FIELD
[0001] The present invention relates to a method of manufacturing a rotor to be used for
a steam turbine.
BACKGROUND
[0002] Steam turbines include those being used for geothermal power generation and also
those to be used for geothermal power generation. For geothermal power generation,
steam stored underground is fed to a steam turbine installed on the ground to drive
the rotor of the steam turbine to rotate by steam force. The temperature of natural
steam that is fed to steam turbines for geothermal power generation is lower than
that of steam that is fed to steam turbines for ordinary thermal power generation
and typically about 200 °C. Natural steam that is fed to steam turbines for geothermal
power generation may contain corrosive gases such as hydrogen sulfide that corrode
metals.
[0003] The rotors being employed for geothermal power generation can give rise to cracking
due to hydrogen embrittlement (to be simply referred to as "hydrogen cracking" hereinafter).
Therefore, materials for forming steam turbines (to be referred to as "rotor materials"
hereinafter) are required to have an ability of not giving rise to cracking that is
attributable to hydrogen embrittlement (to be referred to as "hydrogen cracking resistance"
hereinafter) in addition to satisfactory mechanical properties in terms of tensile
strength, yield strength and tenacity and corrosion resistance.
[0004] Thus, 1 to 2 % Cr-based materials prepared on the basis of 1 % CrMoV steel, which
has a rich record of use as rotor material for steam turbines for thermal power generation,
in order to provide improved toughness are being employed more often than not for
steam turbines for geothermal power generation from the viewpoint of securing the
hydrogen cracking resistance and the corrosion resistance that are required to rotors
of the type under consideration as described above.
[0005] Meanwhile, of the rotor of a steam turbine for geothermal power generation, the regions
that are initially exposed to natural steam may be operated in a highly corrosive
environment if compared with the other region of the rotor. Those regions are accompanied
by a problem that they are particularly liable to give rise to hydrogen cracking and
such hydrogen cracking easily and quickly develops. Therefore, there is a strong demand
for steam turbine rotors that show a particularly improved resistance against hydrogen
cracking.
[0006] JPS6369919 describes a method to manufacture a rotor for a steam turbine having superior creep
strength at a part heated to high temperature during use, by manufacturing a rotor
for a steam turbine with a low alloy steel, hardening a part heated to high temperature
during use and the remaining parts at different temperatures and tempering them at
different temps.
[0007] US6344098 describes a heat treatment process that produces a monoblock, low alloy steel rotor
for use in low pressure steam turbines. The process includes austenitizing the rotor
at substantially uniformly applied treatment temperature of about 840°C, quenching
the rotor, and then differentially tempering the rotor at different axial locations.
[0008] JPS5644722 describes a method to manufacture a rotor shaft in which high pressure parts and
low pressure parts made of the same material are formed in one piece and which satisfy
the characteristics required for the lower pressure parts by increasing its toughness,
by subjecting CR-Mo-V steel to specific heat treatment after it is shaped by forging.
[0009] JPS5538968 describes a method to enhance the high temperature strength of a turbine rotor by
setting a partition plate between the drum of the rotor and each journal and each
coupling to differently heat treat the partitioned parts.
[0010] EP0719869 describes a rotor forging composed to Cr-Mo-V type alloy based on iron being normalizing-treated
at a temperature of from 1000 to 1150°C, the temperature is maintained at 650-750°C
on the way of cooling the temperature from the normalizing-treating temperature to
pearlite transform the microstructure of the rotor forging, the portions of the rotor
forging corresponding to a high pressure or middle pressure portion are quenched at
940-1020°C and the portion corresponding to the low pressure portion is quenched at
850-940°C after the treatment is carried out at 920-950°C once or more times, and
the rotor forging is subjected to tempering at 550-700°C once or more times.
Brief Description of the Drawings
[0011]
FIG. 1 is a schematic illustration of the configuration of an exemplar rotor of a
steam turbine of the first and second embodiments and a peripheral area of the rotor;
FIG. 2 is a table illustrating exemplar materials that can be used for the rotors
of the steam turbines of the first and second embodiments according to the present
invention;
FIG. 3 is a table illustrating the heat treatment conditions of the first and second
embodiments of manufacturing method, some examples of which are according to the present
invention, and some of which are examples useful for understanding the present invention;
FIG. 4 is a table illustrating the results of determining the grain size (G.S. No.),
the residual stress (maximum tensile stress) and the hydrogen cracking resistance
of each of the sample materials that have been subjected to the heat treatments of
the first and second embodiments of manufacturing method, some examples of which are
according to the present invention, and some of which are examples useful for understanding
the present invention;
FIG. 5 is a table illustrating the heat treatment conditions of Comparative Examples;
and
FIG. 6 is a table illustrating the results of determining the grain size (G.S. No.),
the residual stress (maximum tensile stress) and the hydrogen cracking resistance
of the sample materials that have been subjected to the heat treatments of Comparative
Examples.
DETAILED DESCRIPTION
[0012] The problem to be solved by the present invention is to provide a method of manufacturing
a rotor to be used for a steam turbine (to be referred to as steam turbine rotor hereinafter)
that shows an improved hydrogen cracking resistance at predetermined regions that
are the target of improvement.
[0013] The invention is defined in the appended claims. The present invention can improve
the hydrogen cracking resistance at predetermined regions of a steam turbine rotor
that are the target of improvement.
[0014] Now, the present invention will be described in greater detail by referring to the
accompanying drawings that illustrate preferred embodiments of the present invention.
[First Embodiment]
[0015] An exemplar steam turbine rotor, to which the first embodiment of manufacturing method
according to the present invention is applicable, will be described below by referring
to FIG. 1. FIG. 1 is a schematic illustration of the steam turbine rotor of this embodiment
and the configuration of peripheral regions thereof. Note that the lower side of the
rotor blades, the fixed vanes and the casing are omitted in FIG. 1 for the purpose
of easy understanding.
[0016] As shown in FIG. 1, steam turbine 1 is an axial flow type turbo fluid machinery that
has a casing 5, to the inside of which fixed vanes 3 are coupled, and a steam turbine
rotor (to be referred to simply as "rotor" hereinafter) 10 designed to rotate around
a center axis of rotation (indicated by dotted chain line A in FIG. 1) in the casing
5. The rotor 10 has a substantially cylindrical shape and extends in the axial direction.
A plurality of rotor blades 12 is arranged in the circumferential direction of the
rotor 10. As viewed in the axial direction of the rotor 10 (the direction indicated
by dotted chain line A in FIG. 1), the rotor blades 12 are arranged vis-à-vis the
fixed vanes 3. The rotor blades 12 and the fixed vanes 3 form a plurality of turbine
stages.
[0017] As indicated by arrow F in FIG. 1, steam is introduced into the steam turbine 1 from
one axial side (to be referred to as upstream side hereinafter) of the rotor 10. The
steam turbine 1 of this embodiment is employed for geothermal power generation and
hence natural steam is introduced into the steam turbine 1. Natural steam contains
corrosive gases such as hydrogen sulfide that corrode metals. Therefore, the rotor
10 is exposed to natural steam that contains corrosive gases.
[0018] Particularly, regions located near the turbine stages at the upstream side of the
steam turbine 1 are those that are exposed to incoming natural steam as shown in FIG.
1. Therefore, those regions are required to show a particularly high hydrogen cracking
resistance. For example, at the coupling portions 20a, 21a, 22a and 23a where the
turbine blades 12 that form the first through fourth turbine stages (to be referred
to as blade-implanted sections hereinafter, which are indicated by broken lines in
FIG. 1) are coupled, corrosive components of natural steam can be deposited and accumulated
in the gaps surrounding those rotor blades 12. For this reason, hydrogen cracking
is apt to occur at substantially cylindrical portions 20, 21, 22 and 23 (to be referred
to as stage-portions hereinafter, which are indicated by chain double-dashed lines
in FIG. 1) and include the blade-implanted portions 20a, 21a, 22a and 23a respectively,
if compared with the other region of the rotor.
[0019] At the downstream side (not shown) of the steam turbine 1, on the other hand, natural
steam works in the turbine stages at the downstream side and becomes condensed (liquefied)
and drained. For this reason, the corrosive components of natural steam are not deposited
in the blade-implanted portions that are located at the downstream side but discharged
to the outside as they are condensed and drained. Therefore, hydrogen cracking is
not apt to take place in the regions of the rotor 10 that are located at the downstream
side of the steam turbine 1.
[0020] In the rotor 10 of this embodiment, the stage portions 20, 21, 22 and 23 respectively
including the blade-implanted portions 20a, 21a, 22a and 23a of the upstream side
turbine stages of the steam turbine, into which geothermal steam flows, are regions
that are particularly required to show an improved hydrogen cracking resistance. These
regions will be referred to as "target regions" in the following description. Also
in the following description, all the regions other than the predetermined target
regions (stage portions) 20, 21, 22 and 23 will be referred to as "the other region"
[0021] Now, rotor materials that can be used for the method of manufacturing the steam turbine
rotor of this embodiment will be described below by referring to FIG. 2. FIG. 2 is
a table illustrating exemplar rotor materials that can be used for the steam turbine
of this embodiment. Note that FIG. 2 shows the components of the rotor materials that
were assessed (to be referred to as sample materials hereinafter) for the purpose
of the present invention and the components of the rotor material defined in one of
the appended claims for patent of this specification. The results of the heat treatment
operations conducted on the sample materials A, B, C, D, E, F and G for this embodiment
will be described hereinafter.
[0022] As shown in FIG. 2, steam turbine rotors are made of so-called ferritic alloy steels
that contains iron (symbol of element: Fe) as principal component. Ferritic alloy
steels listed in FIG. 2 contains, in mass %(in percentage by mass), carbon (symbol
of element: C): 0.15 % to 0.33 %, silicon (symbol of element: Si): 0.03% to 0.20%,
manganese (symbol of element: Mn): 0.5 % to 2.0 %, nickel (symbol of element: Ni):
0.1 % to 1.3 %, chromium (symbol of element: Cr) : 0.9 % to 3.5 %, molybdenum (symbol
of element: Mo): 0.1 % to 1.5 %, vanadium (symbol of element: V): 0.15 % to 0.35 %
and tungsten (symbol of element: W) as optional component: 1.0 % or less. Note that
a ferritic alloy steel of this embodiment may contains no tungsten.
[0023] Additionally, ferritic alloy steels may contain nitrogen (symbol of element: N):
0.005 % to 0.15 % in mass % if necessary for manufacturing it. Nitrogen improves hardenability
of the material. If applied to a large steel ingot, nitrogen reduces production of
ferrite in center regions of the steel ingot and hence the use of nitrogen is effective
for producing a large rotor made of the rotor material. Furthermore, nitrogen is effective
for raising the degree of strength of the rotor 10 as it incorporates into the matrix
(principal metal, i.e. balance) of the material to produce solid solution and deposits
as a carbonitride of Nb or Nb (C, N). Note that the material may further contain one
or more impurities that unavoidably get into the material in the manufacturing process
(to be referred to as unavoidable impurities hereinafter).
[0024] Now, the manufacturing method (heat treatment method) of this embodiment will be
described below by referring to FIGS. 3 and 4. FIG. 3 is a table illustrating the
heat treatment conditions of the manufacturing method of this embodiment. FIG. 4 is
a table illustrating the results of determining the grain size, the residual stress
(maximum tensile stress) and the hydrogen cracking resistance of each of the sample
materials that were subjected to the heat treatments of the manufacturing method of
this embodiment. Note that examples 1 to 18, 22 to 25, 32, 33 and 38 to 41 are provided
as examples useful for understanding the present invention.
[0025] The above-described rotor material of this embodiment can be obtained by means of
a well-known melting and manufacturing process and the obtained hot alloy steel is
subjected to hot working such as forging. After such a hot working process, the various
heat treatments of this embodiment are executed on the rotor material. The heat treatments
include, for example, a quality heat treatment. A quality heat treatment is a heat
treatment of tempering the material at a relatively high temperature after hardening
the material by quenching in order to stabilize the material. This heat treatment
will be referred to as "quality heat treatment" hereinafter.
(1) Pre-quality heat treatment annealing step
[0026] With the manufacturing method of this embodiment, the rotor material is subjected
to annealing before the execution of the above-described quality heat treatment step.
This annealing step will be described below by referring to FIGS. 3 and 4. In this
step, the steam turbine rotor is heated to a predetermined temperature and held to
the temperature before it is gradually cooled. As a result of the cooling, the rotor
material is softened so that the strain in the inside can be removed. The annealing
step that is conducted prior to the quality heat treatment is referred to as "pre-quality
heat treatment annealing step" hereinafter. The pre-quality heat treatment annealing
step is executed on the entire steam turbine rotor including the above-described target
regions and the other region.
[0027] In the pre-quality heat treatment annealing step of this embodiment, the heating
temperature for the annealing (to be simply referred to as "annealing temperature"
hereinafter) is set to be within the range of 1,050 to 1,300°C. As an example, the
annealing temperature may be set to be equal to 1,150°C or 1,050°C as shown in FIG.
3. In the pre-quality heat treatment annealing step, the rotor material is heated
until the temperature of the rotor material gets to the annealing temperature, which
may be set to be within the range of 1,150 to 1,300°C, and held to the annealing temperature
for a predetermined period of time (to be referred to as holding time hereinafter)
before it is gradually cooled at a sufficiently low cooling rate typically by furnace
cooling or air cooling. Then, in the subsequent quenching step for the quality heat
treatment, the crystal grains of the target regions are refined. Note that 5 hours
is selected for the annealing temperature holding time of this embodiment.
[0028] 1,050°C is selected as the lower limit value of annealing temperature for the purpose
of removing the strain that is given rise to during the hot working step including
forging. Additionally, an annealing temperature not lower than 1,050°C is required
to make course carbides and carbonitrides incorporate into the matrix (principal metal,
i.e. balance) of the material to produce solid solution and obtain a homogeneous structure.
If the annealing temperature is lower than 1,050°C, the quality heat treatment step
that is executed after the pre-quality heat treatment annealing step cannot provide
the quality level and the material characteristics that are required to the rotor
material.
[0029] 1,300°C is selected as the upper limit value of annealing temperature because the
service life of the annealing furnace is remarkably shortened when the annealing temperature
exceeds the above value and hence such a high temperature is inappropriate for actual
manufacturing operations. For the above reasons, the annealing temperature is set
to be within the range of 1,050°C to 1,300°C. For the same reasons, the lower limit
value and the upper limit value of annealing temperature are preferably set to be
equal to 1,100°C and 1,250°C respectively.
[0030] As the pre-quality heat treatment annealing step is executed within the above-described
temperature range, the strain within the rotor material is removed and a pearlite
structure can be produced to a relative high ratio. When the ratio of the pearlite
structure that is produced in the pre-quality heat treatment annealing step is high,
crystal grains are refined to a large extent after the quality heat treatment, which
will be described below. In other words, the step of micronizing crystal grains includes
the above-described pre-quality heat treatment annealing step.
(2) Quality heat treatment
(2-1) Quenching step
[0031] In the quality heat treatment of this embodiment, firstly the structure of the rotor
material is subjected to a quenching step so as to be austenitized. In this step,
the rotor material is heated and held to a predetermined heating temperature (to be
referred to as austenitizing temperature hereinafter), then the rotor material is
rapidly cooled. As a result, the structure of the rotor material is austenitized.
The step of austenitizing the structure by quenching is referred to simply as "quenching
step" hereinafter.
[0032] In the quenching step of this embodiment, the austenitizing temperature of the target
regions of the rotor that are particularly required to show a high hydrogen cracking
resistance is set to a relatively low temperature if compared with the austenitizing
temperature of the other region. More specifically, the austenitizing temperature
of the other region is set to be within the range of 910 to 950°C. On the other hand,
the austenitizing temperature of the target regions is set to be within the range
of 880 to 910 °C that is lower than that of the other region. As the rotor material
is subjected to a quenching step that is executed in the above-described austenitizing
temperature ranges, the crystal grains of the target regions of the rotor can be refined
if compared with the crystal grains of the other region. In other words, the step
of micronizing crystal grains includes the above-described quenching step.
[0033] As shown in FIG. 3, the austenitizing temperature of the other region is set to be
equal to 920°C whereas the austenitizing temperature of the target regions is set
to be equal to 900°C is lower than that of the other region. In the quenching step,
the rotor material is heated until it gets to the above-described austenitizing temperatures,
and the austenitizing temperatures are held for a predetermined holding time before
the rotor material is cooled rapidly by blowing atomized water to the rotor. Note
that 5 hours is selected for the austenitizing temperature holding time of this embodiment.
[0034] If instances where the crystal grains of the target regions are refined are compared
with instances where the crystal grains of the target regions are not refined, the
service life until cracks take place due to hydrogen cracking of the target regions
is same but the rate at which cracks develop (to be referred to as crack developing
rate hereinafter) is reduced in the former instances. As the "crack developing rate"
is reduced, the hydrogen cracking resistance of the target regions is improved.
[0035] While the effect of micronizing crystal grains in the rotor materials can be obtained
if the austenitizing temperature of the target regions is lower than 880°C, carbonitrides
incorporate into the matrix (principal metal, i.e. balance) of the material to produce
solid solution only insufficiently so that the degree of strength and that of toughness
that are required after the tempering step, which will be described hereinafter, are
no longer achievable. Additionally, the use of a low austenitizing temperature can
give rise to relatively large residual (tensile)stress. If, on the other hand, the
austenitizing temperature of the target regions exceeds 910°C, it no longer differs
from the austenitizing temperature of the other region. Then, it is no longer possible
to provide a satisfactory degree of hydrogen cracking resistance that is required
to the target regions. For the above-described reasons, the austenitizing temperature
of the target regions, which particularly require an improved hydrogen cracking resistance,
is set to be within the range of 880 to 910°C. For the same reasons, the lower limit
value and the upper limit value of austenitizing temperature of the target regions
are preferably set to be equal to 890°C and 905°C respectively.
[0036] The above-described austenitizing temperature is held for a predetermined period
of time. Substantially, the rotor is cooled rapidly by spraying water to complete
the quenching step.
(2-2) Tempering step
[0037] In the quality heat treatment of this embodiment, the rotor material is then subjected
to a tempering step. In the tempering step that follows the above-described quenching
step, the rotor material is heated again to a predetermined heating temperature (to
be referred to as tempering temperature hereinafter) that is set to be lower than
the austenitizing temperature and then cooled. As a result, the rotor material can
acquire desired properties including toughness. The step in which the above-described
tempering operations are executed is referred to as "tempering step" hereinafter.
Note that the tempering step is denoted as "the first stage tempering" in FIG. 3.
[0038] In the tempering step of this embodiment, the tempering temperature of the target
regions of the rotor that are required to show a particularly high hydrogen cracking
resistance, is set to a high temperature if compared with the remaining region. The
tempering temperature of the other region is set to be within the range of 600 to
660°C. On the other hand, the tempering temperature of the target regions is set to
be within the range of 660 to 700°C that is higher than the temperature range for
the other region. As the tempering step of this embodiment is executed at the above-described
tempering temperature, the tempering step can reduce the residual stress (tension)
that arises in the target regions of the rotor after tempering if compared with the
other region. In other words, the step of reducing the residual stress of the target
regions relative to the other region includes this tempering step (2-2).
[0039] As shown under "the first stage tempering" in FIG. 3, the tempering temperature of
the target regions of this embodiment is set to be equal to 670°C, while the tempering
temperature of the other region is set to be equal to 630°C. In the tempering step,
the rotor material is heated until the above-described tempering temperatures are
reached and the tempering temperatures are held for a predetermined period of time
before the rotor material is cooled. Note that 20 hours is selected for the tempering
temperature holding time.
[0040] The stress that acts on the rotor material include external stress (external force)
and internal stress (residual stress). As the target regions that particularly require
improvement in the hydrogen cracking resistance is tempered at a temperature within
the range of 660 to 700°C that is higher than that of the other region, the residual
stress (tension) in the target regions is reduced and hence the stress that acts on
the rotor material is reduced so much. Then, as a result, the development of hydrogen
cracking in the target regions can be controlled.
[0041] While the effect of reducing the residual stress (tension) can be obtained for above-described
ferritic alloy steels when the tempering temperature of the target region is not lower
than 660°C, the strength of the rotor falls when the tempering temperature exceeds
700°C. For these reasons, the tempering temperature of the target regions of the rotor
that are required to show a particularly high hydrogen cracking resistance is set
to be within the range of 660 to 700°C. For the same reasons, the lower limit value
and the upper limit value of tempering temperature are preferably set to be equal
to 665°C and 685°C respectively.
[0042] The technique of high-frequency induction heating can be employed as technique for
making the tempering temperature of the target regions of a steam turbine rotor higher
than the tempering temperature of the other region of the rotor. By heating only the
target regions of the rotor, which particularly require an improved hydrogen cracking
resistance, by means of the technique of high-frequency induction heating, the tempering
temperature of the target regions can be made higher than that of the other region.
[Rotor material and heat treatment conditions]
[0043] The method of manufacturing a rotor to be used for the steam turbine (heat treatment
method) of this embodiment will be described below by referring to FIGS. 2 through
6 in terms of the hydrogen cracking resistances of the various rotor materials to
which the method was applied.
[0044] FIG. 2 is a table illustrating exemplar materials that can be used for the steam
turbine rotor of this embodiment. It shows the components of each of the rotor materials
(to be referred to as sample materials hereinafter) that were evaluated. The sample
materials listed in FIG. 2 were provided in the form of three pieces of each of test
sample steel ingots A, B, C, D, E, F and G that were ingoted by means of a vacuum
induction melting furnace (VIM) and forged.
[0045] FIG. 3 is a table illustrating the heat treatment conditions of this embodiment of
manufacturing method of the present invention. FIG. 4 is a table illustrating the
results of determining the crystal grain size (G.S. No.), the residual stress (maximum
tensile stress) and the hydrogen cracking resistance of each of the sample materials
that were subjected to the heat treatments of the first embodiment of manufacturing
method of the present invention. FIG. 5 is a table illustrating the heat treatment
conditions of Comparative Examples. FIG. 6 is a table illustrating the results of
determining the crystal grain size (G.S. No.), the residual stress (maximum tensile
stress) and the hydrogen cracking resistance of the sample materials that were subjected
to the heat treatments of Comparative Examples.
[0046] The crystal grain size (G.S. No.) of each of the sample materials including those
of Comparative Examples was determined by comparing the austenite grain size as defined
in prior JIS(Japanese Industrial Standard, Methods of austenite grain size determination
for steel) with the Grain Size Standard Views for each of the samples. Note that,
as the G.S. No. increases, the crystal grain size decreases.
[0047] For each of the samples, the residual stress (maximum tensile stress) is determined
by means of formula (1) shown below and the X-ray stress measurement method.
In the formula (1),
n: degree of diffraction,
λ: wavelength of X-ray,
d: crystal lattice spacing of material and
θ: angle of diffraction.
[0048] According to Bragg's law for X-ray diffraction as expressed by the formula (1), the
value of d (the crystal lattice spacing of the material) can be determined when θ
(the angle of diffraction) is known. Then, the residual stress can be computationally
determined by determining the strain from the difference between d and the standard
crystal lattice spacing and using Young's modulus to Poisson's ratio.
[0049] Hydrogen cracking resistance refers to the resistance against cracking that is caused
by the pressure of hydrogen gas that is produced due to a phenomenon that hydrogen
produced as a result of corrosion of the material penetrates into the material, diffuses
and concentrates along interfaces between the non-metallic inclusions and the matrix
(principal metal, i.e. balance) so as to become hydrogen gas molecules.
[0050] The test for hydrogen cracking was conducted according to NACE Standard (TM0284,
Evaluation of Pipeline and Pressure Vessel Steels for Resistance against Hydrogen-Induced
Cracking). Each of the test pieces of the sample materials was immersed in a 5 % NaCl
+ 0.5 % acetic acid solution with pH4 for 96 hours at the test temperature of 24 ±
2.8 °C and subsequently cut to evaluate the cross portion and check for existence
or non-existence of cracking. The size of the test pieces was 50 mm x 30 mm x 10 mm
and three test pieces were employed for each of the sample materials. When all of
the three test pieces of a sample material did not show any cracking, the sample material
was rated as "hydrogen cracking resistance : O" and all the sample materials that
were not rated as "O" were rated as "hydrogen cracking resistance: X".
[0051] As shown in FIGS. 3 and 4, when the pre-quality heat treatment annealing step, the
quenching step and the tempering step (the first stage tempering and the second stage
tempering) were executed under the heat treatment conditions of this embodiment, all
the sample materials were rated as "hydrogen racking resistivity: O" at least at the
target regions that are required to show a satisfactory hydrogen cracking resistance.
[0052] However, when at least any one of the pre-quality heat treatment annealing step,
the quenching step and the tempering step (the first stage tempering and the second
stage tempering) was conducted under the conditions where at least one of the heat
treatment conditions of this embodiment was not satisfied, the target regions that
are required to show a satisfactory hydrogen cracking resistance were rated as "hydrogen
crack resistivity: X" as in the case of Comparative Examples shown in FIGS. 5 and
6.
[0053] Thus, it was proved that the method of manufacturing a steam turbine rotor of this
embodiment provides the target regions of the turbine that are particularly required
to show a satisfactory hydrogen cracking resistance with an excellent hydrogen cracking
resistance.
[0054] As described above, the method of manufacturing a steam turbine rotor of this embodiment
comprises a step of reducing the residual stress (2-2) of the target regions of the
rotor if compared with the other region. As the residual stress that arises after
the tempering step is reduced in the target regions than in the other region, the
development of hydrogen cracking in the target regions can be controlled so that the
hydrogen cracking resistance of the rotor can be improved.
[0055] The step of reducing the residual stress of this embodiment includes a tempering
step (2-2) that is a step of tempering the rotor after the quenching the rotor. In
the tempering step, the tempering temperature (the first stage tempering) is set high
for the target regions of the steam turbine rotor if compared with the other region
of the rotor as shown in the inventive examples 19, 21, 27 to 31, 34 to 37 and 42
in FIG. 3, and in examples 4, 5, 7 to 9, 12, 13, 15, 16, 18, 23, 25 and 38 to 40 which
are examples useful for understanding the present invention also in FIG. 3. As a result
of selecting a tempering temperature in the above-described manner, the residual stress
of the target regions can be reduced if compared with the other region.
[0056] Note that the tempering temperature of the target regions of the steam turbine rotor
that is higher than that of the other region of the rotor is reached by high frequency
induction heating in this embodiment. High frequency induction heating can make the
tempering temperature of the target regions that require a particularly improved hydrogen
cracking resistance in the rotor higher than that of the other region.
[0057] This embodiment further includes a step of refining crystal grains in the target
regions if compared with the crystal grains of the other region. Thus, the strength
and the toughness of the target regions can be improved and, at the same time, the
crack developing rate can be reduced by refining crystal grains in the target regions.
[0058] "The step of refining crystal grains" of this embodiment includes a quenching step
(2-1), which is a step where the rotor is subjected to quenching. The quenching step
is executed before the tempering step (2-2). In the quenching step (2-1), the austenitizing
temperature, which is the heating temperature, is set low for the target regions of
the steam turbine rotor if compared with the other region of the rotor as shown in
the inventive examples 19, 21, 27 to 31 and 42 in FIG. 3, and in examples 2, 3, 32
to 37 and 41 which are examples useful for understanding the present invention also
in FIG. 3. As a result of selecting the austenitizing temperature in the above-described
manner, crystal grains can be refined for the target regions if compared with the
other region.
[0059] In this embodiment, the steam turbine rotor is formed by using a ferritic alloy steel
that contains, in mass %, carbon: 0.15 % to 0.33 %, silicon: 0.03 % to 0.20 %, manganese:
0.5 % to 2.0 %, nickel: 0.1 % to 1.3 %, chromium: 0.9 % to 3.5 %, molybdenum:
0.1 % to 1.5 %, vanadium: 0.15 % to 0.35 %, nitrogen: 0.005 % to 0.015 % and tungsten
as optional component: 1.0 % or less as shown in FIG. 2 for the sample materials A,
B, C, D, E, F and G. As a result of using such a ferritic alloy steel for forming
the rotor, it is possible to realize a rotor that hardly produces hydrogen cracking
if exposed to highly corrosive gases such as hydrogen sulfide and, if it produces
hydrogen cracking, cracks hardly develop.
[0060] The heating temperature for the tempering step, or the tempering temperature, is
set to be within the range of 660 to 700°C for the target regions of the steam turbine
rotor as shown in the inventive examples 19, 21, 27 to 31, 34 to 37 and 42 in FIG.
3, and in examples 4, 5, 7 to 9, 12, 13, 15, 16, 18, 23, 25 and 38 to 40 which are
examples useful for understanding the present invention also in FIG. 3. While the
effect of reducing the residual stress (tension) can be obtained for above-described
ferritic alloy steels when the tempering temperature of the target region is not lower
than 660°C, the strength of the rotor falls when the tempering temperature exceeds
700°C. For this reason, the tempering temperature of the target regions of the rotor
that are required to show a particularly high hydrogen cracking resistance is set
to be within the range of 660 to 700°C to effectively reduce the residual stress,
while reducing the extent of fall of the strength.
[0061] While the tempering temperature of the target regions is set to be equal to 670°C
and the tempering temperature of the other region is set to be equal to 630°C for
the tempering step of this embodiment, the tempering temperatures according to the
present invention are by no means limited to the above specified values. For example,
in an example not according to the present invention the tempering temperature of
the target regions may be set to be equal to 630°C, which is equal to the tempering
temperature of the other region.
[0062] The austenitizing temperature in the quenching step (2-1) of the target regions is
set to be within the range of 880 to 910°C as shown in the inventive examples 19,
21, 27 to 31, 34 to 37 and 42 in FIG. 3, and in examples 2, 3, 32, 33 and 41 which
are examples useful for understanding the present invention also in FIG. 3. If the
austenitizing temperature is lower than 880°C, carbonitrides incorporate into the
matrix (principal metal, i.e. balance) of the material to produce solid solution only
insufficiently so that the degree of strength and that of toughness that are required
after the tempering step, which will be described hereinafter, are hardly achievable.
Additionally, the use of a low austenitizing temperature can give rise to relatively
large residual stress. If, on the other hand, the austenitizing temperature exceeds
910°C, it is no longer possible to provide a satisfactory degree of hydrogen cracking
resistance that is required to the target regions. For the above-described reasons,
the austenitizing temperature of the target regions, which particularly require an
improved hydrogen cracking resistance, is set to be within the range of 880 to 910°C
to make it possible to refine their crystal grains, while securing the required degree
of strength and that of toughness.
[0063] Finally, the above-described step of "refining crystal grains include the pre-quality
heat treatment annealing step (1) that is an annealing step to be executed prior to
the quenching step (2-1) and the pre-quality heat treatment annealing temperature,
which is the heating temperature, is set to be within the range of 1,050 to 1,300°C
for the pre-quality heat treatment annealing step (1). As the pre-quality heat treatment
annealing step is executed within the above-described temperature range, a pearlite
structure can be produced to a relatively high ratio. When the rotor material that
shows a high ratio of the pearlite structure is subjected to the quenching step (2-1),
crystal grains can be refined further.
[0064] While the tempering operation is conducted only once in the quality heat treatment
of the above-described embodiment, the mode of conducting of the quality heat treatment
according to the present invention is by no means limited to the above-described one.
For example, the tempering step can suitably be executed twice in the quality heat
treatment. Another mode of quality heat treatment will be described below.
[Second Example]
[0065] The quality heat treatment of this example will be described below. In this example,
the entire steam turbine rotor is subjected to tempering twice. Additionally, the
austenitizing temperature in the quenching step for the quality heat treatment of
this embodiment differs from that of the first embodiment as will be described in
detail below. The arrangements that are common to this example and also to the first
embodiment will not be described below repeatedly.
(2-1B) Quenching step
[0066] In the quenching step not according to the invention, but provided as an example,
the austenitizing temperature is set to be equal for both the target regions of the
steam turbine rotor that are required to show a particularly high hydrogen cracking
resistance and the other region. In other words, the entire rotor including the target
regions and the other region is subjected to quenching at a uniform austenitizing
temperature. The austenitizing temperature is set to be within the range of 910 to
950°C. However, according to the invention, an austenitizing temperature that is lower
than that of the other region is selected for the target regions.
(2-2B) Two-stage tempering step
[0067] A two-stage tempering step is executed for the quality heat treatment of this example.
More specifically, the tempering step of the quality heat treatment of this embodiment
includes the first stage tempering step (2-2B1) and the second stage tempering step
(2-2B2). Note that the tempering temperature in the first stage tempering step is
referred to as "the first stage tempering temperature" and the second stage tempering
temperature is referred to as "the second stage tempering temperature" hereinafter.
(2-2B1) First stage tempering step
[0068] In an example not according to the invention, but useful for understanding the invention,
for the first stage tempering step, the first stage tempering temperature, which is
the heating temperature, is set to be within the range of 600 to 700°C for both the
target regions and the other region of the steam turbine rotor. Note that, in this
example not according to the invention, in particular, a same temperature can be selected
for both the first stage tempering temperature of the target regions and the first
stage tempering temperature of the other region. This will be described below by referring
to FIGS. 3 and 5.
[0069] As shown in FIGS. 3 and 5, 670 °C is selected for the first stage tempering temperature
of the target regions and 630 °C is selected for the first stage tempering temperature
of the other region. In the first stage tempering step, the rotor material is heated
to the first stage tempering temperatures and then the first stage tempering temperatures
are held to a predetermined holding time before the rotor material is cooled. Note
that the first stage tempering temperature holding time is set to be equal to 20 hours.
[0070] Also note that, in an example not according to the invention, the first stage tempering
temperature of the target regions may be set to be equal to 630 °C, which is same
as that of the other region. Alternatively, only the surface layers of the target
regions may be heated to 670 °C, which is the first stage tempering temperature selected
for the target regions.
[0071] For the first stage tempering step, the first stage tempering temperature of the
target regions may be set to be equal to 670 °C, which is the highest allowable temperature
and a technique of so-called gradient heating may suitably be employed. With gradient
heating, the rotor material is heated such that the first stage tempering temperature
is gradually lowered from the target regions toward the other region. Note that, for
the first stage tempering step, the first stage tempering temperature of the other
region is set to be equal to 630°C. When the technique of gradient heating is employed,
it is no longer necessary to execute a heat treatment at "predetermined positions"
of the rotor for the purpose of improving the strength of the rotor at those positions.
After executing the above-described first stage tempering step (2-2B1), the second
stage tempering step (2-2B2) will be executed.
(2-2B2) Second stage tempering step
[0072] For the second stage tempering step, the second stage tempering temperature, which
is the heating temperature, is set to be within the range of 600 to 700°C for both
the target regions and the other region of the steam turbine rotor. In the second
stage tempering step, the rotor material is heated to the second stage tempering temperature
and then the second stage tempering temperature is held for a predetermined holding
time before the rotor material is cooled. Note that the second stage tempering temperature
holding time is set to be equal to 20 hours. Also note that, in particular, a same
temperature may be selected for both the second stage tempering temperature of the
target regions and the second stage tempering temperature of the other region. For
example, the second stage tempering temperature may be set to be equal to 630°C as
shown in FIG. 3.
[0073] The second stage tempering temperature of the target regions may be set to a temperature
level that is higher than that of the second stage tempering temperature of the other
region. For example, as shown FIG. 3, the second stage tempering temperature of the
target regions may be set to be equal to 670°, which is the highest allowable temperature
and a technique of gradient heating may suitably be employed. With gradient heating,
the second stage tempering temperature is gradually lowered from the target regions
toward the other region.
[0074] When the technique of gradient heating is employed for the second stage tempering
step, the target regions are heated to the second stage tempering temperature in the
furnace. At this time, the other region are held to the outside of the furnace because
the other region does not need the second stage tempering. With this arrangement for
the other region, gradient heating of gradually lowering the second stage tempering
temperature from the target regions toward the other region is realized.
[0075] As shown in the examples in FIG. 3, 6 to 8, 10 to 18, 22 to 25, 32, 33 and 39 to
41 which are examples useful for understanding the present invention also in FIG.
3, the tempering step (2-2B) of this embodiment is made to include the first stage
tempering step (2-2B1) of heating the rotor material to the first stage tempering
temperature, which is set to be within the range of 600 to 700°C, for tempering and
the second stage tempering step (2-2B2) to be executed after the first stage tempering
step (2-2B1) of heating the rotor material to the second tempering temperature, which
is also set to be within the range of 600 to 700°C, for tempering.
[0076] The rotor material is mostly turned into a quenched bainite structure in the quenching
step (2-1B). If, however, the residual austenite structure is left, all the residual
austenite structure is not turned into the tempered bainite structure in the next
tempering step but the quenched bainite structure partly remains. Then, for this reason,
the rotor material becomes a mixture of structures containing the tempered bainite
structure in which the strength and the toughness are well balanced and the quenched
bainite structure that shows a high strength and a low toughness. Then, strain is
accumulated between the two structures to increase the residual stress.
[0077] However, the residual stress can be reduced by executing the two-stage tempering
step (2-2B1) and (2-2B2) to completely turning the entire quenched bainite structure
into the tempered bainite structure. This effect can be achieved when both the first
tempering temperature and the second tempering temperature exceed 600°C but the rotor
material does not show the required level of strength when the two tempering temperatures
exceed 700°C. For this reason, both the first tempering temperature and the second
tempering temperature of the target regions whose hydrogen cracking resistance particularly
needs to be improved are limited within the range of 600 to 700°C and the tempering
step is executed in two stages. With this arrangement, the residual stress can be
reduced. Then, as a result, the occurrence of hydrogen cracking and the development
of hydrogen cracking in the target regions can be controlled.
[0078] As shown in the examples 7 to 8, 12 to 18, 22 to 25 and 39 to 40 which are examples
useful for understanding the present invention also in FIG. 3 at least one of the
first stage tempering temperature and the second stage tempering temperature of the
target region is set to show a value higher than that of the other region and the
higher value is set to be within the range of 660 to 700°C. For ferritic alloy steels,
the effect of reducing the residual stress can be achieved when the tempering temperature
is not lower than 660°C. On the other hand, however, the strength of the rotor falls
when the tempering temperature exceeds 700°C. Thus, when the tempering temperature
of at least one of the first stage tempering step (2-2B1) and the second stage tempering
step (2-2B2) is set to be within the range of 660 to 700°C for the target region,
which is required to show a particularly high hydrogen cracking resistance, it is
possible to reduce the extent of possible fall of the strength of the rotor and, at
the same time, reduce the residual stress in the rotor.
[0079] As shown in the arrangement examples 13 to 18 and 22 to 25 in FIG. 3, a technique
of gradient heating of gradually lowering the tempering temperature from the target
regions toward the other region in at least one of the first stage tempering step
(2-2B1) and the second stage tempering step (2-2B2) is employed. When the technique
of gradient heating is employed, it is no longer necessary to execute a heat treatment
at "predetermined positions" of the rotor for the purpose of improving the strength
of the rotor at those positions.
[0080] When the technique of gradient heating is employed for the above-described second
tempering step, the target regions are heated to the second stage tempering temperature
in the furnace for tempering but the other region are held to the outside of the furnace.
"The other region" other than the target regions do not need to be tempered in the
second tempering step. Therefore, "the other region" may be cooled in a state of being
held to the outside of the furnace.
[0081] As shown in examples 6, 10, and 11 in FIG. 3 which are examples useful for understanding
the present invention, a same temperature may be selected for both the first stage
tempering temperature of the target regions and the first stage tempering temperature
of the other region, while a same temperature may be selected for both the second
stage tempering temperature of the target regions and the second stage tempering temperature
of the other region and the second stage tempering temperature may be set to be within
the range of 600 to 660°C. With such a technique, it is possible to secure the hydrogen
cracking resistance of the target regions as shown in FIG. 4.
[Other Embodiment(s)]
[0082] The austenitizing temperature in the quenching step of each of the above-described
embodiment is lower for the target regions that are required to show a particularly
high hydrogen cracking resistance than for the other region. In an example not according
to the invention, but provided to help understand the invention, as shown in the related
tables, a same austenitizing temperature may alternatively be selected for both the
target regions and the other region.
[0083] While the target regions that are required to show a particularly improved hydrogen
cracking resistance are the stage-portions 20, 21, 22 and 23, which include the blade-implanted
portions 20, 21, 22 and 23 respectively, located at the upstream side of the steam
turbine 1 as shown in FIG. 1 in the first embodiment, the target regions are not limited
to such portions for the purpose of the present invention. Any portions of the rotor
may be selected as target regions so long as corrosive components of steam can easily
deposit there and those regions are required to show an improved hydrogen cracking
resistance.
[0084] While certain embodiments have been described, these embodiments have been presented
by way of example only, and are not intended to limit the scope of the invention.
Indeed, the novel embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in the form of the
embodiments described herein may be made without departing from the accompanying claims.