporting the first and the second workpiece within the thermal processing unit.
In accordance with the present invention there is yet further provided a multi cell thermal processing unit comprising:
an air tight common chamber for containing an atmosphere substantially comprising an inert gas;
a loading cell linked to the common chamber via a gas tight door for providing to and receiving from the common chamber a workpiece;
a preheating cell linked to the common chamber via a heat insulating door, the preheating cell for providing a substantially fixed temperature for heating the workpiece to a predetermined temperature;
a first thermochemical processing cell linked to the common chamber, the first thermochemical processing cell for providing a first portion of thermochemical processing conditions of a thermochemical processing process for thermochemical processing the workpiece;
a second thermochemical processing cell linked to the common chamber, the second thermochemical processing cell for providing a second portion of the thermochemical processing conditions of the thermochemical processing process for thermochemical processing the workpiece; and,
a transport mechanism disposed within the common chamber for handling and transporting the workpiece within the thermal processing unit.
In accordance with an aspect of the present invention there is provided a method for thermal processing a workpiece comprising the steps of:
providing a first workpiece to a first thermochemical processing cell linked to a common chamber containing an atmosphere other than ambient air;
thermochemical processing the first workpiece by providing a first portion of thermochemical processing conditions of a first thermochemical process;
transferring via the common chamber the first workpiece from the first thermochemical processing cell to a second thermochemical processing cell linked to the common chamber after elapse of a first predetermined time interval;
thermochemical processing the first workpiece by providing a second portion of the thermochemical processing conditions of the first thermochemical processing process; and,
removing the first workpiece from the second thermochemical processing cell after elapse of a second predetermined time interval.
Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:
a is a simplified flow diagram of a method for thermal processing according to the invention;
b is a simplified flow diagram illustrating a comparison of the timing of a simple process flow divided into three processing steps;
c is a simplified flow diagram illustrating a comparison of the timing of a simple process flow divided into three processing steps;
In the following description the expression workpiece is used to refer to any kind of manufactured metallic component such as springs, valves, piston rings, etc. for thermal processing. Furthermore, the expression workpiece also includes a batch of components, which are treated together and are provided, for example, in a racking. Moreover, a complete process including steps such as preheating, thermochemical processing, quenching etc. is called thermal processing. Whereas, the expression thermochemical processing includes only operations combining the effects of heat and of an active atmosphere such as nitriding, carburizing, nitro-carburizing, or comparable processing steps.
Some of these drawbacks of the prior art are overcome by the thermal processing installation disclosed by Pelissier in U.S. Pat. No. 6,065,964 issued in May 23, 2000. Pelissier teaches a vaccum thermal processing installation for use under a rarefied atmosphere including several processing cells linked to a common air-tight vacuum chamber. By feeding all workpieces through a common vaccum chamber, improved vaccum conditions are achievable within each oven chamber. This has specific advantages to vacuum thermochemical processes, but is of little or no advantage to a nitriding process wherein increased vacuum quality of successive chambers is not necessary. Though, the system of Pelissier, appears similar to the system described herein, it is a system that would not have been considered with foresight in looking toward the inventive solution since, nitriding does not require improved vacuum atmosphere quality. That said, Pelissier does teach a single common low pressure atmosphere chamber for use in loading and unloading of workpieces into ovens for independent processing therein. The main advantage of this installation is the use of only two air-tight doors for operating a plurality of processing cells and a gas quenching cell linked to the common chamber, thus reducing manufacturing costs and improving manufacturing quality.
In order to provide a better understanding of the invention, flow diagrams illustrating processing steps and their possible interconnection during operation of prior art systems will be described first, followed by a comparison with flow diagrams illustrating possible processing flows using the thermal processing unit according to the invention.
Referring to
An improvement of the above prior art systems is obtained using the system disclosed by Pelissier in U.S. Pat. No. 6,065,964 and shown in the flow diagram of FIG. 2. Linking a plurality of thermochemical processing cells, a loading cell and a quenching cell to common chamber provides increased flexibility. Here, one loading cell and one quenching cell are used to serve a plurality of thermochemical processing cells resulting in a more efficient use of the loading and the quenching cell. It allows parallel operation of the thermochemical processing cells and, for example, use of the loading cell and the quenching cell while at a same time workpieces are processed in some of the thermochemical processing cells. Furthermore, it allows expansion of the system by just adding the required components.
Referring to
b and 3c illustrate a comparison of the timing of a simple process flow divided into three processing steps, for example, a thermochemical processing step 1 requiring 30 min, followed by a thermochemical processing step 2 requiring 60 min and a thermochemical processing step 3 requiring 25 min. Provision of one thermochemical processing cell for step 1, two cells for step 2 and one cell for step 3 instead of one cell for all three steps results in considerable time savings as illustrated in
The diagram shown in
Optionally, the method for thermal processing according to the invention includes thermochemical processing steps for different thermochemical processing processes combined in one processing unit and possible interconnection of same. For example, a thermochemical processing cell for nitriding is used for performing a step of a nitro-carburizing process.
Further optionally, the method for thermal processing according to the invention includes other thermal processing steps such as annealing to relieve rolling, forging, or machining strains in a workpiece before thermochemical processing and aging to recover a workpiece from unstable conditions of its structure induced by quenching.
Referring to
The common chamber of the multi-cell thermal processing unit 100 shown in
Referring to
Optionally, sections of the common chamber are separated, for example, by a gas tight door 250. For example, this allows separating the section linked to the thermochemical processing cells 216-222 from the rest of the common chamber reducing the risk of contaminating the atmospheres in the thermochemical processing cells.
Further optionally, the thermal processing unit according to invention comprises a plurality of thermochemical processing cells for providing thermochemical processing conditions for different thermochemical processing such as nitriding as well as carburizing in one thermal processing unit.
Referring to
The multi-cell thermal processing units according to the invention are highly advantageous for modern thermochemical processing applications. For example, changing the operating conditions within a thermochemical processing cell requires a substantial amount of time and energy. Therefore, keeping the operating conditions in each of the thermochemical processing cells constant or varying these conditions only within a range smaller than the range required for a complete thermochemical process provides considerable time as well as energy savings. Moreover, it allows use of thermochemical processing cells, which are operable within narrower operating limits considerably reducing manufacturing and operating costs of each of the thermochemical processing cells. This allows, for example, use of more cells at a same cost further increasing flexibility. Additionally, operating a thermochemical processing cell under substantially constant conditions reduces material fatigue prolonging its lifetime.
Furthermore, operating a thermochemical processing cell under substantially constant conditions considerably facilitates control functions for providing predetermined conditions. This allows a substantially more accurate control of the thermochemical processing conditions which is especially advantageous for reproducibly thermochemical processing workpieces using nitriding processes such as the NITREG® process. Therefore, the multi cell thermal processing unit according to the invention provides the potential to accurately control the conditions for each step of a complex modern nitriding process comprising the steps of activating, nitriding, post nitriding treatment and cooling. Activation of the workpiece is provided in a preheating cell providing a substantially fixed preheating temperature. The workpiece is then transferred to a first thermochemical processing cell for nitriding where the thermochemical processing conditions are provided such that a controlled nitriding potential—expressed as the ratio of ammonia and hydrogen partial pressures—is obtained. Then the workpiece is transferred to a second thermochemical processing cell for post nitriding treatment such as superficial oxidation. Finally, the workpiece is transferred to a cooling cell for controlled cooling to room temperature.
Numerous other embodiments of the invention will be apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2000664 | Hayes | May 1935 | A |
| 3598381 | Schwalm et al. | Aug 1971 | A |
| 3662996 | Schwalm et al. | May 1972 | A |
| 3926415 | Konas et al. | Dec 1975 | A |
| 4653732 | Wunning et al. | Mar 1987 | A |
| 4763880 | Smith et al. | Aug 1988 | A |
| 4951601 | Maydan et al. | Aug 1990 | A |
| 5052923 | Peter et al. | Oct 1991 | A |
| 5868871 | Yokose et al. | Feb 1999 | A |
| 6065964 | Pelissier | May 2000 | A |
| 20010015074 | Hosokawa | Aug 2001 | A1 |
| Number | Date | Country |
|---|---|---|
| 1 193 317 | Apr 2002 | EP |
| 1 229 137 | Aug 2002 | EP |
| 08 178 535 | Jul 1996 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20030116894 A1 | Jun 2003 | US |
