Electric heating – Metal heating – By arc
Reexamination Certificate
2001-02-16
2001-12-25
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121440, C219S121550, C219S121540
Reexamination Certificate
active
06333481
ABSTRACT:
The present invention relates to a plasma-arc work process using a plasma torch supplied with a gas mixture containing hydrogen and nitrogen and/or argon, the amount of hydrogen in the mixture being defined according to one or more parameters chosen from the thickness and/or the grade of the constituent material of the workpiece, the desired work rate and the intensity of the electric current.
A plasma-arc torch that can be used in a cutting, welding, marking or spraying operation, or in any other heat-treatment operation, on a metallic or nonmetallic material usually comprises an electrode made of copper or of a copper alloy carrying a cylindrical insert generally made of hafnium, tungsten or zirconium, on which the electric arc serving to ionize the gas feeding the torch takes root, that is to say the defined flow rate of pressurized gas, called plasma gas, which is delivered between the electrode and the nozzle and which flows out via an orifice of said nozzle toward the workpiece.
The electrode is generally centered above this orifice for ejecting the plasma jet, which is placed axially in the nozzle and forms a constriction diaphragm.
In the particular case of a plasma cutting operation, the plasma cutting device or system usually comprises a plasma torch, a source of electric current, an electric-arc ignition system and one or more sources of fluids, particularly of plasma gas, possibly of shielding gas or of post-injected fluid, and of fluid for cooling the torch, generally distilled water.
Such torches or units are well known to those skilled in the art, since they have already been described in many documents to which reference may be made for further details, especially EP-A-599709, EP-A-872300, EP-A-801882, EP-A-941018, EP-A-144267, EP-A-410875, EP-A-772957, EP-A-902606, EP-A-810052, EP-A-845929, EP-A-790756, EP-A-196612, WO-A-89/11941, U.S. Pat. No. 4,521,666, U.S. Pat. No. 4,059,743, U.S. Pat. No. 4,163,891 and U.S. Pat. No. 5,591,357.
In a known manner, plasma-arc cutting uses the thermal and kinetic effects of a plasma jet to melt the material to be cut and to expel the molten material from the kerf formed after the torch has moved relative to the workpiece.
Moreover, it is also known that, in plasma cutting, the plasma gas or gas mixture used is not the same depending on the nature of the material to be cut.
Thus, a nonoxidizing gas, for example nitrogen, is generally used for stainless steels or for light alloys, whereas it is preferred to use an oxidizing gas, such as oxygen or air, for cutting structural steels.
Conversely, when cutting stainless steels or light alloys, it is generally gas mixtures consisting of argon and hydrogen, or indeed sometimes gas mixtures consisting of nitrogen and hydrogen, which are used.
It is known in fact that the use, in certain cases, of a binary mixture of argon and hydrogen as plasma gas, in concentrations ranging from 20 to 35% hydrogen, makes it possible to reduce the taper and to maintain an unoxidized cut face for large thicknesses.
Moreover, ternary mixtures of argon, nitrogen and hydrogen can be used to cut small thicknesses.
However, these gas mixtures consisting of hydrogen and argon and/or nitrogen are usually premixed, that is to say manufactured according to the chosen contents, before being used to fill suitable containers, such as gas bottles, which are then taken to the operating site, that is to say the point where the cutting has to be carried out.
Now, the fact that these types of gas mixtures must be premixed and contained in gas bottles poses a problem.
Thus, it will be understood that, when such a gas mixture has to be used with different hydrogen contents in the nitrogen and/or argon in order to comply with several different cutting applications or situations, it is necessary to store as many gas bottles as desired contents, that is to say hydrogen
itrogen/argon mixtures having different hydrogen contents.
This therefore requires the use of a large stock of gas bottles of different compositions suitable for the various situations that might arise and also requires many bottle movements and handling operations.
Under these conditions, it is essential to put into place strict procedures for storing and handling each bottle, as well as precise monitoring of the stocks in order to prevent any shortage in terms of a particular gas mixture, something which is difficult to manage and involves relatively large financial investment and operating costs.
The object of the present invention is therefore to solve these problems of bottle storage and movement while making it easier to use gas mixtures having different hydrogen contents.
In other words, the aim of the present invention is to provide a plasma-arc work process using gas mixtures having variable hydrogen contents compatible with various arc-work situations, particularly various material-cutting situations, while at the same time maintaining or improving the cutting performance in terms of rate, flash and/or roughness of the cut faces.
The present invention therefore relates to a process for the plasma-arc working of at least one workpiece, in which:
(a) a plasma torch is supplied with an electric current and with a gas mixture containing hydrogen and at least one compound chosen from nitrogen and argon,
(b) a plasma jet obtained by the ionization of at least part of said gas mixture containing hydrogen and at least one compound chosen from nitrogen and argon by said electric current is delivered by means of said plasma torch,
wherein:
the concentration of hydrogen in the gas mixture containing hydrogen and at least one compound chosen from nitrogen and argon is greater than 0% by volume and less than 50% by volume,
said gas mixture is obtained by the addition of a defined amount of hydrogen to a gas containing at least one compound chosen from nitrogen and argon, said addition of hydrogen being carried out immediately before the gas mixture containing hydrogen and at least one compound chosen from nitrogen and argon is introduced into said torch, and
the amount of hydrogen is defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent material of the workpiece, the desired work rate and the intensity of the electric current.
Depending on the case, the process of the invention may include one or more of the following characteristics:
the process is a plasma cutting or plasma marking, preferably plasma cutting, process;
the concentration of hydrogen in the gas mixture containing hydrogen and at least one compound chosen from nitrogen and argon is between 10% by volume and 40% by volume, preferably between 20% by volume and 35% by volume;
the gas mixture contains nitrogen and argon;
the addition of hydrogen is carried out in at least one gas mixer, preferably said gas mixer including means for controlling or adjusting the hydrogen content;
the defined amount of hydrogen is defined according to the thickness of the workpiece and to at least one parameter chosen from the grade of the constituent material of the workpiece, the desired work rate and the intensity of the electric current;
the thickness of the workpiece is between 0.3 mm and 150 mm, preferably between 0.4 mm and 120 mm;
the desired work rate is between 0.4 m/min and 10 m/min, preferably between 0.5 m/min and 5 m/min;
the intensity of the current is between 10 A and 700 A, preferably between 15 and 600 A;
the grade of the constituent material of the workpiece is chosen from stainless steels, aluminum and light alloys, preferably 304L or 316L stainless steels, or A42 aluminum. A steel may or may not include (non-alloy steel) in addition to iron which constitutes it, other addition elements in variable concentrations. Thus, if the content of addition elements is less than 5% by weight, the steel is called a low-alloy steel, whereas if the content of addition elements is between 5% and 10% by weight, the steel is called a high-alloy steel. A stainless steel includes, in addition to the iron which constitutes it, more than 10% chromium and pos
Augeraud Regis
Borne Andre
Delzenne Michel
L'Air Liquide, Societe Anonyme pour l'Etude et l&Prime
Paschall Mark
Young & Thompson
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