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Steel Designers' Manual - 6th Edition (2003)
200
., 150
U
'a
0.
E 100
0.
Ca
C)
50
-100 -80 -60 -40 -20 0
20 40 60 80 100 120 140 160
Test temperature, °C
Fracture 251
0.8% carbon
Fig. 7.3 The effect of temperature and carbon content on the impact energy of ferritic steels
An important feature of ferritic steels is the transition temperature between
ductile and brittle fracture. Understanding the factors which influence the transition
temperature allows designers to be able to select a material which will be
ductile at the required operating temperatures for a given structure. The traditional
procedure for assessing the ductile to brittle transition in steels is by impact testing
small notched beams. 1 The energy absorbed during the fracture process is a measure
of the toughness of the material and varies from a low value at low temperatures
to a high value as the temperature is raised. The characteristic shape of the impact
energy–temperature graph has led to the terminology of the upper and lower shelf.
The low temperature, brittle behaviour is often referred to as the lower shelf and
the high temperature, ductile behaviour the upper shelf.
The Charpy V-notch test 1 is the most popular impact testing technique and is
described later in this chapter. The transition in impact toughness values obtained
from Charpy tests on carbon steels at different temperatures is shown in Fig. 7.3.
BS EN 10025 2 gives temperatures for minimum toughness values to be obtained for
a range of structural steel grades. Impact transition curves are a simple way of defining
the effect that variables such as heat treatment, alloying elements and effects of
welding have on the fracture behaviour of a steel. Charpy values are useful for
quality control but more sophisticated tests 3,4 are required if the full performance
of a material is to be exploited.
An understanding of the fracture behaviour of steel is particularly important
when considering welded structures. Welding can considerably reduce the toughness
of plate in regions close to the fusion line and introduce defects in the weld