EP0045984B1 - Process for manufacturing an article from a heat-resisting alloy - Google Patents

Abstract

1. Process for manufacturing a workpiece from a creep-resistant, precipitation-hardenable austenitic alloy having a high nickel content and containing a metal oxide as the hardening idspersoid, this process being in accordance with the methods employed in powder metallurgy and in which a metal powder is mixed with a metal oxide powder, is mechanically alloyed, is encased in a metal container, and is densified, by hot-compaction, to 100% of the theoretical density, in a manner such that an easily deformable intermediate material is produced, which has a very fine grain size and is suitable for further processing, terminating in an annealing treatment for developing a coarse grain size, characterised in that the abovementioned intermediate material is converted to the finished workpiece by means of a purpose-designed working operation which embodies the final shaping step, the deformation rate and the degree of deformation being determined in accordance with the intermediate material, which can exhibit deformation which is insufficient, optimum, or excessive, in a manner such that the logarithm of the temperature-compensated deformation rate, expressed as log (.~epsilon DNi--1 m**2 ), where ._epsilon = d[¦In (Ao /Af )¦]/dt, and Ao and Af respectively denote the cross-sectional areas of the workpiece before and after the working operation, while DNi denotes the diffusion coefficient of nickel, which is a function of temperature, lies between the values 16.5 and 20 in case a) - insufficient deformation of the intermediate material, lies between the values 10 and 22 in case b) - optimum deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0 =< epsilon < 0.3, where epsilon denotes ¦In (Ao /Af )¦ and lies, in the range of comparatively high degrees of deformation for the final-shaping step, described by epsilon > 1.0, between the values 15.5 and 20, lies between the values 14 and 18 in case c) - excessive deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0.1 < ¦epsilon¦ < 0.2, and lies, in the range of comparatively high degrees of deformation for the final-shaping step, described by ¦epsilon¦ > 0.8, between 16 and 20, and in that the degree of deformation for the final-shaping step, described by epsilon, reaches a value of not less than 0.5 in case a) - insufficient deformation of the intermediate material, is as small as desired, and can thus also be nil, in case b) - optimum deformation of the intermediate material, increases linearly with the deformation rate, but reaches a value of not less than 0.1, in case c) - excessive deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0.1 < ¦epsilon¦ < 0.6, the above being subject to the restriction that in case a), the values must lie within the shaded region in Figure 5, in case b), the values must lie within the shaded region in Figure 6, and in case c), the values must lie within the shaded region in Figure 7.

Description

The invention relates to a method for producing a workpiece according to the preamble of claim 1.

Oxide dispersion-hardened alloys, in particular those of the nickel-based type, are generally produced by powder metallurgical methods, the technology of mechanical alloying of the powder particles being used to a large extent. In order to achieve the highest possible creep resistance at high temperatures, such alloys must have a coarse-grained structure in the ready-to-use workpiece. The methods of mechanical alloying and the question of the associated further processing of the oxide dispersion-hardened materials are known (e.g. BJP Morse and JS Benjamin, "Mechanical Alloying", New Trends in Materials Processing, pp. 165-199, in particular pp. 177-185, American Society for Metals, seminar October 19/20, 1974). In order to obtain a finished workpiece, the primary material obtained in a first compaction step (powder compaction) must be subjected to further shaping operations. Since both the material and the machining costs of such alloys are very high, this shaping can only be carried out economically by forming. At the end of all processes there is always a heat treatment which serves to convert the finished workpiece into the coarse-grained structure which is best suited for high-temperature operation.

However, the success of such a coarse-grain annealing depends on the entire history of the material. In the first hot compression step of the powder, which is cold-formed by the mechanical alloying, a 100% dense, ultra-fine-grained starting material is obtained, which can be easily deformed in the medium to high temperature range, i. H. has quasi-superplastic properties, so to speak. The primary material can therefore be converted relatively easily into the final shape of the finished workpiece by thermomechanical forming. The only question is whether the required coarse grain can be easily adjusted by additional annealing on the finished end product. Conventional practice now shows that this is by no means guaranteed in all cases. On the contrary, very tight conditions, which are annoying for production, generally have to be observed. The setting options for the coarse grain are known to depend on the available driving forces, the number of bacteria and other physical parameters. It is not indifferent in which way the primary material was created. The latter can be done, for example, by extrusion at high or low temperature or by hot isostatic pressing of the mechanically alloyed, encapsulated powder. The mechanical alloying usually causes a state of the highest possible deformation, that is to say strain hardening driven to the saturation limit, which is more or less broken down in the subsequent thermomechanical deformation steps. Practice shows that there is an optimal deformation state of the primary material for the subsequent formation of coarse grains ("normal"). If, on the other hand, the primary material is insufficiently deformed ("underworked"), i.e. if it has too little work hardening and therefore too little energy for the subsequent recrystallization, the latter is incomplete (mixture of non-recrystallized fine grain with little coarse grain) or is completely absent. However, if the primary material is excessively deformed ("overworked"), ie if it has an excess of energy for later recrystallization, this takes place completely, but, due to the high number of crystallization nuclei, only leads to a relatively fine-grained structure. The latter cannot be converted into coarse grain by any additional heat treatment.

There is therefore a need to break through the constrictions observed in practice in the manufacturing process and to look for methods which enable the production of usable workpieces in a wide range.

The invention is based on the object of specifying a production method for oxide-hardened, heat-resistant workpieces which, regardless of the selected compression step and the resulting state of deformation of the structure of the starting material produced in this way, guarantees a coarse-grained end product that is usable for operation.

This object is achieved by the features of claim 1.

• Fig. 1 das Fließbild (Blockdarstellung) der grundsätzlichen Verfahrensschritte,
• Fig. 2 das Fließbild der Verfahrensschritte für ungenügend verformtes Vormaterial,
• Fig. 3 das Fließbild der Verfahrensschritte für optimal verformtes Vormaterial,
• Fig. 4 das Fließbild der Verfahrensschritte für übermäßig verformtes Vormaterial,
• Fig. 5 ein Diagramm der Verformungsbedingungen zur Erzielung von Grobkörnigkeit für das fertige Werkstück, ausgehend von ungenügend verformtem Vormaterial,
• Fig. 6 ein Diagramm der Verformungsbedingungen zur Erzielung von Grobkörnigkeit für das fertige Werkstück, ausgehend von optimal verformtem Vormaterial,
• Fig. 7 ein Diagramm der Verformungsbedingungen zur Erzielung von Grobkörnigkeit für das fertige Werkstück, ausgehend von übermäßig verformtem Vormaterial.

The invention is described on the basis of the following exemplary embodiments explained by figures. It shows

• 1 shows the flow diagram (block diagram) of the basic method steps,
• 2 shows the flow diagram of the process steps for insufficiently deformed starting material,
• 3 shows the flow diagram of the process steps for optimally deformed starting material,
• 4 shows the flow diagram of the process steps for excessively deformed starting material,
• 5 shows a diagram of the deformation conditions for achieving coarse grain for the finished workpiece, starting from insufficiently deformed starting material,
• 6 shows a diagram of the deformation conditions for achieving coarse grain for the finished workpiece, starting from optimally deformed primary material,
• 7 shows a diagram of the deformation condition to achieve coarse grain for the finished workpiece, starting from excessively deformed raw material.

In Fig. 1, the flow diagram of the basic method is shown in block form. It is generally assumed that metallic powders, which may be in the form of elements and / or master alloys, and metal oxide powders as dispersoids. The powders are very fine-grained, the particle size fluctuates between a few µm and about 60 µm, and the metal oxide powders are usually even finer (below 1 µm). The mixing and mechanical alloying of the powders is generally carried out in a protective gas atmosphere in the attritor. The powder particles are alloyed to homogeneity and mixed with the dispersoid. At the same time, the cold working is driven to the saturation limit, which is reflected in the high hardness, which can reach up to 700 Vickers units. This high degree of cold deformation, which cannot be achieved by any other means, is the prerequisite for the existence of sufficient driving force for the coarseness of the structure that is ultimately desired on the finished workpiece. The mechanically alloyed powder is filled into a ductile metal container, usually soft steel, under vacuum and encapsulated (sealed, sealed can or capsule on all sides). In a subsequent process step, the encapsulated powder is thermally compressed to 100% of the theoretical density. The product is an easily deformable, ultra-fine-grained raw material, which forms the starting material for the further shaping of the workpiece. Depending on the way the performed Warmverformungsschri tes _t results in a primary material, which is deformed insufficiently optimal or excessive in relation to the subsequent recrystallization ( "underworked,""normal,""overworked"). The further processing to the finished workpiece (targeted forming = final shaping) takes place under conditions that take into account the state of deformation of the primary material. The decisive parameters here are temperature, deformation speed and the deformation to be achieved or still necessary in the last forging step, which can be expressed, for example, as a change in cross-section. In any case, a finished workpiece is created, which can be converted into the operational end product by coarse grain annealing.

wobei

die Ableitung des Absolutwertes des natürlichen Logarithmus des Querschnittsverhältnisses (A_o=Querschnittsfläche des Werkstückes vor, A_f=Querschnittsfläche nach der Umformung) des Werkstückes nach der Zeit, sowie D_Ni der temperaturabhängige Diffusionskoeffizient von Nickel bedeutet. Der andere Parameter, der Verformungsgrad, wird zweckmäßigerweise durch

den Absolutwert des natürlichen Logarithmus des Querschnittsverhältnisses des Werkstückes, ausgedrückt. Selbstverständlich kann man auch von der Längenänderung ausgehen und diese dann auf das Querschnittsverhältnis umrechnen.For each state of deformation of the primary material, pairs of values can be specified for the two parameters necessary for subsequent forming into the finished workpiece, which meet the requirements for subsequent coarse grain formation. It is useful to choose the logarithm of the temperature-compensated rate of deformation as the one parameter:

in which

the derivation of the absolute value of the natural logarithm of the cross-sectional ratio (A _o = cross-sectional area of the workpiece before, A _f = cross-sectional area after the forming) of the workpiece after time, and D _{Ni means} the temperature-dependent diffusion coefficient of nickel. The other parameter, the degree of deformation, is expediently determined by

the absolute value of the natural logarithm of the cross-sectional ratio of the workpiece. Of course, you can also start from the change in length and then convert it to the cross-sectional ratio.

2 shows the flow diagram of the process steps for insufficiently deformed starting material. A powder mix was mechanically alloyed and encapsulated in a soft steel can. The Endlegiqrunig had the following composition:

The subsequent hot compression step consisted of extrusion at a temperature of 1075 ° C. Corresponding to the cylinder diameter of the extrusion press of 229 mm and the extrusion diameter of 51 mm, a cross-sectional reduction ratio of 20.25: 1 resulted, which corresponds to a ε = 3. The fine-grained primary material produced in this way had an average particle size of 0.3 µm.

In accordance with the reduction of the cold deformation originally introduced, it was considered to be insufficiently deformed ("underworked"). In general, these materials have an average sub-grain size of 0.25 µm to 0.35 µm. A piece of the bar stock obtained was cut off and in a press from 8 MN press force subjected to forming into a finished workpiece. The degree of deformation s was 1, the logarithmic value corresponding to the rate of deformation

The workpiece was subjected to coarse grain annealing at a temperature of 1220 ° C for 1 h. An average grain size of over 100 µm was found. In general, given these conditions, coarse grain can be understood to mean that grain size which means a coarsening by at least a factor of 100 compared to the fine-grained starting material.

entsprach.3 shows the flow diagram of the process steps for optimally deformed starting material. The starting position corresponded to the exemplary embodiment explained in FIG. 2. The same alloy was used and the same first process steps were used. The extrusion was carried out under similar conditions, but at a temperature of 960 ° C. The reduction ratio likewise gave an e of 3. The fine-grained starting material had a sub-grain size of 0.2 μm. In accordance with the breakdown of work hardening, this material was in the optimal state of deformation ("normal"). The average sub-grain size of these materials generally ranges from 0.15 µm to 0.25 µm. A piece of the primary material was formed on a press by the degree of deformation s = 1.1 with a deformation rate that corresponds to the value

corresponded.

After coarse grain annealing at 1220 ° C / 1 h, an average grain size of 350 µm was determined. Here, too, the grain size had increased by more than 2 orders of magnitude.

4 shows the flow diagram of the process steps for excessively deformed starting material. A powder mixture was mechanically alloyed and encapsulated in a soft steel can. The final alloy had the following composition:

Der Warmverformungsschritt zur Verdichtung des eingekapselten Pulvers auf 100% der theoretischen Dichte bestand in einem heiß-isostatischen Pressen bei einer Temperatur von 950°C während 4 h unter einem Druck von 135 MPa. Die Höhe des ursprünglichen zylindrischen Körpers von 200 mm wurde dabei auf 150 mm reduziert. Das entsprechende ε ergab sich zu 0,3. Das auf diese Weise erzeugte feinkörnige Vormaterial hatte eine Subkorngröße von durchschnittlich 0,14 µm. Entsprechend dem geringeren Abbau der Kaltverfestigung des Pulvers galt dieses Material als übermäßig verformt (»overworked«). Die Subkorngröße derartiger Materialien ist in der Regel ≤0,15 µm. Das Vormaterial wurde auf einer Presse um den Verformungsgrad ε=0,3 mit einer Verformungsgeschwindigkeit umgeformt, welche dem logarithmischen Wert von

entsprach.

The thermoforming step to compress the encapsulated powder to 100% of the theoretical density consisted of hot isostatic pressing at a temperature of 950 ° C. for 4 hours under a pressure of 135 MPa. The height of the original cylindrical body of 200 mm was reduced to 150 mm. The corresponding ε was 0.3. The fine-grained primary material produced in this way had an average sub-grain size of 0.14 µm. Due to the lower breakdown of the work hardening of the powder, this material was considered to be excessively deformed ("overworked"). The subgrain size of such materials is usually ≤0.15 µm. The primary material was formed on a press by the degree of deformation ε = 0.3 at a rate of deformation which corresponds to the logarithmic value of

corresponded.

The workpiece was subjected to coarse grain annealing at a temperature of 1220 ° C for 1 h. An average grain size of over 60 µm was determined, which clearly means coarse grain in this case.

5 shows a diagram of the experimentally determined deformation conditions in order to achieve coarse grain for the finished workpiece in the event that insufficiently deformed starting material (“underworked”) is assumed. The deformation conditions are shown as pairs of values for the deformation speed and the degree of deformation. Each intersection of an abscissa value with an ordinate value represents a specific state that characterizes the deformation condition, but not a functional relationship between the deformation speed and the degree of deformation. If the intersection falls within the hatched area, the conditions for the success of subsequent coarse grain annealing on the finished workpiece are met. If the intersection falls outside the hatched area, coarse grain formation can no longer be expected. Either the recrystallization is then at least partially absent, or a fine-grained structure that is undesirable for operation is formed.

soll zwischen 16,5 und 20 liegen, optimal bei etwa 18 (strichpunktierte Horizontale), während

sein soll. Das günstige Gebiet im Diagramm ist parallel zur Abszisse offen, was bedeutet, daß dem Verformungsgrad nach oben keine Grenzen gesetzt sind.It can be seen from the diagram that the rate of deformation must be kept within fairly narrow limits in order to achieve coarse grain, that an optimum value exists regardless of the degree of deformation and that the latter must not fall below a certain minimum. The value for

should be between 16.5 and 20, optimally around 18 (dash-dotted horizontal), while

should be. The favorable area in the diagram is open parallel to the abscissa, which means that there is no upper limit to the degree of deformation.

Fig. Is a diagram of the experimentally determined deformation conditions to achieve coarseness for the finished workpiece in the event that optimally deformed starting material ("normal") is assumed. The hatched area again represents the totality of the intersection points of an abscissa and ordinate value, for which the coarse grain formation is guaranteed on the occasion of the subsequent annealing.

bis auf

verformt, so wurde nach anschließender Glühung bei 1220°C/lh kein Grobkorn erhalten. Das gleiche Material entsprechend

bis auf

verformt, ergab eindeutig Grobkorn.Has z. B. a raw material according to the characteristics explained in Fig. 3, but with a deformation rate accordingly

until

deformed, no coarse grain was obtained after subsequent annealing at 1220 ° C / lh. The same material accordingly

until

deformed, clearly gave coarse grain.

zwischen 15,5 und 20, optimal etwa 18 entsprechen. Der Wert für s ist hingegen nicht begrenzt, kann also beliebig klein, im Grenzfall auch Null sein (keine weitere Umformung von der Praxis her möglich bzw. erwünscht). Im Bereich niedriger Verformungsgrade für die Endformgebung entsprechend

ist der Bereich für die Verformungsgeschwindigkeit erweitert und erreicht für

Werte, die zwischen etwa 10 und 22 liegen. Das heißt für die Praxis, daß im Falle kleiner Verformungen (z. B. Nachpressen zur Erzielung höherer Genauigkeit und Oberflächengüte des Werkstückes) die Verformungsgeschwindigkeit für zuvor optimal verformtes Vormaterial nicht so kritisch ist wie bei höheren Verformungsgraden.The diagram shows that whenever larger deformations of the workpiece corresponding to ε> 1.0 are necessary, the deformation speed has to be kept within narrow limits, which is the value for

between 15.5 and 20, optimally around 18. The value for s, however, is not limited, so it can be as small as desired, in the limit case it can also be zero (no further transformation possible or desirable in practice). Correspondingly in the area of low degrees of deformation for the final shaping

is the range for the rate of deformation expanded and reached for

Values that are between approximately 10 and 22. In practice, this means that in the case of small deformations (e.g. re-pressing to achieve higher accuracy and surface quality of the workpiece), the deformation speed for previously optimally deformed primary material is not as critical as for higher degrees of deformation.

FIG. 7 shows a diagram of the experimentally determined deformation conditions in order to achieve coarseness for the finished workpiece in the event that excessively deformed starting material ("overworked") is assumed.

liegt der zulässige Wert für

etwa zwischen 14 und 18, für höhere Verformungsgrade entsprechend

zwischen 16 und 20, optimal wieder bei ungefähr 18. Im übrigen besteht im niedrigeren Verformungsbereich entsprechend

etwa ein linearer Zusammenhang mit dem Mittelwert des Logarithmus der Verformungsgeschwindigkeit. Der Verformungsgrad s muß mindestens 0,1 erreichen.The hatched area defined above approaches the ordinate, but does not quite reach it. Correspondingly in the area of low degrees of deformation

is the permissible value for

approximately between 14 and 18, for higher degrees of deformation accordingly

between 16 and 20, optimal again at around 18. Otherwise there is a correspondingly lower deformation range

for example a linear relationship with the mean of the logarithm of the rate of deformation. The degree of deformation s must reach at least 0.1.

bis auf

verformt, so wurde nachträglich kein Grobkorn erhalten, während das Wertepaar 16,6 (Ordinate/0,27 (Abszisse) nach einer Glühung bei 1220° C zu Grobkorn führte.Was a raw material in accordance with the characteristics explained in FIG. 4, but with a deformation rate accordingly

until

deformed, no coarse grain was subsequently obtained, while the value pair 16.6 (ordinate / 0.27 (abscissa) resulted in coarse grain after annealing at 1220 ° C.

5, 6 and 7 it can be seen that for all structural and deformation states of the primary material there is a common optimal range for the logarithm of the deformation speed for the workpiece to be formed, which is at a value of regardless of the degree of deformation to be achieved 18 ± 1.0. The rate of deformation must therefore be kept in a relatively narrow critical range. The only additional condition is that the degree of deformation is high enough if the condition of the primary material is not sufficiently known.

These deformation conditions apply both to a single deformation step and to a complicated forming process consisting of partial steps. In any case, during the implementation of the last step the conditions mentioned above are met. From the above it is clear that ultimately the structural and work hardening condition of the primary material (i.e. the initial conditions) is largely irrelevant. It is always possible to achieve a coarse grain after the final annealing. Forming to the finished workpiece can be done by forging, rolling, pressing, hammering or hot drawing or any combination of these processes.

The starting material can be produced in a conventional manner by hot isostatic pressing or by extrusion.

The method is generally applicable to the alloy type specified in the examples and related dispersion-hardened austenitic superalloys which are suitable for precipitation hardening.

By means of the method according to the invention, the working conditions to be observed for the further shaping of a workpiece from a dispersion-hardened nickel alloy were defined as pairs of values of deformation rate / degree of deformation in order to again achieve a coarse-grained structure which is optimal for operation at high temperatures and clearly represented in diagrams. The process ensures, regardless of the ultra-fine-grained raw material and its degree of work hardening, that coarse grain is obtained in the end product.

Claims (10)

1. Process for manufacturing a workpiece from a creep-resistant, precipitation-hardenable austenitic alloy having a high nickel content and containing a metal oxide as the hardening idspersoid, this process being in accordance with the methods employed in powder metallurgy and in which a metal powder is mixed with a metal oxide powder, is mechanically alloyed, is encased in a metal container, and is densified, by hot-compaction, to 100% of the theoretical density, in a manner such that an easily deformable intermediate material is produced, which has a very fine grain size and is suitable for further processing, terminating in an annealing treatment for developing a coarse grain size, characterised in that the abovementioned intermediate material is converted to the finished workpiece by means of a purpose-designed working operation which embodies the final shaping step, the deformation rate and the degree of deformation being determined in accordance with the intermediate material, which can exhibit deformation which is insufficient, optimum, or excessive, in a manner such that the logarithm of the temperature-compensated deformation rate, expressed as

where

and A_o and A_f respectively denote the cross-sectional areas of the workpiece before and after the working operation, while D_Ni dentoes the diffusion coefficient of nickel, which is a function of temperature,
lies between the values 16.5 and 20 in

case a) - insufficient deformation of the intermediate material, lies between the values 10 and 22 in

case b) - optimum deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0≤ε<0.3, where

and lies, in the range of comparatively high degrees of deformation for the final-shaping step, described by s>1.0, between the values 15.5 and 20,
lies between the values 14 and 18 in

case c) - excessive deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0.1 <|ε|<0.2,

and lies, in the range of comparatively high degrees of deformation for the final-shaping step, described by |ε|>0.8, between 16 and 20, and in that the degree of deformation for the final-shaping step, described by s, reaches a value of not less than 0.5 in

case a) - insufficient deformation of the intermediate material,

is as small as desired, and can thus also be nil, in

case b) - optimum deformation of the intermediate material,

increases linearly with the deformation rate, but reaches a value of not less than 0.1, in

case c) - excessive deformation of the intermediate material in the range of low degrees of deformation for the final-shaping step, described by 0.1 < |ε| < 0.6,

the above being subject to the restriction that in

case a), the values must lie within the shaded region in Figure 5, in

case b), the values must lie within the shaded region in Figure 6, and in

case c), the values must lie within the shaded region in Figure 7.

2. Process according to Claim 1, characterised in that the logarithm of the temperature-compensated deformation rate for the final-shaping step, expressed as

has the value of 18 ±1.0, irrespective of the degree of deformation which is to be achieved, and irrespective of the initial deformation-state of the intermediate material.

3. Process according to Claim 1, characterised in that the easily deformable intermediate material, which has a very fine grain size, is produced by hot-isostatic pressing, or by extrusion.

4. Process according to Claim 1, characterised in that the intermediate material exhibits insufficient deformation, is produced by the extrusion of an encased powder, at a temperature of 1075°C, achieving a reduction ratio of 20 : 1, and possesses a sub-grain size of 0.25 to 0.35 µm.

5. Process according to Claim 1, characterised in that the intermediate material exhibits the optimum deformation, is produced by the extrusion of an encased powder, at a temperature of 950°C, achieving a reduction ratio of 20 : 1, and possesses a sub-grain size of 0.15 to 0.25 µm.

6. Process according to Claim 1, characterised in that the intermediate material exhibits excessive deformation, is produced by hot-isostatic pressing of an encased powder, at a temperature of 950°C, under a pressure of 135 Mpa, for 4 hours, the case undergoing a height-reduction of 30%, this intermediate material possessing a sub-grain size not exceeding 0.15 µm.

7. Process according to Claim 1, characterised in that the creep-resistant austenetic alloy has the following composition:

8. Process according to Claim 1, characterised in that the creep-resistant austenitic alloy has the following composition:

9. Process according to Claim 1, characterised in that the working step, to produce the finished workpiece, is carried out by forging, rolling, pressing, hammering, or hot-drawing.

10. Process according to Claim 9, characterised in that the step in which the intermediate material is worked to produce the finished workpiece is carried out in several successive part-stept, such that the last of them satisfies the deformation conditions specified in Claims 1 and 2, as well as in Claims 4,5 and 6.

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