If die or continuously cast slabs feature surface irregularities that could have a disturbing or damaging effect during further processing, such as casting defects, inclusions, cracks, or burrs, these slabs must undergo a surface treatment. A machining process, such as grinding or face milling, is complicated and hence inefficient. A scarfing process is the better solution here.The scarfing process works on the principle of slab surface heating and subsequent removal of the material that has been locally melted as a result of the heating by a gas jet. The slab surface is heated by means of a torch. Inside the torch the fuel gas flows to the gas nozzle through annularly arranged gas feeding channels. The flame of the torch ignites in the gas nozzle where the fuel gas is mixed with oxygen, fed by a central gas line. The surface defects are eliminated by guiding the gas nozzle mounted in the torch head over the slab surface until it is completely defect-free. The surface treatment costs are mainly determined by the processing time and gas consumption (both oxygen and fuel gas). In the past there was only little potential for a reduction in gas consumption, because the treatment quality had to be maintained or, if possible, even further improved. Egon Evertz KG has several decades of experience in scarfing processes and the associated torch technology. Figure 1 shows a scarfing machine developed by Egon Evertz KG. Development activities at Evertz led to the construction of a new impulse torch which is used in the Evertz scarfing machine type 009. For the scarfing operation the torch, which features a working range of 100 – 450 mm, is guided along the side faces of the slab, where it covers the complete width, then over the complete top and bottom faces in slewing and meandering movements or over selected areas of the slab surface. The machine is operated manually, with the operator having a perfect view of the torch and the slab. This guarantees immediate recognition of faults and the possibility of immediate remedial action. Various scarfing programmes are available to adjust the scarfing depth and speed. The latter can range between 8 and 25 m/min. The advantage of the manual mode over automatic operation is that each individual slab can be processed by the operator in the most effective and Egon Evertz KG has developed an impulse torch capable of considerably reducing the amount of energy consumed during slab scarfing. This new impulse torch is used in Evertz’ type 009 scarfing machine. At the side faces of the slab the torch, featuring a working width of 100 – 450 mm, covers the complete surface area. On the top and bottom faces of the slabs the torch is guided in meandering movements over the complete surface or only over selected areas of the slab surface. Continuous casting - Special equipment MPT International 4/2007 hence most time-saving way. If the scarfing operation was performed fully automatically at a uniform scarfing depth, it would be necessary to select a scarfing depth which ensures that all near-surface defects are captured. A drawback of this is the considerable loss of slab material, because in many surface zones a smaller scarfing depth would be sufficient. A possible alternative would be to measure and optically capture each slab before scarfing in order to determine the position of the various surface defects. This would involve a high technical effort which in most cases would make the scarfing operation uneconomical. Against this background in many situations manual scarfing or semiautomatic scarfing, i.e. areas currently being processed are supervised by monitors, but the scarfing process proper is controlled by the operator, are the better alternative. Impulse torch for pulsating gas jets The innovative impulse torch head uses a new nozzle geometry capable of generating pulsating gas jets. This enables gas consumption of both smaller manual scarfing devices and larger automated scarfing machines to be reduced to a minimum while considerably increasing the effective scarfing performance. The nozzle features two parts. The first part, the shorter one, is characterized by a conical shape tapering down in the direction of flow. At the point where the cross section is smallest the second – longer – part starts. Also this part of the nozzle is conically shaped, however reversely, leading to a widening of the cross-sectional area. Exciting the gas flow by acoustic waves enables the setting of different impulse characteristics of the outflowing gas. These impulse characteristics are optimized with a view to gas consumption, scarfing efficiency and energy requirement. The key feature of this technology is the reduction in the nozzle cross-sectional area down to a critical value followed by an enlargement of the cross section. This conical enlargement is followed by a section featuring a uniform cross-sectional profile acting as a stabilization ring to maintain the created gas flow profile. The described nozzle design generates a pulsating gas jet which attains sonic or supersonic speed at the nozzle outlet. The gas profile is determined, on the one hand, by the ratio between the oxygen pressure at the nozzle inlet and the ambient pressure and, on the other hand, by the ratio between the oxygen pressure at the nozzle outlet and the ambient pressure. The impulse frequency can be controlled by selection of the appropriate geometric parameters and pressure ratios. In a concrete application a broad face of a 45 t slab of 13 m length and 2 m width, i.e. a surface area of 26 m 2 , could be finish-scarfed in approx. 6 min. The torch was – without any interruption - guided over the slab surface in swivelling movements, from the left to the right and vice versa. This technique, which required only onetime ignition, led to a doubling of the scarfing capacity. There were hardly any ridges between the scarfing cuts. The scarfed surface of the slab was completely scale-free. One of these innovative torches was released to the Faculty for Steels and Alloys of the State Polytechnic of St. Petersburg in Russia for test purposes. Prof. Dr. Anatoli M. Sizov supervised the investigation conducted with the test arrangement shown in Figure 2 . An oscillation generator was installed in the provided torch in order to facilitate the transition to impulse scarfing. Within the framework of the tests various oscillation progression curves were set to obtain different impulse characteristics of the outflowing gases. Figure 3 shows the amplitude/frequency characteristics of the outflowing gas. The gas impulse sequence was generated at an oxygen inlet pressure of 1.4 MPa. At the outlet the gas had supersonic speed, which causes the pressure impulse curves typical for incomplete expansion conditions.

MPT International 4/2007