DESCRIPTION
TITLE OF INVENTION
NITRIDING METHOD AND NITRIDED PART PRODUCTION METHOD
TECHNICAL, FIELD
[OOO I]
The present invention relates to a nitriding method and a nitrided part
production method, and more particularly, to a method for nitriding low alloy steels
and a method for producing nitrided parts therefrom.
BACKGROUND ART
[0002]
For steel parts used in motor vehicles, various industrial machines, etc., a case
hardening heat treatment such as carburizing-quenching, induction hardening,
nitriding, or nitrocarburizing is applied to improve their mechanical properties such
as fatigue strength, wear resistance, and seizure resistance. The nitriding process
and the nitrocarburizing process both use a heat treatment in the fenite region at a
heating temperature not more than the A1 temperature without utilizing phase
transformation. As a result, heat treatment-induced distortion can be reduced. For
this reason, the nitriding process or the nitrocarburizing process is frequently used for
parts having high dimensional accuracy and large parts, examples of which include
gears used in automotive transmission parts and crankshafts used in engines. III
particular, the nitriding process requires fewer types of gas for the process than the
nitrocarburizing process, so that atmosphere control therefor is easier.
[0003]
Examples of nitriding processes include the gas nitriding process, the salt bath
nitriding process, and the plasma nitriding process. For automotive parts or the like,
the gas nitriding process, which has high productivity, is widely employed. The gas
nitriding process can result in formation of a compound layer having a thickness of
10 p n or more on the surface of the steel material. The compound layer contains
nitrides such as Fe2.3N and Fe4N, and the hardness of the compound layer is much
higher than that of the base metal of the steel part. Thus, the compound layer
enhances the wear resistance and surface fatigue strength of the steel part at an early
stage of use.
[0004]
However, the compound layer has low toughness and low deformability and
therefore is more likely to experience delamination or cracking during use. For this
reason, nitrided parts processed by gas nitriding are not suitable for use as parts that
can be subjected to impact stresses or high bending stresses. Furthermore, in the
gas nitriding process, although heat treatment-induced distortion is reduced,
straightening is sometimes necessary for long parts such as shafts and crankshafts.
In such an instance, depending on the thickness of the compound layer, cracking may
occur during straightening and this can decrease the fatigue strength of the part.
[OOOS]
Accordingly, there is a need for a gas nitriding process that can provide a
thinner compound layer or even eliminate the compound layer. By the way, it is
known that the thickness of the cornpound layer can be controlled by the process
temperature of the nitriding process and the nitriding potential KN determined by the
following formula using the NH3 partial pressure and H2 partial pressure.
KN = (NH3 partial pressure)/[(H2 partial pressure)3'2]
[0006]
By lowering the nitriding potential KN, it is possible to provide a thinner
compound layer or even to eliminate a compound layer. However, when the
nitriding potential KN is low, ease of nitrogen penetration into the steel is reduced.
In such an instance, the hardened case referred to as a nitrogen diffusion layer will
have reduced hardness and reduced depth. As a result, the nitrided parts will have
reduced fatigue strength, wear resistance, and seizure resistance. Another technique
to eliminate the compound layer is, for example, machine grinding or shot blasting of
the nitrided parts after the gas nitriding process. However, this technique results in
higher production cost.
[0007]
To respond to these problems, one proposed technique is to control the
atmosphere for the gas nitriding process using a nitriding parameter, KN' = (NH3
partial pressure)/[(H;! partial pressure)"'], which is different from the abovelnentioned
nitriding potential, and to thereby fonn a hardened case having a uniform
depth (e.g., Patent Literature 1). Another proposed technique is to use, in the
nitrogen penetration process, a jig having a surface made of a non-nitridable material
for placement of a workpiece to be nitrided in the treatment furnace (e.g., Patent
Literature 2).
[0008]
By using the nitriding parameter proposed by Patent Literature 1, it is possible
to inhibit the formation of the compound layer on the outermost surface in a short
time. However, sometimes, sufficient hardened case depth cannot be obtained for
certain characteristics required. Further, when a non-nitridable jig is prepared to
perform a fluorination process as proposed in Patent Literature 2, there are additional
problems such as selection of a jig and increased man hours.
CITATION LIST
PATENT LITERATURE
[0009]
Patent Literature 1: Japanese Patent Application Publication No. 2006-28588
Patent Literature 2: Japanese Patent Application Publication No. 2007-3 1759
SUMMARY OF INVENTION
[OO 101
An object of the present invention is to provide a method for nitriding low
alloy steels with which the formation of the compou~~lady er can be inhibited and
sufficient case hardness and hardened case depth can be achieved.
[OOl 11
A nitriding method according to the present embodiment includes a gas
nitriding step in which a low alloy steel is heated to a temperature ranging from 550
to 620°C in a gas atmosphere containing NH3, Hz, and Nz, and the gas nitriding step
being performed for a total process time of A ranging from 1.5 to 10 hours. The gas
nitriding step includes a step of performing a high KN value process and a step of
performing a low KN value process. The step of performing a high KN value
process is carried out witli a nitriding potential KNX detennined by Formula (1)
ranging from 0.15 to 1.50 and with an average value K Nof th~e nitr~iding~ pote~ntial
KNXt,h e average value K~xaver anging from 0.30 to 0.80, and the high KNv alue
process being perfonned for a process time of X in hours. The step of performing a
low KN value process is performed after the high KN value process has been
perfonned. The low KN value process is perfomled with a nitriding potential KNY
deternlined by the following Formula (I) ranging from 0.02 to 0.25 and witli an
average value K ~ ~ aovfe th e nitriding potential KNY,th e average value K ~ ~ arvaneg ing
from 0.03 to 0.20, and the low KN value process being perfonned for a process time
of Y in hours. An average nitriding potential value K~avcd etennined by Formula
(2) ranges from 0.07 to 0.30.
KNi = (NH3 partial pressure)/[(H? partial pre~sure)~'~] ... (I)
K~ave= (X X K~xave+ Y X K~yave)/A ... (2)
where i is X or Y.
[OO 121
With the nitriding method of the present embodiment, it is possible to inhibit
tlie fonnation of the compound layer and achieve sufficient hardened case depth.
BRIEF DESCRIPTION OF DRAWINGS
[OO 131
[FIG. I] FIG. 1 is a graph illustrating the relationships between the average value
KNxave of the nitriding potential of tlie high KN value process and the case hardness
and also the compound layer thickness.
[FIG. 21 FIG. 2 is a graph illustrating the relationships between the average value
K~yaveo f the nitriding potential of the low KNv alue process and the case hardness
and also the compound layer thickness.
[FIG. 31 FIG. 3 is a graph illustrating the relationships between the average nitriding
potential value K~avea nd the case hardness and also the compound layer thickness.
DESCRIPTION OF EMBODIMENTS
[00 141
Hereinafter, an embodiment of the present invention will be described in
detail with reference to the drawings. 'The same reference symbols will be used
throughout the drawings to refer to the same or like parts, and description thereof
will not be repeated.
[00 1 51
The present inventors searched for methods to reduce the thickness of the
compound layer, which is formed on the surface of a low alloy steel by a nitriding
process, and also to achieve a deep hardened case. Furtl~ennoret,h ey also searched
for methods to inhibit the formation of pores near the surface of the low alloy steel
due to gasification of nitrogen during a nitriding process (particularly during a
process with a high KN value). Consequently, the present inventors have made the
following findings (a) to (c).
[00 1 61
(a) KN Value in Gas Nitriding Process
Commonly, the KN value is defined by the following fonnula using the NH3
partial pressure and Hz partial pressure in the atmosphere of the furnace where the
gas nitriding process takes place (sometimes referred to as nitriding ahnosphere or
simply as ahnosphere).
KN = (NI-13 partial pressure)/[(Hz partial pre~sure)~'~]
[00 171
The KN value can be controlled by the gas flow rate. However, a certain
period of time is necessary before the KN value of the nitriding atmosphere reaches
an equilibrium after the flow rate is set. Thus, the KN value varies from moment to
moment before the KN value reaches the equilibrium. Also, when the KN value is
changed in the middle of the gas nitriding process, the KN value varies before
reaching the equilibrium.
[00 1 81
The KN value variation described above affects the compound layer, case
hardness, and hardened case depth. Therefore, by controlling the variation range of
the KN value during the gas nitriding process, as well as the average value of the KN
value, to be within a predetermined range, it will be possible to ensure sufficient
hardened case depth and also to inhibit the formation of the compound layer.
[0019]
(b) Colnpatibility of inhibiting Compound Layer Formation and Ensuring
Case Hardness and Hardened Case Depth, in Combination
A more effective way to form the hardened case is to use the compound layer
as a nitrogen supply source. In order to inhibit the formation of the compound layer
and to ensure the hardened case depth, the KN value lnay be controlled so that: the
compound layer can be formed during the first part of the gas nitriding process; and
the compound layer can be decolnposed during the latter part of the gas nitriding
process and substantially disappears at the end of the gas nitriding process.
Specifically, for the first part of the gas nitriding process, a gas nitriding process (a
high KN value process) with a high nitriding potential may be performed. Then, for
the latter part of the gas nitriding process, a gas nitriding process (a low KN value
process) with a nitriding potential lower than that of the high KN value process may
be performed. Consequently, the compound layer formed in the high KN value
process will decompose in the low KN value process, which will promote the
formation of the nitrogen diffision layer (hardened case). As a result, it is possible
to obtain nitrided parts in which the compound layer is inhibited and having a higher
case hardness and a deeper hardened case depth are available.
[0020]
(c) Inhibiting Pore Fonnation
When the compound layer is fonned by the nitriding process with a high KN
value in the first part of the gas nitriding process, a layer containing pores (referred
to as porous layer) sometimes forms. In such an instance, even after the nitrogen
diffusion layer (hardened case) has been formed by the decomposition of nitrides, the
pores sometimes remain as they are in the nitrogen diffusion layer. Pores remaining
in the nitrogen diffision layer will result in a decrease in fatigue strength and
straightenability (probability of cracking in the hardened case due to straightening
operation) of the nitrided parts. By regulating the upper limit of the KN value when
the compound layer is formed in the high KN value process, the formation of the
porous layer and pores can be inhibited to the greatest possible extent.
1002 11
The nitiding method of the present embodiment, which has been
accomplished based on the above findings, includes a gas nitriding step in which a
low alloy steel is heated to a temperature ranging from 550 to 620°C in a gas
atmosphere containing NH3, H2, and N2, and the total process time A ranges from 1.5
to 10 hours. The gas nitriding step includes a step of perfonning a high KN value
process and a step of perfonning a low KN value process. In the step of perfonning
a high KN value process, the nitriding potential KNX. determined by Fonnula (1)
ranges from 0.15 to 1 SO, the average value K~xave of the nitriding potential KNX
ranges from 0.30 to 0.80, and the process time is X in hours. The step of
perfonning a low KN value process is performed after the high KN value process has
been performed. In the low KN value process, the nitriding potential K N ~
determined by Fonnula (1) ranges from 0.02 to 0.25, the average value KN~ave of the
nitriding potential KNY ranges from 0.03 to 0.20, and the process time is Y in hours.
The average nitriding potential value K~aved etennined by Fonnula (2) ranges from
0.07 to 0.30.
KNi = (NH3 partial pressure)/[(H2 partial ... (I)
K~ave= (X X KN~ave Y X K~yave)/A ... (2)
where i is X or Y.
100221
With the nitriding method described above, it is possible to reduce the
thickness of the compound layer to be formed on the surface of a low alloy steel
while preferably inhibiting the formation of pores (porous layer) and further to obtain
high case hardness and a deep hardened case. Consequently, nitrided parts (low
alloy steel parts) produced by carrying out this nitriding process exhibit higher
mechanical properties including fatigue strength, wear resistance, and seizure
resistance and also exhibit higher straightenability.
[0023]
A nitrided part production method of the present embodiment includes a step
of preparing a low alloy steel and a step of performing the above-described nitriding
method on the low alloy steel to produce a nitrided part.
[0024]
A nitriding tnethod and nitrided part production method according to the
present embodiment will now be described in detail.
[0025]
[Nitriding Method]
The nitriding method according to the present embodiinent is designed to
perfonn a gas nitriding process on a low alloy steel. The process temperature for
the gas nitriding process ranges from 550 to 620°C and the process time A for the
entire gas nitriding process ranges from 1.5 to 10 hours.
100261
[Material to be Gas-Nitrided]
Firstly, a low alloy steel, for which the nitriding method of the present
embodiment is intended, is prepared. A low alloy steel as referred to in this
specification is defined as a steel including 93% by mass or more of Fe, or more
preferably, 95% by mass or more of Fe. Examples of low alloy steels as referred to
in this specification include carbon steels for machine structural use specified in JIS
G 4051, structural steels with specified hardenability bands specified in JIS G 4052,
and low-alloyed steels for machine structural use specified in JIS G 4053. The
contents of the alloying elements in the low alloy steel may fall outside the ranges
specified in the JIS standard mentioned above. 'The low alloy steel may fiirther
include, as necessary, an element that is effective in increasing the hardness of the
near-surface portion in the gas nitriding process, e.g., Ti, V, Al, or Nb, or other
elellients than these.
[0027]
[Process Temperature: 5 50 to 620°C]
The temperature of a gas nitriding process (nitriding temperature) largely
correlates with the nitrogen diffusion rate and affects the case hardness and the
hardened case depth. Too low a nitriding temperature leads to a slower nitrogen
diffusion rate, which will result in a lower case hardness and a shallower hardened
case depth. On the other hand, a nitriding temperature exceeding the Acl
temperature leads to formation, in the steel, of the austenite phase (y phase), in which
the nitrogen diffusion rate is slower than in the femte phase (a phase), and this will
result in a lower case hardness and a shallower hardened case depth. Accordingly,
in the present embodiment, the nitriding temperature is within a range of 550 to
620°C. This makes it possible to inhibit the decrease in case hardness and also to
inhibit the reduction in hardened case depth.
[0028]
[Process Time A for Entire Gas Nitriding Process: 1.5 to 10 hours]
In the present enlbodiment, the gas nitriding process is perfonned in an
atmosphere containing NH3, H2, and N2. The time period for the entire nitriding
process, i.e., the time period (process time A) from the beginning of the nitriding
process to the end thereof, correlates with the fonnation and decomposition of the
compound layer and with the penetration of nitrogen, and thus affects the case
hardness and the hardened case depth. Too short process time A will result in a
lower case hardness and a shallower hardened case depth. On the other hand, too
long process time A leads to denitrification, which will result in a decrease in the
case hardness of the steel. Furthermore, too long process time will result in an
increased production cost. Accordingly, the process time A for the entire nitriding
process is within the range of 1.5 to 10 hours.
[0029]
The atmosphere for the gas nitriding process of the present embodiment
inevitably contains impurities such as oxygen and carbon dioxide in addition to NtI3,
H3, and Nz. The atmosphere preferably contains NH3, H2, and N2 in a total amount
of 99.5% or more (by volume).
[0030]
[High KN Value Process and Low KN Value Process]
The above-described gas nitriding process includes a step of performing a
high KN value process and a step of performing a low KN value process. In the high
KN value process, the gas nitriding process is performed with a nitriding potential
KNX that is higher than that for the low KN value process. Further, after the high KN
value process, the low KN value process is perfonned. In the low KN value process,
the gas nitriding process is performed with a nitriding potential KNY that is lower
than that for the high KN value process.
In this manner, the two-stage gas nitriding process (high KN value process and
low KN value process) is performed in the present nitriding method. By using a
high nitriding potential KN value in the first part of the gas nitriding process (high KN
value process), a compound layer is formed on the surface of a low alloy steel.
Thereafter, by lowering the nitriding potential KN value in the latter part of the gas
nitriding process (low KN value process), the compound layer fonned on the surface
of the low alloy steel is decomposed to allow nitrogen to penetrate and diffiise into
the steel. By employing the two-stage gas nitriding process, sufficient hardened
case depth is achieved using the nitrogen resulting from the decomposition of the
colnpound layer while reducing the thickness of the co~npoundla yer.
LO0321
The nitriding potential of the high KN value process is denoted as KNX and the
nitriding potential of the low KN value process is denoted as KNY. Here, the
nitriding potential KNi (i is X or Y) is defined by Formula (1).
KNi = (NH3 partial pressure)/[(H2 partial pressure)3'2] ... (1)
[0033]
The partial pressures of NH3 and H2 in the atlnosphere for the gas nitriding
process can be controlled by regulating the gas flow rate. Accordingly, the nitriding
potential KNi can be regulated by the gas flow rate.
[0034]
When the gas flow rate is regulated to lower the KNv~al ue in the transition
from the high KN value process to the low KN value process, a certain period of time
is necessary before the partial pressures of NH3 and H2 in the furnace are stabilized.
The regulation of the gas flow rate to change the KNi value may be carried out one
time or several times (two or more times) as necessary. After the high KN value
process and before the low KN value process, the KNi value may be lowered once and
then be raised. The point in time at which the KNv~al ue after the high KN value
process falls to 0.25 or less for the last time is designated as the starting time of the
low KN value process.
[0035]
The process time of the high KN value process is denoted as "Xu (in hours)
and the process time of the low KN value process is denoted as "Y" (in hours). The
sum of the process time X and the process time Y is within the range of the process
time A for the entire nitriding process, and preferably equals the process time A.
[0036]
[Conditions for High KN Value Process and Low KN Value Process]
As described above, the nitriding potential in the high KN value process
determined by Fomlula (1) is denoted as "KNx". The nitriding potential in the low
KN value process determined by Fonnula (1) is denoted as "K~y'l. Further, the
average value of the nitriding potential during the high KN value process is denoted
as " K ~ x a v e " and tile average value of the nitriding potential during the low KN value
process is denoted as " K ~ ~ a v e " .
[0037]
Further, the average nitriding potential value of the entire nitriding process is
denoted as "KNave". The average value KNave is defined by Formula (2).
K ~ a v=c (X X K ~ x a v e+ Y X K ~ y a v e ) / A ... (2)
[003 81
In the nitriding method according to the present embodiment, the nitriding
potential KNXo f the high KN value process, the average value K ~ x a v e ,th e process
time X, the nitriding potential KNY of the low KN value process, the average value
KNyave, the process time Y, and the average value KNave satisfy the following
conditions (1) to (IV).
(I) Average value KNxave: 0.30 to 0.80
(11) Average value KNyave: 0.03 to 0.20
(111) KNX:0 .15 to 1.50 and KNY0: .02 to 0.25
(IV) Average value K ~ a v e :0 .07 to 0.30
The conditions (I) to (IV) will be described below.
[0039]
[(I) Average Value KNxave of Nitriding Potential in High KN Value Process]
In the high KNv alue process, the average value K ~ x a v oe f the nitriding
potential ranges from 0.30 to 0.80.
[0040]
FIG. 1 is a graph illustrating the relationships between the average value
KNxave of the nitriding potential of the high KN value process and the case hardness
and also the compound layer thickness. FIG. 1 was obtained from the following
experiment.
[004 1 ]
The gas nitriding process was performed in a gas atmosphere containing NH3,
H2, and N2 using SCr420 (hereinafter referred to as a test specimen), which is a JIS G
4053 low-alloyed steel for machine structural use. In the gas nitriding process, test
specimens were placed into a furnace with atmosphere control capability which had
been heated to a predetermined temperature, and NH3, N2, and H2 gases were flowed
thereinto. During that time, the nitriding potential KNi value was controlled by
regulating the gas flow rate while measuring the partial pressures of NH3 and H2 in
the atmosphere for the gas nitriding process. The KNv~a lue was determined by
Fornlula (1) using the NH3 partial pressure and H2 partial pressure.
100421
The H2 partial pressure during the gas nitriding process was measured, using a
thennal conductivity H2 sensor directly attached to the gas nitriding furnace body, by
converting the thennal conductivity difference between the reference gas and the
measured gas into a gas concentration. 'The H2 partial pressure was continuously
measured during the gas nitriding process. The NH3 partial pressure during the gas
nitriding process was measured with a manual glass tube NH3 spectrometer attached
outside the furnace, by which the partial pressure of the residual NH3 was calculated
and determined every 15 minutes. The nitriding potential KNi value was calculated
every 15 minutes at which the NH3 partial pressure was measured, and the NH3 flow
rate and the N2 flow rate were regulated so as to converge to the target values.
[0043]
In the gas nitriding process, the temperature of the atmosphere was 590°C, the
process time X was 1.0 hour, the process time Y was 2.0 hours, K~yavew as 0.05, all
of which were constant, and K ~ x a v ew as varied within the range of 0.10 to 1.00.
The total process time A was 3.0 hours.
[0044]
The test specimens that had been gas nitrided with various average values
K~xavew ere subjected to the following measurement test.
[0045]
[Measurement of Thickness of Colnpound Layer]
After the gas nitriding process, the cross section of the test specimen was
polished and etched to be observed with an optical microscope. The etching was
carried out with a 3% nital solution for 20 to 30 seconds. The compound layer
exists on the outer layer of the low alloy steel and can be observed as a white nonetched
layer. Using structure micrographs of five visual fields (field area: 2.2 x lo4
pm2) taken with an optical microscope at a magnification of 500x, the thickness of
the compound layer was measured at every 30 pm at four points for each field. The
average value of values measured at the 20 points was designated as the co~npound
layer thickness (pm). When the compound layer thickness is not more than 3 pm,
the occurrences of delamination and cracking are significantly inhibited.
Accordingly, in the present embodiment, the target compound layer thickness was set
to not more than 3 pm.
[0046]
[Measurement of Pore Area Fraction]
Furthermore, the area fraction of pores in the colnpound layer in the cross
section of the test specimen was measured by optical microscope observation. The
measurement was made on five fields (field area: 5.6 x 103pm2) at a magnification of
1000x, and for each field, the percentage of pores (hereinafter referred to as a pore
area fraction) in an area of 25 pm2 at a depth of 5 pm from the outennost surface was
calculated. If the pore area fraction is not less than lo%, the nitrided parts after the
gas nitriding process will have a rough surface roughness, and further, the nitrided
parts will exhibit decreased fatigue strength due to embrittlement of the compound
layer. Accordingly, in the present embodiment, the target pore area fraction was set
to less than 10%.
[0047]
[Measurement of Case Hardness]
Furthermore, the case hardness and effective hardened case depth of the gas
nitrided test specimen were determined by the following method. The Vickers
hardness in the depth direction from the test specimen surface was measured in
accordance with JIS Z 2244 with a test force of 1.96 N. The average value of the
Vickers hardnesses at three points at a position of 50 pm depth from the surface was
designated as the case hardness (HV). Conlmon gas nitriding processes, by which a
compound layer more than 3 pln thick is left, provide a case hardness of 270 to 3 10
HV for JIS Standard S45C or a case hardness of 550 to 590 HV for JIS Standard
SCr420. Accordingly, in the present embodiment, the target case hardness was set
to not less than 290 HV for S45C and not less than 570 for SCr420.
[0048]
[Measurement of Effective Hardened Case Depth]
The Vickers hardness was measured at positions of 50 pm, 100 pm, and every
50 p from 100 pm to 1000 pm depth from the surface and, using the obtained
hardness distribution in the depth direction, the effective hardened case depth was
determined in the following manner. For S45C, in the distribution of Vickers
hardnesses measured in the depth direction from the surface, the depth up to which
the hardness is 250 HV or more was designated as tlle effective hardened case depth
(pm). For SCr420, in the distribution of Vickers hardnesses measured in the depth
direction from the surface, the depth up to which the hardness is 300 HV or more
was designated as the effective hardened case depth (pm).
[0049]
At process temperatures of 570 to 590°C, colnnlon gas nitriding processes, by
which a compound layer 10 pm or more thick is formed, provide an effective
hardened case depth within the range of the value obtained by Formula (A) +20 pm.
Effective hardened case depth (pm) = 130 x {process time A (in hours)} I/' ...
(A)
[0050]
Accordingly, in the present embodiment, the target effective hardened case
depth was set to satisfying Fonnula (B).
Effective hardened case depth (pn) 2 130 x (process time A (in hours)) "' ...
(B)
[005 I]
The results from tlle above-described measurement test indicated that, when
the average value K ~ ~ awvaes 0.20 or more, the effective hardened case depth
satisfied Formula (B) (when A = 3, the effective hardened case depth was 225 pm).
Furthermore, FIG. 1 was generated based on the case hardnesses and compound
layer thicknesses of the test specimens, among the measurement test results, obtained
from the gas nitriding processes with the respective average values K ~ x a v e .
[0052]
The solid line in FIG. 1 is a graph representing the relationship between the
average value K N ~ a v eo f the nitriding potential of the high KNv alue process and the
case hardness (Hv). The dashed line in FIG. 1 is a graph representing the
relationship between the average value KNxave of the nitriding potential of the high
KN value process and the thickness (pm) of the compound layer. Referring to the
graph of the solid line in FIG. 1, provided that the average value K ~ ~ a ovf eth e low
KN value process is constant, the case hardness of the nitrided part significantly
increases with the increase in the average value K ~ x a v ien the high KN value process.
Then, when the average value K ~ x a v he a s reached or exceeded 0.30, the case
hardness reaches or exceeds 570 HV, which is the target for SCr420 test specimens.
On the other hand, when the average value K ~ ~ aivs hei gher than 0.30, the case
hardness remains substantially constant even with krther increase in the average
value KNxave. That is, in the graph plotting the case hardness versus average value
K ~ x n v (es olid line in FIG. I), an inflection point exists around the point of K ~ x a v e=
0.30.
LO0531
Further, refemng to the graph of the dashed line in FIG. 1, the compound
layer thickness significantly decreases with the decrease in the average value K ~ x a v e
from 1.00. Then, when the average value KNxave has reached 0.80, the thickness of
the compound layer reaches or falls below 3 pm. On the other hand, in the range
where the average value K ~ x a v eis not more than 0.80, the thickness of the compound
layer decreases with the decrease in the average value K ~ x a v e b, ut the rate of decrease
in the thickness of the compound layer is smaller than in the range where the average
value KNxave is higher than 0.80. That is, in the graph plotting the case hardness
versus average value KNxave (solid line in FIG. I), an inflection point exists around
the point of KNX,,, = 0.80.
[0054]
Based on the above results, the present embodiment specifies the average
value K ~ x a v oe f 0.30 to 0.80 for the nitriding potential of the high KN value process.
This makes it possible to increase the case hardness of the riitrided low alloy steel
and to inhibit the thickness of the compound layer. Furthennore, it is possible to
achieve sufficient effective hardened case depth. If the average value KNxave is less
than 0.30, the compound production will be insufficient, which results in a decrease
in the case hardness, and therefore it is impossible to achieve sufficient effective
hardened case depth. If the average value KNxave is more than 0.80, the thickness of
the compound layer will exceed 3 Inn, and further, the pore area fiaction can be 10%
or more. A preferred lower limit of the average value KNxave is 0.35. A preferred
upper limit of the average value KNxave is 0.70.
[0055]
[(II) Average Value K ~ y a v oef Nitriding Potential of Low KNV alue Process]
The average value KNyave of the nitriding potential of the low KN value
process ranges from 0.03 to 0.20.
[0056]
FIG. 2 is a graph illustrating the relationships between the average value
K ~ a v oef the nitriding potential of the low KNv alue process and the case hardness
and also the compound layer thickness. FIG. 2 was obtained fiom the following
test.
LO0571
Gas nitriding processes were perfonned on test specimens having a chemical
composition corresponding to that of SCr420, with a nitriding atmosphere
temperature of 590°C, a process time X of 1.0 hour, a process time Y of 2.0 hours,
and an average value K ~ x a v oe f 0.40, each of which is constant, and with average
values K ~ y a v ve a ried from 0.01 to 0.30. The total process time A was 3.0 hours.
After the nitriding process, the case hardness (HV), the effective hardened case depth
(pn), and the compound layer thickness (pm) were measured at each average value
KNyave using the above-described technique. Measurement of the effective
hardened case depths revealed that, when the average value KNyave was not less than
0.02, the effective hardened case depth was 225 pm or more. Further, the case
hardnesses and compound layer thicknesses obtained from the measurement test
were plotted to generate FIG. 2.
[0058]
In FIG. 2, the solid line is a graph representing the relationship between the
average value K ~ ~ a ovf eth e nitriding potential of the low KNv alue process and the
case hardness, and the dashed line is a graph representing the relationship between
the average value K ~ ~ a ovf eth e nitriding potential of the low KNv alue process and
the compou~ldla yer depth. Referring to the graph of the solid line in FIG. 2, the
case hardness significantly increases with the increase in the average value K ~ y a v e
from zero. When KNyave has reached 0.03, the case hardness reaches or exceeds
570 HV. Furthermore, when K ~ ~ a ivs e0. 03 or more, the case hardness remains
substantially constant even with an increase in K ~ y a v e . The above indicates that, in
the graph plotting the case hardness versus average value KNyavc, an inflection point
exists around the point of the average value K ~ ~ a=v 0e.0 3.
[0059]
On the other hand, referring to the graph of the dashed line in FIG. 2, the
thickness of the compound layer remains substantially constant in the average value
K ~ y a v rea nge of from 0.30 down to 0.25. However, the thickness of the compound
layer significantly decreases with the decrease in the average value K ~ ~ a fvroem 0.25.
Then, when the average value K ~ ~ a hvaes reached 0.20, the thickness of the
compound layer reaches or falls below 3 pn. In the range where the average value
K ~ h vies not more than 0.20, the thickness of the compound layer decreases with the
decrease in the average value K ~ ~ a vbeu,t the rate of decrease in the thickness of the
compound layer is smaller than in the range where the average value K ~ y a v eis higher
than 0.20. The above indicates that, in the graph plotting the thickness of the
compound layer versus average value K ~ ~ a vaen, i nflection point exists around the
point of the average value K ~ y a v=e 0.20.
[0060]
Based on the above results, the present embodiment specifies the average
value KNyave of 0.03 to 0.20 for the low KN value process. This makes it possible to
increase the case hardness of the gas nitrided low alloy steel and to inhibit the
thickness of the compound layer. Furthermore, it is possible to achieve sufficient
effective hardened case depth. If the average value K ~ ~ a ivs lee ss than 0.03,
denitrification will occur at the surface, resulting in a decrease in the case hardness.
On the other hand, if the average value KNyave is more than 0.20, decomposition of
the compound will be insufficient, resulting in a shallow effective hardened case
depth and thus a decrease in the case hardness. A preferred lower limit of the
average value K~~aivse 0 .05. A preferred upper limit of the average value KNyave is
0.18.
[006 11
[(111) Ranges of Nitriding Potentials KNX and KNY during Nitriding Process]
In a gas nitriding process, a certain period of time is necessary before the KNi
value of the atmosphere reaches an equilibrium after the gas flow rate is set. Thus,
the KNv~al ue varies from moment to lnolnent before the KNi value reaches the
equilibrium. Furthennore, at the transition from the high KN value process to the
low KN value process, the setting of the KNi value is to be altered during the gas
nitriding process. Also in this instance, the KNi value varies before reaching the
equilibrium.
[0062]
Such variations in the KNv~al ue affect the compound layer thickness and the
hardened case depth. Accordingly, in the high KN value process and low KN value
process, not only the above-described average value K~xave and average value K~~ave
are controlled to be within the above range, but also the nitriding potential KNX
during the high KN value process and the nitriding potential KNY during the low KN
value process are controlled to be within a predetermined range.
100631
Specifically, the present embodiment specifies that the nitriding potential KNX
during the high KN value process be within a range of 0.15 to 1.50 and that the
nitriding potential KNY during the low KN value process be within a range of 0.02 to
0.25.
[0064]
Table 1 shows compound layer thicknesses (pm), pore area fractions (%),
effective hardened case depths (pm), and case hardnesses (HV) of nitrided parts
obtained from nitriding processes performed with various nitriding potentials KNX
and KNY. Table 1 was obtained from the following test.
100651
[Table I]
[0066]
Using SCr420 test specilnens, the gas nitriding processes shown in 'Table 1
(high KN value process and low KN value process) were perfonned on them to
produce nitrided parts. Specifically, for each gas nitriding process of each test
number, the ambient temperature was 590°C, the process tilrie X was 1.0 hour, the
process time Y was 2.0 hours, KNxave was 0.40, and K~yavew as 0.10, all of which
were constant. The high KN value processes and low KN value processes were
perfonned with various minimum KNXv alues K~~miinni,n imum KNYv alues KNymin,
maximum KNXv alues K~~maaxn,d maximum KNYv alues KNylnax ill the gas nitriding
processes. The process time A for the entire nitriding process was 3.0 hours. 'The
co~npoundla yer thickness, pore area fraction, effective hardened case depth, and
case hardness of each nitrided part after the gas nitriding process were measured
using the above-described measurement technique to obtain Table 1.
[0067]
Refening to Table 1, in Tests Nos. 3 to 6 and Nos. 10 to 15, the minimum
value K~xmin and maximum value K~xmax ranged from 0.15 to 1.50 and the minimum
value KN~mina nd maximum value KN~,naxr anged from 0.02 to 0.25. As a result,
their compound layers were thin at 3 pm or less and pores therein were reduced to
less than 10%. Further, their effective hardened case depths were not less than 225
pm and the case hardnesses were not less than 570 HV. In all numbers of tests in
Table 1, the values obtained by Formula (A) (target values for effective hardened
case) were 225 pm, and the effective hardened case depths of the above-mentioned
test numbers were not less than 225 lim while satisfying Formula (B).
[0068]
In contrast, in Tests Nos. 1 and 2, KNxmin was less than 0.1 5 and, as a result,
the case hardness was less than 570 HV. Furthermore, in Test No. 1, KNxmin was
less than 0.14 and, as a result, the effective hardened case depth was less than 225
CLm.
[0069]
In Tests Nos. 7 and 8, K~xmax was more than 1.5 and, as a result, pores
constituted 10% or more of the compound layer. Furthermore, in Test No. 8,
KNxlnax was more than 1.55 and, as a result, the thickness of the conlpound layer was
more than 3 pm.
[0070]
I11 Test No. 9, K~ylninw as less than 0.02 and, as a result, the case hardness
was less than 570 HV. This is considered to be because the low KN value process
not only eliminated the conlpound layer but also caused denitrification at the outer
layer. In Test No. 16, KNymax was more than 0.25. As a result, the thickness of the
compound layer was more than 3 pm. This is considered to be because sufficient
decomposition did not occur due to the KNymax of nlore than 0.25.
[007 11
Based on the above results, the nitriding potential KNX ranging from 0.15 to
1.50 is specified for the high KN value process, and the nitriding potential KNY
ranging from 0.02 to 0.25 is specified for the low KN value process. This makes it
possible to sufficiently reduce the thickness of the compound layer of the nitrided
parts and also to inhibit pores therein. Furthermore, it is possible to achieve
sufficient depth of the effective hardened case depth and obtain high case hardness.
[0072]
If the nitriding potential KNX is less than 0.15, the effective hardened case will
be too shallow andlor the case hardness will be too low. If the nitriding potential
KNX is more than 1 .50, the compound layer will become too thick and/or excessive
amounts of pores will remain.
[0073]
If the nitriding potential KNY is less than 0.02, denitrification will occur,
resulting in a decrease in the case hardness. On the other hand, if the nitriding
potential KNY is more than 0.20, the compound layer will become too thick.
Accordingly, in the present embodiment, the nitriding potential KNX during the high
KN value process is within the range of 0.15 to 1 .50, and the nitriding potential Kw
during the low KN value process is within the range of 0.02 to 0.25.
[0074]
A preferred lower limit of the nitriding potential KNX is 0.25. A preferred
upper lirnit of KNX is 1.40. A preferred lower limit of KNY is 0.03. A preferred
upper limit of Kw is 0.22.
[0075]
[(IV) Average Nitriding Potential Value K~ave throughout Nitriding Process]
The gas nitriding process of the present embodiment further specifies that the
average nitriding potential value K~aved efined by Fonnula (2) be within a range of
0.07 to 0.30.
K~ave = (X X K~xave + Y X K~yave)/A ... (2)
[0076]
FIG. 3 is a graph illustrating the relationships between the average nitriding
potential value K~ave and the case hardness (HV) and also the compound layer
thickness (pm). FIG. 3 was obtained by conducting the following test. Using
SCr420 test specimens, gas nitriding processes were performed thereon. The
specified ambient temperature for the gas nitriding processes was 590°C. Using
various process times X, process times Y, and nitriding potential ranges and average
values (KNx,K NY,K Nxave, K~~aveth),e gas nitriding processes (high KNv alue process
and low KN value process) were performed. The effective hardened case depths,
compound layer thicknesses, and case hardnesses of the gas nitrided test specimens
under the respective test conditions were measured using the above-described
technique. As a result, it was found that, when the average value KNave is not less
than 0.06, the effective hardened case depth satisfies Formula (B). Further, the
resultant compound layer thicknesses and case hardnesses were measured to generate
FIG. 3.
[0077]
The solid line in FIG. 3 is a graph representing the relationship between the
average nitriding potential value K~avea nd the case hardness (HV). The dashed line
in FIG. 3 is a graph representing the relationship between the average nitriding
potential value K~avea nd the thickness (pm) of the compound layer.
LO0781
Referring to the graph of the solid line in FIG. 3, the case hardness
significantly increases with the increase in the average value K~avefr om zero and, at
the average value K~aveo f 0.07, it reaches or exceeds 570 HV. In the range where
the average value K~aveis 0.07 or more, the case hardness remains substantially
constant even with the increase in the average value K~ave. That is, in the graph
plotting the case hardness (HV) versus average value K ~ a v o a, n inflection point exists
around the point of the average value K ~ a v=e 0.07.
LO0791
Further, refemng to the graph of the dashed line in FIG. 3, the compound
layer thickness significantly decreases with the decrease it1 the average value K ~ a v e
from 0.35 and, at the average value KNave of 0.30, it reaches or falls below 3 pm. In
the range where the average value K ~ a v ies less than 0.30, the thickness of the
colnpound layer gradually decreases with the decrease in the average value K ~ a v e b, ut
the rate of decrease in the thickness of the compound layer is smaller than in the
range where the average value K ~ a v eis higher than 0.30. The above indicates that,
in the graph plotting the thickness of the compound layer versus average value K ~ a v e ,
an inflection point exists around the point of the average value KNave = 0.30.
[OOSO]
Based on the above results, the gas nitriding process of the present
embodiment specifies that the average value K ~ a v de e fined by Fonnula (2) be within
the range of 0.07 to 0.30. This makes it possible to obtain gas nitrided parts having
a sufficiently thin conlpound layer. Further, it is possible to obtain high case
hardness. If the average value K ~ a v ies less than 0.07, the case hardness will be low
and the effective hardened case will be shallow. On the other hand, if the average
value K ~ a v ies more than 0.30, the compound layer will be more than 3 pm. A
preferred lower limit of the average value KNave is 0.08. A preferred upper limit of
the average value K ~ a v ies 0.27. When the average value K ~ a v ies 0.06 or more, the
effective hardened case depth satisfies Formula (B).
[OOS I]
[Process Times of High KN Value Process and Low KN Value Process]
The process time X of the high KN value process and the process time Y of
the low KN value process are not particularly limited as long as the average value
KNave defined by Formula (2) is within the range of 0.07 to 0.30. Preferably, the
process time X is not less than 0.50 hours and the process time Y is not less than 0.50
hours.
[0082]
Under the above conditions, the gas nitriding process is perfonned.
Specifically, the high KN value process is performed under the above conditions and
thereafter the low KN value process is performed under the above conditions. After
the low KN value process, the gas nitriding process is tenninated without increasing
the nitriding potential.
[0083]
Nitrided parts are produced by performing the above gas nitriding process.
The produced nitrided parts (made of low alloy steel) have sufficiently high case
hardness and a sufficiently thin compound layer. Further, their effective hardened
case depths are sufficiently deep and the pores in their compound layers are inhibited.
Preferably, nitrided parts produced by performing the nitriding process of the present
embodiment have a case hardness of 570 HV or more (when the nitrided parts are
made of SCr420) or a case hardness of 290 HV or more (when the nitrided parts are
made of S45C), both on the Vickers hardness scale, with a compound layer depth of
not more than 3 pm. Further, they satisfy Fonnula (B). Further, their pore area
fractions are less than 10%.
EXAMPLES
[0084]
A JIS SCr420 steel (JIS G 4053 low-alloyed steel for machine structural use)
and a JIS S45C steel (JIS G 405 1 carbon steel for machine structural use) were each
melted in a 50 kg vacuum hrnace to form molten steels. The molten steels were
cast into ingots. The ingots were hot forged into steel bars having a diameter of 20
mm.
[0085]
The steel bar of SCr420 was subjected to a normalizing treatment to
homogenize the structure and then subjected to quenching and tempering. In the
normalizing treatment, the steel bar was heated to 920°C and held for 30 minutes and
then air cooled. In the quenching treatment, the steel bar was heated to 900°C and
held for 30 minutes and then water cooled. In the tempering treatment, the steel bar
was held at 600°C for one hour.
[0086]
The steel bar of S45C was heated to 870°C and held for 30 minutes and then
air cooled.
[0087]
Test specimens measuring 15 mm x 80 mn x 5 mm were cut from the
produced steel bar by machining.
[0088]
Gas nitriding processes were performed on the cut test specimens under the
following conditions. The test specimens were loaded into a gas nitriding furnace,
and an NH3 gas, a H2 gas, and a Nz gas were introduced into the fiirnace.
Subsequently, high KN value processes under the conditions shown in Table 2 were
performed, which were followed by low KN value processes. The gas nitrided test
specimens were subjected to oil cooling using oil at 80°C.
LO0891

[0090]
[Measurement Test for Compound Layer 'Thickness and Pore Area Fraction]
The cross sections perpendicular to the lengthwise direction of the gas
nitrided test specimens were mirror polished and etched. The etched cross sections
were observed with an optical microscope to measure the compound layer thickness
and investigate whether the pores in the near-surface portion were present. The
etching was carried out with a 3% nital solution for 20 to 30 seconds.
[009 11
The compound layer is identifiable as a wliite non-etched layer present at the
outer layer. Compound layers were observed in structure micrographs of five fields
(field area: 2.2 x 104pm') taken at a magnification of 500x and the thickness of the
compound layer was measured every 30 pm at four points for each field. The
average value of values measured at the 20 points was designated as the compound
layer thickness (pm).
[0092]
Further, the etched cross sections were each observed at five fields at a
magnification of 1000x to determine the proportion of pores in an area of 25 pm2 at a
depth of 5 pm from the outennost surface (pore area fraction, in %).
[0093]
[Measurement Test for Case Hardness and Effective Hardened Case]
Vickers hardnesses of the gas nitrided steel bars of the respective test numbers
were measured at positions of 50 pm, 100 pm, and every 50 pm from 100 pn to
1000 pm depth from the surface, with a test force of 1.96 N, in accordance with JIS
Z 2244. The Vickers hardnesses (I-IV) were measured at three points for each and
the average values thereof were determined. The case hardness was defined as the
average value of values at three points positioned 50 pm from the surface.
[0094]
Based on the measured Vickers hardnesses, effective hardened case depths of
the steel bars of the respective test numbers were determined in the following manner.
For SCr420 (Test Nos. 26 to 30), in the distribution of Vickers hardnesses measured
in the depth direction from the surface, the depth up to which the hardness is 300 HV
or more was designated as the effective hardened case depth (pm). For S45C (Test
Nos. 21 to 25), in the distribution of Vickers hardnesses measured in the depth
direction from the surface, the depth up to which the hardness is 250 1-IV or more
was designated as the effective hardened case depth (pm).
[0095]
Compound layer tl~icknesseso f not more than 3 pm, pore percentages of less
than lo%, and case hardnesses of not less than 290 HV for S45C or not less than 570
HV for SCr420 were evaluated as being good. Further, effective hardened case
depths of not less than 225 HV with Fonllula (B) satisfied were evaluated as being
good.
[0096]
[Test Results]
The results are shown in Table 2. In Table 2, the "Effective hardened case
depth (target)" section lists values (target values) calculated by Fornlula (A) and the
"Effective hardened case depth (actual values)" lists measured values (pm) of the
effective hardened cases. Referring to Table 2, in Tests Nos. 2 1 to 23 and Tests
Nos. 26 to 28, the process temperatures for the gas nitriding processes were within
the range of 550 to 620°C and the process times A were within the range of 1.5 to 10
hours. Further, in the high KN value processes, KNXSw ere within the range of 0.15
to 1.50 and the average values K~xavew ere within the range of 0.30 to 0.80. Further,
in the low KNv alue processes, KNYSw ere within the range of 0.02 to 0.25 and the
average values KNyave were within the range of 0.03 to 0.20. Further, the average
values K~aved etermined by Fonnula (2) were within the range of 0.07 to 0.30. As a
result, in each of the test numbers, after the nitriding processes, the thiclulesses of the
compound layers were not more than 3 pm and the pore area fractions were less than
10%. Further, the effective hardened cases were not less than 225 pm and Formula
(B) was satisfied. Further, S45Cs of Test Nos. 21 to 23 each had a case hardness of
not less than 290 IHV and SCr420s of Test Nos. 26 to 28 each had a case hardness of
not less than 570 HV.
[0097]
In Test No. 24, the maximum KNX value in the high KN value process was
more than 1.50. As a result, the pore area fraction was not less than 10%.
[0098]
In Test No. 25, in the high KN value process, the nlini~numK NXv alue was
less than 0.15 and the average value KNxave was less than 0.30. Further, the average
value K ~ a v we as less than 0.07. As a result, the depth of the effective hardened case
was less than the value defined by Fonnula (R) and the case hardness was less than
290 HV.
[0099]
In Test No. 29, in the low KN value process, KNY was more than 0.25 and the
average value K~yavew as more than 0.20. Further, the average value K ~ a v we as
more than 0.30. As a result, the thickness of the compound layer was more than 3
pm.
[O loo]
In Test No. 30, the average value KNyave in the low KN value process was less
than 0.03. As a result, the case hardness was less than 570 HV.
[OlOl]
In the foregoing specification, an embodiment of the present invention has
been described. However, the above embodiment is merely an illustrative example
by which the present invention is implemented. Accordingly, the present invention
is not limited to the above embodiment, and lnodifications of the above embodiment
may be made appropriately without departing from the spirit and scope of the
invention.

We claim:
1. A nitriding method, comprising a gas nitriding step in which a low alloy steel
is heated to a temperature ranging from 550 to 620°C in a gas atmosphere co~itainirig
NH3, H2, and N,, the gas nitriding step being perfonned for a total process time of A
ranging from 1.5 to 10 hours,
the gas nitriding step including the steps of
perfonning a high KN value process with a nitriding potential KNX determined
by Formula (1) ranging from 0.1 5 to 1.50 and with an average value KNxa,e of the
nitriding potential KNXt,h e average value K~xaver anging from 0.30 to 0.80, the high
KN value process being perfonned for a process time of X in hours, and
perfonning a low KN value process after the high KN value process, the low
KN value process being performed with a nitriding potential KNY detemlined by
Fonnula (1) ranging from 0.02 to 0.25 and with an average value KNyave of the
nitriding potential KNYt,h e average value KNyave ranging from 0.03 to 0.20, the low
KN value process being performed for a process time of Y in hours,
wherein an average nitriding potential value KNave detennined by Formula (2)
ranges from 0.07 to 0.30,
KNi = (NH3 partial pressure)/[(H? partial pressure)3J2] ... (I)
K~ave= (X X K~xave+ Y X K~~ave)lA ... (2)
where i is X or Y.
2. A method for producing a nitrided part, the method comprising the steps of:
preparing a low alloy steel, and
performing the nitriding method according to claim 1 on the low alloy steel to
produce the nitrided part.