Thixoforming : Semi-solid Metal Processing

98j 3 Material Aspects of Steel Thixoforming
temperature of 400 C and a holding time of 1000 h, whereas the conventionally
hardened material decreases to values below 53 HRC. At a fictional service temperature
of 450 C, the difference between the two states becomes even more pronounced,
because the conventionally hardened material exhibits at this temperature
already a very steep decline below 53 HRC after 100 h.
In conclusion, it can be noted that by means of the application of an adjusted heattreatment
strategy, the insertion temperature of the cold working steel X210CrW12
could be increased considerably and could even be inserted at temperatures at which
usually only warm-working steel is used. Depending on the particular application,
service temperatures of up to 400–450 C seem realistic.
3.6.2
Structural Changes and the Properties of the Hypo-eutectic Steel 100Cr6
Due to the chromium and carbon concentrations, the austenite has an Ms temperature
of about 350 C, so that it transforms into martensite during quenching. The
structural images in Figure 3.50 confirm the calculations and reveal a martensitic
structure, at which a light seam is located at the former austenite–grain boundaries.
By close examination of the element distribution images and the line-scan of the
microprobe examinations, it can be assumed that the white phase is a two-phase
structure of retained austenite with finely ingrained carbides, which could be
confirmed by microhardness measurements [70].
Future research should deal with the systematic observation of the structural
development of steel 100Cr6 without a connected carbide network. The transformation
behaviours of both the solid and liquid phases have to be considered to be able
finally to derive a heat-treatment strategy that delivers useful mechanical properties
based on the partial liquid state.
In addition, the volume shrinkage and the concurring pore formation for steel
100Cr6 will be an important factor for the mechanical properties.
3.6.3
Final Evaluation of the Heat-treatment Strategies
The SSM potential of steel concerning the metallic properties was examined by means
of systematic heat treatments on steels X210CrW12 and 100Cr6. After processing in
the partial liquid state, new structures could be produced by means of isothermal
Figure 3.50 Microstructure of 100Cr6 specimens quenched from 1350, 1375, 1400 and 1425 C.