Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of plasma gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto.
When using an inert plasma gas, such as argon, the electrode may include a tungsten element for supporting the arc. The use of inert gases is not always convenient or economical and it is often preferable to use an oxygen gas, such as pure oxygen or air as the plasma gas. Tungsten electrodes cannot be successfully used with oxygen gases, however, because they oxidize too readily in such environments. Accordingly, commonly used emissive materials for oxygen plasma gases include hafnium, zirconium, and their alloys.
A design concern associated with torches of the described type is increasing the service life of the electrode. A significant factor in the service life of the electrode is erosion of the emissive element, which may be further defined to include both arc time erosion and start erosion. The average electrode erosion, therefore, depends on the average duration and frequency of the cuts. Empirical observations as to duration erosion and start erosion indicate that both components are approximately equal for an arc cycle of 30 seconds on followed by 4 seconds off. The present invention is primarily concerned with decreasing start erosion.
While not wishing to be bound by the theory, the inventor presents the following explanation of the start erosion mechanism. During its work as a cathode, the emissive element typically oxidizes, thereby forming a layer of oxide upon the outer surface of the element. In the case of hafnium, the layer includes both stoichiometric hafnium oxide, HfO.sub.2, and a related hafnium oxide, HfO.sub.1.7. Stoichiometric hafnium oxide is a relatively good emitter as well as a good conductor of heat and electricity. However, the oxide retains its conductive properties only when hot; when cold, the oxide becomes an insulator, as recognized by U.S. Pat. No. 5,083,005.
When starting an arc with electrodes of this type, the arc first attaches to the periphery of the emissive element and then migrates to the center of the electrode. The hafnium oxide is initially cold but is rapidly heated. The rapid heating causes thermal "shocks" to the brittle hafnium oxide which cracks as the arc travels to the center of the element. The hafnium oxide layer is thus broken up and removed from the surface of the element, contributing greatly to the start erosion problem. In addition, the particles of hafnium oxide may be deposited on the inner surface of the torch nozzle which provide attachment points for a second arc and increase the possibility of detrimental "double arcing."
This theory of the start erosion mechanism is supported by two additional observations. First, the hafnium oxides are created during each electrode use and are present at the start of the following cycle. The thickness of this oxide layer is approximately the same as the amount of erosion observed for each start. Second, some torches use a tungsten electrode in a hydrogen plasma gas. However, tungsten does not form a compound with hydrogen under these conditions and, indeed, no start erosion is readily observed for a tungsten electrode in a hydrogen plasma gas.
Accordingly, it is desirable to provide an emissive electrode element which exhibits reduced start erosion to promote longer service life and increased performance of the element. Such an element would be useful in an oxygen environment but would suffer to a lesser extent from the detrimental loss of an oxide layer upon starting. In addition, the emissive element should not be prohibitively expensive and would preferably utilize commonly available materials and manufacturing techniques.