DE3006695A1 - Induction heat treatment of railway rail head - produces fine sorbite in mushroom while simultaneously tempering web and foot of rail - Google Patents

Abstract

On leaving a hot rolling mill, a railway rail is cooled(I) until the transformation of austenite into the alpha phase is complete. Cooling(I) is then stopped, and the rail-head or -mushroom is then reheated(II) to form austenite, followed by rapid cooling(III) to form a sorbitic structure. Heating stage(II) is pref. employed simultaneously to temper the web and foot of the rail. After stage(I), the semi-hot rail is pref. fed into the induction heating appts. via guides which hold the rail until stage(III) is completed so the rails are free from distortion. A cold straightening operation may subsequently be used on the rails. The optimum wear-resisting metallurgical structure is obtd. in the mushroom without using costly alloying elements in the rail steel, while using the min. amt. of heating energy to produce the desired structure.

Description

Process for inductive heat treatment of rail heads

The invention relates to a method for inductive heat treatment of rail heads.

In principle, they can be used to produce highly wear-resistant rails Three known procedural routes: The first procedural route is the rails to be rolled from an alloyed rail steel. By coordinating the alloy additives on the normal course of cooling of the rails after rolling, rails can high strength.

Apart from the increase in the price of the rails due to the alloy costs this method has the disadvantage that the highest strength values in the web-foot area of the rails can be adjusted, i.e. in the part of the rails that is less tight but should be tough and insensitive to notches. In addition, these require Naturally hard, high-strength rails require a considerable extra expense with today's usual welding processes.

The second method is to use rails made of standardized rail steels to roll and to accelerate their cooling from the rolling heat so that the rails assume a higher strength. Accelerated cooling can be achieved with the help of Air or Air / water nozzles (OE-PS 2 148 722) or by dipping into a Cooling medium that is not too harsh (oil or boiling water, DE-PS 2 429 338) take place The disadvantage is that these procedures are due to varying final rolling temperatures to fluctuating strength values and thus to uncertainties result in the treatment effect. Another disadvantage is that coarse roll structures are subjected to accelerated cooling and therefore no optimal toughness values result.

These disadvantages are avoided by the third method, which is the higher strength values is set in a heat treatment completely separate from the rail rolling. The rewarming can be safely controlled in terms of temperature and leads to a re-graining of the structure, which therefore also has good toughness properties.

This subsequent heat treatment can be used as full compensation for the whole Track cross-section, but it can also affect those that are subject to wear Restrict part of the Schierenkopf.

High investment and energy costs as well as a limited amount Treatment capacity to the facilities required for this have led to the fact that Heat-treated rails are only used to a limited extent.

It is known that the operational behavior of the rails is mainly depends on their resistance to abrasive and crushing wear. The highly wear-resistant rails get this property from their finest pearlitic, In common parlance it is also called "sorbitic" structure, the possible should not contain free ferrite. Pearlite lamella spacing and thicknesses are decisive for the strength and toughness properties in addition to the cosine size pearlitic steels. But even here an increase is Je: strength with one Reduction of the toughness values verburX. Since the strength values for rails are only available in Vergchlißbereich the head are required, but the bridge and foot area za and should be insensitive to cracks, then the aim of all pest management measures must be this Demands come as close as possible.

In addition, the performance of rails is more recent Findings are largely determined by their internal stress state. High internal stresses can be caused by superimposition of operating and thermal stresses in the track -in particular in the welded track to local excesses of the fatigue strength and thus lead to permanent damage in the rail steel. A relationship between the internal stress state The cause of the corrugation, which has not yet been clarified, is also suspected.

Avoiding high internal stresses in the production of rails is important very difficult because of the unfavorable thermal equilibrium of all rail profiles. Proposals to reduce them consistently lead to a cooling delay after de rolling and thus to a restriction of the capacity of rolling mills.

In addition, the strength utilization of the steel composition prevented by these measures (DE-PS 1 942 929, DE-PS 2 447 0193.

To reduce the stress in heat-treated rails it is known (DE-AS 1 255 689), the rails over their entire length within -the Bend the elastic limit convexly in the cold state, over the rail head to heat the austenitizing temperature, quenching and after passing a predetermined time due to the heat given off by the core of the rail head to leave the skin on itself, to cool down to ambient temperature and then the the clamped splint.

The invention is based on the object of a method for inductive To provide heat treatment of rail heads that is incorporated into the rail rolling can be and, moreover, an energy-saving and cost-effective production heat-treated rails allowed.

This object is achieved according to the invention in that the cooling of the rails from he rolling heat until the end of the / oc conversion takes place, the cooling is then interrupted, the rail head for the production of austenite reheated and then in a known manner for setting a Sorbitol structure is cooled faster.

In a further embodiment of the invention, rewarming takes place at the same time of the rail head, rail web and rail foot, subjected to a tempering treatment, which, as usual, can be around 6000C.

Through these measures is achieved to produce rails, the high Wear and fatigue strength in the rail head with good toughness values at large Connect notch insensitivity in the rail web and rail foot and beyond have extensive freedom from internal stresses. The good weldability standardized Wear-resistant carbon steel rails will not be affected.

The method according to the invention can be carried out, for example, in the manner that the cooling of the carbon steel rails rolled in a known manner in a temperature range between 650 ° C and 450 ", i.e. at the time when the t / o6 conversion is completely complete, and the edge area is interrupted of the rail head to a depth of more than 15 mm austenitized again and is then cooled at an accelerated rate. The rewarming of the rail head edge zone takes place inductively with conventional systems, according to the invention with an additional Equipment for heating the foot part of the bar of the rail are equipped. the According to the invention, cooling of the rails after rolling is thus at the time interrupted, in which the different heat shrinkage in the rail profile is still did not lead to the discarding of the rails.

In a further embodiment of the invention, the to be treated Rails - before the start of their cooling warping in the semi-warm state in the device for heat treatment brought in and held by guides so that that the rails are straight after the treatment, i.e. by means of guides the rails, which are still straight, pass through the device for the heat treatment and held so that the rails are straight after treatment.

Due to the thermal expansion of the reheated rail head this process supports; simultaneous heat supply in the SFeg-FuMbsreich on one Temperature of approx. 6s0 ° C prevents Elgen stress build-up.

During the accelerated cooling of the reheated rail head, the depending on the chosen steel composition with the help of accelerated Air or an air-water mixture can be carried out in the rail head the desired finely lamellar fine-grain pearlite structure of high strength a through the volume reduction, which takes place in parallel, keeps the rail straight, where the still existing tempering temperature in the rail foot the build-up of tensile stresses prevented. The rail thus leaves the treatment facility straight and free of internal stress. the device for heat treatment. The tightest tolerances for straightness and waviness of rails, which are particularly suitable for laying in routes with heavy-duty and / or Schnellstverkshr, can be set in a further Embodiment of the invention an additional straightening take place in the cold state. The straightening work required here is so small that rails manufactured in this way still low in residual stress and significantly compared to rails made up to now are more reliable.

The method according to the invention has the following essential advantages on: The inductive reheating of part of the rail head only bridges another temperature difference of about 3000C, with the vicinity of the Curie point the conversion of electrical energy into heat is still significantly favored. That There is a temperature drop to the non-reheated tail of the rail head small amount and therefore avoids energy losses. The electrical capacity of the induction system could be reduced, if it were not more advantageous, the throughput rate to increase such a system many times over. This makes it possible to adjust the throughput speed of the system to the rolling cycle.

The inventors had recognized that a major advantage of the Heat treatment achieved finest pearlitic structure is based on its fine grain. It is achieved by the graining effect, for which it is, however, the same whether the steel of the first structural transformation had cooled down completely or whether it was immediately was reaustenitized again after the end of the structural transformation after rolling.

Due to the tempering treatment of the bar in the treatment plant and for the rail, the rolling hardness in this part of the rail is reduced and their toughness and notch insensitivity are significantly increased. The operational safety Rails treated in this way are improved, and their risk from incorrect handling for welding and laying work is reduced.

By interrupting the cooling after rolling, the from the Warping of the rails resulting from thermal imbalance and the previously inevitable the resulting straightening work on the cold rail is avoided. The contributor to the Hot-rolled rails in the treatment plant allow bending without significant Effort and without aen build-up of tension. The controlled cooling after the Reheating leads to an almost straight and tension-free splint, which only needs to be corrected slightly.

Claims (4)

Claims 1. Method for inductive heat treatment of rail heads, characterized in that the cooling of the rail from the rolling heat to The / oL conversion is completed, cooling is then interrupted, the rail head reheated to produce austenite and then reheated in a manner known per se is cooled accelerated to set a sorbitol structure. 2. The method according to claim 1, characterized in that at the same time to reheat the rail head, the web and foot part of the rails of a Tempering treatment is subjected. 3. The method according to Arspruch 1 or 2, characterized in that the rails to be treated before the start of their cooling warping in the semi-warm state introduced into the device for the heat treatment and held by guides ensure that the splints are straight after the treatment. 4. The method according to any one of claims 1 to 3, characterized by an additional straightening when the rails are cold.