Ferritic iron-aluminium-chromium alloys

Description

[0001] The present invention relates to ferritic alloys of iron, chromium and aluminium containing a significant amount of hafnium.

[0002] Ferritic iron-chromium-aluminium alloys are known and are in use particularly in environment where resistance to oxidation is of importance. We have found that the oxidation resistance of these alloys and particularly their resistance to oxidation in the presence of sulphur and oxygen, as may be encountered in combustion atmospheres, can be greatly enhanced by small additions of hafnium.

[0003] Accordingly, the present invention provides a ferritic alloy of iron, chromium and aluminium containing:-

0 to 5% nickel, the nickel content not however being so great as to produce significant amounts of a second phase,

0 to 2% titanium, and from 0.5 to 1% hafnium, the balance being iron and incidental amounts of impurities.

[0004] Preferably the alloys contain 15.5 to 25% chromium and 4.5 to 10% aluminium.

[0005] Preferably the alloys contain from 15.5 to 18% chromium.

[0006] Preferably the alloys contain from 4.5 to 6% aluminium e.g about 5%.

[0007] The percentage of nickel is chosen so that it is not so great within the range quoted above as to produce significant amounts of a second phase taking into account the amounts chosen for each of the other ingredients of the alloy. Preferably, the amount of nickel does not exceed 0.5%.

[0008] Typical impurities are yttrium, zirconium, cerium, lanthanum, molybdenum, copper, tungsten and cobalt. Other elements such as sulphur, phosphorus and vanadium may be present as impurities but are not desirable.

[0009] The alloys may be manufactured by the processes normally used for making alloys of this general type. For instance, the alloys may be made by induction melting, either in air or using inert atmosphere or vacuum as appropriate, cast into ingots and subsequently forged or rolled into billet or slab prior to working down to strip, bar, wire or any other commercially sealable form.

[0010] In a typical small scale process for producing an iron-chromium-aluminium steel of the invention, a charge of high purity iron and low carbon ferrochromium is melted down in a basic lined induction furnance, either in air under a basic slag, or under an inert atmosphere or in vacuo, without slag, as is appropriate. When completely melted, the appropriate additions of aluminium, ferrotitanium and hafnium metal are added, in that order, the metal brought to temperature and cast into an appropriate ingot mould.

[0011] The invention will be illustrated by the following examples. Alloys according to the invention were prepared having the compositions shown below by the process described above.

[0012] The size of melt was 10 kg. giving a 2'-_z" (60 mm) sq ingot. This was then heated to about 1100°C and forged under a 10 cwt (508 kg) hammer to produce suitable test bar. For the purpose of comparison, alloys not in accordance with the invention were prepared having the compositions shown below.

[0013] The resistance of these steels to oxidation was compared with the following scaling test procedure.

[0014] Specimens some Z" (13 mm) in diameter by

(30 mm) long were machined from bar and ground to a 120 grit finish. They were washed and cleaned in alcohol prior to test.

[0015] The test was of relatively short duration but involved cycling between ambient and test temperature. The test chamber was an alumina tube 2" (50 mm) internal diameter in which the sample was positioned across an open ended alumina boat. Heating was by means of the concentric electric furnace, the temperature being measured by reference to a noble metal thermo-couple, the hot junction of which was immediately above the specimen. The test amosphere was produced by burning natural gas using excess air over that required for combustion, the flow rates being 1.4 cubic foot and 14.0 cubic foot (0.04 and 0.4 cubic metres) per hour respectively for gas and air. The combustion product, a mixture of nitrogen, oxygen, carbon dioxide and steam was pre-heated to test temperature before passing through the test chamber; test temperature was established prior to inserting the sample so that heating was rapid. Each test cycle was for six hours, after which the specimens were removed from the test chamber and cooled in a closed container so that any oxide scale which became detached was collected. When cold, the specimen was weighed, together with any detached scale and then scrubbed with a stiff bristle brush to remove any loosely adhering oxide prior to re-weighing to get the starting weight for the next cycle. The whole procedure was repeated for a total of seven cycles and the total gain in weight, that is the sum of the individual gains, expressed as milligrams per square centimetre, for the 42 hour period, using the original surface area for the untested specimen, was taken as the scaling index.

[0016] The scaling indexes found for the steels tested are shown in the following Table.

[0017] Samples A to C are in accordance with the invention but do not exemplify its most preferred embodiments. It can be seen that the scaling index of A at 1200°C is considerably improved over that of comparative Sample 1 and is still supeior to that of comparative Sample 2 despite this latter containing much higher amounts of chromium and aluminium. The scaling index of Samples A to C is quite high however at 1250°C. Examples D to K which contain above 4.5% AI and 15.0% Cr show the still more superior characteristics of steels in accordance with the preferred practice of the invention and it can be seen that low scaling indexes are found even at 1250°C.

[0018] It can also be seen that the addition of titanium appears to confer a much smaller but still significant benefit in increased oxidation resistance to the steel containing hafnium. More significantly, the addition of titanium has a substantial effect in reducing the as-cast grain size of the steel and hence greatly improves its hot-working properties. In particular, it was observed that the cast structure of titanium- containing alloys such as B differed from that of alloys A and 2. It was noted that the addition of titanium appeared to have a marked effect on the crystallisation pattern, modifying the coarse columnar crystals of the normal product and giving a more uniform crystal distribution across the section.

[0019] Ingots of the steels were hot worked and it was noted that alloys containing titanium' possessed added ductility and less proneness to surface rupture. Resistance to cracking under thermal stress was enhanced.

[0020] Suitable fields of application for steels according to the invention are those in which resistance to oxidation at high temperature is required. Examples of such uses are in the provision of electric furnace winding material or resistance heating wire generally and in the provision of knitted wire catalyst supports e.g for use in vehicle exhaust systems for reducing emissions. Another field in which such properties are of value is in the construction of furnaces, for instance fluid bed combustion furnaces.

Claims

1. A ferritic alloy of iron, chromium and aluminium containing:-

0 to 5% nickel, the nickel content not however being so great as to produce significant amounts of a second phase, characterised by containing from 0.05 to 1% hafnium, the balance being iron and incidental amounts of impurity.

2. An alloy as claimed in claim 1 containing from 15.5 to 25% chromium and from 4.5 to 10% aluminium.

3. An alloy as claimed in claim 1 containing from 15 to 18% chromium.

4. An alloy as claimed in any preceding claim containing from 4.5% to 6% aluminium.

5. An alloy as claimed in any preceding claim containing about 5% aluminium.

6. An alloy as claimed in any preceding claim containing not more than 0.5% nickel.

7. An alloy as claimed in claim 6 containing from 0.2 to 0.5% nickel.

Revendications

1. Un alliage ferr tique de fer, de chrome et d'aluminium, contenant:

0 à 5% de nickel, la teneur en nickel n'étant cependant pas assez élevée pour produire des quantités notables d'une seconde phase, caractérisé en ce qu'il contient de 0,05 à 1 % de hafnium, le complément étant du fer et des impuretés accidentelles.

2. Un alliage selon la revendication 1, contenant de 15,5 a 25 % de chrome, et de 4,5 à 10 % d'aluminium.

3. Un alliage selon la revendication 1, contenant de 15 à 18 % de chrome.

4. Un alliage selon l'une des précédentes revendications, contenant de 4,5 à 6 % d'aluminium.

5. Un alliage selon l'une quelconque des précédentes revendications, contenant environ 5 % d'aluminium.

6. Un alliage selon l'une quelconque des revendications précédentes, contenant pas plus de 0,5 % de nickel.

7. Un alliage selon la revendication 6, contenant de 0,2 à 0,5 % de nickel.

Ansprüche

1. Ferritische Eisen-Chrom-AluminiumLegierung, enthaltend

wobei jedoch der Nickelgehalt nicht so groß ist, daß bemerkenswerte Mengen einer zweiten Phase gebildet werden, gekennzeichnet durch einen Gehalt an 0,05 bis 1 % Hafnium, Rest Eisen und vernachlässigbare Mengen an Verunreinigungen.

2. Legierung nach Anspruch 1, enthaltend 15,5 bis 25 % Chrom und 4,5 bis 10 % Aluminium.

- 3. Legierung nach Anspruch 1, enthaltend 15 bis 18 % Chrom.

4. Legierung nach einem der vorhergehenden Ansprüche, enthaltend 4,5 bis 6 % Aluminium.

5. Legierung nach einem der vorhergehenden Ansprüche, enthaltend etwa 5 % Aluminium.

6. Legierung nach einem der vorhergehenden Ansprüche, enthaltend nicht mehr als 0,5 % Nickel.

7. Legierung nach Anspruch 6, enthaltend 0,2 bis 0,5 % Nickel.