WO2005080686A1

WO2005080686A1 - Hardening process using laser in railway applications and relative equipment

Info

Publication number: WO2005080686A1
Application number: PCT/IT2005/000092
Authority: WO
Inventors: Sandro Favilli; Pierluigi Siliani
Original assignee: Sandro Favilli; Pierluigi Siliani
Priority date: 2004-02-23
Filing date: 2005-02-21
Publication date: 2005-09-01
Also published as: ITFI20040044A1

Abstract

On-site surface hardening procedure using laser for materials in the railway sector, in particular for rails which, as known, are subject to wear during use due to the guiding mechanism provided by the rim of the wheel on the inner face of the head of the rail.

Description

HARDENING PROCESS USING LASER IN RAILWAY APPLICATIONS AND RELATIVE EQUIPMENT Background of the invention Surface hardening using laser for steels is a heat treatment suitable to improve some mechanical properties of mechanical components obtained with said materials, in particular surface hardness. Laser hardening does not differ fundamentally from conventional surface hardening treatments, as in all these processes the increases in hardness and strength are obtained through rapid cooling, which transforms the austenite produced during the heating phase into martensite. The fundamental difference consists inJhe_factthat_in_the_las„e_r-pro_cess.only.ajhin layer of material is heated to the austenization temperature, before subsequent cooling, leaving the material therewithin unchanged. As steels are essentially good thermal conductors, it is necessary to use intense heat flows, which locally heat the surface layer of the material to the austenization temperature. In the laser process, thanks to selective heating of the surface of the piece and to the high specific power of the laser beam, external cooling means are not required and a hardened zone is obtained with self-quenching phenomena. The depth of the hardened zone, which usually corresponds to the depth of the heat affected zone (HAZ), depends on the hardenability of the material; this depth is sufficient to guarantee the required technological applications. The use of laser technology allows, within the limits of this technology, heating profiles of suitable form and dimension to be obtained through defocusing and scanning of the beam. These heating profiles are a function of the degree of absorption of the surface of the material which can be modified using chemical, mechanical or surface coating treatments. The high heating gradients obtainable with the laser process allow all steels to be hardened, even those which are not hardenable with conventional processes. For this purpose, it is suitable to define a "laser hardenability", normally different from the hardenability usually defined using the Jominy test. The main advantages of laser hardening treatment compared to conventional hardening treatment can be summarized in the following characteristics: > selectivity of the zone to be treated; > practical absence of thermal distortions which means there is no need for subsequent processing; no need for cooling means; > absence of polluting substances; > total repeatability of the treatment; j lowe _energy consumption; reduced management costs; > high degree of safety for operators;

> limited overall dimensions and low costs of the equipment. Summary of the invention The present invention consists in applying the surface hardening procedure using laser in the railway sector, in particular to rails which are subject to considerable wear caused by the monolateral guiding mechanism with play produced between wheelset and track. . The selectivity of the treatment allows an increase in the duration of the surfaces which are subject to the most wear without modifying the global mechanical characteristics of the parts involved. The treatment can also be performed downstream of normal maintenance operations (wheel reprofiling and rail grinding). A first subject of the invention is a process for hardening railway material or the like subject to wear, especially rails, using laser hardening treatment, suitable to perform the treatment with localized laser hardening tracks, characterized in that only the zones subject to wear of said material are treated on-site using a laser hardening process, with moving equipment transiting over said fixed material. The laser treatment can be performed with a single longitudinal hardening track in the zone subject to wear or with high-frequency cyclic orientations with transverse components, or yet again with several longitudinal hardening tracks, adjacent and, if necessary, with superimposed margins. The laser treatment can be preceded by a surface treatment of coating or other chemical or mechanical preparation. A further subject of the invention is a piece of equipment to perform the procedure hereinbefore, which comprises a vehicle which is moving on the track and which is equipped with one or more laser devices positioned to carry out hardening treatment of the rails in the zones subject to wear, during transit of the vehicle at a speed compatible with the typical requirements of the hardening treatment. Said vehicle can be provided with a system to pre- treat the rails suitable to facilitate the subsequent hardening treatments and positioned upstream of the laser device,, with respect to the direction of movement. The equipment can include several laser beam emitting units, staggered in the direction of movement of the vehicle and each suitable to perform the treatment on one or more adjacent hardening tracks. The invention shall be better understood by following the description and accompanying drawing, which shows a non-limiting practical embodiment of the invention. In the drawing: Figure 1 shows a section of a rail and of the wheel transiting on the running surface; Figure 2 shows a diagram of a trolley and of the equipment present thereon, for alternate treatments on the two rails forming the track; Figures 3 and 4 show diagrams of cyclic type laser hardening treatment; (motion comprised of longitudinal advance and scanning with transverse components); Figures 5 and 6 show diagrams of linear treatments, single and respectively with adjacent tracks. Figure 1 shows a section of a rail 1 - exemplificative of a conventional rail - of which 1A indicates the rail head, on the running surface 1B of which the wheel rests, the perimeter profile of which, drawn in a cross section, is indicated with 3; 1C indicates an inner edge or rim of the rail, with which the rim 3A of the wheel cooperates. The zone subject to wear is indicated by the outline E. Monitoring of wear of the rails in specific sections of the rail network is essential for safety and for economic management of the rail network. The invention allows the requirements of safety of tracks in zones subject to considerable wear, such as the materials of points and curves, to be observed economically, also taking into account the different conditions of wear that occur on the inner rim and on the head of the "high" rail and of the

"low" rail. The invention provides for hardening treatment of the rails on-site, for new rails and for used and worn rails, in order to considerably prolong the use thereof without replacements and maintaining the safety conditions established by regulations. To carry out the on--site hardening-treatment on laid rails - both.new or used - according to the invention a railway trolley is used, duly equipped with the components and the equipment required to perform the laser treatment. A trolley, schematized with 10 in Figure 2, for treatment of the rails R of a track is equipped with a laser treatment system, comprising: 1 an assembly 12 for pre-treatment of the zone to be treated on each rail; means for pre-treatment will be provided to perform cleaning, brushing, coating (against possible reflection phenomena), drying and other operations, in order to facilitate subsequent laser hardening operations; this assembly can be positioned upstream of the means for hardening operations, or can be provided on a separate trolley which transits ahead of the trolley for laser hardening. 2 For the hardening treatments, the trolley comprises a voltage source 14, to power the entire system (laser source, movement and various devices). If wished, it is possible to connect the system directly to the power line; therefore, if this is direct current (Italian territory), the use of a DC/AC converter is required. 3 The trolley is also provided with one or more laser sources, with optical path, head and accessories. The or each laser source 16, particularly of the CO₂ type, of the power required to allow treatment, will include all the components necessary for operation thereof, including high voltage power supply units, control and management box, gas box, chiller 16A, gas bottles 16B to supply the laser and the like. 4 The or each optical path 18 will include the assembly of prisms and mirrors suitable to convey, partialize and/or divide the laser beam within the laser head(s). Said path can be "mobile" to allow the laser head(s) the necessary degrees of freedom. 5 The or each head 20 can be of the polar or Cartesian type, according to the type of hardening tracks to be produced, to the method with which they are to be produced and to the dimensions to be obtained. A possible solution is to use a single laser head which can either function as a head of the polar type, to obtain a hardening track through rapid scanning of the surface to be treated according to diagrams in Figures 3 and 4, or as a head of the Cartesian type, where one or more hardening tracks are^~obtained by defocusing of said beam, as schematized in- Figure 5 with a single head or with several beams (Figure 6) suitable to act on adjacent and, if necessary, partially superimposed tracks, along the edges of the individual treated zones, in which there may be tempered zones of the hardened material. The laser hardening treatment performed on site with the vehicle equipped according to the invention can be performed both on new rail and on worn rail and can be performed only on specific sections with a high degree of wear, as in some types of curves, above all and with treatments repeated in time as a function of progressive although limited wear. The depth of martensite transformation due to laser hardening is generally contained within limited values, and wear of the hardened layer is slower compared to untreated material. Therefore, subsequent laser hardening operations according to the invention can be delayed in time in order to act after or immediately before complete wear of the layer treated in the previous operation. Subsequent operations will maintain the advantage of working on the optimum wheel-rail combined profile, resulting from operational wear. This greatly extends the life of the rail, before reaching the limit of wear that necessitates replacement of the rail (or of the components of the points). On-site treatment of the rail with the trolley-mounted system can be preceded by chemical and/or mechanical pre-treatments (brushing, coating and the like) with accessories which can be provided directly on the trolley equipped with the laser system. Nonetheless, it would also be possible to perform pre-treatments using separate trolleys. The laser system will be equipped with optical paths with a series of diverters to have a mobile optical axis with suitable degrees of freedom of the head or heads which can thus act with polar or Cartesian system. The direction of advance of the hardening treatment is provided by the movement of the trolley, which can also be entrusted to trains in transit, bearing in mind that the treatments are sufficient only in limited zones in which there are curves and/or points and not in substantially rectilinear sections. The characteristics of the laser beam, such as intensity, inclination and

-power-density of the beams can- in- any-case be adapted to individuaLneeds,- . as can the instantaneous focusing be easily varied along each repetitive cycle of an excursion, in order to obtain hardening tracks of suitable form and geometry. It is understood that the drawing only shows an example provided by way of a practical arrangement of the invention and that said invention can vary in forms and arrangements without however departing from the scope of the concept on which the invention is based.

Claims

CLAIMS 1) Hardening process for railway material or the like subject to wear, in particular rails, using a laser hardening process, suitable to perform the treatment with localized laser hardening tracks, characterized in that only the zones subject to wear of said material are treated on-site using laser hardening process. 2) Process according to claim 1, characterised in that it is carried out with moving equipment transiting over said fixed material. 3) Process as claimed in claim 1 or 2, characterized in that the , laser treatment is performed with a single hardening track in the zone subject to-wear. _ __ _ _ _ — _ _ . _ 4) Process as claimed in claim 1 or 2, characterized in that the laser treatment is performed with high frequency cyclic orientations with transverse components. 5) Process as claimed in claim 1 or 2, characterized in that laser tracking is performed with several longitudinal hardening tracks, adjacent and, if necessary, with superimposed margins. 6) Process as claimed in one or more of the previous of the previous claims, characterized in that the laser treatment is preceded by a pre-treatment. 7) Process as claimed in claim 6, characterised in that said pretreatment includes a coating surface treatment. 8) Process as claimed in one or more of the previous claims, characterized in that the laser treatment is performed on-site on used rail material. 9) Process as claimed in one or more of the previous claims, characterized in that the laser treatment is performed with treatments repeated in time as a function of progressive although limited wear. 10) Equipment to perform the process according to one or more of claims 1 to 9, characterized in that it is provided on rolling stock suitable to transit on a track. 11) Equipment to perform the process according to one or more of claims 1 to 9, characterized in that it comprises at least a vehicle which moves on fixed railway material and in that it is equipped with at least one laser device positioned to perform the hardening treatment in the zones subject to wear during transit of the vehicle at a speed compatible with the requirements relative to hardening. 12) Equipment as claimed in claim 10 or 11 , characterized in that it comprises a pre-treatment system suitable to act upstream of the laser equipment, with respect to the direction of movement. 13) Equipment as claimed in claim 10 or 11 or 13, characterized in that it comprises several laser beam emitting units staggered in the direction of movement of the vehicle and each suitable to perform the treatment on one or more adjacent hardening tracks.