Electric heating – Metal heating – By arc
Reexamination Certificate
2001-02-20
2002-06-18
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121550, C219S121440
Reexamination Certificate
active
06407358
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a plasma-arc work process using a plasma torch supplied with a gas mixture of an oxidizing gas and nitrogen, the amount of one gas in the mixture with respect to the other 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 which 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. However, in the case of structural steels, even though compressed air has the advantage of being simpler to use than oxygen, since a simple air compressor is sufficient to supply the torch and there is therefore no need to supply gas bottles, it is found that the air used as cutting gas results in cut faces which are of inferior quality to those obtained using oxygen, that is to say it results in the presence of flash and of striations. Oxygen is therefore usually preferred for this reason.
Moreover, the use of mixtures of nitrogen and oxygen in proportions of about 30% nitrogen, as plasma gas, make it possible to increase the cutting speed by about 20% with respect to pure oxygen.
In addition, this same type of nitrogen/oxygen mixture used as a dual flow enables, in some cases, the roughness of the cut faces to be reduced.
However, this nitrogen/oxygen mixture is 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.
SUMMARY OF THE INVENTION
Now, the fact that this type of nitrogen/oxygen mixture must be premixed and contained in gas bottles poses a problem.
Thus, it will be understood that, when a nitrogen/oxygen mixture has to be used with different nitrogen contents in the oxygen 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 nitrogen/oxygen mixtures having different nitrogen 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 mixtures of nitrogen and oxygen of different contents.
In other words, the aim of the present invention is to provide a plasma-arc work process using nitrogen/oxygen mixtures of variable 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, and, more generally, a plasma-arc work process using nitrogen/oxygen-containing gas mixtures.
The present invention therefore relates to a process for the plasma-arc working of at least one workpiece made of structural steel, in which:
(a) a plasma torch is supplied with an electric current and with a gas mixture containing oxygen and nitrogen,
(b) a plasma jet obtained by the ionization of at least part of said gas mixture containing oxygen and nitrogen by said electric current is delivered by means of said plasma torch, wherein:
the concentration of nitrogen in the gas mixture containing oxygen and nitrogen 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 nitrogen to an oxidizing gas containing oxygen, said addition of nitrogen being carried out immediately before the gas mixture containing oxygen and nitrogen is introduced into said torch, and
the defined amount of nitrogen is defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent structural steel of the workpiece, the desired work rate and the intensity of the electric current.
So-called “structural” steels are defined by the Comité Européen de Normalisation [
European Standardization Committee
] as belonging to the families of nonalloy steels, low-alloy steels or high-alloy steels satisfying the EN 10025, EN 10113-2, EN 10113-3, EN 10137-3 et seq. standards.
According to another aspect, the invention also relates to a process for the plasma-arc working of at least one workpiece made of stainless steel, in which:
(a) a plasma torch is supplied with an electric current and with a gas mixture containing oxygen and nitrogen,
(b) a plasma jet obtained by the ionization of at least part of said gas mixture containing oxygen and nitrogen by said electric current is delivered by means of said plasma torch,
wherein:
the concentration of oxygen in the gas mixture containing oxygen and nitrogen is greater than 0% by volume and less than 25% by volume,
said gas mixture is obtained by the addition of a defined amount of oxygen or of a gas containing oxygen in nitrogen, said addition of oxygen being carried out immediately before the gas mixture containing oxygen and nitrogen is introduced into said torch, and
the amount of oxygen or of oxidizing gas containing oxygen is defined according to at least one parameter chosen from the thickness of the workpiece, the grade of the constituent stainless steel of the workpiece, the desired work rate and the intensity of the electric current.
Steels-called “stainless” steels belong, according to the Comité Européen de Normalisation [
European Standardization Committee
] to the family of all
Augeraud Regis
Delzenne Michel
Suzon Serge
L′Air Liquide, Societe Anonyme a directoire et Conseil de
Paschall Mark
Young & Thompson
LandOfFree
Process and unit for plasma-arc working with a gas having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process and unit for plasma-arc working with a gas having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process and unit for plasma-arc working with a gas having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2915268