188 Steels: Metallurgy and Applicationsspiral weld linepipe (5LS series). The API specification for high test linepipe was
introduced in 1948 and at that time included only one grade, X42, with a yield
strength of 42 ksi. Since that time, higher strength steels have been developed
and the specification now includes grades up to XS0 (80 ksi = 551 N/mm2).
Most specifications use yield strength as the design criterion, and as the yield
strength is increased, the wall thickness can be reduced proportionately, using the
same design safety factor. Thus the substitution of X70 by X80 grade can lead to
a reduction in wall thickness of 12.5%, which illustrates very clearly the incentive
for the use of higher-strength steels. However, the measurement of yield strength
in linepipe has been an area of contention between steelmakers and linepipe
operators for some time because of the change in strength that occurs between
as-delivered plate and pipe material. Tensile specimens are cut from finished pipe
and these are cold flattened prior to testing. Due to the method of preparation, the
yield strength measured in such test specimens can be significantly lower than
that obtained on undeformed plate. The differential is due to the well established
Bauschinger effect which leads to a decrease in yield strength when tensile testing
is preceded by stressing in the opposite direction, e.g. during pipe unbending
and flattening. In X70 pipe, the Bauschinger effect can result in a reduction in
yield strength of 69 to 83 N/mm 2 (10 to 12 ksi) and therefore plate material
has to be supplied with extra strength so as to compensate for this apparent
loss in yield strength. However, when the yield strength of pipe material is
measured in a ring tension test, the value is much closer to that measured in
plate material. The Bauschinger effect in linepipe materials is particularly marked
in traditional ferdte-pearlite steels which exhibit discontinuous yielding in the
tensile test. The effect is reduced in steels containing small amounts of bainite
or martensite, and in steels containing a significant amount of lower temperature
transformation products, the unbending and flattening operation can lead to an
increase in yield strength (or 0.2% proof stress) compared with undeformed plate.
These materials exhibit continuous stress-strain curves and the high rate of work
hardening compensates for the loss in strength due to the Bauschinger effect.
Toughness is a major requirement in linepipe materials and detailed consid-
eration has been given to both fracture initiation and propagation. To design
against fracture initiation, the concept of flow stress dependent critical flaw size
is employed. This predicts the critical flaw size, relative to the Charpy toughness
level, for specific pipe dimensions and operating pressures. Above this critical
size of defect, the toughness level required to prevent fracture initiation becomes
infinite and depends solely on the flow stress. The crack opening displacement
test has also been used to determine fracture initiation in linepipe materials,
particularly in relation to the heat-affected zone.
When fracture occurs in an oil pipeline, fluid decompression takes place very
rapidly and therefore the driving force for crack propagation dissipates very
quickly in time and space. However, this is not the case in gas pipelines and
therefore the possibility of developing a long-running crack is a major concern.
However, the avoidance of brittle propagation is generally assured by specifying a
minimum of 85% shear area at the minimum service temperature, in full-thickness
specimens in the Battelle Drop Weight Tear test. Various formulae have also been
derived to specify the minimum Charpy level which will ensure the arrest of a