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Description / Abstract:
INTRODUCTION
Material specifications list the specified minimum yield
strength, minimum and, in most cases, maximum tensile strength and
values for materials and grades covered by the specification. The
tensile properties for a particular material and grade are based on
chemical composition, heat treatment, thickness and production
data. The tensile properties can also be influenced by the amount
of work (reduction) during the rolling process, resulting in higher
values for thin plates than for thick plates. The mill production
data must show that all tensile strength values are within the
specified tensile strength ranges and that the yield strength
values exceed the specified minimum values to make it commercially
acceptable for the producer of that material. Increasing the
specified minimum tensile strength or yield strength would involve
a commercial decision by the material producer, based on his
production data and the expected rejection rate, as to what minimum
specified tensile properties above the specification values are
acceptable to that producer.
Improved melting practices, chemistry controls and rolling
practices can result in improved notch toughness and tensile
properties. Typical production data indicate that often the actual
tensile properties (tensile strength and yield strength)
significantly exceed the specified minimum properties, particularly
in thinner plates. Some industry standards (e.g., API 650, Welded
Steel Tanks for Oil Storage, CODAP) recognize this and include
provisions for use of higher tensile properties. API 650 permits an
increase up to 5 ksi above the specified minimum values for certain
carbon steels. These higher tensile strength values are subject to
agreement between the purchaser and the material producer. The use
of higher guaranteed tensile properties (where this is feasible)
reduces the weight of the vessel component, resulting in savings to
the vessel manufacturer and the owner.