Thixoforming : Semi-solid Metal Processing

38j 2 Metallurgical Aspects of SSM Processing
Figure 2.6 Optical image of the 100Cr6 microstructure after quenching from 1425 C(f L ¼ 0.57) [8].
The presence of austenite would normally be expected to improve the toughness
properties. However, it was shown that the impact energy of such a condition is low
and fracture occurs in an intergranular manner. Intergranular fracture of carbon
steels is commonly observed after overheating during austenitizing and is called
burning when the temperature exceeds the solidus temperature and local melting
occurs at the austenite grain boundaries [10]. The intergranular fracture is then
attributed to a segregation of sulfur into the liquid phase and subsequent sulfide
formation during solidification. Since SSM can be viewed as an extreme form of
burning, it seems reasonable to assume that the intergranular fracture is caused by
segregation of sulfur. As shown in Figure 2.9, the formation of MnS along the grain
boundaries can in fact be observed. Such thin, chain-like MnS precipitates are known
to cause intergranular burning failure [10]. The sporadic presence of carbides in the
austenite grain boundaries might additionally contribute to the intercrystalline
character of the fracture.
Another very important criterion concerning alloy selection and alloy modification
in SSM technology is the terminal freezing range (TFR), that is, the extent of non-
Figure 2.7 BSE micrograph (a) and X-ray mapping [Cr (b); C (c)] of
a 100Cr6 sample quenched from 1400 C [9].