Electric heating – Metal heating – By arc
Reexamination Certificate
1999-09-17
2001-11-06
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130500, C219S137710
Reexamination Certificate
active
06313437
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method in manual or automatic arc welding, of controlling the end of the welding process to ensure that the crater formed in the welding operation is minimised.
BACKGROUND
In arc welding, a depression forms in the molten pool from the pressure of the arc and the material transferred by the arc from the electrode to the work-piece. This depression is often referred to as the crater. At the end of the welding operation, this crater remains in the shape of an end crater. When the energy supply ceases abruptly at the end of the welding operation the molten material solidifies while shrinking. The solidification starts at the weld edges and progresses towards the centre. Particularly in the case of high-energy supply welding with consequential large molten pools, there is a deficiency of material in the centre of the molten pool as the molten material solidifies. As a result, a so called pipe forms, i.e. a hole extending vertically through the end crater. Often, this pipe also is the origin of fissures or cracks extending in the longitudinal direction of the welding bead. Welding defects of this nature impair the weld strength and for this reason various methods have been tested in order to eliminate the end crater.
It is known that in manual welding operations the welder often fills up the end crater by increasing the speed of advancement immediately prior to the end of the welding operation, whereupon he moves the gun back in the molten pool in order to thus produce the crater filling. The method puts considerable demands on the skill of the welder-and its repeatability is poor.
It is further known in automated arc welding to extinguish the arc for a programmed period of cooling and to thereafter re-ignite under reduced welding data conditions in order to produce the crater filling for a programmed filling period (U.S. Pat. No. 4,578,562). The method is time-consuming and disturbing welding spatter occurs in connection with the re-ignition.
It is finally known to successively reduce the size of the molten pool by continuously or in essentially equally large steps reduce the welding voltage and the wire feed rate, respectively. Particularly when the welding is performed under high energy supply in the spray transfer range, such as is the case in MIG/MAG welding, the welding data will, in the fading stage, pass through a range wherein considerable welding spatter and an uneven arc are produced.
OBJECT OF THE INVENTION
The object of the invention is to suggest a novel method in manual or automated arc welding operations that use a melting/consumable electrode, of controlling a source of welding current and the associated wire supply arrangement in a manner ensuring repeatable end-welding operations that in a minimum of time produce satisfactory crater-end filling without causing disturbing welding spatter.
This object is achieved in accordance with the teachings of the invention by fading the arc voltage and the wire feed rate, respectively, said fading comprising arc voltage reduction at the onset of the end of the welding operation for a period that is considerably briefer than the total fading time, during which the arc voltage is reduced to a value below the value at which it is possible to weld during continuous welding and advancement in the joint with otherwise unchanged welding parameters, followed by a further reduction to a final value of arc voltage and fading of the wire feed rate simultaneously with the arc voltage fading to a final value of the wire feed rate. Because the supplied heat is reduced rapidly at the onset of the end of the welding operation while simultaneously more material is supplied, the molten pool is cooled and the size of the molten material is reduced without disturbing welding spatter being generated, since the difficult-to-weld globular transfer range is traversed in a much briefer period of time. In this manner, the end of the welding operation thus becomes very brief and the crater filling satisfactory.
Advantageous modifications of this method are defined in the dependent claims.
According to a further aspect of the invention only the fading time and the arc voltage end values and/or the wire feed rate are adjustable parameters. This simplifies handling and reduces the time consumption further.
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Åberg Per
Soting Tor
Burns Doane Swecker & Mathis L.L.P.
Esab AB
Shaw Clifford C.
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