Thixoforming : Semi-solid Metal Processing

88j 3 Material Aspects of Steel Thixoforming
Figure 3.37 Microstructure after air cooling (a) and CCT diagram
of steel X210CrW12 for a hardening temperature of 980 C (b) [51].
the martensitc matrix and the high content of Cr7C3 carbides, a hardness of about 760
HV10 or 63 HRC is reached. The CCTdiagram for steel X210CrW12 with 11.5% Cr,
0.78% W, 2.03% C, 0.32% Mn, 0.39% Si and 0.11% V in (b) shows that the macrohardness
and the Ms for a hardening temperature of 980 C could be well confirmed
experimentally (hardness 800 HV10; Ms 190 C). Furthermore, inert transformation
behaviour is exhibited at a critical cooling rate of about 2 K s 1 . As a characteristic
orientation factor, the value for the transformation Ac1b (beginning of the
austenitization at heating) is at about 790 C. Therefore, already cooling rates of about
2Ks 1 are sufficient to reach a martensitic transformation of the austenite without
the formation of bainite and pearlite phases.
In the second test series, the influence of forming on the structural development
was examined. Samples that were held isothermally and then quenched in water
exhibit a primary content of globular, former solid phase of about 60% and an average
grain size of 44 mm ( 21 mm). The liquid phase initially existing at 1270 C is dark
and exists at room temperature as finely structured g þ Cr7C3 eutectic, whereas the
light, austenitic primary phase exhibits typical twin contrasts (highlighted by black
arrows). Furthermore, sporadic coalescence develops (highlighted by white arrows).
The examinations show no differences in the structural development of compressed
and uncompressed samples (Figure 3.38). In both cases, a homogeneous structure
exists, which no longer provides any indication of the rolled primary material condition
(no more influence of carbide bands is recognizable). Hardness measurements on the
compressed samples exhibit marginally lower hardness values (Table 3.6).
Figure 3.39 shows a sector of the pseudo-binary cut through the six-component
system with 11.5% Cr, 0.7% W, 0.3% Mn and 0.25% Si. It is readily evident that the
alloying condition point exists in the two-phase area L þ g (liquid þ austenite) at
1270 C and the liquid undergoes a eutectic transformation during cooling. Thermo-
Calc calculations show a phase content of 60 mol% (61.4 vol.%) austenite, or 40 mol%
(38.6 vol.%) for the liquid at 1270 C. At that condition, the chemical composition of the