The preparation of the pipe joint is an essentia! part of pipe welding, as the quality of the weld is affected by the care used in preparing the joint. Indeed, in many instances the failure of the pipe joint can be attributed to faulty joint preparation. The pipe welder must understand and then practice those skills required to prepare the joint properly for welding. This is the first step in making a successful pipe weld.

The preparation of the joint consists of four separate steps:

1. Prepare the edges

2„ Clean the joint surfaces

3. Fit-up the pipes

4. Tack weld the pipes together.

Each step will be treated separately in this chapter. However, first there will be a brief discussion on how to practice pipe welding. Short lengths of pipe, or pipe nipples, should be used to practice weld to economize on the cost of the pipe. However, if the pipes are too short, the cooling rate of the weld may be affected to the extent that the weld will not correctly simulate conditions for welding longer pipe lengths. For this reason, a length of 7 inches is recom­mended for each of the two pipes to be welded. Experience has shown that this length does not affect the cooling rate of the weld significantly.

As mentioned in Chapter 1, two different basic procedures are used to weld pipe. For thin-wall pipe, the downhill method is used; and for thick-wall pipe, the uphill method is used. Except for details involving the size of the joint and welding the tack weld, the procedures for preparing the joint are the same, whether the pipe is thin-wall or thick-wall.

Preparing the Pipe Edges. Single-vee butt joints are usually used to weld sections of pipe together. The elements or parts of the joint have been given standard names, which are shown in Fig. 4-1, These names are used so frequently in pipe welding that they should be committed to memory.

Figure 4-2 shows the standard joint specifications for thick-wall pipes that are to be welded together by the shielded metal-arc welding method. The included angle of this pipe joint is 75 degrees. In order to obtain this angle when the pipes are brought together for welding, the bevel angle on the end of each pipe should be equal to one-half of the included angle, or 37% degrees in this case.

Care should be exercised in making the bevel to leave the correct width of the root face. In Fig. 4-2, the width of the root face for heavy-wall pipe is shown to be 3/& to V8 inch.

Usually the pipes and the pipe fittings are beveled by machining in a shop before they are sent out on the job and no further prepara­tions are required before they are fit-up, other than cleaning. Some­times, however, the pipes are not beveled and this operation must be performed on the job. In this case the bevel is cut with an oxyacety- lene cutting torch and finished by grinding with a hand grinder.

Often the bevel is cut manually, using an ordinary hand oxyacety - lene cutting torch. This requires care and skill. When available, a pipe-beveling machine is used, such as shown in Fig. 4-3. This machine is fastened to the end of the pipe and then is carefully adjusted to cut the bevel. Next, the oxyacetylene cutting torch is fed

around the pipe to cut the bevel. Depending on the design of the machine, the feed may be accomplished by a hand crank or by a self-contained power feed.

The surface produced by the oxyacetylene cutting torch will have a tightly adhering oxide film. If not removed, this film is very detrimental to the quality of the weld. For this reason, the beveled surface should be entirely free of the film before any welding is done over it. The oxyacetylene cutting torch also produces a rather rough surface that is difficult to weld over.

Grinding the bevel following the oxyacetylene cutting operation is done to remove the oxide film and to obtain a smooth, flat contoured surface over which to weld. On very thin-wall pipe (% in.) the entire bevel is sometimes ground with a hand grinder.

Cleaning the Joint Surfaces. Contaminants such as grease, oil, scale, or rust will have a harmful effect on the quality of the weld. As already explained, all traces of the oxide film produced by the oxyacetylene torch must be removed. The welder must also make certain that any other contaminants are removed before starting to weld. This can usually be accomplished by the vigorous application of a wire brush.

Fitting-up the Pipe. The two pipes to be welded together must be accurately aligned prior to welding. The inside surfaces of the pipes
must blend together smoothly, as should the outside surfaces. In many instances the pipes must be aligned so that the finished piping system will be in the correct location. Considerable skill is required to fit-up two pipes in preparation for welding. This is an essential part of the pipe welder’s craft.

Because an understanding of how to fit-up pipe is so important, Chapter 14 is entirely devoted to this subject. The present chapter will treat only the method of fitting-up two short pipe nipples in preparation for practice welding and the reader is referred to Chap­ter 14 for further details on this subject.

Short pipe nipples are not only used for practice welding, but also for jobs in the shop and in the field. The pipe nipples recommended for practice welding are 7 inches long. They are made from 8-inch Schedule 60 mild steel pipe, for which the actual outside diameter is 8.625 inches and the wall thickness is.406 inch.

i/16