EP2999562A1

EP2999562A1 - Method of fabricating a steel part by powder metallurgy, and resulting steel part

Info

Publication number: EP2999562A1
Application number: EP14727770.1A
Authority: EP
Inventors: Gérard Marie Denis RAISSON; Jacques Bellus
Original assignee: Aubert and Duval SA; Areva NP SAS
Current assignee: Aubert and Duval SA; Areva NP SAS
Priority date: 2013-05-22
Filing date: 2014-05-22
Publication date: 2016-03-30
Also published as: FR3005882A1; JP2016526099A; CN105492146A; WO2014187916A1; US20160184895A1; KR20160033076A; FR3005882B1

Abstract

Method of fabricating a steel part (5) by powder metallurgy, characterized in that: - a prealloyed powder (7) is prepared having the composition desired for said part, except regarding the contents of O and N and, optionally, of C, having contents of O and N of at most 200 ppm, and having an Mn content of 0.4 to 2% by weight and a Cr content of less than or equal to 3%; - the powder (7) is placed in a container (1), the walls (2, 3) of which define a space (4), the shape of which corresponds to that of the part (5), a getter (6) being at the periphery of the powder (7), said getter (6) having the ability, at high temperature, to absorb and reduce CO and to absorb nitrogen, and the container (1) is placed under vacuum then sealed; - the container (1) and the powder (7) are brought to a temperature that leads to a sintering of the powder (7) and a densification of said power (7) that does not exceed 5%, an emission of nitrogen and of CO from the powder (7) and the absorption thereof by the getter (6); - a densification of said powder (7) is carried out by hot isostatic pressing in order to obtain said part (5); - said part (5) is separated from the container (1) and from the getter (6); - a scalping, a heat treatment and a machining of said part (5) are carried out. Steel part thus produced.

Description

Process for the metallurgical manufacture of powders of a steel part, and steel part thus obtained

The present invention relates to metallurgy, and more specifically to the manufacture of steel parts by powder metallurgy.

It is known to those skilled in the art that manganese micro-alloyed steels (their Mn content is of the order of 0.4 to 2%) and low alloyed steels substantially free of chromium require a control of their microstructure. to ensure good ductility, especially in resilience. A microstructure exhibiting massive pro-bainitic ferrite ranges is intrinsically fragile while a finer bainitic microstructure limiting the appearance of this pro-bainitic ferrite is a necessary (but not sufficient) condition for ensuring good ductility after income. The parameters controlling the microstructure are chemical analysis, treatment temperatures (austenitization and tempering) and quenching rate after austenitization.

The constituent elements of reactors tanks of nuclear power plants are often made of a manganese steel type 16MND5 which meets the definition above, and whose standardized composition (AFNOR 16 MND 5 standard) is, in percentages by weight (as will all the contents given in the text):

- C <0.22%;

Mn = 1, 15-160%;

- P <0.008%;

- S <0.008%;

- Si = 0.10 - 0.30%;

- Ni = 0.50 - 0.80%;

Cr <0.25%;

Mo = 0.43 - 0.57%;

- V <0,03%, knowing that for the parts to be coated, this maximum content can be reduced to 0,01%;

Cu <0.20%;

- Al <0.04%;

the rest being iron and impurities resulting from the elaboration.

It is also used for this purpose a steel type A508 (ASME SA-508 / S / A-508M grade 3:

- C <0.25%; Mn = 0.5-100%;

- P <0.025%;

- S <0.025%;

- If <0.4%;

Ni = 0.4-100%;

Cr <0.25%;

Mo = 0.45-0.6%;

- V <0.05%;

- Nb <0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- AI <0.025%;

the rest being iron and impurities resulting from the elaboration of the elaboration.

Usually, the elements of these 16MND5 tanks are made by ingot casting and forging. Their mass can, for some, reach several tens, even hundreds of tons.

It would be desirable to be able to achieve at least some of these elements, which can reach and exceed 10 tonnes, by powder metallurgy, so as to reduce production time, make parts easier to inspect and, in general, reduce costs. to their production.

However, tests have shown that parts made by powder metallurgy, even when these parts were of relatively small size, did not allow to obtain satisfactory levels of resilience, despite a relatively thin bainitic microstructure free of probainitic ferrite ranges, for reasons that had not been clarified so far. The problem was even more acute for large parts.

The object of the invention is to propose a process for producing such parts, in particular large parts, in 16MND5 steel and also in other ferrous steels and alloys for which comparable problems would arise, by making use of powder metallurgy, this method nevertheless providing satisfactory mechanical properties to said parts, in particular a resilience at least equal to that obtained on cast and forged parts of the same composition, and having a microstructure bainitic type, such as that which is usually obtained on the parts of the type mainly covered by the invention.

To this end, the subject of the invention is a process for the metallurgical manufacture of powders of a steel part, characterized in that:

a pre-alloyed powder having the desired composition for said part is prepared, except for the contents of O and N and, optionally, at C, with O and N contents of at most 200 ppm, said powder having a Mn content between 0.4 and 2% by weight and a Cr content of less than or equal to 3%;

the powder is placed in a container whose walls define a space whose shape corresponds to that of the part to be manufactured, a getter being positioned at least partially around the periphery of the powder, said getter having the capacity, at high temperature, to absorb and reduce CO and absorb nitrogen by dissolution, and vacuum and seal the container;

the container and the powder which it contains are carried at a temperature resulting in sintering of the powder and a densification of said powder not exceeding 5%, a release of nitrogen and CO from the powder and their absorption by the getter ;

densification of said powder is carried out by hot isostatic compaction by placing said container and the powder in a pressure vessel to obtain said part;

said part is separated from the container and the getter;

and a peeling, a heat treatment and a machining of said part are carried out in order to give it its mechanical properties, its surface state and its exact desired dimensions.

Said part may be made of a composition steel, in% by weight after densification:

- C <0.25%;

Mn = 0.5-1.60%;

- P <0.025%;

- S <0.025%;

- If <0.4%;

Ni = 0.4-100%;

Cr <0.25%;

Mo = 0.43 - 0.6%;

- V <0.05%; - Nb≤0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- AI <0.04%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

Said part may be made of a composition steel, in% by weight, after densification:

- C <0.22%;

Mn = 1, 15-160%;

- P <0.008%;

- S <0.008%;

- Si = 0.10 - 0.30%;

- Ni = 0.50 - 0.80%;

Cr <0.25%;

Mo = 0.43 - 0.57%;

Cu <0.20%;

- Al <0.04%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

- C <0.25%;

Mn = 0.5-100%;

- P <0.025%;

- S <0.025%;

- If <0.4%;

Ni = 0.4-100%; Cr <0.25%;

Mo = 0.45-0.6%;

- V <0.05%;

- Nb <0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- AI <0.025%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

Said getter may be made of a material chosen from titanium, zirconium, hafnium and their alloys, and a stainless steel.

The getter may be of titanium or of titanium alloy and the temperature of the powder during sintering may be between 950 and Ι ΟΘδ'Ό, preferably between 1000 and Ι ΟΘδ'Ό.

Sintering and densification by hot isostatic compaction of the powder can be carried out successively, without intermediate cooling of the powder.

After placing the powder in the space defined by the walls of the container, it can be subjected to cold isostatic compaction at a maximum temperature of 300 ° C and a pressure of 100 to 300 bar.

Said cold isostatic compaction can provide a decrease in the volume of the powder of 1 to 3%.

The wall of the container in contact with the powder may be made of the material constituting the getter.

The getter can be a coating of the container wall.

The getter may constitute a separate piece placed in the vicinity of the wall of the container in contact with the powder.

The subject of the invention is also a steel part, characterized in that it has been obtained by the process, and in that its oxygen content is ≤ 50 ppm, preferably ≤ 20 ppm, its nitrogen content is ≤ 50 ppm, preferably ≤ 25 ppm, and its cumulative oxygen + nitrogen content is 80 80 ppm, preferably 50 50 ppm.

As will be understood, the invention is based primarily on the finding by the inventors that the resilience problems encountered during the manufacture of parts in 16MND5 by powder metallurgy came from too high oxygen and nitrogen contents in the initial primer powder, and that the removal of O and N during sintering of the powder, if at least one of these elements was present at excessive initial content, solved these problems. It should be understood that the application of the process of the invention goes beyond the sole manufacture of parts 16MND5 and other alloys of the same family, in particular the A508, and may concern all ferrous alloys shaped by metallurgy powders which would prove that the contents of O and / or N in the initial powder would pose problems as to the properties of the final piece. Steels containing from 0.4 to 2% Mn and up to 3% Cr in addition to Mn, constitute such alloys.

It is recalled that "pre-alloyed powder" is usually understood to mean a powder whose grains each have, taken separately, the composition intended for the final part (with the exception of the modifications that may occur during the treatment of the powder). This prealloyed powder is defined in contrast to a powder which would consist of grains of various compositions and which, once mixed and sintered, would provide a part whose overall composition would be that targeted, but which could present on the microscopic scale local differences notable composition.

The solution developed by the inventors consists, in particular, in reducing these levels of O and / or N before final densification of the prealloyed powder, using a getter, that is to say a compound which, placed in the the vicinity of the powder present in a container, will capture the oxygen (as CO) and / or nitrogen, because of its higher affinity for these two elements than that of the powder to be treated.

The use of getters is well known when it is desired to reduce the oxygen content of the atmosphere surrounding a material (powder or other) during a heat treatment, in order to avoid contamination of the surface of the material by the oxygen from the ambient atmosphere. However, in the context of the invention, the function of the getter goes much further, in that the inventors have found that it is possible, under certain conditions, to also obtain a deoxidation and / or a denitriding mass of the powder, even when the quantity of powder involved is in hundreds of kilograms, or even in tons.

The getter thus becomes the main agent of a true metallurgical treatment of the powder, and it has been found, surprisingly, that this treatment contributes to a very substantial improvement in the resilience of the parts obtained after hot isostatic compaction. (CIC) of the powder thus treated and a heat treatment of conventional type carried out on the part resulting from the compaction. We thus fall back on mechanical properties of the parts obtained by metallurgy of the powders at least equal to those of the parts obtained by ingot casting and forging, in particular their resilience.

The material constituting the getter is, preferably, titanium, because of its relatively moderate cost, and especially its remarkable ability to absorb quickly and in large quantities both oxygen and nitrogen (several% each). These elements emerge from the powder in the form of CO (resulting from the reduction of the oxides of the powder by the carbon that is present at the beginning) and molecular nitrogen. We will see later why titanium should, however, preferably not be used if a temperature of Ι Οδδ'Ό or more is considered for the treatment of the powder.

Regarding the amount of titanium to use, the most important parameter to consider is the ratio between the titanium surface and the powder mass, due to the high absorption capacity of CO and N ₂ by titanium. An order of magnitude of this ratio is 4 to 20 cm ² of Ti per kg of powder, for a powder typically containing 100 ppm oxygen and 120 ppm nitrogen. The ratio will also be adjusted according to the processing time, which depends mainly on the dimensions of the part. The mass of Ti to be used is, in all rigor, a function of the mass of powder to be treated and the surface of its grains, but experience shows that a Ti sheet of 0.5 mm thickness arranged around the outer surface of the powder mass is sufficient to absorb CO and nitrogen in the desired amounts for most cases. In any case, routine experiments make it possible to check whether the quantity of titanium (or any other material) used to form the getter is sufficient to obtain the absorption of the maximum amount of CO and nitrogen possible for treatment conditions given. The experiment also shows that the transfers of CO and nitrogen to the getter are carried out sufficiently fast and efficiently so that the treatment times necessary for deoxidation and denitriding of the powder are compatible with the requirements of an industrial production.

Other materials than titanium are conceivable to constitute the getter. Zirconium and hafnium would have comparable actions, but their much higher cost makes them economically less attractive, especially for the production of high mass parts. Stainless steels could also be used, with the advantage of being able to withstand higher processing temperatures than titanium, if the composition of the alloy being treated makes such temperatures necessary or useful. But in their case, the absorption kinetics of oxygen and nitrogen is substantially slower than for titanium, which is a serious drawback if several tons of powder. At least in the preferred case of producing parts of 16MND5 nuclear reactor vessels for which it is not necessary to exceed a treatment temperature of 1070 ^, titanium and its alloys constitute, in practice, the materials the more interesting for the implementation of the invention.

If the initial content of the powder in one of the elements O and N is excessive, one could consider using, to form the getter, a material that would only significantly absorb this element in excess and not the other.

If oxygen is present at a high content, for example, about 0.005 to 0.01%, in the initial powder, it is to be expected that the powder will be decarburized by the formation and departure of CO. The lowering of the C content will be substantially equivalent in weight percent decreases of the O content. This may need to be taken into account, that is to say that the final C content of the treated part will generally be lower. to that of the powder. It is therefore possible that a powder having a C content a little too high initially gives rise to a piece whose C content will comply with the requirements of the developed shade. Conversely, a correct initial C content in the powder may no longer be on the final part if the decrease in the C content led to the C content being removed from the requirements of the grade because the departure of CO during treatment would have been excessive, because of an initial O content of the particularly high powder. When choosing the composition of the powder, it is therefore advisable to ensure a certain safety margin on the C content in relation to the final content targeted. This can be done by means of routine experiments and a good control by the powder supplier of the composition of the powder.

The container is adapted to give the pile of powder, before its treatment, a shape and dimensions corresponding very substantially to those of the final piece. Its geometry and its dimensions are to be calculated according to the rules of the art specific to the calculations of the containers for the realization of parts close to their final dimensions by hot isostatic compaction of prealloyed powders. After a hot treatment of the powder which leads to its sintering, the container which contains it is placed in a hot isostatic compaction chamber where the complete densification of the part takes place. After this complete densification, it remains only to take the piece out of the container and heat treat it to give it its final metallurgical structure, and to machine it to perfect its dimensioning and surface condition.

The parameters of the hot isostatic compaction (CIC), and particularly the duration of the temperature and pressure plateau, are fixed according to the usual rules of the art. to obtain a full densification and a uniform temperature in all parts of the room, preferably one hour before the end of the bearing to ensure a sufficient margin of error over the necessary time. The typical times in temperature under 1000 bar are from 2 to 5 hours depending on the massiveness of the part.

Preferably, after placing the powder in the container and before sintering it is practiced cold isostatic compaction, typically leading to a decrease in volume of the powder of 1 to 3%, which corresponds to a pressure of 100 to 300 bar in the case of 16MND5 steel.

The invention will now be described in more detail with reference to the following appended figures:

FIG. 1 which schematically shows the various steps of an exemplary implementation of the method according to the invention

FIG. 2, which shows the variations in the oxygen content measured at the end of the treatment for different quantities of Ti used relative to the mass of powder during the manufacture of 16MND5 alloy geometry slugs of various compositions for containers. 1 10 mm in diameter containing substantially 12 kg of sintered powder 8 hours at 1050 before 3 hours CIC at 1050 ° C. under 1000 bar.

FIG. 3 which shows the variations in the nitrogen content measured at the end of the treatment for different quantities of Ti used relative to the mass of powder during the production of 16MND5 alloy geometry slugs of various compositions.

FIG. 4, which shows the resilience results obtained on different plots of the same geometry as a function of their O content alone;

- Figure 5 shows the resilience results obtained on different plots of the same geometry as a function of their N content alone;

FIG. 6, which shows the resilience results obtained on different pieces of the same geometry as a function of their sum O + N;

FIG. 7, which shows an example of a relatively high mass production device used for the validation tests of the range of operating conditions for the manufacture of industrial parts.

In the following example, we will deal specifically with the manufacture of parts or blocks made of 16MND5 manganese steel, knowing that the process of the invention can be applied to other ferrous alloys, more specifically to low alloyed steels. manganese (ie containing 0.4 to 2% of this element) as families of grades MC5, MND5, MSV5, MSV7, MV7, A508, CDV8, CDV9, and possibly up to 3% of chromium in addition to manganese. A Cr content greater than 3% would lead to the formation of Cr oxides which would not be affected by the process according to the invention, and it would therefore not have the desired efficiency.

In general, the invention finds a preferred application in the case of steels which would have the following composition, resulting from a compromise between the 16MND5 and the A508:

- C <0.25%;

Mn = 0.5-1.60%;

- P <0.025%;

- S <0.025%;

- If <0.4%;

Ni = 0.4-100%;

Cr <0.25%;

Mo = 0.43 - 0.6%;

- V <0.05%;

- Nb <0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- Al <0.04%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

Since 16MND5 and A508 have similar compositions, the treatment conditions that will be described for 16MND5 would also be applicable with advantage to A508.

A container 1 is first prepared, which is for example made of mild steel, composed of two walls 2, 3 separable from one another and defining between them, when assembled, a space 4 whose shape corresponds to to that of the part 5 that one wishes to prepare.

A sheet 6 of titanium T40 of, for example, 1 mm thick, is also prepared, forming the getter with a ratio of, for example, 10 cm ² of Ti per kg of powder, which is set shaped so as to be pressed against one 2 of the walls 2, 3 which define the space 4, during assembly of the container 1.

After this assembly, the shape and the dimensions of the space 4 correspond very substantially to those of the part 5 which it is desired to manufacture by metallurgy of the powders, taking into account the (calculated elsewhere) shrinkage which occurs during densification by hot isostatic compaction, as is conventional in this type of process.

The space 4 is then filled with the pre-alloyed powder 16 of steel 16MND5 intended to constitute the part 5.

This powder typically has the composition:

- C <0.22%;

Mn = 1, 15-160%;

- P <0.008%;

- S <0.008%;

- Si = 0.10 - 0.30%;

- Ni = 0.50 - 0.80%;

Cr <0.25%;

Mo = 0.43 - 0.57%;

Cu <0.20%;

- Al <0.04%;

- Co <0.1%

- Ti 0,0 0.01%

- Nb <0,01%

- Ta <0.01%;

the remainder being iron and impurities resulting from the preparation, in particular of oxygen and nitrogen in variable contents depending on the conditions of preparation of the powder.

By way of example, this powder typically has an N content of 120 ppm and an O content of 100 ppm.

Preferably, before sealing the container to seal, the powder is degassed to remove air and moisture. This degassing, which is conventional in powder compaction operations, is carried out following the rules of the art, for example by a vacuum 70 hours at a temperature in the range of ^'\ 50 ° C. The container 1 is then sealed to the outside air, and the heat treatment is carried out, which will make it possible to carry out the deoxidation and the denitriding of the powder 7, by carrying the container 1 and the powder 7 at a suitable temperature for a period of time. adequate. Experience shows that the diffusion of the gases emerging from the powder is fast enough that they can easily reach the sheet 6. The particle size of the powder is of no significant importance, at least up to the grain sizes usual millimeters for commercial pre-coated powders.

The treatment temperature must be chosen according to the following criteria.

It must be sufficient to cause reactions that will lead to the departure of the oxygen (in the form of CO) and the nitrogen (in molecular form) of the prealloyed powder 7 and their capture by the getter 6, and to give these reactions a kinetics compatible with the requirements of an industrial production. It also causes sintering. But it must not cause very significant densification of the powder 7, so as to allow the CO and N ₂ gas to flow between the powder 7 and the titanium getter 6. In the example described, a minimum temperature of θδΟ ' Ό or, better, 1 000'C, is advisable.

On the other hand, it is very preferable that there are no interactions between the getter 6 and the powder 7 which would significantly cause, for example, diffusion of elements between them or a chemical reaction. Thus, in the example described, it is preferable to avoid exceeding the temperature of the eutectic Fe-Ti which is of the order of Ι Οδδ'Ό. Such an overflow would have the disadvantage of polluting the surface of the future part 5, and therefore require machining to a depth greater than what would be desirable. Another significant disadvantage of exceeding the temperature of this eutectic would be that the product of the eutectic reaction between the titanium and the powder is extremely hard, and could be eliminated only by a long and expensive grinding.

It is therefore generally preferred to choose a maximum treatment temperature located below the eutectic of the main components of the getter 6 and the powder 7, if there is one, and with a sufficient margin of safety to take into account inaccuracies on the furnace temperature and the influence of the alloying elements on the exact temperature of the eutectic, therefore below Ι Οδδ'Ό in the example described. For this example, a range of 950-1065 ° C, preferably 1000-1065 ° C, can be recommended.

The treatment time is essentially a function of the thermal conductivity of the powder in its sintering state, the amount of oxygen and nitrogen that must be removed, and especially the dimensions of the part 5 to be manufactured, in particular its thickness, and 6 getter surface relative to the mass of powder 7. The speed with which one will obtain in all the powder 7 the target treatment temperature and the desired reactions and transformations will depend in particular on all these parameters. As a guide, this treatment time can typically be 8 hours for a cylinder diameter 120 mm to 48 hours for a flat body 250 mm thick.

Due to the low conductivity of the powder, voids appear between it and the container. In order to avoid the possible problems caused by this mechanism (appearance of voids, ...) two solutions can be envisaged, either through a continuous supply of powder, or via a cold isostatic compaction if the latter is preliminary.

The performance, before sintering, of cold isostatic pressing at a temperature of up to 300 ° C. and at a pressure of 100 to 300 bar, typically leading to a decrease in volume of the powder of 1 to 3%, allows to divide these sintering times by 2 or 3.

Then the process is continued by placing the container 1 in a chamber 8 of hot isostatic compaction, where the sintering is carried out in a conventional manner, and especially the densification of the powder 7 under the effect of the pressure. outside the container 1, to obtain the target part 5. The treatment temperature must, again, be chosen preferably to avoid a significant reaction between the getter 6 and the part 5 during compaction, and also to obtain metallurgical structures for the part 5 compatible with the subsequent heat treatments. The pressure and the duration of the treatment are chosen so as to obtain a densification of the satisfactory powder 7 in a suitable time. A bearing time of 1050 at 1000 bar is typically 3 hours for a flat part 250 mm thick.

It is also conceivable to carry out sintering and hot isostatic compaction successively in the same chamber equipped with heating and pressurizing means, by putting the chamber under pressure only during hot isostatic compaction. This solution is economical from an energy point of view, since the treated metal does not cool substantially between the two treatments, whose temperatures are similar. Also, the manipulations of the container 1 are thus limited as much as possible. This same chamber can also be used during a possible cold isostatic compaction that precedes sintering: it is then pressurized, but the heating means are not activated, or weakly so as not to exceed a temperature of 300. ° C. Finally, the piece 5 is out of the container 1 and separated from the getter 6. It undergoes peeling and machining to remove the remains of the getter 6 before the heat treatment that follows.

Since the temperatures maintained during sintering and densification are not adequate for obtaining the desired final metallurgical structure for the part 5, thermal treatments carried out outside the container 1, therefore in the absence of the getter 6, must, indeed, be performed at any time after the separation between the part 5 and the remains of the getter 6. Preferably it is carried out before the final machining, if there is one, so that it can hold account of any deformations experienced by the part 5 during heat treatments.

A 20-hour thermogravimetric study was carried out on the nitriding and oxycarburation of titanium T40, which is a preferred material for constituting the getter 6. It made it possible to determine:

- that the kinetics of nitrogen absorption by the titanium becomes noticeable at levels above 950 ^<C;

- that the kinetics of absorption of CO (and therefore oxygen) by titanium becomes significant from 800-900.

Alternatively, it is possible to perform a slight cold isostatic compaction (CIF) at a temperature below 300 ° C of the powder 7 under a pressure of the order of, for example, 200 bar, in an optimum range of 100 to 300 bar, after the filling of the space 4 and, therefore, before the denitriding and deoxidation treatment. This compaction can lead to a decrease in the volume of the powder of the order of 1% to 3% and can significantly improve the thermal conductivity of the powder 7. The homogeneity of the target temperature for the next heat treatment, which will lead to the deoxidation and the denitruration of the powder, can thus be reached more quickly.

Using the above method, it has thus been realized in the laboratory cylindrical slugs of 12 kg diameter 100 mm and height 180 mm (final dimensions after compaction and peeling) 16MND5 alloy from batches of powder of very comparable compositions, specified in table 1.

C% Mn% If% S% P% Ni% Cr% Mo% 0 initial N initial ppm ppm

Lot 1 0.1 17 1, 41 0.235 0.004 0.007 0.600 0.020 0.480 120 140

Lot 2 0.131 1, 53 0.266 0.004 0.005 0.596 0.128 0.500 56 140

Lot 3 0.168 1, 82 0.302 0.004 0.007 0.573 0.054 0.515 88 140 Lot 4 0.153 1, 63 0.307 0.004 0.004 0.560 0.056 0.504 70 100

Table 1: Compositions of batches of powders used for experiments

In addition, all these batches had Cu contents of 0.012%, V 0.010%, Al <0.003%, Ti <0.003%, As <0.003%, Sn <0.003%, Ca <0, 0005%.

From these powders eleven plots were made according to the following procedure. A container similar in principle to the container 1 of FIG. 1 is prepared, which defines a space 4 of cylindrical shape to give the powder 7 substantially the dimensions targeted for the billet. Against the wall of the container which defines the outer surface of the space, is placed, during the tests carried out according to the invention, a tubular getter T40 titanium thickness of 1 mm. The containers had a diameter of approximately 120 mm and a height of 200 mm and contained a little more than 12 kg of powder. The height of the getter tube is variable for each test according to the ratio between the mass of titanium and the mass of powder that is to be tested. The amount of titanium per kg of powder is given, with the other experimental conditions, in Table 2. The ratio between the getter surface and the powder mass was varied from 10 to 34 cm ² of titanium per kg of powder . Reference tests were also conducted without a getter.

The container containing the getter (when there is one) is filled with powder in the air, then it is placed under vacuum, maintained at 60 ° C. for 70 hours (to be sure of having degassed the powder, as usual in metallurgy powders), and finally sealed.

Then, for the treatments performed according to the invention, the whole is held at 1050 for 8 hours. It is essentially during this step that, if the getter is present, the O and N content of the powder is lowered. At the same time, the powder is sintered without appreciable densification.

Then hot isostatic compaction (CIC) is carried out to densify the slug, placing the assembly in a chamber under pressure at 1000 bar, at a temperature of 100 ° C. for 3 hours.

After compaction, the container and the remains of the getter are removed by peeling, and the billet is cut into three cylindrical portions, of respective heights 87 mm, 87 mm and 5 mm substantially. The 5 mm high part is used for the characterization of the initial state of the slug. The other two parts are heat treated by:

austenitization at 890 ° C. for 2 hours, then oil quenching (TH), at a quenching rate which is high, so that the resilience of the sample which will then be measured will depend only on its oxygen and nitrogen contents and not an effect of the microstructure; the high quenching speed limits the appearance of pro-bainitic ferrite, which is favorable for good resilience for all the carbon contents studied;

- then back to θδΟ'Ό for 4 hours.

Table 2 summarizes the conditions of the different tests.

Table 2: Preparation and treatment conditions of 12 kg slugs The amount of Ti relative to the mass of powder treated was therefore varied during the tests. The results are visible in FIGS. 2 and 3 which show, respectively, the oxygen and nitrogen contents measured at the end of the treatment for different amounts of Ti used, relative to the powder mass.

For the reference tests 4, 5, 6 and 8 that took place without the presence of the Ti getter, the O and N contents in the final sample did not decrease compared to those present in the initial powder.

FIGS. 2 and 3 show that, while the oxygen and nitrogen contents of the sintered and densified sheets by hot isostatic compaction did not vary significantly, the use of a titanium getter in sheet form of 1 mm d thickness decreases their levels, which can fall below 10 ppm for oxygen and below 20- 30 ppm for nitrogen, for amounts of titanium exceeding 10 gr / kg (22 cm ² / kg), with a saturation effect.

After densification by CIC, austenitization and income, these plots were metallographically characterized and mechanically tested.

The grain size is generally 5 ASTM, with some grains of 4 or 4.5 ASTM. This corresponds well to the conventional requirements for 16MND5 used in nuclear reactor vessels. The structure is predominantly bainitic in all cases.

Table 3 summarizes the results of the various mechanical tests carried out, with the O, N and O + N contents of the corresponding slugs.

Table 3: Results of the mechanical tests carried out on the slugs The results of the tensile tests are satisfactory, both at ambient temperature and at 350 ° C. usual temperature of use of nuclear reactor vessels. The ductility, expressed by the elongation at break A and the necking Z, is in accordance with said conventional requirements. The elastic limits Rp _{0 2} and the tensile strengths Rm also, or better than usual for the tensile strength. For each piece tested, the results obtained on the two parts which have been heat treated are presented. The differences between the results obtained on the two parts are generally small, of the order of the dispersions which would have been classically expected. The resilience of the slugs was mainly tested because, as we have seen, it is the characteristic which, a priori, dissuade the skilled person to use powder metallurgy to manufacture the parts mainly concerned with the invention.

Resilience measurements, represented by the Kv impact bending, were carried out at -20, 0 and +20 ° C for the different slugs, whether or not they had undergone the deoxidation and denitriding treatment of the original powder according to the invention whose treatment conditions have been given in Table 2 and the nitrogen and oxygen contents are given in FIGS. 2 and 3. FIG. 4 shows the results obtained as a function of the O content alone, FIG. shows the results obtained as a function of the N content alone, and FIG. 6 shows the results obtained as a function of the sum O + N. Since the process according to the invention simultaneously lowers the O and N contents of the powder, it It is difficult to discriminate the effects of the contents of these elements solely on the basis of the tests carried out.

The target Kv at 0 ° C is at least 60 J for each sample taken individually. A ^ O'C this minimum value is 28 J. At +20 ^q C, this minimum value is 72 J. In Figures 4 and 5, the target Kv values for each sample were reported.

Figure 4 shows that these specifications are all met when O is at most 40 ppm, and O values of less than 20 ppm provide consistently good to excellent Kv values. On the contrary, O contents of 80 to 10 ppm, as in the initial powder, do not allow to reach the minimum Kv required with a sufficient margin of safety.

Figure 5 shows that comparable findings can be made for N content. A content lowered to 40 ppm is often sufficient for acceptable or good Kv values to be obtained, and grades of 25 ppm and below. guarantee good results. The 90 to 140 ppm of the initial powder, on the other hand, is too high for satisfactory Kv values to be reliably obtained.

Finally, if, as in figure 6, we reason on the total O + N, we see again a correlation between a low content and a high Kv. An O + N value of up to 80 ppm, ideally less than 50 ppm, leads to higher Kv's than required, with good reliability.

Such contents are made accessible by the use of a getter according to the invention. They would not be accessible directly by other known methods of making the powder, for example by atomization.

In general, the Kv values obtained during the tests are rather dispersed, since the resilience does not depend only on the composition of the metal, but also on its microstructure, which the conditions of treatment of the samples after sintering contribute to establish. In the present case, a bainitic structure is obtained on the product after sintering, and not a structure with probainitic ferrite that would have been less favorable. Nevertheless, all the tests corresponding to Tables 1, 2 and 3 and to Figures 2 to 6 were carried out for identical heat treatments leading to the same final bainitic final microstructure, the only difference being in the composition of the powder (although it varied only in very small limits on elements other than O and N) and the use or not of a getter.

These tests clearly show that low levels of O and N are, for reasons that are yet to be completely elucidated, conditions essential to obtain the targeted resilience, treatment conditions and metallurgical structure obtained identical. From this point of view, the use of the method according to the invention, in view of the tests carried out on the plots, appears to be particularly advantageous, because of its efficiency and relatively low cost. The influence of the microstructure of the sample on the resilience has been eliminated.

Cross-sectional analyzes of a getter 6 in T40 titanium sheet of 1.2 mm thickness taken from the slug (lopin 10 of Table 2) after its use for 8 hours at Ι ΟδΟ'Ό, between its surface exposed to a 16MND5 steel powder 7, initially having an O content of 80 ppm and an N content of 140 ppm, with a ratio of 20 cm ² of Ti per kg of powder. Its surface was in contact with the 0.15% carbon steel constituting the wall 2 of the container 1. They show :

the distribution of the nitrogen is generally symmetrical between the two surfaces, with a rise in the nitrogen content in the vicinity of each of the surfaces; that is due to the fact that, since the nitrogen equilibrium pressure is high and its absorption kinetics relatively slow, the nitrogen diffuses into the titanium both from the powder 7 and from the wall 2 of the container 1;

the distribution of oxygen, on the other hand, is clearly asymmetrical: only the surface of the titanium which was opposite the powder 7 has significantly absorbed oxygen; the relatively low CO pressure and the rapid kinetics of CO reduction and oxygen uptake mean that the CO does not bypass the titanium sheet 6 to reach its opposite side of the wall 2 of the container 1 ;

- the distribution of carbon is apparently almost symmetrical, unlike that of oxygen; the carbon concentration is even a little higher in the vicinity of the face of the titanium sheet 6 which was facing the wall 2 of the container 1; on the one hand, a portion of the carbon of the steel of the container 1 diffuses in the solid state to the titanium; on the other hand, the absorption of carbon after the reduction of CO escaping from the powder is essentially effected by forming a surface layer of friable TiC; this one tends strongly to detach itself from the sheet 6 of titanium during its manipulations between the experiment and the analysis: the analysis of the sheet 6 does not make, in fact, not well account of the reality of the carbon absorption from the powder 7;

that the nitrogen content of the sheet 6 of Ti is of the order of 2% in the vicinity of its two surfaces, which is far from the maximum solubility of nitrogen at 100 ° C. in titanium (6% );

that the O content of the sheet 6 of Ti is of the order of 2.5% in the vicinity of its surface which was in contact with the powder 7, which is far from the maximum solubility of oxygen at 1 050 ° C in titanium (12%).

Since the sheet 6 is far from being saturated with O and N at the end of the treatment, it is therefore not necessary to provide that the sheet 6 has a high relative mass relative to that of the treated powder 7.

Tubular elements having a height of 287 mm, an internal diameter of 1 mm, an outer diameter of 370 mm and a wall thickness of 1 mm were also tested. They were made with a powder belonging to Lot 4 of Table 1 on a thick tube of austenitic steel (30 mm thick). The Ti getter was placed on the inner wall of the container at the periphery of the cavity where the powder was placed with a ratio of 8.6 g of Ti per kg of powder, representing a surface of 18.2 cm ² of Ti per kg of powder. The method of filling the free space of the container with the powder was identical to that applied to the previous 12 kg plots. First of all, the assembly was held at Ι ΟδΟ'Ό for 18 hours to obtain sintering. Then the hot isostatic compaction was carried out under 1000 bar at Ι ΟδΟ'Ό for 3 hours. The final heat treatment of the tubular element removed from the container and peeled consisted of austenitization, by maintaining at 890 ° C. for 5 h followed by quenching with water at a speed estimated by modeling between Ι, δ ' Ό / β (GOOO / hour at the periphery of the tube and 0 ° C / s (2500 ^q C / hour) in the core of the tube wall.

In all cases, a final N content of less than 3 ppm and a final O content of 3 to 8 ppm on the tube were obtained. Very low levels are thus obtained, which do not really depend on the amount of getter used relative to the powder mass. These results therefore tend to confirm that the influence of the ratio of getter mass / powder is not preponderant, in any case for the manufacture of massive parts. The ratio of the surface of the getter / mass of powder is a more crucial parameter, as well as the duration of the sintering treatment during which most of the getter / powder reaction takes place. Despite low levels of oxygen and nitrogen, resilience was particularly low in areas with low cooling rates during quenching while good in areas with high cooling rates.

The production of two plots 400 mm in diameter and 210 mm in height was also carried out using the device shown in FIG.

The container 8 is generally cylindrical in shape, of external dimensions 400 mm in diameter and height 234 mm (including raised edges which ensure good contact between the various components of the container 8). The thickness of the sheet that constitutes it is 3 mm. It comprises a bottom plate 9, a tubular side wall 10 and a lid 1 1 connected to a pipe 12 which is connected to a pump or the like to allow the reduced pressure of the inside of the container, after its filling and the sealing the lid 1 1 on the side wall 10.

The T40 titanium getter, when present, is composed of three elements:

a plane sheet 13 which lines the bottom plate 9 of the container 8, and which has a diameter of 375 mm and a thickness of 1 mm, its mass is 0.5 kg:

- An annular plate 14 which lines the side wall 10 of the container 8 over part of the height thereof; the sheet 14 has a diameter of 399 mm, a thickness of 1 mm and a height of 95 mm; its lower edge 14 is placed 57 mm from the bottom wall 9 of the container 8; its mass is 0.5 kg; a flat sheet 15 which lines the lid 1 1 of the container 8, and comprises an orifice 16 in line with the pipe 12; it has a diameter of 375 mm, a thickness of 1 mm and a mass of 0.5 kg.

The powder used to prepare the samples is that of Lot 3 of Table 1. A quantity of 147 kg is introduced into the container (therefore a relatively large quantity, which may be representative of what would be the massiveness of some of the industrial parts that it is desired to achieve by the process according to the invention), which provides a ratio by weight of Ti / powder of 8.2 g / kg and a surface ratio of 18.2 cm ² of Ti / kg of powder.

Two slugs were produced with the same batch of 3 powder and substantially the same amount of titanium:

- one with a sintering time of 16 hours

- one with a sintering time of 48 hours

after sintering, the containers were densified by Hot Isostatic Compaction at Ι ΟδΟ'Ό under 1000 bar for a duration of 3 hours.

- After removal of the container and the titanium getter by machining, the slugs (diameter

370 mm height 185 mm) were heat-treated with austenitization at 890 ° C for 5 hours followed by quenching with water and tempering at 680 ° C for 10 hours. The austenitization and tempering times were set according to the dimensions of the plots according to the rules of art. Water quenching resulted in an estimated core cooling rate of 0.8 / s (SOOO / hour), which may be insufficient to eliminate any effect of carbon on resilience.

The microstructures were predominantly bainitic for the two plots.

The results of the dissections of the two plots were as follows:

For the sintered billet for 16 hours, the oxygen concentration ranged from 5 to 90 ppm and that of nitrogen from 3 to 37 ppm in the volume, the highest levels corresponding to the center of the plot and the maximum distance from to the titanium getter.

Resilience values were also very variable, ranging from very bad in zones with high nitrogen concentrations and especially oxygen to very good in low oxygen and nitrogen zones.

For the 48 hour sintered billet, the oxygen and nitrogen concentrations were uniformly very low (3 to 5 ppm for oxygen and <3 ppm for nitrogen) and the Kv values good to very good. For the second billet manufactured using the device of Figure 7, the treatment time was adjusted to ensure that the entire volume of powder reached thermal equilibrium. This was not the case for the first piece, treated for only 16 hours.

The disappointing results of the first of these two tests are attributed to insufficient time for sintering prior to densification. The powder did not have time to homogenize in temperature at least θδΟ'Ό or, better, Ι ΟΟΟ, in all its volume. It was therefore deoxidized and denitrated only in a very partial way before the densification by Isostatic Compaction in Hot.

Between a slug made with the aid of the device of FIG. 7 and the tubular piece previously described, in addition to the quenching speed after austenitization, the analysis of the powder plays an important role on the final microstructure. This role is known to those skilled in the art. It is therefore necessary to adjust the analysis of the powder to ensure the good microstructure and hence the good resilience in the entire volume of the room.

The successive stages of presintering at 950-1065 ° C and densification in this same temperature range can be carried out indifferently in the same chamber which will be put under high pressure only during densification, or in different enclosures with, therefore, the possibility that the container and the powder cools between the two operations, possibly up to ambient. The first solution is the most economically advantageous, especially from the point of view of total energy consumption and the possibility of using only one installation for both operations.

The example which has just been described is particularly suitable for the metallurgical production of the powders of 16MND5 manganese steel parts. However, as has been said, this example is absolutely not limiting. The invention is applicable to the manufacture of parts made of other types of steel with a Mn content between 0.4% and 2% and with a Cr content of less than or equal to 3%, the experience of which would show that the manufacture by metallurgy of powders with satisfactory mechanical properties, especially resilience, would be possible only if the powder used contains only very small amounts of oxygen and nitrogen, comparable to those which have been determined during the tests previously described on the grade 16MnD5.

These amounts are an oxygen content ≤ 50 ppm, preferably ≤ 20 ppm, a nitrogen content ≤ 50 ppm, preferably ≤ 25 ppm, and a cumulative oxygen + nitrogen content ≤ 80 ppm, preferably ≤ 50 ppm.

As has been said, other materials than titanium and its alloys can be optionally used to make the getter 6. This may lead to changing the limits of temperature and duration of treatments that have been given previously, the main thing being that:

these temperatures are compatible with good sintering of the powder 7 of the steel used, which is not necessarily 16MND5;

and preferably, they do not lead to a solidarization of the getter 6 and the powder 7 during sintering, for example by forming a eutectic between the iron and one of the components of the getter 6, so that the separation between the getter and the sintered part can be done easily and a simple peeling is enough to remove from the surface of the room getter residues that may have remained on its surface.

It should be understood that the object of the invention is also a container 1 for sintering and hot isostatic compaction of a metal powder 7, internally coated on at least a portion of its surface which is intended to come into contact of the powder 7 of a getter 6 which has the capacity, in general, to capture, during heating, contaminants contained in the powder 7. Contaminants means elements likely to hinder the smooth running of the sintering and obtaining a part 5 having the mechanical and other properties sought. Oxygen and nitrogen will often be the main contaminants to capture. Oxygen can be captured, in particular, by a reduction mechanism of an oxidized gas such as CO emerging from the powder 7 and oxygen absorption thus obtained.

Advantageously, for the production of bimetallic parts, for example tubes coated externally or internally by a shell of a composition different from that of the tube, provision may be made for the container 1 to include one of the constituents of the bimetallic part. The tube is then integrated into the container 1, and its coating, initially in the form of a prealloyed powder, is applied to it by means of the process according to the invention. The adhesion of the coating to the tube is achieved by diffusion bonding during the treatment. In this configuration, the getter is placed on the surface of the container out of the weld-diffusion surface with the coating.

In general, the invention may, in particular, take the following configurations:

the wall of the container in contact with the powder is made of the material constituting the getter;

the getter (6) is a coating of the wall of the container (1);

- The getter (6) constitutes a separate piece placed in the vicinity of the wall of the container (1) in contact with the powder (7). The invention has been described essentially in the case where the powder is a pralloyed powder of steel weakly alloyed with Mn, and where the contaminants to be removed from the powder before densification are O and N. But as has been said, it is conceivable to apply the invention to the capture of other contaminants and other types of steels containing 0.4 to 2% Mn and 0 to 3% Cr.

Claims

1. A process for the metallurgical manufacture of powders of a piece (5) made of steel, characterized in that:

a pre-alloyed powder (7) having the desired composition for said part is prepared, except for the contents of O and N and, optionally, at C, with O and N contents of at most 200 ppm, said powder having a Mn content between 0.4 and 2% by weight and a Cr content of less than or equal to 3%;

the powder (7) is placed in a container (1) whose walls (2, 3) define a space (4) whose shape corresponds to that of the part (5) to be manufactured, a getter (6) being positioned at least partially at the periphery of the powder (7), said getter (6) having the capacity, at high temperature, to absorb and reduce CO and absorb nitrogen by dissolution, and then vacuum and seals the container (1);

the container (1) and the powder (7) which it contains are carried at a temperature causing sintering of the powder (7) and densification of the said powder (7) not exceeding 5%, a release of nitrogen and CO of the powder (7) and their absorption by the getter (6);

densification of said powder (7) is carried out by hot isostatic compaction by placing said container (1) and the powder (7) in a pressure vessel to obtain said part (5);

said part (5) is separated from the container (1) and the getter (6);

and a peeling, a heat treatment and a machining of said part (5) are carried out to give it its mechanical properties, its surface state and its exact desired dimensions.

2. A process according to claim 1, characterized in that said part (5) is made of a composition steel, in% by weight after densification:

- C <0.25%;

Mn = 0.5-1.60%;

- P <0.025%;

- S <0.025%;

- If <0.4%;

Ni = 0.4-100%;

Cr <0.25%;

Mo = 0.43 - 0.6%; - V <0.05%;

- Nb <0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- AI <0.04%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

3. - Method according to claim 2, characterized in that said piece has the composition, in% by weight, after densification:

- C <0.22%;

Mn = 1, 15-160%;

- P ≤ 0.008%;

- S <0.008%;

- Si = 0.10 - 0.30%;

- Ni = 0.50 - 0.80%;

Cr <0.25%;

Mo = 0.43 - 0.57%;

Cu <0.20%;

- Al <0.04%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

4. - Method according to claim 2, characterized in that said piece has the composition, in% by weight, after densification:

- C <0.25%;

Mn = 0.5-100%;

- P <0.025%;

- S <0.025%;

- If <0.4%; Ni = 0.4-100%;

Cr <0.25%;

Mo = 0.45-0.6%;

- V <0.05%;

- Nb <0.01%;

Cu <0.2%;

- Ca <0.015%;

- B <0.003%;

Ti <0.015%;

- AI <0.025%;

- O≤ 50 ppm, preferably ≤ 20 ppm;

- N <50 ppm, preferably ≤ 25 ppm;

the rest being iron and impurities resulting from the elaboration.

5. - Method according to one of claims 1 to 4, characterized in that said getter (6) is a material selected from titanium, zirconium, hafnium and their alloys, and a stainless steel.

6. - Method according to one of claims 1 to 5, characterized in that:

the getter (6) is made of titanium or titanium alloy;

- The temperature of the powder (7) during sintering is between 950 and Ι ΟΘδ'Ό, preferably between 1000 and 1065 ^< €.

7. - Method according to one of claims 1 to 6, characterized in that the sintering and densification by hot isostatic compaction of the powder (7) are carried out successively, without intermediate cooling of the powder (7).

8. - Method according to one of claims 1 to 7, characterized in that, after placing the powder (7) in the space (4) defined by the walls of the container, it is subjected to cold isostatic compaction at a maximum temperature of 300 ° C and a pressure of 100 to 300 bar.

9. - Process according to claim 8, characterized in that said cold isostatic compaction provides a decrease in the volume of the powder of 1 to 3%.

10. - Method according to one of claims 1 to 9, characterized in that the wall of the container (1) in contact with the powder (7) is made of the material constituting the getter (6).

1 1. - Method according to one of claims 1 to 9, characterized in that the getter (6) is a coating of the container wall (1).

12. - Method according to one of claims 1 to 9, characterized in that the getter (6) is a separate piece placed in the vicinity of the wall of the container (1) in contact with the powder (7).

13. - Steel part, characterized in that it was obtained by the process according to one of claims 1 to 12, and in that its oxygen content is ≤ 50 ppm, preferably ≤ 20 ppm, its content. in nitrogen is ≤ 50 ppm, preferably ≤ 25 ppm, and its cumulative oxygen + nitrogen content is ≤ 80 ppm, preferably ≤ 50 ppm.