Electric heating – Metal heating – By arc
Reexamination Certificate
2001-06-22
2002-10-22
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121390, C219S121590, C266S048000
Reexamination Certificate
active
06469274
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to a unit and a process for cutting ferrous metals, in particular structural steels, in which the metal is locally preheated by a stream of plasma delivered by a plasma torch and the metal is cut by a stream of pressurized oxidizing gas, such as a stream of cutting oxygen, delivered by a delivery nozzle or the like.
At the present time, several processes are known for the automated thermal cutting of metals, said processes having been used for many years on an industrial scale.
By way of examples, mention may be made of oxycutting, plasma cutting and laser cutting, especially for structural steels.
These processes are based on local melting, over the entire thickness, of the material to be cut and on the displacement of the melting front in a path which defines the shape of the cut or kerf that has to be made through the material to be cut.
These various processes cannot actually be considered as competing processes as they are distinguished from one another by different cutting performance and operating and running costs.
Thus, the technique of oxycutting is known for its ability to cut on an industrial scale structural steel thicknesses ranging from 3 mm to 300 mm and to reach, in rarer applications, thicknesses possibly up to 2000 mm.
In this case, although the cost of the cutting tool, that is to say the torch, may be low, an oxycutting process has especially the drawback of being excessively slow overall.
On the other hand, plasma cutting is known for its ability to cut any type of metallic material with a very high productivity.
However, the cost of the cutting tool, namely the assembly consisting of the plasma torch and the current generator, is usually from 30 to 50 times higher than in the previous case, namely in oxycutting.
Moreover, CO
2
laser cutting is known to produce excellent cutting quality, particularly over thicknesses of less than 10 mm, that is to say within a thickness range in which the laser process is also productive.
In contrast, the cost of the cutting tool, namely the assembly consisting of the laser head and the CO
2
laser source, is, here too, 200 to 300 times higher than that in the case of oxycutting.
More generally, the oxycutting technique is based on the use of the thermal energy generated by the combustion of iron, combined with the kinetic energy of the oxygen jet which allows the oxides produced during said cutting to be expelled from the kerf.
However, the combustion of iron requires the presence of preheating flames to initiate it and then sustain it correctly.
To do this, oxycutting torches are conventionally fitted, at their lower end, with a cutting head or delivery nozzle, generally cylindrical in shape, having a central channel for delivering the cutting oxygen which is surrounded, at a certain distance away, by a ring of channels for delivering a mixture of a combustible gas and an oxidizer which are intended to form a heating or preheating flame peripheral to the central oxygen cutting jet.
An oxycutting operation may be described schematically by a cycle comprising the following steps:
(a) opening, by an operator, of the combustible-gas and oxidizer taps so that these gases are fed into the heating orifices of the cutting head;
(b) ignition of the cutting torch, either manually by means of, for example, a lighter flame presented at the exit of the heating orifices of the cutting head, or automatically, for example with the aid of a piezoelectric quartz crystal for creating a spark which ignites a gas pilot, whose flame thus obtained is directed toward the heating orifices of the cutting head, so as to ignite in turn the heating flame of the torch;
(c) adjustment of the combustible gas and oxidizer flow rates, by means of taps provided on the torch, so as to obtain a flame with the chosen stoichiometric ratio or corresponding to the technical requirements of the torch manufacturer;
(d) presentation of the torch at the required point of initiation on the workpiece to be cut;
(e) local heating of the workpiece to be cut until a sufficient temperature is reached, conventionally about 1300° C. in the case of a workpiece made of structural steel, in order for the iron-oxygen reaction to be able to be initiated and sustained;
(f) opening of the cutting oxygen;
(g) drilling of the workpiece over its entire thickness;
(h) movement of the torch by means of the shafts of the cutting machine and execution of the cutting in one or more programmed paths;
(i) end of the cutting operation, stopping the feed of gas to the torch in order to stop the flow of cutting oxygen and the heating or, where appropriate, cutting off the flow of cutting oxygen and continuing the heating in order to move the torch to a new point of initiation.
However, the productivity of oxycutting processes generally suffers from a low rate of propagation of the combustion front of the iron forming part of the composition of the material to be cut and also by the relatively long times to prepare for the actual cutting, that is to say the time to adjust the heating flame and the time for heating the workpiece locally in order to reach the temperature favorable to the iron oxycombustion reaction.
Thus, because of a low heating power density applied to the workpiece, the time needed to raise the material to the required temperature is generally from 5 to 20 seconds and may, in extreme cases, be as long as about 1 minute.
In addition, this heating phase followed by the initiation of the oxycombustion reaction cannot be easily automated because the time needed to reach the correct reaction initiation conditions cannot be accurately predicted.
This is because the factors that can influence this time are, especially, the mass of the workpiece, the thermal conductivity of the grade of material to be heated, the surface state of the material, that is to say for example the possible presence of millscale, grease, paint or another coating on this material, but also other factors associated with the specific heat of the gases used for heating, and their mixing ratio.
In practice, most often the operator carefully monitors the heating operation and manually opens the cutting oxygen when conditions suitable for initiating the oxycombustion reaction seem to him to be achieved.
This practice sometimes leads to ignition “failures”, that is to say ineffective or imperfect ignition, because the temperature of the material is not high enough, or sometimes, on the other hand, to excessively long heating times, for safety's sake, in order to be sure that ignition will take place correctly.
Consequently, the problem which arises is to prevent or minimize ineffective or imperfect ignition and to increase the productivity of oxycutting processes by, in particular, reducing the time needed to prepare for the actual cutting operation, and preferably with effective automation of the entire process.
SUMMARY OF INVENTION
The solution proposed by the present invention relies on coupling an oxycutting process with a process for heating by means of a plasma jet or stream of plasma, and of its operating equipment.
The present invention therefore relates to a process for the plasma oxycutting of at least one metal workpiece containing at least one ferrous metal, in particular iron, in which:
(a) an ignition region of the metal workpiece to be cut is locally preheated by subjecting said ignition region to at least one plasma jet;
(b) at least part of the ignition region at least preheated in step (a) is subjected to at least one stream of oxidizing gas at a pressure of greater than 10
5
Pa;
(c) at least one drillhole is made over the entire thickness of the workpiece to be cut, in at least part of the ignition region subjected to at least preheating by plasma jet in step (a), by melting and/or combustion of the ferrous material contained in said metal workpiece by the reaction of said ferrous material with said stream of oxidizing gas and/or said plasma jet;
(d) the plasma jet and the stream of oxidizing gas are moved in a cutting path in order to
Augeraud Regis
Delzenne Michel
La Soudure Autogene Francaise
Paschall Mark
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