TECHNICAL FIELD
[0001] The present disclosure relates to a plug, and more particularly, to a plug used to
pierce a billet and a method of manufacturing the same.
BACKGROUND ART
[0002] The Mannesmann pipe-making method is widely used to manufacture seamless pipes. The
Mannesmann pipe-making method involves heating a billet to a predetermined temperature
and using a piercing mill to perform piercing/rolling on the billet. The piercing
mill includes a pair of skewed rolls and a plug. The plug is positioned on a pass
line between the skewed rolls. The piercing mill uses the skewed rolls to rotate the
billet in a circumferential direction and, at the same time, pushes it against the
plug to perform piercing/rolling on the billet, thereby producing a hollow shell.
[0003] In the case of a conventional plug, a coating of oxidized scale is formed on the
surface of the base material before the piercing/rolling of a billet. The coating
of oxidized scale is formed by performing heat treatment on the plug. This provides
sufficient heat insulation, lubricity and seizure resistance of the surface of the
plug.
[0004] The coating of oxidized scale gradually wears off due to repeated piercing/rolling.
The coating wears off each time piercing/rolling is performed (i.e. for each pass).
When part of the coating is completely worn off and lost, part of the base material
of the plug is exposed. Then, exposed portions of the base material may be eroded
or the plug may seize on the billet, i.e. opposite material, at which point the life
of the plug expires.
[0005] Particularly, the coating of oxidized scale wears off significantly when billets
made of stainless steel are pierced, leading to a very short life for the plug. When
billets made of stainless steel are pierced, the coating usually wears out in several
passes. Each time a coating wears out, a heat treatment is necessary to produce an
oxidized scale on the surface of the base material of the plug. The heat treatment
typically requires several hours to several tens of hours. Thus, the efficiency with
which a coating of oxidized scale is formed is low.
[0006] To address this, Japanese Patent No.
4279350 proposes forming a coating made of iron and iron oxides on the surface of the base
material of the plug by electric-arc spraying. According to Japanese Patent No.
4279350, the coating is made from iron wire only, and the time required to form the coating
is several minutes to several tens of minutes, which is relatively short. This enables
forming a coating on the surface of the base material at low costs and with high efficiency.
The sprayed coating has a higher adhesion with respect to the base material and a
higher wear resistance than a coating of oxidized scale. This increases the life of
the plug.
[0007] JP 2013-248619 A discloses forming a sprayed coating containing iron and iron oxides on the surface
of the base material of the plug before performing heat treatment on the plug.
[0008] WO 2014/013963 discloses forming, on the surface of the base material of the plug, an Ni-Cr layer
that serves as a foundation for a sprayed coating containing iron and iron oxides.
[0009] JP S61(1986)-286077 A discloses forming a coating on the surface of a mandrel for a rolling mill for steel
pipes by spraying metal-based powder, before performing hot isotropic pressurization
on the mandrel.
[0010] JP H05(1993)-36502 A and
JP H03(1991)-125076 A each disclose a method of forming a sprayed coating, although not applied to a plug
used for piercing a billet.
JP H05(1993)-36502 A discloses forming a sprayed coating of ultrahard alloy on the surface of a base and
forming a plated coating of an Ni-P alloy on the sprayed coating before performing
hot isostatic pressing on the base.
JP H03(1991)-125076 A discloses spraying a wear-resistant material on the surface of a base before performing
a sealing treatment thereon by spraying a powder material capable of closing pores,
and performing hot isostatic pressing on the base.
DISCLOSURE OF THE INVENTION
[0011] A coating formed by arc spraying of iron wire (or steel wire) has a high adhesion
with respect to the base material of the plug and a high wear resistance, which increases
the life for the plug. However, for example, when a billet with high strength made
of a high alloy is to be pierced or when the pierced length of the billet is extremely
long, part of the coating may peel off the surface of the base material during piercing.
If part of the base material is exposed as a result of the peeling of the coating,
the plug may be eroded or the billet may seize on the plug, with the exposed portions
working as initiation points.
[0012] An object of the present disclosure is to provide a plug that achieves prevention
of peel-off of the coating, and a method of manufacturing such a plug.
[0013] A plug according to the present disclosure is used for piercing a billet. The plug
includes a plug body, a body coating, and a surface-layer coating. The body coating
is provided on a surface of the plug body. The body coating contains iron and iron
oxides. The surface-layer coating is provided on the body coating. The surface-layer
coating contains iron and iron oxides. The surface-layer coating has a porosity lower
than that of a region of the body coating adjacent to the surface-layer coating and
having a thickness equal to that of the surface-layer coating.
[0014] The present disclosure is directed to a method of manufacturing a plug. The plug
is used for piercing a billet. The method includes: preparing a plug body; forming
a body coating on a surface of the plug body by performing arc spraying using iron
wire; and forming a surface-layer coating on the body coating by performing arc spraying
using iron wire at a spray distance shorter than that at completion of the formation
of the body coating.
[0015] The plug according to the present disclosure and the method of manufacturing it achieve
prevention of peel-off of the coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[FIG. 1] FIG. 1 is a partial cross-sectional view of a plug according to an embodiment.
[FIG. 2] FIG. 2 is an enlarged view of portion II of the plug shown in FIG. 1.
[FIG. 3] FIG. 3 is an example of a microscopic image of a cross section of the coating.
[FIG. 4] FIG. 4 is a brightness histogram for the microscopic image shown in FIG.
3.
[FIG. 5] FIG. 5 is a brightness histogram for the microscopic image shown in FIG.
3, illustrating how to express the microscopic image using three values.
[FIG. 6] FIG. 6 is a three-value image derived from the microscopic image shown in
FIG. 3.
[FIG. 7] FIG. 7 illustrates a method of manufacturing the plug shown in FIG. 1.
[FIG. 8] FIG. 8 is a graph showing the relationship between the spray distance during
coating formation and the porosity of the coating.
[FIG. 9] FIG. 9 is a graph showing the relationship between the spray distance during
coating formation and the content of oxides in the coating.
[FIG. 10] FIG. 10 is a graph showing the relationship between the spray distance during
coating formation and the tensile strength of the coating.
[FIG. 11A] FIG. 11A illustrates the effects of the plug according to the embodiment.
[FIG. 11B] FIG. 11B illustrates the effects of the plug according to the embodiment.
[FIG. 12A] FIG. 12A illustrates the mechanism through which the coating of a conventional
plug peels off.
[FIG. 12B] FIG. 12B illustrates the mechanism through which the coating of a conventional
plug peels off.
[FIG. 12C] FIG. 12C illustrates the mechanism through which the coating of a conventional
plug peels off.
[FIG. 12D] FIG. 12D illustrates the mechanism through which the coating of a conventional
plug peels off.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0017] The present inventors did extensive research and found out the mechanism through
which the coating of a plug that is piercing a billet peels off. FIGS. 12A to 12D
illustrate the mechanism through which the coating of a conventional plug peels off.
FIGS. 12A to 12D each schematically show a cross section of the plug near its surface.
[0018] As shown in FIG. 12A, before the piercing of a billet, a coating 102 is present on
the surface of the plug body 101. The coating 102 includes pores 103.
[0019] As shown in FIG. 12B, when the piercing of the billet begins, the friction between
the coating 102 and billet causes a load in a direction along the surface of the coating
(i.e. shear direction) to act on the coating. This deforms the coating 102, producing
cracks C on the surface of the coating 102, with the deformed portions working as
initiation points.
[0020] As shown in FIG. 12C, the load in the shear direction acting on the coating 102 causes
cracks C to progress along the interface between the plug body 101 and coating 102.
As a result, part of the coating 102 peels off, as shown in FIG. 12D.
[0021] It occurred to the present inventors that the peel-off of the coating may be prevented
by preventing deformation and cracking of the coating during the piercing of the billet.
The present inventors did further research and completed the plug according to the
embodiment and the method of manufacturing it.
[0022] A plug according to an embodiment is used for piercing a billet. The plug includes
a plug body, a body coating, and a surface-layer coating. The body coating is provided
on a surface of the plug body. The body coating contains iron and iron oxides. The
surface-layer coating is provided on the body coating. The surface-layer coating contains
iron and iron oxides. The surface-layer coating has a porosity lower than that of
a region of the body coating adjacent to the surface-layer coating and having a thickness
equal to that of the surface-layer coating (first arrangement).
[0023] In the first arrangement, on the body coating provided on the surface of the plug
body is further provided a surface-layer coating. The surface-layer coating has a
porosity that is lower than that of a region of the body coating that is located adjacent
to the surface-layer coating. Thus, the surface-layer coating is denser and has a
higher strength than the body coating. This will prevent the surface-layer coating
and the body coating covered with the surface-layer coating from deforming due to
the load in the shear direction, thereby preventing deformation-derived cracks. As
a result, the coatings will be prevented from peeling off the surface of the plug
body.
[0024] The porosity of the surface-layer coating may be not higher than 2.5 % (second arrangement).
[0025] In the second arrangement, the porosity of the surface-layer coating is low enough
to provide a denser surface-layer coating with higher strength. This will further
prevent deformation and cracking of the coatings, thereby preventing peel-off of the
coatings more reliably.
[0026] The thickness of the surface-layer coating may be not larger than 250 µm (third arrangement).
[0027] In the third arrangement, the thickness of the surface-layer coating is small enough
to improve heat removal from the surface-layer coating. This will reduce the temperature
increase in the surface-layer coating during piercing, thereby preventing the billet
from seizing on the plug.
[0028] A method of manufacturing a plug according to an embodiment includes: preparing a
plug body; forming a body coating on a surface of the plug body by performing arc
spraying using iron wire; and forming a surface-layer coating on the body coating
by performing arc spraying using iron wire at a spray distance shorter than that at
completion of the formation of the body coating (fourth arrangement).
[0029] In the fourth arrangement, the surface-layer coating is formed by, after the formation
of the body coating, performing arc spraying with reduced spray distance. This will
reduce the porosity of the surface-layer coating to be lower than the porosity of
a region of the body coating adjacent to the surface-layer coating such that the body
coating will be covered with a surface-layer coating that is dense and has a high
strength. This will prevent the coatings from deforming due to the load in the shear
direction during piercing, thereby preventing deformation-derived cracking. As a result,
the coatings will be prevented from peeling off the surface of the plug surface.
[0030] Further, in the fourth arrangement, both the body coating and surface-layer coating
are formed by arc spraying of iron wire. That is, the body coating and surface-layer
coating are formed from the same material and with the same technique. This will enable
successively forming the body coating and surface-layer coating within the same step.
This will facilitate producing a plug with a body coating and a surface-layer coating.
[0031] The forming the body coating may perform arc spraying while the spray distance is
gradually increased (fifth arrangement).
[0032] For arc spraying, increased spray distance results in increased content of oxides
in the coating. In the fifth arrangement, the spray distance used to form a region
of the body coating adjacent to the plug body is relatively small. Thus, the region
adjacent to the plug body has a high iron content and a low oxide content. This will
improve the adhesion of the body coating with respect to the plug body. On the other
hand, the spray distance used to form a region of the body coating adjacent to the
surface-layer coating is relatively large. Thus, the region adjacent to the surface-layer
coating has a high oxide content, which reduces heat conductivity. This will improve
the heat insulation of the body coating, thereby preventing the billet from seizing
on the plug.
[0033] Embodiments will now be described in detail with reference to the drawings. The same
and corresponding elements in the drawings are labeled with the same characters and
their description will not be repeated. For convenience of explanation, elements may
be simplified or shown schematically in the drawings, and some elements may not be
shown.
[Construction of Plug]
[0034] First, the construction of the plug will be described. As shown in FIG. 1, a plug
10 according to an embodiment includes a plug body 1, a body coating 2, and a surface-layer
coating 3. FIG. 1 shows a cross section of the plug 10.
[0035] The plug body 1 has a circular transverse section with an outer diameter that increases
as it goes from the tip of the plug body 1 toward its rear end. In short, the plug
body 1 is generally shaped like a projectile.
[0036] The body coating 2 is formed on the surface of the plug body 1. The body coating
2 covers the entire surface of the plug body 1 except for the rear end surface of
the plug body 1. The thickness of the body coating 2 may not be constant over the
entire plug. For example, the portions of the body coating 2 that are located on the
tip portion 11 of the plug body 1 have larger thicknesses than the portions located
on the trunk portion 12 of the plug body 1.
[0037] The surface-layer coating 3 is formed on the body coating 2. The surface-layer coating
3 covers the entire the body coating 2. The thickness of the surface-layer coating
3 is smaller than the thickness of the body coating 2. The thickness of the surface-layer
coating 3 is substantially constant over the entire plug. The thickness of the surface-layer
coating 3 is preferably not higher than 250 µm, and more preferably not higher than
200 µm. The thickness of the surface-layer coating 3 is preferably not lower than
50 µm.
[0038] FIG. 2 is an enlarged view of portion II shown in FIG. 1. The body coating 2 and
surface-layer coating 3 contain iron and iron oxides. While the body coating 2 and
surface-layer coating 3 are mainly composed of iron and iron oxides, they may contain
small amounts of elements and/or compounds other than iron and iron oxides. In the
body coating 2, the iron content decreases and the iron-oxide content increases as
it goes from the plug body 1 toward the surface-layer coating 3. The iron content
in the surface-layer coating 3 is higher at least than the iron content in the region
21 of the body content 2 discussed below.
[0039] The body coating 2 includes pores. The surface-layer coating 3 also includes pores,
although small and few. The porosity of the surface-layer coating 3 is lower than
the porosity of the region 21 of the body coating 2. The region 21 is adjacent to
the surface-layer coating 3 of the body coating 2. That is, the region 21 is a region
of the body coating 2 that is located adjacent to the interface between the body coating
and surface-layer coating 3. The thickness of the region 21 is substantially equal
to the thickness of the surface-layer coating 3. The porosity of the surface-layer
coating 3 is preferably not higher than 2.5 %. The lower the porosity of the surface-layer
coating 3, the better; in practice, it is not lower than 0.5 %.
[0040] A method of calculating the iron content and iron-oxide content in and porosity of
the body coating 2 and surface-layer coating 3 will be described.
[0041] First, microscopic images of cross sections of the body coating 2 and surface-layer
coating 3 are obtained. In the microscopic images, the porosity of the region 21 of
the body coating 2 evaluated is that of the portions of the body coating 2 that are
adjacent to the interface to the surface-layer coating 3 and have the same thickness
as the surface-layer coating 3. The porosity of the surface-layer coating 3 evaluated
is that of the entire surface-layer coating 3 captured in the microscopic images.
The area of evaluation as measured in a direction perpendicular to the thickness direction
(i.e. direction parallel to the plug surface) is about 1000 to 1500 µm. Since it is
assumed that pores are basically distributed homogeneously as determined in this direction,
an evaluation with a width of about 1000 to 1500 µm will enable calculation of a porosity
value that is substantially an average one.
[0042] FIG. 3 is an example of a microscopic image (original image) of a cross section of
a coating. Iron, iron oxides and pores in the original image have their respective
color tones. More specifically, the color becomes darker in the following order: iron,
iron oxides, and pores.
[0043] Next, a brightness histogram is created from the original image, as shown in FIG.
4. The brightness histogram is a graph showing the pixel-brightness distribution in
the original image, where the vertical axis represents frequency of occurrence (i.e.
number of pixels) and the horizontal axis represents the brightness value. Detecting
peaks in this brightness histogram provides three peaks derived from iron, iron oxides,
and pores.
[0044] Subsequently, the original image is expressed using three values. As shown in FIG.
5, the thresholds for expression using three values are the middle value M
1 between the brightness value B
1 and brightness value B
2, and the middle value M
2 between the brightness value B2 and brightness value B
3. B
1, B
2 and B
3 are the brightness value of the peak derived from pores, the brightness value of
the peak derived from iron oxides, and the brightness value of the peak derived from
iron, respectively.
[0045] FIG. 6 shows a three-value image obtained by expressing the original image using
three values. In the three-value image, the pixels having brightness values lower
than M
1 in the original image are displayed in black, the pixels having brightness values
not lower than M
1 and lower than M
2 are displayed in gray, and the pixels having brightness values not lower than M
2 are displayed in white. In the three-value image, the black regions are treated as
pore regions, the gray regions as iron-oxide regions, and the white regions as iron
regions, and the number of pixels in the regions of each type is counted. Dividing
the number of pixels in the pore regions, the number of pixels in the iron-oxide regions,
and the number of pixels in the iron regions by the total number of pixels provides
the porosity (%), the content of iron oxides (%) and the content of iron (%), respectively.
That is, the porosity, the content of iron oxides and the content of iron are each
expressed as a value of the proportion of pixels in the original image (i.e. area
ratio).
[Method of Manufacturing Plug]
[0046] A method of manufacturing the plug 10 will be described below.
[0047] First, a plug body 1 is prepared. A body coating 2 and a surface-layer coating 3
are formed on the surface of the plug body 1 by arc spraying.
[0048] Arc spraying may be performed using, for example, arc spraying equipment 4 shown
in FIG. 7. The arc spraying equipment 4 includes a spray gun 41 and a turntable 42.
The spray gun 41 uses an arc to melt wire to be sprayed, and sprays it through a nozzle
by means of compressed air. In the present embodiment, the wire to be sprayed is iron
wire. The iron wire is wire of carbon steel (or common steel) mainly composed of iron
(Fe). The iron wire is typically a so-called common steel mainly composed of Fe and
including carbon (C), silicon (Si), manganese (Mn) and impurities, but may contain
elements such as tungsten (W).
[0049] When the body coating 2 and surface-layer coating 3 are to be formed, the plug body
1 is placed on the turntable 42 of the arc spraying equipment 4. Then, while the turntable
42 rotates the plug body 1 about its axis, iron wire is sprayed on the plug body 1
by arc spraying. Thus, a body coating 2 containing iron and iron oxides is formed
on the surface of the plug body 1. The formation of the body coating 2 is completed
when a desired thickness of material is deposited on the surface of the plug body
1.
[0050] The body coating 2 is preferably formed while the spray distance is gradually increased.
Spray distance refers the minimum distance between the tip of the nozzle of the spray
gun 41 and the surface of the object to which spraying is done. The body coating 2
is formed by placing the spray gun 41 at a predetermined distance from the plug body
1 and initiating arc spraying, and continuing arc spraying while the spray gun 41
is gradually moved away from the plug body 1. Alternatively, the spray distance may
be kept constant during the formation of the body coating 2.
[0051] Immediately after the formation of the body coating 2, a surface-layer coating 3
is formed. That is, after the body coating 2 is formed, arc spraying is continued
without an interruption to form a surface-layer coating 3 on the body coating 2.
[0052] The spray distance during the formation of the surface-layer coating 3 is smaller
than the spray distance during the formation of the body coating 2. More specifically,
the spray distance during the formation of the surface-layer coating 3 is smaller
than at least the spray distance at the completion of the formation of the body coating
2. That is, after the body coating 2 is formed while the spray gun 41 is gradually
moved away from the plug body 1, the spray gun 41 is brought close to the plug body
with a jerk to form the surface-layer coating 3.
[0053] While the surface-layer coating 3 is formed, the spray distance is kept substantially
constant. The spray distance during the formation of the surface-layer coating 3 is
preferably not larger than 200 mm. The formation of the surface-layer coating 3 is
completed when a desired thickness of material is deposited on the body coating 2.
Preferably, the formation of the surface-layer coating 3 is completed before the thickness
of the surface-layer coating 3 exceeds 250 µm.
[0054] Spray distance will be described in more detail. FIG. 8 is a graph showing the relationship
between spray distance and the porosity of a coating. FIG. 9 is a graph showing the
relationship between spray distance and the content of oxides in a coating. FIG. 10
is a graph showing the relationship between spray distance and the tensile strength
of a coating.
[0055] As shown in FIG. 8, the larger the spray distance, the higher the porosity of a coating
becomes. That is, the porosity of the body coating 2 and surface-layer coating 3 may
be controlled by adjusting the spray distance. As discussed above, the spray distance
during the formation of the surface-layer coating 3 is smaller than the spray distance
at the completion of the formation of the body coating 2. The porosity of the surface-layer
coating 3 is lower than the porosity of the region 21 of the body coating 2.
[0056] As shown in FIG. 9, the larger the spray distance, the higher the content of oxides
in a coating becomes. That is, the contents of iron and iron oxides in the body coating
2 and surface-layer coating 3 may be controlled by adjusting the spray distance. As
discussed above, the body coating 2 may be formed while the spray distance is gradually
increased. Thus, in the body coating 2, the iron content decreases and the iron-oxide
content increases as it goes from the plug body 1 toward the surface-layer coating
3. The surface-layer coating 3 is formed by reducing the spray distance after the
formation of the body coating 2. Thus, the content of iron in the surface-layer coating
3 is higher than at least the content of iron in the region 21 of the body coating
2.
[0057] As shown in FIG. 10, the larger the spray distance, the smaller the tensile strength
of a coating becomes. That is, the tensile strength of the body coating 2 and surface-layer
coating 3 may be controlled by adjusting the spray distance. The spray distance during
the formation of the surface-layer coating 3 is smaller than the spray distance at
the completion of the formation of the body coating 2. Thus, the tensile strength
of the surface-layer coating 3 is larger than at least the tensile strength of the
region 21 of the body coating 2.
[0058] After the body coating 2 and surface-layer coating 3 are thus formed, the plug body
1 is removed from the turntable 42 of the arc spray equipment 4. Thus, the plug 10
(FIG. 1) according to the present embodiment is finished.
[Effects]
[0059] In the plug 10 according to the present embodiment, the surface-layer coating 3 with
a low porosity is provided on the body coating 2. This prevents deformation of the
body coating 2 and surface-layer coating 3 due to a load in the shear direction during
piercing, thereby preventing cracks in the body coating 2 and surface-layer coating
3. These effects will be described in more detail with reference to FIGS. 11A and
11B.
[0060] FIG. 11A is a schematic cross section of portions of the plug 10 located near the
surface before the piercing of a billet is initiated. As shown in FIG. 11A, the body
coating 2 on the plug body 1 is covered with the surface-layer coating 3. The surface-layer
coating 3 is formed by performing arc spraying of iron wire at a spray distance smaller
than the spray distance at the completion of the formation of the body coating 2.
Thus, the surface-layer coating 3 has a porosity lower than the porosity of the region
of the body coating 2 located adjacent to the surface-layer coating 3, and is dense
and has a high tensile strength.
[0061] FIG. 11B is a schematic cross section of portions of the plug 10 located near the
surface during the piercing of a billet. As shown in FIG. 11B, when the piercing of
the billet is initiated, a load in the shear direction acts on the surface of the
surface-layer coating 3. However, the surface-layer coating 3 is dense and has a high
tensile strength, and thus cannot easily be deformed by the load in the shear direction.
The body coating 2 cannot easily be deformed, either, since it is covered with the
surface-layer coating 3. Thus, in the body coating 2 and surface-layer coating 3,
no crack occurs that would lead to a peel-off. Thus, the body coating 2 and surface-layer
coating 3 are prevented from peeling off.
[0062] The body coating 2 and surface-layer coating 3 can be easily formed by arc spraying
using iron wire. Further, the body coating 2 and surface-layer coating 3 are successively
formed within one and the same step. Thus, the present embodiment will allow a plug
10 with a coating having a high peeling resistance to be manufactured in a simple
manner.
[0063] The porosity of the surface-layer coating 3 is preferably not higher than 2.5 %.
Thus, the surface-layer coating 3 is denser, providing sufficient tensile strength
of the surface-layer coating 3. This will effectively prevent deformation of and cracks
in the body coating 2 and surface-layer coating 3. This will prevent peel-off of the
body coating 2 and surface-layer coating 3 more reliably. As discussed above, the
lower the porosity of the surface-layer coating 3, the better; in practice, it is
not lower than 0.5 %.
[0064] The thickness of the surface-layer coating 3 is preferably not higher than 250 µm.
This will prevent the temperature of the surface-layer coating 3 from rising during
piercing/rolling. As discussed above, the surface-layer coating 3 has a high iron
content in the coating, and thus has a high heat conductivity. Thus, the surface-layer
coating 3 can easily be heated as it contacts a hot billet during piercing/rolling.
If the thickness of the surface-layer coating 3 is too large, heat can be accumulated
within the surface-layer coating 3, leading to high temperatures in the surface-layer
coating 3. If the surface-layer coating 3 is too hot, the billet can easily seize
on the plug 10. A thickness of the surface-layer coating 3 that is not larger than
250 µm will prevent seizure.
[0065] The thickness of the surface-layer coating 3 is preferably not smaller than 50 µm.
This will prevent the body coating 2 and surface-layer coating 3 from deforming due
to the load in the shear direction during piercing, thereby preventing cracks more
reliably.
[0066] The body coating 2 is formed while the spray distance is gradually increased. Thus,
the iron content is high in a region of the body coating 2 that is adjacent to the
plug body 1, thereby improving the adhesion between the plug body 1 and body coating
2. On the other hand, the iron-oxide content is high in a region of the body coating
2 that is adjacent to the surface-layer coating 2, and thus the heat conductivity
is low, thereby improving heat insulation. This will prevent the billet from seizing
on the plug 10.
[0067] While embodiments have been described, the present disclosure is not limited to the
above-described embodiments, and various modifications are possible without departing
from the spirit of the invention.
EXAMPLES
[0068] The present disclosure will be described in more detail below with reference to examples.
The present disclosure is not limited to the examples below.
[0069] Six steel plug bodies (1) with a maximum diameter of 77.5 mm, with a total length
of 230 mm, and containing 0.15 mass % C and 3.5 mass % W were prepared. Arc spraying
using iron wire formed a body coating (2) on the surface of each plug body (1). During
the formation of the body coating (2), spraying was performed while the spray distance
was changed from 200 mm to 1000 mm. The thickness of the body coating (2) was 1200
µm as measured at the tip (11) of the plug body (1) and 500 µm as measured at the
trunk portion (12).
[0070] For five plug bodies (1), a surface-layer coating (3) was formed on the body coating
(2) by arc spraying using iron wire to provide plugs according to Inventive Examples
1 to 5. The conditions for forming the surface-layer coating (3) are shown in Table
1. For the remaining one plug body (1), no surface-layer coating (3) was provided,
and this provided a plug according to a Comparative Example.
[Table 1]
|
Work conditions |
Results |
Spray conditions and coating thickness |
Proportion of pores in coating |
Life pass number |
Peeling of coating on trunk |
Surface-layer coating |
Body coating *1 |
Surface-layer coating |
Comp. Ex. |
none |
13.8 |
- |
3 |
poor |
Inv. Ex. 1 |
spray distance: 100 mm 300 µ m thick |
9.3 |
1.7 |
1 |
good |
Inv. Ex. 2 |
spray distance: 300 mm 100 µ m thick |
13.8 |
2.7 |
4 |
acceptable |
Inv. Ex. 3 |
spray distance: 100 mm 100 µm thick |
13.8 |
1.7 |
7 |
good |
Inv. Ex. 4 |
spray distance: 100 mm 200 µ m thick |
11.3 |
1.7 |
8 |
good |
Inv. Ex. 5 |
spray distance: 200 mm 200 µm thick |
11.3 |
2.2 |
8 |
good |
*1 The porosity of the body coating evaluated was that of the region adjacent to the
surface layer (i.e. interface between the body coating and surface-layer coating)
and having the same thickness as the surface-layer coating. The value of the comparative
example is the value for the region of 100 µm adjacent to the surface. |
[0071] The plugs according to Inventive Examples 1 to 5 and the Comparative Example were
repeatedly used to perform piercing/rolling on billets made of SUS 304 with a diameter
of 65 mm and a length of 400 mm that had been heated to 1200 °C. For each of Inventive
Examples 1 to 5 and the Comparative Example, the number of piercing rounds performed
until the plug was damaged (i.e. life pass number), and how damaged the plug was,
were determined. The life pass numbers and how damaged the plugs were for Inventive
Examples 1 to 5 and the Comparative Example are shown in Table 1.
[0072] In the plug of the Comparative Example, the coating peeled off the trunk portion
(12) during the third pass. In contrast, in Inventive Examples 3 to 5, the coating
did not peel off even after seven or eight passes. In Inventive Example 1, seizure
occurred during the first pass and piercing rolling was halted at this point, where
the coating did not peel off the trunk portion (12). In Inventive Example 2, the coating
peeled off the trunk portion (12) during the fourth pass. This demonstrates that providing
a surface-layer coating (3) on the body coating (2) prevents peeling of the coating.
In each of Inventive Examples 3 to 5, the deformation of the plug tip (11) after the
life pass number listed in Table 1 exceeded a permitted range, and the piercing/rolling
was halted at this point.
[0073] In Inventive Example 1, the thickness of the surface-layer coating (3) was 300 µm,
which is larger than 250 µm. In Inventive Example 1, the billet seized on the plug
during the first pass. In contrast, in each of Inventive Examples 2 to 5, where the
thickness of the surface-layer coating (3) was not larger than 250 µm, the billet
did not seize on the plug. Thus, the thickness of the surface-layer coating (3) is
preferably not larger than 250 µm to prevent seizure.
[0074] In each of Inventive Examples 1 and 3 to 5, the porosity of the surface-layer coating
(3) was not higher than 2.5 %, meaning that the density and strength of the surface-layer
coating (3) were high. Consequently, in each of Inventive Examples 1 and 3 to 5, the
coating did not peel off. On the other hand, in Inventive Example 2, the spray distance
during the formation of the surface-layer coating (3) was 300 mm and the porosity
of the surface-layer coating (3) was 2.7 %. That is, the density and strength of the
surface-layer coating (3) in Inventive Example 2 were lower than in Inventive Examples
1 and 3 to 5. Consequently, the coating of Inventive Example 2 peeled off during the
fourth pass. These results show that the porosity of the surface-layer coating (3)
is preferably not higher than 2.5 % to prevent peeling of the coating more effectively.