The present invention relates to a protective cap for the nozzle of a plasma arc torch and a plasma torch provided with such a cap.
A plasma torch usable in the cutting, welding, marking, projection or any other operation for thermal treatment of a metallic or non-metallic material, conventionally comprises a copper or copper alloy electrode carrying a cylindrical insert generally of hafnium, tungsten or zirconium, on which the electric arc can take root which serves to ionize the gas supplying the torch, which is to say the predetermined flow rate of gas under pressure, so-called plasmagenic gas, which is distributed between the electrode and the nozzle and which flows through an orifice of said nozzle in the direction of the workpiece.
The electrode is generally centered above this ejection opening of the plasma jet arranged 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 conventionally comprises a plasma torch, a source of electric current, a system for striking the electric arc and one or several sources of fluids, in particular plasmagenic gas, if desired a protective gas or post-injected fluid, and a cooling fluid for the torch, generally distilled water.
Such torches or installations are well known to those skilled in the art because they have already been described in numerous documents which can be referred to for greater detail, particularly 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 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 melted material from the kerf formed consecutive to relative displacement of the torch and the workpiece; the composition or nature of the plasmagenic gas used varies according to the nature of the material to be cut.
The torch nozzle generally has an intermediate electric potential comprised between the electrical potential of the cathode and that of the plate to be worked.
However, for mechanical reasons or for errors of operation, it can happen that the torch comes into contact with the plate.
There is thus created a parasitic arc which instantly destroys the nozzle, which thus translates into a very substantial deterioration of the quality of the cut and/or requires the operator to change the nozzle.
The duration of use of the nozzle is thus reduced, which increases considerably the cost of the process because the nozzles used must be replaced more frequently by new nozzles.
So as to overcome this problem, it has already been proposed to arrange a protective cap about the nozzle of the torch so as to form an insulating mechanical barrier between the nozzle and the plate so as to protect the nozzle, during possible contact with the plate.
To do this, certain documents provide for the use of stumatite caps, which is a natural ceramic, or alumina.
However, it has been noted in practice that if these materials resist thermal shock, they are relatively fragile and can break easily in case of shock against a plate.
Moreover, it is also known to use a zirconia cap.
However, here again, it has been noted on an industrial scale that, if zirconia caps were more durable than stumatite caps or alumina caps, they have, on the other hand, a low resistance to shock and thermal stress.
The problem which accordingly arises is to have a cap for a plasma torch constituted by an insulating material permitting resisting not only high temperature of the plasma jet, which is to say thermal stresses, but also mechanical stresses, particularly shocks against the plate to be worked.
The object of the present invention is thus to solve these problems by providing a protective cap for plasma torch formed of a material having high resilience to resist mechanical shocks, a low coefficient of expansion to resist thermal shocks and which will moreover be a good electrical insulator.
Preferably, the protective cap according to the invention must also be relatively smooth, which is to say without roughness, such that the flag projected during working the metal will not adhere or will adhere the least possible to the cap.
The present invention thus relates to a protective cap for a plasma arc working torch, comprising a cap body comprising an opening for passage of plasma gas, said cap body being comprised of at least one material of ceramic type, characterized in that said ceramic is silicon nitride or aluminum silicate, preferably silicon nitride.
As the case may be, the cap of the invention can comprise one or several of the following characteristics:
said cap body has at least one cylindrical section and an axis of revolution coaxial with said opening, preferably said cap body has the general shape of a sleeve or cup.
the ceramic cap body is at least partially clad with boron nitride deposited on the external surface of the cap body, preferably the thickness of the boron nitride coating is less than 3 mm. The deposit of the layer of boron nitride on the external surface of the cap body is carried out, for example, by sputtering of boron nitride.
the ceramic is aluminum silicate formed of SiO2, Al2O3 and of one or several added constituents selected from TiO2, Fe2O3, CaO, MgO, K2O, Na2O and P2O5, and if desired unavailable impurities, in the following proportions: at least 60% by weight of SiO2, at least 25% by weight of Al2O3 and the rest being essentially one or several of the added constituents selected from TiO2, Fe2O3, CaO, MgO, K2O, Na2O and P2O5. Preferably, the ceramic is aluminum silicate formed by 60 to 80% by weight SiO2, preferably 60 to 70% of SiO2, from 25 to 35% by weight of Al2O3, preferably from 28 to 34% by weight of Al2O3, and for the rest (up to about 100% by weight) one or several additional constituents selected from TiO2, Fe2O3, CaO, MgO, K2O, Na2O and P2O5.
the ceramic is silicon nitride having one or several of the following properties: a density of 2 to 4 kg/dm3, preferably 2.3 to 3.5 kg/dm3; a resistance to thermal shock greater than or equal to 600xc2x0 C.; a volume resistivity of at least 1010xcexa9xc2x7cm, preferably at least 1012xcexa9xc2x7cm; a coefficient of thermal expansion less than 6.10xe2x88x926/xc2x0 C., preferably less than 4.10xe2x88x926/xc2x0 C.; a flectural resistance at 20xc2x0 C. of at least 200 MPa, preferably at least 600 MPa; and/or a compressive strength of at least 650 MPa, preferably at least 1500 MPa.
the body of the cap has a thickness comprised between 2 mm and 10 mm, at the level of or adjacent the opening for passage of the plasma gas.
the body of the cap comprises securement means permitting fixing or arranging securely the cap on a plasma torch about at least one portion of the nozzle of said torch, so as to surround and protect said nozzle.
the cap body is formed of a downstream portion at least partially of ceramic having the orifice, fixed to an upstream portion serving as a support permitting securing said cap body on the plasma torch, preferably of the downstream ceramic portion being fixed to the upstream portion by crimping, cementing or any other suitable technique and said upstream portion is of a metal or a metal alloy, for example brass.
the external wall of the cap body comprises one or several external protuberances, preferably said protuberances have a dimension comprised between 0.5 and 2 mm relative to the surface of the cap.
The invention also relates to a plasma torch comprising a torch body and a torch head provided with an electrode and at least one nozzle, characterized in that it comprises a protective cap according to the invention surrounding at least a portion of said nozzle so as to form an effective mechanical, electrical and thermal barrier about said nozzle.
As the case may be, the torch body and the torch head are separable from each other, as explained in EP0 599 709, to which reference may be had for further details.
Moreover, the invention also relates to an automatic machine, in particular for plasma cutting, comprising a torch according to the invention, as well as to the use of a torch or of a machine according to the invention to cut at least one metal plate by the use of a plasma jet, preferably a steel plate.
Thus, the inventors of the present invention have discovered that, among many ceramics adapted to be used as a material for making a cap for a plasma torch, namely the silicates comprising the steatites and the cordierites, titanium oxides, aluminas and zirconias, the natural ceramics, such as stumatite, certain among them having particularly unexpected properties, which is to say that they have a high resiliency to resist mechanical shocks, a low coefficient of thermal expansion to resist thermal shocks and can be rendered relatively smooth by simple machining.
These particular ceramics are silicon nitride (KERSIT(trademark) or KERNIT(trademark)) and aluminum silicate, which ceramics permit harmonizing all of the above-mentioned requirements.
A protective cap for a plasma torch comprised by one or the other of the particular ceramics according to the present invention, is preferably dimensioned so as to cover at most the nozzle of the torch whilst not interfering with the operation of this latter, which is to say not disturbing the passage of the plasma gas stream.
Thus the distance between the plate and the nozzle being very small, the thickness of the cap must be as small as possible and must permit evacuation of the slag above the plate.
Moreover, protuberances on the cap permit holding the cap spaced from the plate without interfering with the evacuation of slag or of the gas.
Moreover, preferably, an external coating on the cap constituted by boron nitride permits, because of its physical-chemical properties, avoiding slag or molted metal from splashing from the plate during piercing of the latter, for example, not becoming stuck to the protective cap clad with this external cladding.
The invention will now be better understood from the accompanying drawings and from the examples, given by way of illustration but not limitation, of the invention.