Volume 7 Preprint 31
Influence of Protective Coatings and the Service Temperature on Mode I Fracture of EN8 Steel
K.V. Arun and C.S.Venkatesha
Keywords: Corrosive resistant coatings, CVN impact test, Fracture Toughness, Transition temperature, Coating thickness, Micro mechanism
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Volume 7 Paper 31
INFLUENCE OF PROTECTIVE COATINGS AND THE SERVICE
TEMPERATURE ON MODE – I FRACTURE OF EN8 STEEL
Research scholar, Department of Mechanical Engg, University.B.D.T College
of Engg. Davangere-577004, Karnataka, INDIA,
Assistant Professor, Department of Mechanical Engg, University.B.D.T College
of Engg. Davangere-577004, Karnataka, INDIA.
This paper is devoted to the analysis of the influence of the corrosive
resistant coatings and the service temperature on the fracture
toughness behaviour of EN8 steel. This investigation concentrates on
the Zinc, Nickel and powder coatings, which are widely used in the
industrial applications. Experimentations have been carried out on
CVN impact testing machine. The CVN specimens were tested under
different temperatures and coating thickness. The plane strain fracture
toughness and the transition temperature for each coating of the
material is identified. The micro mechanism of fracture is also
identified with the aid of fractography. The test results showed the
existence of a clear correlation between CVN impact energy and the
fracture toughness and also the transition behaviour of EN8 with
respect to the temperature. The specimen preparation and
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science
and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it
should not normally be referenced in published work. © UMIST 2004.
experimentations were carried out according to the ASTM E23
Key Words: Corrosive resistant coatings, CVN impact test, Fracture
Toughness, Transition temperature, Coating thickness,
Metals are seldom found in their pure state. They are usually found in
chemical combination with one or more non-metallic elements. Metal
corrosion is generally defined as “the undesirable deterioration of a
metal or alloy; an interaction of the metal with the environment that
adversely affects the properties of the metal”. Evidence  is available
to show that the majority of metal failures due to corrosion occur
through general, or uniform, modes. The next most common cause is
stress corrosion cracking, followed by pitting corrosion and
intergranular corrosion. These four modes account for about 80% of
the failures examined. The main techniques available for reducing
corrosion are, drying out the environment, e.g., reduce the humidity to
well below 60% such as at a desert destination, use more corrosion
resistant materials such as Monel rather than brass for components
rotating in seawater, alter design to optimise geometry, use organic
coatings such as paints or powder coatings, use metallic coatings,
such as Zinc, Nickel, Hard Chrome, etc.
Altering the environment can retard corrosion, but this is not possible
to use these techniques in all the applications . Industrial finishing
is an integral and important part of most manufacturing processes.
Protective treatments and coatings are used to enhance resistance to
corrosion and abrasion, modify physical or mechanical properties of
the surface material, or enhance the surface finish to improve artistic
appearance and sales appeal. Coating or paint layers are often applied
to the surfaces of metallic, polymeric, or composite structures[3-4].
The problems of forming protective coatings, study of their properties,
and investigations of complex physico-chemical processes, occurring
under a variety of interactions between the substance and the
surrounding medium are attracting the attention of a wide range of
During the fabrication or the service life of a metallic structures, there
are many circumstances capable of giving rise to the appearance of
defects (cracking related to welding, corrosion, coating, fatigue, etc.).
If the size of these defects reaches a critical value, an unstable fracture
of the structure may occur at a nominal stress lower than the yield
stress of the material . Unstable crack propagation is the final stage
in the useful life of the structural component. This stage is governed
by, the material toughness, crack size and shape, and the stress level.
Consequently, unstable crack propagation cannot be attributed only to
material toughness, or only to high stress level caused by inadequate
design, or only to poor fabrication, but rather to a particular
combination of the above factors[6-8].
This study concentrates on the effect of coating thickness on the
energy absorbed and in turn the fracture toughness of the material. In
general, the fracture toughness of structural materials, particularly
steels, increases with increased temperature and decreased load rate.
Each material will have a particular transition temperature where the
fracture behaviour changes from brittle to ductile. This behaviour of
fracture, which depends on the service temperature of the coated
structures, has been investigated. Experimental analysis has been
made on the Zinc, Nickel and powder coated specimens with varied
coating thickness and service temperatures. Since the CVN impact test
is the widely used test method for the analysis of fracture toughness
of steel [9-10], the specimens have been impact tested in a pendulum
type impact testing machine.
2. Materials And Methods
In the present investigation three types of coatings have been
Zinc plating of steel components has been considered the most
economical and viable industrial finishing process for steel,
where sacrificial type corrosion resistance is required. Plating
includes zinc bath in which the cleaned specimen are tied to a
thin wire and immersed fully in to a tank containing the
chemicals which is used to coat or plate the specimen. Here zinc
(anode) is used as a coating material, the composition of
chemicals which are used in the tank are, zinc oxide 60% to 70%,
sodium cyanide 80 gm, caustic soda 50 gm. To form the final
solution all these are mixed with 1000Lt of water. The
specimens are immersed in the solution they act as cathode
(positively charged) and the anode is zinc plate which is
negatively charged, is placed at the two ends of the tank. Then
electrical charge is supplied for 20 minutes for 6 to 8 microns
thickness and the current is varied for different coating
thickness. The thickness of plating is maintained in the range 05
to 25 microns, in steps of 05 microns.
Electroless Nickel plating has been done on the specimen.
Electroless nickel coatings are extensively used in the metal
plating industry, as the physical properties of the coatings
(uniformity, corrosion resistance and lubricity) are better than
electroplated nickel. In electroless plating, metal ions are
reduced to metal by the action of chemical reducing agents,
which are simply electron donors. Electroless nickel plating is a
process whereby a nickel coating is deposited on a surface in a
controlled chemical reduction; the process is termed
“electroless” because the electrons are supplied by a chemical
reducing agent and not electrically.
The catalyst is the
substance which accelerates the electroless chemical reaction,
allowing oxidation of the reducing agent . The metal ion and
reducer concentration must be monitored and controlled closely
in order to maintain proper ratios and to maintain the overall
chemical balance of the plating bath. The thickness of coating is
maintained in the range of 5-20 microns in steps of 5 microns.
Powder coating produces a high specification coating which is
relatively hard, abrasion resistant, corrosion resistant and tough.
The powder coating process used is seven tank process and the
powder used is the matt finish epoxy powder. After applying the
powder in the form of coating, the specimen is taken to the oven
for baking. The temperature of backing is about 4000 C and the
time of backing was 30 – 45 minutes. After backing the
powder gets converted into paste or gel and becomes a coat on
the specimen. The thickness of the powder coating is
maintained at different range, and it depends on the spray
quantity of the powder. The thickness of coating is maintained
in the range 30 to 70 microns, in steps of 10 microns.
Experimentation is carried out on the En8 material which is the widely
used structural material in automobiles, aerospace, ship building,
nuclear power plants, machine tools and general engineering
applications . The chemical composition and the mechanical
properties of the substrate material are given in the Table 1.
Table 1. Chemical composition and Physical Properties of En8
Yield strength = 580 Mpa
Ultimate strength = 653 Mpa
Young’s Modulus = 205 GPa
2.3 Specimen Preparation
The test piece is a square bar of material, 10mm x 10mm x 55mm,
containing a notch cut in the middle of one face. CVN specimen
having notch angle of 45 o , 2mm depth and with a root radius of
0.25mm are prepared by an EN8 rolled bar stock. The desired number
of specimen were coated with Zinc, Nickel and Powder coatings. The
thickness is maintained uniformly through out the specimen surface.
Along with the coated specimen, the normal specimen were also
prepared for the comparison of the results. The CVN specimen were
prepared according to ASTM E23 type A standard.
2.4 Experimentation and Data Analysis
A triaxial state of stress is developed at the root of a notch and hence
notched specimens are used in these tests. The impact test is widely
employed for the testing of ferrous metals and plastics, for the reason
that both are prone to change in their fracture behaviour with changes
in temperature. The specimen used and the testing procedures are as
ASTM E23 standards. The testing is carried out at varied temperatures
starting from 0°C to 225°C in steps of 25°C. The 0°C temperature is
maintained with the aid of ice and refrigerator, the temperatures from
25°C to 225°C in steps of 25°C are maintained by using furnace. The
plain strain fracture toughness for the EN8 steel has been determined
from the CVN impact energy obtained from the experimentation. The
fracture toughness values have been found out for all temperature and
material conditions. The plots of impact energy Vs temperature,
Fracture Toughness Vs temperature have been drawn.
3. Results And Discussions
Past investigations have shown that there will be a change in impact
energy and also in fracture toughness with respect to the service
temperatures [5-9]. In this study fracture toughness is determined by
varying the coating thickness and temperature. Also the micro
mechanism of fracture behavior is identified through SEM
fractographs. The results of which have been discussed below.
3.1 Effect Of
There is an increment in both impact energy and fracture toughness
(K1C ) as the temperature increases, as shown in Figure.1. The strength
of the material will increase up to a certain temperature range only,
beyond which it will decrease drastically. The transition temperature,
where fracture behavior changes from brittle to ductile can also be
seen in the Fig.1.The typical transition temperatures for the tested
material under normal condition are, for En8 = 100O C. The same
behavior has been observed in zinc plated, nickel plated and powder
Impact Energy / Temperature (normal)
Im pact Energy (J/mm )
Temperature (in c)
K1C / Temperature (normal)
K1C (in Mpa (m)1 /2 )
Temperature (in C)
Figure.1. Variation in impact energy and fracture toughness with
respect to the service temperature
Figure.2. Fractured surface of En8 tested at room temperature
Figure.3. Fractured surfaces of En8 tested at 125O C
Fractographic examinations ere carried out to investigate the
damage processes in the brittle and ductile regimes. The fractured
surfaces of all the Charpy specimens that have fractured during the
testing were examined in detail using a scanning electron microscope
(SEM), to identify the type of fracture and the precise brittle to ductile
transition temperature (DBTT). The fractured facets of the specimen
fractured under normal temperature, has shown a brittle fracture
(Figure.2. a&b). As highlighted in the figure.3.2.b, the nature of
fracture is transgranular. It is very clear that (figure.3. a &b) there is a
formation of number of cleavage steps, which have joined and formed
a river pattern. Precise analyses of these river patterns will give the
direction of crack growth. The same material under elevated
temperature (125OC) has shown a ductile fractured facets (Figure.3),
which is fibrous in nature. From this fractographic analysis it is very
clear that, as the service temperature reduces the type of fracture will
be brittle in nature and as the service temperature increases the nature
of fracture will be ductile. But, if the service temperature is very low (
Nil Ductility Temperature NDT) or, if it is more than the BDTT, the
performance of the material will be very poor. From such fractographic
analysis the DBTT of the EN8 steel has been found out as 100OC.
3.2 Effect of Coating Thickness MODE-I Fracture Toughness
With regards to the coating thickness it has been found that as the
coating thickness increases the impact energy and the fracture
toughness of the material will increase, but after a particular level of
coating thickness it will decrease. Coating thickness of each type of
coating selected for the experimentation are based on the practical
applications in the fields of Aerospace engineering, Automobile
engineering, Ocean engineering and Machine tools.
K1C \ Temperature ( Zinc Coated )
K 1C (Mpa(m m) )
Temperature( in C)
Figure.4. Variation of fracture toughness with reference to the Zinc
coating thickness and the service temperature.
K1C /Temperature EN8 (Nickel Coated)
Figure.5. Variation of fracture toughness with reference to the Nickel
coating thickness and the service temperature.
K 1C/Temperature EN8 (Powder Coated)
K 1C(in Mpa(m) )
Figure.6. Variation of fracture toughness with reference to the Powder
coating thickness and the service temperature.
From the experimental investigations it is very clear that the coating
thickness has a great impact on the fracture toughness of the
materials. In case of coated specimens also the effect of temperature
on the fracture toughness of the material remains the same. Maximum
fracture toughness of Zinc coated specimens is obtained with a
coating thickness of 20 microns
(Figure.4). The increment or
decrement in the fracture toughness of the material with respect to the
coating thickness depends on the time duration at which the material
is held in the coating bath. Typically, this duration of time will increase
as the thickness of coating increases. Also one more significant factor
is the temperature of the coating bath and its chemical concentration.
In this experimentation, the results have been derived by considering
both, coating thickness and the service temperature. With respect to
the Nickel coating the maximum fracture toughness is obtained by a
coating thickness of 15 microns (Figure.5). The maximum fracture
toughness of the EN8 is achieved with a powder coating of 60 microns
From the experimental it is found that the maximum fracture
toughness obtained with the Zinc (120 Mpa.mm 1/2) and Powder
coating (118 Mpa.mm1/2) is less than that of the maximum fracture
toughness of normal material (122 Mpa.mm1/2 ). This is mainly because
of the too ductile nature of the coating layer. Also because of the
embitterment of the material by the chemicals and the temperature of
the coating bath in case of Zinc coating. This is because of the backing
temperature in case of the powder coating. But in case of the Nickel
coating the fracture toughness (123 Mpa.mm 1/2) is higher than that of
the normal material, because of the toughness of the Nickel deposit
3.4 Micromechanism of Fracture
Bonding of coatings with metals is a fundamental theoretical and
practical problem in the protective coatings. In this investigation to
analyse the bonding between the coating and the substrate materials,
a microscopic study has been made with the aid of SEM scans.
Microscopic observations of the scans were made to identify
quantitatively the type of fracture as a function of temperature. Also by
observing the fractographs the transition temperature, where the
mode of fracture changes from brittle to ductile can be verified
Figure.7. Shows the nature of fracture in En8 steel under varied
temperature. It is also clear from the figure that at low temperature
(0 OC to 25OC), the nature of fracture is brittle. The fracture observed
was intergranular. Scan(a) shows clearly, a perfect grain boundary
separation and sharp edges which is a clear indication of intergranular
brittle fracture. In the normal service temperature (25OC to 50OC), as
shown in the scan (b) the fracture is again brittle in nature. In higher
temperatures (50O C to 75O C), as shown in scan (c) the fracture is
brittle in nature. But it is clear from the fractograph that the type of
fracture is transgranular. In the temperature between (75O C to 100OC)
as shown in the scan (d) the type of fracture is a combination of bath
ductile and brittle fracture. This indicates that there is a slight
transition in the material behaviour. The majorities of grain boundary
facets are showing dimples and are fibrous in nature (e). This is a clear
indication of ductile fracture. The scan(f) clearly shows that the nature
of fracture is ductile. Compared to scan(e) it can be seen that the
concentration of dimple areas has been increased drastically. Further if
the temp is increased, the fracture
where it can not sustain more load.
will be more ductile in nature
Figure.7. Micromechanism Of Facture Under Varied Temperature
The results of the investigation have shown that, as the coating
thickness and service temperature increases, the impact energy and
the fracture toughness (K1C) will also increase. But the increment in
fracture toughness is achieved only up to certain level of coating
thickness and service temperature, beyond which it will decrease. For
each condition of the material the transition temperature will vary
slightly. The increase or decrease in the fracture toughness of the
coated materials depends mainly on the type of coating, coating
thickness, the environment in which coating is done, the temperature
at which coating process is carried out and the time for which the
material is kept under these conditions during coating. Service
temperature has got a maximum influence on the nature of fracture
and also on the strength of material, which is clearly revealed through
the fractographic examinations in this investigation. If corrosion
protection or decorative items are required one can go for these
coatings with the suitable thickness. This selection of material
condition will also depend on the service temperatures.
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