Patent Application
20160237517
The present invention relates to a method for heat treatment of a stainless member, and a method for producing a forged stainless product.
Priority of this application is claimed based on Japanese Patent Application No. 2013-213754, filed on Oct. 11, 2013 in Japan, the content of which is incorporated herein by reference.
After a stainless member is processed into a predetermined shape by forging or rolling, the forged stainless member or the like may be subjected to solutionizing heat treatment.
For example, PTL 1 discloses technology in which a stainless member forged at a high temperature of 1000° C. to 1300° C. is cooled, and then is subjected to heat treatment at a high temperature of 950° C. to 1125° C. again. In this technology, the heated stainless member is quenched at a cooling speed of 4° C./min to 5° C./min.
PTL 2 discloses technology other than the technology disclosed in PTL 1, which is related to the present invention. In this technology, after an aluminum alloy member is heated for heat treatment, the aluminum alloy member is quenched by blowing a cooling medium to the aluminum alloy member via multiple nozzles. In a case where a metal member is quenched, a high-temperature portion, the temperature of which is easily decreased, and a low-temperature portion, the temperature of which is not easily decreased, are formed in the metal member depending on the shape of the member. As a result, thermal stress and strain occur in the metal member during a cooling process of the metal member. In the technology disclosed in PTL 2, the flow rate of the cooling medium blowing via the multiple nozzles is adjusted to suppress the occurrence of strain in the quenching process of the aluminum alloy member.
[PTL 1] Japanese Unexamined Patent Application, First Publication No. 2012-140690
[PTL 2] Japanese Unexamined Patent Application, First Publication No. 2007-146204
The technology disclosed in PTL 2 refers to technology for an aluminum alloy member. A stainless member has properties different from those of an aluminum alloy member. For this reason, even if the stainless member is heated for heat treatment, and then is subjected to the technology disclosed in PTL 2, it becomes difficult to suppress the occurrence of strain in a cooling process.
An object of the present invention is to provide a method for heat treatment of a stainless member, which is capable of suppressing the occurrence of strain in a process of heating a stainless member for heat treatment and then cooling the stainless member, and to a method for producing a forged stainless product.
According to an aspect of the present invention, in order to achieve this object, there is provided a method for heat treatment of a stainless member, a heating step, in which a stainless member is heated to a temperature within or above a heating phase-transformation temperature range in which the stainless member is phase-transformed when the stainless member is heated, is executed, and a cooling step, in which the stainless member heated in the heating step is cooled to a temperature below a cooling phase-transformation temperature range in which the stainless member is phase-transformed when the stainless member is heated, is executed. In the cooling step, cooling of the stainless member is suppressed in a control temperature range including the cooling phase-transformation temperature range. The stainless member in this application is phase-transformed in the heating step and the cooling step.
In the cooling phase-transformation temperature range, the stainless member is likely to be deformed. In the heat treatment method, cooling of the stainless member is suppressed in a temperature range including the cooling phase-transformation temperature range. As a result, in the heat treatment method, it is possible to suppress temperature differences between portions of the stainless member in the cooling phase-transformation temperature range, and to decrease thermal stress occurring in the stainless member. Accordingly, in the heat treatment method, it is possible to decrease strain of the stainless member.
In the method for heat treatment of a stainless member according to the aspect, in the cooling step, a cooling medium may be supplied to the stainless member.
In a case where the cooling medium is supplied to the stainless member, the flow rate of the cooling medium supplied to the stainless member per unit time may be set to be smaller than those immediately before the temperature of the stainless member reaches the control temperature range and immediately after the temperature passes the control temperature range.
In a case where the cooling medium is supplied to the stainless member, in the cooling step, the length of time from when the cooling of the stainless member is started until when the temperature of the stainless member reaches the cooling phase-transformation temperature range may be obtained in advance. In the cooling step, the flow rate of the cooling medium supplied to the stainless member may be decreased before the length of time obtained in advance has elapsed from when the cooling of the stainless member is started.
In a case where the cooling medium is supplied to the stainless member, a phase-transformation start temperature of the cooling phase-transformation temperature range may be obtained in advance. In the cooling step, the flow rate of the cooling medium supplied to the stainless member may be decreased before the temperature of the stainless member reaches the phase-transformation start temperature.
In a case where the cooling medium is supplied to the stainless member, the flow rate of the cooling medium supplied to the stainless member may be gradually increased from when the cooling step is started until when a predetermined length of time has elapsed, or from when the cooling step is started until when the temperature of the stainless member reaches a predetermined temperature.
In a case where, after the stainless member is placed into a heating furnace, and is heated, the stainless member is extracted from the heating furnace, and is cooled, since an ambient temperature around the stainless member is basically room temperature in the cooling step, the ambient temperature around the stainless member is rapidly decreased from immediately before the end of the heating step to immediately after the start of the cooling step. Accordingly, in the heat treatment method, the flow rate of the cooling medium supplied to the stainless member is gradually increased from when the cooling step is started until when the predetermined length of time has elapsed, or from when the cooling step is started until when the temperature of the stainless member reaches the predetermined temperature. A change in the temperature of the stainless member is suppressed. As a result, in the heat treatment method, it is possible to suppress temperature differences between portions of the stainless member, and to decrease strain of the stainless member.
In the method for heat treatment of a stainless member, in the cooling step, a covering member covering a large surface area portion of the stainless member may be provided on a large surface area portion having a large surface area per unit mass.
In the stainless member, the large surface area portion having a large surface area per unit mass is easily cooled compared to a small surface area portion having a small surface area per unit mass, and the cooling speed of the large surface area portion is higher than that of the small surface area portion. In the heat treatment method, since the covering member covers the large surface area portion which is easily cooled, it is possible to suppress the cooling speed of the large surface area portion. For this reason, in the heat treatment method, it is possible to suppress cooling of the large surface area portion of the stainless member in a temperature range including the cooling phase-transformation temperature range. Accordingly, in the heat treatment method, it is possible to suppress a temperature difference between the large surface area portion and the small surface area portion, and to decrease strain of the stainless member.
In a case where the covering member is provided, the amount of heat dissipation per unit mass from the large surface area portion covered with the covering member may approximate to the amount of heat dissipation per unit mass from a portion not covered with the covering member.
In a case where the covering member is provided, the covering member may be made of the same material as that of the stainless member.
In the heat treatment method, since the thermal expansion coefficient of the stainless member is the same as that of the covering member, the cooling target and the covering member are capable of integrally contracting in a cooling process, and heat transfer between the stainless member and the covering member can be substantially constant. In addition, the stainless member and the covering member have the same thermal properties such as a heat transfer coefficient other than a thermal expansion coefficient. For this reason, in the heat treatment method, it is possible to easily determine various dimensions of the covering member, by which the amount of heat dissipation from the small surface area portion not covered with the covering member is adjusted to be substantially the same as the amount of heat dissipation from the large surface area portion covered with the covering member.
In a case where the covering member is provided, the covering member may be provided on the stainless member before the heating step is started.
In the heat treatment method, when the cooling step is started, it is possible to substantially eliminate a temperature difference between the stainless member and the covering member, and possible to suppress the occurrence of thermal strain based on the temperature difference when the covering member is attached to the stainless member.
In the method for heat treatment of a stainless member, the stainless member may be made of a precipitation hardening stainless steel.
In the method for producing a forged stainless product according to the aspect of the present invention, in order to achieve this object, after executing a forging step, in which a stainless member is processed into a predetermined shape by forging, any one of the methods for heat treatment of a stainless member is executed on the stainless member subjected to the forging step.
In this case, the forged stainless product may be a blade of a steam turbine.
According to an aspect of the present invention, it is possible to suppress temperature differences between portions of a stainless member in a cooling phase-transformation temperature range, and to decrease thermal stress occurring in the stainless member. Accordingly, according to the aspect of the present invention, it is possible to decrease the strain of the stainless member.
Hereinafter, various embodiments and modification examples of the present invention will be described with reference to the accompanying drawings.
First, a first embodiment of the present invention will be described with reference to
In this embodiment, a rotor blade of a steam turbine is produced. As illustrated in
The blade root 19 is mounted on a rotor shaft of the steam turbine. For this reason, the blade root 19 is formed into a Christmas tree shape such that the rotor blade 10 is not disengaged from the rotor shaft during rotation of the rotor shaft. As illustrated in
Hereinafter, a method for producing the aforementioned rotor blade will be described with reference to the flowchart illustrated in
First, stainless members made of a precipitation hardening stainless steel are heated to 1000° C. or greater, and are processed into substantially the same shape as the shape illustrated in
Subsequently, burrs formed on the outer circumferences of the stainless members are removed from the stainless members which have been subjected to the forging step (S1) and cooled to room temperature (S2: burr removing step).
Subsequently, the stainless members, which have been subjected to the burr removing step (S2), are heated again (S3: heating step). As illustrated in
When the heating step (S3) is ended, as illustrated in
A relationship between the temperature and the strain of a precipitation hardening stainless steel, which is the material of the stainless member 10b, will be described with reference to
The room-temperature structure of a precipitation hardening stainless steel is in a martensitic phase α′. The crystalline structure of the martensitic phase α′ is a body-centered cubic lattice. When the precipitation hardening stainless steel is heated to approximately 600° C., gradual phase transformation of the structure from the martensitic phase α′ to an austenitic phase γ starts. When the precipitation hardening stainless steel is further heated at a temperature which is several tens degrees C. higher than approximately 600° C., the phase transformation ends, and the entire structure is in the austenitic phase γ. The crystalline structure of the austenitic phase γ is a face-centered cubic lattice. A heating phase-transformation temperature range Ar refers to a temperature range from a heating phase-transformation start temperature As, which is a phase-transformation start temperature during heating, to a heating phase-transformation end temperature Af which is a phase-transformation end temperature during heating. Even if the precipitation hardening stainless steel is further heated to a temperature of 1000° C. or greater at which the aforementioned solutionizing treatment is performed, the structure is in the austenitic phase γ.
Until the temperature of the precipitation hardening stainless steel reaches the heating phase-transformation start temperature As from room temperature, the temperature and the thermal strain have a substantially proportional relationship, and the thermal strain increases along with the temperature increase. That is, the volume of the precipitation hardening stainless steel expands along with the temperature increase until the temperature reaches the heating phase-transformation start temperature As. In the heating phase-transformation temperature range Ar, there is no much increase in the thermal strain of the precipitation hardening stainless steel along with a temperature increase. That is, in the heating phase-transformation temperature range Ar, there is almost no increase in the volume of the precipitation hardening stainless steel along with the temperature increase. The volume of a body-centered cubic lattice, which is the crystalline structure of the martensitic phase α′, is smaller than a face-centered cubic lattice which is the crystalline structure of the austenitic phase γ. For this reason, during phase transformation from the martensitic phase α′ to the austenitic phase γ, even if the temperature is increased, there is almost no increase in the volume. In a temperature range higher than the heating phase-transformation temperature range Ar, the temperature and the thermal strain of the precipitation hardening stainless steel have a substantially proportional relationship, and the thermal strain increases along with a temperature increase.
When the precipitation hardening stainless steel is cooled to approximately 150° C. from a temperature of 1000° C. or greater at which the aforementioned solutionizing treatment is performed, gradual phase transformation of the structure from the austenitic phase γ to the martensitic phase α′ starts. When the precipitation hardening stainless steel is further cooled to a temperature which is several tens degrees C. lower than an approximately 150° C., the phase transformation ends, and the entire structure in the martensitic phase α′. A cooling phase-transformation temperature range Mr refers to a temperature range from a cooling phase-transformation start temperature Ms, which is a phase-transformation start temperature during cooling, to a cooling phase-transformation end temperature Mf which is a phase-transformation end temperature during cooling.
Until the temperature of the precipitation hardening stainless steel reaches the cooling phase-transformation start temperature Ms from a temperature of 1000° C. or greater at which the aforementioned solutionizing treatment is performed, the temperature and the thermal strain have a substantially proportional relationship, and the thermal strain decreases along with the temperature decrease. In contrast, in the cooling phase-transformation temperature range Mr, the thermal strain of the precipitation hardening stainless steel increases along with a temperature decrease. In a temperature range lower than the cooling phase-transformation temperature range Mr, the temperature and the thermal strain of the precipitation hardening stainless steel have a substantially proportional relationship, and the thermal strain decreases along with a temperature decrease.
The precipitation hardening stainless steel has been described. Basically similar to the precipitation hardening stainless steel, during heating and cooling, phase transformation occurs in martensitic stainless steels, ferritic stainless steels, and austenitic-ferritic two-layer stainless steels. Basically, a relationship between the temperature and the thermal strain of these stainless steels is the same as that of the temperature and the thermal strain of the precipitation hardening stainless steel. In contrast, in a temperature range from room temperature to a temperature at which the solutionizing treatment is performed, phase transformation does not occur in an aluminum alloy member, which is a target for heat treatment and is disclosed in PTL 2 described in the background art section.
A portion, which is easily cooled (in other words, easily heated), and a portion, which is not easily cooled (in other words, not easily heated) may be formed in a metal member depending on the shape of the metal member. Specifically, a portion of the metal member, which is easily cooled, is a large surface area portion that has a large surface area per unit mass. A portion of the metal member, which is not easily cooled, is a small surface area portion that has a small surface area per unit mass. In the embodiment, as illustrated in
In a case where the metal member is heated inside the heating furnace 25, the temperature of the metal member is increased along with an increase in the internal temperature of the heating furnace 25 in which the metal member is disposed, that is, an increase in an ambient temperature. In contrast, in a case where the metal member is taken out of the heating furnace 25, and is cooled, an ambient temperature is room temperature relative to the temperature of the metal member, and a temperature difference between the temperature of the metal and the ambient temperature is large. As a result, basically, a temperature decrease rate during cooling is higher than a temperature increase rate during heating. For this reason, a temperature difference between the high-temperature portion and the low-temperature portion of the metal member becomes small during heating. In contrast, a temperature difference between the high-temperature portion and the low-temperature portion of the metal member becomes large during cooling. Accordingly, suppression of the temperature difference between the high-temperature portion and the low-temperature portion of the metal member during cooling leads to suppression of the occurrence of thermal stress, and suppression of strain.
As described above, in the cooling step (S4) of the embodiment, the flow rate of air blown to the stainless member 10b is controlled.
In the cooling step (S4) of the embodiment, control of the flow rate of the cooling medium will be described with reference to
When the cooling step (S4) is started, as illustrated in
When the first predetermined time (t1) has elapsed from the time (t0) when the driving of the fan 31 is started, the control apparatus 30 sets the amount of driving the fan 31 to be constant. That is, the control apparatus 30 sets the flow rate of air blown to the stainless member 10b per unit time to be constant. A time when the flow rate of air per unit time is set be constant, in other words, a time for the end of the first control temperature range C1 is set to occur before the temperature of the stainless member 10b reaches the cooling phase-transformation start temperature Ms.
When a second predetermined time (t2) has elapsed from the time (t0) when the driving of the fan 31 is started, the control apparatus 30 rapidly decreases the amount of driving the fan 31, and maintains the decreased amount of driving. That is, when the second predetermined time (t2) has elapsed from the time (t0) when the driving of the fan 31 is started, the control apparatus 30 rapidly decreases the flow rate of air blown to the stainless member 10b per unit time, and maintains the decreased flow rate of air. A time (t2) when the flow rate of air per unit time is rapidly decreased is set to occur immediately before a time (t3) when the temperature of the stainless member 10b reaches the cooling phase-transformation start temperature Ms.
When a third predetermined time (t5) has elapsed from the time (t2) when the amount of driving the fan 31 is rapidly decreased, the control apparatus 30 rapidly increases the amount of driving the fan 31 to the amount of driving which has been set before the time (t2) when the amount of driving the fan 31 is rapidly decreased. That is, when the third predetermined time (t5) has elapsed from the time (t2) when the flow rate of air per unit time is rapidly decreased, the control apparatus 30 rapidly increases the flow rate of air per unit time to the flow rate of air which has been set before the time (t2) when the flow rate of air is rapidly decreased. The time (t5) when the flow rate of air per unit time is rapidly increased is set to occur immediately after a time (t4) when the temperature of the stainless member 10b reaches the cooling phase-transformation end temperature Ms.
In the embodiment, a second control temperature range C2 refers to a temperature range including the cooling phase-transformation temperature range Mr, that is, a temperature range from a temperature slightly higher than the cooling phase-transformation start temperature Ms to a temperature slightly lower than the cooling phase-transformation end temperature Mf. In the embodiment, the flow rate of air in the second control temperature range C2 is lower than those immediately before the temperature of the stainless member 10b reaches the second control temperature range C2 and immediately after the temperature passes the second control temperature range C2.
When the amount of driving the fan 31 is rapidly increased (t5), thereafter, the control apparatus 30 maintains the increased amount of driving the fan 31. That is, when the air flow rate of air per unit time is rapidly increased (t5), thereafter, the control apparatus 30 maintains the increased flow rate of air per unit time.
When the stainless member 10b is taken out of the heating furnace 25, and the fan 31 starts blowing air to the stainless member 10b, an ambient temperature around the stainless member 10b is rapidly decreased. As illustrated by the two-dot chain line in
When the temperature of the stainless member 10b is rapidly decreased, a temperature difference between the large surface area portion A and the small surface area portion B of the stainless member 10b is increased, and large strain occurs. In the embodiment, the amount of driving the fan 31 is gradually increased in the first control temperature range C1 set from start time t0 of the cooling step (S4) until when the first time (t1) has elapsed. For this reason, in the embodiment, as illustrated by the two-dot chain line in
Larger strain occurs in the stainless member 10b due to small stress in a phase transformation state than in a non-phase transformation state. For this reason, the occurrence of thermal stress during phase transformation is preferably suppressed by decreasing a temperature difference between the large surface area portion A and the small surface area portion B of the stainless member 10b in a phase transformation state to a level smaller than a temperature difference between the large surface area portion A and the small surface area portion B of the stainless member 10b in a non-phase transformation state.
In the embodiment, as described with reference to
When the cooling step (S4) is ended, and the temperature of the stainless member 10b becomes room temperature, the stainless member 10b is subjected to a finishing process (S5: finishing step). In the finishing step (S5), the stainless member 10b is machine-processed, for example, the stainless member 10b is grinded or polished such that the dimension of each portion of the stainless member 10b is within an allowable dimension. As necessary, the machine-processed stainless member 10b is subjected to a surface treatment.
As such, the rotor blade is produced as a forged product.
In the embodiment, in the cooling step (S4), the initial cooling time zone and strain during phase transformation are decreased by controlling the initial cooling time zone in which the temperature of the stainless member 10b is rapidly changed, and the flow rate of air during phase transformation in which deformation is likely to occur. Accordingly, in the embodiment, it is possible to decrease strain and residual stress of the stainless member 10b after the cooling step (S4) is complete.
In the embodiment, after the cooling step (S4) is complete, the finishing step (S5), for example, machine processing is executed on the stainless member 10b. When residual stress is present in the stainless member 10b before the machine processing is executed, the residual stress is released in the machine processing, and strain occurs due to the release of the residual stress. In the embodiment, as described above, since it is possible to decrease the residual stress of the stainless member 10b after the cooling step (S4) is complete, even if the residual stress is released in machining process, it is possible to decrease strain caused by the release of the residual stress.
When the predetermined length of time has elapsed from when the driving of the fan 31 is started, the control apparatus 30 of the embodiment changes the amount of driving the fan 31 by controlling the time for changing the amount of driving the fan 31. In contrast, in the embodiment, as illustrated in
Hereinafter, a second embodiment of the present invention will be described with reference to
Similar to the first embodiment, also, in the second embodiment, a rotor blade of a steam turbine is produced. Similar to the first embodiment, also, in the second embodiment, the rotor blade of the steam turbine is produced by executing the forging step (S1), the burr removing step (S2), the heating step (S4), the cooling step (S4), and the finishing step (S5). In the embodiment, cooling technique of the stainless member 10b in the cooling step (S4) is different from that in the first embodiment.
In the cooling step (S4) of the embodiment, cooling of the large surface area portion A of the stainless member 10b, that is, a cooling target is suppressed by covering the large surface area portion A with a covering member 40. Specifically, in the embodiment, as illustrated in
The covering member 40 takes the role of decreasing a temperature difference between the small surface area portion B and the large surface area portion A by approximating the amount of heat dissipation from the small surface area portion B not covered with the covering member 40 to the amount of heat dissipation from the large surface area portion A covered with the covering member 40. For this reason, insofar as the covering member 40 is capable of taking the aforementioned role, the covering member 40 may be made of any material. The covering member 40 may be made of an insulating material, steel, an aluminum alloy, a stainless steel, or the like.
Also, in the cooling step (S4) of the embodiment, as illustrated in
In contrast, in the embodiment, since the large surface area portion A of the stainless member 10b, which is easily cooled, is covered with the covering member 40, the amount of heat dissipation from the large surface area portion A approximates to the amount of heat dissipation from the small surface area portion B. For this reason, in the embodiment, as illustrated in
Accordingly, similar to the first embodiment, also, in the cooling step (S4) of the embodiment, it is possible to decrease the initial cooling time zone in which the temperature of the stainless member 10b is rapidly changed, and strain in the temperature range including the cooling phase-transformation temperature range Mr in which deformation is likely to occur. For this reason, also, in the embodiment, it is possible to decrease strain and residual stress of the stainless member 10b after the cooling step (S4) is complete.
The covering member 40 may be attached to a stainless member before the heating step (S4) is started. In this case, when the cooling step (S4) is started, it is possible to substantially eliminate a temperature difference between the stainless member 10b and the covering member 40, and to suppress the occurrence of thermal strain based on the temperature difference when the covering member 40 is attached to the stainless member 10b. The covering member 40 may be made of the same material as that of the stainless member 10b which is a cooling target. In this case, since the thermal expansion coefficient of the cooling target is the same as that of the covering member 40, the cooling target and the covering member 40 are capable of integrally contracting in a cooling process, and heat transfer between the cooling target and the covering member 40 can be substantially constant. In addition, the cooling target and the covering member 40 have the same thermal properties such as a heat transfer coefficient other than a thermal expansion coefficient. For this reason, in this case, it is possible to easily determine various dimensions of the covering member 40, by which the amount of heat dissipation from the small surface area portion B not covered with the covering member 40 is adjusted to be substantially the same as the amount of heat dissipation from the large surface area portion A covered with the covering member 40.
The flow rate of air blown to the stainless member 10b per unit time is adjusted to be constant from the start to the end of the cooling step (S4). In contrast, similar to the first embodiment, also, in the embodiment, the initial cooling time zone, in which the temperature of the stainless member 10b is rapidly changed, may be controlled. The flow rate of air may be controlled during phase transformation in which deformation is likely to occur.
In the embodiments, the heating step (S3) and the cooling step (S4) are executed after the forging step (S1) is executed. In contrast, a rolling step may be executed instead of the forging step (S1), and the same aforementioned cooling step may be executed after the rolling step and the heating step are executed. The heating step and the cooling step may be executed without executing the forging step or the rolling step.
In the embodiments, the rotor blade 10 of a steam turbine is a production target. In contrast, insofar as a stainless member is subjected to the heating step and the cooling step, any object may be used as a target.
In the example illustrated in the embodiments, a stainless member is made of a precipitation hardening stainless steel. In contrast, as described above, basically similar to the precipitation hardening stainless steel, during heating and cooling, phase transformation occurs in martensitic stainless steels, ferritic stainless steels, and austenitic-ferritic two-layer stainless steels. As a result, also, in a case where a stainless member is made of any of the aforementioned materials, the same cooling step as in the embodiments may be executed.
According to an aspect of the present invention, it is possible to decrease strain of a stainless member.
10: ROTOR BLADE
10A, 10B: STAINLESS MEMBER
11, 11B: BLADE BODY
14: BLADE FRONT EDGE
15: BLADE REAR EDGE
31: FAN
30: CONTROL APPARATUS
40: COVERING MEMBER
A: LARGE SURFACE AREA PORTION
B: SMALL SURFACE AREA PORTION
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-213754 | Oct 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/075853 | 9/29/2014 | WO | 00 |
