Fracture Analysis of Chilled Cast Iron Camshaft: China Foundry
Fracture Analysis of Chilled Cast Iron Camshaft: China Foundry
Fracture Analysis of Chilled Cast Iron Camshaft: China Foundry
Abstract: The fracture of a camshaft made of chilled cast iron, installed in a home-made Fukang car, happened
only after running over a distance of 6,200 km. The fractured camshaft was received to conduct a series of
failure analyses using visual inspection, SEM observation of fracture section, microstructure analysis, chemical
composition analysis and hardness examination and so on, while those of CKD camshaft made by Citroën
Company in France was also simultaneously analyzed to compare the difference between them. The results
showed that the fracture of the camshaft mainly results from white section in macrostructure and Ledeburite in
microstructure; the crack in the fractured camshaft should be recognized to initiate at the boundary of coarser
needle-like carbide and matrix, and then propagate through the transverse section. At the same time, the casting
defects such as dendritic shrinkage, accumulated inclusion and initiated crack and abnormal external force might
stimulate the fracture of camshaft as well. Based on failure analysis, some measures have been employed, and as
a result, the fracture of home-made camshafts has been effectively prevented.
2 Results
2.1 Visual inspection
The fractured camshaft is shown in Fig.1. From Fig.1, it is
found that the fracture had taken place in the journal region
between exhaust cam and admission cam of the 2nd cylinder
in the engine. Adherent fins at the molding parting of the
fractured camshaft was not thoroughly trimmed, yet, as shown
in Fig.2. In addition, there was obvious extruded trace on the
interior wall surface of camshaft hole of cylinder head. White
macroscopical section of the fracture zone shown in Fig.3 is
significantly different from the camshaft from France shown in
Fig.4.
Fig.4 Macroscopical fracture section of the camshaft
made by Citroën Company in France
(b)
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200 μm
Fig.6 Graphite shape of the fractured zone of the camshaft 2.4 Chemical composition analysis
The chemical composition of the material from the failed
camshaft, by using a spectroscopic metal analyzer, is reported
in Table 2. At the same time, Table 2 also gives the specified
chemical compositions of the camshaft material. It can be seen
from Table 2 that all of the chemical compositions of the failed
camshaft are within the specified range.
C Si Mn S,max P,max Fe
Failed 3.52 1.78 0.81 0.10 0.07 Bal.
Normal 3.20–3.70 1.70–2.20 0.60–1.10 0.15 0.18 Bal.
200 μm
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1110 for PSA Group of France. The hardness for chilled zone results, visual section characteristic shows nearly white, the
is required to measure within the zones of 2 mm beneath the microstructure of fracture zone is Ledeburite, and its hardness
surface, as shown in Fig. 10(b), where the value specified on is also beyond the range of the standard.
the top of a cam should be in excess of HRC 48, and the value The second reason for it is casting defects such as the
at the other zones of each cam should be higher than HRC inclusion. The existance of the defects increased the initiation
42. And, the hardness values at the non-chilled zones such as and propagation of the crack and decreased the loading
the center of the cam and the transition region of the journal, capacity of the camshaft to impacting force from the outside, i.e.
are specified approximately 170–255 HB. The intermediate increased the inclination of the fracture of the camshaft.
frequency induction furnace with a capacity of 1 ton was Another reason for the fracture of the camshaft is the action
used to melt the molten iron. The pouring temperature was of abnormal force. From the extruded trace in the inner wall
approximately 1,400 ℃. Inoculation treatment was operated in of camshaft hole of cylinder head shown in Fig.3 it can be
a ladle. assumed that the working order of the camshaft was destroyed
since hard particles entered into the clearance between the
camshaft and the corresponding hole of cylinder head before
the fracture failure of the camshaft.
From the above analysis, it can be concluded that the crack
in the fractured camshaft initiated at the boundary of coarser
needle-like carbide and matrix (as shown in Fig.7), and
then propagated through the transverse section, where stress
concentration easily takes place under cyclic bending and
torsion. At the same time, the crack initiated can be rapidly
grown at the boundary of the inclusion and matrix, or passes
through the shrinkage zone within the camshaft (as shown in
Fig.10 Schematic diagram of chilled situation of camshaft
Fig.5). In addition, the extruded force gave rise to the initiation
In order to obtain an excellent wear-resistance of cam of the crack at the removal of the casting fins at the transition
surface of cast camshafts, chills are usually used to increase region surface of the camshaft or the enterance of hard particle
the cooling rate and create a hard ledeburitic structure layer into the engine as well.
on the surface, while the center of the cam still keep grey cast Based on the analyses above, some optimization measures
iron structure [8]. An appropriate depth of the chilled layer taken to prevent the abnormal microstructure at the cam necks
is prerequisite to keeping better resistance and long service can be recommended below:
life of cam. A too-thin chilled depth would result in short (a) Improve casting process design parameters, for example,
service life of cam; however, a too-thick chilled depth would chilled depth of chilled samples has been reduced to 11–13 mm
lead to increase the large brittleness, namely, high inclination from about 15 mm. Furthermore, thermal analysis method has
to fracture of camshaft casting. Transition regions situated been used to rapidly analyze C, Si content and supercooling
between two cams are required to have the approximate degree of melted iron of camshaft for helping effective control
microstructure of grey cast iron and keep good toughness as of chilled depth of chilled samples [9-10].
that of the center of the cam. The chilling tendency of the (b) Purify the melt iron to remove inclusion, and improve
transition regions increases significantly due to the small gating and riser system to provent shrinkage defect and so on.
interval of about 4 mm between the cams, therefore it is very (c) Prevent the crack from initiation by carefully trimming
necessary that casting process of camshaft keep under adequate adherent fins at the parting line of camshaft and avoid hard
control. The measuring method of chilled depth in the sample particle enter into the engine.
is shown as Fig.11. So, the main reason for the fracture Through taking those measures above, the fracture of home-
failure of the camshaft is that an over-chilling impacted in the made camshafts has been effectively prevented.
transition region. As the evidences from the above inspection
4 Conclusions
(1) The main reason for the fracture failure of the camshaft
is that a too strong chilled trend existed in the transition
region. Visual section characteristic shows nearly white, the
microstructure of fracture zone is Ledeburite, and its hardness
is also beyond the range of the standard.
(2) The existance of the defects such as inclusion increased
the potential of the generation and propagation of the crack
and decreased the loading capacity of the camshaft to
impacting force from the outside, i.e. increased the inclination
Fig.11 Measuring method of chilled depth in the sample of the fracture of the camshaft.
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The work was supported financially by the postdoctoral foundation from Yituo Group Co., Ltd., China and
the doctoral foundation from Henan Polytechnic University, Henan, China.
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