EP0087634B1 - Centrifugally cast tube made from spheroidal graphite cast iron, and process for the manufacture thereof - Google Patents

Abstract

The cast tube (T) is subjected internally in a chill-mould (1) to the uniform spraying of water from 1000 DEG C. approximately to 350 DEG C. approximately. Then it is extracted from the chill-mould and in a furnace is subjected to isothermal bainitisation maintenance, after which it is cooled in the atmosphere to ambient temperature. Without it being necessary to add expensive chilling elements, one thus obtains lightened tubes having very good mechanical properties and the ovalisation of which remains acceptable.

Description

The present invention relates to the manufacture of spheroidal graphite cast iron tubes by centrifugal casting and more particularly to a heat treatment subsequent to centrifugal casting aimed at providing the centrifuged tube with a structure allowing its lightening.

The tubes - that is to say the cylindrical pipes of constant thickness - made of spheroidal graphite cast iron currently have, after casting by centrifugation and heat treatment, a ferritic structure which has two advantages: on the one hand, this structure gives them good mechanical characteristics (elastic resistance and ductility); on the other hand, this ferritic structure is easily obtained by heat treatment after centrifugal casting, either in a shell provided internally with a thick coating of a pulverulent mixture of silica and bentonite suspended in water (coating called " wet-spray ”), or in a shell without such a coating.

In the case of the presence of a coating of “wet-spray” on the shell, the tube, extracted from its shell and quickly introduced into an oven before it is too strongly cooled, is subjected to a so-called heat treatment. of "maintaining ferritization" at a temperature of the order of 750 ° C., for a time of the order of 20 to 25 minutes, then it is allowed to cool naturally.

In the absence of a “wet-spray” coating on the shell, the pipe is extracted from its casting shell and it is quickly introduced into an oven where it is subjected to graphitization annealing at a temperature of l 'order of 950 ° C for a time of the order of 20 to 25 minutes, then maintaining ferritization at a temperature of the order of 750 ° C for a time of the order of 15 to 20 minutes.

The Applicant has posed the problem of economically obtaining cast iron tubes poured by centrifugation which are lighter than current tubes, without appreciable loss of mechanical characteristics.

The Applicant sought to obtain this result by conferring on the spheroidal graphite cast iron tube, instead of the usual ferritic structure, a bainitic structure, which has a tensile strength and an elongation characteristic as well as a resilience characteristic. equal to or greater than that of the ferritic structure.

The ferritic structure is obtained in spheroidal graphite cast iron pipes according to patent FR 2 337 765 by centrifuging a cast iron containing, in particular by weight, 3.4% of carbon, 2.6% of silicon, 0.4% of manganese. and 0.04% magnesium, by allowing the centrifuged pipe to cool in its centrifugation mold to a temperature above 800 ° C, by transporting the centrifugation mold containing the still red pipe to a processing station. annealing where the centrifugation mold, driven in rotation, is closed at its ends and where the molded pipe is subjected to the action of hot gases in three temperature zones, successively decreasing up to about 700 ° C., so that the hose cools slowly, and finally unmold the hose and allow it to cool in the open air.

The bainitic structure of spheroidal cast iron has already been sought for cast iron parts cast in a shell, in particular for mechanical components of automobiles, as shown for example by patent FR 1 056 330, because of the good mechanical characteristics conferred by such a structure.

In an article in the review “Hommes et Fonderie” n ° 84 of April 1978, a thermal treatment for obtaining this bainitic structure is described. The heat treatment described is that called "stepped quenching" which allows to reach the bainitic structure through austenitization by successive cooling phases at different speeds, including quenching, starting from the hot room, such that it has just been molded. This treatment has the advantage of not requiring an initial austenitization heating.

However, according to the technique described in this article, given the poor quenchability of spheroidal graphite cast iron, not only is there a very tight control of the carbon, silicon and manganese contents of the cast iron, but also, if relatively thick parts are to be treated, it is necessary to supply elements of expensive alloys such as molybdenum, which are particularly effective, even in modest quantities, in order to increase the quenchability of the cast iron to an extent sufficient to that the stepped quenching prevents the formation of perdite and results in the formation of bainite.

The Applicant has on the contrary posed the problem of obtaining centrifuged tubes in bainitic cast iron with spheroidal graphite without the addition of expensive special elements in small quantities, such as molybdenum.

cette fonte ayant une structure bainitique.To this end, the subject of the invention is a centrifuged tube made of spheroidal graphite cast iron, of the type whose cast iron has the following composition by weight:

this cast iron having a bainitic structure.

To make such a tube, according to the invention, we start, as is known, from a spheroidal graphite cast iron having the indicated composition, this cast iron is poured into a centrifuge shell provided with a refractory coating and cooled externally by l water, the centrifuged tube is allowed to cool in the shell to a temperature of the order of 800 to 1000 ° C to acquire an austenitic structure, then, according to the invention still in the shell, it is cooled vigorously and uniformly over its entire length by spraying water or an air and water mixture on its internal wall, up to about 250 to 450 ° C, so as to give it an austenitic or bainitic structure, then the tube is removed from the shell and placed inside an oven maintained between 250 and 450 ° C in order to create or to maintain a bainitic structure, and the tube is removed from the oven to allow it to cool in air.

Experience shows that the tube according to the invention has a substantially reduced unit weight and a significantly increased operating pressure, at the cost of a higher ovalization under the actual weight of the tube, but remaining within acceptable limits.

• la Fig. 1 est une vue schématique partielle en coupe longitudinale d'une machine à centrifuger des tubes en fonte, équipée d'un dispositif d'arrosage pour la mise en oeuvre du procédé suivant l'invention, la machine étant en position de fin de coulée;
• la Fig. 2 est une vue analogue à la Fig. 1 de la machine pendant la phase d'arrosage du tube en coquille du procédé suivant l'invention;
• la Fig. 3 est une vue en coupe transversale suivant la ligne 3-3 de la Fig. 2;
• la Fig. 4 est une vue schématique en coupe transversale illustrant la phase de maintien de bainitisation, à l'intérieurd'un four, du procédé de l'invention;
• les Fig. 5 et 6 sont des diagrammes comparatifs du traitement thermique du procédé de l'invention (courbes en trait plein) par rapport à des traitements thermiques antérieurs connus, respectivement pour l'obtention d'une structure bainitique avec chauffage d'austénitisation et pour l'obtention d'une structure ferritique dans la fabrication classique des tubes en fonte centrifugés, ces courbes correspondant à des tubes de diamètre nominal 1600 mm;
• Les Fig. 7 et 8 sont des micrographies d'une structure de paroi de tubes centrifugés en fonte à graphite sphéroïdal, respectivement à structure bainitique au grossissement 1000, et à structure ferrito-perlitique au grossissement 100.

The invention is set out below in more detail with the aid of the appended drawings, which show only one embodiment. In these drawings:

• Fig. 1 is a partial schematic view in longitudinal section of a machine for centrifuging cast iron tubes, equipped with a sprinkler device for implementing the method according to the invention, the machine being in the end of casting position;
• Fig. 2 is a view similar to FIG. 1 of the machine during the sprinkling phase of the shell tube of the process according to the invention;
• Fig. 3 is a cross-sectional view along line 3-3 of FIG. 2;
• Fig. 4 is a schematic view in cross section illustrating the bainitization maintenance phase, inside a furnace, of the method of the invention;
• Figs. 5 and 6 are comparative diagrams of the heat treatment of the process of the invention (solid lines curves) with respect to known prior heat treatments, respectively for obtaining a bainitic structure with austenitization heating and for obtaining a ferritic structure in the conventional manufacture of centrifuged cast iron tubes, these curves corresponding to tubes of nominal diameter 1600 mm;
• Figs. 7 and 8 are micrographs of a wall structure of centrifugal tubes in spheroidal graphite cast iron, respectively with bainitic structure at magnification 1000, and ferrito-perlitic structure at magnification 100.

According to the example of execution of FIGS. 1 to 3, the invention is applied to the manufacture of spheroidal graphite cast iron tubes by centrifugal casting.

The process according to the invention consists in starting from a composition of spheroidal graphite cast iron which is as follows, in percentage by weight:

This composition of cast iron was modified compared to that which is usually used for the manufacture of pipe in cast iron with spheroidal graphite with ferrito-perlitic structure compared to elements Ni and Cu which did not exist and by contribution, preferably, of a notable supplement of Mn, the usual basic cast iron containing only 0.1 to 0.2% thereof. The elements Ni, Cu, Mn have the property of improving the hardenability of the cast iron.

This spheroidal graphite cast iron composition is poured by centrifugation into a centrifuge machine shown diagrammatically in FIGS. 1 to 3.

This machine essentially comprises a carriage A movable in translation by means of a jack B. This carriage A carries a metal shell 1 for centrifugation, of axis XX approximately horizontal, by means of rollers C of which at least one is driven by a motor M. The shell 1 offers a cylindrical molding cavity of the same diameter from one end to the other in order to obtain a tube T of constant diameter and wall thickness over its entire length, therefore without interlocking. Lu tube T has for example a length of 6 to 8 m for an internal diameter which can range from 60 mm to 2000 mm depending on the centrifuge machine and the shell 1 used.

The machine is provided, as known, with an external cooling device for the shell 1. It can be water spraying ramps distributed around the shell 1, inside a casing or a body wrapping this shell, or a water envelope flowing from one end to the other of the shell, and outside of the latter, in a closed circuit. For the purpose of diagramming, the external cooling device for the shell 1, whatever it may be, being known per se, has not been shown.

Furthermore, given that the invention applies preferably, but not exclusively, to the manufacture of cast iron tubes of large diameters, that is to say diameters greater than 700 mm and up to 2000 mm, there is shown next to the machine, to the right of FIG. 1, a human silhouette S to clearly show the large diameter of the shell 1 into which the tube T is to be cast.

Although, for the reasons explained below, it is for large diameters that the method of the invention is most advantageous, this method is also applicable to the manufacture of cast iron tubes of small and medium diameters, it is that is, diameters between about 50 and 600 mm.

Inside the shell 1 can penetrate approximately parallel to its axis X-X a pouring channel E provided upstream with a weir G supplied with liquid iron by a tilting pocket H.

The whole of the channel E and of its overflow G is mounted cantilevered on a carriage 2 movable transversely relative to the axis XX, that is to say in an end direction relative to the plane of FIG . 1. The transverse carriage 2 also carries a long rigid pipe or cantilever 3 for spraying water connected to a pressure water supply, not shown. The rigid pipe 3 has a length corresponding to that of the channel E, therefore of the shell 1, and is also approximately parallel to the axis XX of the shell 1. It is mounted on the transverse carriage 2 with an offset with respect to to the channel E by a transverse distance such that by transverse displacement of the carriage 2, when the channel E is inside the shell 1, the rigid pipe 3 is outside and vice versa.

The rigid pipe or ramp 3 is provided over its entire length with pairs of twin nozzles 4 for spraying water. Nozzles 4, op nozzles placed two by two, have adjustable sections and adjusted so as to each provide an appropriate flow of water according to the thickness of the tube, which is substantially constant above all the length of the tube T. The means of adjusting the sections of the nozzles nozzles 4, known per se, are not shown.

The shell 1 is provided, before each casting, with a refractory coating called "wet-spray", that is to say a mixture of silica powder and bentonite suspended in water. This coating has for example a thickness between 0.05 and 0.8 mm. The constituents of this coating mixture are in the following proportions: 500 to 3000 g of silica powder with a particle size between 40 and 100 microns and 10 to 40 g of bentonite per liter of water. The spraying members of this coating, known per se, have not been shown.

It should be noted that in FIG. 1, or the channel E is partly located inside the shell 1, part of the pipe 3 with nozzles 4 is not visible since this pipe is retracted laterally. Consider Fig. 2 to see the pipe 3 with all of its nozzles 4 introduced inside the shell 1 in the watering position; the pouring channel E is then in the laterally retracted position, in front of the plane of FIG. 2, and has been shown only partially, for the sake of clarity. This appears clearly in FIG. 3.

With the help of this installation, a tube T is poured by centrifugation by introducing the casting channel E into the shell and then pouring cast iron through this channel while gradually extracting it from the shell. In accordance with the invention, only a quantity of cast iron is poured into the centrifugation shell 1 capable of giving a centrifuged tube of thickness much smaller than the usual thickness, taking into account the diameter (see below the table of numerical values).

When the casting of the tube T is completed, the latter is subjected to the following heat treatment, which consists of a stepped quenching carried out partly inside the centrifuge shell 1 and partly in a holding oven, in view to obtain and maintain a bainitic structure while avoiding the formation of perlite.

In a first phase of this heat treatment (Fig. 5 and 6, curve entering full), the tube T is left inside the centrifugation shell 1 to make it undergo a bainitization quench, passing through austenitization, without heating , taking advantage of the calories from casting. We therefore start from a tube which has just been centrifuged and solidified and is still at a temperature of the order of 1150 ° C. (after passing from point a to point b on the curve in solid lines in FIGS. 5 and 6).

Because the shell 1 is cooled externally, and the tube T is allowed to rotate on itself, the latter cools slowly from a to b and from b to c, that is to say 1300 ° C to 11 50 ° C and from 1150 ° C to 1000 ° C almost homogeneously; around the point of the curve in solid lines in Figs. 5 and 6 and even below this point, for example up to 800 ° C., there is a slight difference in temperature between the internal wall and the external wall, less than 20 ° C. That is to say T at homogeneous temperature which is thus austenitized, that is to say with an austenitic structure at point c, without contribution of calories, but by the cooling which takes place after the pouring at l inside the shell 1.

From this homogeneous state in temperature and in an austenitic structure, the quenching or rapid cooling heat treatment is carried out inside the centrifugation shell using the spraying boom 3 and the spray nozzles 4, by spraying water or a mixture of air and water.

To this end, immediately after pouring, the pouring channel is retracted laterally by moving the carriage 2, the sprinkler boom 3 with nozzles 4 is completely inserted into the centrifuge shell 1, and sprinkling is carried out of the cavity of the tube T which has just been poured, while continuing to rotate the shell 1. Of course, the watering rate, theoretically constant all along the centrifuged tube, can be adjusted locally, if one notes local irregularities in temperature of the shell 1, although an attempt is made to make the external cooling thereof constant and uniform.

By doing so, the tube T cools homogeneously. This quenching phase is represented by the path c-d on the solid lines of Figs. 5 and 6. The temperature of the tube T thus drops in a few minutes from around 10006C (or less, for example 800 ° C) to around 350 ° C.

The spray water is vaporized inside the rotating pipe and evacuated appropriately by means not shown.

In fact, the end of quenching temperature is between 250 ° C and 450 ° C. In this temperature zone which is either a little above or a little below the value of 350 ° C written on the curves of Figs. 5 and 6, the tube T has sufficient rigidity to no longer risk ovalization outside the centrifugation shell. The tube also obtained by the quenching c-d a structure free of perlite. On the curves of Figs. 5 and 6, the region corresponding to the perlite is located to the right of this curve, at a certain distance from the section c-d.

The second phase of the heat treatment consists of maintaining the temperature to consolidate or fix the bainitic structure (maintenance of bainitization). For this, following the rapid cooling or quenching phase above, the tube T is extracted from the centrifugation shell, either by stopping the rotation of the latter, or by continuing to rotate it during the extraction, according to the extractor device available. As shown in Fig. 4, the demolished tube T is introduced into a tunnel oven 5 with heating nozzles 6, of known type, adjusted so as to maintain the pipe at a constant temperature between 250 ° C and 450 ° C, for example at 350 ° C, for 5 to 120 minutes (section of the quenching turbine of FIGS. 5 and 6), this holding time being approximately the same for all the diameters of tubes, to within 10 minutes.

The temperature retention time aims to obtain a homogeneous bainitic structure offering the optimal mechanical characteristics indicated below.

The tube T is carried in the furnace 5 by a conveyor chain 7 which can be of a type ensuring simultaneously the rotation of the tube around its axis.

The last phase of the heat treatment consists of rapid cooling in the open air: at the end of the bainitisation holding time, the tube T is removed from the holding oven 5 and allowed to cool in the open air according to the section ef of the curves in solid line of FIGS. 5 and 6, which causes rapid cooling, approximately in about ten minutes, approximately to room temperature. The set of sections c-d-e-f of the cooling curve in solid lines represents the stepped quenching of the tube.

Figs. 5 and 6 illustrate the advantages of the heat treatment according to the invention, represented by the curves in solid lines, compared to the known prior treatments, represented by the curves in broken lines. We see a significant saving of time, but it is not the only advantage.

As shown by the dashed line in FIG. 5, the conventional treatment for obtaining a bainitic structure of a statically molded part (which is therefore not a centrifuged tube) comprises a section hjk-1 simular to the section cdef of the process of the invention, but offset in the time of approximately one to two hours due to the two preliminary phases 0-g of austenitization heating, which can last from 20 minutes to 2 hours depending on the applications, and gh of maintaining austenitization at a temperature of approximately 1000 ° C, more generally between 800 and 1000 ° C. The known prior treatment therefore requires a supply of calories to bring the treated parts to the austenitization temperature instead of treating the parts in the mold, immediately after their casting. It is therefore clear that the process of the invention, by saving the austenitization heating, brings a significant energy saving compared to such a treatment.

In Fig. 6, the heat treatment of the invention is compared with the prior technique of heat treatment of ferritization (annealing). The prior heat treatment (broken line curve) has in common the section a-b-c with the solid line curve of the invention. Then, the rest of the curve c-m-n-p-q is substantially different from the curve c-d-e-f of the process of the invention. In the ferritization process, the tube is left inside its centrifugation shell along the abcm curve: this corresponds to cooling at moderate speed, due to the external cooling of the centrifugation shell and the natural internal cooling of the tube centrifugal. From a to c, the austenitic structure is formed. Beyond c- this structure is not maintained, but the cooling continues until m, point where one proceeds to the extraction out of its shell of the sufficiently cooled tube to avoid a notable ovalization. This results in slightly slower air cooling until the tube is introduced into a ferritization annealing oven at a temperature of the order of 750 ° C. As can be seen, a supply of calories is necessary, inside the annealing furnace, for obtaining the ferritic structure, along the ascending part min of the curve, as well as for maintaining the temperature along the section n : 2 .. This intake of calories is appreciably greater than that which is necessary for maintaining bainitization along the section of the curve in solid lines, in the holding oven 5, and this all the more so as the holding temperature of bainitization is much lower (around 350 ° C) than the ferritization holding temperature (around 750 ° C). We note in particular that the bainitisation holding temperature is low enough that the extraction of the tube at this temperature poses no problem and that it is not necessary to reheat this tube during its introduction into the oven 5. In Consequently, compared with the prior art of heat treatment of ferritization of centrifuged cast iron tubes, the method of the invention also allows a significant saving of energy.

Due to the rotation of the tube while it is still inside the centrifugation shell, that is to say during the heat treatment phases represented by the sections a-b-c-d of the solid lines of FIGS. 5 and 6, therefore during natural cooling and during water quenching, the cooling of the pipe is homogeneous.

The bainitic structure makes it possible to reduce the wall thickness, and therefore the unit weight, of the tubes thanks to its good mechanical properties. This appreciable reduction in thickness is also advantageous for the homogeneity of the cooling during the abcd phases, and in particular for the quenchability: it ensures the effectiveness of this quenching according to the phase cd of the heat treatment curve. , through the entire thickness of the centrifuged tube, without the need to add to the composition of the cast iron expensive metallic elements having a quenching effect, that is to say facilitating quenching, such as molybdenum . In other words, the significant reduction in thickness of the centrifuged cast iron tubes brings a significant saving on the composition of the cast iron.

We have also seen that the austenitization and bainitization treatment according to the bcd phases of the heat treatment curve of the tube T inside the centrifugation shell avoids any deformation of the tube, therefore any ovalization while it is still at high temperature: in fact, the centrifugation shell, serving as a support for the tube, maintains its perfectly cylindrical shape, and this despite the notable reduction in thickness which increases its tendency to ovalization. This tendency to ovalization would pose serious problems if the tube were extracted from the centrifuge shell at a higher temperature, for example above 500 ° C.

Carrying out the heat treatment according to the invention, and more particularly the phase of sprinkling or spraying water inside the cavity of the tube along the section cd, is particularly simple and economical compared to a conventional treatment of quenching in salt bath which would also require transport of the tube out of its shell while it is still hot and a manuten-. Immersion of the tube in a salt bath. The method of the invention saves this ma nutention and avoid at the same time the risk of ovalization it involves.

The significant time saving mentioned above makes it possible to increase the production rates of centrifugal tubes in spheroidal graphite cast iron with bainitic structure. Watering the inside of the centrifuged pipe during the quenching phase reduces the residence time of the centrifuged tube in the centrifuge shell. This is what we see by comparing the two curves in FIG. 6, on which the extraction of the tube from the shell is located at point m in the known technique while it is located at point d, 5 to 10 minutes before, in the method of the invention.

Advantageously, this results in a significant reduction in thermal stresses for the centrifugation shell since the calories to be removed are approximately 30 to 40% lower, compared with the prior art of manufacturing ferritic-structure cast iron tubes, due to the 'evacuation of heat by the sprinkling water and the decrease in the quantity of cast iron poured inside the shell. As a result, the life of the centrifuge shells, which are the most important and expensive components of the centrifuge equipment, must be significantly extended compared to the prior art.

Finally, the tube of the invention centrifuged in spheroidal graphite cast iron with a bainitic structure, despite its appreciable reduction in thickness, which provides lightening facilitating its handling, retains mechanical characteristics substantially equivalent to those of anterior ferritic tubes at the cost of greater sensitivity to ovalization, sensitivity remaining tolerable however because the tube does not undergo any handling when it is at high temperature subject to ovalization.

• e = épaisseur en mm de la paroi de tube
• DN = diamètre nominal.
  

With regard to the mechanical characteristics of the tube according to the invention, the table below gives numerical examples of dimensions, weight, guaranteed working pressure and ovalization for tubes intended to be buried under a thickness of earth of 4 m and for large diameter pipes, that is to say greater than 700 mm nominal diameter. The values relating to the bainitic tube of the invention are compared to those of the prior art relating to a ferritic tube and a lightened ferritic tube. In this table, the coefficient K, defined by international standard ISO 2531, characterizes the thickness of the pipe according to the formula: e = K (0.5 + 0.001 DN) in which:

• e = thickness in mm of the tube wall
• DN = nominal diameter.
  

It can be seen in the table above that the gain in unit weight that the invention makes it possible to obtain is greater the greater the diameter of the tube.

• - limite élastique 55 à 75 daN/mm² (au lieu de 30 environ pour la structure ferritique)
• - allongement supérieur à 10% (comme le tube ferritique);
• - résistance à la rupture 70 à 110 daN/mm² (tube ferritique 45 daN/mm² environ).

By way of comparison and to the advantage of the bainitic structure tube of the invention, the mechanical characteristics obtained are the following:

• - elastic limit 55 to 75 daN / mm ² (instead of around 30 for the ferritic structure)
• - elongation greater than 10% (like the ferritic tube);
• - breaking strength 70 to 110 daN / mm ² (ferritic tube 45 daN / mm ² approximately).

Fig. 7 represents a bainitic micrographic structure. In this structure, the black areas that can be seen in the upper and lower left corners are parts of graphite nodules. The elongated forms resembling ferns are zones of ferrite; we can see that they cover most of the surface of the micrograph. The most important white areas correspond to residual austenite; we see that they cover only a small part of the surface of the micrograph. It is the whole of this structure, recognizable only at magnification 1000 and not at magnification 100, which is called "bainitic".

For comparison, according to the micrograph in FIG. 8 at 100 magnification, on NITAL attack, it is a ferritic-pearlitic spheroidal graphite cast iron, with 40% ferrite and 50% perlite, the rest being spheroidal graphite. The round black areas are graphite nodules. The nodules are surrounded by white areas constituting the ferrite. The remaining gray areas are perlite. It is the structure of a conventional type centrifuged tube.

Claims (6)

1. Tube centrifugally cast from spheroidal graphite cast iron, of the type in which the cast iron has the following composition by weight:

this cast iron having a bainitic structure.

2. Tube according to claim 1, characterised in that the cast iron has the following composition:

the remainder being iron.

3. Tube according to one of claims 1 or 2, characterised in that it corresponds to the following table:

4. Method for the manufacture of a tube according to one of claims 1 to 3, of the type in which one starts with a spheroidal graphite cast iron having the composition stated, this cast iron is poured into a centrifugal chill mould provided with a refractory coating and cooled externally by water and of the type in which the centrifugally cast tube is left to cool in the chill mould to a temperature of the order of 800 to 1000°C in order to acquire an austenitic structure, characterized in that, still in the chill mould, it is cooled vigorously and uniformiy over its entire length by spraying water or a mixture of air and water on its inner wall, to a temperature of approximately 250 to 450°C, in order to give it an austenitic or bainitic structure, then the tube is removed from its chill mould and placed inside a furnace kept at a temperature of between 250 and 450°C with a view to creating or maintaining a bainitic structure and the tube is removed from the furnace to allow it to cool in air.

5. Method according to claim 4, craracterised in that an aqueous mixture of silica and bentonite is used as the refraciery coating for the centrifugal casting chill mould.

6. Method according to claim 4, characterised in that during the first stage of cooling to approximately 800 to 1000°C and during the stage of vigorous cooling by wet spraying on the inner wall of the tube from 800 - -1000°C to 250 - 450°C, the tube is set in rotation by means of the centrifugal casting chill mould.

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