Jumat, 02 Maret 2012

Technology Gets to the root of pipe welding

Technology Gets to the Root of Pipe Welding

Open root welds on pipes can be made three to four times faster than GTAW by using the Surface Tension Transfer® process. When integrated with an internal spacer clamp into a new automatic orbital pipe welding system, even faster production is possible, with no lack of fusion.
Pipe welding codes, whether for applications in the field or in the plant, require high-quality root pass welding. To ensure that the joints will not leak, especially for steam or pressurized applications, a weld must penetrate completely through the pipe.
In the past, pipe welding was done by one of three methods, each of which has its advantages and disadvantages. These are the methods that have been used.
Gas tungsten arc welding (GTAW) is popularly known as TIG. Travel speeds are slow, heat input is usually high, and it requires high operator skill level.
Gas metal arc welding (GMAW) - also known as MIG - is a much faster process than GTAW. However, because operator skill level is hight and heat input difficult to control, fusion may not always be 100 percent.
Shielded metal arc welding (SMAW), also known as stick, can be cost effective in terms of equipment but requires high operator skill. Frequent starts and stops are another potential problem.

Smoke and Spatter are reduced when pipe joints are welded by means of the Surface Tension Transfer (STT®) process.

By contrast, the Surface Tension Transfer (STT) process makes it possible to complete open root welds three or four times faster than GTAW, with low heat input and no lack of fusion. The STT process uses high frequency inverter technology with advanced waveform control to produce a high-quality weld with less spatter and smoke. For pipe welding, the process also makes it easier to perform open gap root pass welding, with better back beads and edge fusion. It is easier to operate than other processes, yet produces consistent, X-ray quality welds. The STT process results in a complete back bead without shrinkage from the 12 to 6 o'clock weld positions. Also, because current control is independent of wire feed speed, the process allows greater flexibility under all conditions.

Controlling Spatter and Smoke

STT is a proprietary Lincoln Electric process that makes use of Wave Form Control Technology™ to control current precisely and rapidly during the entire welding cycle. It is unique in that it is neither constant current (CC) nor constant voltage (CV). Instead, the power source adjusts current automatically to the instantaneous heat requirements of the arc.
Spatter and smoke are reduced with this process, whether the arc shielding gas is 100 percent CO2, blends of argon and CO2 or helium mixtures for use with stainless steel. Reducing spatter minimizes final weld surface preparation and allows the operator more welding time before the gun nozzle must be cleaned of accumulated spatter.
In open root pass pipe welding, the STT process controls the wave form of the welidng current for excellent penetration control, fusion, and back bead.
Reduced spatter also translates into significant cost savings because more of the electrode is applied to the weld joint, not as spatter on the pipe and surrounding fixtures. Further cost savings are realized because larger diameter wire can be used.
At the start of the cycle, when the electrode shorts, the current is reduced immediately, eliminating the incipient short. This low-level current is maintained for a short time so that the surface tension forces can begin transferring the drop to the puddle, forming a solid mechanical bridge. A high level of pinch current is then applied to accelerate the transfer of the drop. The necking down or squeezing of the shorted electrode is monitored. When a specific value is reached, the pinch current is reduced quickly to a low value before the fuse separates. When a short breaks, it does so at a low current, which produces very little spatter.
Next, the arc is reestablished and a high current known as peak current is applied. This momentary pulse of current establishes the arc length and causes the arc to broaden and melt a wide surface area, which eliminates cold lapping and promotes good fusion.

Spacer clamp and welding
shed on-site and ready for set up.

Better Pipe Welding Results

The constant voltage GMAW process normally used for pipe welding does not control the current directly. Instead it controls the average voltage. This can cause the weld puddle temperature or fluidity to be too high, and the internal bead may be flat or shrink back into the root. This is known as "suck back." Also, when using conventional short arc GMAW, the operator must concentrate the arc on the lip or leading edge of the puddle to ensure proper penetration and fusion. If the arc is too far back on the puddle, penetration will be incomplete. If the arc is too far ahead, the electrode shoots through the gap and causes whiskers to form inside the pipe.
Because Surface Tension Transfer controls the welding current independently of wire feed speed, the process makes it easy to control the temperature or fluidity of the puddle to ensure proper penetration and fusion. This is what makes it so attractive for open root pipe welding applications. In the 5G position, the operator simply has to stay in the puddle. Experienced pipe welders almost always find the process a welcome improvement, both in ease of welding and comfort. They particularly appreciate the reduction in spatter when welding in the 6 o'clock position.

As the decision process evolves, the vendor and the fabricator will continue working together to determine the appropriate system accessories, including safety devices, the optimal layout for the robotic cell, manpower and training requirements, and service and maintenance requirements (internal vs. outside vendor support).
The STT process is gaining acceptance in pipe welding and similar applications, which require precise control of heat input as well as smoke and spatter reduction. Since the heat is controlled directly, the internal backbead profile is also controlled. Welders find that not only are open root welds easier to make, but their mechanical and metallurgical properties are excellent. Superior welding bead profiles can be attained with improved properties in the heat affected zone. Moreover, open root welds are made without the use of ceramic or copper internal backup. In the case of copper, corrosion is thus eliminated by avoiding the possibility of copper inclusions.
The process is effective for welding mild and high-strength steels, as well as stainless steel and related alloys. On steel, it offers the advantages of low hydrogen and 100 percent CO2 shielding with low spatter. When welding duplex stainless, critical pitting temperature is significantly better with STT than with GTAW, and travel speeds three or four times that of GTAW can be obtained, with much less skill.



Special symbols are used on a drawing to specify where welds are to be located, the type of joint to be used, as well as the size and amount of weld metal to be deposited in the joint. These symbols have been stan­dardized by the American Welding Society (AWS). You will come into contact with these symbols anytime you do a welding job from a set of blueprints. You need to have a working knowledge of the basic weld symbols and the standard location of all the elements of a welding symbol.
A standard welding symbol (fig. 3-43) consists of a reference line, an arrow, and a tail. The reference line becomes the foundation of the welding symbol. It is used to apply weld symbols, dimensions, and other data to the weld. The arrow simply connects the reference line to the joint or area to be welded. The direction of the arrow has no bearing on the significance of the reference line. The tail of the welding symbol is used only when necessary to include a specification, process, or other reference information.     

Weld Symbols
The term weld symbol refers to the symbol for a specific type of weld. As discussed earlier, fillet, groove, surfacing, plug, and slot are all types of welds. Basic weld symbols are shown in figure 3-44. The welding

Figure 3-44.-Basic weld symbols.

Figure 3-45.-Weld symbols applied to reference line.

Figure 3-46.-Specifying weld locations.

Figure 3-47.-Arrowhead indicates beveled plate.

symbol is only part of the information required in the welding symbol. The term welding symbol refers to the total symbol, which includes all information needed to specify the weld(s) required.
Figure 3-45 shows how a weld symbol is applied to the reference line. Notice that the vertical leg of the weld symbol is shown drawn to the left of the slanted leg. Regardless of whether the symbol is for a fillet, bevel, J-groove, or flare-bevel weld, the vertical leg is always drawn to the left.
Figure 3-46 shows the significance of the positions of the weld symbols position on the reference line. In view A the weld symbol is on the lower side of the reference line that is termed the arrow side. View B shows a weld symbol on the upper side of the reference line that is termed the other side. When weld symbols are placed on both sides of the reference line, welds must be made on both sides of the joint (view C).
When only one edge of a joint is to be beveled, it is necessary to show which member is to be beveled. When such a joint is specified, the arrow of the welding symbol points with a definite break toward the member to be beveled. This is shown in figure 3-47.
Figure 3-48 shows other elements that may be added to a welding symbol. The information applied to the reference line on a welding symbol is read from left to right regardless of the direction of the arrow.
In figure 3-48, notice there are designated locations for the size, length, pitch (center-to-center spacing), groove angle, and root opening of a weld. These loca­tions are determined by the side of the reference line on which the weld symbol is placed. Figure 3-49 shows how dimensions are applied to symbols.

Figure 3-48.-Elements of a welding symbol.

Figure 3-49.-Dimensions applied to weld symbols.

Figure 3-50.-Dimensioning of welds.

Figure 3-51.-Supplementary symbols.
Figure 3-50 shows the meaning of various welding dimension symbols. Notice that the size of a weld is shown on the left side of the weld symbol (fig. 3-50, view A). The length and pitch of a fillet weld are indicated on the right side of the weld symbol. View B shows a tee joint with 2-inch intermittent fillet welds that are 5 inches apart, on center. The size of a groove weld is shown in view C. Both sides are 1/2 inch, but note that the 60-degree groove is on the other side of the joint and the 45-degree groove is on the arrow side.

Supplementary Symbols
In addition to basic weld symbols, a set of supple­mentary symbols may be added to a welding symbol. Some of the most common supplementary symbols are shown in figure 3-51.
Contour symbols are used with weld symbols to show how the face of the weld is to be formed. In addition to contour symbols, finish symbols are used to indicate the method to use for forming the contour of the weld.
When a finish symbol is used, it shows the method of finish, not the degree of finish; for example, a C is used to indicate finish by chipping, an M means machin­ing, and a G indicates grinding. Figure 3-52 shows how contour and finish symbols are applied to a weldng symbol. This figure shows that the weld is to be ground flush. Also, notice that the symbols are placed on the same side of the reference line as the weld symbol.

Figure 3-52.-Finish and contour symbols.

Figure 3-53.-Specifying additional welding information.

Another supplementary symbol shown in figure 3-51 is the weld-all-around symbol. When this symbol is placed on a welding symbol, welds are to continue all around the joint.
Welds that cannot be made in the shop are identified as field welds. Afield weld symbol is shown in figure 3-51. This symbol is a black flag that points toward the tail of the welding symbol.
Specifying Additional Information
It is sometimes necessary to specify a certain weld­ing process, a type of electrode, or some type of refer­ence necessary to complete a weld. In this case, a note can be placed in the tail of the reference line. (See

Figure 3-55.-Example of welding symbol in use.
fig. 3-53.) If additional information is not needed, then the tail is omitted.

Multiple-Weld Symbols
When you are fabricating a metal part, there are times when more than one type of weld is needed on the same joint; for example, a joint may require both a bevel groove weld and a fillet weld. Two methods of illustrat­ing these weld symbols are shown in figure 3-54. Note that in each welding symbol, the bevel groove weld is to be completed first, followed by the fillet weld.
Applying a Welding Symbol
Figure 3-55 shows an example of how a welding symbol may appear on a drawing. This figure shows a

Figure 3-56.-Eye protection devices.
steel pipe column that is to be welded to a baseplate. The symbol tells the welder that the pipe is to be beveled at a 30-degree angle followed by a bevel groove weld all around the joint. This is followed by a 1/2-inch fillet weld that is also welded all around the joint. Finally, finish the fillet weld by grinding it to a flush contour. As the field weld symbol indicates, all welds are to be accomplished in the field.
For additional information about welding symbols, refer to Symbols for Welding and Nondestructive Test­ing, ANSI/AWS A2.4-86.