Thixoforming : Semi-solid Metal Processing

92j 3 Material Aspects of Steel Thixoforming
CCT diagram for cooling from 1210 C. Figure 3.42 shows significantly more inert
transformation behaviour for cooling from very high temperatures in comparison with
usual hardening treatments. The pearlite nose dislocates in the direction of longer
times by about half a dimension, that is, in the more advantageous direction. For
interpreting the CCTdiagram, it has to be considered that under certain circumstances
carbide precipitation can already occur at temperatures above Ac1 or Ar1. It has to be
considered here that in equilibrium the carbide content amounts to about 12.8 mol% at
1210 C according to Thermo-Calc calculations, whereas it amounts to about 21 mol%
at 980 C. A pre-eutectic carbide precipitation could, however, not be proven in the
dilatometer. The transformation delay indicates, however, an increased C or Cr content
in the austenite.
The results for specimens annealed at 1270 C and subsequently held in the
dilatometer for 60 min at 1210 C and cooled at different cooling rates are depicted in
Table 3.7 and Figure 3.42. The curves numbered from 1 to 8 represent the different
cooling times from 1210 C to room temperature. Specimens cooled with a cooling
rate lower than 30 K min 1 exhibited a considerable change in length related to the
transformation that occurred, as highlighted by the inset. The measured hardness
values show that the hardness for the quenched state increases from 366 HV10 to the
maximum hardness of 614 HV10 at a cooling rate of about 10 K min 1 , whereas it
drops again monotonously at lower cooling rates.
The metallographic images of the dilatometer experiments in Figure 3.43 show a
continuous transformation of the primary, austenitic phase and a coarsening of the
carbides subject to the cooling time. For cooling times of 10 or 30 min from 1210 Cto
room temperature, the matrix remains austenitic. With increasing cooling time, the
eutectic phase content (g þ Cr7C3) becomes coarser and singular Cr7C3 carbides
form within the grain core of the austenite. At a cooling time of 50 min, a dark phase
forms which was identified as fine needle-shaped pearlite and is treated in detail in the
course of the discussion of the isothermal ageing experiments of the third test series.
The content of this fine pearlite increases with increase in cooling time, and the phase
content of Cr7C3 carbides in the grain core of the retained austenite also increases. In
experiment 8, nearly the whole austenitic matrix structure is transformed.
Table 3.7 Cooling conditions of the dilatometer trials and the corresponding hardness values.
Test
Cooling time,
1210 C–RT (min)
dT/dt
(K min 1 )
t8/5
(min)
1 10 120 2.5 366
2 30 40 7.5 354
3 50 24 12.5 420
4 60 20 15 498
5 90 13.3 22.5 537
6 120 10 30 614
7 180 6.7 50 584
8 270 4.4 67.5 501
Hardness
(HV10)