LU83427A1 - PROCESS FOR IMPROVING MECHANICAL PROPERTIES OF ALLOY PARTS - Google Patents

Description

DESCRIPTIVE MEMORY

FILED IN SUPPORT OF A REQUEST

OF

PATENT

FORMED BY

CHROMALLOY AMERICAN CORPORATION for

Method for improving the mechanical properties of alloy parts.

The present invention relates to a process for improving the mechanical properties of alloys capable of structural hardening having a melting point of more than 1,000 ° C. and, in particular, to a process of treatment under hot isostatic pressure to improve the pro -. mechanical properties of cast alloy parts, for example parts of jet engines whether new or not, for example parts made of an iron-based, nickel-based, cobalt-based or base-based alloy titanium. The inv- CD.ildC .PG. 5 F Çi 6h tion is particularly applicable to the treatment of molded superalloys susceptible to structural hardening.

It is known to apply treatments under hot isostatic pressure to improve the mechanical properties of certain alloys, for example molded alloys which include micropores and / or other structural defects. According to the patent of the United States of America n ° 3.758.34-7î i3110 molded part made of an alloy based on nickel, cobalt, iron or titanium and having internal discontinuities, for example porosity, microcracks, crevices, etc. can be improved by treatment under isostatic pressure at an elevated temperature which is lower than the temperature resulting in a significant degradation of the mechanical properties of the alloy, for a time sufficient for the pores to close and for the walls of the pores to crack and similar defects are not welded diffusion. Certain superalloys are mentioned in particular, for example nickel-based superalloys capable of structural hardening sold under the names of René 80, René 100, etc., the alloy René 80 contains 0.17% carbon, 14% chromium , 5% titanium,; 0.015% boron, 3% aluminum, 4% tungsten, 4% molybene, 955% cobalt, 0.03% zirconium and for the rest of nickel, while the René 100 alloy contains 0, 17% carbon, 9.5% chromium, 4.2% titanium, 0.015% boron, 5.5% aluminum, 3% molybdenum, 15% cobalt, 0.06% * zirconium, 1% vanadium and the rest of the nickel.

According to the aforementioned patent, for the treatment of molded parts of René 80 alloy in an autoclave heated to -1,218 ° C under a gauge pressure of 690 bars, the alloys of the alloy are kept in the oven making enviorori b hours and © have removed after cooling. The test pieces which have undergone the treatment with hot isostatic pressure are compared with others which have not undergone this treatment, after carrying out a heat treatment. The test pieces which have undergone or not the treatment under hot isostatic pressure are subjected to a treatment for forming a solid solution at 1.218 ° C. for 2 hours under vacuum, then suddenly cooled under inert gas to ambient temperature and then heated. at 1.093 ° C; for 4 hours under vacuum and quenched under inert gas to room temperature. After the last cooling, the test pieces are aged at 1.052 ° C for 4 hours, cooled in the oven to 649 ° C and maintained at this temperature for 1 hour before being air-cooled to the temperature ambient. Finally, the test pieces of the two species are heated to 843 ° C for 16 hours in an argon atmosphere and then cooled to room temperature.

The test pieces are then subjected to a stress rupture test at 871 ° C under a stress of 3-103 bars. The results show that the test pieces which have not undergone the treatment under hot isostatic pressure * (two copies) have an average lifespan of approximately 41.5 hours and an average elongation of approximately 2.5% -

The test pieces which have undergone the treatment with hot isostatic pressure (six copies) have an average lifetime of 141 hours and an average elongation of approximately 11.5%.

It is therefore obvious that the hot isostatic pressure treatment applied to the aforementioned nickel-based alloy markedly improves its runture properties under stress.

The elimination of casting defects by treatment with hot isostatic pressure is described in an article entitled "Elimination of Casting Defects Using ΗΓΡ" by G.E. Wasielewski and U.R. Lindhled; Proceedings on The Second International Conference on Superalloys -Processing; Seven Springs, Pa., September 1972 "

According to the above article, the stress-breaking properties and the ductility at ambient temperature of nickel-based superalloys, for example the alloys IÏT-738, René 77 »IR-792, etc., can be improved by treatment under hot isostatic pressure at temperatures of approximately 1.093 to 1.204 ° C for 1 to 10 hours under pressure of approximately 3 ^ 5 to 2.069 bars, a temperature of 1.177 to 1.204 ° C being particularly preferred for 100% densification of the alloy.

Similar improvements are attributed to hot isostatic pressure treatment in an article entitled "Improved Components Through Howmet's ΉΓΡ Process", by T.H. Smith and L. Dardi; in Casting About,

Spring (April) 1974 from Howmet Turbine Components Corporation.

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U.S. Patent No. 4,125-417 (November 14, 1978) describes a process of hot isostatic pressure treatment performed in the same way and for the same purposes as above, except that it is applied to raising and restoring the useful properties of previously used parts which include defects, such as cavities at the grain boundaries or dislocations induced by lelluage at high temperature, in addition to casting defects, such as micropores. After treatment under isostati- pressure (or treatment and formation of a solid and aging solution) bringing the mechanical properties back to their initial values.

The proposal to apply a heat isostatic pressure treatment to improve the mechanical properties of magnesium and aluminum molded parts is made in "US Patent No. 3 * 732,128, according to which the molded part is subjected to the effects of heat and pressure in a container at a temperature of 300 to 600 ° C under a pressure of 6> 90 to 690 "bars for 1 to 72 hours, then cooled rapidly while the pressure is maintained. The molded part thus treated is then subjected to aging at a temperature of 100 to 230 ° C for 1 to 72 hours under atmospheric pressure to improve the mechanical strength of the alloy.

It is therefore known that the execution of a treatment under hot isostatic pressure, comprising the simultaneous exercise of the effects of heat and pressure, on superalloys molded with lost wax significantly improves the mechanical properties at high temperatures. and makes it possible to provide for gas turbines = molded parts of very high quality for a critical application of gas turbines. Lost wax moldings have the advantage of significantly improving the efficiency of gas turbines and making them significantly less expensive. Widespread monetary inflation and the scarcity of fossil fuels have intensified research in this area.

It would be interesting to further improve the properties of alloys susceptible to structural hardening, for example molded superalloys, because of the increasingly stringent criteria imposed on the organs of jet engines exposed to high temperatures, for example turbine blades in the hot engine area.

The object of the invention is to provide an improved technique of treatment under hot isostatic pressure to further improve the mechanical properties of alloys capable of structural hardening having a melting point of more than 1,000 ° C. i It also aims to provide the combination of a hot isostatic pressure treatment and a heat treatment to significantly improve the stress rupture properties of super alloys, for example based on iron, based on nickel or cobalt-based susceptible to structural hardening, as well as titanium-based superalloys.

These and other objects of the invention will emerge from its more detailed description below.

In the drawings: Eig. 1 is a schematic view of a hot isostatic pressure treatment furnace which allows the execution of the invention; i la Eig. 2 is a diagram plotted on the ordinate, on the left, the lifetime until rupture in hours and, on the right, the logarithm of this same quantity and on the abscissa the cumulative probability in. Percent, which makes it possible to compare the lifespan until breaking in hours at a temperature. of the test of ^ 60 ° C under a stress of 5 · δ6ΐ bars on a nickel-based superalloy (René 100) treated in accordance with the invention (black triangles) or having not undergone-. cun treatment under hot isostatic pressure (white triangles) or having undergone conventional hot isostatic pressure treatment (white circles); Fig. 3 is a diagram similar to that of FIG. 2, but comparing the lifetime to rupture at 982 ° C under a stress of 2,000 "bars, and Fig. 4 is a diagram similar to that of Fig. 2, but relating to a nickel-based alloy SEL-15.

In general, the invention relates to a method for improving the mechanical properties of a part - 'made of an alloy capable of structural hardening which comprises structural defects, such as molding micropores and / or microcracks and hollows at the grain boundaries and similar faults occurring during service at elevated temperatures. The alloy capable of structural hardening is an alloy having a melting point of more than 1,000 ° C. and, according to the method, the part made of this alloy is subjected to an autoclave treatment under super atmospheric pressure and at a temperature high solid solution setting of the alloy capable of structural hardening which is greater than 50% of the absolute melting point of the alloy for a period at least sufficient to substantially remove the structural defects by heating and densification, then subject the part made of the alloy with an in situ heat treatment by rapid cooling at a rate of more than 20 ° C. per minute and preferably of at least 25 ° C. per minute, for example at a speed of at least minus about 30 ° C per minute if not more, below the duration of the temperature range; structural alloy, while maintaining the superatmospheric pressure, so that the part thus treated manifests superior mechanical properties over the same part having undergone the conventional heat treatment by rapid cooling outside the autoclave after the treatment under hot isostatic pressure.

By "structural defects" is meant defects in new parts of aviation equipment (for example molded turbine blades which include micropores due to certain lost wax molding techniques) or defects induced during service at high temperature in molded or machined parts, in particular microcracks or cavities at the grain boundaries, with little or no change in dimensions during service, but structural defects may simultaneously include defects initially existing, such as molding micropores, and defects which appeared during creep service or also defects due to cyclic stresses at high temperature, such as fatigue microcracks.

For example, the invention is applicable to the treatment of new molded parts containing micropores which are substantially suppressed by treatment under hot isostatic pressure, while the metallographic properties are brought to the optimum from the point of view of the conduct of the heat treatment due to rapid in situ cooling under superatmospheric pressure before the part having undergone the treatment with hot isostatic pressure is removed from the autoclave. The same part in which defects have appeared by creep or fatigue in service before appreciable modification of the dimensions can also undergo the treatment in accordance with the invention with a view to = restoring the altered properties. Consequently, the invention is applicable to parts which are both new and already put into service.

Often, a molded part comprising micropores T> eut pricorfi ssti Rfnirfi fiïrv Γ.τίΐρρρβ fi o rai CP on ccnri nci example as turbine blades and can therefore be used.

Consequently, these parts present the original micropores and the additional faults due to the high temperature service when they are dismantled during maintenance with a view to reconditioning and restoring their mechanical properties substantially to the initial values. by treatment under hot isostatic pressure. When this is the case, substantially all the defects, regardless of their origin, can be eliminated by treatment under hot isostatic pressure, the part then being cooled rapidly in situ before undergoing another heat treatment outside of the autoclave.

The process of the invention is applicable to many alloys capable of structural hardening which are machined or molded, in particular alloys based on iron, based on nickel, based on cobalt and based on titanium which are susceptible of structural hardening. .

Examples of these various alloys having melting points above 1,000 ° C are given below.

(i) Iron-based alloys Designation Weight composition,% of the alloy -: -

C Mn Si Gr Ni Mo Ti Al B

Alloy 901 0.05 0.10 0.10 12.5 42.5 5.7 2.8 0.2 0.05 A-286 0.05 1.35 0.50 15.0 26.0 1.3 2.0 0.2 0.015

Disc al oy 0.04 0.90 0.80 15.5 26.0 2.7 1.7 0.1 0.005

Per for the rest

Among the above alloys, there are precipitation hardenable stainless steels used for compressor blades of turbojets, as well as for discs and other parts of turbines.

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Denomination Weight composition,% of alloy - C Mn Si Cr lii ïïo V Nb Pe S-816 0.58 1.20 0.40 20 20 4.0 4.0 4.0 4.0 WI-52 0, 45 0.25 0.25 21.0 - - 11.0 2.0 2.0

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The above nickel-based and cobalt-based alloys are suitable for blades, fins, discs and other turbine components.

(IV) Titanium-based alloys Designation by weight,% alloy -—

Al Mo V Sn Zr Ti

Ti-6-4 6-4 - rest

Ti-6-2—42 6 2 - 2 4 rest

Ti-6-2-4-5 6 6 4.0 2 - remainder BETA III 11.5 - 4.5 remainder

Ti-8-1-1 8 1.0 1.0 - - rest

The above alloys are suitable for compressor blades, discs and other parts of aviation equipment.

For the application of the invention, the temperature maintained in the autoclave during the treatment under hot isostatic pressure ranges for the various alloys (homologous temperature) from more than 50% to about 95% of the melting point. absolute, for example from about 60 to 95% and preferably from about 70 to 95% or from 80 to 95% of the absolute fu-1 sion point, provided that this temperature falls within the setting interval solid solution of the alloy and preferably does not exceed the temperature at which melting begins. For the purposes of the invention, the above homologous temperature below 60 ° C. does not mean absolute melting of the alloy eä. the temperature at which mechanical resistance to metal is limited by creep rather than simply by the elastic limit.

Therefore, in the case of "iron-based, nickel-based and cobalt-based superalloys, the temperature for treatment under hot isostatic pressure can range from about 980 ° C to about 1,290 ° C and the manometric pressure is from about 345 to about 3. ^ 48 bars, the temperature and the superatmospheric pressure being chosen according to the alloy treated and the nature of the defects to be eliminated. The duration of the treatment can be range from about 30 minutes to 16 hours and is essentially the reverse of the temperature and pressure selected for a particular alloy, temperature is particularly important to ensure substantially complete removal of defects such as micropores.

At the end of the hot isostatic pressure treatment, the parts are rapidly cooled in situ at a rate of more than 20 ° C per minute, preferably at least about 30 ° C per minute, for example from about 30 to 30 or 60 ° C per minute, if not more.

Fig. 1 schematically represents a form of apparatus which makes it possible to apply the treatment under hot isostatic pressure. Fig. 1 represents an autoclave 10 comprising a bottom plate 11 and cover plates 12 and 13 and housing a pressure-proof container 14 which comprises a pressure-resistant cover 15 and a tightly fitted bottom 16.

the container is provided with an insulating oven jacket 17, an insulating cover of a removable oven 18 and an oven insulating bottom 19. The container is housed inside a cooling jacket 20 which comprises an inlet for re-f 2-oi di ssement 21 and an outlet for cooling water 22.

A perforated heat-resistant support 2¾ mounted inside the container 14, serves as a support for a rack 24 carrying the parts 25 which must be treated, the rack having an open configuration ensuring the regular convection 26 indicated during the treatment under isostatic pressure when hot and during rapid cooling.

The heat source comprises heating elements 27, for example made of graphite, arranged under the support, a blower 28 maintains a forced convection of inert gas heated in the oven around the rack. A fixed thermocouple 29 and flexible thermocouples 29A, 29B and 29C make it possible to continuously record the temperature in the vicinity of the rack and the rooms, the output for the thermocouples being indicated in 30 ·; The apparatus also comprises a source of energy J 27A for the heating elements, a source of inert gas I under pressure 31 and a connector 32 for the adaptation of a vacuum apparatus making it possible to expel the ambient atmosphere! unwanted before pressurizing the autoclave.

i

For the treatment of parts made of a nickel-plated super alloy, the furnace is heated up to approximately 1.315 ° C after the chamber has been filled with an inert gas, such as argon or helium. . Manometric pressures up to 2,069 bar and above can be achieved by the combined effects of compression and thermal expansion. As the pressure of the gas is isostatic, the product finally obtained is substantially free from measurable distortion, provided that the internal structural defects do not have a dimension exceeding a substantial fraction of the section of the product.

Various preferred embodiments of the invention are illustrated by the following examples.

EXAMPLE 1.-

The present example illustrates the importance of rapidly cooling the part made of the alloy in the autoclave by maintaining the isostatic pressure without interruption during rapid cooling down to below 1 - the structural hardening temperature of the alloy, which in this case is the René 100 alloy (0.18% C, 10.0% Cr, 15.0% Co, 3.0% Mo, 4.7% Ti, 5.5% Al, 0.014% B, 0.06% Zr, 1% V, and 'Mi for the rest).

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It should be noted that the blades of superalloy turbines for the hot part of an engine are generally covered with a protective layer of chromium and / or aluminum applied by case hardening in a package. The blades are generally coated at an elevated temperature, which is about 705 to 1,150 ° C, in about 1 to 40 hours, for example at 1,050 ° C in about 4 hours, and then cooled slowly. These coating techniques are described in US Patents 3,257,230, 3-716,358 and 3-999,956.

In order to establish a comparison between the process of the invention and the process not in accordance with the invention, the treatment is carried out under hot isostatic pressure on turbine blades in substance at the same temperature and under the same pressure, except that in in one case the part is quickly cooled in the autoclave to below its structural hardening temperature from the temperature of treatment under hot isostatic pressure and in the other case the part is cooled slowly in the autoclave below its hardening temperature strur-tiinsl of T s msni pt> p tmel 1 p * (1) Method of the invention

The Lené 100 alloy blades are subjected to treatment under hot isostatic pressure by heating in the autoclave to 1190 ° C. for 2 hours under a gauge pressure of approximately 1-931 "bars, then they are rapidly cooled in the autoclave until substantially below the structural hardening interval of the alloy, at a rate of approximately 30 ° C. per minute. The parts are then removed from the autoclave and subjected to a heat treatment in the usual conditions of temperature, duration and cooling rate for case hardening in a packet of the type indicated above, this simulated heat treatment being carried out at 1.052 ° C. for 4 hours, with cooling in the oven at the end of the operation The temperature for treatment under hot isostatic pressure is approximately 95% of the absolute melting point of the alloy. After the heat treatment described above, the parts are subjected to aging at 843 ° C. for 4 hours. , after which we starts to cool in air.

It is thus evident that the particular heat treatment succeeding the treatment under hot isostatic pressure comprises the heat treatment cycle specific to case hardening in packages. The invention is not, however, limited to this aspect. In other words, the heat treatment succeeding the heat isostatic pressure treatment may simply comprise a heat treatment for direct aging outside the autoclave or any other heat treatment desired.

(2) Process not in accordance with the invention

Two distinct treatments are carried out, namely: (A) a treatment under conventional hot isostatic pressure, under iscstatic pressure ε chsud.

(A) Pear the conventional hot isostatic pressure treatment, the parts are kept at 1.190 ° C for 2 hours under a gauge pressure of about 1.896 In, then they are cooled slowly at a rate of less than 15 ° C per minute , after which the vacuum parts are heated to 1,190 ° C for 2 hours and cooled under vacuum to 1,093 ° C in 6 to 10 minutes before keeping them at 1,093 ° C for 4 hours under vacuum and finally cool rapidly in a gas stream. Next, the normal thermal cycle provided for coating the blades, as described above, is carried out, bringing the parts to a temperature of 1.052 ° C. for a period of 4 hours, then cooling them in the oven and subjecting them. then aging at 843 ° C for 4 hours before cooling in air.

(B) For the heat treatment of the parts which are not subjected to the hot isostatic pressure treatment, the parts are subjected first to the simulated heat treatment of the case hardening, that is to say they are heated to 1.032 ° C for 4 hours, then cooled in the oven or in air before aging at 843 ° C for 4 hours and finally air cooled.

After the various treatments above, test pieces are prepared of the parts which have not undergone the treatment under hot isostatic pressure and of the parts which have undergone the treatment under hot isostatic pressure either conventional or according to the invention and they are used for stress rupture tests at 760 ° C under a stress of 5-861 bars, as well as at 982 ° C under a stress of 2000 bars.

The hot isostatic pressure treatment is symbolized by TPIC and the necking or reduction of seci ion is symbolized by the duration is the lifetime.

The results obtained are collated in the following tables.

TABLE I

Stress rupture tests, 760 ° C. 5.861 bars U ° TPIC Diameter Duration Elongation Σ- (mm) (L) (%) (%) AI none 2.057 4-3.1 5.1 7.8 2A none 2.210 69.5 5.7 5.0 3A none 2,032 163.3 3.1 6.0 4A nil 2,057 202.4 6.2 10.0 5A nil 2,591,549 Λ 14.5 14.8

Geometric mean 128.1 5.5 7.7 6A classic 2.261 63.4- 5.5 8.9 7A classic 2.134 75.1 6.0 7.2 8A classic 2.261 111.8 2.8 4.8 9A classic 2.159 132.3 5.3 8.9 10A classic 2.972 221.4 8.5 12.0 11A classic 2.794, 224.9 7.1 18.9 12A classic 2.388 '230.2 8.6 21.7 13A classic 2.210 3H, 8 5.8 6.7 14A classic 2.281 380.8 5.7 13.4 15A classic 35 124 441.6 8.1 10.9

Geometric mean 182.8 6.1 10.3 1 11 invention 2.515 166.1 5.0 9.1 / i »^ invention ^> ^ 40 310.4 5 ^ 0 6.9 3 invention 2.540 374.2 7, 5 8.6 4 the invention '2.565 404.2 5.0 10.0 5 the invention -2.540' Lâ. he

TABLE II

Stress rupture tests, 982 ° C, 2,000 bars U ° TPIC Diameter Duration Elongation 21 (mm) (it) (%) (° / o) IB none 2,261 22.8 11.1 12.9 2B none 2,261 27, 8 8.3 12.6 3B none 2,261 29.0 3.3 6.4

Geometric mean 26.5 7.9 10.6 4B classic 2.235 21.9 4.5 13.1 5® classic 2.261 24.4 18.0 24.1 6B classic 2.286 26.8 6.9 7.9 7B classic 2.286 27.1 13.8 14, o 8B classic 2.184 33.2 11.7 18.3 9B classic 2.261 33.4 6.9 14.5 10B classic 2.261 34.4 5.5 10.7

11B classic 2,286 37.5 13.8 19, Q

Geometric mean 29.8 9.1 15.2 1 invention 2> 565 22.8 12.5 13.7 2- 11 invention 2'5 "> 24.7 12.5 20.2 3 1 invention 2 '^ ° 51.8 15.0 20.2 4 the invention 2> ^ ° 36.9 15.0 21.5

5 - 11 invention ZL ° IM

Geometric mean 30.6 14.0 19.2

It appears from Table I that the treatment under hot isostatic pressure according to the invention leads to a surprising geometric average of the lifetime until rupture by stress at 760 ° C. under 5.861 bars, which is 322.1 hours. against 128.1 hours in the absence of treatment under hot isostatic pressure and 182.8 hours after treatment under an isostatic press press i chauâ c'ja "^ iau =

We can refer to this subject in 2 e Pig · 2.

The process of the invention has no significant effect on the stress fracture properties at 982 ° C (see Table II), but is free from harmful effects and nevertheless leads to a small improvement, as shown by a examination of FIG. 3-EXAMPLE 2, -

Similar hot isostatic pressure treatment tests are carried out on an alloy known as SEL-15, which is a nickel-based alloy capable of structural hardening which contains, on a weight basis, 0.08% C, 0.3% max. . Mn, 0.3% max. Si, 10.3% Cr, 13.5% Co, 6.3% Mo, 1.5% W, 0.5% Mb, 2.5% Ti, 5.5% Al, 0.05% B and for the rest substantially nickel.

(1) Method of the invention

The parts made of the alloy are subjected to treatment under hot isostatic pressure at 1,190 ° C. (approximately 90% of the absolute melting point of the alloy) for 2 hours under a gauge pressure of 2,000 bars, then they are rapidly cools in the autoclave to substantially below the structural hardening interval at a rate of about 30 ° G per minute, after which the parts are removed from the autoclave and subjected to the simulated heat treatment of the case hardening, that is to say they are heated to 1.052 ° C for 4 hours, then left to cool * in the oven. After the last treatment, the parts are subjected to aging at 780 ° C for 4 hours before being allowed to cool in air.

(2) Process not in accordance with 1 ’irver.ticr.

Two separate treatments are carried out, namely: (A) a conventional hot isostatic pressure treatment, and (B) a thermal treatment without treatment under hot isostatic pressure.

(A) For the treatment under conventional hot isostatic pressure, the parts are brought to 1,190 ° C. for a period of 2 hours under a manometric pressure of approximately I.93 I bars at a rate of less than 15 ° C., after which the alloys are returned to a solid solution at 1,190 ° C. for 4 hours and cooled in vacuo. Then, the parts are subjected to the simulated heat treatment representative of case hardening in a package, that is to say that they are heated at 1.052®C for 4 hours, then they are allowed to cool in the oven and then performs aging at Tj> 80oC for 4 hours before air cooling.

(B) For the heat treatment without treatment under hot isostatic pressure, the parts are subjected first to the simulated heat treatment at 1.052 C for 4 hours with cooling in the oven, then to aging at 780 ° C for 4 hours with cooling to the air.

The parts are prepared for a creep test (diameter of 2.54 mm) at 760 ° C under a stress of 5 · 861 bars.

The results are collated in Table III which is also concerned by the Eig. 4.

/ _____

TABLE III

Stress rupture test, 760 ° C, 3-861 Lars

Test No. Duration Extension

Ch) (%)

No TPIC

10 91.8 3.7 2C 170.6 3.7 30 132.6 3.7 4C 104-, 8 3.0 50 64-, 3 3.7

Geometric mean 107.1 3.9

Limit to 98% 50.1 3.0 TPIC classic 6G 64-, 3 2.6 70 8.8 5.5 8C 4-, 5 10.8 90 17.0 3.3 100 52.6 3.4- 110 10.3 8.3 12C 19.9 6.6 130 88.8 6.2 140 7.5

Geometric mean 23.4 - 5.5

Limit to 98% 3.1 2.6 TPIC of the invention 6 200.9- 10.0 7 141.1 6.2 8 89.3 7.5 9 158.4 8.7 10 237.1 18, 0

Geometric mean 156.9 9.4

98% limit 74.1 4.2

It can be observed that the heat isestatic pressure treatment in accordance with the invention leads to a surprisingly high value of the geometric mean of the time until rupture by stress, namely 156 * 9 hours against 25.4 hours for treatment under conventional hot isostatic pressure I and 107 hours in 1 * absence! of treatment under hot isostatic pressure, the process of the invention ensuring with a reliability limit of 98% a duration of 74.1 hours, while the duration is 3.1 hours, - for treatment under isostatic pressure at hot conventional and 50 * 1 hours in the absence of treatment under hot isostatic pressure. In this example, the | j properties manifested at high temperatures by blades of SEL-15 alloy are not improved by the conventional hot isostatic pressure treatment and the heat treatment which follows it, the comparison being established with the same blades undergoing the thermal cycle normally carried out for case hardening before an aging treatment. However, this is not the case in Example 1.

Consequently, a major advantage offered by the invention is that it makes it possible, in a reproducible manner, to clearly improve or completely restore the mechanical properties at high temperatures of very many superalloys, which is not generally the case of heat isostatic pressure treatments carried out in a conventional manner. This distinction emerges from an examination of Example 2 (see Pig. 4) which shows that the alloy SEL-15, when it has not undergone treatment under hot isostatic pressure, is in a superior state to that where it arrives after a conventional hot isostatic pressure treatment and a heat treatment, whereas in Example 1 (see

Fig. 2), the conventional hot isostatic pressure treatment confers on the allied a superiority which it does not manifest in the absence of the hot isostatic pressure treatment. However, in the two examples, the treatment under hot isostatic pressure according to the invention leads to mechanical properties clearly superior to those which are achieved by the treatment under conventional hot isostatic pressure.

Another advantage of the invention is that the method makes it possible to carry out, after the treatment under hot isostatic pressure, simpler heat treatments, which is generally not the case when applying a treatment under conventional hot isostatic pressure. . EXAMPLE 5, -

As already indicated, the invention is applicable to parts made of an iron-based alloy, for example k the so-called A-286 alloy of the following nominal composition: 0.05% C, 1.35% Mn, 0.50% Si, 15.0% Cr, 26.0% Ni, 1.3% Mo, 2% îi, 0.2% Al, 0.015% B, and Ee for the rest. This alloy has a melting range of 1.371-1.399 ° C or an average melting point of about 1.385 ° C. The temperature chosen for the treatment under hot isostatic pressure is approximately 75% of the absolute melting point of the alloy, which is 1.658 Kelvin. The temperature calculated for the treatment under hot isostatic pressure is approximately 970 ° C. which is approximately the temperature of formation of the solid solution of the alloy.

A part made of the iron-based alloy is subjected to treatment under hot isostatic pressure at 970 ° C for 4 hours under a manometric pressure of 1,724 bars, after which the part is rapidly cooled to more than about 30 ° C per minute to below the aging temperature of the alloy. After this treatment under hot isostatic pressure, the alloy is subjected to aging at approximately 720 ° C. for 16 hours, before being cooled in air.

Similar results are obtained with other alloys capable of structural hardening, for example the titanium-based alloy known as Ti-6-2-4-6. An example of the treatment of this alloy is given below. EXAMPLE 4, -

A titanium molded part of the above composition tends to have withdrawal cavities, in this case micropores, but can have a radiographic quality making it acceptable in many cases where the stresses are low. This titanium-based alloy, which contains 6% Al, 2% Sn, 4% Zr, 6% Mo, and for the rest essentially of titanium, has a liquidus temperature of 1.649 ° C. The above melting point corresponds to an absolute melting point of approximately 1,922 Kelvin. For the treatment under hot isostatic pressure, a temperature which is approximately 62% of the absolute melting point of the alloy is chosen, ie approximately 920 ° C. Consequently, the titanium-based alloy part is subjected to the treatment under hot isostatic pressure at approximately 920 ° C. under a gauge pressure of approximately 1,931 bars for approximately 4 hours, after which it is rapidly cooled in situ at a rate from about J> 0 to 40 ° C per minute under superatmospheric pressure up to below the structural hardening interval.

After the above treatment, the alloy is subjected to aging at 593 ° C for 8 hours, then to air cooling to reach the mechanical properties. / * /

As already indicated, the invention is particularly applicable to the treatment of nickel-based and cobalt-based superalloys which are susceptible to structural hardening. An alloy falling within a typical weight range contains up to about 30% chromium, for example about 5 to $ 0% chromium, up to about 20% of a metal of group Mo plus W, "up to about 10% of a metal from the group Nb plus Ta, up to about 1% of carbon (preferably up to about 0.5% of carbon), up to about 10% of a metal from the group Ti plus 11, for example about 0.2 to 10%, the total amount of Ti and Al not exceeding about 12%, up to about 20% iron, up to about 2% manganese, up to about 2% silicon, up to about 0.2% boron, up to about 0.1% zirconium, up to about 2% hafnium, and the rest at least about 45% at least a metal from the group Ni plus Co.

The expression "and for the rest at least about 45% of at least one metal from the group Ni plus Co" means that when the two metals are present, their sum is at least about 45% of the complete composition. Therefore, nickel may be present alone and cobalt may be present as well, each in an amount of at least about 45%. When present together, the amount of each may have any complement value, provided that the sum of the two amounts is at least about 45%, on a weight basis.

Alloys of the above type are generally subjected to a heat treatment by exposure to a solid solution temperature of about 1.080 to 1.125 ° C for about 30 minutes to 16 hours, followed by cooling in the oven or in air. . After the solid solution treatment, the alloy can be subjected to structural hardening (or precipitation hardening), for example by aging at a temperature of about 730 to 870 ° C for a period of up to 24 hours, for example for 4 to 10 hours.

Although various embodiments and details have been described to illustrate the invention, it goes without saying that it is susceptible of numerous variants and modifications without departing from its scope.

/

Claims (8)

3 · - Method according to claim 2, characterized in that the rapid cooling rate in the autoclave is at least about 25 ° C per minute. 4, - Method according to claim 3, characterized in that the part made of the alloy is a part made of a superalloy. .5-- Process according to Claim 4, characterized in that the superalloy is a nickel-based alloy and the temperature for the treatment under hot isostatic pressure is approximately? 0 to 95% of the absolute melting point of l 'alloy. 6.- Method according to claim 3 "characterized in that the temperature for treatment under hot isostatic pressure is about 80 to 95% of the absolute melting point of the alloy. 7-— Process for improving the mechanical properties of a part made of an alloy capable of structural hardening chosen from iron-based alloys, nickel-based alloys, cobalt-based alloys and alloys based on titanium having a melting point of more than 1.000 ° C. which has structural defects such as micropores in casting and / or microcracks or cavities at the grain boundaries which have appeared during service at elevated temperature, characterized in that: the part made of this alloy with the treat- ment JB ent g or s presni.M. hot isostatic in an autoclave under superatmospheric pressure at a high solid solution temperature I of this alloy ranging from more than 50% to about 95% of the absolute melting point of the alloy for a period of time at least sufficient / to bring about the substantial elimination of structural defects by heating and densification, the part made of the alloy is subjected to the in situ heat treatment by rapid cooling at a rate of at least about 25 ° C. per minute up to 'below i. of the structural hardening interval of the alloy by keeping the part under isostatic superatmospheric pressure, and the alloy is subjected to structural hardening after completion of the treatment under hot isostatic pressure, so as to make the mechanical properties of this parts superior to those of the same part having undergone the heat treatment by rapid cooling outside the autoclave and aging after the conventional hot isostatic treatment. 8. Method according to claim 7 * character ized in that the temperature for the treatment under hot isostatic pressure is approximately 60 to 95% of the absolute melting point and the pressure for the treatment under hot isostatic pressure is approximately 345 to 3 * 44-8 bars * 9 * - Method according to claim 8, characterized in that the part made of the alloy is a part made of a superalloy. 10.- Method according to claim 9 »characterized in that the superalloying is a nickel-based alloy and the temperature for treatment under hot isostatic pressure is d1 about 70 to 95% of the absolute melting point of the alloy. 11.- Method for improving the mechanical properties of a part made of a superalloy capable of structural hardening which has structural defects such as micropores of casting and / or microcracks or cavities at the grain boundaries which appeared during ► ► service at a high temperature, the alloy having a melting point of at least about 1,000 ° C, characterized in that: at least one part made of an alloy comprising, on a weight basis, up to about $ 0% chromium, up to approximately 20% of a metal from the group Ho plus W, up to approximately 10% of a metal from the group Nb plus Ta, up to approximately 1% of carbon, up to approximately 10% of a metal from the group Ti plus Al, the total amount of Ti plus Al not exceeding about 12%, up to about 20% iron, up to about 2% manganese, up to about 2 % silicon, up to approximately 0.2% boron, up to approximately 1% zirconium, up to approximately 2% hafnium, and essentially for the rest at least approximately of at least one metal from the group Ni plus Co, the part made of this alloy capable of structural hardening is subjected to a treatment under hot isostatic pressure in an autoclave under superatmospheric pressure and at a high solid solution temperature of this alloy which is greater than 50% of the absolute melting point of the alloy for a sufficient time to cause a substantial elimination of structural defects by heating and densification, the part made of the alloy is subjected to a heat treatment in located by rapid cooling to a% * rate of more than 20 ° C per minute to below the structural hardening interval of the alloy while maintaining the part under isostatic superatmospheric pressure, and the alloy is subjected to hardening structural after completion of the treatment under hot isostatic pressure, so as to make the mechanical properties of this part superior to those of the same part having underwent heat treatment by rapid cooling outside the autoclave and aging after conventional hot isostatic treatment. 12, - Method according to claim 11, characterized in that the gauge pressure for the treatment under hot isostatic pressure is approximately 345 to 3,448 bars and the temperature for the treatment under hot isostatic pressure is approximately 70 to 93 % of the absolute melting point of the alloy. 13-- A method according to claim 12, characterized in that the rapid cooling rate in the autoclave is at least about 25 ° C per minute. 14.- Method according to claim 13, characterized in that the superalloy is a nickel-based alloy and the temperature for the treatment under hot isostatic pressure is approximately 80 to 93% of the absolute melting point of the alloy . A P,