Thixoforming : Semi-solid Metal Processing

Figure 3.12 Heat treatment scheme for steel X210CrW12 [51].
3.3 Alloying Systemsj63
however, concurs with a relatively low toughness [46]. Due to its high carbon content,
the steel grade 1.2434 exhibits a wide melting range with a distinctive plateau, which
is developed due to the eutectic solidification and the overlaid carbide precipitation.
For the steel X210CrW12, the content of eutectic M7C3 carbides amounts to about
14%, where during soft annealing an additional 10% of annealing carbides can be
precipitated, which are also of the type M7C3 [47].
In Figure 3.12, typical heat treatments for the material X210CrW12 are depicted. In
terms of hardness and wear resistance in the hardened state, it is aspired to conduct
the complete finishing in the soft-annealed state up to the net shape. The dimensional
stability of the material, which is influenced by the different volumes of the different
phases, the thermal stress during cooling between border and core and also the
transformation stress as a result of the temporally displaced structural transformation
between border and core, is of essential importance [48]. Depending on the
thickness of the carbide bands, the dimensional change takes place mainly parallel to
the carbide band direction, where in the transverse direction a plastic shortening may
occur. This is irreversible and cannot be adjusted by subsequent annealing [48].
Annealing treatments after the hardening procedure also have a great influence on
the tool s dimensional stability, as pointed out in the double illustration of hardness
and dimensional stability subject to the annealing temperature in Figure 3.13 [46].
With increasing temperature, a volume contraction takes place due to the transformation
of the tetragonal martensite into its cubic modification and then a volume
increase occurs caused by the transformation of the retained austenite into martensite.
With a further temperature increase, another volume contraction takes place due
to the compression of the newly formed martensite by stress relief. The location of the
temperature ranges for the volume changes is influenced by the alloy composition
and the associated retained austenite concentration [49].
Important process-relevant material data such as heat transmission coefficients,
coefficients of thermal expansion, heat capacities and thermal conductivities from
room temperature up to liquidus were determined by means of basic experiments.
Furthermore, flow curves in the partial liquid state were established with casecompression
experiments [4, 50].