US20130299047A1 - Surface treatment of metal objects - Google Patents

Surface treatment of metal objects Download PDF

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US20130299047A1
US20130299047A1 US13/988,254 US201113988254A US2013299047A1 US 20130299047 A1 US20130299047 A1 US 20130299047A1 US 201113988254 A US201113988254 A US 201113988254A US 2013299047 A1 US2013299047 A1 US 2013299047A1
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metal
diffusion
treatment furnace
gas
inert
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Daniel Fabijanic
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Hard Technologies Pty Ltd
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Hard Technologies Pty Ltd
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Assigned to HARD TECHNOLOGIES PTY LTD reassignment HARD TECHNOLOGIES PTY LTD ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FABIJANIC, DANIEL
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding

Definitions

  • 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.
  • 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.
  • 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.
  • HCl and other halide gases are relatively expensive and extensive use of such gases can provide relatively expensive processing of a desired product.
  • 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.
  • 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.
  • 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:
  • the aforesaid method may further include:
  • 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.
  • the inert gas flow in the activation stage may be nitrogen and/or argon.
  • the inert particulate refractory material utilized in the treatment furnace or furnaces might be aluminium oxide or silicon carbide.
  • the diffusion treatment furnace 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.
  • an inert refractory particulate material might be fluidized or at least partly fluidized by vibration means.
  • ammonia is not supplied to the diffusion treatment furnace during the metal diffusion stage.
  • the second period of time is greater than the first period of time.
  • the relatively expensive hydrogen halide gas is used for much shorter periods to achieve the desired diffusion layer on the metal substrate.
  • 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.
  • 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.
  • 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.
  • 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.
  • the hydrogen halide gas flow may be supplied continuously to the activation treatment furnace during the first period of time.
  • the hydrogen halide gas might be pulsed with periods of supply and periods of non supply during the first period of time.
  • 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.
  • an inert carrier gas e.g. nitrogen and/or argon gas
  • the hydrogen gas might be created in the treatment furnace or furnaces by supply of ammonium chloride (NH 4 Cl).
  • 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.
  • 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.
  • nitrogen as an inert gas is introduced into the diffusion treatment furnace during the second period of time.
  • metal substrate is intended to refer to any metal part suitable for heat treatment made from ferrous based metal or ferrous based metal alloys.
  • hydrogen chloride is the halide gas used and chromium metal particles are used to form the diffusion surface layer
  • 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.
  • 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.
  • 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.
  • the white layer will normally be an iron nitride, iron carbide and/or an iron carbonitride, typically either epsilon and/or the gamma form.
  • FIG. 1 is a cross-sectional schematic view of a fluidizable bed furnace capable of use in the performance of the present invention
  • FIGS. 2 and 3 are detailed cross-sectional views of seal arrangements capable of use with the fluidizable bed furnace shown in FIG. 1 ;
  • FIG. 4 is a graph showing Nitrogen (N), Chromium (Cr) and Iron (Fe) wt % concentrations against depth in the treated metal sample of Example 1 produced according to the present invention
  • FIG. 5 is a graph showing Nitrogen (N), Chromium (Cr), Iron (Fe) and Copper (Cu) against depth in the activated chromium coated copper carrier substrate of Example 1;
  • FIG. 6 is a graph showing Nitrogen (N), Chromium (Cr) and Iron (Fe) wt % concentrations against depth in the treated metal sample of Example 1 not produced according to the present invention
  • FIG. 7 is a graph showing Nitrogen (N), Chromium (Cr), Iron (Fe) and Copper (Cu) wt % concentrations against depth in the non activated chromium coated copper carrier substrate of Example 1;
  • FIG. 8 is a graph showing Chromium (Cr), Iron (Fe) and Nitrogen (N) wt % concentrations against depth in the treated metal sample of Example 2 produced according to the present invention
  • FIG. 9 is a graph showing Chromium (Cr), Iron (Fe) and Nitrogen (N) wt % concentrations against depth in the treated activated Chromium sample utilized in Example 2 resulting from carrying out the process of this invention;
  • FIG. 10 is a graph showing Iron (Fe), Nitrogen (N) and Chromium (Cr) wt % concentrations against depth in the metal sample when not treated with a preactivated chromium sample as described in Example 2;
  • FIG. 11 is a quantitative depth profile showing Iron (Fe), Chromium (Cr), Nitrogen (N), Carbon (C) and Oxygen (O) wt % concentrations against depth in the metal sample treated according to the present invention utilising activated chromium powder as described in Example 3;
  • FIG. 12 shows the microstructure of the treated metal sample represented in FIG. 11 (Example 3);
  • FIG. 13 is an x-ray diffraction analysis showing the diffusion layer in the treated metal sample (Example 3) was predominantly CrN;
  • FIGS. 14 and 15 are a quantitative depth profile showing Chromium (Cr), Iron (Fe), Nitrogen (N), Carbon (C) and Oxygen (O) against depth in the respective treated metal sample as described in Example 4; and
  • FIG. 16 shows the microstructure of the treated metal samples as described in Example 4.
  • the apparatus comprises a fluidized bed furnace 10 having an inner retort 11 containing a particulate inert refractory material 12 such as aluminium oxide (Al 2 O 3 ), however, other such inert refractory materials can be employed.
  • the furnace includes an outer insulating layer 13 and a heating zone 14 that might be heated in any conventional manner by combusting a fuel gas, by electrical resistance heating or by any other suitable means.
  • the heating zone 14 is heated by a fuel gas supplied burner 16 .
  • a primary inert gas supply line 17 is provided for fluidizing the refractory material 12 when required.
  • the gas supply line 17 leads to a gas distribution system comprised of a primary distributor 18 and a secondary distributor 19 typically of a porous material construction that is aimed at preventing streaming of the gas flow within the retort and thereby even fluidization and heat treatment.
  • a further gas delivery line 20 is provided so that a halide gas and an inert carrier gas can be introduced into the bottom of the retort via a further distributor 21 separate from the distributors 18 / 19 .
  • a carrier inert gas line e.g. nitrogen and/or argon
  • the amount of inert gas delivered via lines 70 and 17 and the amount of hydrogen halide gas delivered via line 71 may be metered such that the gas quantity delivered to the furnace 10 is known.
  • the distributor 21 might be positioned in the coarse refractory material zone 80 in the lower region of the retort 11 .
  • the delivery line 20 may enter through the bottom of the retort as shown in broken outline or elsewhere subject to the distributor 21 being located in the lower region of the retort. In this arrangement the delivery line 20 might pass upwardly as shown at 20 ′ and include one or more heating coils 81 before returning the halide and inert carrier gas to the distributor 21 in the lower region of the retort 11 .
  • the heating coil(s) 81 are conveniently just above or just within the coarse refractory material zone 80 . It is preferred that the halide gas and the inert carrier gas be thoroughly mixed externally of the retort 11 and further that it be heated before the mixed gases enter the retort. Conveniently heating occurs by heat exchange with a region of the fluidized bed treatment furnace. With the illustrated arrangement in full line, heating of the externally mixed gases occurs as the line 20 passes downwardly through the heated refractory material in the retort. Other arrangements are equally possible. For example one or more coils of the delivery pipe might be provided in the line 20 within the retort. Alternatively, the delivery line 20 might pass through the heating zone 14 with one or more coils located in the zone 14 .
  • premixed inert carrier gas and hydrogen halide gas might enter the furnace directly to be discharged via the distributor 21 without being preheated.
  • Metering and mixing equipment (not illustrated in detail) is used to ensure proper proportions of halide gas and inert carrier/fluidizing gases are used in the activation stage of the treatment process.
  • 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 .
  • particulate metal or metal alloy when used in a treatment process
  • 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 .
  • a first seal means 27 associated with a cover member 29 is provided around the upper access opening 28 leading to the inner zones of the retort 11 .
  • the first seal means 27 enables the retort 11 to be sealed against the ingress of atmospheric air during a treatment process.
  • FIG. 2 or 3 where they are shown operationally with the cover member 29 for the upper access opening 28 .
  • the first seal means 27 comprises a first outer seal part 30 formed by a circumferential flange 31 on the cover member 29 engaging with a seal material 32 positioned between two circumferential and radially spaced flanges 33 , 34 on a member 35 secured to the retort 11 and surrounding the access opening 28 .
  • the first seal means 27 further includes a second inner seal part 36 formed by circumferential flange 37 supported on the member 35 and engaging with a seal material 38 positioned between the outer flange 31 on the cover member 29 and a more inwardly located circumferential flange 39 carried by the cover member 29 .
  • the seal materials 32 or 38 may be any compressible seal material capable of operation at the relevant operating temperatures for the furnace, but may include ceramic fibre or VITON (registered trade mark) rubber material.
  • a gas distributor tube 41 is located in this zone 40 and is fed externally via a line schematically shown at 42 to deliver nitrogen, argon or some other inert gas to the zone 40 at a pressure whereby such gas will leak towards the retort opening 28 if leakage is possible thereby preventing ingress of atmospheric oxygen into the retort 11 .
  • the seal means 27 further includes a third seal part 43 formed by the inner circumferential flange 39 being engaged in a zone 44 containing inert refractory particulate material 45 typically of the same type as contained within the retort 11 .
  • the particulate material 45 may be fluidized by an inert gas supply delivered via line 46 to a distributor 47 therefor to assist at least entry of the flange 39 into the particulate material 45 as the cover member 29 moves to the illustrated closed position.
  • an inert gas supply delivered via line 46 to a distributor 47 therefor to assist at least entry of the flange 39 into the particulate material 45 as the cover member 29 moves to the illustrated closed position.
  • the cover member 29 is removed. This would occur, for example, when a treatment member (e.g. metal substrate) is introduced or withdrawn from the retort.
  • annular flanges 82 , 83 are provided upstanding from the peripheral retort part or member 35 defining a seal zone 84 therebetween.
  • the flanges 82 , 83 are welded or otherwise secured to the retort part 35 and are of differing heights to achieve the seal zone 84 .
  • the upper edges 85 , 86 of the flanges 82 press into and seal with a suitable seal material 87 within an annular recess 88 in the cover member or lid 29 .
  • the upper edge 85 of flange 82 is marginally lower than the upper edge 86 of flange 83 whereby if gas leakage from the seal zone 84 occurs it will preferentially leak towards the inside of the retort 11 rather than externally of same.
  • the seal material 87 might be the same type of material discussed above for seal material 32 , 38 of FIG. 2 a .
  • An inert gas delivery tube 42 is provided to deliver inert gas (eg nitrogen) to a distributor ring 41 within the seal zone 84 such that when the furnace 10 is in use and the cover member 29 is closed, the seal zone 84 is pressurized with an inert gas at a pressure higher than atmosphere and higher than within the retort.
  • Gas leakage from the seal zone 84 “may” occur in both directions past the upper flange edges 85 , 86 but preferentially, if leakage does occur at all, it will occur past the edge 85 back towards the retort. Thus the required atmosphere is maintained within the retort without permitting unwanted oxygen to enter same from the external atmosphere.
  • a further annular flange 89 is provided with a heat insulating material 87 therebetween which can be the same material as the seal material 87 discussed above.
  • Refractory particle material 91 can build up as shown in FIG.
  • the lid or cover member 29 carries a treatment basket (or similar) support device 92 and the cover member 29 is conveniently at least insulated against heat loss.
  • the lid or cover member 29 may also be desirable to include cooling coils or tubes in the lid or cover member 29 to cool down the furnace 10 when desired at the end of a treatment operation.
  • the lid or cover member 29 might also carry optionally, a plug 93 to minimize space above the treatment bed.
  • a metal part (or substrate) to be treated is, subjected to a surface treatment known generally as nitriding or nitrocarburising.
  • 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.
  • 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
  • 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.
  • 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.
  • a carbon bearing gas e.g. natural gas and/or carbon dioxide
  • 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.
  • 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 Al 2 O 3 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.
  • halide gas e.g. hydrogen chloride
  • 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).
  • the metal powders introduced into the bed should be of high purity and conveniently without a surface oxide.
  • 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.
  • the active layer will be chromium chloride.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • CrCl 2 and CrCl 3 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.
  • 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 ⁇ -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.
  • 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 ⁇ -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.
  • 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.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Crucibles And Fluidized-Bed Furnaces (AREA)
US13/988,254 2010-11-17 2011-11-17 Surface treatment of metal objects Abandoned US20130299047A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
AU2010905095 2010-11-17
AU2010905095A AU2010905095A0 (en) 2010-11-17 Surface Treatment of Metal Objects
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AU2011331909B2 (en) 2016-06-23
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EP2640867A1 (en) 2013-09-25
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