Thixoforming : Semi-solid Metal Processing

64j 3 Material Aspects of Steel Thixoforming
Figure 3.13 Hardness and dimensional change of steel
X210CrW12 subject to annealing temperature [46].
3.3.2
Roller-Bearing Steel 100Cr6
The roller-bearing steel 100Cr6 (1.3505) is an hypereutectoidic chromium- and
carbon-rich steel. In contrast to the melting behaviour of X210CrW12, the steel
100Cr6 exhibits no distinctive plateau within the melting range, where, however,
the chemical composition of the specific batch has to be considered in the
examination [18, 19, 52].
Due to the characteristic requirements (high hardness, rolling resistance, wear
resistance, no structural changes and adequate toughness) for this steel, its development
led to a continuous improvement of metallurgical processes for the manufacture
of homogeneous steels with high purity. The reason for this is that mainly
non-metallic inclusions generate local stress peaks in bearing material and thus serve
as the starting position of fatigue cracks [53]. The roller-bearing components made
from the steel 100Cr6 require a minimal hardness of 58 HRC, which can be reached
by means of classical heat treatment by quenching from about 850 C in oil, water or
salt baths to about 200 C and subsequent annealing at 150–180 C. The hardness
constitution is essentially composed of martensite, retained austenite and carbides.
Parts of the retained austenite can also be transformed through annealing with a
second annealing step or by means of freezing. The retained austenite concentration
within the martensitic microstructure amounts to between 7 and 15% and increases
with rising austenitization temperature, which is due to the higher fraction of
dissolved carbon and chromium within the austenite. An overview and discussion
of the transformation structures of 100Cr6 after one- and two-step austenitization