U.S. patent application number 10/333326 was filed with the patent office on 2004-03-04 for piston ring excellent in resistance to scuffing, cracking and fatigue and method for producing the same, and combination of piston ring and cylinder block.
Invention is credited to Inoue, Shigeo, Onuki, Toru, Sasakura, Mitsutaka, Takahashi, Junya.
Application Number | 20040040631 10/333326 |
Document ID | / |
Family ID | 18711547 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040040631 |
Kind Code |
A1 |
Takahashi, Junya ; et
al. |
March 4, 2004 |
Piston ring excellent in resistance to scuffing, cracking and
fatigue and method for producing the same, and combination of
piston ring and cylinder block
Abstract
A piston ring having improved scuffing resistance, cracking
resistance and fatigue resistance, consists of a high-chromium
martensitic stainless steel and a sliding nitriding layer formed on
the surface of said steel. The stainless steel consists of C: 0.3
to 1.0%; Cr: 14.0 to 21.0%, N: 0.05 to 0.50%, at least one of Mo,
V, W and Nb: 0.03 to 3.0% in total, Si: 0.1 to 1.0%, Mn 0.1 to
1.0%, P: 0.05% or less, S: 0.05% or less, the balance being Fe and
unavoidable impurities. The sliding nitriding layer comprises on
its surface hard particles mainly consisting of nitrides in a range
of from 0.2 to 2.0 .mu.m of average particle size, 7 .mu.m or less
of the longest diameter, and from 5 to 30% in area ratio.
Inventors: |
Takahashi, Junya; (Niigata,
JP) ; Onuki, Toru; (Niigata, JP) ; Inoue,
Shigeo; (Niigata, JP) ; Sasakura, Mitsutaka;
(Hyogo, JP) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 710
900 17TH STREET NW
WASHINGTON
DC
20006
|
Family ID: |
18711547 |
Appl. No.: |
10/333326 |
Filed: |
June 17, 2003 |
PCT Filed: |
July 16, 2001 |
PCT NO: |
PCT/JP01/06127 |
Current U.S.
Class: |
148/226 ;
148/232; 148/318 |
Current CPC
Class: |
C22C 38/22 20130101;
C22C 38/26 20130101; C22C 38/002 20130101; C23C 8/26 20130101; C22C
38/18 20130101; C21D 2211/008 20130101; C21D 8/065 20130101; C22C
38/24 20130101; C22C 38/001 20130101; C21D 9/40 20130101 |
Class at
Publication: |
148/226 ;
148/232; 148/318 |
International
Class: |
C23C 008/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2000 |
JP |
2000-216255 |
Claims
1. A piston ring having improved scuffing resistance, cracking
resistance and fatigue resistance, consisting of a high-chromium
martensitic stainless steel and a sliding nitriding layer formed on
the surface of said steel, characterized in that said high-chromium
martensitic stainless steel consists, by weight %, of C: 0.3 to
1.0%; Cr: 14.0 to 21.0%, N: 0.05 to 0.50%, at least one of Mo, V, W
and Nb: 0.03 to 3.0% in total, Si: 0.1 to 1.0%, Mn 0.1 to 1.0%, P:
0.05% or less, S: 0.05% or less, the balance being Fe and
unavoidable impurities; and, further said sliding nitriding layer
comprises on its surface hard particles mainly consisting of
nitrides in a range of from 0.2 to 2.0 .mu.m of average particle
size, 7 .mu.m or less of the longest diameter, and from 5 to 30% in
area %.
2. A piston ring according to claim 1, characterized in that the
grain boundary nitride observed in the longitudinal cross-section
of a nitriding layer is 20 .mu.m or less in size(length).
3. A piston ring according to claim 1 or 2, wherein the N content
of said high-chromium martensitic stainless steel is from 0.05 to
0.20%.
4. A piston ring according to any one of claims 1 through 3,
wherein the hardness of the sliding nitriding layer is in a range
of from Hv 900 to 1400.
5. A method for producing a piston ring having improved scuffing
resistance, cracking resistance and fatigue resistance, by means of
subjecting the high-chromium martensitic stainless steel to
nitriding treatment, characterized in that the high-chromium
martensitic stainless steel consists, by weight %, of C: 0.3 to
1.0%; Cr: 14.0 to 21.0%, N: 0.05 to 0.50%, at least one of Mo, V, W
and Nb: 0.03 to 3.0% in total, Si: 0.1 to 1.0%, Mn: 0.1 to 1.0%, P:
0.05% or less, S: 0.05% or less, the balance being Fe and
unavoidable impurities; and, further the quenching of said
high-chromium martensitic stainless steel prior to bending it into
a ring shape is carried out from a temperature in a range of from
850 to 1000.degree. C.
6. A combination of the piston ring according to any one of claims
1 through 4, and a cast-iron monolithic cylinder.
Description
BACKGROUND OF INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a piston ring used in an
internal combustion engine, particularly, a piston ring consisting
of high chromium martensitic stainless steel with nitriding, having
improved scuffing resistance (seizure resistance), cracking
resistance (failure resistance) and fatigue resistance. The present
invention is also related to a production method of the piston
ring.
[0003] 2. Background Technique
[0004] Along with recent demands for low fuel consumption, weight
reduction and high performance of internal combustion engines, the
piston rings are thinned to reduce weight and to follow the high
rotation of the engine. Material properties of the piston rings,
such as wear resistance, scuffing resistance and fatigue
resistance, and the like must be improved to enable thinning of a
piston ring. The conventional cast-iron piston rings have,
therefore, been replaced with steel piston rings particularly from
a view point of the fatigue resistance and heat resistance.
However, since the scuffing resistance of the steel piston-ring is
inferior to that of the cast-iron piston-ring, any
surface-treatment is usually applied to the sliding surface of
steel piston ring. Steels for piston ring are roughly classified
into carbon steel, silicon-chromium steel, and martensitic
stainless steel. These classifications correspond to different
kinds of surface treatments applied to the respective steels.
Mainly, the chromium plating is applied to carbon steels and
silicon chromium steels. Gas nitriding is applied to martensitic
stainless steels. The chromium plating was the most frequent
surface treatment of the steel piston ring previously, but has been
mostly replaced at present with the nitriding, because the scuffing
resistance of the chromium plating under high load is poor, and,
further, the waste-liquid of the plating must be treated so as not
to incur any environmental problem.
[0005] High chromium martensitic stainless steel mainly used at
present for the piston ring with nitriding is JIS SUS440B
equivalent composition of C: 0.80 -0.95%, Cr: 17.0-18.0%; Si:
0.25-0.50%; Mn: 0.25-0.40%; Mo: 0.70-1.25%; V: 0.07-0.15%; and Fe
in balance. When the steel having this composition is subjected to
nitriding, nitrogen atoms intrude and diffuse from the surface into
the steel and form a nitriding, layer. The nitrides in the
nitriding layer are mainly compounds of Cr, V and Mo, which may
contain the solute Fe. Chromium which is the main component of this
steel, is dissolved in the iron matrix, and is also present in the
form of Cr carbides.
[0006] Since the affinity of Cr for nitrogen is higher than that
for carbon; when nitrogen diffuses from the surface by the
nitriding, the reaction between the nitrogen and Cr carbides occurs
to form the Cr nitrides. Since the Cr content of SUS 440B
equivalent material is as high as 17.0-18.0%, hard Cr nitrides are
dispersed in the nitriding layer in an appropriate area %. The
nitriding layer is, therefore, relatively hard and improves the
wear resistance and scuffing resistance.
[0007] Recently published Japanese Unexamined Patent Publication
No. 11(1999)-80907 proposes martensitic stainless steel with
nitriding, having improved scuffing resistance, which contains Si:
0.25% or less, Mn: 0.30% or less; one or more of MO, W, V and Nb:
0.3-2.5% or Cu: 4.0% or less; Ni: 2.0% or less, and Al: 1.5% or
less.
[0008] Japanese Unexamined Patent Publication 11(1999)-106874
discloses that when the quantity of M.sub.7C.sub.3 carbide in the
microstructure is suppressed to 4.0% or less in area %, not only
scuffing resistance but also workability of the piston ring steel
material are improved.
[0009] Although the wear resistance and scuffing resistance have
been improved by the proposals as described above, when these
piston rings are used in recent internal combustion engines
operated under high revolution, and high power conditions, scuffing
is liable to occur.
[0010] Heretofore, liners are forced into the cylinder block of
Diesel engines. These engines are changed to a monolithic block
cast-iron of narrow bore distance without liners so as to attain
weight reduction and cost saving. The combustion pressure tends to
be increased from the viewpoints of waste gas purification and
power increase. In the microstructure of the cast iron Mono-block,
from relatively large cooling rate difference in casting of the
mono-block, graphite dispersion is not uniform and soft ferrite
phase as the cause of scuffing is unevenly distributed.
[0011] When the cylinder surface having the microstructure
mentioned above is combined with the martensitic stainless-steel
piston ring with nitriding, scuffing is liable to occur in the
initial operation period for the following reasons.
[0012] When the cylinder surface is finished by honing, the
abrasives of a grinding wheel cause clogging due to ferrite phase,
and the surface of the cylinder is liable to be roughened after the
honing. The graphite is covered by the ferrite flowed plastically.
As a result, lubrication and oil reserving effects of the graphite
are lowered since the area % of the graphite decreases. In the case
of high combustion pressure, the back pressure applied to the
piston ring becomes high. Scuffing frequently results from cracks
on the outer peripheral surface of a piston ring, elongating in a
direction perpendicular to the sliding direction. When the
nitriding layer is inspected, cracks are detected along the
lamellar compounds. The compounds are relatively coarse and are
present along grain boundaries of the iron matrix, and is referred
to as the gull phase in the field of the Japanese piston ring
industry. The compound lamella distribute parallel to the surface
of the piston ring.
[0013] In order to solve the problems of the piston rings, the
formation of TiN, CrN and the like is carried out by means of ion
plating. The ion plating can improve the wear resistance and the
scuffing resistance but the production cost is high. The reputation
of ion plating by the users at present is not favorable in the
light of the cost performance.
[0014] It is, therefore, an object of the present invention to
provide a high chromium martensitic stainless steel piston ring
with nitriding and its production method, which ring is
cost-effective and, which incurs neither wear, scuffing, cracking
nor fatigue fracture even when used in a Diesel engine operated at
high revolution and high combustion pressure, particularly, a
cast-iron monolithic block Diesel engine, which is expected, to be
increasingly used in the future because of weight reduction.
SUMMARY OF INVENTION
[0015] According to explanation of "Automotive Piston Ring" edited
by Editing Committee of Automotive Piston Ring, Sankaido Publisher,
page 188, 1997, when load is concentrated on the convexities
(especially of soft phases) of microscopic unevenesses on a sliding
surface, the temperature rises there due to friction heat, and
abnormal softening and melting occurs. This phenomenon results in
scuffing of the piston ring.
[0016] In the high chromium martensitic stainless steel with
nitriding, the microstructure of the nitriding layer generally
shows mainly hard nitrides dispersed in the tempered martensite
matrix. The mechanism of scuffing is strongly dependent upon the
microscopic unevenness on the sliding surface. In the nitriding
layer, hard particles disperse in the relatively soft matrix. The
microscopic unevenness is, therefore, defined by the size and
dispersion state of the hard particles. When the cross section of
the surface layer having such structure is observed, the following
is apparent. The convex hard particles are brought into contact
with an opposite sliding surface, while the relatively soft matrix
is relatively concave. The lubricating oil retained in the concave
portions is subjected to pressure during sliding. Frequency of the
entire direct contact of the steel with nitriding and the opposite
member is low, since the steel with nitriding has the
microstructure as described above. As a result, the contact
pressure between both sliding members is decreased. In addition,
the oil is fed to the convex portions mentioned above. The scuffing
can, therefore, be prevented.
[0017] The hard convex particles can attain the effects as
described above, provided that they are from sub microns to a few
microns in size and dispersed in an amount of 5% by area or more.
In the case in which the hard particles are extremely small and
case in which they are small in quantity, the mechanism according
to action and effect of the convex hard particles mentioned above
cannot be expected.
[0018] Meanwhile, these effects are influenced by the circumstances
of the sliding surface of the opposite member. Specifically, in the
case of the cast iron monolithic cylinder block having a
non-uniform structure as described above, the surface of this block
is liable to be roughened by the grinding. Frequently, the ferrite
phase plastically flows and covers the graphite.
[0019] The sliding surface of even such cast iron is modified by
appropriate sliding referred to by experts as break-in or
compatibility. That is, the following phenomenon occurs. When the
rough inner surface of a cylinder is smoothened during the sliding,
the ferrite is removed and the covered graphite is exposed. Until
the break-in is progressed, an oil film on the sliding surface is
frequently liable to be absent. When the oil film is absent,
friction force applied on the outer peripheral surface of a piston
ring is increased. The large friction force is repeatedly applied
on the outer peripheral surface of a piston ring. The nitriding
layer is, therefore, repeatedly subjected to large stress resulting
in initiation and enlargement of cracks in a direction
perpendicular to the sliding direction. Along with the progress of
the adaptability phenomenon on the inner surface of a cylinder, the
stress applied is lessened, while the cracks propagate with the
lapse of time. As a result, the nitriding layer may locally peel in
the surface, and the inner surface of a cylinder may be damaged.
The scuffing is, therefore, liable to occur in the initial period
of sliding. Since the grain boundary compounds in the nitriding
layer are very brittle, the presence of them promotes the
initiation and propagation of cracks.
[0020] The present inventors found the following essential matters.
A large number of hard particles, mainly Cr nitrides, in proper
size in the nitriding layer should be uniformly dispersed in matrix
in order to decrease the probability of contacts between matrix and
cylinder and to prevent the initial stage scuffing. Especially the
grain boundary compounds formed during nitriding should be fine to
suppress the initiation of cracks in connection with those
compounds. In this fine microstructure, even if cracks initiate,
the development of those can be suppressed.
[0021] When the melt of high-chromium martensitic stainless steel
solidifies, the eutectic Cr carbide (.eta.phase: (Cr,
Fe).sub.7C.sub.3) crystallizes in the grain boundaries of primary
austenite (.gamma.phase). Cr carbides exceeding 20 .mu.min the
largest diameter are present in the high chromium martensitic
stainless steel, which is solidified as above and then hot rolled,
spheroidizing annealed, and finally quenched and tempered.
Regarding the refining of the coarse primary eutectic Cr carbides,
Tetsu and Hagane (Journal of Japan Institute of Iron and Steel),
Vol., 82, No. 4, p.309-314 (1996) reports the refinement of
carbides by addition of 0.25% or more of N. According to this
report, the eutectic Cr carbide in the boundaries of primary
.gamma. disappears and instead, lamellar M.sub.23C.sub.6 and
M.sub.2N (M: Cr, Fe) precipitate around the primary .gamma. grain
boundaries. These lamellar precipitates are finely divided in the
hot rolling. In the subsequent spheroidizing annealing, fine
M.sub.23C.sub.6 newly precipitates at sites different from those of
M.sub.2N. The Cr carbides as a whole become, therefore, fine.
[0022] Netsushori Vol. 36, No. 4, p. 234-238 (1996) reports the
mechanical properties of 16.5% Cr-0.65% C martensitic stainless
steel with the addition of 0.25% of N. That is, the quenching
temperature, at which the highest hardness is obtained, shifts to
lower temperature with the increase in N content. The elongation
also increases with the increase in N content. It is explained that
the solution amount of N in the austenite phase increases and the
austenite phase is stabilized with the increase in quenching
temperature.
[0023] Japanese Unexamined Patent Publication Nos. 9-289053 and
9-287058 disclose the rolling bearing, in which the refining of Cr
carbides due to the N addition is utilized.
[0024] The present inventors have studied the scuffing mechanisms
mentioned above and the influence of relatively large lamellar
grain boundary compounds on cracking in sliding surface of piston
ring and applied the refining technology of Cr carbide using N
addition. As a result, it is found to be desirable that a large
number of nitrides dispersed uniformly in the nitriding layer and
especially grain boundary compounds are fine in size. This fine
microstructure provides a high chromium martensitic stainless steel
piston ring with nitriding having improved scuffing, cracking and
fatigue resistances even when it is used in internal combustion
engines operated under high revolution and high power conditions,
particularly, recent weight reduced cast iron mono-block Diesel
engine, etc.
[0025] The high-chromium martensitic stainless steel piston ring
with nitriding according to the present invention is characterized
in that it comprises the high-chromium martensitic stainless steel,
which consists, by weight %, of C: 0.3 to 1.0%; Cr: 14.0 to 21.0%,
N: 0.05 to 0.50%, at least one of Mo, V, W and Nb: 0.03 to 3.0% in
total, Si: 0.1 to 1.0%, Mn 0.1 to 1.0%, P: 0.05% or less, S: 0.05%
or less, the balance being Fe and unavoidable impurities; and, the
high chromium martensitic stainless steel has a nitriding sliding
layer, which comprises hard particles consisting of carbide,
nitride and carbo-nitride, mainly nitride, and the hard particles
in the surface of the nitriding layer are in a range of from 0.5 to
2.0 .mu.m of average diameter, 7 .mu.m or less of the largest
diameter, and from 5 to 30% in area %. The grain boundary compounds
observed in the longitudinal cross section of the nitriding layer
are 20 .mu.m or less in size (length). The nitriding surface layer
having the microstructural feature mentioned above has hardness in
the range of from Hv 900 to 1400 and has sufficient depth from the
surface.
[0026] The method for producing the high chromium martensitic
stainless steel piston ring with nitriding according to the present
invention comprises: melting the steel having the above composition
followed by adding nitrogen; casting the molten steel into an
ingot; hot rolling; annealing; cold wire drawing; cold rolling to
form an approximate cross sectional shape of the piston ring;
quenching; tempering to provide the wire materials; bending the
wire material into the form of the piston ring; strain-relief
annealing; rough grinding of the side surfaces; nitriding; removal
of surface compound layer; grinding butt ends; finish grinding of
side surfaces; and lapping of the outer peripheral surfaces. Prior
to the bending into the piston ring shape, quenching is carried out
from the temperature of from 850 to 1000.degree. C., which is
relatively low as the quenching temperature of the high chromium
martensitic stainless steel. As a result, the microstructure is
fine and contains a large amount as possible of the dispersed
carbides. The nitriding may be gas nitriding, ion nitriding and
radical nitriding. The nitriding is carried out at in a range of
450 to 600.degree. C. for 1 to 20 hours.
[0027] The present invention is hereinafter described in
detail.
[0028] The components of the high-chromium martensitic stainless
steel according to the present invention are described.
[0029] C is an interstitial solute element in Fe and increases
hardness of matrix. C is easily combined with Cr, Mo, V, W and Nb
and forms carbides. The carbides are converted mainly to nitrides
during the nitriding. In other words, the nitrides enhance the wear
resistance and the scuffing resistance of the sliding surface of a
piston ring. When the C content is less than 0.3%, the hardening
and formation of carbides are not sufficient. On the other hand,
when the C content is more than 1.0%, coarse eutectic Cr carbide
(.eta.phase: M.sub.7C.sub.3 carbide) crystallizes in large amount
during the solidification of the molten steel. This carbide
drastically impairs workability of the material at the subsequent
production processes of wires. The carbon content is, therefore, in
a range of from 0.3 to 1.0%, preferably in a range of from 0.4 to
0.9%.
[0030] Cr is a substitutional solute element in Fe. Cr not only
improves the corrosion resistance but also induces the solution
strengthening and hence improvement in the thermal setting
resistance. Here the thermal setting is a phenomenon that sealing
property is deteriorated by tension decrease due to creep during
operation of a piston ring at high temperature. Cr reacts with C in
steel and forms Cr carbides. These Cr carbides easily react with N,
which intrudes from the surface during nitriding, and are converted
to Cr nitrides. The Cr nitrides are dispersed in the nitriding
layer as the hard particles. The hard particles in the nitriding
layer exceedingly enhance the wear resistance and the scuffing
resistance of the sliding surface of a piston ring. When the Cr
content is less than 14%, the formation of Cr carbides is not
sufficient. On the other hand, when the Cr content is more than
21%, the .delta. ferrite is formed and toughness is hence lowered.
In addition, the Cr concentration in the matrix becomes so high
that the Ms (the starting temperature of martensitic
transformation) is so lowered such that satisfactory quenching
hardness is not obtained. The Cr content is, therefore, in a range
of from 14 to 21%, preferably in a range of from 16 to 19%.
[0031] N is an interstitial element in Fe, as C is. Ternary
Fe--Cr--C phase diagram can be expressed by a pseudo-binary phase
diagram by cutting at, for example, the 17% Cr line. An eutectic
reaction occurs between Fe and C, the concentration of which is
given by the left end of the eutectic line. Meanwhile prior to the
complete solidification, molten steel remains around the grain
boundaries of primary crystals. When the temperature further falls,
the molten steel undergoes the eutectic reaction. When the nitrogen
is added in accordance with the present invention, the C
concentration at the left side mentioned above is higher than that
of the molten steel without nitrogen. Therefore, the eutectic
reaction and hence the formation of .eta. carbide are suppressed.
When the temperature falls lower than the eutectic temperature, the
super saturated C and N precipitate around the primary .gamma.
grains in the form of lamellar M.sub.23C.sub.6 and M.sub.2N
precipitates. When the N content is less than 0.05%, the .eta.
phase crystallizes. On the other hand, when the N content is more
than 0.50%, the amount of M.sub.2N precipitates in the form of a
rod increases, so that the toughness is lowered. The N content is,
therefore. in a range of from 0.05 to 0.50%, more preferably in a
range of from 0.10 to 0.30%. The solute N in the matrix impedes the
diffusion of C and also contributes to refining of the grain
boundary compounds. This is first Fe.sub.3C after casting and is
finally converted to Fe.sub.3N after nitriding treatment. Nitrogen
up to 0.2% can be added under normal pressure. Nitrogen content of
more than 0.2% necessitates melting under pressure N.sub.2
atmosphere. The nitrogen content in a range of from 0.05 to 20% is,
therefore, preferable from the viewpoint of N addition.
[0032] Any one of Mo, V, W and Nb is a carbide former and enhances
wear and scuffing resistances. In addition, Mo prevents softening
during the tempering and nitriding treatments and plays an
important role in attaining the dimension stability of a piston
ring. V promotes nitriding, and therefore the hardness of a
nitriding layer containing V is high. Any one of these elements is
effective for enhancing the properties of a piston ring. When the
total content of at least one of Mo, V, W and Nb is less than
0.03%, their effects are virtually negligible. On the other band,
when the total content of these element(s) is more than 3%, the
workability is seriously impaired and the toughness is lowered. The
total content of at least one of Mo, V, W and Nb is therefore, from
0.03 to 3.0%.
[0033] Si is a deoxidizing additives. Si is also dissolved in Fe
and enhances the softening resistance in tempering. The so-called
thermal setting resistance can therefore, be improved. When the Si
content is less than 0.1% its effect is slight. On the other hand,
when the Si content is more than 1.0%, the toughness is impaired.
The Si content is therefore, in a range of from 0.1 to 1.0%.
[0034] Mn is also a deoxidizing additive. When the Mn content is
less than 0.1%, its effect is slight. On the other hand when the Mn
content is more than 1.0%, the workability is impaired. The Mn
content is, therefore, from 0.1 to 1.0%.
[0035] P forms inclusions with Mn and the like and lowers the
fatigue strength and corrosion resistance. P is an impurity of
steel. The less P, the better. The P content is therefore, 0.05% or
less from a practical point of view. Preferably, P is 0.03% or
less.
[0036] S lowers the fatigue strengh and corrosion resistance, as P
does. Sis an impurity of steel. The less S, the better. The S
content is, therefore, 0.05% or less from a practical points of
view. Preferably, S is 0.03% or less.
[0037] The steel consisting of the composition ranges as described
above is subjected to formation of a microstructure having improved
scuffing resistance, that is, a number of fine nitride particles
are present in the nitride layer. More specifically, the hard
particles consisting of nitrides. i.e., mainly Cr nitride, carbides
and carbonitrides, present in the surface of the nitriding lower
should have an average diameter in a range of from 0.2 to 2 .mu.m,
the largest diameter of 7 .mu.m or less, and area % in a range of
from 5 to 30%. When the average particle diameter is less than 0.2
.mu.m; the convexities of the hard particles are not effective for
preventing scuffing. On the other hand, when the average particle
diameter is more than 2 .mu.m, scuffing is liable to occur when the
load is high. When the largest diameter is more than 7 .mu.m, the
microstructure of the nitriding layer becomes non-uniform so that
scuffing is liable to occur under high load. When the area % is
less than 5%, scuffing is liable to occur. On the other hand, when
the area % of nitrides is more than 30%, the wire drawing and the
bending into the piston ring form after melting become difficult. A
preferable area % is from 10 to 25%.
[0038] The microstructure of the nitriding layer having improved
cracking resistance is such that the grain boundary compounds
observed in the longitudinal cross section of a piston ring are 20
.mu.m or less in size (length). When the longest length is more
than 20 .mu.m, there arises a problem that the cracking is liable
to occur under high load.
[0039] The microstructure of the nitriding layer as described above
according to the present invention is attributable to the
microstructure of stainless steel. First, no coarse eutectic Cr
carbide (.eta. phase: (Cr, Fe).sub.7C.sub.3 carbide) is present in
the steel which has been successively hot rolled, spheroidizing
heat treated, cold wire drawn, quenched and tempered. This is
attained by the nitrogen addition.
[0040] Second, a large number of the fine secondary carbide
(.epsilon. phase, (Cr, Fe).sub.23C.sub.6 carbide) precipitate when
holding at the quenching temperature prior to nitriding. The
Fe--Cr--C phase diagram teaches more and finer carbides
precipitates as the quenching temperature is lower in the
(.gamma.+.epsilon.) region. When the quenching is carried out from
temperature as low as possible in the (.gamma.+.epsilon.) region,
fine .epsilon. carbides can be precipitated in quantity as much as
possible. In addition, the growth of .gamma. crystal grains is
suppressed, so that the quenched steel is of fine grain structure.
When this steel is subjected to nitriding, the grain boundary
compounds becomes also fine. A preferable quenching temperature is,
therefore, in a range of from 850 to 1000.degree. C., from the
viewpoints as described above. When the quenching temperature is
less than 850.degree. C., no hardening occurs and the desired
hardness is not attained because of precipitation of the .alpha.
phase. When the quenching temperature is more than 1000.degree. C.,
the carbides coalesce in the holding step at the quenching
temperature and the .gamma. crystal grains coarsen. As a result,
the coarse carbides are converted to the coarse nitrides. The grain
boundary compounds, which are formed along the coarsened .gamma.
crystal grains in the subsequent nitriding treatment, become
coarse.
[0041] In the present invention, high hardness of from Hv 900 to
1400 is obtained up to a satisfactory depth from the surface by the
nitriding treatment for a relatively short period of time. This
feature is attributable to the relatively fine .gamma. crystal
grains formed at low quenching temperature and thus to the increase
of the grain boundaries which are the main diffusion passages of N
during the nitriding treatment.
[0042] According to the present invention, the nitriding treatment
is carried out in the temperature range of from 450 to 600.degree.
C. In the prior art, the treatment temperature of approximately
590.degree. C., at which the nitrogen solubility in the .alpha.-Fe
lattice is the greatest, has been considered to be advisable.
However, since the present invention utilizes the N diffusion
mainly via the grain boundaries, the treatment temperature is not
limited to approximately 590.degree. C. The lower-temperature
treatment is more advisable from the viewpoint of dimension
stability of a piston ring. However, from a practical point of
view, the nitriding is carried out at 450 to 600.degree. C. for 1
to 20 hours.
BRIEF EXPLANATION OF DRAWINGS
[0043] FIG. 1 is a photograph of back scattered electron image of
surface of sliding nitriding layer observed by a scanning electron
microscope. FIGS. 1(a) and (b) correspond to Example 1 and
Comparative Example 1, respectively.
[0044] FIG. 2 is an optical microscope photograph of the cross
section of a nitriding layer. FIGS. 2(a) and (b) correspond to
Example 1 and Comparative Example 1, respectively.
[0045] FIG. 3 shows a specimen of the scuffing test.
[0046] FIG. 4 shows the movement mechanism of a friction and wear
tester.
[0047] FIG. 5 shows the movement mechanism of a fatigue tester of a
piston ring.
[0048] FIG. 6 is a graph showing the fatigue limit.
[0049] FIG. 7 is a photograph showing a crack formed on the sliding
surface of Comparative Example 13.
BEST MODE FOR CARRYING OUT INVENTION
Examples 1-11 (J1-J11) and Comparative Examples 1 -8 (H1-H18)
[0050] The high chromium martensitic stainless steels having a
composition shown in Table 1 were melted in an amount of 10kg in a
vacuum induction melting furnace. However, less than 0.2% of N was
added to the steel during melting under the normal pressure, while
0.2% or more of N was added to steel during melting under pressure
N.sub.2 gas atmosphere. Wire material having 12 mm of diameter was
obtained by hot working. After acid cleaning, spheroidizing
annealing was carried out at 750.degree. C. for 10 hours. A wire
having a rectangular cross section of 3.5 mm.times.5.0 mm was
produced through working steps. The wire was passed through a
quenching furnace (Ar protective atmosphere) and a tempering
furnace (Ar protective atmosphere). The air quenching was carried
out from 930.degree. C. after keeping approximately 10 minutes at
that temperature. The tempering was carried out at 620.degree. C.
for approximately 25 minutes.
[0051] The wires were cut into 50 mm long samples for the nitriding
treatment. The gas nitriding was carried out at 570.degree. C. for
4 hours. However, the quenching temperature of Comparative Example
1 (H1) was 1100.degree. C. as in the conventional method. The other
conditions are the same as for the Examples and the other
Comparative Examples.
1 TABLE 1 (wt %) C Cr N Mo V W Si Si Mn P S J1 0.65 17.5 0.13 1.5
-- -- -- 0.25 0.35 0.02 0.01 J2 0.41 17.0 0.19 1.0 0.15 -- -- 0.25
0.50 0.02 0.02 J3 0.83 17.8 0.23 -- 0.20 -- -- 0.20 0.30 0.02 0.02
J4 0.59 17.2 0.16 -- -- 0.05 0.20 0.20 0.02 0.02 J5 0.62 17.5 0.15
-- -- -- 0.3 0.20 0.30 0.02 0.02 J6 0.60 14.5 0.15 1.5 0.5 0.1 0.5
0.55 0.65 0.02 0.02 J7 0.60 19.5 0.25 1.0 -- 0.1 -- 0.20 0.30 0.02
0.02 J8 0.35 20.3 0.28 1.0 -- -- 0.3 0.20 0.30 0.02 0.02 J9 0.95
14.9 0.25 -- 0.5 0.1 -- 0.20 0.30 0.02 0.02 J10 0.55 16.5 0.08 --
0.5 -- 0.3 0.35 0.55 0.02 0.02 J11 0.48 18.2 0.42 -- -- 0.1 0.3
0.20 0.20 0.02 0.02 H1 0.81 17.5 0.03 1.0 0.3 -- -- 0.25 0.25 0.02
0.02 H2 0.45 18.0 0.58 1.5 0.5 -- -- 0.20 0.20 0.02 0.02 H3 0.25
17.3 0.16 1.0 0.4 -- -- 0.20 0.30 0.02 0.02 H4 1.12 17.8 0.15 1.2
0.6 -- -- 0.20 0.30 0.02 0.02 H5 0.69 13.2 0.21 1.1 0.5 -- -- 0.20
0.30 0.02 0.02 H6 0.73 22.1 0.22 1.0 0.2 -- -- 0.20 0.20 0.02 0.02
H7 0.65 17.8 0.16 -- -- -- -- 0.20 0.20 0.02 0.02 H8 0.68 17.3 0.15
1.5 1.0 0.5 0.5 0.20 0.20 0.02 0.02
[0052] The wire samples mentioned above were further cut into
lengths of 10 mm for observation of the microscopic structure. The
specimens were embedded in resin and were mirror-finished. The
observation and quantitative evaluation of the microstructure were
carried out using an image analyzer. The back scattered electron
image of the sliding nitriding surface was observed by a scanning
electron microscope with regard to Example 1 (J1) and Comparative
Example (H1). The observed images for Example 1 (J1) and
Comparative Example 1 (H1) are shown in FIGS. 1(a) and (b),
respectively. The cross section of the nitriding layer was observed
by an optical microscope and the observed photographs are shown in
FIGS. 2(a) and (b), respectively, with regard to Example 1 (J1) and
Comparative Example 1 (H1). The hard particles appear black in the
back scattered electron image photograph and white in the optical
microscope photograph. It is apparent that the hard particles
according to the present invention are extremely small in size:
and, the grain boundary compounds observed in the cross section of
the nitriding layer are extremely small in size. The
microstructures of Example 1 through 11 (J1-J11) and Comparative
Examples 1 -8 (H1-H18) were quantitatively evaluated with regard to
the average particle diameter, the largest particle diameter and
area ratio of the hard particles in the sliding nitriding surface
and the longest length of the grain boundary compounds in the cross
section of the nitriding layer. These results are shown in Table 2
together with the hardness of the sliding surface of nitriding
layer.
2 TABLE 2 Hard Particles of Sliding Nitriding Layer Longest Length
of Average Largest Grain Boundary Particle Particle Compound in
Cross Diameter Diameter Area % Section of Nitriding Vickers (.mu.m)
(.mu.m) (%) Layer (.mu.m) Hardness J1 1.6 5 17.2 16 1253 J2 1.3 4
13.0 15 1050 J3 1.0 5 22.5 13 1185 J4 1.7 6 15.9 12 1120 J5 1.6 5
17.1 15 1148 J6 1.5 4 10.7 14 955 J7 0.9 4 21.0 12 1219 J8 1.2 6
18.0 13 1193 J9 1.3 6 13.0 12 984 J10 1.8 5 14.2 17 1031 J11 1.2 5
16.2 14 1083 H1 2.7 15 13.6 28 1065 H2 * * * * * H3 1.5 5 7.5 15
830 H4 * * * * * H5 1.4 5 4.0 14 920 H6 2.2 8 9.1 14 874 H7** 1.6 5
16.5 16 1109 H8 * * * * * *Comparative Examples 2, 4 and 8 (H2, H4
and H8) could not be formed into a wire because of poor
workability. **The post-nitriding dimension was unstable in
Comparative Example 7 (H7).
[0053] The yield is, therefore, low.
[0054] Referring to FIG. 3, a scuffing test sample in the form of
Japanese katakana "]" having 45 mm of the total length is shown.
The wire material was shaped into scuffing test samples of two-pin
integral type. The opposite material was made of FC250 and was in
the form of a disc of 60 mm in diameter and 12 mm in thickness.
[0055] The sliding surface of disc 2 (FIG. 4 ) was adjusted to the
surface roughness (Rz) of from 1 to 2 .mu.m. The scuffing test was
carried out using a friction and wear tester (Product of Riken,
Trade name "Triborik I"). The front ends of the pin (reference
numeral 1, FIG. 4) are convex sliding surfaces having 20 mm of
radius. The front ends were subjected to gas nitriding treatment.
The 5 to 20 .mu.m thick compound layers (white layer) formed on the
front ends were removed by grinding. The front ends were then
mirror-finished by polishing. The surface roughness (Rz) of the
sliding surface of FC250 disc (FIG. 4, Reference numeral-2) used is
adjusted to 1-2 .mu.m The movement mechanism of the friction wear
tester is illustrated in FIG. 4. The testing conditions of scuffing
were as follows.
[0056] Sliding Speed (Disc): 8 m/sec
[0057] Pressing Load: Stepwise increase by 0.2 MPa from the initial
1.0 MPa until occurrence of scuffing
[0058] Lubricating Oil: Motor Oil (Trade Name--Nisseki Motor Oil P
#20)
[0059] Temperature of Lubricating Oil: 80.degree. C. (in the
vicinity of outlet)
[0060] Oil bath: 100.degree. C.
[0061] Feeding Amount of Lubricating Oil: 40 cc/min
[0062] The scuffing surface pressure was calculated from the
scuffing load, and the wear area of the sliding surface. The
scuffing surface pressure obtained is shown with regard to Example
1-11 (J1-11) and Comparative Examples 1-8 (H1-H8)
3 TABLE 3 Scuffing Surface Pressure (MPa) J1 454 J2 443 J3 469 J4
428 J5 458 J6 420 J7 464 J8 430 J9 441 J10 419 J11 452 H1 376 H2 --
H3 340 H4 -- H5 328 H6 297 H7 388 H8 --
[0063] It is apparent that the scuffing resistance of Examples 1-11
(J1-J11) is improved over that of Comparative Examples 1, 3, 5-7.
(H1, H3, H5-H7).
Examples 12-14 (J12-J14) AND COMPARATIVE EXAMPLES 9-11 (H9-H11)
[0064] The materials having the chemical Composition of Example 1
were worked into a wire and air quenched from the temperature shown
in Table 4. The gas nitriding treatment was carried out by the same
method as in Example 1. The microstructure of the nitriding layer
was quantitatively analyzed. The results are shown in Table 4.
4 TABLE 4 Hard Particles of Sliding Longest Length Nitriding Layer
of Grain Boundary Average Largest Compound in Quenching Particle
Particle Area Cross Section of Temperature Diameter Diameter %
Nitriding Layer (.degree. C.) (.mu.m) (.mu.m) (%) (.mu.m) H9* 800
0.3 5 15.4 14 J12 870 0.5 5 19.4 11 J13 920 1.3 6 18.5 15 J14 980
1.8 6 17.4 18 H10 1030 2.3 9 14.7 31 H11 1080 2.8 11 11.5 49 *The
hardness of the nitriding layer of Comparative Example 9 (H9) was
low of Hv 860.
Example 15 and Comparative Example 12
[0065] The steel materials of Example 1 and Comparative Example 1
were subjected to working steps to form a compression ring having a
rectangular cross section. The nominal diameter (d.sub.1) was 95.0
mm, thickness (a.sub.1) was 3.35 mm, and the width (h.sub.1) was
2.3 mm. The quenching was carried out by means of passing through
the quenching furnace at 930.degree. C. for 10 minutes and then
air-cooling. The tempering was carried out by means of the
tempering furnace at 620.degree. C. for approximately 25 minutes.
The continuous quenching and tempering was carried out. The gas
nitriding was carried out at 570.degree. C. for 4 hours. However,
the quenching temperature of Comparative Example 12 was
1100.degree. C. as in the conventional method. The other conditions
are the same as for the Comparative Example 15.
[0066] The produced compression piston ring was tested in a fatigue
tester, the movement mechanism of which is illustrated in FIG. 5.
The butt ends of the compression piston ring were cut at both ends
to widen the dimension of free gap. The so-treated piston ring 3
was set by an adjuster 9 in the tester in such a manner that its
diameter was reduced to the nominal diameter. The eccentric cam 4
was then rotated so as to impart repeated strokes at 40 cycles per
second for further reducing the diameter to less than the nominal
one, until the piston ring 3 fractured. The number of stress
applied at the fracture was obtained. This test was repeated, while
changing the applied stress to the sample of identical
specification. The so-called S--N diagram and finally fatigue limit
diagram shown in FIG. 6 were obtained.
[0067] Referring to FIG. 6, it is apparent that Example 15 is
outstandingly improved over Comparative Example 12.
Examples 16-19 and Comparative Examples 13-14
[0068] The steel materials of Example 1 (Example 16, 17), Example 7
(Examples 18, 19) and Comparative Example 1 (Comparative Examples
13, 14) were subjected to working steps to form a compression ring
(Examples 16, 18 and Comparative Example 13) and the body of a
two-piece oil ring (Examples 17, 19 and Comparative Example 14).
Compression ring had a rectangular cross section. Its nominal
diameter (d.sub.1) was 99.2 mm, thickness (a.sub.1) was 3.8 mm, and
the width (h.sub.1) was 2.5 mm. The body of the oil ring had a
saddle-shape cross section. Its nominal diameter (d.sub.1) was 99.2
mm, thickness (a.sub.1) was 2.5 mm, and the width (h.sub.1) was 3.0
mm.
[0069] The quenching, the tempering and the gas nitriding in
Examples 16-19 were the same as in Example 15. The quenching, the
tempering and the gas nitriding in Comparative Examples 13-14 were
the same as in Comparative Example 12.
[0070] The produced compression rings and oil rings were mounted in
a four-cylinder Diesel engine of 3200 cc displacement. These rings
were mounted on a piston and combined with a monolithic cast-iron
block and operated for 100 hours for the endurance test under the
following condition.
[0071] Number of Revolutions: 3600 rpm
[0072] Power: 75 kW
[0073] Load: full load
[0074] Water temperature: 110.degree. C.
[0075] Oil temperature: 130.degree. C.
[0076] Scuffing occurred after 2 hours 10 minutes in the case of
Comparative Example 13 and after 7 hours 55 minutes in the case of
Comparative Example 14. No trouble occurred during the test in the
case of Examples 16-19. Referring to FIG. 7, the photograph of a
crack on the sliding nitriding surface of Comparative Example 13 is
shown.
INDUSTRIAL APPLICABLITY
[0077] According to the present invention, large amount of fine
nitrides are present in the nitriding layer of the piston ring,
which is made of high chromium martensitic stainless steel with
nitriding. The laminar grain boundary compounds are refined, too.
Such microstructure can be formed by the addition of nitrogen and
the low temperature quenching. The wear resistance, scuffing
resistance, cracking resistance and fatigue resistance are improved
as a result of the microstructure. The piston ring according to the
present invention can, therefore, be advantageously used in
internal combustion engines operated under high rotation and high
power conditions, particularly, the recent light-weight monolithic
block Diesel engine. The piston ring according to the present
invention can also be advantageously used for the piston ring of a
small motor truck, in which the ring fatigue problem is likely to
occur when using the exhaust brake. The piston ring according to
the present invention can be appropriately embodied as the body of
a two-piece oil ring and the rail of a three-piece oil ring.
* * * * *