Thin-wall pipe can be welded by the downhill weld method. Downhill welding is very fast and economical; consequently, it is used extensively to weld cross-country pipelines. In the majority of cases, the wall thickness of the pipes used for cross-country pipelines is within the range of thickness that can be welded by this method.
High-quality welds can be made on thin-wall pipe by the Downhill Weld method. No laxity in the quality of the pipe joint is permissible when the pipeline must transport crude oil, natural gas, and other fuels. Leaks and other defects can present a danger to life and property, as well as be the cause of environmental damage. Most cross-country pipelines are buried underground for almost their entire length. In order to be certain that they are sound, it is normal procedure to test over 50 percent of the welds.
The procedure for downhill welding is different from that for uphill welding. The molten puddle of metal tends to roll down the pipe in the same direction as the arc is traveling. Moreover, the fluid slag in the molten metal also flows in this direction and, unless this can be controlled, there is a danger of slag inclusions occurring.
In order to deposit a sound layer of weld metal, the arc must constantly be kept ahead of the molten pool of metal. This is done by using a high current setting and a fast speed of travel. As a result, the layers of weld metal are thinner, when compared to uphill welding.
Fig. 8-3. A perfect cover pass deposited on small-diameter pipe by uphill
Fig. 8-4. Typical disposition of weld beads on a cross-country pipeline joint.
Figure 8-4 illustrates, a typical disposition of the weld beads in a joint when welding a cross-country pipeline. As usual, four tack welds are made to hold the pipes in place, following which the root bead is welded, starting from the top of the pipe and welding to the bottom. After the root bead has been welded entirely around the pipe, the second pass, called the “hot” pass, is welded. The primary objective of this hot pass is to correct any defects in the root bead; a relatively small amount of metal is deposited by this pass. A third, intermediate, bead is then deposited, which is followed by the cover pass to finish the weld.
For practice-welding, two 7-in. long, 8-in. diameter, Schedule 30 mild steel pipes are recommended. The actual outside diameter of these pipes is 8.625 inches and the wall thickness is.277 inch. The
instructions to follow apply to welding larger-diameter pipes, such as those encountered on cross-country pipelines, as well as the nipples.
Horizontal (2G) Welding. The procedure for welding thin-wall pipe in the horizontal (2G) position is exactly the same as for welding thick-wall pipe. For this reason, this subject will not be treated here and the reader should refer to Chapter 9 for information on horizontal pipe welding.
Outdoor Pipeline Welding. Much of the downhill pipe welding is done outdoors on cross-country pipelines. For this reason it is necessary to mention a few precautions that should be taken when welding under these conditions before treating the actual welding methods.
Courtesy of the H&M Pipe Bexehtix Мм-hine Co. hu
Fig. 8-5. Typical line-up clamps used to clamp and align cross-country pipeline
Weld cracking can be a problem when welding pipes outdoors, particularly when welding large-diameter pipes having a higher alloy content. Several different grades of steel are used in pipeline construction and are designated by the API Standard as X42, X52, X56, X60, and X65. The alloy content increases in these pipes with an increase in number designation. The tensile strength and other mechanical properties also increase in the higher numbered pipes. This progressive increase in properties is the result of the greater alloy contents, principally carbon, manganese, and, perhaps, silicon. Line-up clamps, such as shown in Fig. 8-5, are used to align the pipes and to hold them in place while welding. The root bead is somewhat thin when welded by the downhill method and the pipe joint is weak, until sufficient metal has been deposited in the joint. Short tack welds alone would be apt to crack. For this reason, the clamps are left in place on the pipes until 50 to 100 percent of the root bead has been deposited.
Another form of cracking that occurs, particularly on pipe having a somewhat higher alloy content, is underbead cracking. A more detailed discussion of the nature of this form of cracking is given in Chapter 13; in the following paragraphs some of the precautions that must be taken to prevent its occurrence when welding outdoors are discussed.
Underbead cracking can occur in pipes having a higher alloy content when the weld is cooled too rapidly and when moisture is present. Therefore, the weld joint should not be allowed to cool to the temperature of the surrounding air because the heat retained in the weld will slow down the cooling rate of the next bead to be welded.
In the field, this means that the second layer should be welded as soon as possible after the previous layer is finished, preferably within five to seven minutes. On large-diaraeter pipes it is not possible for a single welder to weld fast enough to maintain the desired interpass temperature in the weld joint. In order to maintain this temperature, it is not unusual to see as many as two to four welders working on a single weld joint simultaneously.
High winds can also cause the weld to cool too rapidly to maintain the desired interpass temperature, especially if the air is cold. Unless a wind shield is placed around the welding area, welding should be stopped if the wind velocity exceeds 25 to 30 miles per hour.
Since moisture can cause under bead cracking, welding should be stopped if it rains or snow is falling. Of course, if a canopy is placed over the weld area to protect the weld joint from rain and snow, welding can be continued. However, the weld joint should be thoroughly dried with an oxyacetylene torch before welding and great precaution should be taken to keep the electrodes dry.
Ingredients in the coating of the electrodes used for downhill welding can contribute to under bead cracking if the weld cools too fast. This is another reason for retaining a fairly high interpass temperature. Ideally, low-hydrogen electrodes should be used because the coating on these electrodes does not contain the harmful ingredients. However, it is not possible to weld downhill with low - hydrogen electrodes because the very fluid blanket of liquid slag obtained will interfere with the weld and will result in a defective bead.
Preparation of the Pipe Joint. The standard joint specifications for thin-wall pipes are given in Fig. 8-6. In the field, the sections of pipe to be welded together have usually been beveled in a shop before
Courtesy of she Hobart Bn и hers Co.
Fig. 8-6. Standard joint specification of a thin-wall pipe joint.
they arrive on the job. Should this not be the case, then it will be necessary to cut the bevel with an oxyacetylene cutting torch and a grinding wheel. The surface of the bevel must be ground smooth and all traces of the tightly adhering oxide coating left by the oxyacetylene cutting torch must be removed.
Before any welding is attempted, the pipe joint must be thoroughly cleaned to remove all foreign matter, such as oil, grease, rust, paint, dirt, etc. On pipeline construction in the field, this work is usually done by a two-man crew working ahead of the welders.
On some jobs a second crew is sent to work ahead of the welders in order to fit-up the pipes in preparation for welding. Line-up clamps, such as are shown in Fig. 8-5, are used to align the pipes and to hold them in place during welding. As already mentioned, these clamps are not removed until a sufficient amount (50 to 100 percent) of the root bead is deposited, to avoid cracking,
For practice-welding, the short pipe nipples are aligned in the same manner as for welding thick-wall pipes, as described in Chapter 4. However, in this case, the diameter of the spacing wire should be Vi § inch, which is equal to the width of the root opening.