IL23129A - Process for the preparation of an iron-aluminium alloy - Google Patents

Description

'jii iDi inD TJinrn m PATENT ATTORNEYS · □ 'DID 3 ' ZJ 111) PATENTS AND DESIGNS ORDINANCE SPECIFICATION A process for the preparation of an iron-aluminium alloy l ian- na na ioao n aan I (we) COMMISSARIAT A L'EIERGIE ATOIIIQUE, of 29, rue de la Fe\ieVation, Paris 15e, France do hereby declare the nature of this invention and in what manner the same is to "be performed, to he particularly described and ascertained in and by the following statement:- - 2 - 23129/2 ^ The present invention consists in a process for the preparation of an iron-aluminium alloy and in alloys obtained by this process.

More particularly, the present invention is an improvement in the process for tie preparation of an iron-alilminium alloy described and claimed in the Patent Specification Uo. 18784 herein referred to as "main patent".

The process according to the main patent comprises: melting an amount of iron which corresponds to a proportion less than 84i¾ by weight of the alloy, adding to the molten iron the other constituenta of, the alloy, casting the melt at a tempera ture slightly above the solidification point of the alloy, cooling the alloy to solidify it in the form of an ingot, and subjecting the ingot to hot-state mechanical working and deformation to destroy the casting structure.

The main patent . already contemplates the introduction in small quantities of less than X of addition elements which are intended to facilitate the trapping of embrittling impurities ( the iron being preferably of high purity so that its carbon content be lower than 1,000 ppm) .

These addition elements may be zirconium, niobium, tiianLum, yttrium, the rare earths or baron.

The present invention consists in a process which endows the alloy from the time of casting with a finer grain and improved machinability as compared with the alloy tfhlch is prepared in accordance with tho main patent.

The invention consists in a process for tho preparation of an ironraluminlum base alloy comprising melting an amount of iron which corresponds to a proportion less than 84$ by weight of the alloy} adding to the molten iron additives selected from the group consisting of yttrium and rare earths} adding aluminium; casting the melt at a temperature slightly above the solidification point of the alloy; cooling the alloy to solidify it in the form of an ingot; and subjecting the ingot to hot-state mechanical working and deformation to destroy the casting structure, wherein said additives are added in proportions such that the total percentage of said additives contained in the solidified alloy is within the range of 1 to 4 .

When the alloy is for use in the nuclear field, the addition element is chosen among those which have the smallest neutron-capture cross-section* i.e. yttrium. In other cases, and for the sake of e conomy , mixtures of rare earths of the type known as "fiischmetal" can be employed.

The invention also provides Improved iron-aluminium alloys obtained by the process referred-to above, containing addition elements seleoted from the group consisting of yttrium and rare earth metals in a proportion from 1 to $ by weight, and having grain dimensions smaller than 20^after mechanical working.

In tie preferred case of alloys which only contain iron, aluminium and possibly also a small percentage of additives?,! the aluminium content is advantageously comprised between 16 and 31$» this last mentioned value corresponding to the appearance of a precipitate of the phase Fe-Alg.

The incorporation of the additive or additives is carried out during th0 melting process, preferably by introducing It or them in the bath of molten iron. During the following phase (up to solidification of the ingot), losses are liable to - 4 - 23129/2 ^ occur; in order that the final content should be comprised between 1 and 4$ by weight, it may therefore bo found necessary to introduce into the molten mass an amount of additives which would correspond to a higher percentage content than that which is sought.

The casting and solidification processes are conducted under conditions which are comparable with those described in the main patent. A substantial reduction in the mean grain size of the ingot is noted, with the advantages resulting from this fact. In the as-cast state under the same conditions of production, the grain size is, for example, reduced by a factor of the order of 10 as compared with th addition of less than Vft> of lanthanum or yttrium. The result thereby achieved is a reduction in brittleness and an improvement in machinability of the as-east ingots; lathe turning can be accomplished under the usual conditions of temperature and in addition products of a quality which is at least equal to that of machining at 400°C which is described in Example II of the main patent, THs reduction in grain size Is retained at the time of roughlng-down, in particular when carried out by the hot extrusion process after cladding as described In the main patent. It is possible in particular to obtain grains having a size of the order of 10 microns after extrusion at 950°C and in general, the grain size remains smaller than 20 microns.

An attempt can be made to explain the achievement of a reduction rathe than an increase in grain size, although it will be understood that the explanation which now follows is given only by way of indication and cannot be considered as having any incidence on the patents the addition of rare earths In pro - ·5· - 23129/^ the precipitation at the grain boundaries of a second phase which is totally different from the main phase which Arms the matrix (Fe - Al phase up to 31$ by weight of aluminium). At the time of extrusion, these precipitates are aligned in the direction of extrusion in fibres having a density which increases with the percentage of rare earths contained in the alloy. The increase in grain size is accordingly prevented by this precipitation.

In the case which lanthanum is employed as an additive, the precipitates observed are Fe - Al - La compounds. This precipitate is brittle, but the mechanical properties of the alloy are nevertheless improved. It is possible that the beneficial effect of lanthanum in this particular case not only lies in its grain-refining action but also in the fact that the matri itself becomes more ductile as a result of the purification which is due to the precipitate or to the lanthanum.

The effect of reduction in grain size is maintained in the case of annealing operations, even at high temperature.

Thus, a level temperature stage of one hour at 1150° does not result in any perceptible increase in grain size sine© the graino are stabilized by the preoipitates.

This stabilization constitutes an appreciable advantage which is of special value for welding purposes. It is in fact possible to weld iron-aluminium alloys having a high aluminium content (for example 40$ in terms of atomic ratio) by conventional methods such as arc welding in an argon atmosphere. However, the welded zone has a casting structure which is much more brittle than that ifthe base alloy. In order to prevent the melting of the alloy, diffusion welding can be performed in - 5a - 23129 2^ · the solid state but, in order to break up the oxide layer and ensure a good weld, the parts to be assembled have to be heated to at least 1100°C fo a period of a few minutes; this treatment produoos a substantial increase in grain size within the ali according to the examples given in the main patent, whereas the alloy which contains 1 to 4 of rare earths retains its fine grain structure and good properties which result therefrom.

Finally, the alloy which is preparctfin accordance with the invention,retains improved machinabillty after extrusion as well as enhanced mechanical properties· Example 1 below shows the properties of a lanthanum alloy according to the main patent while Examples II and III relate to alloys according to the present invention, EXAMPLE II This example refers to a lanthanum alloy which exhibits after roughing-down an elongation of 11?S under tension at normal speed and at room temperature.

T e alloy to be produced has a composition which is comparable with that given in Example I of the main patent, but lanthanum is added instead of zirconium. The melt is prepared fromJ - Electrolytic iron : 3 kg.

- Aluminium of 99.99 ^ purity : 0,960 kg.

- Lanthanum : 40 gr. a) Melting and casting The conditions of melting and casting are similar to those of the process according to the main patent, viz : the iron is melted and brought up to 1600°C in vacuo, aluminium is added, lanthanum is added at the same time as the aluminium, the temperature is reduced to 1 50°C and the casting operation is performed in an ingot-mould which has been pre-heated to 620°C. Finally, the cooling rate is limited to approximately 50° C per hour.

The conditions of casting in vacuo as adopted in the case which is contemplated resulted in a loss of lanthanum and. analysis of the ingot revealed only 0,7 o by weight of lanthanum in addition to the usual traces of carbon, nitrogen, phosphorus and sulphur. b) Roughing-down.

The roughing-down process can consist of a series' of operations which are similar to those described in Example II of the parent patent.

The ingot can subsequently be machined on a lathe by using tools of high hardness. The machined workpiece is clad with a steel jacket a few millimeters in thickness. The composite workpiece obtained is press-extruded and the steel jacket is removed, for example by chemical dissolving in a solution composed of 50 $ water and 50 $ nitric acid. As has been indicated above in a general manner, it is no longer necessary to maintain the alloy at 400° C during the machining operation. c) Cold working The extruded product obtained has a better state of surface than that of the product according to Example II of the parent patent. Moreover, it can be cold-worked and this operation (which is already facilitated by the smaller grain size which is in the vicinity of 10 microns after extrusion.

The machined workpiece can undergo a heat treatment for a period of one hour at 1150° C without any appreciable increase in grain size.

After treatment for a period of one hour at 800° C followed by slow cooling (30° C per hour) , the extruded product has the following characteristics : The above example shows that the invention makes it possible to improve those iron-aluminium alloys in which the proportion of aluminium exceeds 18 fo by weight to an extent which greatly facilitates machining, reduces the wear of tools and lowers the price and duration of the machining operation.

EXAMPLE II : The iron and aluminium contents of the alloy to be produced are the same as in example I, but the final percentage of lanthanum is 3.75 f° by weight.The successive steps are the same. The extruded product has the following characteristics 20° C ; Ultimate tensile strength 66 kg/mm2 Yield strength 58 kg/mm2 Elongation at fracture 1P o the ductility of the alloy is much decreased by the increase in lanthanum content ; on the contrary the elastic limit is increased.

EXAMPIiE III; The iron and aluminium contents are the same as in Example I, but with yttrium, the final proportion of which ia 3.15$· T e successive process steps are the same as above. She product exhibits the following characteristics at 20°C J Ultimate tensile strength 80 kg/mm2 Yield strength 55 kg/mm2 Elongation at fracture 2

Claims (4)

- 9 23125 2

1. A process for the preparation of an iron-aluminium base alloy comprising melting an amount of iron which corresponds to a proportion less than 84$ by weight of the alloy; adding to the molten iron additives selected from the group consisting of yttrium and rare earths; adding aluminium; casting the melt at a temperature slightly above the solidification point of the alloy; cooling the alloy to solidify it in the form of an ingot; and subjecting the ingot to hot-state mechanical working and deformation to destroy the casting structure, wherein said additives are added in proportions such that the total percentage of said additives contained in the solidified alloy is within the range of 1 to 4$.

2. A process according to Claim 1, wherein the iron content ranges from 69$ to 84$ and the aluminium content ranges from 16$ to 31$.

3. A process for the preparation of an iron-aluminium -alloy substantially as herein described.

4. An iron-aluminium alloy obtained by the process according to Claim 1, 2 or 3, containing a proportion ranging from 1 to 4$ by weight of additives selected from the group consisting of yttrium and the rare earths and having grain sizes smaller than 20/^after mechanical working and deformation in the hot state. For the Applicants