Thixoforming : Semi-solid Metal Processing

96j 3 Material Aspects of Steel Thixoforming
consisting of a austenitic–martensitic matrix with high contents of chromium
carbides exhibit unusual wear qualities. The best wear resistance was established
in different examinations to be at a retained austenite content of about 20% [66, 67].
Other examinations show that the smashing of carbides leads to a reduction in the
wear resistance [68]. Because X210CrW12 processed in the partial liquid state
exhibits significantly lower carbide sizes than the conventionally processed material,
this leads to a reduction in carbide smashing and thus a further improvement in the
material properties.
It can be noted that the austenite stability due to the carbon and chromium contents
of the austenite in the partial liquid state (1.23 mass%) is, in comparison with the
austenite using a conventional hardening temperature (0.76 mass%), very much
increased and the phase transformation is displaced towards longer times. Therefore,
SSM tool steels are very transformation inert and even useable for very high component
thicknesses. The hardness increase at lower cooling rates can be explained by the
precipitation of secondary carbides and the concurrent transformation of austenite
into bainite and pearlite. A transformation into martensite could not be observed.
Isothermal holding at high temperatures after quenching from the semi-solid interval
shows a considerable hardening potential, which, at up to 800 HV10 (64 HRC),
is higher than that of conventional hardening (Table 3.8). Due to the destabilization
of the austenite, a microstructure of very fine austenite is formed, as already known
in the literature [65]. In this way, a multiphase structure of transformed eutectic,
pearlite, martensite, retained austenite and sporadically also Widmanst€atten carbides
is formed, which exhibits excellent hardness and wear properties. The measured
hardness changes are, therefore, a result of the complex interplay of different
resulting structures of the austenite transformation: low pearlite and martensite
content at the beginning of the heat treatment (low hardness), optimum pearlite/
martensite composition at the beginning of the heat treatment (maximum hardness)
and high pearlite and low martensite content with over-ageing of the steel and a softer
product structure. The determined ferrite contents correlate very well with the
measured hardness values and support the exhibited results (Figure 3.48).
To verify the transformation kinetics and to determine the rate-determined
mechanism, the activation energy was evaluated using an Arrhenius relationship.
From the temperature–time data of the maximum hardness (595 C/0.6 h, 540 C/
10 h, 490 C/120 h), a value of 285 kJ mol 1 K 1 is obtained, which agrees very well
with published data for the chromium diffusion within austenite (Q ¼ 280 kJ mol 1
K 1 ) of chromium contents of 10–15 mass% [69].
3.6.1.2 Examinations Concerning Long-term Heat Resistance
Long-term annealing at fictional service temperatures of 400 and 450 Cwas executed
for steel X210CrW12 to carry out a benchmarking of the new heat treatment strategy
in comparison with the industrially used reference state. For this, the conventionally
hardened state (980 C/30 min, H2O or air) was compared with different thixo-states
(1270 C or 1300 C/30 min, air or H2O, 540 C/10 h, air). Because no significant
differences concerning the annealing temperature (1270 or 1300 C) or the quenching
medium could be found, in the following only the results based on the state