=Paper=
{{Paper
|id=Vol-2763/CPT2020_paper_s7-6
|storemode=property
|title=Fractographic analysis of fractures of graphitized cast iron using optical microscopy
|pdfUrl=https://ceur-ws.org/Vol-2763/CPT2020_paper_s7-6.pdf
|volume=Vol-2763
|authors=Konstantin Makarenko,Alexander Nikitin,Andrei Parenko
}}
==Fractographic analysis of fractures of graphitized cast iron using optical microscopy==
https://ceur-ws.org/Vol-2763/CPT2020_paper_s7-6.pdf
Fractographic analysis of fractures of graphitized cast iron using optical
microscopy
K.V. Makarenko, A.A. Nikitin, A.S. Parenko
makkon1@yandex.ru| zzzalexzzz95@gmail.com| 3dprintense@gmail.com
BSTU, Bryansk, Russian Federation
Fractography - methods for studying material fractures, the science of the structure of fracture surfaces. Fractography is widely
used to determine the causes of destruction of materials and structures. In materials sciences, fractography is used to study crack
propagation, microstructure studies, and quality control. When investigating the failure of structures, determining the type of crack
(material fatigue, hydrogen embrittlement, stress corrosion cracking, exceeding the permissible load, etc.) makes it possible to determine
the cause of the failure. Fractographic studies include the study of a fracture with the naked eye, using an optical and electron
microscope. Other methods are also often used, such as measuring hardness or studying the distribution of chemical elements using X-
ray spectral microanalysis. Fractographic (or fracture) analysis is the first and obligatory stage of research that must be performed by
an expert of the metallurgical examination laboratory when determining the causes of material destruction. Visual examination of the
fracture (fracture) surface makes it possible to establish the location of the beginning of fracture, the nature of its further course, to
make a judgment about the degree of plasticity of the material and about possible structural defects that led to the initiation and
development of cracks. The use of fractographic analysis methods for reconstructing the fracture surface of graphitized cast iron is
proposed. The influence of various inclusions in the structure of cast iron is considered. A study of the fracture surface of graphitized
cast irons was carried out, and the causes affecting the fracture process were identified.
Keywords: cast iron, graphite, fracture, fractographic analysis.
1. Introduction where ε≈2 – minimum angular resolution of the eye; N –
useful increase; А – lens aperture; n – refractive index; λ –
Fractography as a method for learning fracture’s is
light wavelength. DOF becomes smaller as the numerical
widely used in examinations to identify the causes of the
aperture and microscope increase (for example, for white
destruction of various parts of machines and objects.
light Т=54 μm at А=0,17, N=90 and Т=0,6 μm at А=1,25,
Traditionally, scanning electron microscopy is used for
N=1080). These numbers characterize the height of those
these purposes [1-3]. A distinctive feature of the imaging
structural details that can be examined with a single
of which is the use of secondary electrons. These
microscope setup.
secondary electrons generated by the material make it
Modern digital microscopes based on the use of
possible to obtain an image of objects with complex
traditional optical systems using specialized software make
geometry like a fractures of surfaces.
it possible to solve the problems of fractographic analysis
A traditional optical microscope, unlike a scanning
without the use of expensive electron microscopes [6-8].
electron microscope, does not have a sufficiently extended
The aim of the article is to demonstrate the capabilities
focus depth. This does not allow the traditional microscope
of digital optical microscopy in conjunction with the
to obtain focused images simultaneously for the peaks and
developed image processing algorithms for obtaining
troughs of broken surfaces [4].
fractographic images of fractures of the surfaces of
Depth of field (DOF) of the microscope objective - is
graphitized cast irons.
its ability to maintain the desired image (spatial frequency
at a given contrast) quality without refocusing if the 2. Research methodology
subject is closer and farther from the best focus [5]. DOF
is also applied to objects with complex geometry or Test specimens were made from graphitized cast iron
features of varying heights. DOF (T) can be expressed as smelted in induction furnaces. The melt was modified by
follows: magnesium in an autoclave to obtain spherical graphite in
250 250𝜀𝜀 𝑛𝑛𝑛𝑛 it. Gray cast iron modification was carried out in a ladle by
𝑇𝑇 = 2 + + 2, ferrosilicon. The chemical composition of graphitized cast
𝑁𝑁 𝐴𝐴𝐴𝐴 2𝐴𝐴
irons is presented in table 1.
Table 1. The chemical composition of the cast irons
Content elements %
Cast iron class
C Si Mn S P Cr Ni Mo Cu Mg
high strength 3,5 2,4 1,0 0,02 0,04 0,4 1,3 0,7 0,35 0,05
gray 3,1 1,6 1,3 0,06 0,4 0,3 0,4 - - -
After modification, cast irons were poured into dry ductile and gray cast iron: austenitization temperature
sand-clay molds to obtain solid sleeve castings ∅ 130 mm. 850оС, holding time 60 min; air cooling to room
From castings of high-strength and gray cast iron on a temperatures; tempering 410оС for 60 min.
lathe, blanks of rings with a section of 6×3,5 mm and 6×5 After heat treatment, microstructural research and tests of
mm were cut. The castle was cut in the rings. In order to mechanical properties were carried out (table 2). Bending test
reduce the influence of segregation phenomena, the rings of heat-treated rings was carried out in accordance with the
were subjected to heat treatment. Heat treatment for rings of scheme shown in the fig. 1.
Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY
4.0)
Table 2. The research results of cast iron rings
GOST 3443-87 structure Mechanical properties
Cast iron class Graphite
Metal matrix hardness σ,MPa
Size Form Distribution Amount
High-strength ШГд4 ШГф3 ШГр3 ШГ4 Tempered troostite 280 130,6
Tempered troostite;
Gray ПГд45-ПГд90 ПГф1 ПГр1 ПГ6 270 60,5
phosphide eutectic
Fig. 3. The fracture surface of ductile iron obtained using the
Fig. 1. Loading scheme optical reconstruction program OptiRec3D
For 3-dimensional reconstruction of fracture surfaces, To obtain a fracture image of the surface of graphitized
a layer-by-layer microscopy method was used. The gray and ductile iron, a set of photographs of
method allows to obtain topographic reliefs of volumetric microstructures was used. Image spacing 8 μm for High-
objects based on information from partially sharp images. strength cast iron and 12 μm for gray iron. The pixel size
To do this, using a digital microscope, we recorded images in the images was determined by the increase in the lens,
of the microstructure of the fracture surface with a certain in both cases it was 0.637 μm. The fracture surface of
depth step. The study and registration of the microstructure High-strength cast iron with a surface image (Fig. 4) is
was carried out on an analytical computerized complex, shown in Fig. 5.
which was created on the basis of an inverted Leica DM
IRM metallographic microscope. A number of such
images are presented in Fig. 2.
Fig. 4. The microstructure of the fracture surface of a sample of
High-strength cast iron with spherical graphite, obtained by
summing up areas with high definition from various images of a
number of data (etching with 4% alcohol solution of nitric
acid), × 200
Fig. 2. The initial series of images of fracture of a sample of
high-strength cast iron with spherical graphite with a depth step
of 8 μm, ×200
Optical Reconstruction 3D (OptiRec3D – developer
Ph.D. Chmykhov D.V. (FSBEI BO BSTU)) software was
used to reconstruct the surface of cast iron fracture from
the initial series of previously obtained images. The
program performs volumetric reconstruction of the surface Fig. 5. The fracture surface of a sample of high-strength cast
by a set of photographs of one object photographed with a iron with spherical graphite with an overlaid image of its
given depth step. OptiRec3D has a good source data microstructure
processing speed and high accuracy of reconstructed
models. An example of the volume surface of a fracture of The fracture structure of gray cast iron with lamellar
a sample of High-strength cast iron with nodular graphite graphite after various stages of the surface reconstruction
is presented in Fig. 3. operation is shown in Fig. 6.
fracture surface of high-strength cast iron (Figure 4), it is
noteworthy that the fracture surface passes along the chains
of graphite inclusions. Analysis of the fracture surface shows
that it is precisely along the cluster chains of eutectic cells that
cast iron is destroyed.
The destruction of High-strength cast iron rings takes
place in several stages. At the initial stage, the nucleation
of cracks begins at the boundary “graphite - metal matrix”,
because, despite their compact form, graphite inclusions
separate the metal matrix of cast iron. Then, the process of
separation of the structure is localized by plastic
deformation of the metal matrix. At this stage, the
development of microcracks follows the paths of least
resistance along those sections of the metal matrix that are
between two adjacent graphite inclusions. The weakening
Fig. 6. The fracture surface of gray cast iron with lamellar of the metal matrix occurs as a result of the occurrence in
graphite: a - surface microstructure × 200; b - volumetric the structure of isthmuses between adjacent graphite
reconstruction of the surface with the applied microstructure; c - inclusions combined into cluster chains. Figure 4 shows that
model of fracture surface obtained using OptiRec3D software the graphite inclusions emerging on the fracture surface are
interconnected and form “strings” along which the crack
The fracture surface of gray cast iron (Figure 6, b) is propagates during fracture. Moreover, in the process of
characterized by a rougher structure and a pronounced fracture, graphite inclusions often remain intact, which
relationship with the location of the graphite phase than indicates the existence of a gap between graphite and a
that of a sample of high-strength cast iron. An analysis of metal matrix. At the final stage, the fusion of microcracks
the image of the fracture surface shows that cracks in the begins and the destruction of cast iron occurs.
fracture process mainly formed along the planes of In high-strength cast iron, nonmetalic inclusions are
graphite inclusions (Figure 6, a). The observed picture much smaller than in gray cast iron, so their effect on the
indicates that the destruction passed through the centers of destruction processes is reduced. When studying the
the eutectic colonies, where the graphite inclusions have surface structure of fracture of ductile iron (Figure 5),
the largest transverse dimensions. In these areas of cast there are practically no peaks and depressions, which, as
iron, the metal matrix is strongly fragmented by the previously shown, for gray iron are parts of the structure
graphite phase. where nonmetallic inclusions are located. The reason of
3. The discussion of the results this is that the contained in the ligature magnesium used to
modify cast iron is partially spent on desulfurization,
Analysis of the fracture surface shows that nonmetalic binding sulfur to a compound removed with slag
inclusions also contributed to the destruction of gray cast Research of fracture surfaces of graphitized cast irons
iron. Greater pollution of cast iron by sulfur and additional prove that the destruction processes are influenced not
doping with phosphorus lead to the appearance of sulfide only by the shape, but also by the distribution of the
inclusions and regions with phosphide eutectics in the graphite phase.
structure, which, like graphite, are stress concentrates.
Phosphide eutectic and sulfide inclusions in the 4. Conclusion
structure of cast iron are usually concentrated on the The results of using a modern digital optical
periphery of eutectic colonies. In these areas on the surface microscope in combination with specialized software
model (Figure 6, b) peaks and troughs are observed, presented in the article made it possible to obtain high-
indicating that the destruction occurred precisely on these quality fractograms of fractures of graphitized cast irons.
structural elements. The negative effect of phosphide The results demonstrated a high degree of adequacy and
eutectic is explained by the fact that it, hardening at the last allowed us to assess the relationship between the structure
stage of crystallization, isolates individual eutectic cells, of the material and the processes of destruction The
thereby creating interfaces in the structure. The interface, potential inherent in this method will expand the
as well as graphite plates, violate the continuity and capabilities of traditional optical microscopy and is an
uniformity of the metal matrix. Cementite inclusions excellent alternative to expensive scanning electron
present in phosphide eutectic increase its hardness and microscopy.
contribute to brittle fracture. Thus, a crack in gray cast iron
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About the authors
Makarenko Konstantin V., Doctor of Technical Sciences,
Professor, Department of Engineering and Materials Science,
Bryansk State Technical University. E-mail:
makkon1@yandex.ru.
Nikitin Alexander A., graduate student of the Department of
Tribotechnical Materials Science and Materials Technology,
Bryansk State Technical University. Email:
zzzalexzzz95@gmail.com.
Parenko Andrei S., graduate student of the Department of
Tribotechnical Materials Science and Materials Technology,
Bryansk State Technical University. Email:
3dprintense@gmail.com