GB2207622A - Process for manufacturing thin-walled intermediate products, and their applications - Google Patents

Description

1 - 1 1 e__ 1- 1. - -76 1 PROCESS FOR MACTURING TRIN--WALL STE DIAU

ERMUCTS, AND THEIR APPLICATIONS The invention relates to a process for manufacturing thin-walled intermediate products such as wire or foil of chrome-steel (heating conductor or heating-element material), and to their application.

In apparatus for purifying hot flue gases, and in heating conductors in electrically heated furnaces it is known to use ferritic materials, in particular the steels containing 18 to 26% chromium, 4 to 6% aluminium and about 0.5% silicon, the remainder mainly iron, which are given the numbers 1.4767 and 1.4765 in the "Stahl-Eisen Werkstoffliste% 6th edition, 1977, on pages 102 to 103.

It is also known from the US Patent Specification US-PS 2 061370 that a small addition of rare earths in heating-conductor alloys has a favourable influence on resistance against corrosion or on the oxide layers. An equally good influence is attributed to yttrium, titanium and zirconium. By applying a 2-stage oxidation, columnar crystals of oxide can be grown, as described in the British Patent Specification GB-OS 2 081747.

For bonding ferritic materials of cQnstruction a number of processes are known, for example the sintering of catalyst carriers onto heatingconductor foil at about 1300'C without additives, as described in the German Patent Specification DE-PS 29 47 894, and the application of copper by brazing, subsequently alloyed with aluminium, as described in the US Patent Specification US-PS 3 891784.

Nowadays it is conventional to bond heating conductor parts made of steel cont g aluminium by using nickel-based brazing material.

From the German Patent Specification DE-PS 23 13 040 a catalyst carrier of metal, or of a metal alloy, is known which has at least two layers: 1) a layer of cobalt-nickel-chromium-iron alloy, and 2) a layer of alphaaluminium oxide at least 10pm thick. Layer 1) is applied by flamespraying or plasma-spraying, or by sintering sprinkled powder, by cementation or by applying a dye containing the powder. In order to achieve a welding by diffusion there is applied, before the layer 2), a foundation-layer of nickel either by electro-plating or by applying a nickel dye and sintering under hydrogen, the foil layers being pressed together. The minimal foil thickness is said to be 0.1 mm.

1 2 The disadvantages of this process are as follows:- Firstly, the multiple coating using powders is costly.

Secondly, if dye is used the nickel foundation-layer is uneven.

11firdly, the resulting structure has little strength, unless pressure is applied during the sintering.

Furthermore, by this process it is not possible to make self-supporting coiled structures of foil. To achieve mechanical stability it is necessary to interpose welded-on wires as spacers between the foils. The layers of foil, after coating with nickel, are stacked and sintered under pressure. In this operation the spacer wires must be positioned precisely over each other, otherwise the structure becomes squashed during the sintering. Moreover, the electro-plating with nickel is costly. In the case of ferritic heating conductors containing aluminium, applying the nickel dye with a brash results in inadequate wetting of the surface and, consequently, weak bonded regions. If the metal foil were high in chromium, wettability would be even worse.

The German Patent Specification DE-PS 27 45 188 describes catalyst carriers of iron or steel which are given an aluminium-iron diffusionapplied layer by annealing, the layer being resistant to scaling, strongly adhering and holds attachments well. But this German patent specification provides no solution to the technical problems presented by the use of high-grade chrome-steel as the foundation material, nor does it say how the coiled structures made of high-grade steel can be given mechanical strength. All that this document says it that thickwalled tubes coated with aluminium are induction-welded in the conventional manner. The starting point is said to be a laminated material of steel bonded to an aluminium layer more than 2Ogm thick. The bonding is done by dipping, by roll-cladding, by hot roll-cladding, by alumetizing, alitizing or calorizing. Or the layer is applied by the prior art of the year 1977.

The US Patent Specification US-PS 3 437 605 describes the coating with aluminium powder of metal catalyst carriers of stainless steel. The coated product is then thermally oxidised, resulting in a well-adhering aluminium oxide layer. No solution is proposed for processing high-grade chrome-steel containing more than 12% of chromium, and the method of bonding the foil to form a honeycomb structure is not disclosed.

1 3 The DD-PS 17 711 describes the bonding of metal parts by copper-foil brazing, and the alloying of the steel and the brazing region by precipitation of chromium from the gas phase and chromium diffusion. The British Patent Specification GB-PS 992 321 describes the deposition of a layer containing aluminium, using powders, followed by diffusion. The European Patent Specification EP-PS 0 201 910 describes the coating with aluminium of sheet material more than 0.25 mm thick of aluminium steel. The aluminium layer, which is 12.7 to 76 Kin thick, is produced by dipping the steel sheet in molten aluminium. This is followed by further rolling to form foil, and finally, difflision-annealing.

The coating methods hitherto known have the following disadvantages; costly.

Coating by dipping in molten aluminium produces comparatively thick coats, thicker than 20 gm. But chrome-steels coated with aluminium cannot be rolled to give foil thicknesses of to 70 pm, and wire coated with aluminium cannot be drawn to diameters of 0.1 to 0.05 mm. Consequently, dip-coating of thin heating conductors produces high aluminffirn contents in the material, with consequent brittleness.

Physical and chemical processes for precipitating aluminium from the gas phase are too Lead baths containing aluminium and chromium, as proposed in the US Patent Specification US-PS 4 247 422, clash with the requirement that the environment must be protected.

Coatings of aluminium powder mostly sinter without making metalno-metal contact and, consequently, an even alloy with the subtstrate cannot be formed by subsequent diffusion.

The intention in the invention is to provide a simple and economical process by which one can apply even aluminium coatings to thin-walled chrome-steel intermediate products in such a way that the aluminium coat is mostly metal-to-metal bonded to the substrate.

The problem is solved, according to the invention, by first making an intermediate product of aluminium-free, or almost aluminium-free chromesteel, the thickness of the intermediate product being the thickness intended in the final product, or almost this thickness. The intermediate product is then friction-coated with aluminium, or with a material containing aluminium and then finally, diffusion-annealed, either immediately or subsequently, if desired just before the intermediate product is put to use.

4 It is considerably simpler and more economical to begin by rolling an aluminium-free chrome-steel, containing 12 to 30% of chromium, to form a thin-walled structure, or to begin by drawing wire to a small diameter, and then apply and alloy the aluminium coating, as described in the present invention. This method not only makes much of the intermediate annealing unnecessary, because much higher degrees of deformation can be achieved at each pass between the rolls, or at each draw of die wire, and therefore fewer rolling or drawing steps are required, but also greatly decreases abrasion of the tools.

It has been found that friction-coating with aluminium gives a thin, even layer, most of it making metal-to-metal contact with the substrate. Subsequent diffusion-annealing in vacuo, or under a protective gas, produces an evenly alloyed intermediate product. The resulting coated thin foil can then, if desired, be embossed and coiled to form a compact structure, which is finally diffusion-annealed in vacuo at about 130TC. The diffusion and the bonding of -the layers of foil to each other are therefore both accomplished in a single thermal process step.

Measurements have shown that friction-coating is a mechanical process in which, in the first place, the surface of the intermediate product is abraded clean, producing a fresh surface.

Simultaneously the surfaces of the aluminium particles are also abraded clean, producing fresh surfaces on them, not contaminated by oxygen andlor nitrogen. The hard, abrasive materials used in the process produce a true metal-to-metal contact between the aluminiurn powder and the substrate. It should he observed that the resulting bond is not a result of the development of heat by friction.

In regard to the nature of the abrasive powder, it has been found that silicon carbide, aluminium oxide and the like conventional abrasives are suitable, or a hard metal powder, preferably containing aluminium as an alloy constituent. The abrasive powder is mixed with aluminium powder, the mixture being projected against the surface of the intermediate product by a device similar to a sand-blasting apparatus, the powder particles being carried in a stream of protective gas, the process being conducted in an atmosphere of protective gas.

Alternatively, if desired, aluminium powder alone can be used in the process of the invention. Or there can be used a powder mixture of, for example, aluminium mixed with an iron-aluminium alloy as the hard material. In drawing wire, in the last draw the powder mixture can be fed into the gap between the wire and the wall of the mouth of the drawing die. Friction between the die and the FeAl alloy powder, and between the FeAl powder particles. as well as the simultaneous deformation of the wire and the thrusting of the powder particles into the surface of the wire, produce a coating with predominantly metal-to-metal contact between the aluminium and the substrate surface of the intermediate product.

1 1 1 i A further, particularly good method which can be used for friction- coating consists in using a rotating steel brush in an atmosphere of protective gas. The brash is applied to the surface of a rotating aluminium rod, rubbing away particles of aluminium from the rod, the brush also being applied to the surface of a travelling steel foil. Most of the aluminium particles are collected as a powder, which is screened. The screened powder is then preferably used to form a slurry, whch can subsequently be used for dip-coating intermediate products such as catalyst carriers. The dip-coating is then sintered and oxidised. This produces a rough coating of aluminium oxide on the intermediate product which protects the substrate material and forms a well-adhering base for taking further coatings.

For forming textured coatings of oxides, which are mainly aluminium oxide, and for improving the adhesion of the oxide layers to the substrate, it has been found useful to alloy the aluminium coating material with a small quantity of zirconium. Alternatively, one can alloy with titanium, rare earths or yttrium.

In particular, yttrium improves adhesion of the oxide layer. But in the conventional processes for making yttrium-doped heating conductor materials large quantities of yttrium, which is an expensive substance, are lost, for example as slag or roll-scale. Aluminium alloys with 0.001 to 0.1% of yttrium can be coated and alloyed with less loss. Frictioncoating with aluminium containing yttrium has a further advantage that after diffusion-annealing the yttrium is found only near the surface of the heating conductor, where is has a good effect, and not evenly distributed in the substrate material, where it is not wanted.

The invention will now be described with the help of examples.

6 t Exw-ple 1 Chrome-steel wire of Steel No 1.4510 (C 0.03. Si 0.49, Aln 0.56, P 0.021, S 0.003, Cr 18.27, Ti 0.51) was drawn in the conventional manner down to 0.09 mrn diameter. The wire was introduced into a diamond drawing die of at least aperture about 0.084 mm, which was positioned with its axis vertical. Its mouth-diameter, into which the wire was inserted, has a diameter of 0.13 nun. With the help of a vibratory feeder and a funnel a powder was fed into the inlet between the wire and the drawing die, the powder consisting of. a) in one case aluminium with 0.2% yttrium, b) in another case aluminium with 0.4% zirconium, in each case with up to 40% ferro-aluminium, the grain size in each case being k 15 gm. When the wire was drawn through this mixture in the mouth of the die, the powder became rubbed into the wire, giving it an alumin3ilm-ricb surface layer of thickness about 3 to 5 gm. The coated wire was heated to 10500C in H2/N2 reaction gas for 8 minutes, blackening the wire by the formation of nitride. The interior of the wire became.alloyed with about 5% of aluminium. The wire was then allowed to oxidise in air at 120TC, which gave it an oxide layer showing, in both cases a) and b), an excellent resistance to fluctuating temperatures. Equally good results are obtained by using a slurry of powder in an organic solvent.

Example 2

A sand-blast nozzle was positioned over an aluminium rod (80 nun diameter, 1 % zirconium) with a sloping end. A blast of powder was produced, carried in a stream of argon.

A strip of foil (width 50 nun, 0.04 mm thick, of Steel No 1.4510) was positioned under the nozzle and conveyed along at a speed of about 1 m/minute. The powder was ferro-aluminium of grain size between 50 and 150 pm diameter, which was blown against the rod material dt an acute angle with high velocity, the particles of powder then impacting on the foil. By the end of 5 minutes the powder, which was continuously re-cycled, had picked up enough aluminium on its surface to coat the foil with an aluminium layer about 1.5 pm thick. -By this method objects of even quite complex shapes, instead of foil, could be given a thin coat of aluminium.

Q, 7 ExMle 3 A foil of thickness 45 Km of Steel No 1.4510 was brushed on both sides under argon with a steel brush (wire 0.2 mm diameter, the brush of diameter 200 mm turning at 3000 revolutions per minute) loaded with aluminium containing 2% of Zr as a compact powder. The foil became coated (solder coated) on both sides with a layer 1.5 [LM thick. The excess powder, which also contained some abraded material, was collected and screened to give grains of size > 0.05 mm diameter. The resulting foil was then passed between the pair of embossing rolls with arrow-like projecting teeth (6 tooth-point angle) and with corrugations (wave height 0.89 mm, spacing about 5.5 mm). Coiling the resulting foil gave a structure showing channels with open cross sections of dimensions about 0. 85 x 5.0 rnm. The coiled foil was equipped with a slotted tubular jacket of Steel No 1.4841 with silicon carbide rings and heated in a high vacuum for 15 minutes at 124TC, with a holding period of 15 minutes at 12OTC. Alternatively one can use rings or other materials which have thermal expansions about the same as silicon carbide, such as aluminium oxide, molybdenum, the material known as UM, or others. It is advisable to pretreat the slot of the jacket tube with a nickel-base solder.

After cooling and removal of the silicon carbide rings, the product was a stable honeycomb structure contained by a butt welded jacket tube which is also firmly bonded to the honeycomb structure. Polished sections of the jointing regions of the honeycomb structureshowed typical soldered or brazed joints formed by the formation of a liquid phase. Enrichment with aluminium is not marked except the 1601p in the contact zones. In contrast, sintered regions at the joints of ferritic heating-conductor foils (5.5% AI, 24% Cr, the remainder mainly iron) show decreased aluminium and increased chromium.

The product was then dipped in an organic slurry containing 60g11 of the aluminium abrasion powder collected during the friction coating, together with 40 gIl of a ferro-aluminium powder of grain size -, 70 gm. After dipping, the product was blown with compressed air and dried, leaving about 50 grams of deposit per kg of foil. The product was then heated in H2/N 2 reaction gas in a soldering furnace ( Oew-point - 4TC) to 10STC, producing a black, rough and firmly adhering coating. The product was then annealed in air for 5 hours at 900 to 130TC, resulting in a formation of a grey, firmly adhering, rough oxide layer showing short whiskers. FinaIlly, the product was given, as is conventional, a wash coat of y-alum' ium oxide, with promoter additives, and an impregnation of noble metal. The resulting powder is used as a 3-way catalyst reactor. Subsequent measurements showed about 3% aluminium in the material.

1 8

Claims (9)

1. CLAIMS i 1. A process for manufacturing thin-walled intermediate products

   in the form of wire or foil of steel containing chromium (heating-conductor material), characterised in that an aluminium-free steel is first deformed down to its final thickness, or to nearly the final thickness, after which the surface of the intermediate product is friction-coated with aluminium, or with a material containing aluminium, after which the intermediate product is finally diffusion-annealed.
2. 2. Process as claimed in Claim 1, characterised in that the frictioncoating is done before, or simultaneously with, the last cold deformation.
3. 3. 'Process as claimed in Claim 2, characterised in that a wire is coated, before the last drawing operation, with a powder containing aluminium, after which the wire is drawn to its final dimensions.
4. 4. Process as claimed in Claim 1, characterised in that the original intermediate product is a foil, which is friction-coated by means of a rotating mechanical device.
5. 5. Process as claimed in Claim 4, characterised in that the rotating mechanical device is a steel brush or a leaf-fan abrader, which picks up coating material from an aluminium rod.
6. 6. Process as claimed in one of the claims 1 to 5, characterised in that the coating material is an aluminium alloy containing 0.1 to 3% zirconium, or 0.001 to 1% yttrium, the intermediate product being of a steel containing 12 to 30% chromium.
7. 7. Process as claimed in one of the Claims 1 to 6, characterised in that several layers of the intermediate product are laminated together using the applied coating as a solder.
8. 8. Process as claimed in Claim 7, characterised in that after die soldering the intermediate product is slurry-coated with a powder containing aluminium and dried, after which the coating is sintered under a protective gas, after which it is, either immediately or subsequently, thermally or chemically oxidised.
9. 9. Ile use of an intermediate product made as claimed in Claims 1 to 8 for manufacturing catalyst carriers, heat exchangers or soot filters.

   Published 1988 at The P itent O-Tice, State House, 66.171 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Crkv, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.