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Submitted by Jim Mullins, Technical Director
Questions
- What is
good melting practice for gray and ductile iron?
- How do I know what
chemistry to aim for in ductile iron production?
- Describe controls
for good ductile iron treatment practice?
- What
are the basics for inoculation of cast iron?
- What
sulfur and manganese do I need in gray iron?
- Describe
the usual tramp elements in ductile iron and how to control them?
Answers
1.
Melting practice for gray & ductile iron. (Note melting for
ductile iron and gray iron are the same)
The charge will include pig iron, return scrap, steel scrap, silicon
carbide and carbon. It is important to remove as much slag from the
furnace prior to charging, since carbon will be caught in the slag. A
rinse heat may be necessary after an alloyed metal heat. This is
especially true with a ductile base iron heat.
The charge should be melted quickly, except that we do not want to
exceed 2800 F. Slagging the furnace may be required if more carbon needs
to be picked up. Carbon will not go through a slag layer. So bath must be
clean.
Wedge tests can help determine whether or not the carbon as graphite is
in solution. (Carbon exists in two forms in cast irons; graphite and
combined carbon such as carbide - primarily as part of the pearlite). In
graphitic cast irons such as gray iron, ductile iron, and compacted
graphite iron we need to have the graphite in the proper form such as
flakes, nodules, etc to obtain the correct properties. The wedge value is
high when there is an excess of carbide in the fracture.
Chemistry samples need to be taken after good stirring of the top of
the metal bath. Do not want to get loose carbon in the sample, as it will
give erroneous readings.
Usually silicon is added after the carbon is in solution, but should
have at least 1% silicon in the base while melting in. Silicon will reduce
the carbide promoting effect and reduce the wedge value.
The iron should not be held for long periods of time especially at
higher temperatures (>2750 F), because the wedge value will increase
with holding time. Adjustments to wedge (pig iron and silicon will reduce
the wedge value and must be done prior to tapping, if wedge is high.
Holding for as little as 15 minutes at higher temperature will destroy
nucleation in the iron and wedge.
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2. Metallurgy of
ductile iron
The rate of solidification, melting and processing dictate what the
chemistry should be, to determine the microstructure in a certain area of
the casting. It is very important to have the chemistry correct not only
to obtain the right grade of ductile iron but also to avoid problems in
the casting and microstructure.
Thin section (1/2 to 1 inch) ferritic irons can tolerate higher carbon
(3.7%) and silicon (2.7%) contents, but must be low in all other elements
to achieve good toughness. For maximum toughness silicon can be lowered to
2.2% while phosphorus should be a maximum of 0.02%.
In most cases Cr, V and other hardening elements must be kept below
0.04%. Usually only Cu with some Mn (residual of 0.20%) is used for
strengthening. An addition of 0.30% Cu in this case will usually harden a
65-45-12 iron to an 80-55-06 iron in thin sections.
Elements such as B, Ti and Mo should be kept very low as they can
segregate in the last areas to freeze and form carbides.
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3. Treatment
practice for ductile iron
The control of the treatment step is critical to obtaining consistent
magnesium recovery. In order to control it the following items must be
carefully monitored:
- Sulfur and oxygen levels of base iron prior to treatment
- Treatment temperature of the metal (variations will give variable
recovery)
- Rate of metal transfer from the ladle into the treatment device (as
fast as possible)
- Measuring and controlling the inlet and outlet hole sizes of the
device
- Placement of the treatment alloy & temperature of the alloy
(cold as possible)
- Sizing of the alloy compared to the weight of the tapped metal
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4.
Inoculation practice for gray & ductile iron
All graphitic cast irons need to be inoculated. The amount added is
very important. Too much can cause high cell counts and more shrinkage in
gray iron. Gray iron requires between 0.1-0.3% additions depending upon
the section size (cooling rate of the casting). Faster cooling requires
more inoculation than slowly cooled thick sections.
Ductile iron needs considerably more inoculation due to the removal of
most of the sulfur and oxygen from the melt during the treatment step and
the carbide - promoting tendency of the magnesium alloy. Additions can
range from 0.4 to 1.0%.
The strength of the inoculant must also be considered. Most inoculants
have some Al and Ca. Stronger inoculants, which have additional elements
added such as Sr, Ba, etc.will allow reduced additions and still give the
same effect. The stronger inoculants for gray and ductile are different
and should not be interchanged. Sr base inoculants are not used in ductile
iron as the Ca in the metal retards the effectiveness of the Sr.
The addition method can be any good way to distribute the inoculant
evenly and get it dissolved uniformly in the metal. This is usually done
in the pouring ladle (with ductile, after the treatment step). It can also
be done on a reduced scale (never more than 0.2%) in the pouring box of
the mold.
All inoculant effects fade with time after addition. Usually 20 minutes
is the maximum time to insure any residual effect. For this reason, many
foundries do some inoculation in the ladle and some in the mold, as well
as control the time of the ladle on the floor. Un-inoculated irons and
those with faded inoculation behave much differently and can have unwanted
graphite structures, carbides, and changes to the matrix structure.
Shrinkage can also be affected.
Inoculants must be carefully weighed and be kept dry as they can pick
up some moisture. Water in inoculants will produce large carbides in the
metal.
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5.
Melting practice and detailed chemistry control for gray irons
Sulfur and manganese are very important in gray cast iron. Sulfur is an
active element - promoting more graphite and good type A graphite flakes,
when it is in the proper range of 0.06 -0.09%. Sulfur can be added to the
charge in the form of pyrite (25% S) or as small briquettes (30% S). Low
sulfur (<0.04%) causes more chill (carbides) to form and can make the
iron more difficult to inoculate well.
Manganese is necessary to form MnS in the melt, which is stable. The
normal ratio of Mn to S is: (1.7 X %S) +.30 Mn = total minimum Mn content
(usually kept about 0.60% for normal S levels). If the manganese is low
for the amount of S present, the excess sulfur combines with iron and
causes FeS (iron sulfide) to form and can cause slag like defects.
Manganese contents over the minimum required to tie up the S act as
pearlite stabilizers to strengthen the iron up to about 0.80%. At higher
levels of Mn (>0.80%), graphite flakes can grow large reducing the
tensile strength.
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6.
Metallurgy and tramp elements control in ductile iron
Cerium - this element is usually added to MgFeSi treatment alloys. The
effect of Ce is similar to Mg in forming spheroidal graphite and so the
effects of the two are additive. Increasing Ce allows for lower Mg
additions. A small amount of Ce (about 0.005% is good) will promote good
nodularity. However, it can be very detrimental in heavy sections (over
1.5 - 2 inches thick). In these sections chunky graphite will form. This
greatly reduces strengths and elongation. Cerium must be closely
controlled. The main advantage of Ce is to reduce the harmful effects of
tramp elements such as Pb, Sb, and Ti, which can enter through the scrap
stream.
Lead and antimony - Both of these elements promote flake graphite in
ductile iron. Their effects are eliminated with a small Ce addition, but
good scrap inspection and chemistry control should be practiced to keep
them out as much as possible.
Titanium - produces vermicular (CG) graphite and is also a carbide
stabilizer in heavy sections. It is not as strong as other elements, so up
to about 0.03% can be tolerated with a small Ce addition. Ti comes with
pig iron and steel scrap and is not lost upon melting without oxygen
additions, so it tends to carry over.
Magnesium - Must be kept as low as possible and while still producing
good graphite shape. In sections of ˝ to 1" a final Mg of 0.03%
should be okay. Mg levels will increase slightly until at about 2"
section, it should be about 0.045%.
Sulfur - Level in base ductile iron should be 0.010- 0.015%. Too little
will promote carbides in the final structure, also because Mg can be
higher. Too much requires a larger Mg addition and the possibility of
reduced nodularity.
Metallurgy of ductile iron - Nodularity checks
All separate ladles of ductile iron should be checked for nodularity,
by one of the approved methods from the end of the pouring ladle. The
sample is then polished sufficiently to reveal the graphite structure. The
nodularity should be > 85% in the test sample. Also examination of the
nodularity and nodule counts in fractured 1" test bar specimens
should also show good nodularity and nodule counts > 100 mm˛. Good
nodule shape and higher nodule counts are indicative of correct chemistry
and inoculation practice. This micro-examination should be done frequently
to insure quality. Irons with good nodule counts and shape will have less
shrinkage and carbide problems.
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