is achieved by solution strengthening of the ferrite matrix through higher silicon contents. To the user of the castings, these materials provide the advantages of better machinabil- ity, and uniform hardness and strength throughout the cast- ing. At the same time, it provides the possibility of reduc- ing the wall thicknesses (light-weight construction) with the intention of saving energy and raw materials. An important benefit for the foundries is that it becomes less difficult to comply with hardness tolerances.
When the new DIN EN 1563 standard became effective, very little knowledge was available as to the production and properties of the new ferritic, solution strengthened grades. The German and Austrian governments sponsored a re- search project which had the objective of filling this gap of material-related and casting technological information.
Experimental Procedure
Within the framework of this project, alloys were produced in a 150 kg induction furnace (medium frequency). The ba- sic analysis of the alloys was C: 3.5-3.6 %; Si: 2.3-2.5%; Mn: 0.15-0.2 %; P: <0.02 %; S: <0.009 %; Mg: 0.04-0.05%. This basic analysis was used as reference to determine the effect of higher Si contents. During tundish cover treatment with FeSiMg, the pre-alloy was covered by very finely cut steel sheet. The tapping temperature from the induction fur- nace was set at 2,768°F-2,804°F, the casting temperature was between 2,516°F and 2,534°F. Inoculation took place in the pouring basin during casting. The melting tests were all conducted with the same basic charge–40% steel and 60% Sorel metal. The melting parameters were the same for all tests. The silicon contents were all set using FeSi 90, with the carbon content being adjusted by means of electrode graphite to a CE value of ~4.3. Except for specific inocula- tion tests, the amount of inoculants was 0.3% for all tests. The inoculants were added to the pouring stream of the mag- nesium-treated melt while it was being poured into the basin. The stoppers of the pouring basin were pulled upon reach- ing a casting temperature of 2,516°F-2,534°F. This made the casting temperature and the casting speed of all test melts comparable. A pattern from previous projects was used for casting radial speci- mens for the basic material- technological examinations. For this project, a 0.20-in.- thick plate was added to the ra- dial pattern (Figure 1).2
In ad-
dition to this radial specimen, technological test pieces were cast in a mould for standard specimens of the dimensions specified in DIN EN 1563 as
36
well as Y2 and Y4-keel block. The two moulds were filled with metal from the same pouring basin in order to guarantee that the metallurgical preconditions are the same. Addition- ally, three shrinkage specimens were cast on the same mould plate. For complementary shrinkage tests, additional plate-, cylinder- and cube-type castings were produced (not in the figure shown). Tensile test pieces and metallographic sec- tions were taken from the thermal center of the radial speci- men. The results of the examinations were documented and evaluated.
The fatigue tests were carried out under rotation bending conditions within minimum 15 samples (stress ratio R=-1). The test frequency was 200 Hz and the investigations were conducted at room temperature. The sample length and dia- meter were 6.30 in. and 0.28 in. respectively.
Test Results
Influence of the Si Content on the Static Mechanical Properties
Melting tests were conducted with Si contents between 2.4 and 6%. From the cast specimens, test bars were taken and tested. When the Si content is increased starting from 2.4%—which is the usual Si content in EN-GJS—the tensile strength values increase up to a maximum, which is reached at an Si content of 4.3%. After surpassing this Si content, the tensile strength drops from the peak of 89,000 psi down to 72,500 psi at a Si content of 5% (Figure 2). The yield strength only starts to drop at a Si content of about 4.6% (Figure 3), an effect known from ductile cast iron grades, such as EN-GJS-400-18. In these grades, growing genera- tion of embrittling elements or graphite degeneration first lead to decreasing tensile strength and elongation, before the yield strength decreases due to further advancing em- brittlement and graphite degeneration. At 5% Si, the yield strength and the tensile strength values coincide. In the tests,
Figure 1. Geometry of the radial sample (left) and the Y-blocks and one type of the shrinkage test samples (right).
International Journal of Metalcasting/Volume 8, Issue 2, 2014
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