mechanical break produced in the lab.
Within the dendritic zone of some of
the fractures were rounded dendrite
arms (Figure 3). The rounding indicates
a shrinkage defect caused by an isolated
area of liquid that was frozen off from
the riser system because it was not properly fed. When the liquid was consumed
during solidification, the dendrite arms
could not continue to grow, leaving the
blunted tips. Dark areas, which were
identified as oxide films formed from
exposure to air during casting solidification or heat treatment, were also found
on the lab fractures. This proves the
fractures were present before the casting went into service.
Inclusions and porosity also were visible. The inclusions were mold sand,
slag and oxides. The grains were very
large compared to wrought product
forms—about 1/4 to over 1/2 inch ( 6 to
13 millimeters). The grain boundaries
were the intersection of the dendrites
formed during solidification. One leak
path was cross-sectioned (the photomicrograph in Figure 4 shows the continuation along a grain boundary).
In the as-cast condition, segregation
and second phases could be seen in the
secondary electron SEM image (Figure
5). The heavier average atomic mass
produces a lighter image while the
lighter average atomic mass produces a
darker image. The darker dendritic
regions are enriched in the lighter elements, chromium and nickel, while the
lighter interdendritic regions contain
higher concentrations of the heavier element molybdenum. The second phases
are undesirable because of reduced
toughness and susceptibility to preferential corrosion attack.
Proper solution heat treatment is
required to reduce the segregation and
second phases to a minimal level.
Impact testing on an as-cast sample
of CW2M did show excellent impact
toughness. The results were 100 and
112 foot pounds (ft-lb) (136 and 152
joules or J) at ambient temperature;
114 and 124 ft-lb (155 and 168 J) at
600° F (315° C); and 115 and 130 ft-lb
(156 and 176 J) at 1,200° F (650° C).
During solidification, however, when the
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