Thixoforming : Semi-solid Metal Processing

80j 3 Material Aspects of Steel Thixoforming
Figure 3.30 Carbon, chromium, silicon and manganese contents
of the line-scan measurement of a sample quenched from
1425 C.
A comparison of the chromium and carbon distributions (Figure 3.29) and a close
inspection of the line scan (Figure 3.30) indicate that carbides precipitated in the
segregated grain boundary areas whereas most of the segregated zones consist of
retained austenite. In the areas in which higher chromium contents and marginally
higher manganese contents without simultaneously raised carbon concentrations
exist, retained austenite should be present in a metastable form due to a concurrent
reduction of the Ms temperature. In the right part of the line scan, detection of carbide
can be assumed, due to higher chromium and carbon concentrations, with simultaneous
depletion of manganese. It is clear from the manganese distribution that the
manganese concentration in the neighbourhood of the carbide is slightly raised and
the carbide is, therefore, surrounded by austenite with higher stability. A detailed
discussion of the structural development is presented with regard to the resulting
mechanical properties in Section 3.6.
3.4.2.3 EBSD Measurements of Quenched 100Cr6 Specimens
Figure 3.31a shows the BSE image of a specimen quenched from 1425 C. The black
areas are fine cracks formed due to the high cooling rate. The image in (b) shows that
retained austenite is present in the probably former liquid-phase areas. These can be
taken as the lower boundary of the liquid-phase content and can provide a rough
indicator of the former structure in some samples. The orientation measurement in
(c) shows a random distribution of the martensitic structure. An accurate differentiation
of the formerly solid and liquid sample areas is, however, not possible.
The quenching experiments on steel 100Cr6 confirm the tendency of the thermodynamic
calculations that in comparison with the partial liquid X210CrW12 no
liquid-phase determination is possible due to the diffusion of the elements in the