NO301262B1 - Procedure for middle welding of fine-perlite rails - Google Patents

Abstract

The method concerns insert-cast welding of fine-pearlitic rails by pouring aluminothermally produced steel into a mould surrounding both rail ends. It is characterised by the following facts: (a) after the joint has cooled down, the running track and adjoining regions are shock-heated from the top by means of a welding torch for a period of 50 to 150 seconds; (b) the torch is supplied with oxygen at least at 10000 litres/hr and with propane or acetylene at 3500 litres/hr; and (c) the torch distance (a) measured in mm is equal to 0.1-1 times the numerical value of the wt. in kg/m of the rail.

Description

The invention relates to a method for intermediate goods welding of finely pearlitized rails by filling in aluminothermally produced steel in a mold which surrounds the two rail ends.

The invention particularly relates to a method for intermediate goods welding of fine pearlitized rails, where the rail head areas adjacent to the weld joint on both sides have an increased rail strength, approximately that of fine pearlitized rails.

The wear and tear of a rail is determined by given traffic loads mainly by the rail strength or hardness. At present, naturally hard rails with a minimum tensile strength of 900 N/mm2 are generally used in fully welded rail tracks. The rail manufacturer uses the alloying elements carbon and manganese to achieve the rail strength. In the case of greatly increased stresses, as is the case for example on the outer rail in curves, it is also relevant to use the naturally hard, additionally with chrome and/or vanadium alloyed special qualities with a minimum tensile strength of 1100 N/mm2.

As an alternative to the naturally hard, alloyed special qualities, it is also possible to achieve firmness in connection with a heat treatment of the rails after rolling. Usually the heat treatment is limited to the running surface areas of the rails. Such rails have a chemical composition that roughly corresponds to that of rails with a minimum tensile strength of 900 N/mm<2>, and the pearlite structure will, as a result of the heat treatment, be very finely laminated, with associated high hardness/strength. The present invention relates to an improvement of the hardening process in aluminothermically provided welding points in the head area of this type of rail.

As a fusion welding method, the aluminothermic welding method will give a characteristic design of the welding area in the form of an intermediate area, consisting of aluminothermicly produced steel and dissolved rail steel, and which is centrally located in the originally existing welding space between the two rail ends, as well as a respective heat influence zone to the right and left of the weld seam.

Fig. IA shows these areas by means of a partially cut-through perspective view of a finely pearlitized (head-hardened) rail, purely schematically. An intermediate goods area 1 consists of aluminothermically produced steel and melted rail steel. Bounded by the melting lines 2, there follows on each side a heat-influenced zone 3. These merge into the heat-unaffected rail steel areas denoted by the reference numerals 4. The finely pearlitized rail head area is denoted by 5.

For wear resistance during use, this design is very important. The hardness and thus the firmness of the weld seam and thus also its resistance to wear can be influenced quite accurately at a given cooling rate by means of the chemical composition of the aluminothermic steel. The hardness in the intermediate goods area will thus not pose any problem in connection with the rail welding, with regard to the conditions of wear and tear. However, the conditions are completely different inside the two heat influence zones. Here it will be the chemical composition and the cooling rate that together determine the hardness distribution. Seen in the rail's longitudinal direction, the hardness inside the heat-affected zones decreases with increasing distance from the center of the weld seam in the rail's longitudinal direction X, until a minimum value at the border with the heat-unaffected rail steel is passed. This is where the so-called soft annealing zone is located. As the wear resistance decreases with decreasing hardness, during operation, i.e. load on the rails, you will have to expect increased wear and tear, particularly in the heat-affected zones.

The lower hardness in the heat-affected zones is due to metal-physical phenomena. In the largest part of the heat-influenced zone, which borders the intermediate goods area, austenitization of the rail steel takes place, while in the part of the heat-influenced zones further away from the weld seam, a maximum temperature in the rail of from 600 to 700°C is achieved. During the cooling of the weld, various grain structures will appear in the heat-influenced zones: at the melting line, i.e. in the transition intermediate goods area/-

heat influence zone, for example a coarse-grained, hard one

perlite;

at the transition heat-affected zone/heat-unaffected rail steel, i.e. at the end of the heat-affected zones, a shaped spherical, soft pearlite.

As the cooling rate of the aluminothermic welding is given in advance, in the same way as the chemical composition of the rails, depending on the rail steel quality, a specific grain structure will arise and thus a specific hardness distribution in the heat-affected zones, seen in the longitudinal direction of the rail.

When welding naturally hard rails, which are therefore not subjected to any heat treatment at the rail manufacturer after rolling, the hardness distribution within the heat-influenced zones will form depending on the rail steel's chemical composition. The alloying elements C, Mn, V, Cr and others influence the hardness level via the rail steel's conversion ratio and/or via carbide formation. The hardness of the heat-unaffected rail is likewise controlled only by these two mechanisms during cooling after rolling. When welding such naturally hard rails, regardless of the rail steel analysis, the same differential value will almost always occur between the hardness of the unaffected rail and the hardness progression in the heat-affected zones. This differential value can be tolerated in the case of such aluminothermic welding in terms of wear technology.

When it comes to a heat-treated, finely pearlitized rail, where the hardness level is "artificially" raised after rolling, a larger differential value for the hardnesses will arise, because the originally fine laminar pearlite structure is destroyed and the hardness distribution in the heat-affected zones after welding will then automatically occur in the same way as in a welded, naturally hard rail with a minimum tensile strength of 900 N/mm2. This is due to the identical chemical composition of the two rail steel qualities.

In the lower part (fig. 1B) of the schematic representation in fig. 1 shows the hardness course at the running surface, calculated in the rail's longitudinal direction X. The hardness course in a weld between fine pearlitized (head-hardened) rails is denoted by I, while II denotes the hardness course in a weld between naturally hard rails with a minimum tensile strength of 900 N/mm<2>. The running rafts on the finely pearlitized rails will be subjected to relatively stronger wear in the area of the heat-affected zones compared to the weld material and the unaffected rail than is the case in a weld between naturally hard rails. For this reason too, greater demands are placed on the welding technique for fine pearlitized rails.

In practice, there are two methods for aluminothermic intermediate goods welding that have proven to be particularly suitable. In this connection, it concerns the methods specified in DE-PS 22 08 692 and DE-PS 21 61 134.

The method according to DE-PS 22 08 692 is characterized by the combination of the following features: a) the width of the opening between the two rail ends is set to a value of 1 to 2 times the rail step thickness; b) the cavity in the mold is selected so that a width of 2 to 4 times and a depth of 0.15 to 0.6 times the rail step thickness appears for the weld bead; c) the width of the preheat burner flame band is set to 1.5 to 2.5 times the rail step thickness.

Such execution of the rail welding requires a preheating time for the rail ends that are surrounded by the mold halves of approx. 6-10 minutes.

The method according to DE-PS 21 61 134 is characterized in that the two rail ends to be welded together are preheated within a period of a maximum of 2 minutes to a temperature between at least 300°C and a maximum of 700°C, and that the weight of the aluminothermic mixture amounts to 0 .15 to 0.25 parts by weight of the rail's meter weight. This procedure is referred to as welding with short preheating (SkV).

The steep temperature gradient in the longitudinal direction of the rail associated with the short preheating in the SkV method results in a smaller expansion of the heat influence zones compared to the method with normal preheating. The hardness reduction in the heat-influenced zones is therefore limited by the SkV method to a smaller area set in the rail's longitudinal direction, which has a favorable effect on the wear conditions during use. For this reason, the SkV method is usually preferred when welding fine pearlitized rails. For railways, where the normal method is to use a welding methodology with a required preheating time of 6 to 10 minutes, this means that for the narrow heat influence zones desired when welding fine pearlite rails, a second method is used, namely the SkV method with short preheating. The necessary equipment in this connection is an economic disadvantage, and it would therefore be desirable if one could connect finely beaded rails also by means of welding methods with long preheating times, without associated disadvantages.

An improvement in the course of hardness in the heat-influenced zones can also be achieved in a known manner by means of an additional heat treatment of the rails in the welding area. Such a heat treatment is usually carried out - regardless of whether an aluminothermic welding method or a burn-off butt welding is used - when the weld has cooled to ambient temperature or at least to a temperature below 700°C, i.e. to a temperature where the austenite-pearlite transformation has ended. The weld is austenitized again using a suitable gas burner device and is then accelerated cooled using compressed air, a compressed air-water mixture or a similar cooling medium. However, this method has the disadvantage that it is difficult to carry out in a laid rail track and that it also requires a relatively large amount of equipment, which must be transported to the welding site.

The purpose of the present invention is to provide a heat finishing method which enables an improvement in the course of hardness in the heat influence zones in the area close to the driving surface, without it being necessary to use a cooling medium. This would be too complicated. Such a new method must be able to be carried out with the least possible procurement of equipment at the rail welding site in the laid rail track and must also provide a high degree of operational reliability. The economy of the procedure also plays a significant role.

According to the invention, this is achieved by the fact that, after the weld has cooled, the running surface at the weld and the adjacent heat-affected zones from above are heated shock-like over a period of from 50 to 150 seconds. by means of a burner, the burner being charged with at least 10,000 litres/hour of oxygen and 3,500 litres/hour of propane or acetylene, and the burner head's burning surface is kept at a distance, measured in millimeters, from the running surface of the weld and the heat-influenced zones corresponding to 0.1 to 1 times the numerical value of the meter weight of the rail used, measured in kilograms.

The method according to the invention enables a fine pearlitization of the area adjacent to the rail running surface of the weld and the heat influence zones in the rail track without the use of an external coolant. On the contrary, the cooling from the austenite temperature takes place through the rail itself. It was surprising that when using the new method, a cooling rate specific to the fine pearlitization and the desired hardness distribution are set in the weld and the heat-influenced zones and thus in the entire treatment area.

Methods are known for increasing the wear resistance of rail running surfaces by means of heat treatment. If a cooling medium is dispensed with, then it concerns methods that are used in connection with naturally hard, i.e. not finely pearlitized, rails, as is the case for example for the one described in the magazine "Railway Engineering and Maintenance", February 1937, on pages 97-99 method. An increase in hardness is achieved here, and thus one can also expect better wear conditions, but no further consideration is given to the fact that at the end of the heat-treating area, a soft annealed spot will have to form, with associated selective wear. This is due to the formation of shaped, spherical and soft pearlite from the originally lamellar pearlite.

However, in the case of an aluminothermic welding in finely pearlitized rails, it will be necessary, firstly, to strive for a high hardness on and below the running surface - at a depth of at least 10 mm below the running surface level, for wear engineering reasons - and secondly, the design of a new The hardness valley with formed pearlite is not displaced in the longitudinal direction, but in principle is suppressed or reduced as much as possible. This dual objective, with the premise that one must renounce a cooling medium, is not mentioned in the aforementioned article in the trade journal.

It has been shown that, according to the invention, the very rapid supply of heat to the rail running surface is necessary to achieve the desired fine pearlitization of the weld and its adjacent heat influence zones, with the associated increase in hardness. Best suited is a powerful gas burner 11, whose flames 9 can in the shortest time bring the area close to the running surface of the rail up to austenite temperature. However, the largest part of the head, step and foot in the weld or the rail remains cold. As soon as the burner flame is removed, the cold parts will quickly draw heat from the austenitized surface area. As a result, the desired fine pearlitic structure with correspondingly high wear resistance will form in the area close to the running surface of the weld and the heat-influenced zones, without a new lowering of hardness occurring to an undesirable degree at the edge of the heat-affected area or elsewhere. Decisive for the good result will be that the depth of the finely pearlitized zone is firstly large enough as a result of the general expected wear and tear during use, while on the other hand the depth must be limited. Namely, if the austenitized area extends too far into the rail head, the heat will not be able to be drawn away quickly enough through the remaining rail cross-section. Despite this limitation, it has surprisingly been shown that the fine pearlitization can still be carried out with a sufficient depth in the rail head in terms of wear technology.

The desired cooling rate for the formation of the fine lamellar structure in the weld and the heat-affected zones down to a depth of at least 10 mm, with the simultaneous suppression of a large hardness reduction at the edge of the heat treatment, is effected by a rapid introduction of heat over a precisely determined period of time and thus by means of of a defined temperature gradient in the vertical direction, and a determined depth for the austenitization emanating from the rail running surface.

Propane and oxygen should preferably be mentioned as gas for the burner. The working pressure of the oxygen must be as large as possible, thereby enabling a rapid supply of heat. Alongside propane and oxygen, one can also think of combinations such as acetylene/oxygen or petrol/oxygen and others.

Particularly advantageous (see fig. 2) is a vertical supply of the gas flame 9 directly towards the running surface 8 of the rail. For the defined heat supply, there exists an optimal burner distance a to the running surface 8 of the rail 13. In accordance with the present invention, the burning surface of the burner head is placed at a distance from the running surface in the weld and the heat influence zones which, measured in millimetres, correspond to 0.1 to 1 times the numerical value of the meter weight of the rail used, expressed in kilograms. In the case of a rail with a meter weight of 60 kilos, the distance between the burning surface will therefore be 6 to 60 mm.

Of great importance for the achievement of the fine pearlitization is, as already mentioned, also the duration of the heat supply, which must be adjusted depending on the type of burner used. When using a five bar oxygen and 1.5 bar propane driven burner with 78 gas outlet holes, each with a diameter of 1 mm, in the burner head and a consumption of 18,000 liters of oxygen/hour and 6,000 liters of propane/hour, for example a heating time in 10 sec. be too short, as the austenitizing temperature will not be reached. It is also not recommended to achieve the austenitizing temperature only briefly, because the cooling rate will then be too great, so that a separation with an undesirable hardening structure occurs. Times of more than 180 seconds will also be unfavorable for such a burner, because the deep heat in the rail will be too great and the subsequent cooling will take place too slowly. There is thus a favorable time duration for the heat supply, all depending on the burner type and burner gas/pressure. For powerful burners, this will mostly be a time of between 50 and 150 seconds.

A preferred embodiment of the method according to the invention consists in choosing 80 to 120 seconds as the heating time for rails with a meter weight of from 45 to 70 kg, and that the burning surface of the burner head is kept at a distance measured in millimeters corresponding to 0.3 to 0 .7 times the corresponding numerical value of the rail's meter weight, measured in kilograms.

The composition of the intermediate goods area must correspond to the composition of the finely pearlitized rail to be welded, so that you thus have the same conversion ratio and an even hardness distribution can be achieved. The welding portion must also be similarly alloyed. In accordance with this, the welding will first produce an intermediate area, which in terms of hardness roughly corresponds to a weld with naturally hard rails with a minimum tensile strength of 900 N/mm2. The heat-affected zones also correspond to a weld on such rails, since the finely pearlitized rail in its chemical composition corresponds to the rail with a minimum tensile strength of 900 N/mm<2>. After the weld 10 has cooled to ambient temperature, the burner 11 is started. The gas flames 9 will cover the weld 10, the two heat-influencing zones and also the areas 12 that are not affected by the weld 10 (fig. 2); i.e. an area which is greater than the distance between the two hardness valleys (Fig. IA and

IB ).

Consideration must be given to the conditions prevailing at the welding site, so that the various weather conditions prevailing there do not have any influence on the quality of the fine pearlitization. In particular, for example, sharp cold winds will lead to excessive cooling rates. This can be avoided by using corresponding wind protection.

Heat treatment methods of the type described advantageously now also enable the use of aluminothermic welding methods with longer preheating times for fine pearlitized rails, without disadvantages, because the larger heat influence zones are also involved in the heat treatment and are thus hardened.

That a sharp reduction in hardness does not again occur at the edge of the heat treatment must be declared by the short time that this area is exposed to temperatures from 600 to 700°C. As a result of the short time, the pearlite structure cannot shape itself and assume the spherical, soft shape. This is made possible as a result of the short but rapid heat supply, which nevertheless provides a sufficiently large depth for the through-hardening of the driving surface. In order to achieve the desired properties, one must find optimal values for parameters such as gas pressure, flow rate, torch distance to running surface and the chemical composition of the weld metal or portion. Also of importance here is the rail's size and mass, because larger or heavier rails require a greater heat supply, while with lighter rails you have to make sure that you have a smaller heat supply.

Below, the method according to the invention will be explained in more detail in the form of an example and with reference to fig. 2.

A rail 13 of finely pearlitized quality with the following chemical composition in weight # is used:

residual Fr and melt-related contaminants.

This rail 13 has an average running surface hardness of 345 HV. The heat treatment takes place from above towards the rail running surface 8 after a cooling of the weld 10 to ambient temperature. The heating takes place with the already described oxygen/propane burner (working pressure 5 bar oxygen, 1.5 bar propane, burner head 11 with 78 bores 14, each with a diameter of 1.0 mm, consumption 18,000 liters of oxygen/hour and 6,000 liters of propane/ hour). The distance a between the burner head 11 and the rail running surface 8 amounts to 25 mm. The time for the heat supply is 100 seconds. The austenitized running surface 8 is then cooled under a protective cover.

The diagram in fig. 3 shows the result of the heat treatment carried out according to the invention. Recorded is the hardening of the running surface in the weld and the heat influence zones in the longitudinal direction of the rail (curve IV). Compared to an untreated weld, which is produced according to the state of the art using the SKV method (curve III) and using a high-alloy portion, one can see that as a result of the heat treatment only very modest hardness valleys are obtained.

Claims (2)

1. Method for intermediate goods welding of finely pearlitized rails by filling in aluminothermally produced steel in a mold that surrounds the two rail ends, characterized in that, after the weld has cooled, the running surface in the weld and the adjacent heat-affected zones are shock-heated from above using a burner over a period of from 50 to 150 seconds, whereby the burner is charged with at least 10,000 litres/hour of oxygen and 3,500 litres/hour of propane or acetylene, and the burner head's burning surface is placed at a distance, measured in millimeters, from the running surface in the weld and the heat influence zones corresponding to 0.1 to 1 times the numerical value of the meter weight of the rail used, measured in kilograms. 2. Method according to claim 1, characterized in that 80 to 120 seconds is chosen as the time for the heating of rails with a weight per meter of from 45 to 70 kg, and that the burning surface of the burner head is placed at a distance measured in millimeters corresponding to 0.3 to 0.7 times the numerical value of the rails' meter weight, measured in kilograms.