US3131055A

US3131055A - Alloy based on iron, containing nickel, chromium and aluminium, and process for obtaining same

Info

Publication number: US3131055A
Application number: US93521A
Authority: US
Inventors: Behar Isaac
Original assignee: Societe Metallurgique dImphy SA
Current assignee: Societe Metallurgique dImphy SA
Priority date: 1960-03-11
Filing date: 1961-03-06
Publication date: 1964-04-28
Anticipated expiration: 1981-04-28

Description

Aprll 28, 1964 I. BEHAR 3,131,055

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, AND PROCESS FOR OBTAINING SAME Filed March 6. 1961 4 Sheets-Sheet 1 FIg.'l

I. l I I I I I I I I I B 0 T 7 I -I "I I l I I I I I i II I I l [I l I: l

l I I II I I I I II I I 1 I AI I 1 I l I l I c c c c AAI April 28, 1964 1. BEHAR 3,

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, AND PROCESS FOR OBTAINING SAME Filed March 6, 1961 4 Sheets-Sheet 2 Fig.2

Apnl 28, 1964 l. BEHAR 3,

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, AND PROCESS FOR OBTAINING SAME Filed March 6, 1961 4 Sheets-Sheet 5 l. BEHAR 3,131,055 ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND AND PROCESS FOR OBTAINING SAME 4 Sheets-Sheet 4 L m 9 M 9 1 h com 2 1 M p A United States Patent M The present invention relates to alloys based on iron, nickel, chrome and aluminium and has for an object such an alloy which is rich in iron. The invention also relates to a process for obtaining such an alloy.

Another object of the invention is the manufacture of an alloy which can be worked when cold or hot and which, after the final hardening treatment, has constant physical characteristics from one casting to another and, in particular, a particularly high elastic limit and breaking strain.

In industrial practice hardening processes of different types are used. The hardening of steels which are rich in carbon is known; in this case the austenite is decom posed in the course of cooling to give rise to harder structures (martensite, bainite). The hardness of the products of decomposition of the austenite in the course of cooling is higher when the carbon content is greater. This type of hardening, which allows high mechanical characteristics to be obtained, has, however, a certain number of drawbacks, such as the deformation resulting from hardening from a high temperature, irregularity in its characteristics, the relative fragility of carbon-rich steels, or welding dhliculties, for example.

The structural hardening of stable austenites is also known, the development of which is a result of research undertaken since 1928. This type of hardening has shown itself to be particularly flexible, it can be carried out by tempering from a relatively low temperature and this allows pronounced deformations and the risks of ordinary hardening to be avoided. However, the degree of hardness obtained by structural hardening of the stable austenite is much smaller than that which can be obtained by the hardening of steels.

Certain authors have studied the structural hardening of stable ferritic alloys; in this case also the hardening is not very great and, apart from this, these alloys are relatively fragile.

The structural hardening of stable austenites results from the variation in the solubility of the hardening elements as a function of the temperature, and I have determined that a change in the crystallographic structure of the alloy permits structural hardening in general and structural hardening by aluminium in particular to be raised.

Reference will now be made to the accompanying drawings showing representative graphs, and in which:

FIGURE 1 shows curves representing the different solubilities of aluminium under different conditions,

FIGURE 2 shows the variations in the Vickers H hardnesses for different kinds of castings,

FIGURE 3 shows a graph of proportions of alloyed nickel and chromium, and

FIGURE 4 shows the variation of the mechanical characteristics of the alloy when hot as a function of the temperature.

In order to define the nature of the alloy, according to the invention, reference is now made to FIGURE 1 which shows diagrammatically the variation in the solubility of aluminium in a stable austenitic structure (curve E) and in an alloy having a gamma-alpha transformation on cooling (curve EDBA) at the temperature T If the stable austenitic alloy and the alloy having a gamma-alpha 3,131,055 Patented Apr. 28, 1964 transformation upon cooling are both quenched from the temperature T and then aged at the temperature T the hardening will be proportional to the length C C in the case of austenite and C C' in the case of the alloy which has a gamma-alpha transformation. Referring now to FIGURE 2, the curves 1, 2, 3 and 4 represent the variations in the Vickers H hardnesses determined at 2.5 kilograms, as a function of the tempering temperature for 3 hours for three stable austenitic castings and for one casting which has a gamma-alpha transformation upon cooling, the compositions of which are given in the following table.

C Si Mn Ni C1 A1 The hardness of these alloys have been determined in the quenched state, after an intense cold-working of 87.5% followed by ageing carried out in the cold-worked state. It was observed that the structural hardening of the casting 4 upon ageing at 450 C. to 500 C. is particularly great. The alloy according to the invention is therefore one which has a gammaalpha transformation upon cooling in such a way that the hardening element is dissolved at a high temperature in the gamma state and then precipitated in the alpha phase after the gammaalpha transformation.

According to the invention the proportions by weight in the alloy of nickel and chromium correspond to the area defined by the outline ABCDE shown in FIGURE 3 of the accompanying drawing and the alloy which is free from 6 ferrite has, moreover, a proportion by weight of aluminium ranging from 1% to 6% and a content of carbon at the most equal to 0.15%, this alloy being subjected on the one hand to an annealing treatment followed by quenching which transforms the greatest part of austenite into an a structure of martensitic appearance and leaves almost all the aluminium in super-saturation and on the other hand to a hardening tempering precipitating a compound which is rich in aluminium into the alloy.

The thermal treatment conditions leaving the aluminium in super-saturation or, in a general way, leading to a softened state as against a hardened state will hereinafter he referred to as super-hardening.

The alloy transformed by the super-hardening can afterwards be hardened by tempering at a sufficiently low temperature which does not involve oxidation or deformation.

An essential characteristic of the alloy according to the invention is that it does not contain free 6 ferrite and that by reason of the relation of the proportions between the alpha-producing elements and the gamma-producing elements, this ferrite cannot appear in the course of thermal treatments. In these conditions the property of aluminium to favour the transformation of the austenite into on ferrite of martensitic appearance and the property of alu minium of being very slightly soluble in its a phase, whilst it is very soluble in its 7 phase, can be fully used.

In the course of heating at high temperature the alloy is austenitic and it is thus possible to cause a considerable proportion of aluminium to pass into solid solution. As this autenite is not stable it is transformed into or ferrite of martensitic appearance in the course of cooling.

The dissolving or softening treatment is effected at a temperature extending from 750 to 1250 C. The duration of maintenance is variable according to the temperature and the size .of the pieces to be treated. It amounts to a few minutes in the case of transitory treatments at a temperature of 1150 C. for example and can extend to 20 hours when the treatment is at a relatively low temperature. This softening can be carried out in several stages: It can for example be convenient to carry out a standardized treatment at a high temperature and then a softening treatment at a lower temperature. Usually a treatment of 30 minutes at 800 C. is sufficient.

The cooling which follows the dissolving can take place, as desired, in air, oil or water.

The hardening tempering can he carried out at a temperature ranging from 200 C. to 700 C. according to the desired characteristics and according to the treatment to which the alloy has been subjected before tempering (super-hardening or super-hardening followed by cold working). The duration of the tempering can be contained between a few minutes and 24 hours. The hardening can also be carried out by repeated ageing treatments, each one of these treatments being followed by cooling at the ambient temperature or at a lower temperature.

During this tempering the aluminium originally in super-saturation in the a ferrite of martensitic appearance formed in the course of cooling, is precipitated in the martensite in the form of an aluminium-rich compound producting a high degree of structural hardening.

The hatched area defined by the outline ABCDE in FIGURE 3 determines the proportions of nickel and chromium which can be used according to the invention, the representative point of the alloy being found of necessity inside this outline. In the drawing, the abscissa indicates the chromium constituent and the ordinate indicates the nickel constituent.

The alloy according to the invention contains moreover 1% to 6% of aluminium which can be partially replaced by titanium and/ or molybdenum, the aluminium content not going below 0.5%. The addition of titanium and/ or molybdenum can have as its efiect the modification of the composition, the crystallographic structure or the kinetic of precipitation of the aluminium-rich compound.

The alloy can also contain up to 2% silicon and up to 4% manganese.

The carbon content must be limited at the most to 0.155%.

Bearing in mind the influence of the different elements of the alloy on the formation of 6 ferrite, it is also necessary that the sum of the percentages by Weight Cr+1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi12 C0.2 Mn should at the most be equal to 10.

Furthermore the alloy can contain different additions in small quantities which however do not modify its nature so much regarding the balance of the alpha- 'producing and gamma-producing elements as regarding the mechanism of the structural hardening of the alloy by precipitation of an aluminium-rich compound and/or a titanium-rich compound and/or a molybdenum-rich compound in the course of hardening tempering.

The alloys according to the invention have a greater amenability to hardening and a greater elastic limit, due to the precipitation in the on phase of an aluminium-rich and/ or titanium-rich and/ or molybdenum-rich constituent, than the alloys which are free from these elements such as alloys with 18% of chromium and 8% nickel.

Certain known alloys assaying about 17% of chromium and 7% of nickel contain a relatively small proportion of aluminium. These alloys have free 6 ferrite. The alloys according to the invention, by reason of the balance of the chromium and nickel elements defined by the hatched area ABCDE, do not contain 6 ferrite. They therefore have a higher aluminium content which can be contained between 1% and 6% and is generally higher than 2.5%, when the highest mechanical characteristics are to be obtained.

The absence of ferrite is extremely important in this type of alloy.

The austenite of the alloy which is very unstable is decomposed in the a phase of martensitic appearance in the course of cooling as far as the ambient temperature which follows the treatment at high temperature: the residual austenite, if there is any, can be transformed by refrigeration at a lower temperature or by cold working. The hardening of the alloy being connected with the variation of solubility of the aluminium in the 'y and a phase and the composition of the alloy being such that free 6 ferrite does not appear, all the matrix can participate in the structural hardening due to the precipitation of an aluminium-rich compound in super-saturation in the o: ferrite. The existence of a certain quantity of free 6 ferrite can therefore in certain known alloys reduce the possibility of structural hardening.

As aluminium, apart from its action on the amenability to hardening, favours the formation of 6 ferrite, it is essential that the chromium and nickel contents be contained within the hatched area ABCDE and that the sum of the percentages by weight Cr+ 1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi12 C-0.2 Mn be at the most equal to 10 in order to avoid the risks of formation of 6 ferrite. In the case of known alloys, assaying about 17% of chromium, 7% of nickel and 1.2% of aluminium, small variations in the composition of the castings can bring about considerable variations in the free 8 ferrite content, and, as a result, a divergence in the mechanical characteristics from one casting to another.

Among the other advantages of the claimed alloy, in connection with the absence of 6 ferrite, may be mentioned the following:

The super-hardening temperature can be raised in order to dissolve a greater aluminium content without the risk of producing a high proportion of free ferrite.

The absence of the risk of formation of 6 ferrite allows the super-hardening temperature to he fixed Without any high degree of precision. In the alloys containing a certain proportion of 6 ferrite on the other hand a variation in the softening temperature would bring about an important variation in the free 8 ferrite content and, as a result, variations in the mechanical properties of the alloy.

The absence of the formation of 6 ferrite in the alloy according to the invention allows the drawbacks related to the dendritic segregation to be reduced. In the alloys containing 6 ferrite these drawbacks are particularly accentuated when the ingots are of large dimensions, the centre of the ingots being then very rich in ferrite.

The alloy having at high temperatures, an austenitic structure free of 6 ferrite, the transformations of the alloy when hot (forging and rolling) do not present any difficulty.

To sum up, an absence of free 5 ferrite in the alloy according to the invention allows structural hardening of the alloy to be raised, allows a good reproducibility of the mechanical characteristics from one casting to another to be obtained, allows the drawbacks connected with the dendritic segregation to be reduced, and allows the transformation of an alloy in the hot state to be facilitated.

The alpha phase obtained after super-hardening is plastic enough for the alloy to be easily moulded, cold rolled, drawn or swaged as the results, recorded in the following examples, show.

The invention will now be described with reference to castings of different compositions and with different degrees of treatment. The castings are preferably melted in vacuum, but it is also possible to melt them in air.

In the examples which follow, indications will be given concerning the possibility of replacing certain elements of the alloy by equivalent elements.

Example 1 A casting was made having, in percentage by weight, the following composition:

C 0.025 Cr 7.01 Si 0.14 Al 3.69 Mn 0.59 Fe to The treatments and characteristics of the alloy are summarized in the following table:

The super-hardening treatments were carried out on pieces of 7 millimetres in diameter and 65 millimetres long and the traction test pieces were taken from among forged plates.

E is the 0.2% yield strength.

R is the ultimate tensile strength.

A is the elongation.

and 2 is the contraction of cross-section.

Example 2 A casting was carried out having the following composition:

C 0.03 Cr 10.91

Si 0.14 Al 2.03 Mn 0.53 Fe to 100.

The treatments and characteristics of the alloy are summarized in the following table:

Treatments E 0.2, R, 2,

kg/n'nn. kgJmm. percent percent 30 111111., 800 Oil-cooled 65.1 103. 5 11.5 74.1 30 min., 800+480, 1 hour air-cooled 134. 1 149. 5 12. 9 47. 1 30 min., 800+500, 1 hour air-cooled 146. 2 160. 1 10. 4 51.9 30 min, 800+550, 1 hour air-cooled 130. 7 147.0 11.0 45. 2

E, R, A and 2 being as before. It is noted that the replacement of a portion of the aluminium weight for weight by titanium and/ or molybdenum gives alloys having important properties, the aluminium content being at least equal to 0.5%.

Example 3 By way of example there are hereafter indicated the mechanical characteristics obtained with a casting E, R, A and 2 being as before.

Example 4 It has been determined that the replacement of a portion of the chromium weight for weight by molybdenum, always with a maximum molybdenum content in the alloy .of 4.5%, allows the mechanical characteristics when cold to be increased. Apart from this the molybdenum allows the benefit of hardening at a moderately high temperature to be retained. A casting was carried out having the following composition:

C 0.01 Cr n 4.36 Si 0.14 Al 3.51

Mn 0.57 Mo 2.40 Ni 15.10 Fe to 100.

This casting is derived from that of Example 1 by replacing about 2.5% of the chromium by 2.5 of

molybdenum.

Treatments E 0.2, R, A, 2,

kg./mm. kg./mm. percent percent 30 min, 800 oil cooled 108. 0 126. 8 9.0 71. 9 30 111111., 800+3 hr., 500 aircooled 182. 9 194. 7 8. 1 26. 7 30 min., 800+3 hr., 520 aircooled 186. 1 194. 8 9. 2 42. 1 30 min., 800+3 hr., 550 aircooled 177. 6 184. 2 7. 9 51.0 30 min.,'800+3 hr., 58L aircool (1 170.8 175.4 7.2 52.8

E, R, A and 2 being as before.

The characteristics obtained after a super-hardening followed by tempering are very clearly superior to those obtained on alloys hardened by aluminium, assaying about 15% of chromium, 7% of nickel and 2% of molybdenum.

It should be noted that after quenching from 800 C. and tempering for 3 hours at 550, it is possible to obtain an elongation of 9% and a reduction of area of 40% whilst the ultimate tensile strength is 195 kilograms per square millimetre and the 0.2% yield strength is 185 kilograms per square millimetre. The corresponding resilience is K =2.2 kilograms per square centimetre whilst the resilience of the alloys assaying about 15% of chromium, 7% of nickel, 2% of molybdenum and 1.2% of aluminium is lower than 1 and generally of the order of 0.5 kilogram per square centimetre, for a tensile strength of the order of kilograms ,per square millimetre.

The technical superiority of the alloy of the present invention over the known alloys derived from alloys with 18% of chromium and 8% of nickel and hardened by aluminium, clearly appears in the curves in FIGURE 4 which represent the variation of the mechanical char acteristics when hot as a function of the temperature at which the rapid traction test was carried out.

In this figure, the characteristics of the reference alloy are indicated in unbroken lines While the characteristics of the alloy according to the invention are indicated in broken lines.

The reference alloy had the following composition:

C 0. 08 Cr 15.10 Si 0.47 Al 1.30 'Mn 0.76 Mo 2.60 Ni 7.18 Fe to 100.

The test pieces were subjected to the following treatment before traction tests:

1 hour at 1050 C. oil-cooled+1 /z hours at 760 C., Water-cooled at 0 C.+maintained for /2 hour at 0 C.+1 hour at 570 C., air-cooled.

The alloy according ,to the invention. had the following composition:

C 0.025 Cr 4.17 Si 0.01 A1 3.28 Mn 0.57 Mo 2.20 Ni 16.2 Feto 100.

Before the hot traction tests, the test pieces were treated for 3 hours at 800 C., oil-cooled+3hours at 550 C.

air-cooled.

It is pointed out that at every temperature the 0.2% yield strength and the tensile strength of the alloy according to the invention are clearly higher and that the percent elongation and the percent reduction of area are equal to or higher than those of the reference alloy.

Creep tests confirm the technical superiority of the alloy according to the invention.

The creep tests were carried out at 400 C. using the same castings and the same thermal treatments as those used for the rapid traction tests above room temperature.

Reference alloy Alloy oi the invention Longitu- Load, kg./ruu1. dinal Longitu- Liie in expansion Life in dinal hours up to hours expansion breaking, to breaking,

percent percent It is also possible to replace a portion of the chromium weight for weight by molybdenum. In this case abscissa of FIGURE 3 corresponds to the sum Cr-l-Mo, molybdenum being added in amounts equal to or less than 4.5%.

Similarly it is possible to replace a portion of the nickel by manganese and by carbon in quantities so that the sum Ni% +30 (3% +0.5 Mn% is equal to the nickel content corresponding to the extent defined by the outline ABCDE in FIGURE 3. In any case the manganese content must not exceed 4% and the carbon content must not exceed 0.15%.

In the case of the substitution of the chromium by molybdenum and/ or aluminium and of the nickel by carbon and/or manganese, it is necessary that the sum Cr+1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi-12 C-0.2 Mn remains lower than 10.

The alloy according to the invention can be plastical- 1y deformed when cold without difficulty before final hardening. This property can be made use of in particular for obtaining metal foil and wire.

It is possible to obtain a reduction in thickness of more than 90% by rolling when cold and of more than 75% by drawing without any intermediary softening.

It is also possible to obtain deep swaging on the superhardened metal.

For example on a blank of 0.24 millimetre thickness transiently super-hardened at 1150 C. and having the following composition:

C 0.04 Cr 7.03 Si 0.01 Al 3.54 Mn 0.47 Fe to 100.

the swaging depth carried out in the course of an Erichsen test was 9 millimetres, the diameter of the ball being 20 millimetres and the diameter of the jaws 27 millimetres.

It was also determined that if an intense cold-working was carried out after super-hardening, a tempering treatment afterwards carried out at a mean temperature which is often of the order of 400500 C. considerably raised the hardening of the alloy.

By way of example a cold-working of 87.5% by rolling when cold and a tempering of 3 hours at 450 C. allows there to be obtained, on a foil of 0.10 millimetre having the same composition as that of the plate of 0.24 millimetre which has just been mentioned, an elastic limit at 0.2% of 240 kilograms per square millimetre and a breaking strain of 250 kilograms per square millimetre.

On a wire of the same casting, cold worked by drawing to 73%, the characteristics obtained after ageing for 3 hours at 450 C. are 230 kilograms per square millimetre for the conventional elastic limit at 0.2% and of 238 kilograms per square millimetre for the breaking strain.

The particularly high mechanical characteristics, obtained after super-hardening and tempering or after superhardening, cold working and tempering of an alloy according to the invention, allow it to be used in the field of the manufacture of springs and in a general way in the field of aeronautics and ballistic machines. With regard to this it is interesting to note that the density of the alloy is relatively small, of the order of 7.5 and that the Elastic limit Density relation is therefore particularly high.

The alloy according to the invention has moreover a good resistance to corrosion. Taking into account the nickel, chromium and aluminium contents, the alloy does not rust in water and is not attacked by immersion in a 3% solution of sodium chloride or in solutions, at variable concentrations, of acetic acid.

Apart from this the chromium-rich alloys corresponding to the area defined by the outline ABCDE have a good resistance to solutions of nitric acid.

Five consecutive tests each of 48 hours were carried out at the ambient temperature on an alloy having the composition:

C 0.04 Cr 11.87 Si Traces Al 3.59 Mn 0.31 Fe to 100.

The breaking strain after cold working and hardening tempering at 500 C. were 241 kilograms per square millimetre.

The resistance to corrosion is important 'for many applicat-ions and in particular the alloy according to the invention is very suitable for the manufacture of motor springs intended for watch making, where corrosion is the forerunner of breakage.

Because of the high content of nickel, chromium and aluminium, an alloy according to the invention does not rust if it is exposed to the atmosphere.

A high quality motor spring must also develop a high torque and this implies good mechanical characteristics. These high mechanical characteristics are obtained by intense cold working followed by tempering at a relatively low temperature, generally between 400 C. and 550 C.

Bearing in mind the hardening process of the alloy, the manufacture of motor springs for watchmaking is more simple than that of carbon-steel springs. Apart from this, springs manufactured from this alloy have certain advantages over springs manufactured of carbonsteel:

(1) It is not necessary to take particular precautions to avoid corrosion either for the manufacture or for the stocking or despatching of the springs, the alloy being substantially un-tarnishable.

(2) The manufacturing process requires only a tempering on a previously cold worked material, the treatment furnaces are simple and the characteristics obtained with the springs can be very regular. In the case of carbon-steels, on the other hand, hardening installations are necessary and the eiiiciency of the hardening is never very regular.

(3) Since the cold worked alloy retains a good malleability the fashioning of the springs is easy and does not involve an abnormal Wear in tools.

It is also unnecessary to soften one of the ends of the spring in order to form the retaining eye. After the hardening treatment this eye there-fore remains rigid and does not deform in service in the watch.

(4) Since the carbon content of the alloy remains small the shackle can be spot welded without difliculty.

(5) Springs manufactured from the alloy also have the following advantages:

(a) They are unbreakable in the watchm-aking sense.

(b) They develop high torques.

(0) They are stable. The torques of the springs do not decrease due to fatigue nor upon prolonged maintenance at the ambient temperature contrarily to that which is observed in the case of carbon-steel springs.

The cold working necessary to develop the hardening of the alloy can be carried out in different ways according to the transformation process used. A. The alloy is transformed into bands by cold rolling.- The cold rolled bands are strongly cold worked by cold rolling: according to the thickness of the original hot rolled bands and the thickness of the final bands one or more intermediary softening treatments may be necessary in order to carry out a determined cold-working rate which is most often higher than 80%.

B. The alloy is hot rolled in the machine wire state. After calibrating the machine wire by drawing the latter is rolled cold in order to obtain small hands, the width of which, which depends upon the diameter of the original machine wire, is often greater than millimetres. One or more intermediary softenings may be necessary in order to achieve an adequate cold working rate which is often higher than 80%.

In these two transformation processes the eyes necessary for the manufacture of springs are clipped into the band or the litle band and the edges are mechanically polished.

C. The alloy is hot rolled in the machine wire state. Instead of attempting to calibrate the wire it is possible to carry out the drawing process until a wire is obtained the diameter of which is such that by final rolling the necessary strip is obtained for the manufacture of the spring. Since the wire having small diameter and the cold working by rolling are then insufiicient it is necessary in order to obtain good characteristics in the springs to combine the cold working by drawing and cold working by rolling.

Whatever the transformation technique used, the alloy according to the invention allows good motor springs to be obtained and retains the properties mentioned above.

I claim:

1. An alloy consisting essentially of iron, nickel, chromium and aluminium, the proportion by weight in the alloy of nickel and chromium corresponding to the area ABCDE in the FIG. 3 of the accompanying drawings, the aluminium content by weight being of 1 to 6% and the maximum carbon content being 0.15% including as possible components Si, Ti, Mo and Mn in which the sum Cr+1.5 Si+2.5 Al-|-2.5 Ti+0.8 Mo-Ni12 C0.2 Mn is at the most equal to 10, the balance being iron, said alloy being free of 6 ferrite and susceptible to annealing at 750 C. to 1250 C. and quenching through phase transformation to a substantially aluminium-soluble martensite-like structure and thereafter to precipitation hardening at from 200 C. to 700 C. by an aluminium rich compound, the component Ni+30 C+0.5 Mn being sub stitutable for Ni and the component Cr-l-Mo being substitutable for Cr, Mn then having a maximum value of 4% and Mo having a maximum value of 4.5%

2. An alloy as described in claim 1, in which the aluminium is partially replaced by a metal selected from the group consisting of titanium, molybdenum and mixtures thereof, the aluminium content remaining at least equal to 0.5%.

3. An alloy as described in claim 1, including a maximum of 2% silicon by weight.

4. An alloy as described in claim 1, including a maximum of 4% manganese by Weight.

5. A process for hardening an alloy based on iron, including the steps of forming an alloy free of 5 ferrite as described in claim 1, then annealing the alloy at about 750 C. to 1250 C., then quenching the same thus achieving a soft substantially aluminium-soluble martensite-like structure and then ageing the alloy at least once at a temperature ranging from 200 to 700 C. to precipitate therein an aluminium rich compound.

6. A process as described in claim 5, in which the quenching is carried out in a medium selected from the group consisting of air, oil and water.

7. A process as described in claim 5, in which the tempering is preceded by cold working.

References Cited in the file of this patent UNITED STATES PATENTS 1,538,360 Smith May 19, 1925 1,941,648 Armstrong Jan. 2, 1934 2,048,164 Filling et a1. July 21, 1936 2,505,762 Goller May 2, 1950 FOREIGN PATENTS 378,478 Great Britain Aug. 15, 1932 404,876 Great Britain J an. 25, 1934

Claims

1. AN ALLOY CONSISTING ESSENTIALLY OF IRON, NICKEL, CHROLMIUM AND ALUMINIUM, THE PROPORTION BY WEIGHTIN THE ALLOY OF NICKEL AND CHROMIUM CORRESPONDING TO THE AREA ABCDE IN THE FIG. 3 OF THE ACCOMPANYING DRAWINGS, THE ALUMINIUM CONTENT BY WEIGHT BEING OF 1 TO 6% AND THE MAXIMUM CARBON CONTENT BEING 0.15% INCLUDING AS POSSIBLE COMPONENTS SI, TI, MO AND MN IN WHICH THE SUM CR+1.5 SI+2.5 AL+2.5 TI+ 098 MO-NI-12 C-0.2 MN IS AT THE MOST EQUAL TO 10, THE BALANCE BEING IRON, SAID ALLOY BEING FREE OF $ FERRITE AND SUSCEPTIBLE TO ANNEALING AT 750*C. TO 1250*C. AND QUENCHING THROUGH PHASE TRANSFORMATION TO A SUBSTANTIALLY ALUMINUM-SOLUBEL MARTENSITE-LIKE STRCTURE AND THEREAFTER TO PRECIPITATION HARDENING AT FROM 200*C. TO 700*C. BY AN ALUMINIUM RICH COMPOUND, THE COMPONENT NI+30 C+0.5 MN BEING SUBSTITUTABLE FOR NI AND THE COMPONENT CR+MO BEING SUBSTITUTABLE FOR CR, MN THEN HAVING A MAXIMUM VALUE OF 4% AND MO HAVING A MAXIMUM VALUE OF 4.5%.