US3519497A - Method for the thermal treatment of steel rails - Google Patents

Description

July 7, 1970 J. POMEY I 3,519,497

METHOD FOR THE THERMAL TREATMENT OF STEEL RAILS Filed April 28, 1966 3 she e ts sh eet 1 J ce'uis Ponev Arron/ens METHOD FOR THE THERMAL TREATMENT OF STEEL RAILS J. POMEY July 7, 1970 s sheets-s eet 2 Filed April 28, 1966 INVENTOR: J}! co dEs POMEY Arromvs 7S Jul 7, 1970 J. POMEY 3,519,491

METHOD FOR THE THERMAL TREATMENT OF STEEL RAILS Filed April 28, 1966 3'Sheet s- S heet 5 .ZNvEN'roR: Jkcquss PaMEY V ATTORNEY:

United States Patent US. Cl. 14814 2 Claims ABSTRACT OF THE DISCLOSURE Hot steel rail taken directly from rolling mill and completely immersed in a fluidized bed of refractory powder. Rail arranged in bed with its head on the bottom, its flange on top, and the flat face of the flange horizontal. Temperature of bed maintained between 560 and 620 C. To produce isothermal perlitic transformation, or between 380 and 460 C. to produce isothermal bainitic transformation. Rails may contain 0.4 to 1.0% C, 0.5 to 2.5% Mn, 0.02 to 1.8% Si, up to 1.5% Cr, up to 0.5% Mo, up to 0.4% V, and up to 0.25% Nb.

The present invention relates to a method and an installation intended to improve generally the properties of steel rails.

Attempts have already been made to obtain this result by a judicious choice of the chemical composition of the steel employed in the manufacture of the rails, and in applying heat treatment to the rails at the output side of the rolling mill.

Thus, in certain previous installations, the head of the rail passing continuously out of the rolling mill, was sprinkled with a moderate quantity of water; after this watering zone, the surface layers were re-heated by the heat diffused throughout the body of the head, which remained hot. By this means, it was hoped to obtain a sorbitic temper and to produce better properties, but because the non-isothermism was very considerable, the risk of shrinkage cracks was also very great, and the structure was ill-defined and heterogenous, so that this method was abandoned.

Trials were then made using rails of harder steel, for example by increasing the contents of carbon and manganese in the steel. However, in the absence of special precautions during cooling after rolling, the steel showed an ill-defined micrographic structure. In fact, in the regions which were richest in carbon and manganese, the rate of cooling was too high to permit the completion of the perlitic conversion, and this continued at a lower temperature, given a little bainite and a little martensite. This resulted therefore in two inherent defects, namely very great fragility and the presence of in ternal cracks and flakes. Thus, it became necessary to carry out a very slow cooling of the rails at the outlet of the rolling mill.

However, even when completed in this way, the method still has numerous disadvantages. In fact, taking account of the high production of rolling mills, it is desirable to have available cooling pits of very large capacity, which are bulky and expensive. Furthermore, the operation of loading the rails into the pits is accompanied by deformation of the rails due to the handling necessary, and results in irregular cooling, so that it becomes necessary to straighten the rails. Finally, it is found that the inherent quality of the steel is very dispersed, so that it is in fact necessary to restrict the content of carbon and manganese in the steel, which does not permit its life to be increased as much as would be desirable.

The method in accordance with the invention enables ice k in particular all the defects of the previous techniques to be remedied.

To this end, according to the invention, while the rail is still hot at the outlet of the rolling mill, it is immersed in a cooling medium maintained at constant temperature, so that the steel undergoes a complete isothermic transformation, perlitic or preferably 'bainitic.

In accordance with a preferred form of embodiment, the cooling medium is a fluidized bed of refractory pow der maintained at a constant temperature, and the rail is introduced into this bed in such manner that the head of the rail is at the bottom, the flange being uppermost and the flat face of this latter being horizontal, so that the rising flow which ensures the fiuidization of the bed comes first into contact with the head of the rail.

It should be noted that it is known to utilize fluidized powdered beds in carrying out the tempering of metallic parts in stages, taking advantage of the very great aptitude of fluidized powdered beds for the transmission of heat. In addition, this technique makes it possible to operate at any chosen temperature, which is distributed in a very uniform manner throughout the whole of the bed. Thus, the part which was previously heated to a high temperature is cooled very rapidly in the fluidized powder bed which constitutes a tempering medium. This cooling is furthermore limited to the temperature to which the medium was previously heated, so that the part is rapidly brought up to this temperature throughout its whole mass, and is maintained at that temperature.

According to the invention, on the contrary, there are utilized simultaneously two contradictory properties of powdered media, by virtue of the shape of the rail and the orientation of the rail in the powdered medium. In fact, in powdered media at rest and non-fluidized, heat is transmitted very badly and the media thus behave in the same way as heat-insulating media. In consequence, a part immersed while hot in a non-fluidized powdered medium of refractory materials cools extremely slowly. This property is contrary to that of fluidized powder beds, although it is actually obtained with the same medium. Now, according to the invention, the surface of the rail flange, which will constitute the lower surface when in use on railway tracks, becomes the horizontal upper surface during the course of the treatment, due to the fact that the rail is turned through at the moment of its immersion while hot in the fluidized powder bed.

The result is that the powder bed which covers this face becomes a stagnant zone. In consequence, the powdered medium at rest which covers what is now the upper surface of the flange, has become a heat-insulating medium which tends to prevent the cooling from this surface.

Another special property of the profile of the rail is that, while the flange represents approximately the same mass as the head of the rail, its surface area is substantially double. Since, with the method according to the invention, the whole surface which has become the upper horizontal face is protected against cooling, the surface area which continues to be cooled is of the same order for the flange and for the head. Since the masses are of the same order of magnitude, the average rate of cooling is substantially the same for the flange and for the head, contrary to that which occurs in natural cooling in air or in a liquid (in which the flange would be cooled approximately twice as fast as the head). The result of the equality of rates of cooling with the method accord ing to the invention is that the rail remains straight during cooling, and that in consequence it is free from internal stresses.

Finally, the method in accordance with the invention utilizes in some places the good calorific conductivity of fluidized powder media, and at other places the good heat insulation of powder media in a state of rest, this combination of effects being associated with the shape of the profile of the rail and with the orientation given to the said rail. The uniformity of the rate of cooling is furthermore simultaneously related to the ratio of the surface areas and to the ratio of the masses of the flange and the head of the rail.

The temperature of the cooling medium and the time for which the rails remain in this medium will of course depend on the composition of the steel and on the temperature of the rails as they pass out of the rolling mill.

In the case of isothermal perlitic transformation, the temperature could be chosen, in accordance with the foregoing considerations, between 560 and 620 C., with an immersion time corresponding to a temperature-level period of 200 to 800 seconds, the hardness being comprised between 23 and 43 Rockwell Cone, depending on the composition and the treatment.

In the case of isothermal bainitic transformation, which is the process preferred, the temperature may be chosen between 380 and 460 C., with an immersion time corresponding to a temperature-level period of 300 to 900 seconds, the Rockwell Cone hardness being comprised between 34 and 44, according to the composition and the treatment.

This latter isothermal transformation provides additional advantages, since it makes it possible to obtain increased hardness, a higher resistance to wear, and a higher limit of endurance to alternating stresses. Furthermore, the more moderate temperature of the fluidized bed and of the apparatus gives greater economy and a greater degree of strength. Finally, the structure and the characteristics of the steel are more uniform. The lower dispersion of this structure and of its characteristics results from a reduced effect of variations in composition, either between the head and the foot of a billet or from one pouring to another.

The invention also relates to the rails obtained by the above method, and which have advantageous characteristics as compared with rails manufactured by previous techniques. These characteristics may be considered from various aspects, namely the chemical composition, the structure, the mechanical properties, the external appearance and the properties of use. In fact, the product according to the invention is always distinguished from previous rail steels by at least one of the properties specified above, and in the majority of cases by a number of these simultaneously.

In the standard method of manufacture with natural cooling at the outlet of the rolling mill, either in free air on the laying grid, or in a pit, the cooling at high temperatures would, in the case of a hyper-eutectoid steel be slow enough to permit the cellular separation of the pro-eutectoid carbide (cementite), which is a cause of fragility. Taking account of the major segregation of the billet which increases the carbon content in the central region, recourse is therefore had to a highly hypo-eutectoid overall composition.

In the method according to the invention on the con trary, it is possible to use a steel which is richer in carbon on the average. In fact, the rapid cooling at the highest temperatures prevents the separation of the proeutec toid carbide, which remains very finely divided in an intra-cellular manner, which increases resistance to wear without risk of fragility. From this fact alone, the composition, the structure, the mechanical characteristics and the properties of use are different and improved.

With the standard methods, the content of the addition elements Mn, Ni, Cr, M0, is extremely limited and these elements can only be added if the carbon content with respect to eutectoid carbon steel is reduced in a corresponding manner, in order to prevent the appearance of the constituents of temper: bainite and martensite in a mixture, during the course of natural cooling on,the grid. In fact, martensite is a cause of fragility and increases 4 the risk of formation of flakes. If, on the contrary, the cooling takes place slowly in a pit, the alloy elements have no longer a favourable effect.

On the other hand, when the cooling of the rails at the outlet of the rolling mill is carried out according to the method of the invention, the rails can take advantage of the full and favourable effect of the addition elements. In consequence, the rails according to the invention can contain 0.4 to 1% C, 0.5 to 2.5 Mn, 0.02 to 1.8% Si, and they may also contain 0 to 1.5% Cr, 0 to 0.5% Mo, 0 to 0.4% V, 0 to 0.25% Nb, without any resulting disadvantage, and obtaining the benefit of these additions.

According to a further particular feature of the rails according to the invention, the structure is uniform throughout the whole mass, and in consequence over the whole of any section, which is not the case with rails which are cooled naturally. With these latter, the centre of the head which is cooled more slowly has a structure in which the pro-eutectoid elements are definitely separate, and in wich the perlite is coarser, the steel being less hard at its heart than at its surface. In addition, in the surface layers and in the thinnest parts in which the cooling is more rapid, if the steel is fairly well charged with alloy elements, there is a very unfavourable formation of bainite and martensite.

Similarly, in the case of rails treated locally, the structure is naturally heterogenous, and in rails having undergone a tempering operation followed by annealing, the structure is sorbitic, different from bainite in its method of formation, and it is obtained by a more costly process. These products are therefore different from the products according to the invention, in which not only the structure and the hardness are uniform, but also the structure is constituted by pure lower bainite, without any trace of martensite, this bainite being an extremely fine and hard mixture of ferrite and carbides.

According to a further particular feature, during the course of the cooling control by the fluidized powder bed, the rail remains straight and is free from residual stresses and internal tensions, whereas in methods employing temper and annealing, the rail is deformed and, after straightening retains large internal stresses which have a detrimental effect.

A further characteristic feature of the product according to the invention is that the resistance to shock of the rails is greater than or at least equal to that of tempered and annealed rails, for the same carbon content and the same hardness.

In the case where the liquid steel has not been treated under vacuum in order to drive-off hydrogen, if the rail steel is highly alloyed, with the prior technique it exhibits flakes which result in temper shrinkage cracks after treatment, or which result in the formation of fissures during straightening. On the contrary, for the same chemical composition, the steel treated according to the invention does not contain either flakes or temper cracks or straightening fissures. In particular, when carrying out an examination by ultra-sonic tests, no continuity defects are revealed.

Finally, the characteristic mechanical properties of the product according to the invention, as compared with rails manufactured in accordance with the usual methods, can be summarized as follows:

For the same carbon content, the steel obtained is more resilient, harder, more enduring against alternating stresses and more resistant to wear;

For the same hardness, the steel obtained is more resilient, has greater endurance to alternating stresses and is more resistant to wear;

For the same method of operation at the steel works, that is to say for the same content of inclusions and the same hydrogen content, the rails obtained are sounder when examined by ultra-sonic tests, have a better resistance to shock and a better resistance to alternating stresses.

By applying solely the treatment according to the invention, it is possible to obtain sound rails of eutectoid steel with a fine lamellar perlitic structure having a tensile breaking load comprised for example between 90 and 110 hectobars, or rails of steel having a pure bainitic structure showing a tensile breaking load comprised for example between 120 and 140 hectobars.

In addition, all other things being equal, the steels treated in accordance with the invention, have generally speaking a better performance in service. Interruptions of service for oval flaws, upsetting of the head, scaling, fissuration, etc. are much less frequent and permit a longer period of service under more severe conditions of utilization.

The invention also extends to cover an installation for carrying the above method into effect.

An installation according to the invention will comprise more precisely handling means capable of lifting the rail at numerous points at the outlet of the rolling mill and of turning it on its own axis, preferably parallel to the rolling axis, so that the head of the rail is at the bottom, the flange being at the top with its flat face substantially horizontal, a receptacle fitted with a lower grating and containing the powdered refractory bed capable of being fluidized by a flow of air or of any other suitable gas discharged through the lower grating by means of a windbox, means for ensuring a constant temperature of the fluidized bed, and lifting means for extracting the rail from the fluidized bed so as to bring it into a subsequent cooling area.

The means for ensuring a constant temperature of the bed may comprise heating means and cooling means, a temperature regulator actuated by a thermometer probe and a servo-mechanism acting on the said heating and cooling means.

The heating means may utilize the Joule effect, combustion, etc., but a re-heating can also be effected by means of the fluidization gas itself. With regard to the cooling means, these may work by air circulation, by means of mist or water, etc., an injection of atomized water into the fluidization gas being also capable of use for this purpose.

An installation of this kind having a small overall size can be interposed on the cooling area of a high production rolling mill. In any case, for the same rate of production, it necessitates less room than cooling pits. It may of course be wholly mechanized and made automatic by bringing into play conventional techniques of handling, automation and regulation.

The description which follows below with reference to the accompanying drawings given by way of example only, will make it quite clear how the invention may be carried into effect. In these drawings:

FIG. 1 is a diagrammatic view with cross-section of an installation according to the invention;

FIG. 2 shows the position of a rail in the fluidized powder bed;

FIG. 3 is an alternative form of FIG. 1, for the case in which the cooling time of the rail in an isothermal manner is greater than the production rate of the rolling mill;

FIG. 4 is substantially similar to FIG. 1, and shows in addition the combination of a fluidized bed ensuring rapid cooling and homogenization of the rail with a furnace ensuring the isothermal transformation and the maintenance of the rail at a constant temperature;

FIG. 5 is an alternative form of the device of FIG. 3, with two wind-boxes.

FIG. 1 is a transverse section of the installation, in which the development in length normal to the plane of the drawing corresponds to the length of the longest rails to be manufactured. As in the other figures, the mechanical handling devices have not been shown, since they may be of very varied types and their provision presents no difllculties for those skilled in the art.

There has been indicated at 1 a roller track located at the output side of a rolling mill. A hot straight rail A which is located on the track 1, or which has been slid laterally in order to be freed from the rolling mill, is gripped at a large number of points in order to avoid any deformation due to the effect of its weight and is lifted at B by lifting means. It is then turned over at C and brought above a fluidized bed at D. It is in this position, with the head of the rail downwards and the flat face of the flange arranged horizontally that the rail is introduced into the fluidized bed, in which it is immersed as indicated at E.

The fluidized refractory powder bed, prepared in a manner well known per se, is contained in a receptacle 2 which comprises at its lower portion a horizontal grating 3 designed in such manner that the grains of the bed cannot pass through the openings of the said grating. The receptacle 2 is extended below the grating by a wind-box 4, into which a fan 5 blows air (or any other suitable gas), the air or gas being cold or heated as may be necessary. The rate of flow of this gas is of course kept within the limits essential to ensure the fluidization of the medium.

In addition, means are provided to ensure constant temperature of the fluidized bed, these means comprising as required heating means and cooling means. For example, the heating means 6 may utilize the Joule effect or combustion, while the cooling means 7 may be constituted by a circulation of air, of mist or of water. In an alternative form, the heating may be ensured by the fluidization gas itself, brought-up to a suitable temperature and the cooling may be effected by injection of water sprayed into the fluidization gas.

In order that the temperature may be constant, there has been provided a temperature regulator (not shown), acting on the heating means and on the cooling means through the intermediary of a servo-mechanism, this regulator being in turn controlled by a thermometer probe 8 which can be of the expansion, contact electromotive force, or resistance type, etc.

The rail immersed at E is cooled and the steel is transformed isothermally, since the temperature of the medium is maintained constant. After cooling to the temperature of the medium, and after complete isothermal transformation of the steel, the rail is removed from the bath into the position shown at F. It can then be shifted laterally to G and laid down at H on the final cooling area 9.

When the rail occupies the position shown at E in FIG. 2, in the heart of the fluidized bed, it is found that the heat convection currents which result from the confused agitation, with inter-granular collisions of the powder in the fluidized medium, do not exist on the horizontal face of the flange, on which the powder is deposited in a compact manner and at rest, following the angle of repose, as indicated at e.

FIG. 3 completes FIG. 1 in the case where the period of immersion of the rails in the bed is longer than the rate of production of the rolling mill. For example, if the rolling mill produces one rail per minute and if the time of cooling is less than or equal to three minutes, the time of maintenance at temperature being ten minutes so as to ensure the transformation of the steel, or a total immersion time of thirteen minutes, it is necessary that there should be thirteen rails simultaneously immersed. If it is assumed that the flange has a width of 15 cm. and that the space necessary for the circulation of the fluidized bed is 25 cm., or a total of 40 cm. between centres of two adjacent rails, the width of the fluidized bath must be equal to 40 13=520 cm. In this case, the rail is immersed from the position D to the position E and then is displaced laterally in the powder bath from E to E. The rail is then removed from the bath from the position E to the position F, and is then moved sideways at G and laid down at H on a cooling grid 9,

on which the rails cool down side by side, up to the position I at which they are taken for despatch.

With a device of this kind, the heating and cooling elements may be immersed in the bath, either below the level of the rails if the lateral moving device is located above in free air, or above in the contrary case. These elements 10 may for example be arranged in tubes parallel to each other, located longitudinally or transversely with respect to the length of the tank. Additional cooling tubes may also be arranged lengthwise on the input side, below the position E of the rails and the adjacent positions, at which it is necessary to extract heat so as to cool the rails to the desired temperature, and not to provide cooling tubes in the remainder of the bath in which the rail is kept at a constant temperature and in which, on the contrary, the losses of heat must be compen-sated.

The rail may also be left in the fluidized bath for just the time necessary for its cooling to the desired temperature and the homogenization of the temperature, after which it is extracted from this medium and is passed into a complementary device at which it is maintained at a constant temperature for the time necessary to complete the isothermal transformation. An alternative arrangement of this kind is shown in FIG. 4, in which there is again seen, on the left-hand side, an installation similar to that of FIG. 1. This installation is followed, for example, by a pusher furnace 11 or a pusherstove, in which the rails travel up to the outlet position J, the rails then passing on to the grid 9 for final cooling in free air.

FIG. 5 shows an alternative form of FIG. 3, in which two wind-boxes 12 and 13 are provided instead of a single wind-box. The first box 12 is located on the side at which the hot rails arrive to be cooled rapidly, and it is supplied with air or cold gas delivered by a fan 14. The second wind-box 13 receives hot gases at the actual temperature of stabilization of the rails and of isothermal transformation of the steel. In addition, cooling tubes 15 may be provided, immersed in the fluidized bed on the incoming side of the rails.

The hot gases may be supplied to the front box 13 from any suitable source. For example, they may be obtained from a fan 16 and pass into a heating chamber 17, kept at the desired temperature by a burner 18, with mixture of combustion gases and regulation of the temperature by means of a pyrometer probe 19.

In all cases, it is seen that the controlled cooling device according to the invention, for a high production rolling mill, occupies relatively little space and that it can be arranged in a cooling bay between the roller track at the outlet of the rolling mill and the natural cooling grid in free air. On the other hand, the succession of the various operations can be carried out in a wholly automatic manner, without causing any special difficulty for persons skilled in the art.

It will of course be understood that the forms of embodiment described have especially been given by way of example only, and that modifications may be made thereto without thereby departing from the scope of the invention.

What I claim is:

1. A method of heat treatment for improving the properties of steel rails, comprising the steps of providing a fluidized refractory powder bed, maintaining the bed at a constant temperature between 560 and 620 C., taking a hot rail as it leaves a rolling mill and immersing the rail completely in the bed; the immersion time corresponding to a stationary-temperature period of 200 to 800 seconds, whereby a complete and uniform perlitic transformation is achieved, the rail being oriented within the bed with its head at the bottom, its flange at the top, and the flat face of the flange horizontal, so that the gas flowing upwardly to fluidize the bed first encounters the head of the rail, and operating the bed so that it stagnates directly above the horizontal flange face and the powder particles come to rest upon and cover said face to insulate it, whereby said rail top and rail flange cool at substantially the same rate despite the greater surface area of the flange.

2. A method of heat treatment for improving the properties of steel rails, comprising the steps of providing a fluidized refractory powder bed, maintaining the bed at a constant temperature between 380 and 460 C., taking a hot rail as it leaves a rolling mill and immersing the rail completely in the bed; the immersion time corresponding to a stationary-temperature period of 300 to 900 seconds, whereby a complete and uniform bainitic transformation is achieved, the rail being oriented within the bed with its head at. the bottom, its flange at the top, and the flat face of the flange horizontal, so that the gas flowing upwardly to fluidize the bed first encounters the head of the rail, and operating the bed so that it stagnates directly above the horizontal flange face and the powder particles come to rest upon and cover said face to insulate it, whereby said rail top and rail flange cool at substantially the same rate despite the greater surface area of the flange.

References Cited UNITED STATES PATENTS 368,132 8/1887 Coffin 266-6 X 879,634 2/1908 Hadfield 148--143 1,827,616 10/1931 Sandberg et al. 148143 2,116,070 5/ 1938 Hoffman et al 148-15 2,576,223 11/1951 Hoffmann l48l2.4 3,053,704 9/1962 Munday 148--13.1 X 3,197,346 7/1965 Munday l4813.1 X

CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 148--134

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