GB2070642A - Ferritic iron-aluminium- chromium alloys - Google Patents

Abstract

A ferritic alloy of iron, chromium and aluminium containing:- 10 to 25% Chromium 1 to 10% Aluminium 0 to 0.15% Carbon 0 to 3% Silicon 0 to 2% Manganese 0 to 5% Nickel, the nickel content not however being so great as to produce significant amounts of a second phase, up to 2% of rare earth metals selected from Yttrium, Hafnium, Zirconium, Cerium and Lanthanum, And from 0.1 to 2% Titanium, the balance being iron and incidental amounts of the other alloying elements. The alloy may be used for resistance heating elements, furnace construction, or as a knitted wire catalyst support in motor vehicle exhaust systems.

Description

SPECIFICATION Ferritic iron-aluminium-chromium alloys The present invention relates to ferritic alloys of iron, chromium and aluminium containing a significant amount of titanium.

Ferritic iron-chromium-aluminium alloys are known and are in use particularly in environments where resistance to oxidation is of importance. These alloys tend to have a relatively large ascast grain size which is detrimental to the subsequent working of the alloy into forms in which it can be utilised.

We have now found that the as-cast grain size of such alloys can be reduced and their hot working properties improved by the addition of a small amount of titanium.

According, the present invention provides a ferritic alloy or iron, chromium and aluminium containing: 10 to 25% chromium 1 to 10% aluminium O to 0.15% carbon O to 3% silicon O to 2% manganese O to 5% nickel, the nickel content not however being so great as to produce significant amounts of a second phase, up to 2% of rare earth metals selected from yttrium, hafnium, zirconium, cerium and lanthanum, and from 0.1 to 2% titanium, the balance being iron and incidental amounts of the other alloying elements.

Preferably the alloys contain from 1 3 to 18% chromium.

Preferably the alloys contain from 1 to 6% aluminium e.g. about 4.5%.

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%.

It is known that addition of yttrium can improve the oxidation resistance of iron-chromiumaluminium alloys and accordingly, in one preferred aspect of the invention, the alloys contain yttrium in an amount up to 1%. Other rare earth metals may be used in a similar way to improve the oxidation resistance of the alloys and accordingly, it is alternatively preferred to provide hafnium in an amount up to 1%, zirconium in an amount up to 2% or more preferably in an amount up to 1% or the commercially avaiable alloy known as mischmetall in an amount up to 1%. Naturally, the principal ingredients of mischmetall, cerium and lanthanum may be used individually if desired.

The presence of incidental amounts of molybdenum, copper, tungsten and cobalt above the impurity level can be tolereated provided these elements are not present in excess. Other elements such as sulphur, phosphorus and vanadium may be present as impurities but are not desirable.

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 atmospheres 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 saleable form.

In a typical process for producing an iron-chromium-aluminium steel of the invention, a primary charge consisting of iron, ferro-chromium and suitable scrap if avaiable would be introduced into a high frequency induction furnace with a basic lining, e.g. a 6 ton furnace. The first slag would be removed when the metal is clean melted, a clean reducing slag made up and sufficient aluminium added to approximate to the composition desired. An analysis sample would be taken and the composition adjusted by suitable final additions of chromium and/or aluminium. The temperature would be adjusted and the appropriate amount of ferro-titanium added. As soon as this was melted in, the metal would be tapped into a ladel and then teemed into ingots of around 1 ton in weight.The ingots would then be suitably forged to a desired size or alternatively to blooms which might subsequently be rolled to billet or slab for bar wire or strip production.

The invention will be illustrated by the following example. Alloys according to the invention were prepared having the composition shown below.

Sample % Chromium % Aluminium % Titanium % Hafnium A 14.2 4 0.27 Nil B 15.9 48 0.34 0.46 For the purpose of comparison, two alloys not in accordance with the invention were prepared having the composition shown below.

Sample % Chromium % Aluminium % Titanium % Hafnium C 12.6 4.3 Nil Nil D 14.1 4.6 Nil 0.53 The cast structure of titanium-containing alloys A and B was compared with the reference materials C and D. It was noted that the addition of titanium had 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.

Ingots of the steels were hot worked and it was noted that those containing titanium (A and B) possessed added ductility and less proneness to surface rupture. Their resistance to cracking under thermal stress was enhanced. The net effect of these changes in practice would be a higher yield of servicible billet or slab with a lower conversion cost and lower rectification charges.

The resistance of these four steels to oxidation was compared by the foilowing scaling test procedure.

Specimens some " (13 mm) in diameter by 14111 (30 mm) long were machined from bar and ground to a 1 20 grit finish. They were washed and cleaned in alchohol prior to test.

The test was a 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 atmosphere was produced by burning natural gas using excess air over that required for combustion, the flow rates being 1.4 cubic foot and 1.14 cubic foot (0.04 and 0.4 cubic metres) per hour respectiveiy 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 centimetres for the 42 hour period, using the original surface area for the untested specimen, was taken as the scaling index.

The scaling indexes found for the four steels tested at 1200"C were as follows: A 233 B 0.56 C 403 D 0.99 It can be seen that the addition of titanium to a steel in the absence of hafnium (comparing A and C) results in a marked increase in oxidation resistance. It can also be seen that the increase in oxidation resistance is still brought about when titanium is added to a hafnium containing iron chromium aluminium alloy (comparing B and D) although in this case the addition of hafnium has brought about a very large increase in oxidation resistance in itself.

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 (11)

1. A ferritic alloy of iron, chromium and aluminium containing: 10 to 25% chromium 1 to 10% aluminium O to 0.15% carbon O to 3% silicon O to 2% manganese O to 5% nickel, the nickel content not however being so great as to produce significant amounts of a second phase, up to 2% of rare earth metals selected from yttrium, hafnium, zirconium, cerium and lanthanum, and from 0.1 to 2% titanium, the balance being iron and incidental amounts of the other alloying elements.

2. An alloy as claimed in claim 1 containing from 13 to 18% chromium.

3. An alloy as claimed in claim 1 or claim 2 containing from 1 to 6% aluminium.

4. An alloy as claimed in any preceding claim containing about 4.5% aluminium.

5. An alloy as claimed in any preceding claim containing less than 0.5% nickel.

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

7. An alloy as claimed in any preceding claim containing yttrium in an amount up to 1%.

8. An alloy as claimed in any one of claims 1 to 6 containing hafnium in an amount up to 1%.

9. An alloy as claimed in any one of claims 1 to 6 containing zirconium in an amount up to 2%.

10. An alloy as claimed in claim 9 containing zirconium in an amount up to 1%.

11. An alloy as claimed in any one of claims 1 to 6 containing mischmetall in an amount up to 1%.

1 2. An alloy as claimed in claim 1 substantially as hereinbefore described in the Example.