FIELD OF INVENTION
The present invention relates to the repair of shafts and their restorations by thermal spray coatings.
More particularly, the present invention relates to a method for dimensional restoration of shaft
using iron based metallic powder deposited by the high velocity oxygen fuel (HVOF) based
thermal spray coating process.
BACKGROUND OF INVENTION
Machinery in operation in industry, transportation etc., undergo mechanical contact and eventually
experience wear and deterioration of the components. The wear process, by definition, causes
material damage and, as a result, some geometrical characteristics of the component are lost. The
results can range from a slight change in dimension of the worn part to more problematic situations
in which the shape of the component is altered completely. In some cases, over machining of shafts
also makes the shafts under dimensional and unfit for the application. In some applications, a new
spare part is too expensive, while in other cases new spare parts cannot be found because of the
age of the equipment. Therefore, there is a need of restoration of the components back to their
initial dimensions and specifications very reliably and in a cost effective way.
Though hard chromium plating is used to restore worn out shafts, such technology presents
harmful effects on the environment and the public health and it exhibits certain intrinsic technical
limitations. In this pursuit, thermal spraying is becoming an increasingly popular and effective
solution for the repair and restoration of worn components. Moreover, the HVOF (high-velocity
oxy-fuel) thermal spraying process is more environmental friendly than the chromium plating
process.
PRIOR ARTS OF INVENTION
A number of approaches have been undertaken in the recent times for repair and restoration of the
shafts. These include laser cladding, welding and electroplating and thermal spray processes.

Additionally, hard chromium plating is usually used to restore to the original dimensions of the
worn out surfaces of gas turbine shafts.
In a literature titled “Comparative study of wear behavior of Thermal Spray HVOF coating on 304
SS”, the coatings of tungsten carbide and chromium carbide sprayed by HVOF are studied. Both
the coatings included the same binder with equal percentage. The corresponding mechanical and
structural characterizations has been performed with the help of scanning electron microscope
(SEM), image analyzer, x-ray diffractometer (XRD), energy dispersive spectroscopy (EDS) and
pin-on-disc wear testing machine. The results indicated that the wear rate of tungsten carbide
coating is much lower than that of chromium carbide coating in presence of different loads at room
temperature. Moreover, thermal conductivity and the porosity of the coating material were found
to have a marked influence on the wear rate.
In another prior art titled “Investigation of HVOF processed carbide based coating on AISI-4340”,
a comparative analysis involving tungsten carbide and chromium carbide-based coatings deposited
by HVOF technique on AISI 4340 steel has been conducted. The structural and mechanical
properties were characterized with the help of energy dispersive x-ray analysis (EDAX) and SEM.
The Vickers micro hardness tester is used to evaluate the hardness of coating. Wear resistance of
the base material is analyzed using Pin-on-Disc testing method. The effect of surface modification
on different attributes like hardness, surface morphology and wear behavior has been examined.
The results confirmed the coating with tungsten carbide exhibited enhanced performance in all
aspects of attributes compared to that shown by chromium carbide.
It may be noted that the materials used in HVOF coating are tungsten carbide or chromium carbide
particles in a metallic alloy matrix consisting of various combinations of Co, Ni or Cr. The two
most common carbide coatings are WC-Co and Cr3C2-NiCr. However, these coatings are very
hard and are not machinable in nature. Moreover, their properties differs from that of the base
material as well.
OBJECTS OF THE INVENTION
It is an object of the present subject matter to overcome the aforementioned and other drawbacks
existing in the prior art systems and methods.

It is one object of the present subject matter to use a coating for restoration of the dimensionality
of the shaft such that the coating is easily machinable.
It is another object of the present subject matter to select a metallic powder for coating such that it
possesses similar elemental composition as that of most of the alloy steels.
It is still another object of the present subject matter to select a metallic powder for coating such
that it resembles similar properties as that of the base material.
It is yet another object of the present disclosure to implement an iron based metallic coating on the
surface of the shaft for restoration of the dimensionality of the shaft.
It is even another object of the present disclosure to implement an iron based metallic coating on
the surfaces of the shaft by means of a HVOF based thermal spray process.
These and the other objects and advantages of the present subject matter will be apparent to a
person skilled in the art after consideration of the following detailed description taking into
consideration with accompanied drawings in which preferred embodiments of the present subject
matter are illustrated.
SUMMARY OF THE INVENTION
This summary is provided to introduce concepts related to a method of coating of the shaft material
of machineries in industries by an iron based metallic powder following the principle of HVOF
thermal spray process. The concepts are further described below in the detailed description. This
summary is not intended to identify key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed subject matter.
A method of shaft restoration by an iron based metallic powder coating by a oxygen fuel based
thermal spray process; the method comprising of preparing the surface of a base material by the
process that includes grit blasting; fixing said base material samples to a fixture; loading said iron
based metallic powder for coating said base material samples; setting the process parameters in a
control unit as required for coating with said iron based metallic powder wherein the coating is
based on a high velocity oxygen fuel thermal spray process; coating in passes with said iron based
metallic powder till the required thickness is achieved; and air cooling in between the passes after
completion of said coating. Herein, the base material includes a plurality of CrMoV steel plates

and the roughness value (Ra) of the surface of the base material ranges between 11 µm to 15 µm.
Iron based metallic powder is loaded to the equipment in powder feeder and said movement of the
robot arm is achieved by programming. The thickness per pass of said coating ranges between 50
µm to 70 µm and the total thickness of the coating ranges from 3 mm to 5 mm. Moreover, the
porosity and the adhesion strength of the coating ranges from 0.5 % to 1.5 % and from 9200 psi to
9800 psi, respectively. Further, the iron based metallic powder coating has a value of abrasion
resistance comparable to that of said base material. Additionally, the iron based metallic powder
coating has a higher machinability than that of a tungsten carbide coating, wherein the
machinability of said iron based metallic powder coating is in line with said base material.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the
present subject matter and are therefore not to be considered for limiting of its scope, for the
invention may admit to other equally effective embodiments. The detailed description is described
with reference to the accompanying figures. In the figures, a reference number identifies the figure
in which the reference number first appears. The same numbers are used throughout the figures to
reference like features and components. Some embodiments of system or methods or structure in
accordance with embodiments of the present subject matter are now described, by way of example,
and with reference to the accompanying figures, in which
Figure 1 depicts the flow diagram illustrating the coating process in accordance with the present
disclosure.
Figure 2 illustrates the XRD pattern of different material and coating in accordance with the
present disclosure.
Figure 3 depicts HVOF coating microstructure in accordance with the present disclosure.
Figure 4 depicts the hardness values in accordance with the present disclosure.
Figure 5 depicts the abrasion test results in accordance with the present disclosure.

DETAILED DESCRIPTION OF INVENTION WITH REFERENCE TO THE
DRAWINGS OF THE PREFERRED EMBODIMENTS
The following is a detailed description of embodiments of the disclosure depicted in the
accompanying drawings. The embodiments are in such detail as to clearly communicate the
disclosure. However, the amount of detail offered is not intended to limit the anticipated variations
of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the present disclosure as defined by the appended
claims.
While the embodiments of the disclosure are subject to various modifications and alternative
forms, specific embodiment thereof have been shown by way of example in the figures and will
be described below. It should be understood, however, that it is not intended to limit the disclosure
to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications,
equivalents, and alternative falling within the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are
intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises
a list of components does not include only those components but may include other components
not expressly listed or inherent to such system, or assembly, or device. In other words, one or more
elements in a system or device proceeded by “comprises… a” does not, without more constraints,
preclude the existence of other elements or additional elements in the system or device.
In this work, iron based metallic powders have been applied using thermal spray process as an
alternating method for restoration of shafts. Iron based powders have the composition close to steel
compositions. Moreover, the high velocity oxygen fuel (HVOF) based coating of iron based
metallic powders is tested for suitability for the shaft restoration by evaluating the properties of
the coating like porosity, bond strength, hardness, abrasion resistance etc.
In the present disclosure, in an embodiment, suitable iron based metallic powder is selected to suit
its deposition on various alloy steel shafts. The iron based metallic powder is selected in such a
way that it possess similar elemental composition as present in most of the alloy steels. This is due

to the fact that coating resulted from deposition of this iron based metallic powder can yield similar
mechanical properties.
Figure 1 depicts the flow diagram illustrating the coating process. HVOF (High velocity oxygen
fuel) coating is a thermal spray process. In an embodiment, the fuel burns in the combustion
chamber thereby producing high temperature and high pressure combustion gas, which comes out
of the chamber. The metallic powder is supplied simultaneously by the powder feeder to the high
temperature gas stream. Consequently, the supplied powder particles get partially melted and come
along with the combustion gas at high velocity and settles on to the base material surface forming
a dense coating. Thus the selected iron based metallic powder is deposited into the base material
through this process.
In an embodiment, the parameters used for the coating are listed below.
• Spray Distance : (250-290) mm
• Spray angle : nearly 90ᵒ
• Spray Velocity : (420-480) mm/s
• Fuel (kerosene) flow rate : (20-22) l/h
• Oxygen flow rate : (770-830) nlpm
• Powder Feed rate : (60-80) g/min
• Carrier gas flow rate : (7-10) nlpm
In a preferred embodiment, the parameters used for the coating process are as follows:
• Spray Distance : 270mm
• Spray angle : 90ᵒ
• Spray Velocity : 450mm/s
• Fuel (kerosene) flow rate : 20 l/h
• Oxygen flow rate : 800 nlpm
• Powder Feed rate : 70g/min

• Carrier gas flow rate : 8 nlpm
• Spray gun : WOKA 610
Figure 2 illustrates the X-ray diffraction (XRD) pattern of different material and coating. In an
embodiment, X-Ray diffraction patterns are generated for CrMoV steel, selected iron based
metallic powder and HVOF coated sample. The corresponding XRD patterns of all these three
components are plotted on the same graph. The graph depicts the presence of ferrite and Cr1 Fe1
compound in the base CrMoV steel, iron based powder and also in HVOF coating. Therefore, the
coating is having the compound similar to the base material.
Figure 3 depicts HVOF coating microstructure. In an embodiment, optical micro graphs of HVOF
coated samples are taken to observe microstructure, thickness, porosity and coating interface. The
porosity of the coating ranged from 0.5 % to 1.5 %. In a preferred embodiment, the porosity of the
coating was observed around 1.08%.
In an embodiment, hardness of HVOF coated sample and chosen base material (CrMoV steel) is
measured on HV0.3 scale. The corresponding hardness values are depicted in Figure 4. The
hardness values of the HVOF coating, CrMoV steel base material and WC are shown in the graph
below in Table 1. It may be noted from the figure that the values of hardness for WC is very high
and therefore it is not machinable unlike the iron based metallic coating disclosed in the present
subject matter.
Table 1. Comparative studies of the hardness values
Scale Base (CrMoV steel) Iron based metallic Tungsten carbide (WC)
coating using HVOF
HV0.3 240 – 270 480 - 520 1150-1200
In an embodiment, abrasion test is conducted on the samples as per ASTM G-65 standard. In this
test, sample is subjected to sliding wear test. Before the abrasion test, coated samples are finely
ground for making the surface smooth and the samples are weighed before and after the test. The
corresponding weight loss is calculated and converted into volume loss (mm3). The test samples
for the abrasion test included Iron based HVOF coating and chosen CrMoV steel sample.

In an aspect, tungsten Carbide (WC) coated sample is also included for the abrasion test for a
comparative study. The comparative results are also shown in Figure 5.
In an aspect, the volume loss of the samples under test i.e. the base material, WC coated by HVOF
technique and metallic iron based power coated by HVOF technique is depicted. It is observed
from the figure that WC coated material exhibited a negligible loss in volume. However, the
volume loss in the base material and the metallic iron based powder coated by HVOF technique
are more or less similar. Therefore, it is ascertained that the properties of the base material are
retained after HVOF coating by metallic iron based powder.
In an embodiment, adhesion strength (bond strength) of the iron based metallic coating is tested
with pull-off adhesion tester. The adhesion strength of the coating ranges from 9200 psi to 9800
psi.
In a preferred embodiment, a sample of 50 mm X 50 mm is used for the test. After grit blasting, a
dolly of 8 mm diameter is glued with a special adhesive paste meant for bond strength testing
followed by temperature curing.
Further in a preferred embodiment, adhesion strength values of iron based metallic coating freshly
after deposition and after a machining process i.e. milling are mentioned below in Table 2. The
results indicate that iron based metallic coating can adhere efficiently on the surface of the base
material.
Table 2. Pull-off adhesion strength results
Adhesion strength (psi)
Coating process Iron based metallic coating
As coated (before machining) 9578.8
After milling 7454.6 (glue failure)
In an embodiment, milling operation is performed on coated flat samples and the factors
considered are surface roughness, chip morphology and temperature induced on tool and work
piece during operation keeping all other parameters constant.

In a preferred embodiment, the parameters used for the milling process during the testing phase
are tabulated in Table 3.
Table 3. Milling process parameters
Parameters Value
Tool material Cemented carbide
Tool RPM 113
Depth of cut 0.3mm
Feed 0.6mm/s
It is to be observed from Table 3 that tool RPM, depth of cut and feed rate are constant parameters
during the process of milling.
Working of the invention
The present subject matter aims towards a cost effective and machinable coating on the surfaces
of the shaft for restoration of its dimension and efficient operation. In the present disclosure, as
already mentioned, metallic iron power based coating material has been chosen in lieu of WC in
order to ensure enhanced machinability. In a preferred embodiment, the coating of the base
material with the metallic iron based powder using the HVOF technique is performed in terms of
the following steps:
a) The surface of the base material (CrMoV steel plates) is prepared by a process that includes grit
blasting. The corresponding surface roughness values have been maintained to Ra ≈ 13μm.
b) The base materials samples are fixed to the fixture.
c) The powder is loaded in the powder feeder and programming for the robot arm movement has
been processed for coating process. The corresponding process parameters in the control unit of
the facility are set for the coating.
d) The powder is then coated in passes till the required thickness is achieved with air cooling in
between the passes.

Moreover, in a preferred embodiment, the value of thickness per pass is in the range of 50 μm-70
μm.
Test results
In a preferred embodiment, the chip morphology and surface roughness are compared for
machined iron based metallic coating, WC coating and chosen base (CrMoV steel). The
corresponding results are given in Table 4.
Table 4. Comparative study indicating the change in parameters for different types of
coating

Surface Temperature rise Chip MRR
roughness-μm morphology (mm3/s)
(After milling)

CrMoV steel Ra=8.15
Rz=36.45 8°C-9°C – work
piece
4°C-6 °C – tool Large or lengthy 7.5-8
HVOF Ra=2.24 5°C-7°C – work Medium size 7.5-8
coating Rz=12.57 piece
1°C-3°C – tool rolls
WC coating Broken surface 11°C-13°C–work Powder and Non-uniform
piece 6°C-8°C - tool damaged tool machining
It could be observed from the data given in Table 4 that temperature rises while machining process
remains lower in iron based metallic coated sample than chosen base (CrMoV steel) and WC
coated sample. In an embodiment, it is further observed that surface roughness value of iron based
metallic coated sample after machining is lower than that of CrMoV steel.
In an aspect, it is noted that WC coated sample could not be machined properly as it is mentioned
in Table 4. Instead of machining, WC coating chipped off non-uniformly from surface. Moreover,

the machining tool tip is completely worn out and broken at cutting edge. This brings out the
conclusion that iron based metallic coating has better machinability than WC coating.
Moreover, as already mentioned in Figure 5, the volume loss in WC coated material is negligible
as compared to the base material. However, the volume loss in the iron powder coated material is
comparable to that of the base material. This ensures similarities in the properties of the coating
material with that of the base material.
Advantages of the invention
Additionally, the metallic iron power based HVOF coating used in the present disclosure is
advantageous in the following ways:
a) Good machinability
b) Easy and quick to cut
c) Less consumption of power
d) Easy obtainment of a good finish in the coating
e) Less wearing of the tooling material
f) Cost effective in nature
It should be noted that the description and figures merely illustrate the principles of the present
subject matter. It should be appreciated by those skilled in the art that conception and specific
embodiment disclosed may be readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present subject matter. It should also be
appreciated by those skilled in the art that by devising various arrangements that, although not
explicitly described or shown herein, embody the principles of the present subject matter and are
included within its spirit and scope. Furthermore, all examples recited herein are principally
intended expressly to be for pedagogical purposes to aid the reader in understanding the principles
of the present subject matter and the concepts contributed by the inventor(s) to furthering the art
and are to be construed as being without limitation to such specifically recited examples and
conditions. The novel features which are believed to be characteristic of the present subject matter,
both as to its organization and method of operation, together with further objects and advantages
will be better understood from the following description when considered in connection with the
accompanying figures.

Although embodiments for the present subject matter have been described in language specific to
package features, it is to be understood that the present subject matter is not necessarily limited to
the specific features described. Rather, the specific features and methods are disclosed as
embodiments for the present subject matter. Numerous modifications and adaptations of the
system/device of the present invention will be apparent to those skilled in the art, and thus it is
intended by the appended claims to cover all such modifications and adaptations which fall within
the scope of the present subject matter.
It will be understood by those within the art that, in general, terms used herein, and especially in
the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms
(e.g., the term “including” should be interpreted as “including but not limited to,” the term
“having” should be interpreted as “having at least,” the term “includes” should be interpreted as
“includes but is not limited to,” etc.). It will be further understood by those within the art that if a
specific number of an introduced claim recitation is intended, such an intent will be explicitly
recited in the claim, and in the absence of such recitation no such intent is present. For example,
as an aid to understanding, the following appended claims may contain usage of the introductory
phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such
phrases should not be construed to imply that the introduction of a claim recitation by the indefinite
articles “a” or “an” limits any particular claim containing such introduced claim recitation to
inventions containing only one such recitation, even when the same claim includes the introductory
phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or
“an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true
for the use of definite articles used to introduce claim recitations. In addition, even if a specific
number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize
that such recitation should typically be interpreted to mean at least the recited number (e.g., the
bare recitation of “two recitations,” without other modifiers, typically means at least two
recitations, or two or more recitations). Furthermore, in those instances where a convention
analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended
in the sense one having skill in the art would understand the convention (e.g., “a system havin g at
least one of A, B, and C” would include but not be limited to systems that have A alone, B alone,
C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.).
In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in

general such a construction is intended in the sense one having skill in the art would understand
the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited
to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C
together, and/or A, B, and C together, etc.). It will be further understood by those within the art
that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether
in the description, claims, or drawings, should be understood to contemplate the possibilities of
including one of the terms, either of the terms, or both terms. For example, the phrase “A or B”
will be understood to include the possibilities of “A” or “B” or “A and B.”
It will be further appreciated that functions or structures of a plurality of components or steps ma y
be combined into a single component or step, or the functions or structures of one-step or
component may be split among plural steps or components. The present invention contemplates
all of these combinations. Unless stated otherwise, dimensions and geometries of the various
structures depicted herein are not intended to be restrictive of the invention, and other dimensions
or geometries are possible. In addition, while a feature of the present invention may have been
described in the context of only one of the illustrated embodiments, such feature may be combined
with one or more other features of other embodiments, for any given application. It will also be
appreciated from the above that the fabrication of the unique structures herein and the operation
thereof also constitute methods in accordance with the present invention. The present invention
also encompasses intermediate and end products resulting from the practice of the methods herein.
The use of “comprising” or “including” also contemplates embodiments that “consist essentially
of” or “consist of” the recited feature.

WE CLAIM:
1. A method of shaft restoration by an iron based metallic powder coating by oxygen fuel
based thermal spray process; the method comprising:
- preparing the surface of a base material;
- fixing said base material samples to a fixture;
- loading said iron based metallic powder for coating said base material samples;
- setting the process parameters in a control unit for coating with said iron based
metallic powder;
- coating in passes with said iron based metallic powder till the thickness is
achieved; and
- air cooling in between the passes after completion of said coating.

2. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claim 1, wherein the base material includes
a plurality of CrMoV steel plates.
3. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-2, wherein the surface of the base
material is prepared by grit blasting, wherein roughness value of the surface of the base
material ranges between 11 µm to15 µm.
4. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-3, wherein the iron based metallic
powder is loaded in a powder feeder and a robot is provided for execution of the coating
process, wherein the coating is based on a high velocity oxygen fuel thermal spray process.
5. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-4, wherein thickness per pass of
said coating ranges between 50 µm to 70 µm .
6. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-5, wherein porosity of said coating
ranges from 0.5 % to 1.5 %.

7. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-6, wherein thickness of said coating
ranges from 3 mm to 5 mm.
8. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-7, wherein adhesion strength of
said coating ranges from 9200 psi to 9800 psi.
9. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-8, wherein parameters involved
for said iron based metallic powder coating are as follows:

- a spray distance in the range of 250 mm to 290 mm;
- a spray angle nearly at 90 degree;
- a spray velocity in the range of 420 mm/s to 480 mm/s;
- a fuel flow rate in the range of 20 l/h to 22 l/h;
- an oxygen flow rate in the range of 770 nlpm to 830 nlpm;
- a powder feed rate in the range of 60 g/min to 80 g/min; and
- a carrier gas flow rate in the range of 7 nlpm to 10 nlpm.
10. The method of shaft restoration by the iron based metallic powder coating by the oxygen
fuel based thermal spray process as claimed in claims 1-9, wherein said iron based metallic
powder coating has a higher machinability than that of a tungsten carbide coating, wherein
the machinability of said iron based metallic powder coating is in line with said base
material.