Thin-wall pipes are categorized as having a wall thickness less than 5/ie inch. The diameter of thin-wall pipes may be as small as I inch and as large as 42 inches, or larger, such as that used for cross-country pipelines.

When the pipeline is horizontal, or the pipes are in the 5G position, either the uphill or the downhill welding procedure can be used. Because it is a faster method, downhill welding is preferred for welding cross-country pipelines, where the pipes are usually made of low-carbon mild steel and this method of pipe welding will be emphasized in this chapter.

Not only the wall size but the diameter of the pipe affect the welding method that can be employed. Small-diameter pipes with thin walls present a problem because the heat resulting from the weld does not dissipate rapidly, thereby causing the heat to build up in the weld zone. In this case the heat input into the weld must be controlled.

To control the heat input, for welding small-diameter pipe the current setting is less, and a smaller-diameter electrode is used as compared to that used for large-diameter pipe having the same wall thickness. While welding small-diameter pipe, the welder must pay careful attention to the electrode angle and to the arc length. The electrode angle is more difficult to control on small-diameter pipe because the position of the electrode changes rapidly in order to maintain a uniform electrode angle while welding around the pipe, A drastic change in the electrode angle can have an adverse effect on the quality of the weld. While watching the electrode angle, the welder must also prevent the arc length from becoming too large as

(1) this will increase the heat input into the weld, and (2) if excessive, it will decrease the weld penetration. Considerable practice is usually required of the welder before the technique of welding small-diameter pipe is mastered.