Valve Magazine - Spring 2012

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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