Steels_ Metallurgy and Applications, Third Edition

114 Steels: Metallurgy and Applications

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Plunger Drop weight (crushedcup)

method method

Figure 1.113
Henning 173)

Schematic diagrams illustrating two forms of an SCWE test (After

tensile machine, or it may be dropped from a suitable height, usually 1 metre. In
each case, the test is carried out at various temperatures below room temperature
and a temperature is established at which a brittle crack is first formed. This
temperature, known as the ductile to brittle transition temperature (DB'Iq'), is a
measure of the brittle tendency of the steel and decreases as the tendency for
brittle behaviour decreases. It is found, however, that the transition temperature
is influenced by the drawing ratio used to produce the cup and increases with
increasing drawing ratio. 172
In a variant of the above test, a cylindrical cup is also used but the weight
is dropped onto the side of the cup, as also illustrated in Figure 1.113. Again
the DB'I~ is defined as that at which brittle cracks first form. The third method
involves bending. (173) Suitable strips of steel are bent through 180" and then
opened at various temperatures below room temperature using five samples at
each temperature. At a sufficiently low temperature, the samples crack open in
a brittle manner and the transition temperature is defined as the temperature at
which all five specimens crack in this brittle manner. The three methods do not
give the same transition temperature for the same steel but there is a correlation
between the results. Figure 1.114 shows, for example, a correlation between the
transition temperature from the bend test and that from the side crush test. It is
useful to note that the transition temperature from the bend test is higher than that
from the side impact test. This implies that the bend test is more severe. It has
recently been suggested that the bend test could become more widely used. 174
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As indicated earlier, the concern about SCWE covers IF steels and particu-
larly those containing phosphorus. 175'176 In the absence of interstitial carbon, the
phosphorus may segregate to the grain boundaries during annealing and promote
intergranular fracture by causing grain boundary embrittlement. The effect is
greater for batch-annealed steel than for continuously annealed steel 177 since
slow cooling gives more time for segregation. Carbon in solution reduces the
effect, 17s but the main way of controlling the condition is by the addition of
boron. 176 A very small quantity, such as 0.0005%, is sufficient to reduce the