Thixoforming : Semi-solid Metal Processing

Figure 3.38 Microstructures after annealing within the
solidification interval at 1270 C for 20 min without (a) and with
subsequent transformation of 100% and air cooling (b). The light
arrows highlight the coalescing globulites, and the twin
boundaries are marked by dark arrows.
austenitic solid phase exhibits, with 9.4% Cr, 1.31% C, 0.39% W, 0.28% Si and 0.24%
Mn, significantly higher contents of the austenite-stabilizing elements chromium and
carbon than at conventional hardening temperatures, so that a martensite start
temperature of 147 C results. Therefore, the appearance of the metastable austenitic
phase is confirmed by thermodynamic calculations and rules of experience.
Figure 3.40 allows a detailed appraisal of the stability of the austenitic hightemperature
phase. It shows the dependence of the carbon and chromium contents
on the annealing temperature (a). According to these values, the martensite start
temperature can be calculated depending on the annealing temperature (b). Therefore,
the maximum solubility of carbon and chromium is reached at a process
temperature of the semi-solid, forming above the solidus temperature, that is in the
three-phase area L þ g þ Cr 7C 3, so that the austenite stability is the highest there
and Ms is reduced to values below 150 C. With a temperature window suitable for
thixoforming between 1260 and 1290 C, the development of metastable austenitic
structural areas is therefore expected.
Thermo-Calc calculations of the transformation temperatures in equilibrium show
that during cooling at the Ac1 temperature ( 780 C) which is g ! a, transformations
Table 3.6 Measured macro- and microhardnesses of the different structures.
Heat treatment
3.6 Microstructure Analysis and Material Propertiesj89
Hardness
(HV10)
Primary grain
(HV0.1)
Eutectic
(HV0.1)
1270 C, 20 min, air 370 9 310 6 620 150
1270 C, 20 min, air, Deformation 366 11 295 8 534 109
970 C, 12 min, oil 774 20 — —