[Technical Field]
[0001] The present invention relates to a sheath heater, and more particularly to a glow
plug.
[Background Art]
[0002] A glow plug includes a sheath heater, and is used as an auxiliary heat source for
an internal combustion engine of a compression ignition system (such as a diesel engine).
The glow plug is required to have durability in usage environment within a combustion
chamber or the like. To satisfy such characteristics, various combinations of materials
are proposed. For example, as a material for a sheath tube which houses a heat generating
coil and an insulator (for example, MgO), a nickel-based heat-resistant alloy (for
example, INCONEL 601 (INCONEL is a registered trademark)), an austenitic stainless
steel (Fe-Cr-Ni alloy, for example, SUS310S), or the like is used (for example, Patent
Document 1).
[0003] The nickel-based heat-resistant alloy and the austenitic stainless steel have a stabilized
face-centered cubic (fcc) crystal structure by containing nickel. When the crystal
structure is fcc, the diffusion of oxygen into alloy becomes slow and thus the oxidation
resistance becomes high. In contrast, a ferritic stainless steel (Fe-Cr alloy) that
does not contain nickel has a body-centered cubic (bcc) crystal structure. Accordingly,
the ferritic stainless steel is inferior in oxidation resistance under high-temperature
environment and is less frequently used as the material for the sheath tube.
[Prior Art Document]
[Patent Document]
[Summary of the Invention]
[Problem to be Solved by the Invention]
[0005] The problem of the above-described prior art is that there is still room for improvement
in durability. For example, when a nickel-based alloy is used as the material for
the sheath tube, exposure to high temperature may cause a clearance
[0006] (hereinafter referred to as "crack") within the insulator or between the sheath tube
and the insulator due to the thermal expansion difference between the sheath tube
and the insulator. The occurrence of the crack locally deteriorates the heat transfer
between the heat generating coil and the sheath tube. As a result, the temperature
of the heat generating coil may be partially increased, which occasionally leads to
meltdown of the heat generating coil. The durability to be focused on in this application
refers to the properties that do not cause such meltdown of the heat generating coil.
[0007] Such a crack may occur also by thermal contraction of the sheath tube. The thermal
contraction means a decrease in volume due to phase transformation caused by a temperature
rise. The sheath tube compresses the insulator when thermal contraction occurs. The
sheath tube after compressing the insulator may be elastically deformed to be pressed
up from the inside by a reactive force against this compression. This elastic deformation
causes a crack occurring between the sheath tube and the insulator when the temperature
decreases.
[Means for Solving the Problems]
[0008] The present invention has been made to solve the above-mentioned problem, and can
be achieved as embodiments below.
(1) According to one aspect of the present invention, the sheath heater includes:
a tubular sheath tube whose one end is closed; a heating unit that is arranged inside
the sheath tube and that generates heat by transmission of electricity; and a magnesium
oxide that is arranged between the sheath tube and the heating unit and is filled
directly in contact with the sheath tube. The sheath tube is not thermally contracted
when a temperature is increased from 20°C to 1200°C, and an average thermal expansion
rate is equal to or more than 13 × 10-6/K and equal to or less than 18 × 10-6/K when the temperature is increased from 20°C to 1200°C. According to this aspect,
the durability at 20°C to 1200°C is improved. Since the sheath tube is not thermally
contracted, elastic deformation of the sheath tube caused by the thermal contraction
is prevented. Furthermore, since the average thermal expansion coefficient of the
sheath tube is equal to or more than 13 × 10-6/K and 18 × 10-6/K, this average thermal expansion coefficient has a value close to the average thermal
expansion coefficient of the magnesium oxide as an insulator. As a result, the occurrence
of a crack caused by a temperature change is restrained.
(2) The average thermal expansion rate of the sheath tube may be equal to or less
than 17 × 10-6/K. According to this aspect, the occurrence of a crack caused by a temperature change
is further restrained. This is because, according to this aspect, the average thermal
expansion coefficient of the sheath tube has a value closer to the average thermal
expansion coefficient of the magnesium oxide as the insulator.
(3) The average thermal expansion rate of the sheath tube may be equal to or more
than 15 × 10-6/K. According to this aspect, the occurrence of a crack caused by a temperature change
is further restrained. This is because, according to this aspect, the average thermal
expansion coefficient of the sheath tube has a value closer to the average thermal
expansion coefficient of the magnesium oxide as the insulator.
(4) The average thermal expansion rate of the sheath tube may be equal to or more
than 16 × 10-6/K. According to this aspect, the occurrence of a crack caused by a temperature change
is further restrained.
(5) The sheath tube may contain nickel as a main component and contain chrome. According
to this aspect, the numerical range of the above-described average thermal expansion
rate is likely to be achieved.
(6) The sheath tube may contain at least one of aluminum, silicon, iron, and molybdenum.
According to this aspect, the numerical range of the above-described average thermal
expansion rate is likely to be achieved.
(7) In the sheath tube, an aluminum content rate may be equal to or more than 0.5
mass% and a silicon content rate may be equal to or more than 0.2 mass%. According
to this aspect, the oxidation resistance is improved. This is because the oxide coatings
of aluminum and silicon are formed on the surface of the sheath tube and suppresses
the oxidation inside the sheath tube.
(8) In the sheath tube, an aluminum content rate may be equal to or less than 2.0
mass% and a silicon content rate may be equal to or less than 2.0 mass%. According
to this aspect, the numerical range of the above-described average thermal expansion
rate is likely to be achieved.
(9) In the sheath tube, an iron content rate may be equal to or less than 10.0 mass%.
According to this aspect, the numerical range of the above-described average thermal
expansion rate is likely to be achieved.
(10) In the sheath tube, an iron content rate may be equal to or less than 2.0 mass%.
(11) In the sheath tube, a molybdenum content rate may be equal to or more than 6.0
mass%. According to this aspect, the numerical range of the above-described average
thermal expansion rate is likely to be achieved.
(12) In the sheath tube, a molybdenum content rate may be equal to or less than 12.0
mass%. According to this aspect, the reduction in oxidation resistance can be restrained.
(13) The sheath tube may satisfy at least one of: a chrome content rate is equal to
or more than 12.0 mass%; and a chrome content rate is equal to or more than 10.0 mass%
and an aluminum content rate is equal to or more than 0.3 mass%. The sheath tube may
contain iron as a main component. According to this aspect, the phase transformation
from bcc to fcc is restrained and thus the thermal contraction is restrained.
(14) In the sheath tube, an aluminum content rate may be equal to or more than 1.0
mass%. According to this aspect, the oxidation resistance is improved. This is because
the oxide coating of aluminum is formed on the surface of the sheath tube and suppresses
the oxidation inside the sheath tube.
(15) In the sheath tube, an aluminum content rate may be equal to or less than 7.0
mass%. According to this aspect, the deterioration in workability of the sheath tube
is restrained. This is because, for example, forming by swaging processing becomes
difficult when the aluminum content rate is more than 7.0 mass%.
(16) In the sheath tube, a chrome content rate may be equal to or less than 30.0 mass%.
According to this aspect, the deterioration in workability of the sheath tube is restrained.
This is because a σ phase is easily deposited when the chrome content rate is more
than 30.0 mass%. The σ phase is an intermetallic compound between iron and chrome,
and is brittle.
[0009] The present invention can be embodied by various aspects other than the above-described
aspect. For example, the present invention can be embodied as a glow plug that includes
the above-described sheath heater and a metal shell holding the sheath heater.
[Brief Description of the Drawings]
[0010]
[FIG. 1] FIG. 1 is a sectional view and an external view of a glow plug.
[FIG. 2] FIG. 2 is a sectional view of a sheath heater.
[FIG. 3] FIG. 3 is a table showing test conditions and test results of a durability
test of a heat generating coil.
[FIG. 4] FIG. 4 is a table showing test conditions and test results of the durability
test of the heat generating coil.
[FIG. 5] FIG. 5 is a table showing test conditions and test results of the durability
test of the heat generating coil.
[Description of Embodiments]
[0011] FIG. 1 illustrates a glow plug 10. FIG. 1 illustrates the external configuration
on the right side of an axial line O on the paper and illustrates the cross-sectional
configuration on the left side of the axial line O on the paper. The glow plug 10
functions as a heat source that assists ignition at the start of a diesel engine.
[0012] The glow plug 10 includes a center rod member 200, a metal shell 500, and a sheath
heater 800 that generates heat by transmission of electricity. These members are assembled
along the axial line O of the glow plug 10. In this description, the sheath heater
800 side in the glow plug 10 is referred to as a "front end side" while the opposite
side is referred to as a "rear end side."
[0013] The metal shell 500 is formed into a tubular shape and made of carbon steel. The
metal shell 500 holds the sheath heater 800 at an end portion on the front end side.
The metal shell 500 holds the center rod member 200 at an end portion on the rear
end side via an insulating member 410 and an O-ring 460. A position of the insulating
member 410 in the axial line O direction is secured by crimping a ring 300 that is
in contact with a rear end of the insulating member 410 to the center rod member 200.
The insulating member 410 insulates the rear end side of the metal shell 500. The
metal shell 500 incorporates a part of the center rod member 200 from the insulating
member 410 to the sheath heater 800. The metal shell 500 includes an axial hole 510,
a tool engagement portion 520, and an external thread portion 540.
[0014] The axial hole 510 is a through hole formed along the axial line O, and has a diameter
larger than the center rod member 200. In a state where the center rod member 200
is arranged in the axial hole 510, a space is formed between the axial hole 510 and
the center rod member 200 so as to provide an electrical insulation therebetween.
The sheath heater 800 is press-fitted and joined to the front end side of the axial
hole 510. The external thread portion 540 fits an internal thread formed at an internal
combustion engine (not illustrated). The tool engagement portion 520 engages a tool
(not illustrated) used for installation and removal of the glow plug 10.
[0015] The center rod member 200 is made of a conductive material in a cylindrical shape.
The center rod member 200 is assembled along the axial line O while being inserted
into the axial hole 510 of the metal shell 500. The center rod member 200 includes
a center rod member front end portion 210 formed at the front end side and a connecting
portion 290 disposed at the rear end side. The center rod member front end portion
210 is inserted to the inside of the sheath heater 800. The connecting portion 290
is an external thread projected from the metal shell 500. The engaging member 100
is fitted to the connecting portion 290.
[0016] FIG. 2 is a sectional view illustrating a detailed configuration of the sheath heater
800. The sheath heater 800 includes a sheath tube 810, a heat generating coil 820
as a heating unit, a control coil 830, and insulating powder 840.
[0017] The sheath tube 810 is a tubular member that extends in the axial line O direction
and has a closed front end. The sheath tube 810 is made of metal, whose composition
will be described in detail with reference to FIG. 3. The heat generating coil 820,
the control coil 830, and the insulating powder 840 are arranged inside the sheath
tube 810. The sheath tube 810 includes a sheath tube front end portion 811 and a sheath
tube rear end portion 819. The sheath tube front end portion 811 is an end portion
formed to a rounded shape toward the outside at the front end side of the sheath tube
810. The sheath tube rear end portion 819 is an end portion open at the rear end side
of the sheath tube 810. The center rod member front end portion 210 of the center
rod member 200 is arranged at the inside from the sheath tube rear end portion 819
to the sheath tube 810. A packing 600 and the insulating powder 840 electrically insulate
the sheath tube 810 from the center rod member 200. The packing 600 is an insulating
member sandwiched between the center rod member 200 and the sheath tube 810. The sheath
tube 810 is electrically connected to the metal shell 500.
[0018] The control coil 830 is a coil made of a conductive material that has a temperature
coefficient of specific electric resistance larger than a material forming the heat
generating coil 820. As the conductive material, nickel is preferable. Other than
this, for example, the conductive material may be an alloy containing cobalt or nickel
as a main component. The main component according to this embodiment is a component
having the highest content rate (mass%).
[0019] The control coil 830 is disposed inside of the sheath tube 810. The control coil
830 controls electric power supplied to the heat generating coil 820 according to
the temperature. The control coil 830 includes a control coil front end portion 831
at the end portion on the front end side, and a control coil rear end portion 839
at the end portion on the rear end side. The control coil front end portion 831 is
electrically connected to the heat generating coil 820 by being welded to a heat generating
coil rear end portion 829 of the heat generating coil 820. The control coil rear end
portion 839 is electrically connected to the center rod member 200 by being bonded
to the center rod member front end portion 210 of the center rod member 200.
[0020] The insulating powder 840 is powder having electrical insulating properties. As the
insulating powder 840, for example, powder of magnesium oxide (MgO) is used. In this
embodiment, the magnesium oxide content rate in the insulating powder 840 is equal
to or more than 85.0 mass%. Other than magnesium oxide, calcium oxide (CaO), zirconia
(zirconium dioxide, ZrO
2), or the like is contained in the insulating powder 840, for example. The insulating
powder 840 is filled inside the sheath tube 810. The insulating powder 840 electrically
insulates respective clearances of the sheath tube 810, the heat generating coil 820,
the control coil 830, and the center rod member 200. After filling up with the insulating
powder 840, the outer diameter of the sheath tube 810 is adjusted by swaging processing.
The insulating powder 840 is compressed due to the use of the glow plug 10, and then
loses fluidity. As a result, the above-mentioned crack (the clearance inside the insulating
powder 840 or the clearance between the insulating powder 840 and the sheath tube
810) may occur at the insulating powder 840.
[0021] The heat generating coil 820 contains, for example, iron or nickel as a main component,
and may contain at least any of aluminum, chrome, and tungsten (see FIG. 3). The heat
generating coil 820 is disposed along the axial line O direction on the inner side
of the sheath tube 810, and generates heat by transmission of electricity.
[0022] The heat generation by the heat generating coil 820 allows a rapid temperature rise.
The rapid temperature rise means that the surface temperature of a predetermined portion
of the sheath tube 810 reaches 1000°C from a normal temperature within 2 seconds.
The above-described predetermined portion is at the position moved by 2 mm from the
front end of the sheath tube 810 to the rear end side in the axial line O direction.
The front end of the sheath tube 810 is identical to the front end of the sheath tube
front end portion 811. For the rapid temperature rise, the electric power equal to
or more than a predetermined value is supplied to the heat generating coil 820.
[0023] The heat generating coil 820 includes a heat generating coil front end portion 821
at the end portion on the front end side, and the heat generating coil rear end portion
829 at the end portion on the rear end side. The heat generating coil front end portion
821 is electrically connected to the sheath tube 810 by being welded to a part in
the vicinity of the front end of the sheath tube 810.
[0024] FIGS. 3, 4, and 5 show test conditions and test results of durability tests of the
heat generating coil 820 as tables. FIG. 3 shows the case where the main component
of the sheath tube 810 is iron, and FIGS. 4 and 5 show the cases where the main component
of the sheath tube 810 is nickel. However, the main component of the test piece No.
1 shown in FIG. 3 is platinum.
[0025] The sign "-" shown in FIGS. 3, 4, and 5 means that the content rate is zero or a
value within an error range. The content rates of the sheath tube 810 and the heat
generating coil 820 have values in a region except for a region where the constituent
changes due to welding with the above-mentioned sheath tube 810.
[0026] As shown in FIG. 3, the sheath tubes 810 (the test pieces Nos. 2 to 16) that contain
iron as the main component contain chrome. The sheath tubes 810 of the test pieces
Nos. 3, 4, 6, and 8 to 16 contain aluminum.
[0027] As shown in FIGS. 4 and 5, the sheath tubes 810 (the test pieces Nos.17 to 73) that
contain nickel as the main component contain chrome. Furthermore, the sheath tubes
810 that contain nickel as the main component contain at least one of silicon, aluminum,
molybdenum, and iron. Furthermore, some of the sheath tubes 810 that contain nickel
as the main component contain at least one of manganese, cobalt, titanium, niobium,
tantalum, and yttrium. The value shown in "OTHER" in FIG. 4 is indicated by mass%
of the subsequent chemical symbol. For example, "0.2Ti, 4Nb+Ta" of the test piece
No. 18 means that the titanium content rate is 0.2 mass% and the sum of the niobium
content rate and the tantalum content rate is 4.0 mass%. The sheath tubes 810 of the
test pieces Nos. 1 to 73 may contain other impurities.
[0028] The compositions of the heat generating coils 820 shown in FIGS. 3, 4, and 5 show
chemical symbols of the main components and the other components. The other components
are shown by mass%. For example, "Fe20Cr5Al" in the test piece No. 1 means that the
main component is iron, the chrome content rate is 20.0 mass%, and the aluminum content
rate is 5.0 mass%.
[0029] The parameters changed as the test condition are the composition and thermal expansion
rate of the sheath tube 810, the composition of the heat generating coil 820, the
temperature, and the atmosphere gas.
[0030] The thermal expansion rate of the sheath tube 810 (hereinafter simply referred to
as "thermal expansion rate") is an average value of the thermal expansion rate during
the temperature rise from 20°C to 1200°C. The method for obtaining the thermal expansion
rate is as follows. After a length L
20 of a test piece at room temperature is measured, the temperature of the test piece
is increased and a length L
1200 of the test piece at 1200°C is measured. The thermal expansion rate is calculated
by (L
1200 - L
20)/(L
20 × 1180 K). In this embodiment, the length of the test piece was measured using a
thermo-mechanical analyzer (TMA) while the temperature was gradually increased. Accordingly,
the length at a medium temperature between 20°C and 1200°C was also measured. Thus,
whether or not thermal contraction occurs during the temperature rise from 20°C to
1200°C in the above-described test can be also determined. In this embodiment, however,
the length at the medium temperature was not used for calculating the thermal expansion
rate as described above.
[0031] The durability test was carried out by repeating heating and cooling of the heat
generating coil 820 while energizing the heat generating coil 820 in the air and by
counting repetitions (breaking cycles) until the wire breaking of the heat generating
coil 820 occurred. The heating was performed for 10 minutes to reach 900°C, 1100°C,
or 1150°C. These temperatures are the surface temperatures of the glow plug 10, and
the conditions of measurement are as follows. When a monochromatic radiation thermometer
was used, an emissivity ε = 1.0 at the time of measurement, and a measurement spot
diameter was 2 mm, a measuring position was set to the position by 2 mm from the sheath
tube front end portion 811 of the sheath tube 810 to the rear end side in the axial
line O direction. The cooling was performed for 2 minutes by air cooling in the atmosphere.
[0032] Regarding the test pieces Nos. 2 and 3, the tests were also carried out in nitrogen,
in addition to the atmosphere, in accordance with the above-described procedure. In
the case where the heating temperature was 900°C or 1100°C, an evaluation A was determined
when the count of breaking cycles was equal to or more than 20 thousand, an evaluation
B was determined when the count of breaking cycles was equal to or more than 10 thousand
and less than 20 thousand, and an evaluation C was determined when the count of breaking
cycles was less than 10 thousand. In the case where the heating temperature was 1150°C,
an evaluation A was determined when the count of breaking cycles was equal to or more
than 10 thousand, an evaluation B was determined when the count of breaking cycles
was equal to or more than 7 thousand and less than 10 thousand, and an evaluation
C was determined when the count of breaking cycles was less than 7 thousand. However,
it was impossible to assemble the test piece No. 15, and therefore the durability
test was not able to be carried out for the test piece No. 15 (details will be described
later).
[0033] Based on the above-described count of breaking cycles, the comprehensive evaluations
of the respective test pieces were determined. The comprehensive evaluation was determined
by six levels from a comprehensive evaluation 1 to a comprehensive evaluation 6 while
the comprehensive evaluation 1 was ranked as the most preferable evaluation. The specific
determination method of the comprehensive evaluation is as follows. In the following
description of the determination method, the test was carried out in the atmosphere
unless otherwise stated.
[0034] Under the condition at 1150°C, the test pieces 30 to 33, and 36 with the evaluation
A were determined as the comprehensive evaluation 1. For the test pieces Nos. 28 and
29, the test under the condition at 1150°C was not carried out but the composition
of the sheath tube 810 was identical to that of the test piece No. 30. Accordingly,
the test pieces Nos. 28 and 29 were determined as the comprehensive evaluation 1.
For the test pieces Nos. 34 and 35, the test under the condition at 1150°C was not
carried out but the composition of the sheath tube 810 was identical to that of the
test piece No. 36. Accordingly, the test pieces Nos. 34 and 35 were determined as
the comprehensive evaluation 1.
[0035] After the test pieces with the comprehensive evaluation 1 were excluded, the test
pieces 17 to 26 and 37 to 39 with the evaluation A under the condition at 1100°C were
determined as the comprehensive evaluation 2. For the test piece No. 27, the composition
of the sheath tube 810 was identical to that of the test piece No. 26. Accordingly,
the test piece No. 27 was determined as the comprehensive evaluation 2.
[0036] After the test pieces with the comprehensive evaluations 1 and 2 were excluded, the
test pieces 4, 6, 8 to 12, 16, and 40 to 67 were determined as the comprehensive evaluation
3. The test pieces 4, 6, 8 to 12, 16, and 40 to 67, which satisfied at least any of:
the evaluation B under the condition at 1100°C; and the evaluation B under the condition
at 1150°C, were determined as the comprehensive evaluation 3. The test pieces Nos.
13 and 14 did not satisfy any of the above-described two conditions but the composition
of the sheath tube 810 was identical to that of the test piece No. 12. Accordingly,
the test pieces Nos. 13 and 14 were determined as the comprehensive evaluation 3.
[0037] The test piece No. 7 with the evaluation B under the condition at 900°C was determined
as the comprehensive evaluation 4. Furthermore, the test piece No. 3 with the evaluation
B under the condition in nitrogen and at 1100°C was also determined as the comprehensive
evaluation 4.
[0038] The test piece No. 15 was determined as the comprehensive evaluation 5. The test
piece No. 15 was not able to undergo the test as described later in detail, but the
thermal expansion rate was 18 × 10
-6/K. Accordingly, the test piece No. 15 was determined to be more preferable than the
following comprehensive evaluation 6.
[0039] The test pieces except for the above-described pieces were determined as the comprehensive
evaluation 6. That is, the test pieces Nos. 1, 5, and 68 to 73 determined only as
the evaluation C were determined as the comprehensive evaluation 6.
[0040] Both in the case where the thermal expansion rate was 11 × 10
-6/K (in the test piece No. 1) and in the case where the thermal expansion rate was
19 × 10
-6/K (in the test pieces Nos. 68 to 73), a large crack occurred and the comprehensive
evaluation 6 was determined. In contrast, in the case where the thermal expansion
rate was equal to or more than 13 × 10
-6/K and equal to or less than 18 × 10
-6/K, the comprehensive evaluation 5 or a higher rank was determined. Accordingly, the
thermal expansion rate is preferred to be equal to or more than 13 × 10
-6/K and equal to or less than 18 × 10
-6/K.
[0041] The thermal expansion rate of the sheath tube 810 is preferred to be equal to or
more than 13 × 10
-6/K and equal to or less than 18 × 10
-6/K because this thermal expansion rate is close to 15.7 × 10
-6/K that is the thermal expansion rate of the insulating powder 840 according to this
embodiment. This causes reduction in the size or suppression in occurrence of the
above-mentioned crack even when heating and cooling are repeated.
[0042] The test piece No. 5 had a thermal expansion rate of a value (15 × 10
-6/K) within the above-described preferred range, but was determined as the comprehensive
evaluation 6. This is considered to be because the sheath tube 810 was thermally contracted.
This thermal contraction is considered to occur at, for example, 840 to 890°C. As
described above, the thermal contraction of the sheath tube 810 may cause a crack
of the insulating powder 840 and meltdown of the heat generating coil 820. It is considered
from the comparison with the test pieces Nos. 3 and 7 that the thermal contraction
occurred in the test piece No. 5 because any of the following (a) and (b) was not
satisfied.
- (a) the chrome content rate in the sheath tube 810 is equal to or more than 10.0 mass%
and the aluminum content rate in the sheath tube 810 is equal to or more than 0.3
mass%
- (b) the chrome content rate in the sheath tube 810 is equal to or more than 12.0 mass%
[0043] Accordingly, when the main component is iron, the numerical ranges shown as (a) and
(b) are preferred. When any of (a) and (b) is satisfied, the occurrence of thermal
contraction is restrained. This is considered to be because the phase transformation
from bcc to fcc for iron contained as the main component is restricted.
[0044] The test piece No. 3 had a thermal expansion rate of a value (14 × 10
-6/K) within the above-described preferred range, but was determined as the comprehensive
evaluation 4. This is considered to be because the sheath tube 810 had a hole due
to the durability test in the case where the atmosphere was the air. Since the evaluation
on breaking of wire in the test piece No. 3 in the case where the atmosphere was nitrogen
was the evaluation B, the cause of the hole of the sheath tube 810 in the test piece
No. 3 is considered to be the oxidation of the sheath tube 810.
[0045] On the other hand, the test piece No. 6 was determined as the evaluation B in the
case of 1100°C in the test in the air. Compared with the sheath tube 810 of the test
piece No. 3, the sheath tube 810 of the test piece No. 6 contained the same element
as the main component and had the same chrome content rate while having a higher content
rate (1.0 mass%) of aluminum. Accordingly, the condition where the aluminum content
rate in the sheath tube 810 is equal to or more than 1.0 mass% is considered to suppress
the occurrence of the hole due to the oxidation of the sheath tube 810 and thus is
preferred.
[0046] Incidentally, the test piece No. 2 was determined as the evaluation B in the test
at 900°C. Accordingly, it is considered that, under the usage environment up to 900°C,
the hole is not generated due to the oxidation and the test piece No. 2 is durable
in use, even if aluminum is not contained.
[0047] The test pieces where the main component of the sheath tube 810 was iron and the
thermal expansion rate was equal to or more than 15 × 10
-6/K and equal to or less than 17 × 10
-6/K (the test pieces Nos. 4, 6, and 8 to 14) were determined as the comprehensive evaluation
3, except the test piece No. 5 where thermal contraction occurred and the test piece
No. 7. When the main component of the sheath tube 810 is iron, the thermal expansion
rate is preferred to be equal to or more than 15 × 10
-6/K and equal to or less than 17 × 10
-6/K. The thermal expansion rate is preferred to be equal to or more than 15 × 10
-6/K and equal to or less than 17 × 10
-6/K because it is considered that the thermal expansion rate is closer to the thermal
expansion rate (15.7 × 10
-6/K) of magnesium oxide and therefore the occurrence of a crack is further restrained.
[0048] The test piece No. 7 had the thermal expansion rate of 15 × 10
-6/K, but was determined as the comprehensive evaluation 4 because it is considered
that the oxidation resistance of the sheath tube 810 in the test piece No. 7 is inferior
to those of the other test pieces. The oxidation resistance of the sheath tube 810
in the test piece No. 7 is inferior because the aluminum content rate in the sheath
tube 810 is approximately zero. When the main component of the sheath tube 810 is
iron and the thermal expansion rate is equal to or more than 15 × 10
-6/K and equal to or less than 17 × 10
-6/K, the aluminum content rate in the sheath tube 810 is preferred to be equal to or
more than 1.0 mass%, for example, like the test pieces Nos. 4, 6, 8 to 14, and 16.
[0049] For the test piece No. 15, the durability test was not able to be carried out as
mentioned above. This is because, in the case of the test piece No. 15, the workability
of the sheath tube 810 was poor and swaging processing of the sheath tube 810 was
not able to be properly carried out. It is considered that the workability was poor
because the aluminum content rate was 10.0 mass%. In the test pieces except the test
piece No. 15, there was no problem with the workability of the sheath tube 810 and
the aluminum content rate was equal to or less than 7.0 mass%. Accordingly, when the
main component of the sheath tube 810 is iron, the aluminum content rate in the sheath
tube 810 is preferred to be equal to or less than 7.0 mass%.
[0050] When the main component of the sheath tube 810 is iron, the chrome content rate in
the sheath tube 810 is preferred to be equal to or less than 30.0 mass%. This is because
a σ phase is deposited when the chrome content rate in the sheath tube 810 exceeds
30.0 mass%. The σ phase is an intermetallic compound between iron and chrome, and
is brittle. Accordingly, the deposition of the σ phase makes the production of the
sheath tube 810 difficult.
[0051] In the test pieces in which the main component of the sheath tube 810 was iron and
which were determined as the comprehensive evaluations 3 and 4, the content rate of
iron was equal to or more than 61.0 mass%. In addition, in the test pieces in which
the main component of the sheath tube 810 was iron and which were determined as the
comprehensive evaluations 3 and 4, thermal contraction did not occur in the above-described
tests where the temperature was increased from 20°C to 1200°C.
[0052] As shown in FIGS. 4 and 5, the test pieces Nos. 17 to 67 where the main component
of the sheath tube 810 was nickel and the thermal expansion rate was equal to or more
than 16 × 10
-6/K and equal to or less than 18 × 10
-6/K were determined as the comprehensive evaluation 3 or a higher rank. Accordingly,
when the main component of the sheath tube 810 is nickel, the thermal expansion rate
is preferred to be equal to or more than 16 × 10
-6/K and equal to or less than 18 × 10
-6/K.
[0053] The test pieces Nos. 17 to 39 where the main component of the sheath tube 810 was
nickel and the thermal expansion rate was equal to or more than 16 × 10
-6/K and equal to or less than 17 × 10
-6/K were determined as the comprehensive evaluation 2 or a higher rank. Accordingly,
when the main component of the sheath tube 810 is nickel, the thermal expansion rate
is preferred to be equal to or more than 16 × 10
-6/K and equal to or less than 17 × 10
-6/K.
[0054] As mentioned above, when the main component of the sheath tube 810 was iron, all
the test pieces (the test pieces Nos. 2 to 16) were determined as the comprehensive
evaluation 3 or a lower rank. On the other hand, when the main component of the sheath
tube 810 was nickel, some test pieces were determined as the comprehensive evaluation
2 or a higher rank as described above. Such a difference occurs because the crystal
structure is bcc when the main component of the sheath tube 810 is iron, while the
crystal structure is fcc when the main component of the sheath tube 810 is nickel.
The crystal structure of fcc is more excellent in high-temperature strength compared
with the case where the crystal structure is bcc.
[0055] In all of the test pieces Nos. 17 to 73 where the main component of the sheath tube
810 was nickel, the sheath tube 810 contained chrome. When the main component of the
sheath tube 810 is nickel, it is considered that the chrome content rate in the sheath
tube 810 facilitates obtainment of a desired thermal expansion rate. Accordingly,
when the main component of the sheath tube 810 is nickel, the sheath tube 810 is preferred
to contain chrome.
[0056] When the main component of the sheath tube 810 was nickel and the thermal expansion
rate was equal to or more than 16 × 10
-6/K and equal to or less than 18 × 10
-6/K, the test pieces Nos. 18, 26, and 40 were determined as the evaluation C in the
test at 1150°C. The test pieces Nos. 18, 26, and 40 were determined as the evaluation
B or a higher rank at 1100°C. Thus, the cause of the evaluation C is considered that
the test pieces Nos. 18, 26, and 40 are inferior to the other test pieces in oxidation
resistance under a high-temperature condition.
[0057] The oxidation resistance under the high-temperature condition depends on the content
rates of silicon and aluminum. It is understood that, from the comparison between
the test piece No. 40 and the test piece No. 41, the evaluation at 1150°C improves
from the evaluation C to the evaluation B when the silicon content rate is increased
from 0.1 mass% to 0.2 mass%. Accordingly, the silicon content rate is preferred to
be equal to or more than 0.2 mass%.
[0058] On the other hand, the test piece No. 18 where the silicon content rate was 0.2 mass%
was determined as the evaluation C at 1150°C. This is considered to be because the
aluminum content rate is 0.2 mass% from the comparison with the test piece No. 41
where the aluminum content rate is 0.5 mass%. Accordingly, the aluminum content rate
is preferred to be equal to or more than 0.5 mass%.
[0059] As discussed above, when the main component of the sheath tube 810 is nickel, the
oxidation resistance is preferably suppressed in the case where the silicon content
rate is equal to or more than 0.2 mass% and the aluminum content rate is equal to
or more than 0.5 mass%.
[0060] As shown in FIG. 4, in all the test pieces Nos. 28 to 36 determined as the comprehensive
evaluation 1, the silicon content rate was 0.2 mass% and the aluminum content rate
was equal to or more than 0.5 mass% as described above. Also for this reason, when
the main component of the sheath tube 810 is nickel, it is preferred that the silicon
content rate be 0.2 mass% and the aluminum content rate be equal to or more than 0.5
mass%.
[0061] On the other hand, the test pieces Nos. 37 to 39 were determined as the comprehensive
evaluation 2 even when the main component of the sheath tube 810 was nickel, the thermal
expansion rate was equal to or more than 16 × 10
-6/K and equal to or less than 17 × 10
-6/K, the silicon content rate was 0.2 mass%, and the aluminum content rate was equal
to or more than 0.5 mass%, similarly to the test pieces Nos. 28 to 36. This is considered
to be because the molybdenum content rate was 13.0 mass% from the comparison with
the test pieces Nos. 28 to 36. On the other hand, the test pieces Nos. 28 to 36 were
determined as the comprehensive evaluation 1, in which the molybdenum content rate
was equal to or less than 12.0 mass%. Accordingly, when the main component of the
sheath tube 810 is nickel, the molybdenum content rate is preferred to be equal to
or less than 12.0 mass%. The cause of the comprehensive evaluation 2 when the molybdenum
content rate was 13.0 mass% is considered to be because a large content of molybdenum
was oxidized.
[0062] When the main component of the sheath tube 810 was nickel, in all the test pieces
Nos. 17 to 39 with the comprehensive evaluation 2 or a higher rank, the thermal expansion
rate was equal to or more than 16 × 10
-6 and equal to or less than 17 × 10
-6 and the molybdenum content rate was equal to or more than 6.0 mass%. In contrast,
when the main component of the sheath tube 810 was nickel, in all the test pieces
Nos. 40 to 73 with the comprehensive evaluation 3 or a lower rank, the thermal expansion
rate was equal to or more than 18 × 10
-6 and equal to or less than 19 × 10
-6 and the molybdenum content rate was equal to or less than 3.0 mass%. Accordingly,
when the main component of the sheath tube 810 is nickel, the molybdenum content rate
is preferred to be equal to or more than 6.0 mass%. It is considered that the above-described
test results are caused by the phenomenon where the thermal expansion rate is reduced
when the molybdenum content rate is high.
[0063] As shown in FIGS. 4 and 5, in the test pieces Nos. 18 to 67 in which the main component
of the sheath tube 810 was nickel and which were determined as the comprehensive evaluation
3 or a higher rank, the thermal expansion rate was equal to or more than 16 × 10
-6/K and equal to or less than 18 × 10
-6/K and the iron content rate was equal to or less than 10.0 mass%. In contrast, in
the test pieces Nos. 68 to 71 determined as the comprehensive evaluation 6, the thermal
expansion rate was 19 × 10
-6, and the iron content rate was equal to or more than 11.0 mass%. Accordingly, when
the main component of the sheath tube 810 is nickel, the iron content rate is preferred
to be equal to or less than 10.0 mass%. It is considered that the above-described
test results are caused by the phenomenon where the thermal expansion rate is reduced
when the iron content rate is low. Even if the iron content rate is 18.0 mass%, however,
the comprehensive evaluation 2 can be obtained when a thermal expansion rate of 17
× 10
-6/K, like the test piece No. 17.
[0064] As shown in FIGS. 4 and 5, in the test pieces Nos. 23 to 44, 46 to 52, and 62 to
67, the main component of the sheath tube 810 was nickel, the comprehensive evaluation
3 or a higher rank was determined, and the iron content rate was equal to or less
than 2.0 mass%.
[0065] Although the main component of the sheath tube 810 was nickel and the iron content
rate was approximately zero, the test piece No. 72 was determined as the comprehensive
evaluation 6 and had a thermal expansion rate of 19 × 10
-6. This is considered to be because the aluminum content rate was 2.1 mass%. In contrast,
in all the test pieces Nos. 17 to 67 that were determined as the comprehensive evaluation
3 or a higher rank, the aluminum content rate was equal to or less than 2.0 mass%.
Accordingly, when the main component of the sheath tube 810 is nickel, the aluminum
content rate is preferred to be equal to or less than 2.0 mass%.
[0066] Although the main component of the sheath tube 810 was nickel and the iron content
rate was approximately zero, the test piece No. 73 was determined as the comprehensive
evaluation 6 and had a thermal expansion rate of 19 × 10
-6. This is considered to be because the silicon content rate was 2.1 mass%. In contrast,
in all the test pieces Nos. 17 to 67 that were determined as the comprehensive evaluation
3 or a higher rank, the silicon content rate was equal to or less than 2.0 mass%.
Accordingly, when the main component of the sheath tube 810 is nickel, the silicon
content rate is preferred to be equal to or less than 2.0 mass%.
[0067] In all the test pieces Nos. 17 to 73, thermal contraction did not occur. In the test
piece No. 17, inconel HX was used as a material for the sheath tube 810. In the test
pieces Nos. 18 to 22, inconel 625 was used as the material for the sheath tube 810.
In the test pieces Nos. 26 and 27, inconel 617 was used as the material for the sheath
tube 810. In the test pieces Nos. 69 to 71, inconel 601 was used as the material for
the sheath tube 810.
[0068] The present invention is not limited to the above-described embodiment, and may be
practiced in various forms without departing from the scope of the invention. For
example, the technical features in the embodiment corresponding to the technical features
in the respective aspects described in Summary of the Invention may be, as necessary,
replaced or combined to solve a part or all of the above-described problems or to
achieve a part or all of the above-described advantageous effects. The technical features
may be, as necessary, omitted unless the technical features are explained as necessary
features in this description. The following describes examples.
[0069] The above-described sheath heater may be used for, for example, a heater, a cooker,
or the like, other than the glow plug. The length of a test piece at a medium temperature
may be taken into consideration in calculating an average thermal expansion rate.
For example, the least squares method or integration may be used. When integration
is used, for example, an area value in a strain-temperature relationship may be obtained
and the tangent of the approximated rectangular triangle may be obtained as a value
of the thermal expansion rate. The approximated rectangular triangle is the rectangular
triangle that has the area identical to the above-described area value and uses the
temperature range of the measurement target as a length of the base. The sheath tube
may contain nickel as impurities even when iron is contained as the main component.
[0070] The sheath heater may not include the control coil. When the control coil is not
included, the heat generation of the glow plug may be controlled by a glow controller.
[0071] The magnesium oxide content rate in the insulating powder may be high than 85.0 mass%.
For example, the magnesium oxide content rate may be higher to the extent that the
thermal expansion rate of the insulating powder has the value approximately identical
to the thermal expansion rate of pure magnesium oxide. The approximately identical
value is, for example, equal to or more than 13.0 × 10
-6/K and equal to or less than 18.0 × 10
-6/K. The lower limit value is more preferably equal to or more than 15.0 × 10
-6/K. The upper limit value is more preferably equal to or less than 17.0 × 10
-6/K, and even more preferably equal to or less than 16.0 × 10
-6/K. To obtain such a thermal expansion rate, for example, the magnesium oxide content
rate in the insulating powder may be set to be equal to or more than 98.0 mass%.
[Description of Reference Numerals]
[0072]
- 10:
- Glow plug
- 100:
- Engaging member
- 200:
- Center rod
- 210:
- Center rod member front end portion
- 290:
- Connecting portion
- 300:
- Ring
- 410:
- Insulating member
- 460:
- O-ring
- 500:
- Metal shell
- 510:
- Axial hole
- 520:
- Tool engagement portion
- 540:
- External thread portion
- 600:
- Packing
- 800:
- Sheath heater
- 810:
- Sheath tube
- 811:
- Sheath tube front end portion
- 819:
- Sheath tube rear end portion
- 820:
- Heat generating coil
- 821:
- Heat generating coil front end portion
- 829:
- Heat generating coil rear end portion
- 830:
- Control coil
- 831:
- Control coil front end portion
- 839:
- Control coil rear end portion
- 840:
- Insulating powder
- O:
- Axial line