Thixoforming : Semi-solid Metal Processing

94j 3 Material Aspects of Steel Thixoforming
Figure 3.45 Optical interference–contrast image of a specimen
annealed at 595 C for 30 min after quenching from 1270 C (a)
and scanning electron microscope image (b) [with enlarged
eutectic carbides (C), needle-shaped pearlite (P) and partially in
martensite (M) transformed austenite (g)].
shows the formation of needle-shaped, acicular pearlite and martensite needles
within the not yet transformed austenite, as depicted for an annealing temperature of
595 C and an annealing time of 30 min (Figure 3.45) or 75 min (Figure 3.46). After a
holding time of 30 min, when the maximum hardness is reached, about half of the
retained austenite is transformed into fine, needle-shaped pearlite whereas the rest of
the austenite partially transforms into martensite. After a holding time of 75 min, that
is, in the over-aged state, a high pearlite content and a significantly lower retained austenite
content exist, which transforms completely into martensite. At ageing temperatures
of 490 and 540 C, a similar behaviour can be observed. The only difference
is the formation of very low amounts of Widmanst€atten carbides within the austenite.
The development of this form of carbide can be found increasingly at 490 C, at which
it always concurs with pronounced martensite formation, as depicted in Figure 3.46b.
For all ageing temperatures, the morphology of the acicular pearlite is very fine and
Figure 3.46 Optical interference–contrast images of specimens
annealed at 595 C for 75 min after quenching from 1270 C
(a) and annealed at 490 C for 96 h (b) [with coarsened eutectic
carbides (C), Widmanst€atten carbides (CW), needle-shaped
pearlite (P) and partially in martensite (M) transformed
austenite (g)].