A SOLIDIFICATION TIME-BASED METHOD FOR RAPID EVALUATION OF THE MECHANICAL PROPERTIES OF GREY IRON CASTINGS P. Ferro , T. Borsato, and F. Bonollo Department of Engineering and Management, University of Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy S. Padovan Fonderie di Montorso, Via Valchiampo 62, 36050 Montorso, VI, Italy Copyright Ó 2018 American Foundry Society https://doi.org/10.1007/s40962-018-0290-8 Abstract Designers often need to know the mechanical properties of different zones of a casting. This is because such properties are often very different from those declared in the standard classification of the cast iron used or derived from sepa- rately cast specimens. At constant chemical composition, the mechanical properties of a casting will depend on the microstructure, which in turn is ruled by the cooling rate at each point of the component. In this work, a method developed to rapidly predict mechanical properties in each zone of a cast iron casting, which uses only results from moulding–solidification numerical simulation, is proposed. Such approach, applied to real cast irons components, was found to be in tune with experimental results. Keywords: grey iron, finite element, thermal analysis, mechanical properties, EN-GJL 300 Introduction The mechanical properties of a cast iron component are often very different from those declared in the standard classification of the alloy used. This is because a cast iron grade is classified according to values obtained from sep- arately casted samples whose thermal and microstructural history is different from that of the casting itself. As a matter of fact, geometrical variations of the casting induce different cooling rates from one area to another, which in turn are related to different microstructure and mechanical properties. 1,2 For this reason, designers often force the foundry to obtain castings with controlled mechanical properties verified with tensile tests, which requires sam- ples taken from particular zones of the casting itself and not separately cast. This methodology is cost and time demanding, since on the one hand, it implies a decrease in the regular production of castings, on the other, it requires tensile tests beyond standards that sometimes, according to the position and thickness of the casting, may be difficult to obtain. Furthermore, static and fatigue strength of heavy section iron castings is not standardized yet, and this is the reason why in recent literature new works were published about mechanical characterization of such large cast iron components. 3–7 A possible solution to this problem comes from numerical simulation that is able to predict the mechanical properties of the casting according to the chemical composition of the alloy and process parameters. 8–11 For instance, Jakob Olofsson and Ingvar L Svensson demonstrated in their work 12 that it is possible to foresee the mechanical properties of a cast iron component through casting process simulation and stress–strain simulations. A particular simulation strategy called ‘a closed chain of simulations for cast components’, 8 which uses solidifica- tion and solid-state transformation models to predict microstructure formation and mechanical behaviour on a local level throughout the component, is proposed. Simi- larly, two years before, Donlean carried out a model to predict microstructure and mechanical properties of ferritic ductile iron components. 13 The numerical model was applied to a heavy section casting with a satisfactory cor- relation between numerical and experimental results. In the same year (2000), Italian researchers, Calcaterra, Campana International Journal of Metalcasting
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A SOLIDIFICATION TIME-BASED METHOD FOR RAPID EVALUATIONOF THE MECHANICAL PROPERTIES OF GREY IRON CASTINGS
P. Ferro , T. Borsato, and F. BonolloDepartment of Engineering and Management, University of Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy
S. PadovanFonderie di Montorso, Via Valchiampo 62, 36050 Montorso, VI, Italy
Copyright � 2018 American Foundry Society
https://doi.org/10.1007/s40962-018-0290-8
Abstract
Designers often need to know the mechanical properties of
different zones of a casting. This is because such properties
are often very different from those declared in the standard
classification of the cast iron used or derived from sepa-
rately cast specimens. At constant chemical composition,
the mechanical properties of a casting will depend on the
microstructure, which in turn is ruled by the cooling rate at
each point of the component. In this work, a method
developed to rapidly predict mechanical properties in each
zone of a cast iron casting, which uses only results from
moulding–solidification numerical simulation, is proposed.
Such approach, applied to real cast irons components, was
Figure 5 shows the location of the thermocouple that was
in the middle of the 30-mm-thickness step of sample A
(Figure 3), while Figure 6a shows the obtained result and
the definition and procedure used to calculate the solidifi-
cation time.
Numerical Model and Parameters Calibration
The numerical model for moulding and solidification
simulation was carried out by NovaFlow & Solid. In the
proposed approach, the simulation is used to calculate the
solidification times in the different parts of the casting
whose mechanical properties have to be determined. It is
mandatory that input parameters used in the model have to
be correct and thus validated by experiments. A conver-
gence analysis, resulting in a 4.7-mm element size
dimension, was performed in order to optimize the mesh
density and the corresponding computational times. The
parameters calibration of the model was obtained by
comparing the thermal history measured with the
Figure 3. Mould and samples geometry used in the experiments (mm). Specimenswith the same colour have the same diameter highlighted in the legend.
Figure 4. Valve housing (a) and front cover (b).
Figure 5. Thermocouple location.
International Journal of Metalcasting
thermocouple and the one resulting from the simulation.
Input parameters were taken from the software database,
and little variations (with reference to above all thermal
contact resistance at interface between the mould and the
molten metal) were made necessary in order to overlap the
two curves as shown in Figure 6b. The alignment between
experimental and numerical results assured a good cali-
bration of the model parameters.
Results and Discussion
UTS Versus Thickness
Figure 7 summarizes the tensile test results (in terms of
UTS) as a function of the thickness (or diameter) of the
sample where specimens were taken from. It is noted that
despite the scattering of the results, typical of the analyzed
quasi-brittle material, an inverse relation is found between
thickness/diameter of the sample and its UTS value. The
greater the thickness/diameter, the lower the tensile
strength. However, this relation seems not to be true for
samples taken from 10-mm steps. This apparent anomalous
behaviour is due to the high cooling rate and the conse-
quent overcooled microstructure that can be detected in
that zone of the step-form sample. It is worth mentioning
that the scattering of results is also due to the different
microstructure that may be present in the specimens taken
from the same step thickness but with different solidifica-
tion times. For the same reason, the microstructure, and
thus the mechanical properties, of specimens coming from
step-form and cylindrical form samples will be different.
This is the main reason why the ‘master curve’ must refer
to the solidification time rather than the casting thickness or
diameter as made in the past or standards.
Master Curve, UTS Versus Solidification Time
By using the calibrated numerical model, the solidification
time was calculated for each specimen by referring to the
central point of the step where it was taken from. The little
variation of solidification times across the section of the
specimen was thus neglected. By using only UTS values
coming from step-form samples, Figure 7 is thus converted
in a more useful plot, UTS versus solidification time,
named ‘master curve’ (MC) (Figure 8). Data were statis-
tically elaborated by using a lognormal distribution, and
survival probabilities (the probability that the sample
will break at lower load values) (Ps) of 50%
(UTS = 2341.5t-0.348) and 95% (UTS = 1899.5t-0.348)
were obtained (Figure 8).
Figure 6. Measured temperature history inside the30-mm thick step (a) and comparison between experi-mental and numerical (FEM) temperature history evalu-ation (b).
Figure 7. UTS of EN-GJL 300 GCI as a function of sample thickness/diameter.
International Journal of Metalcasting
It is observed that despite the three thickness values of the
step-form moulds and because of the different position of
the 30-mm-thick step in the step-form mould A compared
to that in the step-form mould B (Figure 3), different
solidifications times were calculated, as expected
(Figure 8).
Rapid Calculation of Casting’s UTS
Assuming that sound castings without relevant macro-de-
fects are obtained, the MC (Figure 8) can be used to
rapidly estimate the UTS of the casting by using the
solidification times coming from simulation. By referring
to the calculated solidification times of the zones of interest
of the front cover and valve housing shown in Figure 4, the
estimated UTS values with a survival probability of 50%
were 272 MPa, 260 MPa and 217 MPa, respectively. If the
real UTS values obtained by means of tensile tests are now
inserted in the MC, it is easy to observe that they fall over
the calculated survival probability of 95% and in particular
they are close to the Ps 50% values (Figure 9) (Table 2).
The same results were obtained for the mechanical
Figure 8. Master curve of EN-GJL 300 GCI.
Figure 9. Comparison between predicted and experimental UTS values as afunction of the solidification time.
International Journal of Metalcasting
properties of the cylindrical form samples, as shown in
Figure 9. Finally, Figure 9 also shows the obtained
microstructures as a function of the solidification time. The
lower the solidification time, the higher the graphite
coarsening linked to a UTS reduction. However, at solid-
ification times lower than about 187 s, the overcooled
microstructure as well as the finer pearlitic matrix makes
the material more brittle and sensitive to defects. (The
scattering of results increases.) This is also confirmed by
‘hardness versus thickness’ relation with Brinell hardness
measured values of 193, 215 and 230 corresponding to
thicknesses of 30 mm, 15 mm and 10 mm, respectively.
Conclusions
A method was described for a rapid mechanical properties
estimation of different zones of a casting made of EN-GJL
300 grey cast iron. The proposed approach is based on the
intrinsic relation between microstructure-solidification
time and mechanical properties. A master curve, describing
the tensile property as a function of the solidification time,
was calculated by means of tensile tests performed with
specimens taken from step-form samples and moulding–
solidification numerical simulation. The main idea is that
for sound castings without relevant solidification defects,
the master curve depends only on the chemical composi-
tion of the analyzed alloy. As a matter of fact, for each cast
iron composition, the solidification time is the most pow-
erful parameter influencing the microstructure. Process
parameters such as casting temperature, inoculation and
thickness influence the solidification time that is just
included in the master curve. It was demonstrated that with
a rapid numerical calculation of the solidification times of
the zones of interest of an industrial casting, the master
curve is able to rapidly estimate the ultimate tensile
strength of such zones. The advantage of the method
consists in overcoming the problems related to mechanical