Serial
Production of Compacted Graphite Iron
Methods
for producing CGI
Compacted
graphite iron can be produced by treating base iron with magnesium alloys
in a ladle in the same way as ductile iron is produced. However, the
magnesium level must be kept within a very narrow process
"window", usually less than ± 0.002%g. Both a max and a min
limit need to be considered, not just a min level as with ductile iron. In
order to obtain prime quality compacted graphite it is not enough just to
consider the amount of elemental magnesium in the melt. The level and type
of oxides, sulphides and other compounds that can not be detected by a
spectrometer are equally important. Without a metallurgical process
control method, high scrap levels and large variation in quality can be
expected. Commercial ladle treatment methods for CGI that use a two-step
approach start with an initial Mg-treatment. Then the status of the melt
is analysed by thermal analysis or oxygen activity measurement. A second
corrective treatment is made based on the results. Such methods work but
involve many procedural steps and are time-consuming. NovaCast’s PQ-CGI
Ladle process is a further development of ladle treatment methods. The
PQ-CGI Ladle process is based on an analysis of the base iron. The
treatment is performed in a one step operation, which reduces several of
the problems encountered with batch treatment processes. The PQ-CGI Ladle
method is suitable for short to medium series and larger casting weights.
However, all ladle or batch treatment methods have problems associated
with fading of magnesium, which makes it difficult e.g. to use pouring
furnaces. A more suitable method for production of castings in large
serial production (engine blocks, cylinder heads, bedplates, liners etc.)
is needed. PQ-CGI InMold is the answer!
The
PQ-CGI InMold process
PQ
means Prime Quality. Our scientifically based processes are aimed at
obtaining prime quality compacted graphite iron for demanding castings,
especially for the automotive industry. Making the treatment with
magnesium inside each mould has been used for decades in producing ductile
iron. Good ductile iron can be obtained given that a minimum level of
magnesium is maintained. This allows a safety margin to be used in
traditional InMold. There is a very narrow process "window" for
CGI where both a min and a max level must be maintained throughout the
pouring sequence, which involves variations in the pouring rate. The
PQ-CGI InMold process uses a new design of the gating system (pat.
pending) and reaction chamber to meet the more stringent demands needed
for CGI. The system is individually adapted to each type of casting using
NovaCast Foundry Tech II and NovaFlow simulation software.
The
base iron must be conditioned before pouring in order to achieve
consistent nucleation properties for precipitation of compacted graphite.
The PQ-CGI processes are therefore based on careful metallurgical
conditioning of the base iron (pat. pending). The conditioning uses an
advanced thermal analysis system to monitor the total oxygen level
(information about dissolved oxygen is insufficient) as well as other
relevant thermodynamic properties. The PQ-CGI software is based on an
expert system that analyses these data as well as the chemical
composition. Necessary alloy additions are calculated in order to meet the
specifications for the casting to be produced. The specification for each
type of casting is achieved during the calibration of the system and
stored in a database.
The
PQ-CGI system produces a recipe for conditioning the base iron. This
recipe can be used until the furnace has been emptied or refilled.
Normally it is sufficient to take a sample for each new melt or for each
hour when an intermediate holding furnace is used.
When
the base iron is ready for use it can be poured directly into the reaction
chamber located in the drag mould. The filling can be made fully automatic
using a scale and a laser system to verify that the alloy has been added
to the chamber. Only about 0.3 to 0.5% alloy is used, so a very small
space is required for the reaction chamber. The total yield can even be
higher than compared to batch methods, since there are no metal losses due
to deslagging after treatment as with other methods
Due
to the short time span (< 2 seconds) between treatment and filling into
the casting cavity and thanks to reaction kinetics there is no need to use
any inoculant. The risk for primary carbides even in very thin sections is
negligible. The casting quality is verified by 100% testing with NDT
methods such as resonant frequency or ultrasonic.
The
PQ-CGI software
The
process control system uses information from chemical and thermal
analysis.
The
PQ-CGI thermal analysis system is based on simultaneous evaluations of two
samples with and without a deoxidant. The alloy database contains all
limit values and targets both for base and final iron. A separate database
stores data about the alloys used, as well as the yield, etc.
The
samples are automatically evaluated and the system estimates total oxygen
as well as oxygen combined as SiO2 and FeO. The SPC
function allows a statistical evaluation of the process at any time.
Data
from the thermal analysis are combined with chemical data about the base
iron in order to calculate the ideal additions to the base iron, to
condition the iron for the InMold treatment. The final recipe can be
transferred to a separate external screen or to alloy feeders.
 The
PQ-CGI control cabinet
Advantages
of the PQ-CGI InMold process
1.
Casting quality
Each casting has the same magnesium level since there is no fading and
hence there is less variation in properties. Less magnesium can be used
Ethe yield is >80%. This means less risk for micro shrinkages and
less problems with dross. Few and easily controllable steps mean a more
reliable process already before the final quality assurance inspection of
the castings.
3.
Use of pouring furnaces
If
treated iron is kept in a pouring furnace there is always a fading of
magnesium, which is not easy to control. The pouring must be stopped when
a correction needs to be made and that causes downtime. Storing magnesium
treated iron also causes problems with the refractory lining. The PQ-CGI
InMold process completely eliminates these problems as the magnesium
treatment is made in each mould. This means that the base iron can be
stored in a pouring furnace without any problems. The same base iron can
be used for a wide range of CGI
specifications (various casting modulus) as the magnesium level is
determined by the alloy in the reaction chamber
4.
Environment
The
PQ-CGI InMold process offers several features that reduce the
environmental load. The iron temperature during holding can be only
slightly above the pouring temperature, at least 70°C less than with
other methods. The lower temperature means direct energy savings. There
are no fumes from MgO and Mg3N2 as is the case
during ladle treatment. No extra ventilation is needed, which means
further considerable energy savings!
5.
Economy
The
PQ-CGI InMold process offers the lowest production costs compared to any
other commercial method. The main contributing factors are: low energy and
alloy consumption, no inoculant, less manpower, less refractory
maintenance, low scrap level and improved machinability.
6.
Adaptive learning
The process is adaptive, which means that the software uses a learning
algorithm that allows fine-tuning of the process through a feedback from
lab results and final inspection of castings. The process accuracy and
reliability will therefore improve over time. It is also easy to tailor
the process for various conditions.
Limitations
with PQ-CGI InMold
A
system design and calibration must be made for each type of casting in
order to use the process. This limits the method to castings for large
serial production. Another
limitation is weight, since the maximum cast weight per mould should not
exceed 500 kg.
The
PQ-CGI InMold system includes flow and pressure regulators as well as
filters to ensure an even adsorption of the magnesium alloy. The photo
above shows part of a mould with the reaction chamber with alloy in the
middle.
Pouring
PQ-CGI InMold Eno visible reaction.
Both
fully pearlitic and ferritic structures can be achieved with the PQ-CGI
processes. The photo shows a typical structure for an engine block in
pearlitic CGI
NovaSolid
is used to simulate and optimize the mould filling sequence of the PQ-CGI
system.The data are used when designing the system.
NovaCast AB
Creativity
& Experience
Soft Center, SE-372 25 Ronneby,
Sweden
Tel:
+ 46 457 386 300 Fax: + 46 457 156 22 |
