it was found that small amounts of Cu can counteract the effect of low level addition of manganese.3
On the contrary,
the addition of 1 wt. % of copper definitely had a pearlite promoter effect. The present series of experiments was in- tended at providing further experimental data for small in- crements in copper contents from 0.11 up to 1 wt. % so as to lead to a clearer understanding of its effect on the eutectoid transformation. For that purpose, use has been made of ther- mal analysis during cooling of nodular cast irons with vari- ous amounts of Cu, either straight after solidification or after an austenitizing treatment.
Experimental
Melts were prepared in medium frequency induction fur- naces of 6 and 10 t capacity using metallic charges made from 25% of pig iron, 25% of automotive steel scrap and 50% of classified returns. After melting, carbon and silicon contents were corrected as a function of thermal analysis tests performed using the Thermolan®
system9 and of spec-
trometry analysis on a metal sample for other chemical ele- ments. Additions of C and Si to the melt were performed using electrode graphite and FeSi 75% master alloy, respec- tively. At the same time, SiC was added into the induction furnace to increase the nucleation potential of the melt. The temperature of the melt was then increased to 1490-1500ºC (2714-2732°F) after the corrections have been made, and its surface was skimmed. The spheroidization treatment was achieved with a magnesium alloy (46.7% Si, 5.2% Mg, 2.2% Ca, 1.8% Re and 1.1% Al, in wt. %) at about 1440-1470ºC (2624-2678°F) using the tundish-cover method into a ladle of 2 t capacity. The change in Cu content was realized by adding various amounts of Cu (99.9% of purity) together with the steel cover for magnesium treatment. After the reac- tion was completed, the slag was removed from the melt sur- face and the batch was transferred to a pouring furnace. To check the chemical composition of the treated melts, metal samples were taken just before picking up the iron from the pouring furnace and were analysed by combustion tech- niques using Leco CS 244 equipment for determining C and S contents, gravimetric procedures for Si and optical emis- sion spectrometry (OBLF QS750) for other elements. From the melts cast, one ferritic grade (NF3) and seven pearlitic grades (NP1 to NP7) obtained by adding different amounts of Cu will be considered in the present work. The composi- tions of the alloys are listed in Table 1 where the expected accuracy for each element is indicated.
The liquid-solid and solid-solid transformations of as-cast materials were investigated by means of standard thermal analysis (TA) cups with geometric modulus 0.62-0.63 cm. Before pouring the cups, 0.20% (of the weight of the sample) of a commercial inoculant (0.2-0.5 mm in size with chemical composition given as: 70-78% Si, 3.2–4.5% Al, 0.3–1.5% Ca, 0.5% Re, in wt. %) was added into the cups. The cool- ing curves were recorded in the range of 1210-600ºC (2210- 1112°F) and were analysed using the Thermolan®
system.9 52
For studying solid state transformation after austenitiza- tion, cylindrical samples of 30 and 17 mm in diameter and 30 and 20 mm in length were machined out from the TA cups. These samples were then introduced in a tempera- ture-controlled tubular furnace in which they were first heated for holding at a temperature in the range 850 to 1000°C (1562 to 1832°F) and then cooled. Four different cooling rates (Vcool
) were achieved by switching off the
power and varying the position of the sample along the axis of the furnace. These four cooling conditions will be labelled U1 to U4. The temperature versus time evolution of the material was recorded using K-type thermocouples located in the centre of the samples. Finally, cylinders of 3-4 mm in diameter and 2 mm in length were obtained from the as-cast cups and analysed using a TA SDT 2960 instrument. In these experiments, aluminium oxide was used to fill the reference holder. The samples were heated to 950ºC (1742°F), held at that temperature for 20 min, and then cooled down to room temperature. Three cooling rates were used, 5, 10 and about 38 K/min, the latter being obtained as the maximum achievable cooling rate of the DTA-TGA apparatus.
The evaluation of the experimental nodule counts (denoted as N) and the characterisation of solid-state structures were carried out through metallographic analysis by means of op- tical microscopy made on as-cast and austenitized samples. Nodule counts and ferrite contents were evaluated on dif- ferent micrographs obtained at a magnification of ×100 in the central area of the samples using Leica image analysis software. For each sample, measurements were performed on three different fields.
Carbon content (wt%)
Figure 1. Schematic isopleths section of the Fe-C-Si phase diagram in the temperature range of the eutectoid reaction. Solid lines correspond to the stable system ferrite/austenite/graphite and dotted lines are related to the metastable system ferrite/austenite/cementite.
International Journal of Metalcasting/Winter 10
Temperature (ºC)
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