Abstract:
The invention relates to precipitation hardened alloy compositions comprising the following elements, with the contents expressed in % by weight: 
     
       ______________________________________ 
     
     nickel:             ≧52.00%chromium:           20.50%-22.50%iron:               7.00%-13.00%molybdenum:         5.50%-7.0%copper:             1.00%-3.50%niobium:            2.65%-3.50%titanium:           1.0%-2.0%cobalt:             0-3.00%aluminum:           0-0.75%tungsten:           0-0.50%silicon:            0-0.20%manganese:          0-0.20%phosphorous:        0-0.03%carbon:             0-0.02%nitrogen:           0-0.02%magnesium:          0-0.005%sulfur:             0-0.005%______________________________________ 
     the elements satisfying the following four relationships: 
     X=(2.271% Ti+1.142% Cr+0.957% Mn+0.858% Fe+0.777% Co+0.717% Ni+2.117% Nb+1.550% Mo+1.655% W+1.90% Al+1.90% Si+0.615% Cu)≦93.5, the percentages for this relationship being in atomic %; 
     Y=(% Mo+% W+% Cu)≦9, the percentages for this relationship being in % by weight; 
     A=(0.65% Nb+1.25% Ti+2.20% Al)≧4.4, the percentages for this relationship being in % by weight; ##EQU1## the percentages for this relationship being in % by weight. The invention also relates to a process for transforming this alloy and to very large pieces of up to several tonnes for the oil industry.

Description:
FIELD OF THE INVENTION 
     The invention relates to a precipitation hardened Ni-Cr-Fe based alloy composition with good mechanical characteristics up to 500° C., also good resistance to different types of corrosion and hydrogen embrittlement. This novel alloy is specially adapted to die forging very large pieces of up to several tonnes (i.e. metric tons), for very deep oil and gas wells, including offshore wells. 
     The tables and text in the present application use the following abbreviations: 
     R m  =maximum strength 
     R p0 .2 =conventional 0.2% yield strength 
     E 5d ,4d =% elongation over 4d and 5d bases (d=test piece diameter) 
     Z=necking 
     RCH=Rockwell hardness 
     E=elongation 
     ε=strain rate 
     L/L 0  =degree of drawing; L 0  is the undrawn length and L is the drawn length; 
     KV=impact bending energy at break using a test piece with a V notch 
     HV 30  =Vickers hardness at 30 kg load. 
     TECHNOLOGICAL BACKGROUND 
     Continuous exploitation of fossil energy, gas and oil, has led to a demand for materials which have both high corrosion resistance and sufficient mechanical strength to stand up to the conditions they encounter, but also which are very malleable so that they can be easily transformed using existing techniques to produce pieces which may have complex shapes which necessitate handling several tonnes of these materials, for example &#34;Christmas trees&#34; or offshore well heads, pump bodies, or valves. 
     The types of corrosion encountered in the fossil energy extraction field, can be classified into three categories: the first, into which the majority of cases fall, being corrosion by CO 2  gas; the second, which is quite frequent, being corrosion by a combination of CO 2  and H 2  S gases; and the third class of corrosive environments, which is still in the minority since it is encountered in deep wells, being corrosion by simultaneous concentrations of highly aggressive substances H 2  S, CO 2 , and chloride salts mixed with water, methane, and hydrocarbons. These hydrocarbons can sometimes be in a minor proportion in extracted mixtures which are rendered highly acidic by the presence of H 2  S and its chemical reactions. 
     The increase in pressure and temperature with depth renders these environments even more aggressive against metallic materials: deep wells drilled onshore and offshore in northern USA, in Europe, and in Australia, for example, have experienced high rates of corrosion and premature breakage of ordinary metallic materials. Offshore drilling adds to that the external corrosion of pieces by seawater. 
     The combination of such very severe conditions means that the materials used must have very good resistance to corrosion in all of its forms. However, generalised corrosion resistance is relatively easy to obtain, and it is necessary to ensure that precautions are taken against localised corrosion, stress corrosion in chloride environments, and against hydrogen embrittlement. Although the oil industry has few corrosion problems in terms of frequency, it encounters just the specific problems described above during offshore production, combined with difficulties in overcoming or evaluating actual deaeration of hot corrosive environments, which are highly acidic and under pressure, and which can lead to stress corrosion cracking (SCC) of usual alloys such as stainless steels. Stress corrosion must be distinguished from hydrogen embrittlement which is connected with the presence of H 2  S, although those two mechanisms can occur simultaneously. 
     Among metallic materials, alloys based on Ni--Cr systems and Ni--Fe--Cr systems are used under such conditions because of the excellent compromise they offer between corrosion resistance and cost compared with other, more costly materials such as titanium. Ni--Cr based alloys with a high nickel content are known to be sensitive to hydrogen, and the severe conditions described above thus direct the choice to Ni--Fe--Cr alloys which have a high iron content which also limits materials costs. 
     Ni--Fe alloys with nickel contents of over about 40% by weight can be protected against stress corrosion in chloride environments in limited circumstances which depend on complementary additions, in particular of the elements chromium, molybdenum and copper. 
     In general, nickel based alloys containing the elements Mo, Cu and Fe are suitable for non oxidising corrosive environments, in particular non aerated environments: the synergistic effect of molybdenum and copper is recognised for countering corrosion in reducing acidic environments which are rich in chlorides, the effect of the molybdenum being preponderant. 
     However, in the most severe oil industry environments described above, which can be partially, temporarily, or even accidentally, oxidising or aerated, chromium, an element which resists oxidation makes an essential contribution to the corrosion resistance of Ni--Fe--Cr--Mo--Cu alloys. 
     The molybdenum content has a marked effect on alloys containing at least 40% of nickel and 20% of chromium, as regards stress corrosion in oil industry environments containing CO 2 , H 2  S, Cl -  and elemental sulfur. Three important criteria can be distinguished in these environments: temperature, acidity, and chloride concentration. In particular, increasing the molybdenum content can increase the maximum service temperature above which stress corrosion occurs. 
     Similarly, raising the molybdenum content means that higher acidity and higher chloride concentrations can be tolerated, with due allowance being made for localised corrosion of Ni--Fe--Cr--Mo--Cu alloys. 
     With the aim of satisfying the demand for good mechanical characteristics and creep strength, Ni--Fe--Cr based alloys exist which are precipitation hardened by precipitation of phases which confer high strength while ductility remains good. In the majority of these alloys, used in particular in the aircraft industry in the hot zones in engines or in turbines, the elements niobium, titanium, and aluminum, in suitable proportions, participate in hardening reactions by which the alloy attains its hardness during an aging treatment carried out in a temperature range where those addition elements are supersaturated in the austenitic matrix, which has previously been homogenised at a higher temperature, and which then becomes a metastable solution. 
     French patents FR-A-2 154 871 and FR-A-2 277 901 indicate that elements Al, Ti and Nb which are supersaturated in austenite form intermetallic compounds with nickel which produce the desired hardening, of which there are two types. Aluminum causes the formation of a face centered cubic phase with crystallographic structure L 12  with an A 3  B type chemical composition where A represents nickel and a small fraction of the other elements of the austenitic matrix (iron, cobalt, chromium, . . .) and B represents aluminum; that phase is termed the γ&#39; phase. Titanium can substitute all or part of the aluminum in the γ&#39; phase and can increase its hardening effect at higher temperatures. 
     Niobium causes the formation of an intermetallic phase with a Do 22  type centred tetragonal structure, also with a composition A 3  B where this time B represents niobium, the phase being termed the γ&#34; phase. 
     Such alloys are hardened by precipitation of one or other of those phases, or both at once, between 600° C. and 800° C., which constitutes an improvement claimed in FR-A-2 154 871 which is applied to Ni--Fe--Cr based alloys comprising 15% to 25% by weight of iron, 15% to 25% of chromium, 2.5% to 9% of molybdenum, 1.5% to 6.5% of niobium and/or tantalum, 0.5% to 1.5% of titanium, 0.3% to 1.5% of aluminum and 0.03% to 0.2% of carbon. The alloys claimed in that patent are characterized by very precise titanium, aluminum, and niobium (+tantalum) contents, such that their sum in atomic % must be in the range 4% to 6% and such that the (Ti+AI)/(Nb+Ta) ratio of the sums in atomic % is over 0.8, with the precise aims firstly of obtaining good precipitation hardening during an aging cycle carried out in two temperature stages, and secondly of suppressing the overaging effect which occurs in such alloys during high temperature service, and finally to provide good ductility both at room temperature and when hot, for example in creep. 
     The invention described in FR-A-2 154 871 is based on the following discovery: the claimed properties are due to a particular and very stable morphology of the γ&#39; and γ&#34; precipitates in which the six faces of the cubic γ&#39; phase precipitates formed during the highest temperature aging stage are covered with precipitates of platelets of the γ&#34; phase formed during the second temperature stage, at a lower temperature than the first. 
     According to that invention, simultaneous precipitation of carbides in the grain boundaries can further increase the strength at high service temperatures. 
     European patent EP-A-0 262 673 claims a precipitation hardened nickel based alloy which is resistant to hydrogen embrittlement and to corrosion in chloride media containing H 2  S. That alloy contains 15% to 25% by weight of Cr, 5% to 28% of Fe, 6% to 9% of Mo, 2.5% to 5% of Nb, 0.5% to 2.5% of Ti, up to 0.5% of Al and 54% to 64% of Ni: the claims are directed towards applications in oil industry environments. Supplementary conditions give more precise values for the alloying element contents. The alloys described in EP-A-0 262 673 do not include the addition of copper, or copper addition is limited to less than 1%, but they can be hardened by precipitation of two phases: γ&#39; then γ&#34;. 
     EP-A-0 247 577 claims another nickel based alloy which is precipitation hardened by γ&#39; and γ&#34; phases and which is resistant to corrosion in media containing H 2  S and Cl -   at high temperatures; that alloy contains 16% to 24% of chromium, 7% to 12% of molybdenum, less than 4% of tungsten, 2% to 6% of niobium, less than 1% of aluminum, 0.5% to 2.5% of titanium, 0 to 3% of copper, less than 20% of iron, more than 55% of nickel and controlled amounts of carbon, cobalt, silicon, manganese, boron, zirconium, nitrogen, phosphorous and sulfur. Further, the sum (Cr+Mo) is limited to 31% by weight while the sum of the atomic percentages of elements Nb, Ti and Al is kept between 3.5% and 5%. 
     All of the alloys described in the patents referred to above, in addition to high strength and good corrosion resistance, are easy to work and are insensitive to precipitation of harmful intermetallic phases. 
     However, manufacturers of pieces from such alloys and from super-alloys in general, whether involved in smelting, forging, or using heat treatment units, encounter insurmountable problems with ingots and semi-finished products weighing several tonnes and with large cross sections. 
     Typically forging produces pieces by thermo-mechanical transformation of ingots: for nickel based alloys containing the elements titanium, aluminum, or chromium, which have a high affinity for oxygen, ingots are produced in two steps. The first step is the production of electrodes under vacuum and the second step is one or more remelting steps using electrodes using vacuum arc remelting (VAR) or electroslag remelting (ESR). Those two remelting processes affect the quality of the metals by completing primary production (complementary purification and improving inclusion cleanliness); they also affect solidification control by reducing defects and minimising segregation of the alloying elements. 
     The ESR process currently results in better desulfuration, and is used especially for the production of large ingots where it is irreplaceable. Controlling the remelting parameters can produce the best structural homogeneity in the ingots. 
     However, as the quantity of alloying elements which are prone to segregation increases, and when the size of the remelted ingots increases, which is possible nowadays with furnaces with ever larger diameters, the risk of defects on solidification grows: defects which can occur in remelted ingots are listed in American Standard Test Method ASTM A604. 
     There is an absolute limit on the size of a remelted ingot for a given alloy and once the remelting parameters have been optimised, above which the degree of segregation is such that the properties in service can no longer be guaranteed at all points in its volume. The hardening elements titanium, aluminum, and niobium are known to be very limiting as regards the maximum diameter of ingots if a no-defect situation is desired. 
     In extreme cases, once solidification has finished, segregation of alloying elements generally causes the formation of particular eutectoid or eutectic compositions the nature of which depend on the segregated elements: niobium forms Laves phases with iron (Fe 2  Nb), in which silicon is concentrated; molybdenum produces a p phase (Ni 7  Mo 6 ) with nickel, and the sigma phase with chromium and iron, while titanium and aluminum produce massive Ni 3  (Ti,Al) type eutectic compositions. Further, all of those elements can form nitrides, carbides, or borides in the presence of nitrogen, carbon, and boron respectively. 
     Such compositions are harmful since they produce heterogeneous properties in the alloy and fix the alloying elements in the service and forging temperature ranges. Further, the majority have low melting points, which limits the homogenisation and forging temperatures of the ingots. 
     A further very constricting phenomenon of too much addition of elements titanium, aluminum, niobium, and molybdenum is heat hardening of a solid solution (Mo, W, Nb) or premature precipitation hardening of secondary phases (Ti, Al) which increases as the temperature falls: such hardening effects are such that thermo-mechanical transformation operations require energy which is not available with existing tools in the case of large ingots. This phenomenon limits the forging range to a minimum temperature below which the metal is too hard. 
     Increasing the addition of alloying elements thus has the effect of reducing the range over which alloys can be subjected to thermo-mechanical transformation, both at low temperatures and at high temperatures. 
     Finally, when alloys require solution heat treatment and rapid quenching followed by age hardening by precipitation of secondary phases, the size of pieces is again limited, and above that size the rate of cooling during rapid quenching is insufficient at the core of the piece to avoid the onset of coarse and disordered precipitation of the hardening phases: the addition of more alloying elements, which accentuates matrix supersaturation, accelerates precipitation kinetics which are thus out of control and thereby reduces the maximum size of pieces that can be treated. 
     SUMMARY OF THE INVENTION 
     In order to overcome such very difficult problems and allow easy working of die forged pieces weighing several tonnes, while retaining the desired properties, the inventors have developed novel precipitation hardened Ni--Cr--Fe alloy compositions where all of the elements are linked by restrictive formulae so as to obtain: 
     ingots with low segregation, including those with a mass of at least 8 tonnes; 
     a wide range of homogenisation temperatures, extending up to 1250° C.; 
     a wide range of thermo-mechanical transformation temperatures, in which the material can easily be worked by forging, drop forging, and rolling using normal existing tools; 
     controlled supersaturation of the alloying elements in the dissolved matrix, to retain a metastable austenite which can be age hardened, including in the cores of massive pieces; 
     combining high hardness, of over 34 RCH after treatment, with a high yield strength (R p0 .2 ≧700 N/mm 2 ) and good ductility (E 4d  ≧20%), even in the core of large pieces; 
     good resistance to the various types of corrosion encountered in oil environments. 
     The essential elements in this nickel based alloy are chromium, molybdenum, niobium, titanium, aluminum and copper, iron being the make-up as that element is less expensive and helps to limit sensitivity to hydrogen embrittlement in Ni--Cr alloys. 
     The broadest composition ranges claimed are, in percentages by weight: 
     
         ______________________________________Aluminum      ≦0.75Carbon         ≦0.020Niobium       2.65-3.50Cobalt        ≦3.0Chromium      20.5-22.5Copper        1.0-3.5Iron           7.0-13.0Magnesium      ≦0.0050Manganese     ≦0.20Molybdenum    5.50-7.0Nitrogen       ≦0.020Nickel         ≧52.0Phosphorous    ≦0.030Sulfur         ≦0.0050Silicon       ≦0.20Titanium      1.0-2.0Tungsten      ≦0.50______________________________________ 
    
     Further, the claimed alloy compositions must satisfy the following relationships in order to guarantee all of the desired properties: 
     X=[2.271% Ti+1.142% Cr+0.957% Mn+0.858% Fe+0.777% Co+0.717% Ni+2.117% Nb+1.550% Mo+1.655% W+1.90% Al+1.90% Si+0.615% Cu] ≦93.5, in atomic %; 
     Y=(% Mo+% W+% Cu)≦9, in % by weight; 
     A=(0.65% Nb+1.25% Ti+2.20% Al)≧4.4, in % by weight; and ##EQU2## in % by weight. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The role of each of the elements in the alloy is explained along with the reasons leading to defining the above relationships, and also the limits on the contents of each of the controlled elements. 
     In contrast to FR-A-2 154 871, which claims nickel based alloys comprising 0.03% to 0.2% of carbon to improve mechanical behaviour at high temperature, in particular creep behaviour, the alloy of the present invention for low temperature applications (&lt;400° C.) claims a very low carbon content for applications where corrosion resistance is paramount, for the following reasons. Carbon has a high tendency to segregate during solidification of nickel based alloys, in which this element is poorly soluble even at concentrations as low as 0.03%; in the presence of carbide-forming elements such as niobium, titanium, or molybdenum, which also have a tendency to segregate, carbon forms massive carbides in eutectic clusters, in the interdendritic regions at the end of solidification. These clusters of segregated carbides are extremely harmful from a variety of aspects. In ingots, because of their low melting point, they cause constitutional melting on reheating before homogenisation or forging at temperatures well below the solidus of the alloy, which means that effective homogenisation treatments for the metallic addition elements cannot be used, and they are a source of cracks during high temperature thermo-mechanical transformations. Further, pieces are found to contain aligned carbides which contribute to structural heterogeneities and which constitute weak points, in particular as regards corrosion resistance. 
     Finally, carbon is well known to be extremely harmful, regarding intergranular corrosion resistance of steels and alloys aged between 600° C. and about 900° C., since it fixes chromium and molybdenum in M 23  C 6  or M 6  C carbides, thereby depleting grain boundaries in these two elements which are essential to corrosion resistance, and it also weakens precipitation hardening of γ&#39;Ni 3  (Al,Ti) and γ&#34;Ni 3  Nb phases since, in this case also, carbon fixes the essential elements titanium and niobium in stable carbides over a wide temperature range. 
     For these reasons, the carbon content in the claimed alloy is limited to 0.02%, and preferably to less than 0.015%. 
     The very precise proportions of the addition elements titanium, aluminum, and niobium ensure the desired hardening by a solution treatment followed by double aging in two temperature stages which precipitate the γ&#39; and the γ&#34; phases in succession. Calculation of the degree of precipitation of the hardening phases has allowed us to adjust the titanium, aluminum, and niobium contents. In order to obtain a room temperature yield strength of 700 N/mm 2  or more, the following relationships have been defined: 
     
         A=(0.65% Nb+1.25% Ti+2.20% Al)≧4.4% by weight, and ##EQU3## 
    
     The metal is hardened after solution treatment and rapid quenching by a two-stage double aging treatment, the first stage at a temperature in the range 700° C. to 760° C. for first rapid hardening in less than 8 hours by precipitation of a first phase γ&#39; Ni 3  (Ti,Al) while the second stage, at a lower temperature, typically 600° C. to 675° C., ensures complementary hardening by precipitation of the γ&#34; Ni 3  Nb phase on the γ&#34; phase seeds. 
     Aluminum, which causes a very rapid hardening reaction, is not suitable for treating large pieces in which the heating and cooling kinetics are necessarily slow since disordered precipitation of Ni 3  Al could commence during rapid quenching after solution treatment, or overaging could occur during the first hardening stage. For this reason, hardening with titanium is preferred, the aluminum content being limited to 0.75%, preferably in the range 0.4% to 0.6%. Further, hardening in the first stage only becomes appreciable with the addition of titanium, as we demonstrate in the examples. 
     
                                           TABLE 1__________________________________________________________________________Characteristics of test castingsCastings A   B   C   D   E   F   G     H     I     J     K__________________________________________________________________________Ingot weight    35 kg        35 kg            35 kg                35 kg                    35 kg                        35 kg                            1 tonne                                  1 tonne                                        8 tonnes                                              8 tonnes                                                    8 tonnesProduction process    VIM VIM VIM VIM VIM VIM VIM + ESR                                  VIM + ESR                                        VIM + ESR                                              VIM + ESR                                                    VIM + ESRCarbon (wt %)     0.005         0.006             0.008                 0.005                     0.0037                         0.0037                             0.015                                   0.0088                                         0.0064                                               0.017                                                     0.015Silicon (wt %)    &lt;0.10        &lt;0.10            &lt;0.10                &lt;0.10                    &lt;0.10                        &lt;0.10                            &lt;0.10 &lt;0.10 &lt;0.10 &lt;0.10 &lt;0.10Manganese (wt %)    &lt;0.02        &lt;0.02            &lt;0.02                &lt;0.02                    &lt;0.02                        &lt;0.02                            0.023 &lt;0.02 &lt;0.02 &lt;0.02 &lt;0.02Sulfur (wt %)    0.0010        0.0010            0.0011                0.0014                    0.0010                        0.0009                            0.00017                                  0.00013                                        0.00016                                              0.00021                                                    0.00036Phosphorous (wt %)    &lt;0.003        &lt;0.003            &lt;0.003                &lt;0.003                    &lt;0.003                        0.003                            0.013 0.0054                                        &lt;0.003                                              &lt;0.003                                                    &lt;0.003Nickel (wt %)    53.1        52.4            52.6                52.5                    58.5                        55.5                            55    52.9  54.7  54.3  54.4Chromium (wt %)    21.54        21.35            21.43                21.22                    21.41                        21.10                            21.70 21.74 21.96 21.55 21.28Molybdenum    5.52        5.51            5.53                5.53                    5.64                        6.53                            5.77  6.63  5.92  6.62  6.59(wt %)Aluminum (wt %)    0.75        0.80            0.50                0.47                    0.52                        0.52                            0.45  0.43  0.43  0.49  0.47Cobalt (wt %)    &lt;0.02        &lt;0.02            &lt;0.02                &lt;0.02                    &lt;0.02                        &lt;0.02                            &lt;0.02 &lt;0.02 &lt;0.02 &lt;0.02 &lt;0.02Copper (wt %)    1.17        1.18            1.15                1.21                    1.32                        3.96                            1.30  1.33  1.36  1.43  1.40Titanium (wt %)    2.72        1.98            1.25                0.76                    1.47                        1.20                            1.58  1.40  1.56  1.41  1.36Iron (wt %)    14.50        15.50            14.50                14.50                    5.85                        8.06                            11.22 12.58 11.11 11.01 11.25Niobium (wt %)    0.65        1 .20            2.97                3.74                    2.81                        3.03                            2.78  2.65  2.88  3.08  2.79Tungsten (wt %)    &lt;0.10        &lt;0.10            &lt;0.10                &lt;0.10                    2.58                        &lt;0.10                            &lt;0.10 &lt;0.10 &lt;0.10 &lt;0.10 &lt;0.10Nitrogen (wt %)    --  --   --  --  --  --  0.0083                                   0.00057                                         0.00074                                               0.0033                                                     0.00088Oxygen (wt %)    --  --  --  --  --  --  0.0018                                  0.0024                                        0.00087                                              0.0017                                                    0.00050RelationshipsX (atomic %)     94.0         93.3             92.9                 92.6                     93.2                         92.2                             92.9  93.2  93.2  93.4  92.9Y (wt %) 6.69        6.69            6.68                6.74                    9.54                        10.49                            7.07  7.96  7.28  8.05  7.99A (wt %) 5.47        5.01            4.59                4.41                    4.81                        4.60                            4.77  4.41  4.77  4.84  4.55B (wt %) 11.95        5.43            1.38                0.82                    1.63                        1.34                            1.64  1.57  1.55  1.42  1.51__________________________________________________________________________ VIM: Vacuum induced melting ESR: Electro Slag Remelting 
    
     Castings A to F were not in accordance with the present invention and are used as comparative examples throughout the present application. 
     Castings G, H, I, J and K were in accordance with the present invention. So far as the inventors are aware, casting I constitutes the best mode of producing the alloy composition of the invention. 
     Castings A, B and C, the characteristics of which are shown in Table 1, were produced under vacuum. With the aim of illustrating the invention, they showed the fundamental hardening effect of titanium. To this end, HV 30  hardness measurements were made after different aging processes. The results obtained are shown in Table 2 below. 
     
                       TABLE 2______________________________________Aging response for castings A, B and C forged then solution treatedand rapidly quenched   Castings (solution treatment)     A            B        CAging     (1050° C./                  (1020° C./                           (1000° C./(temperature/     2 h/water)   2 h/water)                           2 h/water)time at   (Hardness    (Hardness                           (Hardnesstemperature)     HV.sub.30)   HV.sub.30)                           HV.sub.30)______________________________________650° C./5 h     240          245      230650° C./10 h     260          250      250650° C./20 h     275          255      247700° C./15 h     280          280      278700° C./10 h     305          285      280700° C./20 h     310          290      287750° C./5 h     315          320      317750° C./10 h     355          315      304750° C./20 h     350          320      305800° C./5 h     335          308      305800° C./10 h     330          300      290800° C./20 h     342          310      277______________________________________ 
    
     It can be seen that, despite the inversely proportional increase of niobium with decreasing titanium content, the high titanium castings hardened substantially during simple aging limited to industrially practical periods (&lt;1 day) through the whole temperature range which was in the range 650° C. to 800° C. 
     However, the increase in the titanium content gradually leads to poor performance as shown by the tests carried out on bars forged from castings A to E, the results of which are shown in Table 3 below. 
     
                                           TABLE 3__________________________________________________________________________Characteristics of square section bars forged from test castings A, B, C,D and EProducts/ Characteristic        Castings testedCondition measured        A     B      C    D    E__________________________________________________________________________17 × 17 mm Hardness        410   310    295  not forged                               not forgedas forged HV.sub.3045 × 45 mm        222   214    210  223  215as forged                      (cracks)                               (cracks)17 × 17 mm Hardness        310   260    200  --   --annealed Impact bending         25    55    &gt;294 --   --950° C./1 h KV (J) Precipitation        Ni.sub.3 Ti              Ni.sub.3 Ti                     Rare structure        abundant and              inter-granular                     Ni.sub.3 Ti        generalisedPrecipitate Solvus 1040-1060              1000-1020                     980-1000                          --   --dissolution temperaturerange (° C.) Hardness HV.sub.30        160   175    190  --   --17 × 17 mm Average grain         4     5      7   --   --fine grains index (ASTM(index ≧ 10) E112)__________________________________________________________________________ 
    
     It can be seen that the solvus temperature of the γ&#39; phase rises; this imposes a higher and higher solution treatment temperature, which is deleterious, firstly because the grain size of the material which has undergone thermo-mechanical transformation tends to increase greatly and more and more rapidly with solution treatment temperature, and secondly because carbides are dissolved, liberating residual carbon, which is deleterious during aging treatment which encourages intergranular reprecipitation of harmful carbides of chromium and molybdenum. 
     Metal hardening is more and more rapid on cooling from the upper range of solution treatment temperatures, as shown by the final hardness measured on as forged bars from castings A to E which include a variety of titanium contents: castings which are richest in titanium start to harden during the forging operations, as shown by their increased hardness which is more heterogeneous through the volume of the products. 
     In order to contain this rapid and expected hardening effect within acceptable limits during anisothermal thermomechanical transformations, which causes great deformation difficulties, or during rapid quenching of massive pieces, which risks causing quenching cracks in the pieces and disordered hardening, the claimed titanium content in the alloy is limited to 2%. 
     Niobium is the element which allows complementary hardening of the claimed alloy: with less than 2% of titanium (castings B and C), Table 2 shows that the desired hardness (&gt;320 HV 30 ) can not be regularly reached by precipitation of only the γ&#39; phase between 700° C. and 760° C. Addition of niobium such that 
     A=0.65%Nb+1.25% Ti+2.20% Al≧4.4 (castings B and C), combined with a second aging stage of practical industrial duration between 600° C. and 675° C., can further increase hardness, as can be deduced from Table 4 below which shows the results of mechanical tests carried out on castings A, B and C in optimized conditions. 
     
                                           TABLE 4__________________________________________________________________________Mechanical characteristics of castings A, B and Cin optimized conditions, in the form of square section bars                   Room temperature tension                                   BreakCasting/ Heat treatments              Hardness                   Rm   Rρo.sub..2                             E4d                                Z  impactProducts S.Q.  D.A.   HV.sub.30                   (N/mm.sup.2)                        (N/mm.sup.2)                             (%)                                (%)                                   KV (J)__________________________________________________________________________A     1040° C./2 h       750° C./4 h/              320  1197 724  29.5                                42 9617 × 17 mm       I.C. 25°/h toforged      620° C./8 h/Air 1050° C./2 h       760° C./4 h/              350  1225 733  27 34 55       I.C. 40°/h to       620° C./8 h/AirB     1000° C./2 h       750° C./4 h/              312  1153 669  29 41 8417 × 17 mm       I.C. 25°/h toforged      620° C./8 h/Air 1020° C./2 h       750° C./8 h/              315  1183 681  26.5                                40 78       I.C. 40°/h to       620° C./8 h/AirC      980° C./2 h       720° C./4 h/              334  1168 754  29.5                                48 11817 × 17 mm       I.C. 25°/h toforged      620° C./8 h/Air       750° C./4 h/              355  1208 822  25.5                                43 70       I.C. 25°/h to       620° C./8 h/Air 1000° C./2 h       730° C./4 h/              330  1153 729  31 48 135       I.C. 40°/h to       620° C./8 h/AirE     1000° C./2 h       730° C./4 h/              285  1048 640  41 57 13545 × 45 mm       I.C. 40°/h toforged      620° C./8 h/Air__________________________________________________________________________ S.Q. = Dissolution + rapid quenching D.A. = Double Aging with intermediate cooling (I.C) 
    
     In addition, double aging in two stages can be optimised using a single heat treatment which comprises a first aging stage between 700° C. and 760° C., and slow cooling to the temperature of the second stage which is then maintained for the second aging stage. This cycle, which produces the desired hardening, is particularly suitable for large pieces where the cooling kinetics are slow. So far as the inventors are aware, this process constitutes the best way of carrying out the invention. 
     However, increasing addition of the highly segregating element niobium results in ingots of the alloy of the invention (even if they are alloyed with a small amount of molybdenum, another highly segregating element), and in the formation of an increasing quantity of Laves phases at the end of the solidification step, combined with the element iron. In addition to a reduction in the burn temperature of ingots by liquation of these Laves phases which are rich in niobium and iron, their forgeability is drastically degraded at the top of the usual range of mechanical transformation operations, as shown by the hot forgeability tensile tests carried out on castings A, B, C and D in a molded or homogenised condition (tension at 1200° C., ε=6.5 s -1 ), the results of which are shown in Table 5 below. 
     
                       TABLE 5______________________________________Hot forgeability tensile tests on castings in molded andhomogenised condition (tension at 1200° C., ε = 6.5s.sup.-1)           CastingsHomogenisation      Characteristics                 A      B    C    D    F______________________________________1200° C.      Rm (N/mm.sup.2)                 210    200  190  185  &lt;10for        E (%)      61     57   39    7    016 h       Z (%)      51     61   44    5    01250° C.      Rm (N/mm.sup.2)                 --     --   196  --   216for        E (%)      --     --   55   --    716 h       Z (%)      --     --   53   --    10______________________________________ 
    
     It also becomes impossible to obtain satisfactory homogenisation of the segregated elements at low temperatures, below the burn temperature. 
     In casting example D for which the sum of the hardening elements A=0.65% Nb and 1.25% Ti+2.20% Al is limited to the minimum theoretical value of 4.4 necessary for the desired hardening, the 3.74% niobium content already causes unacceptable segregation in the small 35 kg ingot and greatly affects its forgeability at 1200° C.: the products are very sensitive to heat cracking while the forgeability range is limited to less than 1150° C. 
     In contrast, alloy G, for example, as claimed in our invention, remains forgeable up to 1235° C. as is shown by the results of hot forgeability tensile tests on forged semi-finished products shown in Table 6 below. 
     
                       TABLE 6______________________________________Hot forgeability tensile tests on forged semi-finished products fromindustrial castings G and J  Casting G         Casting J    Strain rate Strain rate Strain rateTest     ε = 0.15 s.sup.-1                ε = 6.5 s.sup.-1                            ε = 6.5 s.sup.-1temperature    Rm       E      Rm     E    Rm     E(° C.)    (N/mm.sup.2)             (%)    (N/mm.sup.2)                           (%)  (N/mm.sup.2)                                       (%)______________________________________ 900     396      54     438    43   --     -- 950     325      70     425    42   --     --1050     207      85     370    64   --     --1100     160      98     313    67   --     --1150     122      98     --     --   260    731170     --       --     240    63   --     --1180     --       --     --     --   228    581200      94      92     215    39   213    501225     --       --     187     4   170     21235      24      31     --     --   --     --1250     --       --      5      0   --     --______________________________________ 
    
     As a sequence, the niobium content in the alloys of the invention is limited to 3.50%, preferably to 3.25%. 
     Provide that the contents of hardening elements as indicated above are satisfied, the alloy of the invention is characterized by a low solvus temperature of the hardening phases, of the order of 1000° C., by a wide homogenisation range, up to 1250° C., and by a disordered solid solution which is metastable, but sufficiently stable on cooling from temperatures above the solvus of γ&#39;, to satisfy two essential aims on application to large pieces: 
     the absence of disordered hardening precipitation on cooling from the solid solution region means that all of the hardening potential on aging can be retained with sufficient homogeneity in large pieces; 
     after forging at a temperature above the solvus of the hardening phases, the metal does not harden during cooling, and thus remains very ductile, which allows a variety of cooling modes to be used without risking quench cracking. 
     In certain cases with pieces of limited dimensions, the alloy of the invention can be aged directly after thermomechanical transformation, so as to obtain high strengths, while preserving good ductility as shown by mechanical tests carried out on a variety of forged or rolled products from industrial castings G to K of the invention, the results of which are shown in Table 7 below. 
     
                                           TABLE 7__________________________________________________________________________Mechanical tests on forged or rolled products from castings G to K                                    BreakCasting/Products         Room temperature tension                                    impactHardness in      Heat treatments                    Rm   Rρ.sub.0.2                              E4d                                 Z  at 20° C.unfinished condition      S.Q.  D.A.    (N/mm.sup.2)                         (N/mm.sup.2)                              (%)                                 (%)                                    KV (J)__________________________________________________________________________Casting G  None  750° C/4 h/                    1254 1014 18.1                                 39 --1/2 product      I.C. 25°/h tosquare section   620° C/8 h/Air250 × 160 mmforged at 1160° C.(250 HV.sub.30)Casting G  980° C./2 h            750° C./4 h/                    1148 703  30 43 --1/2 product      I.C. 25°/h tosquare section   620° C./8 h/Air250 × 160 mmforged at 1160° C.(250 HV.sub.30)Casting G  None  750° C./4 h/                    1285 1035 18.7                                 36 --1/2 product      I.C. 25°/h tosquare section   620° C./8 h/Air250 × 160 mmforged at 1010° C.(280 HV.sub.30)Casting G  980° C./2 h            750° C/4 h/                    1139 721  34 46 --1/2 product      I.C. 25°/h tosquare section   620° C./8 h/Air250 × 160 mm forgedat 1010° C. (280 HV.sub.30)Casting G  None  715° C./4 h/&#34;DN 32&#34; valve    I.C. 25°/h to                    1152 811  31 42 106drop forged at   620° C/10 h/Air1100° C. (200 HV.sub.30)Casting G  975° C./6 h            745° C./4 h/                    1149 768  31 .5                                 47 65&#34;DN 32&#34; valve    I.C. 25°/h todrop forged at   620° C./10 h/Air1100° C. (200 HV.sub.30)Casting G  None  715° C./4 h/                    1131 776  33.5                                 49 89&#34;DN 32&#34; valve    I.C. 25°/h todrop forged at   620° C./10 h/Air                    1218 899  29 47 1031030° C. (220 HV.sub.30)Casting G  975° C./6 h            745°C./4 h/                    1150 768  26.5                                 47 65&#34;DN 32&#34; valve    I.C 25°/h todrop forged at   620° C./10 h/Air1030° C. (220 HV.sub.30)                    1119 695  34.5                                 46 67Casting G  None  720° C./4 h/                    1248 846  31 44 47.O slashed. 55 mm bar            I.C. 25°/h torolled at 1160° C. (235            620° C./6 h/AirHV.sub.30)Casting G  975° C./2 h            740° C./4 h/                    1200 805  28 41 44.O slashed. 55 mm bar            I.C. 25°/h torolled at 1160° C. (235            620° C./10 h/AirHV.sub.30)Casting H  980° C./2 h            750° C./4 h/                    1122 714  32 28 291/2 product      I.C. 25°/h tosquare section   620° C./8 h/Air250 × 160 mm forgedat 1160° C. (265 HV.sub.30)Casting H  975° C./6 h            745° C./4 h/                     912 701  14 19 22&#34;DN 32&#34; valve    i.C. 25°/h todrop forged at   620° C./12 h/Air1100° C.Casting H  1015° C./6 h            745° C./4h/                    1074 721  36 41 111&#34;DN 32&#34; valve    I.C. 25°/h todrop forged at   620° C./12 h/Air1030° C.          1079 649  36 42 97Casting J  1000° C./4 h            750° C./6 h/                    1118 802  20.5                                 21 592&#34; valve         I.C. 40°/h todrop forged at   620° C.12 h/Air1160° C.Casting J  990° C./5 h            750° C./6h/                    1146 797  31 40 77forged bar       I.C. 40°/h to.O slashed. 245 mm            620° C./12 h/AirCasting J  990° C./5 h            750° C./8 h/                    1162 782  32.5                                 42 71forged bar       I.C. 40°/h to.O slashed. 145 mm            620° C./12 h/AirCasting J  990° C./1 h            750° C./4 h/                    1215 776  33.5                                 47 54bar .O slashed. 20 mm            I.C. 40°/h torolled           620° C./2 h/AirCasting K  1010° C./9 h            740° C./6 h/                    1032 700  33 38 96well head        I.C. 40°/h todrop forged      620° C./12 h/AirCasting K  990° C./1 h            750° C/4 h/                    1202 744  34.5                                 49 64bar .O slashed. 20 mm            I.C. 40°/h torolled           620° C./12 h/AirCasting I  1000° C/9 h            750° C./6 h/                    1016 750  20 24 75well head        IC. 40° /h todrop forged at   620° C./12 h/Air1160° C.          1036 763  20.5                                 27 78Casting I  1000° C./4 h            750° C./6 h/2&#34; valve drop forged            I.C. 40°/h to                    1074 769  31.5                                 33 106at 1160° C.            620° C./12 h/AirCasting I  990° C./5 h            750° C./6 h/                    1118 742  32.5                                 34 89forged bar       I.C. 40°/h to.O slashed. 245 mm            620° C./12 h/AirCasting I  990° C./5 h            750° C./6 h/                    1135 726  34 41 88forged bar       I.C. 40°/h to.O slashed. 145 mm            620° C./12 h/AirCasting I  990° C./1 h            750° C/4 h/                    1182 735  34.5                                 47 78bar .O slashed. 20 mm            I.C. 40°/h torolled at 1160° C.            620° C./12 h/Air__________________________________________________________________________ S.Q. = solution treated + rapid quenching D.A = Double aging with intermediate cooling (I. C) 
    
     Having set the conditions which must be satisfied by the hardening elements titanium, aluminum, and niobium in the claimed alloy, we define the relationship which takes all of the elements of the alloy into account and which represents the character of each concerning its tendency to form harmful intermetallic phases. The relationship is as follows: 
     X=2.271% Ti+1.142% Cr+0.957% Mn+0.858% Fe+0.777% Co+0.717% Ni+2.117% Nb+1.550% Mo+1.655% W+1.90% Al+1.90% Si+0.615% Cu, in atomic %. 
     This formula has been defined in order to predict the tendency of alloys to precipitate the sigma phase as a function of the value of X. 
     In the context of the present invention in order to guarantee the absence of the sigma phase, the value X in the above relationship as applied to the residual composition of the austenitic matrix of the claimed alloys, after aging which consumes the elements Ni, Ti, Al and Nb by precipitating the γ&#39; and γ&#34; phases, must remain below 91.5. 
     Calculation based on empirical observations allows us to evaluate the composition of the matrix of an aged super-alloy. All of castings A to K shown in Table 1 were balanced such that their aged matrix, enriched in chromium, molybdenum, iron, and copper, satisfied the relationship X&lt;91.5. 
     An essential characteristic of the present invention is thus that, if the relationship above, applied to the overall composition of the alloy before aging, gives a value of X≦93.5 (taking into account inevitable minor segregation in large ingots), this alloy will not give rise to sigma phase precipitation during industrial aging cycles between 600° C. and 900° C. The industrial castings G to K of Table 1 satisfy this limit. 
     It can be seen that ingots of alloys of the invention, which all contain a certain fraction of Laves phase and/or sigma phase at the end of solidification, can be totally re-homogenised at a very high temperature to a single austenitic phase when their overall composition satisfies X≦93.5. 
     Copper appears to be a determining element in this respect since, of all of the elements in the above relationship, it is the element with the smallest coefficient, which means it has a very low tendency to form the sigma phase: in this, it counter-balances the strong ability of other elements of the alloy to stabilise this phase. 
     The concentrations of each of the alloying elements can thus be selected as a function of their own effects: 
     The chromium content is in the range 20.5% to 22.5%, giving the alloy good general corrosion resistance in a variety of environments. 
     Molybdenum is the element which results in the best corrosion resistance in acidic reducing environments containing chlorides and the gases H 2  S and CO 2 . However, its content is limited to between 5.5% and 7% in the alloy claimed, firstly because molybdenum strongly encourages sigma phase formation; and age hardening, which precipitates the γ&#39; Ni 3  (Ti,Al) and γ&#34; Ni 3  Nb phases, depletes the aged matrix in nickel, titanium, aluminum, and niobium and, as a consequence, enriches it in molybdenum in particular. Secondly, these contents are sufficient for the material to be resistant in oil industry environments, as the results of slow strain rate or constant load NACE tensile tests show in Tables 8, 9 and 10 below. 
     
                                           TABLE 8__________________________________________________________________________SSC (Sulfide Stress Corrosion) test in accordance with NACErecommendation TM 0177-90-Method AConditions: Constant load tensile test, suspended in standardde-oxygenated medium,at temperature of 23 ± 3° C., for a strain of 100% of the yieldstrength R.sub.ρ0.2                                       Corrosion by coupling                            Free corrosion                                       with a steel      Mechanical characteristics                            Solution                                  Result                                       Solution                                             ResultCasting    Rm   Rρ.sub.0.2                E.sub.4d                    Z  Hardness                            (pH)  (3 tests                                       (pH)  (3 testsproductCondition      (N/mm.sup.2)           (N/mm.sup.2)                (%) (%)                       (RCH)                            Start                               End                                  of 720 h)                                       Start                                          End                                             of 720__________________________________________________________________________                                             h)I    990° C./1 h      1182 735  34.5                    47 35.1 2.8                               2.9                                  3 non-                                       2.8                                          3.7                                             3 non-bar  +                                 ruptures   ruptures.O slashed.750° C./4 h/               no SSC     no SSC20 mmI.C. 40° C./hJ    to 620° C./      1215 776  33.5                    47 35.8 2.8                               2.9                                  3 non-                                       2.8                                          3.8                                             3 non-bar  12 h/Air                          ruptures   ruptures.O slashed.                            no SSC     no SSC20 mmK          1202 744  34.5                    49 35   2.8                               2.9                                  3 non-                                       2.8                                          3.8                                             3 non-bar                                    ruptures   ruptures.O slashed.                            no SSC     no SSC20 mm__________________________________________________________________________ 
    
     
                                           TABLE 9__________________________________________________________________________SSC (Sulfide Stress Corrosion) test in accordance with NACErecommendation TM 0177-90-Method CConditions: Constant load test with C-ring test pieces in standardde-oxygenated medium,at temperature of 23° C. corrosion for 720 h            Mechanical characteristics            Rm   R.sub.ρ0.2                      E.sub.4d                          Z  Hardness                                  Applied stress                                         ResultCasting/product  Condition (N/mm.sup.2)                 (N/mm.sup.2)                      (%) (%)                             RCH  (N/mm.sup.2)                                         (two 720 h__________________________________________________________________________                                         tests)Casting G  975° C./2 h +            1200 805  28  41 35.6 710    2 non rupturesBar .O slashed. 55 mm  740° C./4 h/I.C. 25°/h                                  (R.sub.ρ0.02)  to 620° C./10 h/AirRolled 720° C./4 h/            1248 846  31  44 38   820    2 non ruptures  I.C. 25°/h to            (R.sub.ρ0.02)  620° C./6 h/AirCasting J  1000° C./4 h +            1118 802  21  21 --   802    2 non ruptures2&#34; valve  750° C./6 h/I.C. 40°/h                                  (R.sub.ρ0.2)Casting I  to 620° C./12 h/Air            1074 769  31.5                          33 --   769    2 non ruptures2&#34; valve                               (R.sub.ρ0.2)__________________________________________________________________________ 
    
     
                                           TABLE 10__________________________________________________________________________Slow strain rate tensile testSSRT (Slow Strain Rate Tensile) in accordance with NACE recommendation(SSRT for SCC in Sour Oilfield Service)Test in Level VI medium of Nace MR 0175 (Table I) at 175° C.and a strain rate of 4 × 10.sup.-6 s.sup.-1 :Partial pressure of H.sub.2 S: 3.5 MPa (508 psia)Partial pressure of CO.sub.2 : 3.5 MPa (508 psia)Total pressure: 8.2 MPa (1185 psia)NaCl concentration: 20% by weight          SSR in air at 175° C.                    SSR in medium          Time to   Time to   Sensitivity indices          break              E  Z  break                        E  Z  TTF E  ZCasting/product  Condition          TTF (h)              (%)                 (%)                    TTF (h)                        (%)                           (%)                              TTFalr                                  E air                                     Z air                                        Remarks__________________________________________________________________________I      990° C./1 h          26.3              36 43 26.2                        36 35 1.0 1.0                                     0.80                                        no SCC.O slashed. 20 mm bar  +35.1 RCH  750° C./4 h                    27.0                        36 35 1.03                                  1.0                                     0.80                                        &#34;J      I.C. 40°/h/to          24.7              34 41 26.3                        36 37 1.0 1.0                                     0.84                                        &#34;.O slashed. 20 mm bar  820° C./12 h/Air35.8 RCH                 24.4                        33 33 0.92                                  0.92                                     0.75                                        &#34;K              27.5              38 49 25.2                        34 33 0.94                                  0.94                                     0.75                                        &#34;.O slashed. 20 mm bar35.0 RCH                 25.6                        34 33 0.94                                  0.94                                     0.75                                        &#34;__________________________________________________________________________ 
    
     With 7% of molybdenum in the alloy, the aged matrix can contain up to 8.0% of that element, which is sufficient. 
     Pitting corrosion resistance is gradually improved with the % of molybdenum as shown by tests for determining the pitting temperatures carried out on forged or rolled products from castings A, B, C, E, G, H, I and K using two conventional tests, the ASTM G48 test and the &#34;Green Death&#34; test. 
     ASTM G48 Pitting Corrosion Test 
     The pitting temperature was determined in an aqueous medium containing 10% by weight of crystallised iron perchloride (FeCl 3  -6H 2  O), using parallelipipedal test pieces polished with SiC paper under water (120 grade), using the following methodology: 
     a succession of several immersion periods of 72 h at increasing temperatures; 
     the test temperature was increased by 5° C. after an immersion period of 72 h; the test pieces were re-polished for each new exposure. 
     &#34;Green Death&#34; Pitting Corrosion Test 
     The pitting temperature was determined in an aqueous medium constituted by: 
     11.5% H 2  SO 4  -1.2% HCl - 1% FeCl 3  - 1% CuCl 2  (% by weight) (or 7% by volume H 2  SO 4 , 3% by volume HCl, 1% by weight FeCl 3 , 1% by weight CuCl 2 ) using parallelipipedal test pieces polished with SiC paper under water (120 grade) using the following methodology: 
     a succession of several immersion periods of 96 h at increasing temperatures; 
     the temperature was increased by 5° C. after an immersion period of 96 h; the test pieces were re-polished for each new exposure. 
     The results of these two tests are shown in Table 11 below: 
     
                                           TABLE 11__________________________________________________________________________Pitting corrosion resistance of forged or rolled products in tworepresentative media  Condition      ASTM                     &lt;&lt;Green                          Y = % Mo + % WCastings/  Solution       G48A                     Death &gt;&gt;                          +% Cu/Products  treated        Aging    test                     test % Mo__________________________________________________________________________A      1050° C./2 h        760° C./4 h                 --  &lt;50° C.                          6.69/square section        i.C. 40°C./h                          5.5245 × 45 mm        to 620° C./8 h/airB      1020° C./2 h        750° C./8 h                 --  &lt;50° C.                          6.69/square section        i.C. 40° C./h                          5.5145 × 45 mm        to 620° C/8 h/airC      1000° C./2 h        730° C./4 h                 --  ≈50° C.                          6.68/square section        I.C. 40° C./h                          5.5345 × 45 mm        to 620° C./8 h/airG      none  720° C./4 h                 65-75                     60-70.O slashed. 55 mm        I.C. 25° C./h                          7.07/        to 620° C./6 h/air                          5.77        740° C./4 h  975° C./2 h        I.C. 25° C./h                 65-75                     60-70        to 620° C./10 h/airE      1000° C./2 h        730° C./4 h                 --  65   10.49/square section        I.C. 40° C./h                          5.6445 × 45 mm        to 620° C/8 h/airH      975° C./6 h        745° C/4 h                 85  75        I.C. 25° C./h                          7.96/        to 620° C./12 h/air                          6.63DN 32 valves  1015° C./6 h        745° C./4 h        I.C. 25° C./h        to 620° C./12 h/airI      1000° C./9 h        750° C./6 h                 80  75   7.28/drop forged  I.C. 40° C./h                          5.92well head    to 620° C./12 h/airK      1010° C./9 h        740° C./6 h                 90  85   7.99/drop forged  I.C. 40° C./h                          6.59well head    to 620° C./12 h/air__________________________________________________________________________ 
    
     Addition of copper is favorable for the reasons described above: in contrast, tertiary Ni--Cr--Cu and Ni--Mo--Cu phase diagrams include regions of immiscibility for low copper contents, which are extensions of regions of immiscibility in binary Cr--Cu and Mo--Cu systems, which are also observed in the Fe--Cu system. 
     In addition, pseudo-binary eutectics of low melting point exist in the Ni--Ti--Cu system: as a result, copper addition must be strictly controlled in Ni--Cr--Fe--Mo--Ti alloys, in particular those which are rich in molybdenum, if forgeability is to be retained. As an example, it can be seen from Table 5 above that the alloy of casting F which contains 6.5% of molybdenum and 4% of copper (not in accordance with the invention) has very greatly degraded behavior during thermo-mechanical rough forging of ingots, probably due to partial liquification of the boundaries of segregated grains; deep homogenisation of the ingot improves this behavior, but not sufficiently. The alloy of the invention thus claims a maximum copper content of 3.5% in its broadest composition, preferably limited to 2.5%, in the case of severe thermo-mechanical transformation (rolling), and such that 
     % Mo+% Cu+% W≦9% by weight. 
     Iron is not an element which is essential to the properties of the alloy, but this element is considered to improve the tolerance of nickel based alloys to hydrogen and is inexpensive. In contrast, in combination with niobium and silicon, iron increases the formation of unwanted Laves phases. For these reasons, its content in the alloy is limited to between 7% and 13%. 
     The addition of cobalt is not necessary to the alloy of the invention but a residual content of up to 3% can be tolerated so as to allow recycling of materials containing cobalt. 
     Tungsten is an element which, to a lesser extent than molybdenum, encourages corrosion resistance in reducing environments. However, this element induces a solid solution hardening effect which is marked at high temperatures: this effect can be seen for casting E which contains 2.6% of W substituting for iron, and for which heat hardening has rendered forgeability very difficult (see Table 3). As a result, tungsten content is limited to 0.5% in the alloy of the invention, with the aim of accepting raw materials which may contain it; preferably, however, its addition is not desirable. 
     The residual element contents are strictly controlled in the following cases: 
     Silicon, which is always found in trace amounts in nickel based alloys, is a particularly harmful element in several respects: it has a strong tendency to form intermetallic phases, among them the sigma phase, Laves phases, and titanium silicides in the alloys of the invention, whether they are ingots at the end of solidification with its strong tendency to segregate or whether during age hardening where it can also give rise to the formation of Ni 3  Si type phases which are unwanted within the context of the present invention. The silicon content in the alloy which is claimed is thus limited to 0.2%. 
     Magnesium is a deoxidising element which can be included in small quantities on production; in some cases this element has a beneficial effect on forgeability of nickel based alloys by a particular action which cannot be clearly explained. In the alloy which is claimed, magnesium addition is not necessary and the residual content of this element is at most 0.005%, which means that its deoxidation properties can be used if necessary. 
     Phosphorous and sulfur in particular are impurities which are considered to be extremely harmful to the alloy as they strongly affect the behavior of the metal on forging and encourage hydrogen embrittlement. These two elements are thus limited to very low contents, i.e., S≦0.0050% and P≦0.0300% and, preferably, S≦0.0020% and P≦0.0100% when better thermomechanical transformation behavior is desired. 
     Given the low solubility of nitrogen in austenitic nickel based alloys and the high reactivity of this element with titanium, the nitrogen content is limited to a maximum of 0.020%, well within its theoretical solubility in the liquid metal of the claimed alloy, which is about 0.15% to 0.20%: this necessitates vacuum production means. 
     The alloy of the invention can be produced and transformed using known techniques which are in current use for the commercial production of nickel based alloys. Production under vacuum is necessary in order to prepare an electrode with low gas contents and a very low carbon content, which is then remelted so as to obtain an alloy of the required quality. The alloy is readily heat transformable using conventional thermo-mechanical transformation techniques following very high temperature homogenisation treatment at temperatures which do not exceed 1250° C. 
     The thermo-mechanical transformations are carried out after solution treatment/reheating in a temperature range which is in the range 975° C. to 1175° C. 
     However, low pre-heating temperatures, which then require high energy during transformations, are reserved for specific applications where grain size growth is to be avoided, such as final drop forging of pieces of average to small dimensions. 
     In all cases, the temperature at the end of the thermo-mechanical transformation step must be over 950° C. if the better malleability of the alloy is to be available. 
     The low solvus temperatures of the hardening phases of this alloy, 950° C. to about 1025° C., allow low dissolution temperatures to be used, which is advantageous since they limit the effect of abrupt grain size growth as encountered at higher temperatures. 
     However, these low dissolution temperatures compete to limit the rate of carbide dissolution, which attenuates the rate of intergranular reprecipitation of carbide which are harmful for subsequent aging. 
     Ingots of industrial castings G to K were transformed into a variety of products using the above precepts. The tables above show the mechanical characteristics obtained with the alloys of the invention. Tables 12 and 13 below more particularly concern tests on products from casting G. 
     
                                           TABLE 12__________________________________________________________________________Characteristics of a .O slashed. 30 mm rolled bar as a function ofdifferentcold drawing rates - Casting G solution treated for 1 hour at 980°C.Mechanical characteristics in drawn                 Mechanical characteristics in drawncondition        and aged condition (D.A)Draw rateTensile test at 20° C.                 Tensile test at 20° C.L    R.sub.m     Rρ.sub.0.2          E.sub.5d              Z  R.sub.m                      Rρ.sub.0.2                           E.sub.5d                              ZLo   (N/mm.sup.2)     (N/mm.sup.2)          (%) (%)                 (N/mm.sup.2)                      (N/mm.sup.2)                           (%)                              (%)__________________________________________________________________________1.0   866 411  46  69 1230 824  27.5                              551.15 1013 713  34  65 1361 1162 21.5                              481.23 1101 886  27  62 1451 1302 17.5                              481.31 1183 1006   16.5              60 1531 1426 12.5                              401.41 1281 1124 14  55 1601 1520  8.5                              40__________________________________________________________________________ D.A = Double aging; 720° C./4 h/I.C. 50°/h → 620° C./8 h/Air 
    
     Table 13 Mechanical Characteristics of Hot Rolled Products 
     Casting G, rolled .O slashed.55 mm products at 1160° C., solution treated-rapidly quenched at 975° C./2 h and aged as follows: 740° C./4h/l.C. 25° C./h to 620° C./10 h/air 
     
         ______________________________________      Tension characteristicsTest temperature        R.sub.m  Rρ.sub.0.2                            E.sub.4d                                  Z(° C.)        (N/mm.sup.2)                 (N/mm.sup.2)                            (%)   (%)______________________________________ 20          1200     805        28    41250          1168     789          31.5                                  49550          1116     795        29    42600          1130     776        30    46______________________________________ 
    
     Despite a gradual reduction in ductility in the core of the pieces when their dimensions increased, the values were acceptable in the case of well heads of up to 2.5 tonnes, drop forged in one pressing from a parallelipipedal semi-finished product. The strength characteristics were retained up to 550° C., well beyond the service range in oil industry environments. 
     Clearly, the different implementations of the process of the invention which have been described have been given by way of purely illustrative and non limiting example only, and that a number of modifications can readily be made by the skilled person without departing from the ambit of the invention.