Thixoforming : Semi-solid Metal Processing

ageing or controlled, continuous cooling due to the element distribution in the liquid
and solid phases. Due to these, mechanical properties can be achieved that are
superior to those of conventional hardening. For example, the application temperature
of steel X210CrW12 can be raised considerably by means of the described heat
treatments.
To consolidate the industrial application of steel 100Cr6 in the thixoformed state,
further systematic examinations have to be conducted and applicable heat-treatment
strategies have to be developed. Important for this is the avoidance of a connected
carbide network on the grain boundaries.
Based on the examinations up to this point, a transformation of the segregated
grain-boundary areas has to take place in such a way that a structural compound of
austenite, martensite and finely distributed carbides can form. The necessary
transformations can be reached either by means of specific cooling directly following
the transformation process or by means of a special heat treatment by heating from
the cooled state, so that no degradation of the global mechanical properties of the
material occurs.
Table 3.9 shows the element contents of the separate phases with a fraction of
50% solid and 50% liquid phase. The M s temperatures calculated from this are given
in the last column. For the purpose of comparison, the respective values for the
conventional hardening of the materials are also given. In Table 3.10, the summarized
measured microhardness of the separate structural components of steels
X210CrW12 and 100Cr6 are given. The values in these two tables clarify that due
to the strong reallocation of the elements chromium and carbon in both materials, a
multiphase structure is formed which exhibits potential for new material properties
due to its different transformation behaviour. For steel X210CrW12, this was already
Table 3.9 Thermo-Calc calculated carbon and chromium contents
of the solid and liquid phases at equilibrium with phase contents
of 50 mol% and of the austenite at conventional hardening
temperatures; calculated Ms temperatures as a function of the
annealing temperature using the empirical equation
M s ¼ 635 – 475[mass% C(g)] 17[mass% Cr(g)] 33[mass%
Mn(g)] [64].
C
(mass%)
3.6 Microstructure Analysis and Material Propertiesj99
Cr
(mass%)
Mn
(mass%)
Ms
( C)
X210CrW12 Solid (austenite) 1.23 9.16 0.23 112.6
1285 C Liquid 3.07 14.85 0.37 (eutectic
transformation)
f s ¼ 50%
Near 970 C Austenite 0.76 4.62 0.30 185.5
100Cr6 Solid (austenite) 0.55 1.36 0.24 342.7
1418 C Liquid 1.46 1.65 0.36 98.5
f s ¼ 50%
850 C Austenite 1.00 1.50 0.30 124.6