AU1838201A

AU1838201A - Coatings produced by thermal powder-cladding

Info

Publication number: AU1838201A
Application number: AU18382/01A
Authority: AU
Inventors: Milan Brandt; Mahnaz Dr Jahedi
Original assignee: NISSAN CASTING AUSTRALIA Pty L
Current assignee: NISSAN CASTING AUSTRALIA Pty Ltd
Priority date: 2000-02-09
Filing date: 2001-02-09
Publication date: 2001-08-16

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title COATINGS PRODUCED BY THERMAL

POWDER-CLADDING

NISSAN CASTING AUSTRALIA PTY. LTD.

ACN 004 663 156 Applicant: o *oo io o o The following statement is a full description of this invention, including the best method of performing it known to me: W:\mary\RNCWORKX583925a.doc COATINGS PRODUCED BY THERMAL POWDER-CLADDING This invention relates to coatings produced by thermal powder-cladding. In particular, the invention relates to the use of thermal powder-cladding in providing a protective coating on a surface of a steel component, such as a component which is to be contacted by molten aluminium alloys, and to a steel component having a protective cladding coating.

Molten aluminium alloys attack steels, such as tool steels, resulting in the production of brittle intermetallic compounds. In die casting an aluminium alloy, the alloy can attack the tool steel of which the die tools are made to give rise to soldering, and result in defective castings and a shortened tool life. The soldering occurs only in some areas of the die tools, depending on temperature and alloy solidification time.

:"Laser cladding is one of the techniques tried for local application of a coating to prevent soldering inside the die cavity of high pressure die casting tools. Laser cladding also has been used in that context to prevent erosion. Under procedures for such laser cladding, nickel and cobalt have been used as cladding matrix materials in which fine particles of a wear resistant material is dispersed. However like iron, nickel and cobalt produce soldered layers when they come in contact with molten aluminium alloys.

The present invention seeks to provide an improved process for protecting the S 25 surface of a steel component, such as a component to be contacted by molten aluminium alloy.

The use of nickel and cobalt as laser cladding matrix materials has, in part, reflected a recognition of the unsuitability of iron as the matrix material. That is, it is iron in the steel of die tools which is attacked by aluminium alloy, and iron as the cladding matrix material also would give rise to soldering, probably to a greater extent. It therefore is surprising that, as we have found, iron powder can MR W:nmary\RNCNODEL583925A.doc be used in providing a matrix material to form a protective coating. However this is possible if, as we have discovered, at least the exposed surface of the iron of the coating matrix is converted to a suitable oxide Specifically, we have found that molten aluminium alloys do not give rise to soldering if at least the surface of the iron content of the coating is oxidised to provide oxide which comprises, or contains a sufficient level of, magnetite (Fe30 4 Thus, according to the present invention, there is provided a process for producing a steel component having a protective coating, the process including the steps of: forming on a surface of the component, by a suitable hardfacing process, a coating having an iron or iron-base matrix material in which fine particles of another material are dispersed; and 15 oxidising the iron of at least an outer surface layer of the matrix material whereby resultant iron oxide comprises, or contains a sufficient or required level of, magnetite.

In the process of the invention, the hardfacing process may comprise thermal powder-cladding. At least where this is the case, iron powder preferably is used as the source for the matrix material. The iron powder may be of relatively high level of purity. However, this is not necessary and the iron powder can be of any suitable grade. That is, the powder can contain carbon and other elements at impurity or at least minor alloying levels. Thus, the source can be iron powder or iron-base powder.

The thermal powder-cladding preferably comprises laser cladding. However other procedures can be used, including thermal spraying processes used for hard facing by powder spraying. Thus, the cladding can be formed, for example, by a single spray-step version, or the usual two-step, of the spray and fuse process, by plasma spraying or by detonation-gun spraying. In preferred MR W:\mary\RNCNODEL\583925A.doc thermal cladding procedures able to be used in the invention, particles of the powder constituents are heated to a plastic or molten state by a gas flame, plasma arc or gases resulting from detonation of an explosive gas mixture, with the heated particles propelled by a gas jet onto the substrate to form a required coating thereon.

In many applications, such as with die casting die tools, the dispersed material will be required to impart protection against erosion by molten alloy. In such applications, the dispersed material is chosen to impart wear resistance, and powders for example of tungsten, tungsten carbide, silicon carbide and boron nitride are suitable for this purpose. However, dispersed particles suitable for providing other properties can be used if required.

In one arrangement, the coating may be formed by application of a flame, 15 plasma arc or laser to melt a mixture of powders pre-placed onto the surface of the component. Laser cladding with a pre-placed powder mixture has the benefit of simplicity and ease of application. However, it can be difficult to keep the powder in place during processing, especially if a shielding gas is used to protect the melt pool from oxidising, and when the laser cladding is over a three- 20 dimensional surface.

In one preferred arrangement, the coating is formed by a mixture of powders being blown into a molten pool created on the component by application of a laser, with the mixture of powders being blown by entrainment in a suitable 00 25 carrier gas. The carrier gas may be any suitable inert or protective gas such as nitrogen, hydrogen, argon, helium and mixtures thereof.

In use of the spray and fuse process, whether as a single spray-step or as combined spray and fuse steps, the powder may be propelled into the flame by compressed air or an aspirating gas, or it may be fed directly into the flame under gravity. In plasma spraying, the powder preferably is fed directly into the MR W:\maryRNCNODEL%583925A.doc plasma whereas, with detonation gun spraying, the powder is propelled by combustion gases such as resulting from detonation of acetylene.

As an alternative to thermal powder-cladding, the hardfacing processing used to form a coating having an iron or iron-base matrix containing dispersed particles can comprise a suitable welding process. Gas tungsten arc welding (GTAW), often called TIG (tungsten inert gas) welding, and plasma arc welding (PAW) are forms of welding able to be used. In each of GTAW and PAW, the filler metal used to form the coating needs to be in the form of a rod or wire. Suitable rod or wire can be produced with a required overall composition corresponding to that of a suitable powder mix used for thermal powder-cladding. Powder can be rolled in strip steel to form a tube, or packed into a preformed hollow steel tube, after which the tube can be swaged or drawn to form rod or wire of a required cross-section. In such case, the powder may comprise or .i 15 predominantly consist of the fine particles which are to be dispersed in a matrix formed at least in part from the steel. Alternatively, the rod or wire may be formed by extruding a suitable powder mix containing a binder which confers sufficient green strength after extrusion, or the powder may be formed into a sintered billet which then is extruded to provide the rod or wire.

Oxidation of the iron of the coating can proceed by any suitable means.

However, it is preferred that oxidation is achieved in a practical period by S* heating the component and its cladding in a suitable atmosphere. One preferred arrangement is to conduct the heating in steam. In an alternative, the 25 heating is conducted in a suitable gas mixture, such as carbon dioxide mixed with a small proportion of hydrogen sufficient to maintain a low partial pressure of oxygen.

The iron of the coating is to be oxidised to a suitable form and to an extent which substantially prevent soldering when the coating is contacted by and provide protection against molten aluminium alloy. The oxide preferably has at least a major proportion of magnetite (Fe 3 0 4 which is found to be particularly MR W:\mary\RNCNODELX583925A.doc 6 effective in preventing soldering. However, the oxide can be or include haematite (Fe 2 0 3 wistite (FexO where x varies from slightly below 1, up to 1), or mixtures of these oxides with magnetite. Most preferably the oxide is substantially magnetite.

Prior to oxidation, the iron can be present in a variety of phases, depending on its composition and rate of cooling of the coating on application. Preferably the hardfacing process by which the coating is formed causes minimal melting of the surface of the component sufficient to ensure satisfactory bonding of the applied coating. Also, the coating itself usually is relatively thin. Thus, the applied coating can be expected to cool rapidly as a result of the steel component on which it is formed acting as a heat sink. As a consequence, the iron can be present in a martensitic or related phase resulting from rapid cooling. Also, depending on composition, the iron can be present as austenite.

15 However the actual phases that are present is a matter of secondary importance to the ability of the iron to be oxidised satisfactorily. Of relevance to the ability to be oxidised and also the choice of the source of the iron in the case of use of an iron-base powder, is the need to avoid use of iron having alloy additions imparting substantial resistance to oxidation.

EXAMPLE 1 S. In trials in accordance with the present invention, high pressure die casting tool samples of H13 tool steel were provided with a protective coating of tungsten particles dispersed in an iron matrix. In these trials, an iron powder used to form the matrix had a maximum particle size of 106 m, a minimum particle size of and a composition of 0.2% carbon, 0.5% manganese, 0.04% phosphorous and a balance of iron. The tungsten powder had a particle size of 6ptm and a purity level of 99.99%.

In the trials, the coating was formed by laser cladding. For this, different mixtures of the iron and tungsten powders were used. The weight ratios of iron powder to tungsten powder used were 70:30; 50:50 and 30:70, respectively.

MR W:\mary\RNCNODEL\583925A.doc 7 In each of a first set of trials using those powder mixtures, a laser beam was scanned over a respective bed of powder mixture pre-placed on a surface of the H13 die casting tool sample. The laser was a 500W average power Nd:YAG laser delivered to the component through a 10m length of 0.6mm diameter stepindex optical fibre. The output end of the optical fibre was terminated with a lens train of 100mm nominal focal length. The lens train with its housing was secured to the Z-axis of computer numerical control system that was used to vary the position of the focal spot of the focused laser beam relative to the casting tool surface sample to which the cladding was being applied.

With the trials, the coatings provided by laser cladding of the steel samples comprised tungsten powder dispersed in a matrix of iron and ranged from about 500[m to 750im in thickness. The coatings then were ground to remove 15 excessive roughness, after which the samples were steam tempered to oxidise the iron matrix of the coatings. The steam tempering involved: heating the coated samples to about 4000C in a reaction chamber; injecting into the chamber steam at a line pressure of about 610OkPa; 20 increasing the temperature of the samples to about 5600C; holding respective samples at about 5600C for intervals of 1 hour, 2 hours and 4 hours; and venting the chamber after the holding period and removing the samples.

25 After the steam tempering, the iron of at least a substantial surface thickness of the matrix material was found to be substantially fully oxidised. The oxide produced was found to be mainly magnetite (Fe 3 0 4 but with some haematite (Fe 2 3 Coated samples which were not subjected to steam tempering and, hence, having the iron matrix substantially free of oxide, were found to be prone to soldering when the coating was contacted with molten ADC12 aluminium alloy.

MR W:AmarYRNCNODELZ83925A.doc In contrast, coated samples subjected to steam tempering were found to have substantial resistance to soldering when the coating was contacted with the molten alloy. The durability of the resistance to soldering was tested by mounting the samples in a rig on which the samples were immersed in the molten alloy. Even after 4 hours immersion of the samples, no reaction between the coating and the molten alloy was detected.

EXAMPLE 2 Further samples of H13 tool steels were coated in the same manner as for each of the series of trials of Example 1. The samples of each series were then subjected to an alternative oxidation procedure. In that procedure the Samples were heated to 5500C in a sealed muffle furnace. During the heating and holding times as in Example 1, the furnace atmosphere was provided by a flow of gas, from a cylinder containing a mixture of CO2 and H 2 in a ratio of 95:5, to .i 15 create in the furnace an oxygen partial pressure of 10-23 atmospheres. On completion of the holding times, the samples were cooled to room temperature in the furnace while maintaining the C0 2

/H

2 flow.

.9 oo *9999* Oxidation of at least outer surface layer of the iron content of the coating of the samples was found to be similar to that obtained in Example 1. However, with heating in the flow of C0 2

/H

2 mixture, the oxide formed was found to be substantially pure magnetite.

9o The further samples of the present Example were contacted with molten ADC12 25 aluminium alloy in a manner similar to that used in Example 1. Again, it was found that even after an immersion time of 4 hours in the molten alloy, no reaction between the coating and the alloy was detected.

As an alternative to the procedure of Example 1, using the same powder mixtures and further H13 tool steel samples, a respective coating could be formed by injected powder laser cladding on each sample. The laser can be as detailed above for use with pre-placed powder. The directed injection may be MR W:\naryVRNCNODEL\583925A.doc 9 conducted using a powder storage and delivery unit and an injection nozzle, each built for the purpose. These units could be designed to deliver the powder mixture in a controlled, near laminar flow with minimal fluctuation. The units also could be designed to deliver the powder mixture in a stream having a width not greater, but preferably less, than the diameter of the focussed laser beam spot on the steel sample being coated, in order to maximise the powder utilisation efficiency. The storage and delivery unit could, for example, be capable of mass flow delivery as low as Ig/min and, with its use, powder flow could be controlled independently of the entraining gas flow by the powder mixture being pushed from a rotating disc with an annular slot, by means of a suitably shaped scraper.

In the laser cladding by powder injection, the powder may be delivered by the nozzle into a melt pool established by the laser beam. The nozzle could employ 15 an inner stainless steel tube able to deliver powder under gravity into the melt pool, and a co-axial outer aluminium tube for delivering shielding gas to the S"surface of the steel sample. Inner tube diameters from 1.1mm to 2.4mm are likely to be suitable, with 1.4mm likely to be optimum. The outer tube could have an internal diameter of 8mm.

Finally, it is to be understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.

MR W:\maryRNCNODELa583925A.doc

Claims

1. A process for producing a steel component having a protective coating, the process including the steps of: forming on a surface of the component, by a suitable hardfacing process, a coating having an iron or iron-base matrix material in which fine particles of another material are dispersed; and oxidising the iron of at least an outer surface layer of the matrix material whereby resultant iron oxide comprises, or contains, a sufficient or required level of magnetite.

2. The process of claim 1, wherein the hardfacing process comprises 15 thermal powder-cladding.

3. The process of claim 2, wherein the thermal powder-cladding comprises laser cladding.

4. The process of claim 2, wherein the thermal powder-cladding comprises a thermal spraying processes able to use for hard facing by powder spraying. The process of claim 4, wherein the cladding is formed by a single spray- step version, or two-step version of a spray and fuse process, by plasma spraying or by detonation-gun spraying.

6. The process of claim 4 or claim 5, wherein particles of powder constituents to form the coating are heated to a plastic or molten state by a gas flame, plasma arc or gases resulting from detonation of an explosive gas mixture, with the heated particles propelled by a gas jet onto the substrate to form the coating thereon. MR W:\mary\RNCNODELX583925A.doc 11

7. The process of any one of claims 4 to 6, wherein the coating is formed by a spray and fuse process, whether as a single spray-step or as combined spray and fuse steps, and powder to form the coating is propelled into a flame by compressed air or an aspirating gas or is fed directly into the flame under gravity.

8. The process of claim 1 or claim 2, wherein the cladding may be achieved by application of a flame, plasma arc or laser to melt a mixture of powders pre- placed onto the surface of the component.

9. The process of any one of claims 1 to 3, wherein the cladding is formed by blowing a mixture of powders into a molten pool created on the component by application of a laser, with the mixture of powders being blown by entrainment in a suitable carrier gas. '9

10. The process of claim 9, wherein carrier gas is a suitable inert or 99 9o protective gas such as nitrogen, hydrogen, argon, helium and mixtures thereof.

11. The process of claim 1, wherein the hardfacing process comprises a 20 suitable welding process. .welin

12. The process of claim 11, wherein the welding process comprises gas tungsten arc welding (GTAW) or TIG (tungsten inert gas) welding, or plasma arc welding (PAW), in which the filler metal used to form the coating is in the form of a rod or wire.

13. The process of claim 12, wherein the rod or wire has a required overall composition corresponding to that of a suitable powder mix used for thermal powder-cladding and is produced by powder being rolled in strip steel to form a tube, or by packing the powder into a preformed hollow steel tube, after which the tube is swaged or drawn to form rod or wire of a required cross-section. MR W:nmary RNCNODEL\583925A.doc

14. The process of claim 12, wherein the rod or wire is formed by extruding a suitable powder mix containing a binder which confers sufficient green strength after extrusion, or the powder is formed into a sintered billet which then is extruded to provide the rod or wire. The process according to any one of claims 1 to 14, wherein oxidation of the iron of the matrix material is achieved by heating the component and its coating in a suitable atmosphere.

16. The process of claim 15, wherein the heating is conducted in steam.

17. The process of claim 15, wherein the heating is conducted in a suitable gas mixture, such as carbon dioxide mixed with a small proportion of hydrogen sufficient to maintain a low partial pressure of oxygen. 0.

18. The process of any one of claims 1 to 17, wherein the iron of the coating is oxidised to a suitable form and to an extent which substantially prevent soldering where the steel component, but for the cladding, would be contacted 9 by molten aluminium alloy. *oa

19. The process of any one of claims 1 to 18, wherein the oxide has at least .I a major proportion of magnetite (Fe 3 0 4 The process of claim 19, wherein the oxide is substantially magnetite.

21. The process of any one of claims 1 to 19, wherein the oxide includes haematite (Fe 2 O 3 wstite (FexO where x varies from slightly below 1, up to 1), or mixtures of these oxides, and magnetite.

22. The process of any one of claims 1 to 21, wherein the coating is formed using iron powder as the source for the matrix material. MR W:\marykRNCNODEL583925A.doc 13

23. The process of any one of claims 1 to 22, wherein the coating is formed using a powder mixture containing iron or iron-base powder and powder of another material providing the dispersed particles.

24. The process of claim 23, wherein the dispersed particles impart wear resistance to the coating. The process of claim 24, wherein the dispersed particles comprise at least one of tungsten, tungsten carbide, silicon carbide and boron nitride.

26. The process of claim 1, substantially as herein described with reference to Example 1 or Example 2.

27. A steel component having a protective coating produced by the method 15 of any one of claims 1 to 26. V"606 S6 28. A steel component according to claim 27, wherein the component is a die 06 tool for use in die casting of an aluminium alloy. S 20 DATED: 9 February 2001 PHILLIPS ORMONDE FITZPATRICK Patent Attorneys for: NISSAN CASTING AUSTRALIA PTY. LTD. MR W:\mary\RNCNODEL\583925A.doc