Patent Application
20130299047
The present invention relates to methods and apparatus for treating a metal substrate to achieve a diffusion surface layer on the substrate.
Metal surface treatments have traditionally comprised forming a nitrided surface on the substrate followed by a physical vapour deposition of a coating such as titanium, 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. Further HCl and other halide gases are relatively expensive and extensive use of such gases can provide relatively expensive processing of a desired product. Also the halide gas can react instantly at low temperatures with ammonia forming solid 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.
International patent application no. PCT/AU2006/001031 discloses treatment methods and treatment apparatus enabling a desired diffusion layer to be formed on a metal substrate product, the methods disclosed supply halide gas throughout a lengthy period of the processing and while the methods work satisfactorily, the processing cost is quite expensive due to the required volume of halide gas utilized.
The objective therefore of the present invention is to provide a method of forming a diffusion surface layer on a metal substrate in a more economical manner than with prior art processes while still retaining a reliable and safe processing of the metal substrate.
Accordingly, the present invention provides in a first aspect, a method of forming a diffusion surface layer extending inwardly of an outer surface of a metal substrate, said method including:
Conveniently, the aforesaid method may further include:
In a preferred construction the aforesaid method may be carried out in a single treatment furnace where the diffusion treatment furnace also acts as the activation treatment furnace. The method can however, be carried out in different furnaces acting as the activation treatment furnace and the diffusion treatment furnace.
Preferably, the inert gas flow in the activation stage may be nitrogen and/or argon. Conveniently, the inert particulate refractory material utilized in the treatment furnace or furnaces might be aluminium oxide or silicon carbide.
Conveniently, when the diffusion treatment furnace contains an inert refractory particulate material, it is fluidized by a flow of an inert gas during the metal diffusion stage. Alternatively, such an inert refractory particulate material might be fluidized or at least partly fluidized by vibration means. Preferably, ammonia is not supplied to the diffusion treatment furnace during the metal diffusion stage.
In a particularly preferred embodiment, the second period of time is greater than the first period of time. In this manner, the relatively expensive hydrogen halide gas is used for much shorter periods to achieve the desired diffusion layer on the metal substrate. During the diffusion stage, the hydrogen halide gas might not be utilized at all but small amounts of the hydrogen halide gas could be used for short periods of time to reactivate the metal based material, if required. Typically, if required, the hydrogen halide gas might be pulsed for periods of no hydrogen halide gas provided in the retort and at least one period of hydrogen halide gas provided during the diffusion stage.
Conveniently an inert gas flow may be provided to the diffusion treatment furnace during the second period of time, the inert gas flow being variable from a zero flow rate to a flow rate at or above a minimum fluidization velocity for the diffusion treatment furnace.
Conveniently the operating temperature for the first and second periods for the treatment furnace or furnaces during the activation stage and the diffusion stage is between 500 and 750° C.
Preferably, in one embodiment the hydrogen halide gas flow may be supplied continuously to the activation treatment furnace during the first period of time. In a possible alternative, the hydrogen halide gas might be pulsed with periods of supply and periods of non supply during the first period of time. Conveniently, the hydrogen halide gas used might be selected from hydrogen chloride gas, hydrogen bromide gas, hydrogen fluoride gas or hydrogen iodide gas. The hydrogen halide gas when supplied to the activation treatment furnace or the diffusion treatment furnace is preferably mixed with an inert carrier gas (e.g. nitrogen and/or argon gas) externally of the treatment furnace or furnaces. Conveniently, when supplied, the hydrogen halide gas and the inert carrier gas enter the treatment furnace or furnaces at a lower region thereof.
In a further embodiment, the hydrogen gas might be created in the treatment furnace or furnaces by supply of ammonium chloride (NH4Cl). Ammonium chloride might be supplied in solid or pellet form through a delivery tube or pipe whereby it is heated in the delivery pipe or tube to disassociate into nitrogen gas and hydrogen chloride (HCl) gas. An inert gas such as nitrogen or argon could also be supplied via the delivery tube such that the HCl gas is at least partly mixed with the inert gas by the time it enters the furnace. Such a delivery system might be used in either the activation stage or, if required, in the diffusion stage. If this delivery system is used, the operating temperature of the furnace might be close to 700° C. or even higher.
The metal based material for forming the diffusion surface layer may be chosen from at least one of:
The metal substrate is conveniently a ferrous based metal or a ferrous based metal alloy.
Conveniently nitrogen as an inert gas is introduced into the diffusion treatment furnace during the second period of time.
The term “metal substrate” is intended to refer to any metal part suitable for heat treatment made from ferrous based metal or ferrous based metal alloys.
In accordance with the method of this invention where hydrogen chloride is the halide gas used and chromium metal particles are used to form the diffusion surface layer, it is believed that hydrogen chloride causes an active chromium chloride layer on the surface of the aluminium oxide (inert fluidizing media) as well as on the chromium metal particles in the fluidizable bed furnace during the activation stage. During the metal diffusion stage of the process, a solid-state interaction between the activated chromium chloride and a nitrogen-rich ferrous surface of the metal substrate occurs to form the diffusion surface layer on the substrate. This occurs when the treatment furnace, typically a fluidizable bed furnace is substantially not fluidized by a flow of inert gas and also when the bed is fluidized. Fluidization of the bed can occur either by a suitable gas flow or by some vibration means as is known in the art. The process has considerable economic advantages as the hydrogen halide gas, typically hydrogen chloride is expensive and minimizing its use provides a much more economical process.
It is generally desirable that the outer portion of the diffusion zone (the white layer), be substantially free from porosity. The white layer will normally be an iron nitride, iron carbide and/or an iron carbonitride, typically either epsilon and/or the gamma form.
A preferred embodiment of the process of this invention will now be described with reference to the accompanying drawings, in which:
Reference will now be made to the drawings which schematically illustrate relevant parts of a fluidized bed treatment apparatus according to a preferred form of this invention, it being understood from the preceding disclosure that at least the pre-treatment stage of the heat treatment process need not be completed in fluidized bed heat treatment equipment and any other known heat treatment equipment could be used in this stage. Moreover, although it is desirable that the activation stage and the diffusion stage be carried out in the same fluidized bed heat treatment furnace, it is equally possible for separate fluidized bed heat treatment furnaces to be used for the activation and diffusion stages.
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 may be provided to scrape such materials, preferably back into the collection box 23. Other approaches could be utilized rather than the illustrated physical scraper. For example, pulsed bursts of inert gas might be used from time to time to break up or move material in the exhaust passage 22 back into the retort 11. Conveniently particulate metal or metal alloy (when used 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 or metal coated particulate material into the passage 22. The scraper mechanism 24 if used or some pusher device might then be used to push this metal into the retort 11 when required. This is preferably done when the bed is slumped (i.e. 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 pre-treatment 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, desirable 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 including the lack of porosity therein.
When producing a nitrided or nitro carburised structure, two zones are produced. The first inner zone is the diffusion zone where nitrogen diffuses into the substrate through the diffusion zone from the substrate surface and increases the hardness of the substrate, and the second outer zone is the white layer which can consist of either the epsilon and/or the gamma layer as illustrated, for example, in international patent application no. PCT/AU2006/001031.
When the pretreatment stage of this process is carried out in a fluidized bed heat treatment furnace, control of same requires the supply to the bed of ammonia/nitrogen (for nitriding) and a carbon bearing gas (e.g. 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 pre-treatment stage is completed satisfactorily, the part or substrate to be processed needs to be treated 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 may be followed:
In the activation stage of the process, the metal or metal based material to be surface diffused may be 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° to 700° C., typically about 575° C. The bed itself may include an inert refractory particulate material such as Al2O3 with the desired metal to be diffused into the surface in particulate or powder form in the bed or alternatively coating the inert refractory particles. Such metal should preferably comprise between 5 to 30 weight percent of the bed materials, i.e. the balance being the inert refractory material. The bed is then fluidized by a flow of halide gas (e.g. hydrogen chloride) and inert gas for a first period of time without the metal substrate to be treated. The inert gas may be argon and/or nitrogen in the presence of a separately introduced halide gas (e.g. HCl) premixed into an inert carrier gas stream (e.g. nitrogen and/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 inert 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 pretreatment 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 hydrogen halide gas used may typically be a technical grade HCl although other hydrogen halide gases might be used.
The hydrogen halide gas typically will constitute between 0.2 and 3 percent of the total gas flow to the fluidized heat treatment bed furnace. The hydrogen 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 hydrogen halide gas may be preheated before it enters the bed to ensure that it is in its most reactive stage when it enters the bed. Preheating of the halide gas and the inert carrier gas has the benefit of enabling a further reduction in the amount of hydrogen halide gas required. The first period might typically be between 45 and 120 minutes, preferably between 60 and 90 minutes to produce an active layer on the diffusion metal and on the inert fluidizing media (aluminium oxide) in the bed. When chromium is used and the hydrogen halide gas is hydrogen chloride, the active layer will be chromium chloride.
At the end of this initial activating period, the pretreated metal substrate (pretreated as described above) is immediately introduced into the furnace bed or a furnace bed containing the activated metal based material and the flow of halide gas is then stopped. During this subsequent metal diffusion stage, the metal substrate on which the diffusion layer is to be formed is then held within the preactivated bed for a second period (typically 1 to 8 hours and preferably 4 to 8 hours) under an inert gas atmosphere. The bed is conveniently held at a temperature below 750° C. and conveniently in the range of 500° C. to 700° C., typically about 575° C. The fluidized bed in the metal diffusion stage may have minimal inert gas flow such that it is substantially slumped up to a high inert gas flow such that it is highly fluidized. The inert gas might be nitrogen. In some cases it might be desirable to include a pulsed halide gas flow during the second stage, if it is deemed the bed needs some reactivation.
It is generally desirable during treatment processes to maintain relatively uniform temperature levels in the bed, i.e. between the various heights in the bed. This may be achieved by including temperature monitoring means and varying the flow of the inert gases to the bed in response to sensed temperatures.
The metal or metal based material used to provide a metal to be diffused into the diffusion surface layer of the metal substrate to be treated may be chosen from at least one of a solid metal or metal alloy either in particulate form or one or more solid block members, a metal or metal alloy coated on a substrate carrier where the substrate carrier is in particulate form or as one or more solid block members where the substrate carrier will not, within the treatment conditions, react with the coating metal or metal alloy or the metal substrate being treated, a metal halide particle or powder (anhydrous or hydrated), and a metal halide material (anhydrous or hydrated) coated on a substrate carrier where the substrate carrier is in particulate form or as one or more solid block members where the substrate carrier will not, within the treatment conditions, react with the coating material or the metal substrate being treated. Conveniently the metal of the metal based material used to provide a metal to be diffused can be selected from chromium, titanium, vanadium, niobium, tantalum, tungsten, molybdenum, manganese, and alloys thereof including ferrous based alloys. Conveniently, the above referred to metal halides may be comprised of a selected metal as set out above and a halide selected from chlorine, bromine, iodine or fluorine. For example, CrCl2 and CrCl3 are soluble in water and ethanol to form a slurry whereby it could be painted on a suitable carrier substrate or the carrier substrate could be dipped into the slurry to form a suitable coating.
Several examples of preferred embodiments of the process of this invention will be described in the following.
A specimen of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of this specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of this nitrocarburised sample was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.
A 38 mm diameter 5 mm thick piece of pure copper was polished to a 1200 grade SiC finish prior to electrolytic hard chromium plating from a commercial supplier. A 2 micron pure chromium layer was produced by this method. Copper was chosen as a substrate carrier as Cr and Cu are essentially insoluble, and therefore the chromium layer will not decompose by diffusion into the copper specimen during heating. This chromium-plated sample was then immersed in a fluid bed heat treatment reactor of diameter 90 mm and depth 250 mm containing 3 kg of 99.99% purity alumina oxide powder of average particle size 125 microns. This fluid bed was heated to 575° C. under nitrogen and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% of the total gas flow. This “activation” stage continued for a duration of 1 hour. After this activation stage the chromium plated copper sample was cooled to room temperature in a flow of nitrogen.
Immediately after removal from the fluid bed reactor into ambient air conditions the hydrogen chloride activated chromium plated copper sample was physically coupled to the nitrocarburised sample, and a clamping pressure applied. This coupling was then placed in a fluid bed furnace and heated to 575° C. under nitrogen flow, held at this temperature for 4 hours, then cooled to room temperature under a nitrogen flow. This experiment was repeated, where the coupling consisted of chromium-plated copper without hydrogen chloride treatment as aforesaid and a nitrocarburised specimen. Upon uncoupling the two contacting surfaces were analysed for chemical composition using Glow Discharge Optical Emission Spectroscopy (GDOES).
It was found that by activating the surface of the chromium plated copper sample by use if hydrogen chloride gas, this surface reacted with the nitrocarburised specimen. Chromium transferred from the activated chromium-plated sample to the nitrocarburised specimen (
A specimen of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of this specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of this nitrocarburised sample was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.
A 38 mm diameter 5 mm thick piece of 99.99% purity chromium was polished to a 1200 grade SiC was immersed in a fluid bed reactor of diameter 90 mm and depth 250 mm containing 3 kg of 99.99% purity alumina powder of average particle size 125 microns. This fluid bed was heated to 575° C. under nitrogen and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage continued for duration of 1 hour. After this activation stage the chromium sample was cooled to room temperature in a flow of nitrogen.
Immediately after removal from the fluid bed reactor in to ambient air conditions the hydrogen chloride activated chromium sample was physically coupled to the nitrocarburised sample and a clamping pressure applied. This coupling was then placed in a fluid bed furnace and heated to 575° C. under nitrogen flow, held at this temperature for 4 hours, then cooled to room temperature under a nitrogen flow. This experiment was repeated, where the coupling consisted of chromium without hydrogen chloride treatment and a nitrocarburised specimen. Upon uncoupling the two contacting surfaces were analysed for chemical composition using Glow Discharge Optical Emission Spectroscopy (GDOES).
As per Example 1, by activating the surface of chromium by use if hydrogen chloride gas, this surface reacted with the nitrocarburised specimen. Chromium transferred from the activated chromium sample to the nitrocarburised specimen and nitrogen diffused to the surface to create a peak coinciding with the chromium peak (
Two specimens of hardened and tempered (1020° C. autenitised and air cooled, double tempered at 575° C.) AISI H13 hot work tool steel with a diameter of 38 mm and thickness of 5 mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of each specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of the nitrocarburised samples was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.
In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina powder of average particle size 125 microns.
This fluid bed was heated to 575° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature a sample of nitrocarburised AISH13, as prepared above, was immersed in the heated fluidising powder for a period of 4 hours. The sample was cooled in the fluid bed to 350° C. under nitrogen flow and cooled in air. No chromium enrichment of the nitrocarburised surface was experienced as a result of this process.
In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina powder of average particle size 125 microns. This fluid bed was heated to 575° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage was for a duration of 1 hour. After activation a sample of nitrocarburised AISH13, as prepared above, was immersed in the heated fluidising powder with the hydrogen chloride gas flow being stopped, the heat treatment being at 575° C. for a period of 4 hours. The sample was then cooled in the fluid bed to 350° C. under nitrogen flow and cooled in air. In this trial significant chromium-enrichment (about 70 wt %, refer to quantitative depth profile in
To assess the potential to increase the process temperature above 575° C. two steel grades were selected having higher tempering resistance than AISI H13 hot work tool steel. Specimens of hardened and tempered (1050° C. autenitised and oil quenched, double tempered at 575° C.) powder metallurgy tool steel Crucible CPM 1V® and conventional ingot metallurgy Bohler-Uddeholm QRO® 90 with a diameter of 38 mm and thickness of 5 mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60% nitrogen atmosphere for 3.5 hours at 575° C. Prior to nitrocarburising the surface of each specimen was prepared using 1200 grade SiC abrasive to ensure good surface finish. This produced a surface structure consisting of a 1 micron oxygen-rich surface layer directly above a 10 micron compound layer composed of ε-iron carbonitride, and finally an inner diffusion zone of 70-90 microns. The surface of the nitrocarburised samples was then wet grit blasted to remove the oxide layer, while retaining the compound layer and diffusion zone. The composition of chromium in the compound layer was determined to be about 4 wt %.
In a fluid bed reactor of diameter 90 mm and depth 250 mm 380 g of 99.99% purity chromium powder of average particle size 80 microns was mixed with 3.4 kg of 99.99% purity alumina oxide powder of average particle size 125 microns. This fluid bed was heated to 625° C. under high purity nitrogen with sufficient flow for fluidisation and at this temperature hydrogen chloride gas was added to the input gas stream to a concentration of 1% flow. This “activation” stage was for a duration of 1 hour. After activation one nitrocarburised sample of each grade, as prepared above, was immersed in the heated fluidising powder for a period of 4 hours under high purity nitrogen. The samples were cooled in the fluid bed to 350° C. under nitrogen flow and then removed from the fluid bed reactor and cooled in air. In this trial significant chromium-enrichment (about 70 wt %, refer to quantitative depth profile in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010905095 | Nov 2010 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU11/01479 | 11/17/2011 | WO | 00 | 7/19/2013 |
