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Technological Parameters Choice of Rail Flash Butt Welding
addtime:2009-04-11click:1425


Reasonable technological parameters are the guarantee of high quality flash butt welding joint. Reasonable technological parameters can not only ensure the high quality welding joint of low carbon steel workpiece, but also can make some metal workpiece with poor weldability get satisfactory quality of joint.

Welding Voltage and Current      

Reasonable welding current is the necessary condition of stimulating flash and maintain stable process of flash, usually we use current density (the current which through unit binding surface) represents the welding condition level. Usually, flash butt welding current density is 10-25ampere/millimeter. If current density is too small, rate of heating is low, flash butt can not be stimulated quickly or the process of flash butt is not steady. If current is too large, the process of flash butt welding will be too fierce, there will be deeper fire-pit on welding edge face, when upset, it is difficult to seal up them, and thereby there is defects. If current is too large, the process of flash butt welding will be too fierce, this can make the temperature field of welding   excessive, when upset, it is not easy to make plastic deformation and then reduce the quality of joint. When volume-produce mild steellow alloyed steel joint, usually choose upper limit value of current density.  After the butt-welding machine is definedthe current of the joint is determined by voltage.  Change the gear of closing primary coil tap of welding transformator, then change the welding voltage and current (sometimes stepless regulation of the welding voltage).

Burn-off rateFeed rate      

The burn-off rate of flash butt welding is determined by welding current densitypreheat degree and some other factors of workpiece chemical constituents. The heat produced on the interface between two workpieces when flash butt welding, the temperature of workpiece enhances with the increase of flash and the burn-off rate enhances with the increase of workpiece’ temperature . The feed rate must compatible with burn-off rate of the workpiece. If the feed rate is greater than burn-off rate, the two interfaces of the workpiece will short-circuited. Because the burn-off rate enhances with the increase of workpiece’ temperature, the feed-in of workpiece should be accelerated motion, and the motion curves should be nearly constant around the parabola. 

Combustion Allowance      

Combustion allowance is the lost length of workpiece in the process of flash-butt injection. Combustion lost must be considered when butt welding to ensure the length of the workpiece meets the design requirement. The basis which determines the combustion is: the allowance must be enough to ensure when the process of combustion finished, there should be even heating on all end faces of the two weldments, and the extent of heating should make plastic deformation in the process of upsetting to get high quality joints. The larger interfacial areas of the weldments, the slower homogenization rate of the temperature on the contact face, and the flash (combustion) time is longer, combustion allowance is larger. If pre-heating mean is carried out, the temperature of interface is higher, and there is temperature homogeneous diffusion to the whole interface, in the process of flashing, the time of temperature homogenization will be shortened, and then the combustion allowance will be smaller. To the steel parts of which section is round in shape, when the continuous flash welding is adopted, the combustion allowance (the sum of burned value of the two workpieces) normally is (0.6—0.7) d, when the preheating flash butt welding is adopted, the combustion allowance is (0.4—0.5) d, (D is the diameter of the workpiece).

Upset Allowance

Upset allowance is the shortening of the workpiece in the process of upsetting.

      (1) Molten metal and metallic oxide must be crowded to the surface of the workpiece

      (2) Metals of the two interfaces must close contact at the whole cross section. That is the contact parts of the two workpieces should produce sufficient plastic deformation. If the upset allowance is smaller, it is the main cause for the defects of shrinkage cavity inside the joint, cast structure, grey speck, oxide inclusion and incomplete penetration. If the upset allowance is larger, excessive plastic metal will be squeezed, and increases the difficulty in eliminating joint burr. In order to make sure the hot metal (liquid state) of the binding surface not is oxidized, and supplement the subcooling in the process of flash butt, the power should not be cut off before the upsetting, but in the process of upsetting. The upset process before the power is cue off is called charging upset; the subsequent process is called electroless upset. So in fact, the upset allowance is the sum of charging upset and electroless upset, and charging upset accounts for the whole upset allowance of 30%--40%.

Upset Speed

If there is flash butt welding, a normal tendency is for larger upset speed to crush out the molten metal before the solidification, and upset in the optimum breach when the metal has good plasticity. High upset speed will reduce the risk of oxidizing hot metal to get high quality welding joints. Usually, the upset speed is no less than 20mm / s.  Hydraulic rotary transmission welding machine generally has large upset speed, its upset speed can reach 60 mm / s.

Extension

Extension is the protruding length of workpiece from the clamp jaw (conductor). After fixturing, the distance between two conductors is the whole extension; its value is the sum of the extensions of two workpieces. When the section surface and material trademark are same, its extension is the half of the whole extension. When weld different materials workpieces, the speed of melting will be different in the process of flash butt because of different conductivity, thermal coefficient and melting point. If fixturing at the moment, the extension of the fast melting workpiece is larger than the extension of the other one.  When weld metal piece of which cross section is round in shape, the whole extension is normally (1—1.4) d, (D is the diameter of the workpiece).



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