Basic Pipe Welding Procedures
When the pipe is in the 5G position, with its axis horizontal as in Fig. 1-3, positions on the pipe can readily be identified by their likeness to the numbers on the face of a clock. Thus, the top of the pipe is in the 12 o’clock position and the bottom is the 6 o’clock position.
Two different welding procedures are used when the pipe is in the horizontal position: downhill and uphill pipe welding. The choice of the method is not affected by the diameter of the pipe; it depends primarily on the wall thickness and the alloy content of the pipe, as explained in the following section.
Fig. 1-3. The identification of the welding positions around the pipe joint by the numbers on the face of a clock.
Downhill Pipe Welding. Regardless of the method used, the pipes must first be tack welded together. For downhill welding (Fig. 1 -4), the weld is started in the 12 o’clock position and the bead is welded
progressively downward around the pipe until the 6 o’clock position is reached. Starting again at the 12 o’clock position, the bead is welded around the other side of the pipe to close with the first bead at the 6 o’clock position.
Downhill pipe welding is used primarily to weld thin-wall mild steel pipe having a wall thickness of % to 5A« inch. The relatively thin wall of the pipe retains the heat longer than thick metal would. This causes the metal in the area of the weld to cool slowly if the speed of welding and the heat input are the same. Slow cooling is START STOP
Fig. 1-4. (Left) General procedure for downhill pipe welding. Fig. 1-5. (Right) General procedure for uphill pipe welding.
desirable because a softer and more ductile grain structure is then obtained in the metal in the area of the weld.
When welding mild steel pipe, the slower cooling rate of the thin-walled pipe does make it possible to deposit the weld at a faster rate without harmful effects to the welded joint. For this reason, downhill welding is preferred when welding thin-wall mild steel pipe. The ductility of the metal in the weld and in the surrounding area can be further improved by depositing several beads around the weld. Each succeeding bead heats the previous bead, which cools relatively slowly.
The fabrication of cross-country transmission pipelines and other low pressure storage vessels are examples where the downhill method of arc welding (SMAW) is used. Since such fabrications will be of materials less than 3/8 of an inch thick, the downhill technique allows faster welding speeds with less tendency to burn through the root of the joint. In contrast, thicker materials and many of the alloy steel materials require the uphill welding method.
Downhill welding requires the use of fast-freezing, lightly coated electrodes such as 6010, 6011, 6012, 7010, and 7014 that produce minimum slag. As the electrode is moved down along the joint the molten puddle and it’s slag covering will tend to smother the arc causing porosity and slag inclusions in the weld. The use of the proper
electrodes, proper rod angle, and adequate travel speed, keeping ahead of the molten slag will insure sound welds. In some instances the use of straight polarity (electrode negative) along with the downhill method will eliminate burn though when poorly fitted joints are encountered.
The use of the heavier coated electrodes such as 7024 and the iron powder low hydrogen types are not suitable in downhill welding as the problems of slag entrapment, porosity, and cold lapping become insurmountable. Such electrodes will also require higher operating currents thus increasing the chance of burn though.
Early in the twentieth century the oxy-acetylene welding (OAW) process was used to join the low and medium carbon steel pipe material used at the time. Later, in the 1930's, when electric arc welding (SMAW) came into general use in pipe fabrication, the higher welding temperatures generated by the arc caused cracking problems, particularly in the root or first stringer pass.
Since that time pipe manufacturers have refined and improved the metallurgy of their products to take advantage of the newer welding processes and electrodes that were becoming available.
In recent years as the demand for larger diameter pipe with thicker
Fast moving welding crew.
walls for transporting grade oil over very long distances and natural gas at higher pressures, the pipe industry has improved the mechanical properties of the pipe by additions of manganese and silicon, along with more rigidly controlled amounts of carbon. Also small amounts of columbium and vanadium have been used, usually in the range of
0. 20-0.26 percent. Such materials are known as “low alloy” piping because attempting to increase strength by excessive alloying causes additional problems in welding such as cracking and embrittlement of the heat effect zone (HAZ).
Increasing the carbon, manganese, and silicon content to achieve these higher level mechanical properties may seem reasonable. However, it will not be in the interest of joining pipe edges by welding without risking faulty welds. Even small-quantity increases in carbon can have great effects, increasing both the tensile strength and hardness. Likewise, manganese will increase both toughness and ductility, but will suffer from not having the necessary value to resist cracking.
Today’s pipelines, and all of the Lx60 and Lx65 grade pipe materials, are being made by alloying techniques, other than simply increasing carbon and manganese levels, to the limits appropriate for carbon steel. In most instances, columbium or vanadium are added to a steel containing 0.20-to-0.26 percent carbon and 1.0-to-1.35 percent manganese, and. a “hot roll” practice within the critical range is responsible for grain refinement and adequate mechanical properties.
Pipelines welders are on the go all day. They are provided with welder helpers who are responsible for grinding and wire brushing each layer of weld deposit They are also responsible for adjusting the welding current as instructed by the welders, keeping the welder supplied with electrodes, and handing the welders an electrode each time the one in use is consumed. The welder’s helpers play a very supportive role because of their knowledge and experience in grinding and brushing welds complements the welder’s effort in making perfect welds.
The welder helper’s responsibility goes even further. After welds have been brushed and grinding is completed, the welder helper is often the first to discover defects such as surface porosity, poor fusion, poor tie-in, and inadequate filter metal on the weld groove before capping. The helper then informs the welder, who will provide instruction about what additional steps are needed to correct such defects. Because of the many variables on pipeline construction, especially with welding procedures and standards, welder helpers should be given training, or brought up to date, before pipeline construction
The majority of pipelines constructed today use low alloy grade materials, although it is very possible higher strength alloy pipe material will be introduced in the future. Meanwhile, because of their role with preheating, interpass temperature, screening welds from wind and rain, and observing surface defects, welder helpers should be formally trained. Note: Chapter 8 continues the discussion of pipeline welding.
Uphill Pipe Welding. After the pipe has been tack welded, the weld is started at the lowest spot on the pipe or the 6 o’clock position (Fig. 1 -5), and the bead is deposited upward around the pipe until the І 2 o’clock position is reached. The second half of the pipe is then welded by again starting at the 6 o’clock position and welding upward around the other side to the І 2 o’clock position, where the joint is closed.
This method is preferred for welding heavy-wall pipe and pipe made of alloy steel. The thicker pipe wall acts as a “heat sink” by withdrawing the heat more rapidly from the weld area than does a thin-wall pipe. The faster cooling rate causes the metal in the weld area to become more brittle in mild steel pipe. In alloy steel pipe the tendency toward brittleness is greatly increased.
Cross country pipeline laying.
To overcome this tendency the cooling rate in the weld area must be reduced. This can be accomplished by decreasing the welding speed and by depositing a heavier bead. Both of these objectives — slower welding speed and a heavier bead — are achieved by welding the pipe in the uphill direction.
Horizontal Pipe Welding. When the pipe is in the 2G position, with its axis vertical, the weld joint connecting the two pipes is in a horizontal plane, and a horizontal position (2G) weld must be made around the pipe. Horizontal pipe welding will be treated in detail in a later chapter.
There are, of course, cases where the weld must be made in still other positions, such as the 6G, or inclined, position. Welding in these positions is usually done by using one of the methods just described. Sometimes a combination of procedures is required and the welder must exercise good judgment in selecting the best one.