Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1997-09-12
2001-05-08
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S058100, C249S086000
Reexamination Certificate
active
06227282
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the aluminothermic welding of the rails with controlled alloying of the weld metal in the rail head region. The object of the process is purposefully to set a desired property pattern in the weld in accordance with the requirements of the track.
DESCRIPTION OF THE PRIOR ART
The aluminothermic welding process (THERMIT process) is the only one of the known fusion-welding processes which exploits a chemical reaction for generating the hot molten filler material.
In this case, the high affinity of aluminum for oxygen is utilized for the reduction of heavy metal oxides, preferably iron oxide.
The process which operates highly exothermically can be described as
heavy metal oxide+aluminum→heavy metal+aluminum oxide+heat
or for the iron reaction
Fe
2
O
3
+2 Al→2 Fe+Al
2
O
3
+849 kJ.
After punctiform ignition using a small pilot rod, the aluminothermic reaction proceeds in a crucible within a few seconds, with vigorous evolution of heat. The hot reaction products at approximately 2500° C. then separate from one another, the lower-density slag (Al
2
O
3
) floating on the iron.
Granulated steel particles for damping the reaction and, depending on the base material to be welded, steel formers such as C, Mn, Cr, V, Mo are mixed into the base constituents of the THERMIT fraction—iron oxide and aluminum of small grain size.
The hot molten THERMIT steel of defined quality, smelted in the reaction mixture, is outstandingly suitable for welding process purposes.
The welding process can be characterized by the following working steps:
aligning the workpieces laid with a gap depending on the welding cross section and the process;
forming a refractory mold around the welding point;
preheating the ends of the workpieces by means of a special burner with gas/air, gasolene/air, gasolene/oxygen, acetylene/oxygen or preferably propane/oxygen mixtures;
pouring of the hot molten steel into the mold and welding of the workpiece ends by intercasting and casting-in.
Using this process, workpieces of any type with any desired cross-sections can be welded together during construction or repair. Because its procedure is simple and independent of external energy sources, the THERMIT welding process has found its most widespread application in rail welding.
The aluminothermically generated steel, serving here as the welding material, should correspond in its strength properties as far as possible to the rail steel.
This demand is met by the known aluminothermic mixtures—also called welding portions—by adding alloying elements such as, in particular, carbon, manganese, chromium, silicon, vanadium and titanium to the base mixture consisting of aluminum and iron oxides. For damping and cooling, iron or scrap steel are also mixed into the aluminothermic mixture, whereby the steel yield is increased at the same time.
Thus, very specifically alloyed rail grades are sometimes used in various countries, where there is an interest in concentrating these specific alloying elements under control in the rail head in defined concentrations corresponding to the rail composition, without the rail foot being alloyed.
In recent years, however, head-hardened rails have increasingly been used in rail traffic. The reason for this trend is that, on the one hand, the stresses on the rails increase which leads to intensified wear in conventional rails and, on the other hand, there are increasingly economic imperatives, so that longer replacement cycles of rails are desired.
For example, particularly tight radii (<300 m), extreme gradients or the increasingly rising axle loads, in particular in countries with predominantly heavy-load traffic, such as in North America, South Africa, Asia and Australia, represent an increasing stress on the rail, which must be taken into account both for the rail and for the joining of rails.
The increased use of head-hardened rails of course also makes it necessary to adapt the required joining technologies.
It would here be of particular interest for the track operation, to achieve an increase in hardness in the head region and, in relation thereto, increased ductility in the foot of the rail.
In addition, grain-refining alloying additives are also frequently used where controlled concentration in the head would be desired.
Hitherto, the thermit welding portions have thus been modified in newly developed rail grades, so that the entire rail joint was adapted to the changed rail grade but itself showed a uniform property pattern.
This means that, in conventional aluminothermic welding technology, hardening additives are mixed into the welding portion in order to effect the required hardness in the head region, but that simultaneously the same hardness is obtained in the entire rail profile, that is to say also in the rail foot region, where ductility is rather more desired.
CH-PS 658 817 describes a process for the aluminothermic composite welding, in which two cast steel alloys are made up from two welding materials of different composition in such a way that they give in each case, from two crucible chambers, one hard and wear-resistant steel for the rail head and a tough ductile steel, flowing first into the mold, for the welding of the web and rail foot.
This means that this process is a two-stage process. This procedure is very time-consuming and, for use in practice on the track, so complicated that it has not been able to gain acceptance in the face of the conventional thermit joint-welding known worldwide. In addition, further interfaces are formed between the two thermit steels, it being possible for undesired defects to arise in the weld.
From DE-PS 898,989 it is known that the iron formed in the aluminothermic reaction can be alloyed with steel-refining metals or metalloids which are located in an excavation in the upper parts or on the bottom of the mold surrounding the material to be welded.
It is the object of this invention to provide a thermit steel which is as homogeneous as possible and to avoid possible losses of required alloying additives via the slag as far as possible. However, it is pointed out that intimately and thoroughly mixed steel is desired.
Moreover, it is expensive in production technology and in addition difficult in the case of alloying the rail head to incorporate these metals or metalloids in the mold, since the distance between the mold and the running surface, which is to be alloyed, of the rail head is large. The metal/metalloid must overcome this distance solely by diffusion.
There was thus a need for the simplest possible welding process which comprises only one process step or reaction step, to provide the metals or metalloids to be alloyed in a simple but also reliable manner, that is to say both in the preparation of the required consumable materials and in the installation of the thermid weld, and also makes it possible to concentrate these alloying additives under control in the head of the weld. It is the object of the welding process to be developed to produce a joint which is in accordance with the property pattern of the rails to be welded and in particular achieves a hard weld material, which is as fine-grained as possible, in the rail head, and at the same time ensures a foot which is less prone to breakage and is as ductile as possible.
By means of such a welded joint, the wear resistance and the associated economic advantages would be realized and the quality of the weld would be improved because of the higher hardness and a microstructure of higher load-bearing capacity due to the finer grain.
Depending on the alloying element or combination of alloying elements and on the quantity to be alloyed in, a different property pattern—chemical composition of the steel, mechanical properties, microstructure and the like—can be set. These different properties depend in part on one another, according to the nature and quantity of the additive.
There was a particular need for the simplest possible, most reliable and reproducible methods in the alumin
Kuster Frank
MacRae Donald
Mulder Gerhardus Johannes
Steinhorst Michael
Brown & Wood LLP
Elektro-Thermit GmbH
Lin Kuang Y.
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