Thixoforming : Semi-solid Metal Processing

3.6 Microstructure Analysis and Material Propertiesj87
Cr, 0.81% W, 0.33% Mn and 0.21% Si were used. Cuts of 30 mm length underwent
different heat treatments and forming experiments. As reference state, a first test
series was austenitized on the basis of the norm at a temperature of 970 C and a
holding time of 30 min and subsequently quenched in oil. In the following, some of
the specimens were annealed at 300 C for 2 h. To examine the influence of a
transformation on the structural development, the cold-rolled primary material was
annealed at 1270 C for 20 min and heated to 900 C, then compressed by 100% and
subsequently cooled with air. For the comparison with conventionally hardened
material, a third test series was executed, in which steel X210CrW12 was held at
1270 C for 20 min in the partial liquid state and subsequently cooled in air. Some of
the specimens of this test series were subsequently isothermally annealed for 1–168 h
at 490 C, for 1–24 h at 540 C and for 30–75 min at 595 C. Additionally, small
dilatometer samples with a length of 10 mm and a diameter of 5 mm were examined
concerning their transformation behaviour and their microstructure at a temperature
of 1210 C and at continuous cooling rates of 4.4, 6.7, 13.3, 20, 24, 40 and 120 K min 1 .
The experiments were carried out slightly beneath the solidus temperature at 1210 C,
because the specimens would have been distorted by the spring force of the clamping
device at higher temperatures. The length changes were established by means of an
inductive position transducer, which offers a precision of about 10 nm. The macrohardness
(HV10) and the microhardness (HV0,1) were determined.
3.6.1.1 Thermodynamic Preliminary Considerations and Microstructural
Examinations
Preliminary examinations of the structural development of steel X210CrW12 rapidly
cooled from the solidification interval show that the structure existing at room
temperature consists mainly of an interpenetrating two-phase network formed by an
austenitic primary phase and a g þ Cr 7C 3 eutectic. The primary phase equals the
solid phase located in the solidification interval, whereas the eutectic phase content
reflects the original liquid phase. In contrast, when applying a conventional heat
treatment the austenitic phase transforms to a considerable extent into martensite,
whereas an annealing treatment at significantly higher temperatures stabilizes the
austenite so much that it remains the dominant phase at room temperature. In the
following, the material scientific aspects of the structural development are, therefore,
examined more closely also in respect of the kinetic phenomena during cooling from
high annealing temperatures.
The pseudo-binary phase diagram shows a carbon content of 2.1 mass% for a
recommended hardening temperature of 970 C (Figure 3.11). For these conditions,
Thermo-Calc calculations show a phase content of 79 mol% austenite and 21 mol%
Cr7C3 carbide. The chemical composition of the austenitic phase is made up of 4.6%
Cr, 0.76% C, 0.45% W, 0.30% Si and 0.28% Mn. According to the empirical equation
Ms = 635 – 435%C 17%( C) the calculated Ms is about 196 C [64], so that with
sufficiently rapid cooling the austenite transforms into martensite and the structure
of the hardened steel is characterized by a martensitic matrix with embedded Cr 7C 3
carbides at room temperature. Here, carbides in the sub-micrometer range and up to
a size of 50 mm are formed (Figure 3.37). Due to the relatively high carbon content in