Patent Application
20140212323
The present invention relates to a multilayer overlay for forming a Cr-containing steel overlay on a surface with which bearings of a high Cr steel turbine rotor come into contact.
Since a high Cr steel is excellent in high-temperature strength and low-temperature toughness, the steel has been increasingly used as a material for high-pressure and medium-pressure turbine rotors of generators. However, the contact surface with the bearings of the high Cr steel turbine rotor is prone to develop seizure at the bearings during its use, so that there is a concern of inviting damage. Therefore, there has been proposed a method for preventing the generation of seizure by overlay welding a low alloy steel on the bearing part of the rotor (e.g., see Patent Document 1).
Heretofore, with regard to such a kind of overlay welding, mainly welding materials and welding methods for submerge arc welding have been developed.
For example, Patent Document 2 proposes a welding material to be applied to a journal part of a turbine rotor and, in consideration of weld residual stress, discloses one using a welding material having low strength and large linear expansion coefficient as a first layer welding material as compared with a low alloy steel of an upper layer and a rotor substrate.
Moreover, in the invention described in Patent Document 3, it is intended to reduce a difference in the level of strength for the purpose of an increase in fatigue strength. Patent Document 4 discloses intention of an improvement in fatigue strength by using a welding material having a high Cr content as a first layer.
However, since the welding material used in the first layer welding material described in Patent Document 2 contains no Cr, accumulation of strain occurs in the first layer at shape discontinuous parts such as a thrust part during stress relief annealing and there is a possibility of generation of defects such as cracks in some cases, depending on the strength of the upper layer.
Also, in the execution with the welding material having a Cr amount of 1.0% for the first layer defined in the invention described in Patent Document 3, there is a concern that the Cr content of the welding section decreases and further there is a concern that diffusion of C and the like occurs and the strength at the welding metal side decreases due to the difference in the Cr amount between the base material and the welding metal during the stress relief annealing after welding.
As above, in the conventional technologies, since the Cr content of the welding material which comes into contact with the rotor substrate is low and strength balance among the rotor substrate, the first layer, and the upper layer is not considered, the residual stress induced by welding is high. Also, in the shape discontinuous parts such as the thrust part at which stress concentration is prone to occur due to the shape, there is a concern of occurrence of defects. In Patent Document 4, although a welding material having a high Cr content is used for the first layer, it is supposed that stress relief annealing crack susceptibility increases through component dilution by welding depending on the components of the rotor base material, so that there is room for improvement.
Accordingly, an object of the present invention is to provide a combination of overlay welding materials for the first layer or the upper layer and a first layer or upper layer welding section obtained therefrom, and a process for producing a multilayer overlay welding section, which satisfy the strength and toughness required for a high Cr steel turbine rotor bearing part and are used for avoiding cracks during the stress relief annealing.
Namely, a first invention relates to a first layer welding section among a multilayer overlay welding section formed on a bearing contact surface of a high Cr steel turbine rotor, and the first layer welding section includes, in terms of % by weight: C: 0.05 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.5%, Cr: 4.0 to 7.7%, and Mo: 0.5 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, V: 0.2% or less, Ni: 0.3% or less, Co: 1.5% or less, B: 0.005% or less, W: 1.5% or less, and Nb: 0.07% or less in terms of % by weight based on the first layer welding section.
A second invention relates to the first layer welding section among the multilayer overlay welding section formed on the bearing contact surface of the high Cr steel turbine rotor, and in the first invention, the following expression (1) is satisfied:
Pcr(1)=(a Cr amount in the first layer welding section)×0.65−(a Cr amount of the high Cr steel turbine rotor−the Cr amount in the first layer welding section)×0.35>0.7 (1).
A third invention relates to an overlay welding material for a first layer welding section for obtaining the first layer welding section according to the first or second invention, among the multilayer overlay welding section formed on the bearing contact surface of the high Cr steel turbine rotor, and the welding material includes, in terms of % by weight: C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.2%, Cr: 2.0 to 5.5%, and Mo: 0.1 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and a sum of one or more kinds selected from the group consisting of Ni, Nb and Ti is 0.2% or less in terms of % by weight based on the overlay welding material for the first layer welding section.
A fourth invention relates to an upper layer welding section formed on the first layer welding section according to the first or second invention, among the multilayer overlay welding section formed on the bearing contact surface of the high Cr steel turbine rotor, the upper layer welding section includes, in terms of % by weight: C: 0.05 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 2.5%, Cr: 1.0 to 4.0%, and Mo: 0.5 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, V: 0.15% or less, Ni: 0.3% or less, and Nb: 0.07% or less in terms of % by weight based on the upper layer welding section.
A fifth invention relates to the upper layer welding section of the high Cr steel turbine rotor, and in the fourth invention, an amount of V contained in the upper layer welding section is smaller than an amount of V contained in the first layer welding section according to the first or second invention.
A sixth invention relates to the upper layer welding section of the high Cr steel turbine rotor, and in the fourth or fifth invention, the following expression (2) is satisfied:
Per(n)=(a Cr amount in the upper layer welding section at n-th layer)×0.65−{a Cr amount in the upper layer welding section at (n−1)-th layer−the Cr amount in the upper layer welding section at the n-th layer}×0.35>0.7 (2),
in which when N represents the number of layers constituting the multilayer overlay welding section, 2≦n≦N.
A seventh invention relates to an overlay welding material for an upper layer welding section for obtaining the upper layer welding section according to any one of the fourth to sixth inventions formed on the first layer welding section of the multilayer overlay welding section formed on the bearing contact surface of the high Cr steel turbine rotor, and the welding material includes, in terms of % by weight: C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 3.0%, Cr: 1.0 to 2.5%, and Mo: 0.1 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and a sum of one or more kinds selected from the group consisting of Ni, Nb and Ti is 0.2% or less in terms of % by weight based on the overlay welding material for the upper layer welding section.
A eighth invention relates to a process for producing a multilayer overlay welding section of a high Cr steel turbine rotor, the process including:
forming the first layer welding section according to the first or second invention on the bearing contact surface of the high Cr steel turbine rotor by welding using an overlay welding material for the first layer welding section including, in terms of % by weight: C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.2%, Cr: 2.0 to 5.5%, and Mo: 0.1 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and a sum of one or more kinds selected from the group consisting of Ni, Nb and Ti is 0.2% or less in terms of % by weight based on the overlay welding material for the first layer welding section, and
forming the upper layer welding section according to any one of the fourth to sixth inventions on an upper layer of the thus-formed first layer welding section by welding using an overlay welding material for the upper layer welding section including, in terms of % by weight: C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 3.0%, Cr: 1.0 to 2.5%, and Mo: 0.1 to 1.5%, with a remainder including Fe and unavoidable impurities, in which the unavoidable impurities contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and a sum of one or more kinds selected from the group consisting of Ni, Nb and Ti is 0.2% or less in terms of % by weight based on the overlay welding material for the upper layer welding section.
According to the upper layer welding section of the present invention, ferrite is not formed at the boundary part between the first layer and the upper layer, also it is possible to suppress stress relief annealing cracks, and bearings can be prevented from seizing with satisfying strength and toughness required for the bearing part.
Furthermore, the strength balance among the base material, the first layer, and the upper layer is made most suitable by combining the first layer welding section according to the first invention and the second invention as mentioned above with the upper layer welding section, so that the strain concentration to the first layer can be prevented during stress relief annealing. Thereby, the execution becomes possible without generating stress relief annealing cracks even for the welding of the thrust part which may develop large residual stress at welding. Also, by considering the difference in the Cr amount in each layer, an overlay-strengthened high Cr steel turbine rotor having stable quality can be provided with preventing the formation of pro-eutectoid ferrite.
a) to 4(d) are schematic views showing a ring crack test piece for evaluating stress relief annealing crack susceptibility at the first layer welding section.
The following will describe embodiments of the present invention in detail but the invention is not limited thereto and can be implemented in any variations.
The components of the first layer welding section according to the invention contain C: 0.05 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.5%, Cr: 4.0 to 7.7%, and Mo: 0.5 to 1.5% in terms of % by weight and a remainder includes Fe and unavoidable impurities. The following will describe the meanings of these components and, when the first layer welding section has the above-mentioned composition, a difference in the level of strength among the base material and the overlay first layer or subsequent overlay layers can be suppressed to a degree at which problems do not practically arise and thus stress relief annealing cracks in the first layer can be suppressed. The contents in the following are all shown in terms of % by weight.
Since C is a necessary additive element from the viewpoint of securing the tensile strength of the welding section, a lower limit thereof is set to 0.05%. On the other hand, in view of a decrease in an impact value and an increase in weld crack susceptibility, an upper limit thereof is set to 0.2%.
Since Si is an element necessary as a deoxidizer or for securing strength, a lower limit thereof is set to 0.1%. However, since an excessive content of Si promotes cracks such as stress relief annealing cracks and invites a decrease in toughness, an upper limit thereof is set to 1.0%. For the same reason, the lower limit thereof is preferably set to 0.25% and the upper limit is desirably set to 0.7%.
Since Mn is an element necessary as a deoxidizer or for securing strength similarly to Si, a lower limit thereof is set to 0.3%. However, since an excessive content of Mn invites a decrease in toughness, an upper limit thereof is set to 1.5%. For the same reason, the upper limit is set to desirably 1.2%, more desirably 1.0%.
Cr is an important element for securing strength and toughness. In order to suppress the strain concentration to the first layer, suppress the difference in Cr from the base material, and prevent the formation of ferrite, a lower limit thereof is set to 4.0%. However, since an excessive content of Cr increases hardenability and enhances weld crack susceptibility, an upper limit thereof is set to 7.7%. For the same reason, the upper limit is desirably set to 6.7%.
Since Mo precipitates as a carbide during the stress relief annealing and enhances temper softening resistance, it is an important element for obtaining strength after the stress relief annealing. In order to suppress the strain concentration during the stress relief annealing, a lower limit thereof is set to 0.5%. However, since an excessive content of Mo enhances cracking ability and invites a decrease in toughness, an upper limit thereof is set to 1.5%. For the same reason, the upper limit is desirably set to 1.0%.
The essential constituting elements of the first layer welding section are as mentioned above. Also, the remainder substantially contains Fe and unavoidable impurities resulting from dilution from the base material. As the unavoidable impurities, the first layer welding section may contain P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, V: 0.2% or less, Ni: 0.3% or less, Co: 1.5% or less, B: 0.005% or less, W: 1.5% or less, and Nb: 0.07% or less in terms of % by weight based on the whole first layer welding section. The following will describe reasons thereof.
P: 0.015% or less
P is an impurity element that mixes in from a raw material during the smelting of a metal material. Since P has a possibility of decreasing toughness, it is desirable to reduce it as far as possible. Therefore, the content of P is set to 0.015% or less.
S: 0.015% or less
Similarly to P, S is an impurity element that mixes in from a raw material during the smelting of a metal material. Since S has a possibility of decreasing toughness, it is desirable to reduce it as far as possible. Therefore, the content of S is set to 0.015% or less.
Cu: 0.2% or less
Since Cu has a possibility of decreasing the toughness of the welding section, an upper limit of the content is set to 0.2% or less.
V: 0.2% or less
V is known as an element for increasing the temper softening resistance to obtain strength after stress relief annealing. However, from the studies of the present inventors, it has been found that V extremely elevates the stress relief annealing crack susceptibility. Therefore, the content of V is limited to 0.2%, preferably 0.1% at most.
Ni: 0.3% or less
Since an excessive content of Ni has a possibility of causing temper embrittlement, the content of Ni is limited to 0.3% at most.
Co: 1.5% or less
In the case where the base material contains Co, the content of Co should be suppressed to an increase by dilution/fusion from the base material during welding and is limited to 1.5% at most.
B: 0.005% or less
W: 1.5% or less
Nb: 0.07% or less
These elements are generally known as elements that improve temper softening resistance during the stress relief annealing and secure room-temperature strength. However, from the studies of the present inventors, it has been found that an excessive content of each element decreases toughness and also degrades welding ability. Therefore, in the invention, as for the contents of these components, upper limits are determined to the above values, respectively.
The Cr amount contained in the first layer welding section according to the invention preferably satisfies the following expression (1):
Pcr(1)=(Cr amount in first layer welding section)×0.65−(Cr amount in high Cr steel turbine rotor−Cr amount in first layer welding section)×0.35>0.7 (1)
With an increase in the Cr content of the first layer welding section and a decrease in the difference from the Cr amount in the high Cr steel turbine rotor, the ferrite formation during the stress relief annealing is suppressed. When the Pcr (1) value represented by the above expression (1) is a value exceeding 0.7, the ferrite formation is suppressed at the boundary between the base material and the first layer welding section and an overlay-strengthened high Cr steel turbine rotor having stable quality can be obtained. Incidentally, the Cr amounts in the above expression (1) are all represented in terms of % by weight.
In the welding material for obtaining the above first layer welding section, the dilution of the components occurs with the base material during the overlay welding on the base material and also the dilution of the components occurs with the upper layer to be welded on the upper layer. The dilution of the components occurs through the fusion of a part of the neighboring layers during welding and the migration of the components from a layer having high concentration of the components to a layer having low concentration thereof. It is generally sufficient that the base material is a high Cr steel to be commonly used as a turbine rotor for thermal power generation and particularly, the material thereof is preferably 12Cr steel, more preferably one so-called new 12Cr steel in which W, Co, and B are added thereto.
With considering the aforementioned dilution, the welding material for the first layer welding section is defined for obtaining the above composition of the first layer welding section. Thereby, the action and effect of the first layer welding section can be exhibited.
By using the welding material for the first layer welding section as an overlay first layer in the high Cr rotor substrate, a difference in the level of strength between the base material and the overlay first layer or subsequent overlay layers can be suppressed to a degree at which no problem arises practically and the stress relief annealing cracks can be suppressed in the first layer. Moreover, for the welding material, by suppressing the difference in the Cr content between the base material and the first layer welding metal so that the first layer welding section satisfies the above expression (1), the formation of ferrite can be suppressed.
Specifically, the welding material for the first layer welding section according to the invention includes C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.2%, Cr: 2.0 to 5.5%, and Mo: 0.1 to 1.5% in terms of % by weight, and the remainder includes Fe and unavoidable impurities. The unavoidable impurities are desirably P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and the sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti is 0.2% or less in terms of % by weight based on the welding material for the first layer welding section.
The following will specifically describe individual components.
The C content of the first layer welding section according to the invention ranges from 0.05 to 0.2% and there is a case where the base material component exceeds the upper limit of the C content range of the first layer welding section. In consideration of the dilution/fusion with the base material component during welding, it is desirable that a lower limit of the C content in the welding material is set to 0.03% and an upper limit thereof is set to 0.2% in view of welding workability and the like, for making the C content of the first layer welding section fall within the above range.
The Si content of the first layer welding section according to the invention ranges from 0.1 to 1.0%. In consideration of the dilution/fusion with the base material component during welding, the Si content in the welding material desirably ranges from 0.1 to 1.0%, for making the Si content of the first layer welding section fall within the above range.
The Mn content of the first layer welding section according to the invention ranges from 0.3 to 1.5% and there is a case where the base material component exceeds the upper limit of the Mn content range of the first layer welding section. In consideration of the dilution/fusion with the base material component during welding, it is desirable that a lower limit of the Mn content in the welding material is 0.3% for assuring the effect as a deoxidizer and an upper limit thereof is 1.2% so as not to exceed the upper limit of the component range of the first layer welding section.
The Cr content of the first layer welding section according to the invention ranges from 4.0 to 7.7%. In consideration of the dilution/fusion with the base material component during welding, the Cr content in the welding material desirably ranges from 2.0 to 5.5% for obtaining the component range of the first layer welding section.
The Mo content of the first layer welding section according to the invention ranges from 0.5 to 1.5%. In consideration of the dilution/fusion with the base material component during welding, the Mo content in the welding material desirably ranges from 0.5 to 1.5% for obtaining the component range of the first layer welding section.
As the impurities contained in the welding material, there may be mentioned P, S, Cu, V, Ni, Nb, Ti, W, Co, B, and the like.
As the impurities of the first layer welding section defined in the invention, the following are allowed: P: 0.015% or less, S: 0.015% or less, and Cu: 0.2% or less. These are components that deteriorate mechanical properties and welding ability of both the welding section and base material and, also in the welding material for the first layer, the same component range is desirable as that of the first layer welding section.
V: 0.1% or less
V is a component contained in the base material. For obtaining the range, i.e., 0.2% or less, of the first layer welding section according to the invention, the V content in the welding material is desirably 0.1% or less in consideration of the dilution/fusion during welding.
The sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti being 0.2% or less
Ni and Nb are elements contained in the base material. For obtaining Ni: 0.3% or less and Nb: 0.07% or less that are the ranges of the first layer welding section according to the invention, the contents thereof in the welding material are desirably as low as possible in consideration of the dilution/fusion during welding.
Moreover, Ti is an element that is usually hardly contained in the base material. However, since Ti increases the formation of non-metal inclusion when remains, the content thereof in the welding material is desirably as low as possible. Therefore, the sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti is desirably 0.2% or less.
W, Co, and B are components that may be contained in the base material. For obtaining W: 1.5% or less, Co: 1.5% or less, and B: 0.005% or less that are the ranges of the first layer welding section according to the invention, the contents thereof are desirably as low as possible within the ranges unavoidably contained by a usual process for producing the welding material.
The components constituting the upper layer according to the invention contain C: 0.05 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 2.5%, Cr: 1.0 to 4.0%, and Mo: 0.5 to 1.5% in terms of % by weight and the remainder includes Fe and unavoidable impurities.
The following will describe reasons for defining the components of the upper layer welding section of the invention. Incidentally, the contents in the following are all represented in terms of % by weight.
Since C is a necessary additive element for imparting a necessary strength to the bearing surface, a lower limit thereof is set to 0.05%. However, since the impact value is lowered and the weld crack susceptibility becomes high when the content thereof exceeds 0.2%, an upper limit thereof is set to 0.2%.
Since Si is an element necessary as a deoxidizer or for securing strength as shown in the first layer overlay welding metal, the content thereof is limited to 0.1% at least. However, since an excessive content of Si promotes cracks such as stress relief annealing cracks and invites a decrease in toughness, an upper limit thereof is set to 1.0%. For the same reason, the lower limit is desirably set to 0.3% and the upper limit is desirably set to 0.7%.
Since Mn is an element necessary as a deoxidizer or for securing strength similarly to Si, a lower limit thereof is set to 0.3%. However, since an excessive content of Mn invites a decrease in toughness, an upper limit thereof is set to 2.5%. For the same reason, the lower limit is desirably set to 0.7% and the upper limit is desirably set to 2.0%, and further, the lower limit is more desirably set to 1.0%.
Cr is an important element for securing strength and toughness. In order to suppress the difference in Cr from the first layer and prevent the formation of ferrite, a lower limit thereof is set to 1.0%. However, since strain is concentrated to the first layer owing to too high strength when the content exceeds 4.0% and a crack may be generated in some cases during the stress relief annealing, an upper limit thereof is set to 4.0%.
Since Mo precipitates as a carbide during the stress relief annealing and enhances temper softening resistance, it is an important element for obtaining strength after the stress relief annealing. In order to suppress the strain concentration during the stress relief annealing, a lower limit thereof is set to 0.5%. However, an excessive content thereof enhances the stress relief annealing crack susceptibility and invites a decrease in toughness, an upper limit thereof is set to 1.5%.
The essential constituting elements of the upper layer welding section are as mentioned above and the remainder substantially contains Fe. However, minute amounts of unavoidable impurities such as S, P, and Ni may be further contained in the range where the above properties are not inhibited. Seizure resistance can be made excellent by using the above upper layer welding section as an upper layer in the high Cr steel turbine rotor substrate, and the formation of ferrite can be suppressed by suppressing the difference in the Cr content between the first layer and the upper layer welding section.
As the unavoidable impurities, there may be contained P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, V: 0.15% or less, Ni: 0.3% or less, and Nb: 0.07% or less in terms of % by weight based on the upper layer welding section. The following will describe details.
P: 0.015% or less
P is an impurity element that mixes in from a raw material during the smelting of a metal material. Since P has a possibility of decreasing toughness, it is desirable to reduce it as far as possible. Therefore, the content of P is set to 0.015% or less.
S: 0.015% or less
S is also an impurity element that mixes in from a raw material during the smelting of a metal material. Since S has a possibility of decreasing toughness, it is desirable to reduce it as far as possible. Therefore, the content of S is set to 0.015% or less.
Cu: 0.2% or less
Since Cu has a possibility of decreasing the toughness of the welding section, an upper limit thereof is set to 0.2% or less.
V: 0.15% or less
V is an element for increasing the temper softening resistance to obtain strength after the stress relief annealing. On the other hand, in the case of considering that the prevention of the stress annealing crack is important, the content of V is limited. Since V improves the temper softening resistance, the strain concentration to the first layer occurs and thus a crack may be generated during the stress annealing in some cases. In order to avoid it, the V content is limited to 0.15% or less.
Incidentally, a welding material containing more than 0.15% of V may be used in combination, for example, with avoiding the stress annealing cracks by controlling the V content of the welding section to 0.15% or less using a welding material having a low V content as the first layer of the upper layer and with controlling the V content of the welding section of each of the second and subsequent layers or at least the product surface layer part that is the outermost surface of the upper layer to 0.15 to 0.3%. In that case, the Cr content of welding section of the product surface layer part is desirably 2.5% or less for preventing seizure. Although a mechanism between the Cr amount and the generation of seizure is not solved, it is empirically well known for one of ordinary skill in the art that a risk of the generation of seizure becomes high when the Cr amount exceeds 2.5%.
Moreover, the amount of V contained in the upper layer welding section is preferably smaller than the amount of V contained in the first layer welding section from the viewpoint of no occurrence of excessive stress concentration to the first layer.
Ni: 0.3% or less
Since an excessive content of Ni has a possibility of causing temper embrittlement, the content thereof is limited to 0.3% at most.
Nb: 0.07% or less
Nb improves the temper softening resistance during the stress relief annealing and secures the room-temperature strength. However, since an excessive content thereof may decrease toughness and also degrade welding ability, an upper limit thereof is defined to be 0.07% in the invention.
The Cr amount contained in the upper layer welding section preferably satisfies the following expression (2):
Pcr(n)=(Cr amount in upper layer welding section at n-th layer)×0.65−{Cr amount of upper layer welding section at (n−1)-th layer−Cr amount of upper layer welding section at n-th layer}×0.35>0.7 (2),
in which when N represents the number of layers constituting the multilayer overlay welding section, 2≦n≦N.
By controlling Pcr (n) represented by the above expression (2) to a value exceeding 0.7 with considering the difference between the Cr amount of the upper layer welding section at the n-th layer and the Cr amount of the upper layer welding section at the (n−1)-th layer, the ferrite formation at the boundaries of individual layers is suppressed. In this regard, Per (2) means a calculation value satisfying the expression (2) of an upper layer that is the 2nd layer (the 1st layer of the upper layer) on the first layer welding section that is the 1st layer. In this case, in the expression (2), the upper layer welding section at the (n−1)-th layer corresponds the first layer welding section. Incidentally, the Cr amounts in the above expression are all represented in terms of % by weight.
With considering composition variation by the occurrence of dilution of the components with the first layer welding section during the overlay welding on the first layer welding section, the overlay welding material for obtaining the above upper layer welding section is defined for obtaining the above composition of the upper layer welding section. By the welding using the overlay welding material, the above upper layer welding section can be obtained, and the action and effect of satisfying strength and toughness and avoiding cracks during the stress relief annealing can be exhibited.
Specifically, the overlay welding material for the upper layer welding section contains C: 0.03 to 0.2%, Si: 0.1 to 1.0%, Mn: 0.3 to 3.0%, Cr: 1.0 to 2.5%, and Mo: 0.1 to 1.5% in terms of % by weight, with the remainder including Fe and unavoidable impurities. The unavoidable impurities desirably contains P: 0.015% or less, S: 0.015% or less, Cu: 0.2% or less, and V: 0.1% or less, and the sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti is 0.2% or less in terms of % by weight based on the overlay welding material for the upper layer welding section.
The following will specifically describe the above components.
The C content of the upper layer welding section according to the invention ranges from 0.05 to 0.2%. In consideration of the dilution/fusion with C contained in the first layer welding section according to the invention, it is desirable that a lower limit of the C content in the welding material is set to 0.03% and an upper limit thereof is set to 0.2% in view of welding workability, in order not to exceed the upper limit of the C component range of the upper layer welding section.
The Si content of the upper layer welding section according to the invention ranges from 0.1 to 1.0%. In consideration of the dilution/fusion with Si contained in the first layer welding section according to the invention, the Si content in the welding material desirably ranges from 0.1 to 1.0% for obtaining the Si component range of the upper layer welding section.
The Mn content of the upper layer welding section according to the invention ranges from 0.3 to 2.5%. In consideration of the dilution/fusion with Mn contained in the first layer welding section according to the invention, the Mn content in the welding material is desirably ranges from 0.3% to 3.0% for obtaining the Mn component range of the upper layer welding section.
The Cr content of the upper layer welding section according to the invention ranges from 1.0 to 4.0%. In consideration of the dilution/fusion with Cr contained in the first layer welding section according to the invention, a lower limit of the Cr content in the welding material is desirably 1.0% so as not to be lower than the lower limit of the Cr content of the upper layer welding section. An upper limit thereof is desirably 2.5% so as not to exceed the upper limit of the Cr content of the upper layer welding section.
Moreover, the Cr content of the upper welding section as the product surface layer part that is the outermost surface of the upper layer is also desirably 2.5% or less for preventing seizure. Similarly, an upper limit of the overlay welding material for the upper layer welding section as the product surface layer part is also desirably 2.5%.
The Mo content of the upper layer welding section according to the invention ranges from 0.5 to 1.5%. In consideration of the dilution/fusion with Mo contained in the first layer welding section according to the invention, the Mo content in the welding material desirably ranges from 0.1 to 1.5% for obtaining the component range of the upper layer welding section.
Unavoidable impurities: P, S, Cu, V, Ni, Nb, Ti, W, Co, B
As the impurities contained in the upper layer overlay welding material according to the invention, the following are allowed: P: 0.015% or less, S: 0.015% or less, and Cu: 0.2% or less in terms of % by weight based on the overlay welding material for the upper layer welding section. These are components deteriorating mechanical properties and welding ability, and the same component ranges are desirable as those of the upper layer welding section according to the invention.
V: 0.1% or less
As for V, 0.2% or less of V may be contained in the first layer welding section by the dilution/fusion from the base material. Therefore, for obtaining 0.15% or less as the range of the V content of the upper layer welding section according to the invention, the V content in the welding material is desirably 0.1% or less in consideration of the dilution/fusion during binding.
The sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti being 0.2% or less
Ni and Nb are elements that may be contained in the base material. For obtaining Ni: 0.3% or less and Nb: 0.07% or less that are the ranges of the upper layer welding section defined in the invention, the contents thereof in the welding material are desirably as low as possible.
Moreover, Ti is usually hardly contained in the base material but, since Ti increases the formation of non-metal inclusion when remains in the welding section, the content in the welding material is also desirably as low as possible. Therefore, the sum of one or more kinds selected from the group consisting of Ni, Nb, and Ti is desirably 0.2% or less.
W, Co, and B may be contained in the ranges of W: 1.5% or less, Co: 1.5% or less, and B: 0.005% or less in the first layer welding section by the dilution/fusion from the base material. However, the upper layer welding section does not necessarily contain these components and, in view of costs, it is sufficient that the contents fall within the ranges unavoidably contained by a usual process for producing the welding material.
With regard to the composition of the welding section, since the material to be welded is generally melted to a degree of 20 to 40% and is diluted/fused with the welding material during welding, the elemental composition of the welding material may be determined with taking the dilution/fusion into account.
In the invention of the present application, a high Cr steel turbine rotor is a target of the overlay welding. The high Cr steel turbine rotor is constituted by a high Cr steel and, for example, a steel containing 8 to 13% of Cr is exemplified. The composition of the turbine rotor in the invention is not limited to specific one and a high Cr steel capable of being used as the turbine rotor is sufficient.
The following will exemplify a typical turbine rotor composition (% by weight).
C: 0.05 to 0.25%,
Si: 1.0% or less,
Mn: 1.5% or less,
Ni: 1.0% or less,
Cr: 8 to 13%,
Mo: 2.0% or less,
V: 0.05 to 0.4%,
Nb: 0.01 to 0.1%,
N: 0.01 to 0.05%,
W: 0.05 to 5.0%,
Co: 0.05 to 5.0%, and
B: 0.015% or less,
with the remainder being Fe and unavoidable impurities.
A high Cr steel turbine rotor 1 has a journal part 2 and a thrust part 3 as bearing parts, and the overlay welding section can be formed by welding to one or both of the journal part 2 and the thrust part 3 using the welding material according to the invention.
At the formation of the overlay welding section, it is desirable to form the first layer welding section according to the invention and then to form the upper layer welding section thereon using the welding material according to the invention. In the formation of the above first layer and upper layer welding sections, it is desirable to perform the welding by TIG (Tungsten Inert Gas) welding, submerge arc welding, or the like. The welding method and welding conditions in the welding are not particularly limited in the invention and a known method can be conducted under known conditions.
At the welding of an under layer, it is desirable to determine the Cr content of the first layer so as to satisfy the following expression (1).
Pcr(1)=(Cr content in first layer welding section)×0.65−(Cr content of high Cr steel turbine rotor−Cr content in first layer welding section)×0.35>0.7 (1)
Also, at the welding of overlaying, it is desirable to determine the Cr content of the upper layer so as to satisfy the following expression (2).
Pcr(n)=(Cr content in upper layer welding section at n-th layer)×0.65−{Cr content in upper layer welding section at (n−1)-th layer−Cr content in upper layer welding section at n-th layer}×0.35>0.7 (2),
in which when N represents the number of layers constituting the multilayer overlay welding section, 2≦n≦N.
The following will describe Examples of the invention.
Using a 12Cr rotor substrate having a component composition (the remainder being Fe and unavoidable impurities) shown in Table 1 on the supposition of a high Cr steel turbine rotor, overlay welding wires each having the component composition (the remainder being Fe and unavoidable impurities) shown in Table 2 were used as welding materials for first layer welding sections of Examples or Comparative Examples and further, overlay welding wires each having the component composition (the remainder being Fe and unavoidable impurities) shown in Table 3 were used as overlay welding materials for upper layer welding sections of Examples or Comparative Examples.
The component analysis of the rotor substrate was conducted in accordance with the prescription of JIS G 1253 (2010) after a test piece was sampled from an arbitrary portion. The component analysis of the overlay welding wires as the overlay welding materials for the first layer welding section or for the upper layer welding object was conducted in accordance with the prescription of JIS Z 3317 (2010).
Using each of the above welding materials, the first layer and the upper layer was overlay-welded by TIG welding under the welding conditions shown in Table 4. Thereafter, a test piece was sampled from the check analysis position 20 shown in
Furthermore, as shown by α in
Then, in order to evaluate the stress relief annealing crack susceptibility with the Cr amount, a high-temperature low strain rate tensile test was conducted at a strain rate of 6.7×10−6/s using Cr variation materials in which the Cr content was varied by changing welding conditions. A value of reduction in area (%) was used for the evaluation.
Table 5 shows results of check analysis of the first layer and
Next, Table 6 shows results of check analysis of the first layer welding section after overlay welding (the remainder being Fe and unavoidable impurities) and Table 7 shows results of check analysis of the upper layer welding section (the remainder being Fe and unavoidable impurities). Moreover, Table 8 shows weld combinations with which the high-temperature low strain rate tensile test was performed and test results thereof.
The check analysis and the high-temperature low strain rate tensile test were performed in the same manner as mentioned above.
From Table 6, Cr contained in the first layer welding sections according to the invention is from 4.0 to 7.7% by weight but the contents of Cr are small in the first layer wires Nos. 1, 2, and 5 and thus the contents fall out of the above range. Moreover, as compared with the range of the Si content contained in the first layer welding sections according to the invention, the content is small in the first layer wire No. 2 and, as compared with the range of the V content, the content is large in the first layer wire No. 5.
In Table 8, since the accumulation of strain into the first layer is thought to be a cause of generation of the stress relief annealing cracks, the case where a test piece was broken at the first layer and the reduction in area at break is 10% or less is shown as (x), the case where a test piece was broken at the first layer and the reduction in area at break is more than 10% and less than 30% is shown as (Δ), and the case where a test piece was broken at the upper layer and the reduction in area at break is 30% or more is shown as (0). As is apparent also from Table 8, with wires of Examples which satisfy the requirements of the invention, test pieces are broken at the upper layer welding metal or the reduction in area at break exceeds 10% even when they are broken at the first layer section, so that no accumulation of strain into the first layer is observed or a sufficient reduction in area at break is exhibited even when the accumulation of strain into the first layer is observed.
In the case where the difference in the Cr amount contained is large as in the case of the first layer welding metal (first layer wire No. 6) and the upper layer welding metal (upper layer wire No. 8), Pcr (2) was 0.66 in the following expression and minute ferrite was formed at the boundary of the welding sections. Since ferrite induces a local decrease in strength and there is a possibility of concentration of strain during the stress relief annealing treatment. Therefore, it is preferable to prevent the ferrite formation, so that the Cr amount of each layer desirably satisfies a value larger than 0.7 in the expression shown below. Moreover, from the following expression (1), the precipitation of ferrite at the boundary between the base material and the first layer can be prevented when the Cr amount of the first layer welding section is 4.1% or more.
Pcr(n)=(Cr amount in metal at n-th layer)×0.65−{Cr amount in metal at (n-1)-th layer−Cr amount in metal at n-th layer}×0.35>0.7 (1),
in which n=0 represents the base material and n=1 represents the first layer.
[0088]
After welding was performed with a welding material for the first layer welding section where the welding material was selected from Table 2 based on the results in Table 8, a ring crack test was conducted, which is a test for evaluating the stress relief annealing crack susceptibility of the first layer welding section.
In
b) shows a side view of the ring crack test piece 12 and
The ring crack test piece 12 has a cylindrical shape having an inner diameter of 5 mm, an outer diameter of 10 mm, and a length of 20 mm and a slit 12a running through in a diameter direction and having a gap of 0.3 mm is formed along an axis direction in the side wall. Also, on the outer peripheral wall at the opposite side of the slit 12a, a U notch 12b having a width of 0.4 mm, a depth of 0.5 mm, and a bottom part whose cross-sectional shape is a curved shape having a curvature of 0.2 mm is formed along an axis direction.
In order to exclude the influence of the welding thermal cycle, after one layer is build up on the base material with an arbitrary welding material, the ring crack test piece 12 is sampled with adjustment so that the U notch comes to an unaffected zone, in a welded state without further treatment (
By imparting a force to the sampled test piece in arrowed directions as shown in
After the restrained and welded ring crack test piece 121 was subjected to a stress relief annealing treatment at 630° C.×10 hours, the presence of cracks was evaluated using two test pieces (N−1, N−2) by observing three cross-sections per one test piece, i.e., six cross-sections in total. Table 9 shows the results. Also, the Pcr(1) value was calculated based on the expression (1) and shown in Table 9.
The case where no crack is generated was shown as “O” and the case where crack(s) were generated was shown as “x”.
Here, the compositions of elements constituting the wires Nos. 3, 4, and 6 fall within the range of the first layer welding section according to the invention and the wires Nos. 1 and 5 have compositions falling outside the range of the first layer welding section according to the invention.
From the results in Table 9, the stress relief annealing crack susceptibility varied depending on the Cr content and the stress relief annealing crack susceptibility decreased in the case of a high Cr material.
Table 10 shows results collecting the above Examples. In the over-all evaluation, it was evaluated so that the case where all the individual evaluation items were shown as “O” was marked “OO”, the case where “Δ” was present was marked “O”, and the case where “x” was present in any item was marked “x”. The case where the over-all evaluation is marked “OO” can be judged to be sufficiently usable and the case where the over-all evaluation is marked “O” can be judged to be usable.
From the results, the welding sections and welding materials according to the invention exhibit low stress relief annealing crack susceptibility of the first layer and no accumulation of strain in the first layer was observed in the high-temperature low strain rate tensile test, so that breakage occurs at the upper layer welding metal. Furthermore, by considering Cr in the base material, the first layer, and the upper layer, the precipitation of ferrite can be also suppressed.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
The present application is based on Japanese Patent Application No. 2011-178628 filed on Aug. 17, 2011, and the contents are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-178628 | Aug 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/070923 | 8/17/2012 | WO | 00 | 2/14/2014 |
