In the fin tube welding process rolled steel strip is continuously welded in spiral form on the outside diameter of a tube. The weld produced in this process is a true forge, blacksmith weld. This type of weld is comprised of a fusion between two portions of parent metal without the introduction of a filler material. The weld is simply produced by heating the interfaces to be joined to a plastic state and applying pressure.
The process Tulsa Fin Tube uses differs from other methods of electrical, resistance welding in that through the use of high frequency current the amount of material heated in order to produce the weld is minimized and localized to the area where the forge weld occurs. This localization is accomplished by introducing high frequency current directly into the material by means of sliding contacts located prior to the weld point. The incoming fin material converges toward the tube in the form of a “V”.
High frequency current seeking the path of least impedance, flows from the contact on the fin to the apex of the “V” and back through the surface of the tube to the opposite contact. Due to the high frequency skin effect and the proximity effect of adjacent conductors, the high frequency current tends to crowd the inner most edge of the “V”. This high concentration of current preheats the inner edge of the fin and outer surface of the tube directly under the fin so they are at welding temperature when they abut at the apex. The temperature of the material reaches a maximum at the apex of the “V” which is coincidental with the forge point.
In making a fin-to-tube weld, there is a distinct difference in the thermal balance or heat sumps on the two sides of the “V” as compared to ordinary pipe or tube welding. It is desirable to put more electrical energy into the tube than into the fin.
The difference in thermal sump requires an adjustment in the contact arrangement that is introducing the high frequency current. The adjustment involves placement of the contacts on the two surfaces with a longer current path in the tube surface than in the edge of the fin. By the displacement of the two contacts, each unit area on the surface of the tube has current flowing into it for a longer period of time than the equivalent unit area of the edge of the fin. Hence, more total heat will be put into the tube in order to compensate for the increased thermal flow away from the weld area. This means that both the tube and the fin can be brought to the weld point at proper welding temperature.
The high frequency welding process depends upon two phenomena associated with HF current, Skin Effect & Proximity Effect. Skin Effect is the tendency of HF current to concentrate at the surface of a conductor.
The second electrical phenomenon which is important in the HF welding process is Proximity Effect. This is the tendency of the HF current in a pair of go / return conductors to concentrate in the portions of the conductor surfaces which are nearest each other. The physics behind Proximity Effects depends on the fact that the magnetic field surrounding the go / return conductors is more concentrated in the narrow space between them than it is elsewhere. The magnetic lines of force have less room and are squeezed closer together. It follows that Proximity Effect is stronger when the conductors are closer together. It is also stronger when the side facing each other are wider.
Spume is the name generally given to the small particles of metal that are ejected from the welding “V” during the high frequency welding operation as a result of electromagnetic forces produced by the weld current. Stainless steel spume tends to be in the form of very small, spherical particles of metal and refractory oxides. It is extremely important to always have coolant directed at the weld point and in between the deforming wheels to prevent spume build-up.
The optimum weld power is the minimum power at which a satisfactory weld is obtained. Adjustment of the weld power is generally done by observing the spume which occurs at the apex of the weld. The spume, which appears as yellow or orange droplets of small diameter, usually spurts downstream from the weld area. A moderate, continuous spume is the best indication of adequate weld power and steady mill conditions. Uneven spume is an indication of uneven welding conditions and usually of poor weld quality. It can be caused either by mechanical irregularities in the mill, such as deforming wheels, fin tilt roller, tips or excessive weld power.
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