Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 189

propagating ductile fracture. These indicate that higher Charpy energy values are
required for higher operating pressures and for pipes with higher strengths, larger
diameters and heavier wall thicknesses. However, there still appears to be some
concern about the adequacy of Charpy values for the prediction of crack arrest
behaviour.
The weldability of linepipe materials is important, firstly in relation to pipe
fabrication and secondly with regard to the girth welds that are used in the field
for pipeline construction. Obviously the latter represent the more arduous welding
requirements, particularly in low-temperature environments, but, in general, the
use of carbon equivalent formulae appears to be adequate in ensuring crack-free
welds. Traditionally, the International Institute of Welding (IIW) formula has
been used to assess the weldability of materials:

Mn Cr + Mo + V Cu + Ni

Carbon equivalent = C + --~ + 5 + 15

However, there is the general feeling that the llW formula is not adequate to
define the behaviour of modem steels with low carbon contents and the following
relationship by Ito and Bessyo is sometimes preferred:

Si

Carbon equivalent -- C 4- ~-~ -t-

Mn + Cu + Cr Ni Mo V

20 + g6 + + =+(s It) •

As indicated earlier, thermomechanical processing has permitted the development
of high-strength steels with low carbon contents and this has contributed greatly
to improved weldability in linepipe steels.

Linep~pe manufacturing processes

Japan is a leading producer of linepipe and a summary of the processes and size
ranges available in that country in 1981 is given in Figure 2.23. This indicates that
linepipe is produced as seamless and welded tubing, the former being restricted
to relatively small-diameter, thick-walled tubing. Welded pipes are produced by
electric resistance welding (ERW) and submerged arc welding (SAW), the latter
being used for both longitudinal and spiral welded pipe. ERW pipes are produced
in sizes up to 600 mm (24 in) in diameter and up to 19 mm (0.75 in) wall
thickness. Longitudinal welded pipes are produced mainly by the U-O process and
account for most of the pipes used for oil and gas transmission lines. However,
a small amount of longitudinal welded pipe is also produced by roll bending.
As illustrated in Figure 2.23, very large diameter pipe is produced by the spiral
welded process in wall thickness up to 25 mm (1 in).

Steel compositions for Unepipe

A feature of the AP! 5LX specification is that it lays down very broad require-

ments for chemical composition, specifying only the maximum permitted levels

for carbon, manganese, sulphur and phosphorus. On the other hand, customer

specifications are much more restrictive in composition so as to obtain high

levels of toughness and weldability at a specific level of yield strength. Even

so, steelmakers can still exercise various options in terms of composition and