Distortion in Pipe Welding
Distortion caused by welding can seriously affect the alignment and the locational accuracy of a pipe installation unless preventive measures are taken to avoid these problems. While in the last chapter the basic causes of distortion in weld joints were treated, this chapter will show how the distortion in the weld joint can affect the fabrication of the pipe installation and explain the steps necessary to overcome this.
On the job, the pipe welder must always be aware of the distortion and must take those steps required to prevent this from affecting the quality of his work. The amount of distortion cannot be calculated, even by engineers. However, this should not deter the welder from estimating the direction and possible magnitude of the distortion. Having done this, he should plan his work in advance to minimize the effect of the distortion. The following examples will describe how this is done.
When cross-country pipelines were discussed in Chapter I, it was pointed out that line-up clamps are used to hold the pipes in place while welding the root bead. Their primary purpose in pipeline welding is to prevent the root bead from cracking as it is being deposited, but these clamps also serve to align the pipes.
Line-up clamps are also used on other types of pipe-fabricating jobs. Here their main purpose is to align the pipes and to hold them in position. A sufficient amount of the root bead is deposited to assure alignment of the pipes after they are welded and to prevent the root bead from cracking before removing the alignment clamps. These clamps can be obtained in several different styles and sizes (see Fig. 8-5).
When welding longer lengths of pipeline, where several standard lengths of pipe are welded together, the pipes will bend under their own weight. It is important then to provide an adequate means of support under the pipes to prevent them from sagging while they are being welded together.
A typical welded-pipe fabrication is shown in Fig. 12-1, Several welds are required to fabricate the installation, which presents some distortion problems that are frequently encountered in pipe welding. The header, Fig. 12-2, has four branch pipes that are welded along the top of the header pipe, as shown. Weld joints, such as made by these branch lines, are sometimes called “weldolets.”
Simply welding the branch lines to the header pipe would cause it to bend, as shown in the lower view of Fig. 12-2. To prevent this from occurring, a strongback is attached to the header on the side opposite the branch pipes. The purpose of the strongback is fo add to the stiffness of the header pipe, and to resist the tendency of this pipe to bend as a result of heat input, in making the welds.
Fig. 12*1. A typicai welded-pipe fabrication.
Fig. 12-2. (Top) Use of a strongback to prevent distortion. (Bottom) Pipe distorted (bent) as a result of welding.
A strongback may be a channel section or an I-beam of adequate size in relation to the header pipe. On miid steel pipe it may be attached to the header by a series of tack welds. After the branch pipes have been welded in place, the header is removed with an oxyacetylene cutting torch. On some jobs this procedure for attaching the header is not permissible, especially when the header pipe is made from a higher alloy steel. In this case the strongback is attached by using heavy-duty clamps.
The branch lines should not be welded in the alphabetical order shown in Fig. 12-2. To minimize distortion, they should be welded in the following order: C, A, D, and B. By welding the branch pipes in this sequence, the amount of heat put into the area surrounding a header at one time will be reduced. While the temperature of the weld must be high enough to obtain fusion, the heat will diffuse more rapidly and only a small area will attain a temperature high enough to cause serious distortion. It is the expansion and contraction of the metal surrounding the weld, as well as the weld metal itself, that causes the distortion. The overall effect of using this sequence, together with the strong back, is to reduce the amount of distortion that occurs, or to eliminate it entirely.
To be able to more fully understand how distortion occurs when the metal adjacent to the weld is heated to a high temperature, a schematic example will be given. The bar of steel in Fig* 12-3 is to have a small section heated to a high temperature where plastic flow can readily occur. This can be likened to heating this part of the bar to the forging temperature* Moreover, heating a small section of the bar to this temperature can be compared to the condition of the bar when welding a single bead.
Assume that the section of the bar to be heated can be cut out and that this section will fit tightly into the resulting slot, as in Fig.
12- 3A. If the section is removed from the bar and then heated, it will expand and no longer fit into the slot, as in Fig. 12-3B.
In order that it again may fit tightly into the slot, the removed section must be upset by applying a compression force, as shown in Fig. 12-3C. This section is then placed back in the slot, Fig. 12-3D, and allowed to cool to room temperature. When this has occurred, the upset section will shrink and, as shown in Fig. 12-3E, it will fit loosely in the slot.
Two things occur to make the loose section fit tightly in the slot again; these are shown in Fig. 12-3F. The loose section is stretched by pulling on it in tension and the sides of the slot are pulled inward. However, pulling inward on the sides of the slot will tend to cause the large bar of steel to bend. Figure 12-3G shows how the bar of
steei may be bent permanently when the loose section is back in the slot, fitting tightly.
Heating a part of the bar of steel or a part of a pipe only will result in a situation that is very similar to the schematic example discussed in the preceding paragraphs. When the small part of the bar or pipe is heated, it will be upset by the unyielding colder metal that surrounds it. As the upset metal cools to room temperature it contracts and, if left by itself, it would be even shorter than before because it was upset while hot. In contracting, the upset metal pulls on the surrounding colder metal while, at the same time, it is being pulled by the colder metal. By pulling on the surrounding colder metal, the heated part of the bar tends to cause the entire bar to bend as it cools.
Actually, the bar may bend very noticeably or very little bending may take place. The amount of permanent bending that occurs will depend on the amount of heat input, the size of the area heated, the size of the bar of steel, and its shape.
Fig. і 2-3. Schematic drawings showing how a bar of metal is bent as a result of heating a small volume of metal on one surface. A. Original bar; B. Small section of bar removed and heated to a forging temperature; С Heated section upset by compression; D. Upset section placed in notch with a tight fit; E. Upset section after cooling to room temperature; F. Forces on the bar and the upset section required to obtain a tight fit; G. Bar bent to close notch in order to obtain a tight fit of the upset piece; FI. Strongback prevents the bar from bending. When upset piece is attached to ends of notch, it will be elongated or
If a strongback is added to the bar of steel to increase its stiffness, Fig. І2-ЗН, the amount of permanent bending will be reduced or, perhaps, it can be prevented entirely. In this case the bar of steel is stiff enough to resist bending and the tensile forces pulling on the heated section will elongate or “iron out” this section as it cools.
It should be remembered that heating and cooling a small area of the bar will set up residual stresses within the metal. These occur whether or not a strongback is used. When the strongback is removed, the residual stresses will usually cause a slight amount of bending, but often this is a negligible quantity. The residual stresses can be removed only by heating the entire bar of steel to a temperature that is just below the lower critical temperature.
When welding branch lines onto a pipe, where the branch lines are not in the same plane, the welder must plan to weld the branches in the correct sequence in order to minimize the effect of distortion. In Fig. 12-4, the branch line С is to be welded on the opposite side of A and B.
Because they are located further from the center of the pipe, the welds at A and В will cause less distortion and bending than the weld at С» which is located near the center of the pipe. For this reason, branches A and В should be welded first. If the pipe is bent as a result of the welds at A and Bt welding the branch at С will tend to straighten the pipe out again because this weld has the greatest bending effect on the pipe.
Fig. 12-4. Correct welding sequence for welding branch lines that are in different planes along the header pipe.
To maintain alignment when welding and after the completion of a weld, a branch pipe must not be welded by depositing the root bead completely around the circumference of the pipe at one time.
The central axis of the pipe is called the neutral axis. In order to balance the distortion resulting from the shrinkage of the weld metal when it cools (thereby maintaining the alignment of the branch pipe), the welds should be balanced around this neutral axis, as much as possible, as shown in Fig. 12-5.
The procedure to follow is to weld two short tack welds on opposite sides of the branch pipe and two more 90 degrees from the first two. Then weld a short section of the root bead on one side of the pipe as in Fig. 12-5, and another short section of the root bead on the other side. After this the root bead is welded on the remaining sides.
While it is most important to weld the root beads in sequence, it will also help to minimize distortion and to maintain the alignment of the branch pipe4f the intermediate layers are welded in the same sequence. The cover layer may be welded by going entirely around the circumference of the pipe.
While the weld must be built up to have enough strength to withstand any load to which it might be subjected, it is a mistake to overweld the joint. This can cause distortion and will reduce the elasticity of the joint. Particularly when the piping system is to contain hot fluids, some elasticity in the piping system is required to allow for the temperature changes that will occur. Elasticity is also required for an outdoor piping system if it is subjected to large seasonal temperature variations.
Hg. І2-5. Method of balancing the root beads around the neutral axis of a branch pipe to minimize distortion and maintain alignment.
Frequently, short sections of pipe, as in Fig. 12-6, must be welded together so that they will align with two or more pipes. They must be carefully aligned when they are fit-up and they must be welded together in the correct sequence; otherwise distortion can cause the vertical pipe to be misaligned with respect to the pipe to which it is attached.
Fig. 12-6. Correct welding sequence for welding the root beads of the pipe
Hot metal deposited in the weld joint will shrink as it cools to room temperature. For a given metal, such as steel, the amount of shrinkage will depend upon the size of the deposit of weld metal and the freedom of the pipes to move. After the tack welds have been made to hold the pipes in place, the root bead should be welded in place on all of the joints in Fig. 12-6 before the remaining beads are deposited. The root beads are relatively narrow and cause less distortion than the wider intermediate and cover beads. Moreover, they stiffen the entire assembly, thereby reducing the distortion resulting from the shrinkage of the wider beads.
It is equally important to weld the root beads in the correct order. The last root bead should always be welded in the joint at E and the other root beads should be welded in a balanced sequence on either side of E. In this way, a misalignment of the vertical pipe can be corrected by heating the tack welds at E and drawing the misaligned pipe into position before the root bead is deposited in this joint.
The first step in fabricating the pipe assembly is to fit-up the pipes. Since this subject is treated in detail in Chapter 14, it will only be necessary to mention here that the pipes must be aligned in the specified position and tack welded in place. Then the root beads are welded in all pipe joints and in the correct sequence. The correct sequence is the alphabetical order given in Fig. 12-6, i. e., joints A, В, С, Д and E. By following this procedure, the root beads are alternately welded in joints on one side and then on the other side of E, and joint E is welded last. The remaining beads should also be welded by welding the joints in this sequence.
Right-angle pipe joints, Fig. 12-7, are frequently encountered in pipe welding. The 90-degree turn is made by welding a pipe elbow in
place as shown. As always, the pipes must be ht-up correctly; however, a good fit-up job can be spoiled by using an incorrect welding procedure. As shown in Fig. 12-7B, distortion caused by welding can cause the pipes to move so that they no longer are at right angles after they are welded together.
To prevent this from happening, the elbow should first be tack welded to the pipe that is in the fixed location, which, in this case, is the horizontal pipe. If the vertical pipe is in a fixed location, the
Fig. 12-7, A. Using a support to maintain the correct alignment of a right-angle pipe joint; B. Distortion of a right-angie pipe joint caused by welding.
elbow should first be tacked to this pipe. After the elbow has been tack welded to the horizontal pipe, the angularity of the pipe should be checked by placing a level across the unwelded face of the elbow. If necessary, the elbow can be aligned by heating the tack welds and bending it slightly-.
The vertical pipe is then placed in position and aligned with the elbow. The correct spacing of the root opening can be obtained with a bent piece of wire. Four tack welds are then deposited around this joint in the usual manner and the alignment of the assembly is checked.
Before welding the remainder of the root bead, a support is added to hold the pipes in position. This can be in the form of an “angle iron” (correctly called a “steel angle”) which is tack welded to the two pipes to which the elbow is attached. The angle iron should be welded at approximately 45 degrees, with respect to the elbow, to form a right triangle (disregarding the curvature of the elbow). This will provide the maximum stiffness to the joint for holding the pipes in place while welding.
With the pipes held in the correct angular relationship to each other, the root beads are deposited in both weld joints, after which the intermediate and cover beads are welded. When the welds in both joints are finished, the angle iron is removed with an oxyacety - lene cutting torch. It is easy to visualize how joints can be welded at other angles, as in Fig. 12-8, by using this procedure.
In summary, while pipe welds tend to distort, errors caused by distortion can be avoided by planning ahead and by using the
Fig. )2~8. Using a support to maintain the correct angular alignment when
welding a pipe joint.
correct welding procedures. The real craftsman not only can make good welds, he can fabricate pipe installations that meet all of the dimensional requirements.
Following are some suggestions that will help to prevent distortion:
1. Plan how each job is to be done before starting. Determine the best sequence in which the pipe joints are to be welded.
2. Check the position and the alignment of each joint before the filler layers are deposited. Usually, this should be done after tack welding.
3. Balance the welds, especially the tack welds, about the neutral axis of the pipe. Weld short root beads opposite each other.
4. Keep to a minimum the heat input in any section along the pipe. Do not allow the heat to build up but distribute it as evenly as possible.
5. Clamp or weld temporary supports to the pipes in order to maintain the alignment while welding.
6. Make allowances for the contraction of the weld when lengths of pipe are welded together.
7. Never weld the pipe joint to completion unless the pipes are known to be in correct alignment and in the correct position. Before starting to weld, make sure that the job will be right when it is finished.