Electric heating – Metal heating – By arc
Reexamination Certificate
2000-05-10
2001-03-20
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S061000, C219S1370WM
Reexamination Certificate
active
06204478
ABSTRACT:
The present invention relates to a method of pipe welding and more particularly the method of using a particular welding wire with a specific power supply known in the pipe welding industry as the STT electric arc welder for welding the open root between pipe ends.
INCORPORATION BY REFERENCE
Over the last decade, the art of welding large plates together has adopted a short circuit electric arc welder for many specific welding applications. This welder sold by The Lincoln Electric Company of Cleveland, Ohio under the trademark STT is disclosed in Stava U.S. Pat. No. 5,742,029 for use in a specific wallpapering application. The unique short circuit electric arc welder is now the power supply of choice for electric arc welding of the spaced ends of pipe sections when laying pipe lines in the field. The implementation of a pipe welding method utilizing the unique short circuit welder is disclosed in Parker U.S. Pat. No. 5,676,857. Since these two patents and the material disclosed therein define the STT welder by The Lincoln Electric Company and its application to welding heavy plates, such as pipes, the patents are incorporated by reference herein as background information so that this known technology need not be repeated. Certain concepts regarding the use of a cored electrode and reverse polarity welding by the STT welder is disclosed in a copending application by Elliott K. Stava, Ser. No. 200,594 filed Nov. 27, 1998. This prior copending application is incorporated by reference herein as background information and for technology which also need not be repeated to understand the present invention.
BACKGROUND OF INVENTION
When pipe welding in the field, the joint between the ends of pipe sections is essentially the same as a joint between two heavy plates, except the pipe joint to be welded includes an open root where the ends of the pipe are spaced slightly. This open root is a gap normally created by bringing the two pipe sections into abutting relationship, followed by a withdrawal of a selected amount to define the minimum open root in the joint. It is essential that this open root be welded together with a quality weld throughout the total thickness of the bottom area of the pipe joint; however, when laying the first bead in the open root, it is also necessary that the molten weld metal does not protrude inwardly of the pipe section to any substantial distance. The pipe must be clear so that a pig and other cylindrical devices can move through the pipe section without encountering inwardly protruding weld metal created during the open root first welding. As another consideration, the heat of the open root weld cannot be too high causing metal shrinkage and, thus, draw back into the gap forming the open root. To accomplish a quality pipe open root weld, without substantial inward protrusion of molten metal or metal draw back, a short circuit arc welding method of the type made possible by use of an STT electric arc welder has been adopted. This pipe welding process controls the initial welding pass of the pipe welding procedure to fill the open root. Although this type of welding process is extremely advantageous, a substantial amount of development work has been required to select the welding wire to be used during the short circuit welding process. It has been found that a cored electrode has substantial advantages when used with an STT electric arc welder in welding the joint between pipe sections; however, the open root pass weld bead presents unique welding challenges. It has been found that the root pass weld bead is best accomplished by using a solid wire with the characteristics of the ANSI-AWS A 5. 1895 solid welding wire. This type of welding wire is used with a shielded gas and has the following specification.
TABLE I
Percentage
Carbon
.06-.15
Manganese
.90-1.40
Silicon
.45-.75
Phosphorous
0-.025
Sulfur
0-.035
Copper
0-.50
Ni/Cr/Mo/V
0-.50
This standard gas shielded welding wire has been selected as a welding wire which provides a good appearance and allows the advantages of the STT electric arc welder during the open root welding pass. Although the weld bead appearance is usually acceptable with standard solid wire, a substantial improvement in bead appearance is obtained on the top and bottom of the bead by using the present invention.
THE INVENTION
After substantial experimentation and costly investigation it has been found that a quality weld is produced consistently with an excellent appearance and highest travel speed because of better flow characteristics when phosphorous is maintained at only a trace and sulfur is maintained at a high level in the specific range of over 0.015% by weight and less than 0.035% by weight in the welding wire. By controlling and maintaining these limits for phosphorous and sulfur in the welding wire, a consistent good appearing open root weld is accomplished. Consequently, in accordance with the present invention there is provided a method of welding the ends of two pipes at the gap or open root between the spaced ends. This method comprises selecting a welding wire having 0.06-0.15% by weight of carbon, 0.90-1.40% by weight manganese, and 0.45-0.75% by weight silicon. In addition, the wire includes phosphorous, copper, stainless steel alloys and sulfur. The method involves maintaining the sulfur at a given percentage level of the selected wire in the specific range of 0.015 -0.35% by weight and maintaining the phosphorous at a given percentage level of the selected wire in the specific range of less than about 0.015%. Indeed, the phosphorous is normally at a trace or tramp level of 0.006-0.008% by weight of the welding wire. In this invention, the selected welding wire is advanced at a given wire feed rate toward the open root between the spaced pipe ends to weld the sections together by filling the open root in a first weld pass, creating a welding current with a controlled wave form, which wave form includes a succession of welding cycles each having a short circuit portion and a plasma arc portion with the plasma arc portion including in sequence a plasma boost segment, a tailout segment and a background current segment. The welding wire is moved along the open root as the welding current is passed through the wire to melt the wire and transfer the wire by surface tension transfer to the pipe ends thereby filling the open root. The current wave form is formed by a rapid succession of current pulses created by an oscillator at a rate of at least 18 kHz and with a width controlled by a pulse width modulator. By using the invention, a high quality weld bead is deposited in the open root during the first pass of a welding process. Thereafter, another welding wire, such as a flux cored wire can be used to fill the remainder of the joint. Thus, the root pass is filled by an optimized weld procedure and the remaining joint is filled by a procedure tailored to high deposition demands.
The primary object of the present invention is the provision of a method of filling the open root in a pipe welding process, which method employs a specific type of short circuit welding process and a solid welding wire having certain maintained levels of phosphorous and sulfur.
Yet another object of the present invention is the provision of the present invention is the provision of a method as defined above, which method consistently produces a quality open root weld in a pipe welding procedure.
REFERENCES:
patent: 5676857 (1997-10-01), Parker
patent: 5742029 (1998-04-01), Stava et al.
patent: 5945014 (1999-08-01), Crockett et al.
patent: 5961863 (1999-10-01), Stava
patent: 6093906 (2000-07-01), Nicholson et al.
Nicholson Peter
Stava Elliott K.
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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