Patent Application
20090297725
The present invention relates to methods and apparatus for heating a metal substrate to achieve a diffusion surface layer on the substrate.
Duplex surface treatments have traditionally comprised forming a nitrided surface on the substrate followed by a physical vapour deposition of a coating such as titanium or chromium nitride or carbon nitrocarburising onto the surface as an adhered coating. Some work has also been carried out where the surfacing material is diffused into the surface zone of the substrate simultaneously as nitrogen diffuses towards the surface making a chromium or titanium nitride or carbon nitride layer on the surface. The published patent specification of European Patent Nos. 0471276, 0252480, 0303191 and an International Publication Number WO/47794 disclose such treatment methods. Such methods are capable of providing a better performing surface treatment because, the surface layer is a diffusion layer and not simply a coating layer adhered to the substrate, however, practical control of the required materials and parameters to achieve this desirable result has proven to be quite difficult. The use of a halide gas such as HCl mixed with a reactive gas or a combustible gas such as hydrogen and/or ammonia leads to problems in the construction of the mixing gas panel. The halide gas can react instantly at low temperatures with ammonia forming ammonium chloride which may block the gas pipes and even leak back into the solenoid valves and flow meters of the gas delivery equipment causing blockages and potential damage to the equipment. Thus it is desirable to isolate the possible reactive gases from the halide gas until they are ready to mix in the fluidized bed furnace and then react with the metal powders to achieve the desired metal diffusion. However, when gases are separately introduced into a fluidized bed, it is difficult to get uniform mixing of the gases within the bed and then uniformity of treatment of the product being treated is difficult.
The objective therefore, of the present invention is to provide methods, and apparatus for use in the methods, that will enable diffusion surface layers to be reliably, safely and economically formed on metal substrates.
According to a first aspect of this invention there is provided a method of forming a diffusion surface layer on a surface of a metal substrate, said method including:
In one preferred embodiment, the treatment steps (ii) might include mechanical treatment such as polishing the surface to remove any said surface oxide. In an alternative to the aforesaid, the substrate from the first stage may be transferred to the second stage while maintaining an inert atmosphere surrounding the substrate. In yet another possible alternative any existing surface oxide is removed by treatment in the second stage of the process. Such treatment in the second stage of the process may include treating the surface of the substrate with a combination of the halide gas and hydrogen.
Whilst it is generally preferred that the first stage of the process be carried out in a fluidized bed furnace, conveniently separate from the fluidized bed furnace used in the second stage of the process, this is not essential and the first stage of the process might be performed in any one of a salt bath, gas heat treatment equipment, or vacuum plasma equipment. It is possible also for the first and second stages of the process to be carried out in the same fluidized bed furnace but at different times.
In a further preferred aspect, the first stage of the process may be carried out with the supply of ammonia gas to the fluidized bed furnace, the ammonia gas being less than 20% of the complete gas flow to the fluidized bed furnace. Conveniently the ammonia gas comprises between 5 and 10% of the total gas flow to the fluidized bed furnace utilized in the first stage of the process.
Conveniently, the metal substrate may be made from metal materials such as a ferrous based metal including steel and steel alloys, as well as from titanium, aluminium, and alloys of titanium and aluminium. Preferably the metal substrate is a preformed or pre-machined metal product.
Preferably the halide gas might be achieved by introducing a halogen salt or acid to the fluidized bed furnace. The halide gas might be formed from HCl. Conveniently, the aforesaid halide gas is mixed with an inert carrier gas before entry into the fluidized bed furnace, the halide gas and said inert carrier gas entering the inert particulate refractory containing zone of the fluidized bed furnace, enters at a lower region thereof. Preferably the halide gas comprises between 0.2 and 3% of the inert carrier gas fluidizing the fluidized bed furnace and is preferably heated before entry into the fluidized bed furnace. The aforesaid heating of the halide gas may occur by heat exchange with at least a portion of the fluidized bed furnace, such as for example, by passing the delivery means for the halide gas/inert carrier gas through the zone of the fluidized bed furnace containing the particulate material to the lower region of the fluidized bed furnace. The inert carrier gas for the halide gas may be the same or different inert gas to that which is used to fluidize the particulate material in the fluidized bed furnace. The inert gas used to fluidize the fluidized bed furnace may be selected from inert argon or inert nitrogen.
Preferably, the particulate (powdered) metal or metal alloy introduced into the particulate refractory material in the fluidized bed furnace is selected from metals of group IVA, VA, VIA or VIIA, iron or alloys of these metals. The particulate metal or metal alloy might be selected from chromium, titanium, vanadium, niobium, tantalum, tungsten, molybdenum and manganese, or alloys of these metals including ferrous based alloys.
Conveniently the percentage of the particulate metal relative to the particulate refractory material in the fluidized bed furnace is between 5 and 30 weight percent. Preferably the temperature of operation of the fluidized bed furnace is below 750° C. and preferably below 700° C. The operating temperature of the fluidized bed furnace is between 500 and 700° C. The time of treatment of the substrate within the fluidized bed furnace is between 1 and 16 hours, more preferably between 3 and 8 hours.
According to a further aspect, the present invention also provides a method of forming a diffusion zone extending inwardly of a metal substrate surface in which nitrogen has been diffused to form a nitride or carbo nitride inner zone and an outer white layer, said method including placing the metal substrate in a fluidized bed furnace operated at a temperature of no greater than 700° C., and supplying ammonia gas in an amount no greater than 20% of a total gas flow to said fluidized bed furnace. Preferably, the ammonia gas flow to the fluidized bed furnace may be between 3 and 15%, preferably between 5 and 10% of the total gas flow to the fluidized bed furnace.
In accordance with a second aspect, the present invention also provides a fluidized bed furnace for heat treatment of metal substrates, said fluidized bed furnace including:
Preferably, the first seal means of the aforesaid fluidized bed furnace includes a first inner peripheral seal and a second outer peripheral seal each surrounding said retort access opening and defining a first seal zone therebetween, means being provided to introduce an inert gas under pressure into said first seal zone whereby, in use, any inert gas in said first seal zone tends to leak in a direction towards the access opening providing access to said retort. Conveniently, the first seal means includes a third peripheral seal located inwardly of said first inner peripheral seal, said third peripheral seal including a peripheral flange portion positionable in a region containing inert particulate refractory material when the cover means is positioned to close said access opening, a gas flow supply means being provided to said region to fluidize the inert particulate refractory material therein at least when the cover means is being moved to a position closing said access opening.
The cover means may include an opening or closing mechanism, the opening or closing mechanism enabling the cover means to be moved in an opening direction away from the access opening in an axial direction and then rotated about a pivot axis parallel to and spaced from a longitudinal axis of the retort with a reverse movement occurring when the cover means is moved to close said access opening. The cover means may be housed within an intermediate chamber through which access to and from said access opening is achieved, said intermediate chamber including an intermediate access opening aligned with the access opening to said retort, said intermediate chamber providing a sealed zone surrounding said cover means except for the intermediate access opening and the access opening to said retort.
The fluidized bed furnace may further include a transfer container means defining an internal holding zone accessed through a transfer container access opening, second seal means cooperable between said transfer container means and said intermediate chamber when the transfer container access opening is positioned adjacent the intermediate access opening of the intermediate chamber. Conveniently, the second seal means is configured similarly to the first seal means. The transfer container means may include a transfer container cover means to selectably open or close said transfer container access opening, third seal means being provided operable between said transfer container cover means and the transfer container access opening when the container cover is moved to a position closing said transfer container access opening. Conveniently, the third seal means is configured similarly to said first seal means. The transfer container means may further include an operating mechanism to open or close same, said operating mechanism enabling the transfer container cover means to be moved in an opening direction away from the transfer container access opening in an axial direction and then rotated about a pivot axis parallel to and spaced from a longitudinal axis of the transfer container with a reverse movement occurring when the transfer container cover means is moved to close said transfer container access opening.
Preferably when the second seal means is operationally engaged, the retort access opening, the access opening to the intermediate chamber and the access opening to the transfer container are aligned enabling a substrate to be treated to be transferred to and from said retort to the transfer container. Conveniently an inert gas supply means is provided to selectably supply inert gas to said transfer container to enable an inert gas atmosphere to be maintained therein when the transfer container cover means is closed or when it is open but the second seal means are engaged while a substrate to be treated is transferred from said transfer container to said retort. The inert gas supply means may also be arranged to selectably supply inert gas to said intermediate chamber.
The fluidized bed furnace may further include an exhaust gas flow path leading from said retort through a grit collection means to an exhaust gas treatment means, said gas flow path including scraper means to scrape solid deposits from said path and to move same into said collection means. In another possible arrangement, the fluidized bed furnace may further include an exhaust gas flow path leading from said retort, and a metering means to deliver the particulate metal or metal alloy in predetermined quantities into said retort via said exhaust gas flow path, said delivery occurring when minimal or no exhaust gas flow is occurring.
According to another aspect of this invention, there is provided a fluidized bed furnace for treatment of metal substrates, said fluidized bed furnace including:
Conveniently the first cooperable seal engaging surfaces of the first seal means separate the first seal chamber from said retort. The second cooperable seal engaging surfaces of the first seal means may separate the first seal chamber from atmosphere. Conveniently, inert gas from the first seal chamber is arranged to preferentially leak past said first cooperable seal engaging surfaces towards the retort.
A further preferred feature of this aspect of the invention provides for the first seal means to include a second seal zone surrounding said access opening being inwardly located relative to the first seal chamber, said second seal zone having a peripheral flange portion positionable in a region containing inert particulate refractory material when the cover means is positioned to close said access opening, a gas flow supply means being provided to said region to fluidize the inert particulate refractory material therein at least when the cover means is being moved to a position closing said access opening.
In accordance with a still further preferred aspect of this invention, there is provided a fluidized bed furnace treatment arrangement including
A number of preferred embodiments will hereafter be described with reference to the accompanying drawings in which:
a and 2b are cross-sectional views of seal arrangements usable in the apparatus of
As used herein “white layer” is intended to refer to a metal nitride, metal carbo nitride or mixtures thereof formed on the surface of a metal during a nitriding or nitrocarburising process. In the case of ferrous metal substrates, the white layer will be an iron nitride or an iron carbo nitride, typically either the epsilon and/or the gamma form.
Reference will now be made to
As illustrated in
An exhaust passage 22 leads from an upper region of the retort 11 whereby exhaust gases can escape in a controlled manner and be treated downstream (not shown) for safety purposes. It is possible for some of the refractory material to escape along this path and this material is conveniently collected in a grit collection box or container 23. From time to time it is possible for certain reaction products to solidify in this passage 22 which might lead ultimately to the passage becoming blocked. A scraper mechanism 24 is therefore provided to scrape such materials, preferably back into the collection box 23. Conveniently particulate metal or metal alloy (for use in a treatment process) can also be introduced via the exhaust passage 22. A storage zone 25 for such particulate metal is provided with a metering valve or the like 26 to deliver a desired quantity of metal powder into the passage 22. The scraper mechanism 24 might then be used to push this metal into the retort when required. This is preferably done when the bed is slumped (ie not in operation) such that there is no or minimal gas flow in an outward direction along the passage 22.
As shown in
In the seal arrangement shown in
The process of this invention according to a number of preferred aspects will now be described. In a first stage, a metal part (or substrate) to be treated is, subjected to a surface treatment known generally as nitriding or nitrocarburising. This can be achieved in a variety of different apparatus including salt baths, gas heat treatment apparatus, vacuum plasma equipment and fluidized bed furnaces. It is, however, important that the so-called white layer established via this first stage is substantially without significant porosity. Other desirable factors also relate to the concentration, depth and microstructure of the white layer.
When producing a nitrided or nitro carburised structure, two zones are produced. The first zone is the diffusion zone 83 where nitrogen diffuses into the substrate through zone 6 from the substrate surface 85 and increases the hardness of the substrate 86, and the second zone is the white layer 84 which can consist of either the epsilon and/or the gamma layer.
When the first stage of this process is carried out in a fluidized bed furnace, control of same requires the supply to the bed of ammonia/nitrogen (for nitriding) and a carbon bearing gas (eg natural gas and/or carbon dioxide) for nitrocarburising. During nitrocarburising, it is important that some oxygen is involved in the process which may be contributed by a hydrocarbon gas, carbon dioxide and/or oxygen. Once this first stage is completed satisfactorily, the part or substrate to be processed needs to be heated to ensure a surface oxide does not exist on the surface into which a metal is to be diffused. To obtain (or maintain) a suitable surface finish, one of the following options needs to be followed:
In the second stage of the process, the part or substrate is placed into and held in a fluidized bed furnace operated at a temperature below 750° C. and preferably no higher than 700° C. Conveniently the temperature is in the range of 500° C. to 700° C. The bed itself should include an inert refractory particulate material such as Al2O3 and the treatment should occur with the desired metal to be diffused into the surface in particulate or powder form in the bed. Such metal should preferably comprise between 5 to 30 weight percent of the bed materials, ie the balance being the inert refractory material. Treatment to diffuse the desired metal into the nitrogen based layer or zone of the substrate conveniently occurs with the bed being fluidized by an inert gas flow such as argon or nitrogen in the presence of a separately introduced halide gas (eg HCl) premixed into an inert carrier gas stream (eg nitrogen or argon).
Preferably, the metal powders introduced into the bed should be of high purity and conveniently without a surface oxide. Thus measures need to be taken to prevent air contact before the powders enter the bed and while they remain in the bed itself. The gases used also need to be of high purity. Common gases capable of use in the process are high purity nitrogen (less than 10 ppm oxygen), high purity argon (less than 5 ppm oxygen), and for the first stage processing, technical grade ammonia which has no more than 500 ppm water vapour and is further dried, for example by passing same through a desiccant before use. The halide gas used may typically be a technical grade HCl.
The halide gas typically will constitute between 0.2 and 3 percent of the gas flow to the treatment bed. The halide gas flow needs to be closely regulated and mixed thoroughly with the inert carrier gas before it enters the bed. This is important to avoid non uniformity within the bed. The halide gas should preferably be preheated before it enters the bed to ensure that it is in its most reactive stage when it contacts the part to be heated. Preheating of the halide gas and the inert carrier gas has the benefit of enabling a reduction in the amount of halide gas required.
It has been observed that the refractory powder (aluminium oxide powder) may also, over time, become contaminated and this can have an adverse affect on the process.
Equipment to carry out the process of this invention will desirably include first and second fluidized bed furnaces each with an upper intermediate isolation chamber as described previously, with a movable transport container or hood as described previously capable of moving between the two fluidized bed furnaces, also to and from a loading bay, and to and from a quenching fluidized bed. One of the two fluidized bed furnaces is intended to be used as a nitriding/nitrocarburising furnace and can, as discussed previously, be replaced by other equipment for achieving similar effects. The following description will, however, assume that two fluidized bed furnaces are used with some reference to
A possible first stage of the process is shown in
The intermediate purge chambers 51 are designed to:
As described previously, the fluidized bed furnaces are designed to allow the introduction of gases including ammonia, nitrogen, carbon dioxide, and HCl gas although the gas supply for the different furnaces 10 may be different. In some cases, a small amount of oxygen might also be supplied to the fluidized bed 10′ furnace undertaking the first stage 80, that is the nitriding or nitrocarburising process. The quenching bed 70 may be a standardized fluidized bed except that the top design is such to allow a fast purge between the hood 54 and the quenching bed 70. In consequence the quenching bed 70 may be modified to allow nitrogen not only to be injected through the bed for fluidizing the refractory media (typically aluminium oxide), but also injected over or above the bed to reach the ten volume changes in the space between the top of the refractory media and the underneath of the seal with the transfer hood 54. With the fluidized bed furnace 10 for performing the second stage 81 of the process, the halide gas supply is a separate one and it enters the bed through a separate distributor to the distributor system for the fluidizing gases. To obtain good and relative uniform mixing, the halide gas is pre-mixed with an inert carrier gas before it is introduced through the separate distributor. Moreover, preheating the halide and inert carrier gas flow appears to avoid a problem that can occur in the bottom 25 mm of a bed immediately above the distributor. With cold gas entering at this position, by products may be formed which in a worst case scenario may cause sintering of the metal powder and the refractory media of the bed. In consequence, pre-heating the inert carrier gas containing the active halide gas and the premixing arrangement and delivery systems avoids possible sintering by product effects as aforesaid, reduces the quantity of halide gas required, improves uniformity of processing and eliminates possible by products in the gas mixing panel as well as in the furnace.
a) of the annexed drawings shows the transfer hood 54 at the loading station 71 ready to pick up the load 72 to be treated. A pick up mechanism 63 within the hood 54 enables the load 72 to be picked up and moved into the hood 54. Once this has occurred, the cover member 58 seals the access opening 56 to the hood 54 and the internal volume of the hood 54 is purged with an inert gas such as nitrogen as shown in
In a potential alternative equipment design, one or two treatment fluidised bed furnaces might be arranged close to one another, possibly with a further fluidized bed capable of acting as a quenching device when required, also located close to the treatment fluidized bed furnace or furnaces. Each such fluidized bed furnace and quenching bed will have a cover structure and seal arrangement as described to enable a desired atmosphere to be maintained in the furnace during a treatment stage. A sealed chamber is configured above the array of fluidized bed furnaces which houses any desired pick up, introduction and transport apparatus to allow a metal part (or parts) to be treated to be introduced into a furnace for a treatment stage, to be removed from the furnace after the treatment stage and to be moved, when desired between furnaces for different treatment stages of the overall treatment process. The sealed chamber provides a volume within which the pick up, introduction and transport apparatus works and the volume includes means to introduce and maintain an inert gas atmosphere therein as previously defined for the intermediate chamber 51 and transport hood 54 of the previously described arrangements. The volume of the sealed chamber should be maintained as low as possible to minimize the utilization of inert gases and thereby minimize costs. The sealed chamber will of course require an access system that is capable of being opened and resealed to introduce metal products to be treated and to remove metal products after treatment. In addition, it is believed desirable to include a viewing panel or panels in the sealed chamber so that an operator can maintain some visual contact with the processes being undertaken.
It is believed the weight percent of metal powder to be diffused into a substrate surface might comprise between 5 and 30 weight percent of the refractory material in the bed. The amount of halide gas might be varied between 0.2 and 3% flow with the balance of the carrier/fluidizing gas being an inert gas such as argon or nitrogen. The processing temperature should be below 750° C. and preferably below 700° C. More preferably, the processing temperature should be in the range of 500 to 700° C. The processing time should be between one and sixteen hours.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005903894 | Jul 2005 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU2006/001031 | 7/20/2006 | WO | 00 | 7/7/2009 |
