Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2000-01-20
2001-09-25
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S055000, C219S091100
Reexamination Certificate
active
06294752
ABSTRACT:
TECHNICAL FIELD
The claimed invention pertains to flash-butt welding, predominantly to welding of items of a large cross-section, and can be applied in various industries where flash butt welding is used.
BACKGROUND ART
Known is a process of flash-butt welding with a rigid program control of flashing speed (<<Technology and equipment of flash-butt welding>> B. D.Orlov, Yu. V.Dmitriev, et al., Moscow <<Mashinostrojenie>> 1975, see (p.280), where the value of the allowance for welding and upsetting is programmed. Such systems are operating reliably under the conditions in which self-regulation of flashing is ensured in a broad range of parameters, i.e. in welding of parts of a comparatively small cross-section (for sections of up to 1000-1500 mm
2
) and in welding with preheating. The program of speed variation is usually accomplished by means of cams and is predetermined.
The main disadvantage of the systems of programming by travel is the need to ensure an accurate preparation of the end faces of the parts to be welded, prior to welding. Inaccurate positioning of the parts prior to welding, non-uniform gap between the parts, change of the geometrical dimensions of edge preparation—all these deviations require an appropriate correction of the optimal allowance for flashing and change of the assigned program, especially in welding of large cross-sections where the process of flashing excitation is especially difficult and requires high power consumption.
Known is a method of continuous flash-butt welding in which a coefficient of negative feedback by current is used, which is changed during welding with the change of voltage. The process of flashing by this method is described on pages 55-59 of the book edited by S. I.Kuchuk-Yatsenko “Equipment for flash-butt welding of rails and its operation”, Kiev. “Naukova Dumka”. 1974. This method is used in continuous flash-butt welding of parts of a large cross-section. The control of the change of the transformer voltage and of the flashing speed is set as a function of flashing time. In welding with the above prior art method, the coefficient of negative feedback by welding current is set at the start of welding, and after some time, at the moment of the welding voltage switching, the negative feedback coefficient is changed. With this method the change of the negative feedback coefficient is connected with the change of welding voltage.
In this method the change of the negative feedback coefficient is not connected with heating of the parts being welded. With this method the speed of the parts drawing together and the coefficient of negative feedback by current is set prior to welding. After the parts contact each other, the welding current flows through the welding circuit and flashing starts. The flashing speed (actual speed of drawing together) is determined both by the set speed of the parts drawing together, and by the coefficient of negative feedback by current. At the start of welding, when the part end faces are cold, the flashing process requires a high power. As the parts being welded are heated, the welding current drops, if the set speed of drawing together is low (of the order of 0.1-0.4 mm/s). Lowering of welding current (it is due to heating of the parts being welded) leads to reduction of power consumption and heating intensity. In this case also the intensity of heating the parts being welded, is, lowered as the losses of energy consumed in the parts heating become greater with the increase of the flashing speed. Lowering of heating intensity increases the welding time. Now if the set speed of the parts drawing together is high, excitation of the welding process is problematic. Transition to a short-circuit is possible at the start of the process. This method envisages lowering of welding voltage and change of the negative feedback coefficient at the moment of voltage switching. This technique somewhat increases the heating intensity. This measure, however, is not effective enough, as lowering of welding voltage leads to reduction of the set power of the welding machine, as well as of the useful power.
This method does not allow assessment during flashing of the change of the physical condition of the end faces of parts being welded. This essentially influences the quality of the welded joint and the stability of production of a sound joint. This method does not envisage automatic selection of the most favorable moment for change of voltage and performance of upsetting.
Known is a method of flash-butt welding in which upsetting is switched on when the specified temperature of the parts being welded has been reached (USSR A.C. 241567). The Curie point is taken as the specified preheating temperature, whereas the command pulse is given with an electromagnetic device measuring the size of the zone of preheating of the end faces being welded.
The quality of welded joints in flash-butt welding is determined not only by the preheating zone, but also by many other factors, for instance gradient of the temperature field of the preheating zone, temperature of end faces heating, mains voltage variation, flashing nature, speed of flashing in heating and prior to upsetting, upset value and many others. The above method takes into account only the width of the heating zone, but it does not provide the required stability of the welded joint quality.
Known is a flash-butt welding method (A.C. 313628) in which the flashing process is changed to accelerated flashing with subsequent upsetting. The value of flashing speed is a signal for switching on accelerated flashing and upsetting.
This method does not envisage control of the welding process at the stage of heating, i.e. during the longest period of welding.
In welding of parts this method does not permit assessment of the degree of heating of the parts being welded, or switching to accelerated flashing and upsetting at the most favorable moment which will ensure a high quality of welded joint.
Known is a method of regulation of the process of flash-butt welding (USSR A.C. 593857) by the action of the machine mobile plate, depending on the mismatch signal generated as a result of comparison of the actual and assigned value of the parameter characterizing the flashing process. In this case the energy evolved in the butt is used as the parameter characterizing the flashing process, in order to stabilize the quality of the joints at the arising disturbances by voltage or other energy parameters of the process.
This method provides the stability of heating in welding, as heating of parts in this case does not depend on random changes of the welding mode parameters, which determine the heating.
However, in welding of parts with a large cross-section, it is impossible to achieve the required heating of the parts being welded, and this welding process does not envisage correction of the welding parameters (for instance, welding voltage, feedback coefficient, flashing speed).
Known is a device and method of flash-butt welding (U.S. Pat. No. 4,101,753), predominantly of the strip ends in continuous rolling lines and the corresponding device for implementation of this method, in which energy fed into the joint zone, is measured continuously, and the current is switched off at the upsetting stage when the specified energy value has been achieved. The device consists of the drive of the parts movement, current supplies, upset drive, ignitron contactor connected into the primary winding of the welding transformer and machine control system. This system consists of the memory block connected with the device of energy measurement at the flashing stage, block of comparison of the energy supplied during the entire welding cycle and the reference value of energy, as well as an output logic block which forms the command for the contactor switching off. In this case, the amount of energy supplied at the upsetting stage depends on the energy supplied at the flashing stage. Also provided is a device for recording and storing the values of energy in each stage of the process, this
Bogorskiy Mikhail V.
Didkovskiy Alecsandr V.
Gorishnyakov Alexei I.
Krivenko Valeriy G.
Krivonos Vadim P.
Cooke Colleen
Dunn Tom
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