The present invention relates generally to electrochemical machining, and, more specifically, to machining of long tubes.
Petroleum undergoes pyrolysis in cracking furnaces for producing useful products such as ethylene. The cracking furnace uses long tubes through which the petroleum is channeled and heated for the pyrolysis thereof.
Fluted process tubes are desired for increasing the available surface area and the corresponding rate of pyrolysis. Such fluted tubes may be formed by extruding suitably malleable metal, yet such metal is subject to undesirable creep at the high temperature required in the pyrolysis cracking process.
Accordingly, a nickel and chromium based superalloy having substantial high temperature creep strength is preferred for the process tubes. However, such high strength material cannot be extruded, and the flutes must be otherwise formed.
For example, electrochemical machining (ECM) may use a cathode tool in the desired shape of the internal flutes of the tube for electrochemically forming the flutes as an electrolyte flows between the tool and the tube when powered under high electrical current defining respective cathode and anode. ECM chemically erodes the initially circular unfinished surface defining the tube bore to form internal flutes being complementary with the external flutes of the cathode tool.
However, since the individual tubes are long, and the cathode tool must be driven completely through the tube, the electrolyte contained in the tube can effect stray ECM in the finished bore as the tool continues its travel through the tube. And, the electrolyte becomes heated during operation and affects the ECM process.
Accordingly, it is desired to provide an improved ECM apparatus and process for locally isolating the cathode tool during operation, as well as providing internal cooling of the tube in a preferred embodiment.
An ECM apparatus includes a fluted cathode tool for being driven through a tubular workpiece for electrochemically forming internal flutes therein. The cathode tool is sealed at one end for isolating electrolyte over the flutes thereof to seal against flow of the electrolyte past the tool and into the finished fluted bore.