WO2017122044A1

WO2017122044A1 - Equipment for ion nitriding/nitrocarburizing treatment comprising two furnace chambers with shared resources, able to run glow discharge treatment continuously between the two chambers

Info

Publication number: WO2017122044A1
Application number: PCT/IB2016/050144
Authority: WO
Inventors: Andrés BERNAL DUQUE; Santiago VARGAS GIRALDO
Original assignee: Ion Heat S.A.S
Priority date: 2016-01-13
Filing date: 2016-01-13
Publication date: 2017-07-20

Abstract

This invention relates to an equipment for ion nitriding/nitrocarburizing treatment comprising two treatment chambers that can share the resources for gas supply, plasma generation, and vacuum system, furthermore involving special hardware and software configurations which enable continuous glow discharge treatment between the two chambers. Each chamber has two operating modes; one relates to the heating of work pieces to the temperature necessary for nitriding/nitrocarburizing treatment, which is done by convective heating in controlled atmosphere at positive pressure with forced circulation. The other one relates to the glow discharge treatment, carried out in controlled atmosphere with gas supply, temperature and vacuum control which, combined with an applied external electrical field generates the glow discharge between work piece (cathode) and furnace wall (anode). Two production orders can run, one in each chamber, simultaneously, such that the heating in one chamber can be completed while the other chamber is performing glow discharge treatment.

Description

Equipment for ion nitriding/nitrocarburizing treatment comprising two furnace chambers with shared resources, able to run glow discharge treatment continuously between the two chambers.

FIELD OF THE INVENTION

This invention relates to an ion nitriding treatment plant or equipment in which ionized gases, generated by an applied electrical field, are used to form nitrides at the surface of different materials with the aim of producing local hardening of the same.

BACKGROUND OF THE INVENTION

Ion nitriding treatment is generally carried out at temperatures between 200-600 °C in a vacuum furnace. At this temperature, the parts to be nitrided are subjected to a glow discharge, generated by an applied electrical field. The parts to nitrided are connected as cathode and the furnace walls act as the anode. When nitrogen containing gas is released in the vacuum furnace, it is ionized and the atoms accelerate toward the cathode (work piece) creating a bombardment of ions on its surface, thereby forming a nitride containing layer on the work piece. In this way, the plasma nitriding process uses the active species that are generated during an electrical discharge in gases at low pressures to obtain a nitriding potential. The processes of ionization and recombination of the plasma generate active species that allow the formation of nitrides. After the sought-after nitride layer is obtained, the parts are cooled in the furnace in a controlled atmosphere to minimize dimensional deformations and the formation of oxides.

Accordingly, the ion nitriding treatment can be divided into three steps consisting of heating in a controlled atmosphere (1), glow discharge treatment for plasma cleaning and nitriding/nitrocarburizing (2), and cooling (3). Traditionally, heating of the pa rts was carried out by the glow discharge in a vacuum, however later systems incorporated heating elements elements in the exterior furnace walls to heat its interior, thereby increasing the thermal efficiency of the process and reducing the problems of arcing and degasing of the vacuum chamber. The heating elements can thus be used to provide step 1 of the ion nitriding process. When practicing ion nitriding treatment, it becomes clear that the process has a fourth step: loading/unloading of the parts. For treating large volumes, more than one load is often needed and to improve process efficiency, nitriding systems with two furnace chambers have been developed, often referred to as tandem systems. These provide the possibility of running a treatment in one chamber while the other chamber is being loaded, therefore saving the time for this fourth step between two treatments. However, although the second chamber can be loaded and sealed while treatment in the first chamber is running, currently it cannot be heated until the cooling step begins in the first chamber. The reason for this is that to be able to run the heating step in the second chamber while the first one is still under glow discharge treatment, it is necessary to design and build a number of features in the hardware and the software that allow this.

According to the above, the state of the art include a plurality of disclosure related to this kind of equipment or treatment process, such as document EP 0159222, which teaches the treatment, which endows steel or cast iron parts with resistance to wear and to corrosion, being characterized in that the surface of the parts is subjected successively to ionic nitriding, ionic oxynitriding and ionic oxidation.

Similarly, document US 4194930 discloses an ion-nitriding process wherein a workpiece having at least one aperture is subjected to a DC voltage in an atmosphere of nitrogen-containing gas, characterized in that a first nitriding step is carried out under a vacuum which is strong enough to suppress arc discharge on the workpiece, and a second nitriding step is carried out under a weaker vacuum as compared to that in the first step such that glow discharge is produced even in the aperture of the workpiece.

On the other hand, document US 4109157 divulges an apparatus for ion-nitriding comprising: (a) a vacuum chamber sectioned into a heating area having insulating chamber walls and a discharge nitriding area having electrically conductive chamber walls; (b) a first transferring means disposed in said heating area for transferring a workpiece; (c) heating means disposed in said heating area for pre-heating the workpiece up to the temperature at which the workpiece can be glow discharge nitrided; (d) electrically conductive transferring means disposed in said discharge nitriding area for receiving the pre-heated workpiece from said first transferring means; (e) heat- retaining means disposed in said discharge nitriding area for maintaining the pre-heated workpiece at a temperature suitable for glow discharge nitriding; and (f) discharge means to generate glow discharge between said conductive chamber wall of said discharge nitriding area as anode and said electrically conductive transferring means as cathode.

Furthermore, another document related to this technology is US 4179617 which discloses an ion- nitriding apparatus wherein heating and nitriding of workpieces can be carried out at high thermal efficiency, with excellent uniformity and in a short time by the combined use of glow discharge and heat generated by a heat-producing element. Heating efficiency can be raised in heating of workpieces by lessening the heat value to be released out of the furnace, and overheating of workpieces can be prevented during nitriding by increasing the heat value to be released out of the furnace in proportion to the increase of glow discharge output, and thus the nitriding efficiency can be raised.

A similar process is disclosed in document US 4212686 which generally teaches an ion-nitriding process in which workpieces are subjected to a two-step glow discharge in a nitrogen and hydrogen atmosphere, there being a larger nitrogen to hydrogen ratio in the second step.

In the same manner, document US 4460415 discloses a method for nitriding materials using a glow discharge in an atmosphere of nitrogen or gas mixture at a suitable pressure, wherein the nitriding treatment can be combined with a plasma aided coating process and the temperature control during both processes can be achieved with the aid of separate filament. Thus, the nitriding unit can be a separate rig or a part of the coating unit. Moreover, the method can be used to increase the wear resistance of a work piece by increasing the hardness of its surface. Because of the low pressure used in the nitriding process the same equipment can be used to produce a separate hard and wear resistant compound or alloy coating on the nitrided surface to further increase the hardness of the uppermost surface. The main goal of the method is in increasing the wear and corrosion resistance of machine parts and tools.

In addition to the above, another document in the prior art corresponding to US 4704168 discloses a surface of a steel substrate which is nitrided without external heating by exposing it to a beam of nitrogen ions under a low pressure, wherein the pressure is much lower than that employed for ion-nitriding, and an ion source is used instead of a glow discharge. Accordingly, both of these features reduce the introduction of impurities into the substrate surface.

Now, document US 5176760 describes a hard wear-resistant and corrosion- and oxidation- resistant stainless steel article made by precision machining a work piece of approximately the size and shape of the desired article, then subjecting the resulting cold worked article to ion bombardment until the article is nitrided to a depth of about 0.002 inch. Finally the nitrided article is subjected to an atmosphere of argon, nitrogen, and oxygen until the resulting ion bombardment has penetrated to a depth in the article surface of about 0.0001 inch. On the other hand, document US 6179933 discloses a process of manufacturing a rolling-element bearing component from pre-hardened steel, wherein as a final step in the manufacturing process the component is subjected to a plasma (ion) nitriding treatment by exposing the component to plasma created by subjecting a gaseous atmosphere composed of substantially 2 % N₂ and 98 % H₂ to electrical energy for a short time duration of about 1 to 2 hours, to create a hardened outer surface case which is devoid of any compound layer.

Finally, document US 2003141186 divulges a system for performing Physical Vapor Deposition (PVD) of metallic nitride, wherein the improved performance is provided by a method of increasing the partial pressures of nitrogen or other active gases near the wafer surface through initial introduction of the argon or other neutral gases alone into an ionized metal plasma PVD chamber through an upper gas inlet at or near the target, initiating the plasma in the presence of argon or other neutral gases alone, after which nitrogen or other active gases are introduced into the chamber through a lower gas inlet at or near the wafer surface to increase deposition rates and lower electrical resistivity of the deposited metallic layer. This document also discloses an apparatus for carrying out the invention including a source of argon near the target surface and a source of nitrogen integral to the substrate support thereby delivering nitrogen near the substrate surface.

SUMMARY OF THE INVENTION

In view of the above mentioned deficiency of the current tandem nitriding systems, the present invention provides a tandem nitriding system which aims at increasing the efficiency of the process by allowing continuous glow discharge treatment between two furnace chambers in a tandem nitriding plant, in part by sharing the resources used for gas flow control, vacuum lines, and glow discharge generation. Furthermore, the invented system has the following features which makes this possible: 1) Special software algorithm which allows simultaneous control of the atmospheres in the two chambers. 2) Special hardware configuration which allows gas flows to the two chambers at the same time. 3) Special software algorithm which allows control of the heating ramp in both chambers at the same time. 4) Specia l hardware configuration with SCR controls which allow electrical power to be fed to independent heating elements of both chambers at the same time. 5) Special software algorithm which controls the use and availability of each of the shared resources making the system a tandem plant. The combination of these features allow the glow discharge treatment to run in chamber 1 while chamber 2 heats the parts until these reach the temperature necessary for glow discharge treatment, such that when chamber 1 goes into the cooling step, the glow discharge treatment can start immediately in chamber 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The function and advantages of the present invention can be understood further by the detailed description made with reference to the drawings wherein:

FIG. 1 depicts a descriptive diagram of the equipment.

FIG. 2 depicts overlapping curves of the processes running in the two chambers in a temperature against time graph.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the tandem nitriding plant with its two furnace chambers (0), each consisting of a fixed base plate (1) and a removable case (2), is shown. Elements 3 are locking rings which seal each of the chambers with the base plate. Each chamber contains a rack with three levels or treatment zones; elements 4 are supporting plates on which the load is placed, elements 5 are temperature sensors that generate the signals that are sent to the controller which in turn regulates the heating (6) and cooling (7) devices. Elements 8, 9, and 10 correspond to an internal ventilator, pressure sensor, and ventilation valves, respectively. Selection of nitrogen input in each chamber is done through automatic valves (11) on each tube line of this gas, and the flow of nitrogen and other gases from their respective tanks (12) is regulated by flow controllers (13).

Elements 14, 15, and 16 correspond to isolating valves for the vacuum line, the vacuum pump, and a conductance controlling valve, respectively. To generate the plasma glow discharge, the system uses a source of controlled potential (17), which allows generation of the electrical glow discharge between the load to be treated on the plates (element 1, cathode) and the furnace wall (element 2, anode). The glow discharge will take place in chamber 1 or 2 according to the configuration of the potential selecting switches (18).

As to the function of the invention, each furnace chamber has two operating modes: A) Convection heating furnace with controlled atmosphere at positive pressure with forced circulation.

B) Ion nitriding or ion nitrocarburizing furnace at low pressure (1-10 Torr).

In operating mode A, convection heating is achieved by the combination of the external heating devices (6), the inlet of pressurized nitrogen gas through the lower valves (11), and a forced circulation caused by element 8. This element is responsible for pushing the cold nitrogen gas up through the furnace walls for its heating, and at the same time it draws down heated nitrogen from the top part of the furnace through the central zone, improving in this way the temperature homogeneity of the chamber. The internal pressure sensor (9) measures the pressure changes due to the intake of nitrogen and the outlet of air through the ventilation valve (10). In operating mode B, the vacuum pump (15) and the isolation valves (14) to the chamber are used to reduce the pressure used during the heating step. The pressure control is done via the control of the vacuum line conductance with the throttle valve (16). Gas mixtures of nitrogen, hydrogen, methane and argon, are introduced into the furnace chamber in the upper part and regulated by flow controllers (12). When controlled atmosphere conditions are established, element (17) is activated and directed to the concerned chamber by the potential selecting switches (18) to generate the plasma glow discharge, according to the above description.

Each batch of work pieces ran in each chamber is called a production order (PO). Each production order has several steps governed by a recipe. Each step of the recipe demands the use of different shared resources of the plant, i.e. vacuum pump (15), mass flow controllers (12) and plasma generator (17), according to the recipe parameters set by the user. The software algorithm allows the user to run two production orders, one in each chamber, simultaneously and predicts when to start or change from one step to the next one according to the usage of the shared resources previously described. It calculates the ideal point in time to start heating the second PO in mode A, in its corresponding chamber, as to comply with the graph in Fig 2, i.e. matching the point in time where the first PO reaches the cooling starting point and the second PO reaches the set process temperature according to its respective recipe.

In a preferred embodiment of the invention, figure 2 shows the functioning of the equipment of the present invention, in which the solid line corresponds to one of the chambers (0) and the dotted line corresponds to the other chamber (0), wherein once the process temperature is reached by the first chamber, that temperature is kept during the entire process. When the first chamber is ready to begin the cooling stage the other chamber has already reached the process temperature, as can be seen in the graph.

In this regard, it is clear that one of the chambers starts the heating process while the other one is still under glow discharge in order to decrease the floor to floor time from batch to batch of the system, thereby increasing its productivity.

Claims

An equipment or plant for ion nitriding/nitrocarburizing treatment comprising two furnace chambers (0), each consisting of a fixed base plate (1) and a removable case (2); locking rings (3) which seal each of the chambers (0) with the base plate (1), each chamber containing a rack with three levels or treatment zones; supporting plates (4) on which the load is placed; temperature sensors (5) which generate the signals being sent to the controller which regulates the heating (6) and cooling (7) devices; an internal ventilator (8), pressure sensor (9), and ventilation valves (10), wherein the selection of nitrogen input in each chamber (0) is made through automatic valves (11) located on each tube line of this gas, and the flow of nitrogen and other gases from their respective tanks (12) is regulated by flow controllers (13); isolating valves (14) for the vacuum line, the vacuum pump (15), and a conductance controlling valve (16); a source of controlled potential (17), which allows generation of the electrical glow discharge between the load to be treated on the plates (4) and the furnace wall (2), wherein the glow discharge is carried out in chamber 1 or 2 according to the configuration of the potential selecting switches (18), and wherein one of the chambers starts the heating process while the other one is still under glow discharge in order to decrease the floor to floor time from batch to batch of the system, thus increasing its productivity.

The equipment or plant according to claim 1, wherein each furnace chamber has two operating modes defined as: a) convection heating furnace with controlled atmosphere at positive pressure with forced circulation; and b) ion nitriding or ion nitrocarburizing furnace at low pressure (1 ¹⁰ Torr).

The equipment or plant according to claim 2, wherein convection heating is achieved by the combination of the external heating devices (6), the inlet of pressurized nitrogen gas through the lower valves (11), and a forced circulation caused by element 8.

The equipment or plant according to claim 2, wherein in operating mode B, the vacuum pump (15) and the isolation valves (14) to the chamber reduce the pressu re used during the heating step, wherein the pressure control is carried out via the control of the vacuum line conductance with the throttle valve (16).

The equipment or plant according to claims 1 to 4, wherein gas mixtures of nitrogen, hydrogen, methane and argon, are introduced into the furnace chamber in the upper part and regulated by flow controllers (12) and when controlled atmosphere conditions are established, element (17) is activated and directed to the concerned chamber by the potential selecting switches (18) to generate the plasma glow discharge.

The equipment or plant according to claim 5, in which the two operating modes defined in claim 2 can run simultaneously, one in each chamber.

The equipment of claim 6 further including a switch controlled system that allows an immediate change from operating mode A to mode B in one chamber and vice versa in the other one.

The system of claim 7, wherein ion nitriding and/or ion nitrocarburizing treatment can run continuously between the two chambers, starting in one chamber immediately after the end of treatment in the other.

The equipment of claim 8, further containing a software algorithm that allows the user to run two production orders, one in each chamber, simultaneously and predicts when to start heating the load in chamber two or change from one step to the next one according to the usage of the shared resources defined in claim 1 as elements 12, 15 and 17.