Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium

Definitions

• This invention relates to hafnium-containing alloy steels treated with rare-earth additions to enhance the physical properties of the alloy.
• the invention also concerns the production of foil manufactured from such alloys and especially foils for use as, for example, substrate material in catalytic converters employed to clean the emissions of vehicle exhausts.
• European Patent 35369 discloses a ferritic alloy containing between 0.05 and 1.00% hafnium. It has now been found, however, that the high temperature properties of alloy steels can be significantly enhanced by treatment with rare earth additions to give in the final steel specified contents of cerium and/or lanthanum.
• steels in accordance with this invention can advantageously be used in the production of foils for use as substrate material in catalytic converters for cleaning the emissions of vehicle exhausts.
• the service conditions of such substrate material are onerous and include thermal cycling up to 800°C with occasional excursions up to temperatures in excess of 1100°C in addition to exposure to highly corrosive atmospheres.
• stringent requirements are laid down by the end-users regarding the maximum possible weight gain achieved after the foil substrate has been subjected to an oxidation generating atmosphere at a high temperature for a considerable length of time.
• One such criterion specifies a maximum weight gain of 6% after oxidation for times exceeding 300 hours at 1100°C in air.
• Other criteria are set down regarding the physical properties of the substrate material.
• the present invention sets out inter alia to provide a steel from which such foils can be produced.
• a steel alloy of composition containing by weight: 10 to 25% chromium; 1 to 10% aluminium; 0.5% maximum carbon; up to 3% silicon; up to 2% manganese; 0.010% maximum sulphur; up to 5% nickel; up to 2% titanium; from 0.01 to 1% hafnium; 0.01 to 0.10% zirconium; up to 0.05% nitrogen; up to 1% molybdenum; up to 1% copper; 0.1 max phosphorous; rare earth additions to give a cerium and/or lanthanum content of between 0.01 to 0.20%; the balance being iron and incidental amounts of impurities.
• a preferred composition contains from 18 to 25% chromium, from 5.0 to 6.5% aluminium, and a rare earth addition to give from 0.02 to 0.15% cerium and/or lanthanum.
• the percentage of nickel is chosen so that its presence does not produce significant amounts of a second phase taking in account the amounts chosen for each of the other ingredients of the alloy.
• the amount of nickel does not exceed 0.5%.
• the rare earth additions may take the form of Misch metal.
• a foil for use for example, in calatytic converters, the foil having a thickness within the range of 45 to 55 microns (or ⁇ m) and comprising a ferrous alloy of a composition by weight percent including:- C 0.01 to 0.10; Hf 0.01 to 1.00; Cr 18.00 to 25.00; Al 4.00 to 6.00; and Ce and/or La 0.01 to 0.20.
• Alloys in accordance with the invention are preferably produced by a route which includes melting a suitable feedstock within an induction furnace or an electric arc furnace; subjecting the melt or an ingot produced therefrom to secondary refining; and rolling the ingot to gauge (e.g. 0.05mm foil) for high temperature service.
• gauge e.g. 0.05mm foil
• One example of a product of an alloy steel is a catalytic converter substrate material used at temperatures of up to 1250°C for cleaning the exhausts of vehicles.
• a charge of high purity iron and low carbon ferrochromium is melted down in a basic lined induction furnace, either in air or under a basic slag, and the appropriate additions of aluminium, ferro-titanium, hafnium and misch metal made, in that order, to the melt.
• the melt is subsequently cast into an AOD vessel and subsequently subjected to secondary refining.
• the alloy steel ingot had the following composition by weight per cent: C 0.015; Si 0.45; Mn 0.29; Ni 0.24; Cr 20.80; Mo 0.02; S 0.001; P 0.023; Al 5.15; Cu 0.06; Zr 0.047; Hf 0.055; Ce 0.025; Ti 0.04; N 0.006; remainder Fe apart from incidental inclusions and impurities.
• the ingot was hot charged and slabbed, the slab being rolled to hot band of approximately 3mm thickness, annealed, shot blasted and pickled, and then cold rolled to the final gauge using a sequence of cold rolling and annealing. Finally, the rolled material was processed to foil of thickness 52 ⁇ m.
• a sample foil approximating to 25 mm x 25 mm was taken and its surface area and weight measured carefully after a thorough degreasing treatment consisting of ultrasonic agitation in chlorinated solvent, followed by forced drying in hot air.
• This oxidation sample was placed in a platinum crucible in a furnace set at 1100°C. The sample was positioned so as to allow free access of air to its surfaces, After 200 hours the sample was removed, weighed, examined visually and replaced for a further 24 hours. This sequence was repeated for up to 320 hours. Examination of the surface of the as-rolled and as-oxidised sample was carried out on the scanning electron microscope (SEM). Chemical analysis of the scale was performed on the SEM by energy dispersive analysis of x-rays (EDA).
• the weight gain result for the sample was as follows:- Alloy Thickness Exposure Time, h 200 224 248 Weight Gain Weight Gain Weight Gain mg/cm2 % mg/cm2 % mg/cm2 % BF5CeHf 52 ⁇ m 0.638 3.65 0.689 3.94 0.732 4.18 Alloy Thickness Exposure Time, h 272 296 320 Weight Gain Weight Gain Weight Gain mg/cm2 % mg/cm2 % mg/cm2 % BF5CeHf 52 ⁇ m 0.760 4.35 0.782 4.47 0.818 4.67
• the weight gain result is also presented in graphical form in Figures 1 and 2.
• the oxides formed on the sample after 200 hours exposure were very good, displaying the desired characteristics of compactness and uniformity. On further exposure, the oxide coating deteriorated gradually with small areas of more voluminous brown/black coloured oxides becoming apparent There was no visual indication that oxide spalling from the samples's surface had occurred. Oxide spalling would not be acceptable to the end user.
• the first oxide type was alumina-rich and showed variously sized nodules. These nodules varied in composition but generally had a high aluminium content with small amounts of other elements and occasional significantly higher levels of titanium and hafnium, e.g. 19% and 14% respectively.
• the large grained oxide was high in iron oxide, 93%, but also showed some chromium.
• the sample satisfied the criteria set for such by the end user.
• the most important factors influencing the onset of the breakaway stage are therefore the matrix chromium content, the foil thickness, the matrix aluminium content, the presence of surface defects and the adherence of the protective alumina layer, which is improved by active metal additions.
• foil made from alloy steels in accordance with the invention meet the industry's requirement of a maximum weight gain of 6% after oxidation for 300 h at 1100°C in air.
• the alloy examined formed a compact, adherent alumina oxide layer under the stipulated oxidising conditions. Only after prolonged exposure did a small proportion of iron oxide form and it also appeared to be protective.

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Citations (3)

• 1988
  • 1988-11-01 GB GB8825536A patent/GB2224288B/en not_active Expired - Fee Related
• 1989
  • 1989-11-01 EP EP89311327A patent/EP0370645A1/de not_active Withdrawn

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