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Efeito da Pre Inoculação com SiC

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AFS Transactions 01-064 (Page 1 of 15)
Pre-Inoculation Effect of SiC in Thin-Wall Ductile Iron Production
K.W. Copi,
EXOLON ESK, Pembroke, Georgia
Y.S. Lerner and N. Laukhin
University of Northern Iowa, Cedar Falls, Iowa
Copyright 2001 American Foundry Society
ABSTRACT
Pre-inoculation effect of SiC furnace additions on microstructure, magnesium recovery, and castability in thin wall
ductile iron plates were studied and compared with alternative FeSi75.
Obtained results showed that pre-inoculation with SiC provides higher residual magnesium and better magnesium
recovery (61.7% for SiC vs. 55.3% for FeSi75) when the same amount of FeSiMg-masteralloy was used. When the amount
of FeSiMg-masteralloy for experimental heats using SiC was reduced by 11% the residual magnesium remained higher
(0.033% vs. 0.032%) than in experimental heats using FeSi75 and standard amount of FeSiMg. Chill tendency in thin wall
ductile iron plates was lower even in those experimental heats using SiC with residual magnesium higher 4.1% than in
experimental heats using FeSi75 with the same carbon equivalent. Castability was found to be equal for both materials.
INTRODUCTION
Previous research [1] aimed to investigate the influence of pre-inoculation effect of different grades of SiC in
comparison with FeSi75 on microstructure and solidification behavior of gray iron, showed that 85 - 90% metallurgical SiC
gave the best results.
Studies [2,3] have been done to assess pre-inoculation with SiC by evaluating the cooling curves, structure and
shrinkage tendency of ductile iron as a function of holding time in comparison with FeSi75. It was found that 97-98%
crystalline SiC produced the best pre-inoculation effects in ductile iron.
A literature review has revealed the fact that the published information on SiC pre-inoculation does not involve
studies on thin wall ductile iron castings. At the same time, the advancement in production of thin wall iron castings, for an
overall weight reduction of industrial parts, particularly in powertrain components, plays significant role in automotive and
other industries [4].
The objective of this study was to investigate pre-inoculation effect of SiC as furnace additions in comparison with
FeSi75 additions by assessing fluidity/castability, microstructure of thin wall ductile iron plates, and overall magnesium
recovery in the process of ductile iron production.
EXPERIMENTAL PROCEDURES
Charging and Melting of Base Iron. Base iron for experimental heats was produced in a medium frequency coreless
induction furnace from a charge mix consisted of nodular grade pig iron, ductile iron returns, AISI 1010 steel punchings, and
carbon riser. SiC (30x50mesh) containing 67.9% silicon and 30% carbon or FeSi75 (3/8x12 mesh) containing 76% silicon
were used as furnace additions. The molten base iron was tapped from the furnace into the flow-through Mg-reaction
chamber, containing treatment additions.
Treatment and Inoculation of Base Iron. The flow-through technique (Figure 1) was used to treat base iron with FeSiMg-
masteralloy (3/8x18 mesh), containing 3.65% Mg and 45.49% Si. Along with the magnesium- masteralloy, FeSi75 in the
standard amount of 0.75% (1.87 lb) was placed in the flow-through reaction chamber. The tapping and treatment
temperatures, percentage of pre-inoculation additions (FeSi75 or SiC), masteralloy additions (FeSiMg), and inoculant
additions (FeSi75) are shown in Table 1. No late additions (stream or mold) of any inoculant were used.
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AFS Transactions 01-064 (Page 3 of 15)
Figure 1. The schematic showing tapping of base iron from the induction furnace and the flow-through
Mg-treatment process: 1--coreless induction furnace; 2--base iron; 3--charging window; 4--reaction chamber;
5--FeSiMg-treatment alloy; 6--inoculant; 7--treated iron; 8--ladle.
The aim of the first series of experiments was to set the baseline of iron quality, when using FeSi75 in amount of
0.75% (1.875 lb.) as furnace additions. In these series, FeSiMg-masteralloy and FeSi75 inoculant were placed in the flow-
through reaction chamber.
The second series of experiments investigated pre-inoculation effect of SiC furnace additions in amount of 0.86%
(2.15 lb.), which ensured the same silicon content in the base iron as when FeSi75 was used. The standard additions of 1.8%
(4.5 lb.) FeSiMg-masteralloy and FeSi75 inoculant in the amount of 0.75% (1.87 lb.) were placed in the flow-through
reaction chamber. Magnesium recoveries were found to be considerably higher, than in the baseline heats, leading to much
higher than anticipated residual magnesium levels (Table 2).
Table 2. Experimental parameters of pre-inoculation experiments after Mg-treatment
Iron temperature,
°F/°C
Chemical composition after treatment, %
Index Series of experiment After
treatment
Before
pouring CE,% C Si Mn P S Mg
1
Pre-inoculation with
FeSi75 (standard additions
of FeSiMg)
2598/
1425
2505/
1373
4.60 3.66 2.71 0.22 0.016 0.007 0.032
2
Pre-inoculation with SiC
added into the furnace
(standard additions of
FeSiMg)
2532/
1388
2432/
1333
4.65 3.85 2.50 0.28 0.023 0.010 0.039
3
Pre-inoculation with SiC
added into the furnace
(reduced by 11% additions
of FeSiMg)
2581/
1416
2566/
1407
4.67 3.85 2.74 0.38 0.022 0.010 0.033
4
Pre-inoculation with SiC
added into the furnace
(reduced by 22% additions
of FeSiMg)
2592/
1421
2546/
1401
4.70 3.91 2.68 0.42 0.022 0.010 0.029
AFS Transactions 01-064 (Page 4 of 15)
Therefore, the third and the fourth series of experiments also investigated SiC pre-inoculation capabilities as furnace
additions in the same amount of 0.86% (2.15 lb.) as in series two, but the quantity of FeSiMg was reduced by 11% from
1.8% (4.5 lb.) to 1.6% (4.0 lb.) in the third series of experiments, and by 22% from 1.8% (4.5 lb.) to 1.4% (3.5 lb.) in fourth
series of experiments, attempting to produce a final iron with residual Mg content closer to that made in series one with
FeSi75. For consistency, each series of experiments was repeated at least three times.
Experimental Molds and Test Castings. Test castings for this study were poured in no-bake, un-coated sand molds,
assembled of seven 7 ¼" diameter and ¾" thickness cores, made of silica sand mesh 57x62. Figure-2 illustrates the core
boxes and the experimental cores. The cores were stacked together with pouring cup glued on the top of the assembly
(Figure 3). The typical test casting consisted of three levels of plates, creating an "experimental tree" (Figure 4). Each level
comprised of four 2" (50 mm) long and