GB2082631A

GB2082631A - Ferritic iron-aluminium-chromium alloys

Info

Publication number: GB2082631A
Application number: GB8006739A
Authority: GB
Original assignee: Firth Brown Ltd
Current assignee: Firth Brown Ltd
Priority date: 1980-02-28
Filing date: 1980-02-28
Publication date: 1982-03-10
Also published as: DE3168563D1; EP0035369B1; EP0035369A1

Description

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GB 2 082 631 A

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SPECIFICATION

Ferritic iron-aluminium-chromium alloys

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

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 10 cornbustion atmospheres, can be greatly enhanced by small additions of hafnium.

Accordingly, the present invention provides a ferritic alloy or iron, chromium and aluminium containing lOto 15% chromium

1 to 10% aluminium

Oto 0.15% carbon 15 Oto 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,

Oto 2% titanium, and hafnium in an amount up to 1%,

20 the balance being iron and incidental amounts of the other alloying elements.

Preferably the alloys contain from 13 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 25 ingredients of the alloy. Preferably, the amount of nickel does not exceed 0.5%.

Other rare earth metals may be used in addition to hafnium, e.g. yttrium, zirconium in an amount up to 2% or more preferably in an amount up to 1% or the commercially available 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.

30 The presence of incidental amounts of molybdenum, copper, tungsten and cobalt above the impurity level can be tolerated 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 atmosphere or vacuum 35 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 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 furnace, either in air under a basic slag, or under an inert atmosphere or in vacuo, without slag, as is appropriate. When 40 completely melted, the appropriate additions of aluminium, ferrotitanium and halfnium metal are added, in that order, the metal brought to temperature and cast into an appropriate ingot mould.

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

The size of melt was 10 kg, giving a 21/2"(60 mm) sq ingot. This was then heated to about 1100°C and 45 forged under a 10 cwt hammer to produce suitable test bar.

Sample % Chromium % Aluminium % Titanium % Hafnium

A 14.1 4.6 Nil 0.53

50 *

B 15.9 4.8 0.34 0.46

For the purpose of comparison, an alloy not in accordance with the invention was prepared having the 55 composition shown below.

Sample % Chromium % Aluminium % Titanium % Hafnium C 12.6 4.3 Nil Nil

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

Specimens some W (13 mm) in diameter by 1V4" (30 mm) long were machined from bar and ground to a 120 grit finish. They were washed and cleaned in alcohol priortotest.

65 The test was of relatively short duration but involved cycling between ambient and test temperature. The

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GB 2 082 631 A

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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 5 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 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 an^1 10 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 tq 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 15 taken as the scaling index.

The scaling indexes found for the steels tested at 1200°C were as follows:

A 0.99 20 B 0.56

C 403

It can be seen that the addition of hafnium (comparing A and C) results in a very substantial increase in 25 oxidation resistance whilst the addition of titanium appears to confer a much smaller but still significant benefit to the steel containing hafnium.

Suitable fields of application for steels according to the invention are those in which resistance to oxidation at high temperatures 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 30 e.g. for use in vehicle exhaust systems for reducing emissions. Anotherfield in which such properties are of value is in the construction of furnaces, for instance fluid bed combustion furnaces.

It was observed that the cast structure of titanium-containing alloy B differed from that of alloys A and C. It was noted that the addition of titanium appeared to have had a marked effect on the crystallisation pattern, modifying the coarse columnar crystals of the normal product and giving a more uniform crystal distribution 35 across the section.

Ingots of the steels were hot worked and it was noted that the containing titanium B possessed added ductility and less pronenessto surface rupture. Its resistance to cracking under thermal stress was enhanced.

Claims

CLAIMS 40 1. A ferritic alloy or iron, chromium and aluminium containing:- 10 to 25% chromium

1 to 10% aluminium

Oto 0.15% carbon 45 0 to 3% silicon

0 to 2% manganese

Oto 2% titanium

50 and hafnium up to 1%,

the balance being iron and incidental amounts of the other alloying elements.

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

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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 claim 1 substantially as hereinbefore described in the Example.

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Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.