As stated earlier, the usual joint to be welded in pipings is the circumferential butt joint. To weld such a joint by the MMA process, the pipe edge can be square or slightly chamfered when the wall thickness is below 5 mm for carbon steel, and below 3.2 mm for stainless steel. Thick­nesses greater than these and up to 22 mm should have their edges prepared as at (a) in Fig.

11. 1, while thicknesses greater than 22 mm should have edge preparation as at (b) in the same figure.

In critical applications where carbon and low-alloy steel piping stainless steel piping and most non-ferrous piping is to be TIG welded, joint preparations including consumable insert rings as shown in Fig. 11.2 are used. In all the cases shown, U or flat-land bevel prepa­rations are employed, because they help to minimise excessive shrink.

For butt joints between unequal wall thicknesses (for example, between a pipe and a cast steel fitting or valve body), codes recommend that a smooth taper be provided on the edge of the thicker member.

Fillet-welded joints are often used for pipe sizes 50 mm in diameter and smaller, and for joining pipe to flanges, pipe to valves and pipe to socket joints. Three examples are shown in Fig. 11.3.

70 1/8"

Square butt (Flat land)

4

■ 1/16" clearance

Fig. 11.3 Examples of fillet-welded joints