TECHNICAL FIELD
[0001] The present invention relates to a crankshaft and a method of
manufacturing such a crankshaft.
5 BACKGROUND ART
[0002] Some crankshafts are nitrided prior to use in order to improve
fatigue strength and wear resistance. A nitriding process that is
commonly used for crankshafts is gas nitriding, which offers high
productivity.
10 [0003] JP 2018-70928 A, WO 2018/066667 A1 and JP 2013-221203 A
each describes controlling the nitriding potential during a nitriding
process to form a dense compound layer made of γ′ phase on the surface
of the steel, thereby providing both high fatigue strength and
bend-straightening performance.
15 [0004] Japanese Patent No. 5898092 describes a method of
manufacturing a driving cam that includes performing a soft nitriding
process on a sliding surface of a driving cam to form a hardened layer
and a compound layer and then removing the compound layer such that
the hardened layer is present at the surface of the sliding surface.
20 [0005] In addition to fatigue strength and wear resistance, seizure
resistance is required of crankshafts. It has been proposed to improve
seizure resistance by controlling the surface geometry of frictional
parts.
[0006] JP 2017-218951 A teaches that the surface roughness Ra of a
25 crankshaft for a freezing-machine compressor is to be not higher than
0.05 μm. WO 2016/072305 A1 teaches that, in a rotary slide bearing
composed of a bearing and a shaft, the surface roughness Ra of the
shaft is to be not higher than 0.10 μm. Japanese Patent No. 5199728
teaches that the surface roughness of the martensite layer or nitride
30 layer of a crankshaft is to be lower than the surface roughness of the
associated journal bearing.
PRIOR ART DOCUMENTS
PATENT DOCUMENTS
35 [0007] [Patent Document 1] JP 2018-70928 A
3
[Patent Document 2] WO 2018/066667 A1
[Patent Document 3] JP 2013-221203 A
[Patent Document 4] Japanese Patent No. 5898092
[Patent Document 5] JP 2017-218951 A
5 [Patent Document 6] WO 2016/072305 A1
[Patent Document 7] Japanese Patent No. 5199728
NON-PATENT DOCUMENTS
[0008] [Non-Patent Document 1] Dieter Liedtke et al., “Nitriding and
10 Nitrocarburizing on Iron Materials”, AGNE Gijutsu Center Inc., pp. 23
and 72, 2011
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
15 [0009] In recent years, lubricating oils with lower viscosities and
crankshafts with frictional parts constituted by thinner shafts have
been developed to improve fuel efficiency and, accordingly, even better
seizure resistance is required of a crankshaft.
[0010] An object of the present invention is to provide a crankshaft
20 with improved seizure resistance and a method of manufacturing such
a crankshaft.
MEANS FOR SOLVING THE PROBLEMS
[0011] A crankshaft according to an embodiment of the present
25 invention is a crankshaft having a journal and a pin, including a
compound layer containing iron and nitrogen on a surface thereof,
wherein, in the compound layer, for each of the journal and pin, a
porosity area ratio of a thinner one of a region from the surface to a
depth of 3.0 μm and a region across a total thickness of the compound
30 layer is not higher than 10.0 %, and each of the journal and pin has
such a surface geometry that an arithmetical mean deviation of a
primary profile, Pa, is not larger than 0.090 μm.
[0012] A method of manufacturing a crankshaft according to an
embodiment of the present invention is a method of manufacturing the
35 above-described crankshaft, including: an intermediate grinding step
4
for grinding a journal and a pin of an intermediate product of the
crankshaft; after the intermediate grinding step, an intermediate
lapping step for lapping the journal and pin of the intermediate
product; after the intermediate lapping step, a nitriding step for
5 nitriding the intermediate product; after the nitriding step, a grinding
step for grinding the journal and pin of the intermediate product; after
the grinding step, a rough-lapping step for lapping the journal and pin
of the intermediate product using a film coated with alumina abrasive
grains; and after the rough-lapping step, a finish-lapping step for
10 lapping the journal and pin of the intermediate product using a film
coated with diamond abrasive grains.
EFFECTS OF THE INVENTION
[0013] The present invention provides a crankshaft with improved
15 seizure resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] [FIG. 1] FIG. 1 is a schematic cross-sectional view of a
near-surface structure of a steel that has been subjected to a typical gas
20 nitriding process.
[FIG. 2] FIG. 2 shows an exemplary primary profile.
[FIG. 3] FIG. 3 shows an exemplary roughness profile.
[FIG. 4] FIG. 4 is a schematic view of a crankshaft according to
an embodiment of the present invention.
25 [FIG. 5] FIG. 5 is a flow chart illustrating an exemplary method
of manufacturing the crankshaft of FIG. 4.
[FIG. 6] FIG. 6 is a schematic view of evaluation equipment
used for seizure testing.
[FIG. 7] FIG. 7 is a schematic view of a bearing and nearby
30 portions of the evaluation equipment of FIG. 6.
[FIG. 8] FIG. 8 schematically illustrates changes over time in
the surface pressure applied to the test shaft.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
35 [0015] A nitriding process for a crankshaft is intended to improve wear
5
resistance and/or fatigue strength. Meanwhile, sufficient research has
not been conducted on the relationship between a nitriding process and
seizure resistance. Particularly, the relationship between the
compound layer formed during nitriding and seizure resistance has not
5 been systematically investigated, although it has been pointed out that
a compound layer with a high porosity area ratio works as an oil basin
to contribute to an improvement in seizure resistance (Dieter Liedtke et
al., “Nitriding and Nitrocarburizing on Iron Materials”, AGNE Gijutsu
Center Inc., p. 27, 2011).
10 [0016] FIG. 1 is a schematic cross-sectional view of a near-surface
structure of a steel that has been subjected to a typical gas nitriding
process. On the surface of the steel is formed a compound layer 50
with a thickness of about several tens of micrometers. Below the
compound layer 50, a nitrogen diffusion layer 60 is formed by nitrogen
15 diffusing on the surface of the steel. The compound layer 50 includes a
porous layer 51 located near the surface and having a high porosity
area ratio, and a dense layer 52 located between the porous layer 51
and nitrogen diffusion layer 60 and having a low porosity area ratio.
[0017] It is known that the compound layer 50 includes ε phase
20 (Fe2-3N), γ′ phase (Fe4N), and α phase (αFe). Particularly, it is known
that a compound layer 50 including such a porous layer 51 as
mentioned above is mainly composed of ε phase. As disclosed in JP
2018-70928 A and WO 2018/066667 A1, mentioned above, it is also
known that controlling the nitriding potential during nitriding can form
25 a less porous compound layer mainly composed of γ′ phase.
[0018] The present inventors conducted detailed research on the
relationship between the compound layer and seizure resistance.
Specifically, they evaluated the seizure resistance of (1) a steel with a
compound layer mainly composed of ε phase and including a porous
30 layer and a dense layer; (2) a steel with a compound layer mainly
composed of ε phase and including only a dense layer after removal of
the porous layer; (3) a steel with an exposed nitrogen diffusion layer
after removal of the compound layer; and (4) a steel including a
compound layer mainly composed of γ′ phase.
35 [0019] The research showed that steels (2) and (4) had better seizure
6
resistances than steels (1) and (3). This shows that the presence of a
compound layer is advantageous in improving seizure resistance and
that a compound layer with a lower porosity area ratio is advantageous
in improving seizure resistance.
5 [0020] To improve seizure resistance, the surface geometry of frictional
parts provided with a compound layer is also important. The present
inventors have discovered that the seizure resistance of a nitrided
crankshaft can be significantly improved over conventional crankshafts
if the porosity area ratio of the near-surface portion of the compound
10 layer is not higher than 10.0 % and the arithmetical mean deviation of
the primary profile, Pa, is not larger than 0.090 μm.
[0021] JP 2017-218951 A and WO 2016/072305 A1, mentioned above,
each specify a surface geometry using the arithmetical mean deviation
of the roughness profile, Ra (hereinafter referred to as “average
15 roughness Ra”). However, specifying a geometry using the average
roughness Ra has the following problems.
[0022] FIGS. 2 and 3 show an exemplary primary profile and an
exemplary roughness profile, respectively. The surface geometry of an
industrial product, such as a crankshaft, includes not only short-period
20 components (i.e., roughness), but also non-negligible levels of
long-period components (i.e., undulations) caused by, for example,
vibrations of the grinding machine. The average roughness Ra is
based on the roughness profile (FIG. 3), which results from removal of
the undulating components through a high-pass filter, and thus it
25 cannot be said, without reserve, that the value exactly reflects the
actual surface geometry. Further, the value of the average roughness
Ra significantly varies depending on the cutoff value λc of the high-pass
filter used to obtain the roughness profile. Actually, even for about the
same level of average roughness Ra, seizure resistance significantly
30 varies depending on the magnitude of undulations. Thus, an
evaluation parameter using the primary profile (FIG.2) as an assessed
profile would be a more suitable indication used to control seizure
resistance.
[0023] Generally, a nitriding process disadvantageously increases the
35 surface roughness by about 1.5 to 2 times (Dieter Liedtke et al.,
7
“Nitriding and Nitrocarburizing on Iron Materials”, AGNE Gijutsu
Center Inc., p. 72, 2011). Thus, to reduce the arithmetical mean
deviation of the primary profile Pa of a steel after nitriding, the steel
must be polished after nitriding to provide a neat surface geometry.
5 Meanwhile, the thickness of a compound layer that can be formed in an
industrially realistic time is about several tens of micrometers. The
small amount that can be polished means that, in order to reduce the
arithmetical mean deviation of the primary profile Pa to 0.090 μm or
below while leaving some compound layer, a sufficiently smooth surface
10 geometry must be provided prior to polishing. This requires sufficient
polishing prior to nitriding, not just after nitriding.
[0024] The present invention was made based on the above-described
findings. Embodiments of the present invention will now be described
in detail with reference to the drawings. The same or corresponding
15 elements in the drawings are labeled with the same reference
characters, and their description will not be repeated. The size ratios
between the components shown in the drawings do not necessarily
indicate the actual size ratios.
[0025] [Crankshaft]
20 FIG. 4 is a schematic view of a crankshaft 10 according to an
embodiment of the present invention. The crankshaft 10 includes
journals 11, pins 12, and arms 13.
[0026] The journals 11 are coupled to a cylinder block (not shown).
The pins 12 are coupled to connecting rods (not shown). The arms 13
25 connect the journals 11 and pins 12.
[0027] The crankshaft 10 may be made from a steel for machine
structural use, for example. Although not limiting, the crankshaft 10
may be made from a carbon steel for machine structural use in
accordance with JIS G 4051:2009; or an alloyed steel for machine
30 structural use in accordance with JIS G 4053:2008, for example.
Especially S45C, S50C and S53C in accordance with JIS G 4051:2009
and SMn438 in accordance with JIS G 4053:2008 are suitable, and such
steel materials to which S has been added to improve machinability are
particularly suitable.
35 [0028] The crankshaft 10 may have a chemical composition (i.e.,
8
chemical composition of a base material excluding the compound layer
and nitrogen diffusion layer) including, for example, in mass %: 0.30 to
0.60 % C; 0.01 to 2.0 % Si; 0.1 to 2.0 % Mn; 0.01 to 0.50 % Cr; 0.001 to
0.06 % Al; 0.001 to 0.02 % N; up to 0.03 % P; and up to 0.20 % S, in
5 addition to Fe and impurities. The chemical composition of the
crankshaft 10 may include other elements. The chemical composition
of the crankshaft 10 may include, for example, in mass %: 0 to 0.50 %
Mo; 0 to 0.50 % Cu; 0 to 0.50 % Ni; 0 to 0.050 % Ti; 0 to 0.050 % Nb; 0 to
0.005 % Ca; 0 to 0.30 % Bi; and 0 to 0.20 % V.
10 [0029] A compound layer containing iron and nitrogen is formed on the
surface of the crankshaft 10. The compound layer is mainly composed
of an iron–nitrogen compound, but may contain small amounts of
elements other than iron and nitrogen. Preferably, in the compound
layer, the total content of the other elements other than iron and
15 nitrogen is not higher than 10 mass %.
[0030] The compound layer typically covers the entire surface of the
crankshaft 10. However, the compound layer is only required to cover
the surfaces of the frictional parts, i.e., journals 11 and pins 12, and
need not necessarily cover the entire surface of the crankshaft 10.
20 [0031] The compound layer may be mainly composed of ε phase
(Fe2-3N), or may be mainly composed of γ′ phase (Fe4N). The
compound layer may be a mixture of ε and γ′ phases.
[0032] In one implementation, the crankshaft 10 may include a
compound layer in which the proportion of ε phase as represented by a
25 cross-sectional area ratio is not lower than 80 %. ε phase has a
crystalline structure composed of a close-packed hexagonal lattice and
has a better fatigue strength and a better wear resistance than γ′ phase,
and is thus convenient in applications where mechanical strength
properties are important. Further, the coefficient of self-diffusion of ε
30 phase is 10 times that of γ′ phase or higher under the same temperature
conditions, which means that ε phase can easily be produced. Thus, a
crankshaft including a compound layer with high proportion of ε phase
is more advantageous in manufacturing than a crankshaft including a
compound layer with high proportion of γ′ phase. The cross-sectional
35 area ratio of ε phase is more preferably not lower than 90 %.
9
[0033] In another implementation, the crankshaft 10 may include a
compound layer in which the proportion of γ′ phase as represented by a
cross-sectional area ratio is not lower than 80 %. γ′ phase has a
crystalline structure composed of a face-centered cubic lattice and has a
5 coefficient of cubic expansion lower than that of ε phase by about 30 %,
and is thus convenient in applications where thermal stability, such as
thermal-shock resistance, is important. The cross-sectional area ratio
of γ′ phase is more preferably not lower than 90 %.
[0034] The proportions of ε phase, γ′ phase and α phase in the
10 compound layer can be determined by electron backscatter diffraction
(EBSD). Specifically, an EBSD measurement is performed on a cross
section of the compound layer, followed by mapping of the ε phase, γ′
phase and α phase to determine the area ratio of these phases. It is
appropriate that EBSD measurements for about 10 fields of view are
15 performed with a magnification of approximately 4000 times.
[0035] In the compound layer, for both the journals 11 and pins 12, the
porosity area ratio of a region from the surface to a depth of 3.0 μm is
not higher than 10.0 %. However, if the thickness of the compound
layer is smaller than 3.0 μm, it is permitted that the porosity area ratio
20 measured for the entire thickness be not higher than 10.0 %. Porosity
area ratio of the thinner one of a region from the surface to a depth of
3.0 μm and a region across the entire thickness of a compound layer will
be hereinafter referred to as “surface-layer porosity area ratio” of the
compound layer.
25 [0036] Although the mechanism is not clear, the lower the
surface-layer porosity area ratio of a compound layer, the better seizure
resistance. The surface-layer porosity area ratio of the compound
layer of the journals 11 and pins 12 is preferably not higher than 5.0 %,
and more preferably not higher than 3.0 %.
30 [0037] Surface-layer porosity area ratio can be measured in the
following manner: A cross section of the compound layer is
photographed by scanning electron microscopy (SEM) with a
magnification of about 5000 times; 12 lines are drawn that are spaced
apart from each other by 0.25 μm and parallel to the surface of the
35 compound layer, together with 92 lines spaced apart from each other by
10
0.25 μm and perpendicular to the surface of the compound layer; and
the proportion of those intersections of such lines which are in voids are
treated as the surface-layer porosity area ratio.
[0038] The compound layer may have any surface-layer porosity area
5 ratio for the crankshaft portions other than the journals 11 and pins 12.
The compound layer over the entire crankshaft may have low
surface-layer porosity area ratios, or only the layer portions over the
journals 11 and pins 12 may have low surface-layer porosity area ratios.
[0039] The thickness of the compound layer for the journals 11 and
10 pins 12 is preferably 1.0 to 50 μm. A lower limit for the thickness of
the compound layer for the journals 11 and pins 12 is more preferably
2.0 μm, and yet more preferably 3.0 μm. An upper limit for the
thickness of the compound layer for the journals 11 and pins 12 is more
preferably 30 μm, and yet more preferably 20 μm, and still more
15 preferably 8 μm. The compound layer may have any thickness for the
crankshaft portions other than the journals 11 and pins 12.
[0040] The thickness of the compound layer can be measured in the
following manner: A cross section of the compound layer is polished,
etched by nital solution, and observed by optical microscopy. The
20 compound layer, which appears as a white, uncorroded layer, is
observed. 5 fields of view, in microstructure photographs taken by
optical microscopy at a magnification of 500 times, are observed. For
each field of view, the thickness of the compound layer is measured at 4
points that are horizontally arranged and spaced apart from each other
25 by 30 μm. The average of the thickness values at the 20 measured
points is treated as the thickness of the compound layer.
[0041] The hardness of the compound layer for the journals 11 and pins
12 is preferably HV500 to HV1000. A lower limit for the hardness of
the compound layer for the journals 11 and pins 12 is more preferably
30 HV700, and yet more preferably HV800. The compound layer may
have any hardness for the crankshaft portions other than journals 11
and pins 12.
[0042] The journals 11, as well as the pins 12, have such a surface
geometry that the arithmetical mean deviation of the primary profile
35 Pa is not larger than 0.090 μm. As used herein, arithmetical mean
11
deviation of a primary profile Pa is as defined by JIS B 0601:2001.
[0043] More specifically, the arithmetical mean deviation of the
primary profile Pa is measured in the following manner: Test specimens
are taken from the crankshaft 10 at locations for measurement (i.e., on
5 the journals 11 and pins 12), and a contact-type roughness tester is
used to obtain a measured primary profile. The contact roughness
tester used has a stylus with a tip radius of 2 μm and a cone with a
taper angle of 60°. The scan rate is 0.5 mm/s or lower, and the length
for measurement is 5 mm or larger.
10 [0044] A low-pass filter with a cutoff value λs is applied to the
measured primary profile to obtain a primary profile. As shown in
FIG. 2, using the primary profile as an assessed profile, the average of
the absolute values of Z(x) for a length for evaluation l is calculated,
which is treated as the arithmetical mean deviation of the primary
15 profile Pa. Here, Z(x) is the vertical coordinate at a location x; the
cutoff value λs is 2.5 μm, and the length for evaluation is 5 mm.
[0045] As the surface-layer porosity area ratio of the compound layer is
not higher than 10.0 % and the arithmetical mean deviation of the
primary profile Pa is not higher than 0.090 μm, seizure resistance will
20 be significantly improved over conventional techniques. The
arithmetical mean deviation of the primary profile Pa is preferably not
higher than 0.080 μm.
[0046] [Method of Manufacturing Crankshaft]
An exemplary method of manufacturing the crankshaft 10 will
25 now be described. The manufacturing method described below is
merely illustrative and by no means limits the method of
manufacturing the crankshaft 10.
[0047] FIG. 5 is a flow chart illustrating an exemplary method of
manufacturing the crankshaft 10. The manufacturing method
30 includes a material preparation step (step S1), a hot forging step (step
S2), a heat treatment step (step S3), a machining step (step S4), an
intermediate grinding step (step S5), an intermediate lapping step (step
S6), a nitriding step (step S7), a grinding step (step S8), a rough-lapping
step (step S9), and a finish-lapping step (step S10). These steps will
35 now be described in detail.
12
[0048] A material for a crankshaft is prepared (step S1). The material
for a crankshaft is not limited to any particular chemical composition,
and may be a steel for machine structural use mentioned above, for
example. The material may be produced by, for example, continuously
5 casting or blooming a steel melt having such a chemical composition as
specified above.
[0049] The material is hot forged into a roughly shaped crankshaft
(step S2). The hot forging process may be divided into rough forging
and finish forging.
10 [0050] The roughly shaped crankshaft product produced by the hot
forging may be subjected to a heat treatment, such as quenching,
tempering and/or normalizing, as necessary (step S3). The heat
treatment step (step S3) is an optional step, and may be omitted
depending on the crankshaft properties required or other factors.
15 [0051] The roughly shaped crankshaft product is machined (step S4).
Machining processes include cutting, grinding and hole drilling. This
step results in an intermediate crankshaft product having a shape
similar to that of the final product.
[0052] The journals and pins of the intermediate crankshaft product
20 are subjected to intermediate grinding and intermediate lapping (steps
S5 and S6). As discussed above, in the crankshaft according to the
present embodiment, the arithmetical mean deviation of the primary
profile Pa is not higher than 0.090 μm while some compound layer is
left. This requires that the arithmetical mean deviation of the primary
25 profile Pa of the journals and pins be reduced prior to the nitriding step
(step S7). Preferably, the intermediate grinding and intermediate
lapping reduce the arithmetical mean deviation of the primary profile
Pa for both the journals and the pins to 0.15 μm or below.
[0053] The intermediate crankshaft product subjected to the
30 intermediate grinding and intermediate lapping is nitrided (step S7).
The nitriding process is performed in an atmosphere containing NH3,
H2 and N2, for example. The nitriding process may be performed in an
atmosphere containing CO2 in addition to NH3, H2 and N2. The
process temperature is 550 to 620 °C, for example. The process time is
35 1.5 to 10 hours, for example.
13
[0054] At this time, the nitriding potential Kn = PNH3/(PH2)
3/2 may be
controlled to control the proportions of ε and γ′ phases in the compound
layer. PNH3 and PH2 indicate partial pressures of NH3 and H2,
respectively. Specifically, increasing the nitriding potential Kn
5 increases the proportion of ε phase, whereas reducing the nitriding
potential Kn increases the proportion of γ′ phase.
[0055] After nitriding, the journals and pins are ground again to
provide a neat surface geometry (step S8). If the compound layer
formed during nitriding includes a porous layer (denoted by numeral 51
10 in FIG. 1), this grinding step removes the porous layer.
[0056] Subsequently, the journals and pins are lapped (steps S9 and
S10). This lapping process is divided into a rough-lapping step and a
finish-lapping step, where the rough-lapping step uses a lapping film
coated with alumina abrasive grains whereas the finish-lapping step
15 uses a lapping film coated with diamond abrasive grains. This reduces
the arithmetical mean deviation of the primary profile Pa to 0.090 μm
or below while leaving some compound layer. It is difficult at this
point to reduce the arithmetical mean deviation of the primary profile
Pa to 0.090 μm or below while leaving some compound layer if the
20 above-discussed intermediate grinding and intermediate lapping were
not sufficient.
[0057] To reduce the arithmetical mean deviation of the primary profile
Pa to 0.090 μm or below, it is necessary to reduce both the roughness
and undulations during the intermediate grinding step (step S5) and
25 grinding step (step S8). Especially the roughness depends on the size
of the abrasive grains used for grinding. In view of this, it is
preferable to use abrasive grains that are as small as possible for
grinding.
[0058] On the journals and pins, undulations of periods of several
30 hundreds of micrometers to several millimeters are present that have
been caused by feeding and vibration of the tool during the machining
step (step S4). Even if the roughness is sufficiently reduced (i.e., the
arithmetical mean deviation of the roughness profile Ra is sufficiently
reduced), the arithmetical mean deviation of the primary profile Pa
35 does not decrease if undulations remain. In view of this, at the
14
intermediate grinding step (step S5) and grinding step (step S8),
undulations must be sufficiently removed by continuing grinding even
after the deviation Ra has decreased.
[0059] Further, at the lapping steps (steps S6, S9 and S10), it is
5 preferable to perform the following processes, (1) to (4), to prevent
formation of a centrally recessed shape with a concave central portion:
(1) polishing occurs while a lapping film with a small width is fed in the
axial direction, which makes it easier for lubricating oil to reach the
central portion of the lapping film; (2) abrasive grains with sizes that
10 are as small as possible are used, which results in a small cutting depth,
mitigating excessive grinding; (3) the rotating rate of the workpiece is
increased while the pressing force is reduced, which increases the
thickness of the oil (or water) film between the lapping film and
workpiece; and (4) the amount of lubricant oil (or water) is increased,
15 which increases the thickness of the oil (or water) film between the
lapping film and worikpiece.
[0060] Further, for each of the intermediate lapping step (step S6),
rough-lapping step (step S9) and finish-lapping step (step S10), the feed
rate of the lapping film in the axial direction of the crankshaft is to be
20 as low as possible. This removes small undulations, thereby further
reducing the arithmetical mean deviation of the primary profile Pa.
[0061] An exemplary construction of the crankshaft 10 according to an
embodiment of the present invention and an exemplary method of
manufacturing the same have been described. The embodiments
25 provide a crankshaft with improved seizure resistance

We claim:
1. A crankshaft having a journal and a pin, including a
compound layer containing iron and nitrogen on a surface thereof,
5 wherein, in the compound layer, for each of the journal and pin,
a porosity area ratio of a thinner one of a region from the surface to a
depth of 3.0 μm and a region across a total thickness of the compound
layer is not higher than 10.0 %, and
each of the journal and pin has such a surface geometry that an
10 arithmetical mean deviation of a primary profile, Pa, is not larger than
0.090 μm.
2. The crankshaft according to claim 1, wherein the compound
layer has a hardness of HV500 to HV1000.
15
3. The crankshaft according to claim 1 or 2, wherein, for each of
the journal and pin, the compound layer has a thickness of 1.0 to 50 μm.
4. The crankshaft according to any one of claims 1 to 3, wherein
20 a proportion of ε phase in the compound layer as represented by a
cross-sectional area ratio is not lower than 80 %.
5. The crankshaft according to any one of claims 1 to 3, wherein
a proportion of γ′ phase in the compound layer as represented by a
25 cross-sectional area ratio is not lower than 80 %.
6. A method of manufacturing the crankshaft according to any
one of claims 1 to 5, including:
an intermediate grinding step for grinding a journal and a pin of
30 an intermediate product of the crankshaft;
after the intermediate grinding step, an intermediate lapping
step for lapping the journal and pin of the intermediate product;
after the intermediate lapping step, a nitriding step for
nitriding the intermediate product;
35 after the nitriding step, a grinding step for grinding the journal
21
and pin of the intermediate product;
after the grinding step, a rough-lapping step for lapping the
journal and pin of the intermediate product using a film coated with
alumina abrasive grains; and
5 after the rough-lapping step, a finish-lapping step for lapping
the journal and pin of the intermediate product using a film coated with
diamond abrasive grains.