Patent Application
20200010930
The present invention relates to a super heat resistant Ni-based alloy and a method of manufacturing the Ni-based alloy.
A super heat resistant Ni-based alloy, such as an Inconel (registered trademark) 718 alloy, has been used for a heat-resistant product for an engine of an aircraft or a gas turbine for power generation. The heat-resistant product has been required to have a heat-resistance to a higher temperature due to an increase in performance and a reduction in fuel consumption of the gas turbine. In order to improve the heat resistance (or high-temperature strength) of the Ni-based alloy, it is most effective to increase an amount of gamma prime phase (hereinafter also referred to merely as “γ′ (gamma-prime)” phase) which is an intermetallic compound mainly composed of Ni3Al and effects as a precipitation-strengthening phase. When the Ni-based alloy includes higher amounts of Al, Ti and Nb which are γ′ generating elements, high-temperature strength of the Ni-based alloy can be further improved. In the future, a Ni-based alloy including a larger amount of γ′ phase will be demanded in order to achieve the high heat resistance and the high-temperature strength.
However, it has been known that the Ni-based alloy including a larger amount of γ′ phase has higher deformation resistance during hot working and thus has lower workability. In particular, when the amount of γ′ phase in the Ni-based alloy is not less than 35-40 mol %, the Ni-based alloy has particularly reduced workability. For example, alloys such as Inconel (registered trademark) 713C alloy, IN939, IN100 and Mar-M247 include a particularly large amount of γ′ phase and thus can not be subjected to plastic working. Accordingly, these alloys are typically used as-cast, i.e. they are as-cast alloys.
As a proposal for improving hot plastic workability of such a super heat resistant Ni-based alloy, WO 2016/129485A1 discloses a method of manufacturing the super heat resistant Ni-based alloy. The Ni alloy ingot having a composition including a γ′ phase of not less than 40 mol % is cold worked at a working rate of not less than 5% and less than 30%, and the cold-worked alloy is then heat treated at a temperature higher than a γ′ solid solution temperature. In this method, a combination of the cold working step and the heat treatment step provides a recrystallization rate of not less than 90% at which hot working can be applied to the Ni-based alloy.
Furthermore, such cases have been increasing in recent years that a heat-resistant product of the Ni-based alloy including a large amount of γ′ phase is repaired or the heat-resistant product of the Ni-based alloy is produced by three-dimensional forming. In such cases, a wire of the Ni-based alloy has been required as a raw material for the forming. The wire product can also be processed e.g. into a spring. The wire product of the Ni-based alloy has a small diameter of e.g. not more than 5 mm, or further not more than 3 mm. Such a wire product is efficiently produced, for example, by plastic-working an intermediate product of a “wire material” having a diameter of not more than 10 mm. If the “wire material” which is the intermediate product can also be produced by plastic-working, a wire product of the super heat resistant Ni-based alloy can be efficiently produced.
As a method of manufacturing such a wire product of a super heat resistant alloy, a method has been proposed (see U.S. Pat. No. 4,777,710 A), starting from a cast wires having a diameter of not less than 5 mm and hot-extruding a bundle of the cast wires and then separate it thereafter.
PATENT LITERATURE 1: WO 2016/129485 A1
PATENT LITERATURE 2: U.S. Pat. No. 4,777,710 A
A super heat resistant Ni-based alloy which includes a larger amount of γ′ phase has lower hot plastic workability as described above. The method of U.S. Pat. No. 4,777,710 A is applicable to a limited composition for effectively manufacturing a wire product, but it is extremely difficult to produce a wire product by hot plastic working a super heat resistant Ni-based alloy having a composition in which an amount of γ′ phase is “not less than 35 mol %” (described later). Furthermore, the method of U.S. Pat. No. 4,777,710 A has problems since the process is complicated and takes high cost.
The method of WO 2016/129485 A is effective in manufacturing a super heat resistant Ni-based alloy to which hot working is applied. For the manufacturing of the Ni-based alloy, however, the ingot should be cold worked at a working rate of not less than 5% and less than 30% and then further heat treated.
An object of the present invention is to provide a super heat resistant Ni-based alloy which has excellent plastic workability by a new approach different from a conventional approach and a method of manufacturing the Ni-based alloy. Another object of the present invention is to provide the Ni-based alloy to which plastic working at a high working rate can be performed without performing hot working and the method of manufacturing the Ni-based alloy. Further object of the present invention is to provide the Ni-based alloy to which plastic working at a high working rate can be performed without performing heat treatment during the plastic working and a method of manufacturing the Ni-based alloy. Further object of the present invention is to provide a new method of manufacturing a wire material and a wire product of the super heat resistant Ni-based alloy.
According to an aspect of the present invention, provided is a super heat resistant Ni-based alloy having such a composition that an amount of precipitated gamma prime phase in equilibrium at 700° C. is not less than 35 mol %. The Ni-based alloy has a cross-sectional structure including grains having a maximum grain size of not more than 75 nm.
According to an embodiment, the Ni-based alloy preferably has a hardness of not less than 500 HV.
According to an embodiment, the cross-sectional structure preferably includes not less than 5 grains having a maximum grain size of not more than 75 nm per 1 μm2.
According to an embodiment, the Ni-based alloy preferably includes, by mass %, 0 to 0.25% of C, 8.0 to 25.0% of Cr, 0.5 to 8.0% of Al, 0.4 to 7.0% of Ti, 0 to 28.0% of Co, 0 to 8% of Mo, 0 to 6.0% of W, 0 to 4.0% of Nb, 0 to 3.0% of Ta, 0 to 10.0% of Fe, 0 to 1.2% of V, 0 to 1.0% of Hf, 0 to 0.300% of B, 0 to 0.300% of Zr, and the balance of Ni and impurities.
According to an embodiment, the Ni-based alloy preferably has such a composition such that an amount of precipitated gamma prime phase in equilibrium at 700° C. is not less than 40 mol %.
According to an embodiment, the Ni-based alloy preferably includes, by mass %, 0 to 0.03% of C, 8.0 to 22.0% of Cr, 2.0 to 8.0% of Al, 0.4 to 7.0% of Ti, 0 to 28.0% of Co, 2.0 to 7.0% of Mo, 0 to 6.0% of W, 0 to 4.0% of Nb, 0 to 3.0% of Ta, 0 to 10.0% of Fe, 0 to 1.2% of V, 0 to 1.0% of Hf, 0 to 0.300% of B, 0 to 0.300% of Zr, and the balance of Ni and impurities.
According to another aspect of the present invention, provided is a method of manufacturing the super heat resistant Ni-based alloy. The method includes: a preparation step of preparing a raw material of the Ni-based alloy having the composition; and a working step of plastic working the raw material multiple times at a temperature of not higher than 500° C. so that a cumulative working rate is not less than 30%.
According to an embodiment, the raw material has a form of a bar material and the plastic working reduces a cross-sectional area of the bar material. The plastic working preferably includes a step of compressing the bar material from a peripheral surface toward an axis of the bar material.
According to an embodiment, preferably, no heat treatment is performed between plastic workings.
According to another aspect of the present invention, provided is a method of manufacturing or working an super heat resistant Ni-based alloy having such a composition that an amount of precipitated gamma prime phase in equilibrium at 700° C. is not less than 35 mol %. The method includes: a preparation step of preparing an alloy material having a hardness of not less than 500 HV and having a cross-sectional structure including grains having a maximum grain size of not more than 75 nm; and a working step of plastic-working the alloy material at a temperature of not higher than 500° C., thereby producing an alloy having a hardness of not less than 500 HV.
According to an embodiment, the working step is preferably repeated multiple times.
Furthermore, no heat treatment is preferably performed between working steps.
According to an embodiment, the cross-sectional structure of the alloy material and the alloy preferably include not less than 5 grains which have a maximum grain size of not more than 75 nm per 1 μm2.
According to an embodiment, the Ni-based alloy preferably has such a composition that an amount of precipitated gamma prime phase in equilibrium at 700° C. is not less than 40 mol %.
Furthermore, the Ni-based alloy preferably has the composition described above.
Further features, advantages, and details of the present invention will be apparent from the following description of non-limiting embodiments with reference to the accompanying drawings.
The present invention is truly innovative, and provides a super heat resistant Ni-based alloy with excellent plastic workability by a new approach, but not by conventional hot plastic working.
The inventors have studied plastic workability of a super heat resistant Ni-based alloy including a large amount of γ′ phase. As a result, they have found a phenomenon that plastic workability of the Ni-based alloy is dramatically improved by generating “nano-grains” in a structure of the Ni-based alloy. Then, they have found that the generation of the nano-grains can be achieved by “cold plastic working” under predetermined condition, and thus reached the present invention.
A super heat resistant Ni-based alloy according to the present invention has such a composition that an amount of precipitated gamma prime (γ′) phase in equilibrium at 700° C. is not less than 35 mol %, and the Ni-based alloy has a cross-sectional structure including grains which has a maximum grain size of not more than 75 nm. The Ni-based alloy includes grains which has a maximum grain size of not more than 75 nm and thus has excellent plastic workability. With regard to the plastic workability, the Ni-based alloy according to the present invention has, in particular, excellent cold plastic workability.
The amount of γ′ phase of the Ni-based alloy can be indicated by a numerical indicator such as a “volume ratio” or an “area ratio” of the γ′ phase. In the specification, the amount of γ′ phase is indicated by “γ′ mol %”. The γ′ mol % indicates a stable amount of precipitated gamma prime phase in equilibrium in a thermodynamic equilibrium state of the Ni-based alloy. The amount of precipitated gamma prime phase in equilibrium, expressed in “mol %”, is determined by the composition of the Ni-based alloy. The amount in mol % can be obtained by analysis through thermodynamic equilibrium calculation. The amount can be obtained correctly and easily by the analysis using various kinds of thermodynamic equilibrium calculation software.
In the present invention, the γ′ mol % of the Ni-based alloy is indicated by an “equilibrium precipitation amount at 700° C.”. High-temperature strength of the Ni-based alloy depends on the equilibrium precipitation amount of gamma prime phase in the structure. As the Ni-based alloy has higher high-temperature strength, it is more difficult to perform hot plastic working. In general, the temperature dependence on the equilibrium precipitation amount of gamma prime phase in the structure becomes small and becomes approximately constant at or below about 700° C. Thus, the equilibrium precipitation amount at “700° C.” is used.
As described above, it is typically more difficult to hot-plastic-working the super heat resistant Ni-based alloy, as the Ni-based alloy has a higher γ′ mol %. On the contrary according to the present invention, high γ′ mol % of the Ni-based alloy greatly contributes to improvement of cold plastic workability of the Ni-based alloy. The Ni-based alloy of the present invention includes “nano-grains” in the cross-sectional structure, which dramatically improve cold plastic workability. The nano-grains are likely to be generated at an interface between an austenitic phase (gamma (γ)) which is a matrix of the Ni-based alloy, and the gamma prime phase. The high γ′ mol % of the Ni-based alloy increases the phase interface, and thus contributes to the generation of the nano-grains. When the γ′ mol % has reached 35%, the generation of the nano-grains is accelerated. The amount of precipitated gamma prime phase in equilibrium at 700° C. is more preferably not less than 40 mol %, still more preferably not less than 50 mol %, and further still more preferably not less than 60 mol %. In particular, the amount of precipitated gamma prime phase in equilibrium is preferably not less than 63 mol %, more preferably not less than 66 mol %, and still more preferably not less than 68 mol %. While an upper limit of the amount of precipitated gamma prime phase in equilibrium at 700° C. is not particularly limited, its practical value is approximately 75 mol %.
The Ni-based alloy of a precipitation strengthening type, which has the amount of precipitated gamma prime phase in equilibrium at 700° C. is not less than 35 mol %, preferably has a composition, for example, including, by mass %, 0 to 0.25% of C, 8.0 to 25.0% of Cr, 0.5 to 8.0% of Al, 0.4 to 7.0% of Ti, 0 to 28.0% of Co, 0 to 8% of Mo, 0 to 6.0% of W, 0 to 4.0% of Nb, 0 to 3.0% of Ta, 0 to 10.0% of Fe, 0 to 1.2% of V, 0 to 1.0% of Hf, 0 to 0.300% of B, 0 to 0.300% of Zr, and the balance of Ni and impurities.
Alternatively, the Ni-based alloy preferably has a composition including, by mass %, 0 to 0.03% of C, 8.0 to 22.0% of Cr, 2.0 to 8.0% of Al, 0.4 to 7.0% of Ti, 0 to 28.0% of Co, 2.0 to 7.0% of Mo, 0 to 6.0% of W, 0 to 4.0% of Nb, 0 to 3.0% of Ta, 0 to 10.0% of Fe, 0 to 1.2% of V, 0 to 1.0% of Hf, 0 to 0.300% of B, 0 to 0.300% of Zr, and the balance of Ni and impurities.
Each content of a preferable composition as an embodiment of the Ni-based alloy of the present invention will be described below (the content is expressed by “mass %”).
Carbon is conventionally included for improving casting ability of the Ni-based alloy. In particular, since the Ni-based alloy including a large amount of γ′ phase is typically produced as a cast component since it is difficult to be plastic-worked, it includes a certain amount of carbon. Carbon remains as carbides in a structure of the cast Ni-based alloy, and a part of carbon forms coarse eutectic carbides. When the Ni-based alloy is subjected to plastic working, in particular at a room temperature, the coarse eutectic carbides become a starting point and progress route of cracking, and adversely affect plastic workability of the Ni-based alloy.
Therefore, a reduction of the amount of carbon is extremely important for Ni-based alloy according to the present invention, since the present invention has an object of providing the Ni-based alloy including a large amount of γ′ phase and having excellent plastic workability, not as a cast component. However, the Ni-based alloy of the present invention may include carbon in almost the same amount as that in the cast component, since it includes “nano-grains” in the cross-sectional structure to dramatically improve cold plastic workability. In the present invention, the carbon content is preferably not more than 0.25%, more preferably not more than 0.1%, still more preferably not more than 0.03%, still more preferably not more than 0.025%, further still more preferably not more than 0.02%, and particularly preferably less than 0.02%.
Carbon is an element to be limited, and thus the carbon content is preferably controlled to be lower for the Ni-based alloy of the present invention. When carbon is not intentionally added (i.e. it is inevitable impurity), a lower limit of carbon may be 0 mass %. Even if carbon is not intentionally added, approximately 0.001% for example of carbon is measured.
Chromium (Cr) improves oxidation resistance and corrosion resistance. However, an excessive amount of Cr results in formation of a brittle phase such as a σ (sigma) phase to deteriorate strength and hot workability during raw material production. Therefore, the Cr content is preferably 8.0 to 25.0% for example, and more preferably 8.0 to 22.0%. A lower limit of the Cr content is preferably 9.0%, more preferably 9.5%, and still more preferably 10.0%. An upper limit of the Cr content is preferably 18.0%, more preferably 16.0%, still more preferably 14.0%, and particularly preferably 12.5%.
Molybdenum (Mo) contributes to solid solution strengthening of a matrix, and has an effect of improving high-temperature strength. However, an excessive amount of Mo results in formation of an intermetallic compound phase to deteriorate high-temperature strength. Therefore, the Mo content is preferably 0 to 8% (or may be not intentionally added (i.e. inevitable impurity)), and more preferably 2.0 to 7.0%. A lower limit of the Mo content is preferably 2.5%, more preferably 3.0%, and still more preferably 3.5%. An upper limit of the Mo content is preferably 6.0%, and more preferably 5.0%.
Aluminum (Al) forms a γ′ (Ni3Al) phase, which is a strengthening phase, and improves high-temperature strength. However, an excessive amount of Al deteriorates hot workability during the raw material production and causes a material defect such as cracks during processing. Therefore, the Al content is preferably 0.5 to 8.0%, and more preferably 2.0 to 8.0%. A lower limit of the Al content is preferably 2.5%, more preferably 3.0%, still more preferably 4.0%, further still more preferably 4.5%, and particularly preferably 5.1%. An upper limit of the Al content is preferably 7.5%, more preferably 7.0%, and still more preferably 6.5%.
In relation to the above Cr content, when the Cr content is reduced, the Al content is allowed to be increased for complementing the reduced amount of Cr content in order to ensure hot workability during the raw material production For example, when the upper limit of the Cr content is 13.5%, the lower limit of the Al content is preferably 3.5%.
Titanium (Ti) forms a γ′ phase and increases high-temperature strength through solid solution strengthening of the γ′ phase, similarly to Al. However, an excessive amount of Ti makes the γ′ phase unstable and coarse at a high temperature and forms a harmful η (eta) phase to deteriorate hot workability during the raw material production. Therefore, the Ti content is preferably 0.4 to 7.0% for example. Considering a balance with other γ′ generating elements and an Ni matrix, a lower limit of the Ti content is preferably 0.6%, more preferably 0.7%, and still more preferably 0.8%. An upper limit of the Ti content is preferably 6.5%, more preferably 6.0%, still more preferably 4.0%, and particularly preferably 2.0%.
Optional elements that may be added to the Ni-based alloy of the present invention will be described below.
Cobalt (Co) improves stability of the alloy structure, and can maintain hot workability during the raw material production even if the alloy includes a large amount of strengthening element Ti. On the other hand, Co is expensive and thus increases cost of the alloy. Accordingly, Co is an optional element and it may be included, for example, in a range of not more than 28.0% according to a combination with other elements. When Co is added, a lower limit of the Co content is preferably 8.0%, and more preferably 10.0%. An upper limit of the Co content is preferably 18.0%, and more preferably 16.0%. Considering a balance with γ′ generating elements and a Ni matrix, if Co is not intentionally added (i.e. it is inevitable impurity in a raw material), the lower limit of Co is 0%.
Tungsten (W) is an optional element which contributes to solid solution strengthening of a matrix, similarly to Mo. However, an excessive amount of W results in formation of a harmful intermetallic compound phase to deteriorate high-temperature strength. Therefore, an upper limit of the W content is, for example, 6.0%. The upper limit of the W content is preferably 5.5%, and more preferably 5.0%. In order to more reliably achieve the above effect of W, a lower limit of the W content is preferably 1.0%. Addition of both W and Mo in combination is more effective in achieving the solid solution strengthening. In the case where both W and Mo are added in combination, the W content is preferably not less than 0.8%. Due to the addition of a sufficient amount of Mo, if W is not intentionally added (i.e. it is inevitable impurity in a raw material), the lower limit of W is 0%.
Niobium (Nb) is an optional element which forms a γ′ phase and increases high-temperature strength through solid solution strengthening of the γ′ phase, similarly to Al and Ti. However, an excessive amount of Nb results in formation of a harmful δ (delta) phase to deteriorate hot workability during the raw material production. Therefore, an upper limit of Nb is, for example, 4.0%. The upper limit of the Nb content is preferably 3.5%, and more preferably 2.5%. In order to more reliably achieve the above effect of Nb, a lower limit of the Nb content is preferably 1.0%, and more preferably 2.0%. Due to the addition of other γ′ generating elements, if Nb is not intentionally added (i.e. it is inevitable impurity), the lower limit of Nb is 0%.
Tantalum (Ta) is an optical element which forms a γ′ phase and increases high-temperature strength through solid solution strengthening of the γ′ phase, similarly to Al and Ti. However, an excessive amount of Ta makes the γ′ phase unstable and coarse at a high temperature and forms a harmful η (eta) phase to deteriorate hot workability during the raw material production. Therefore, the Ta content is, for example, not more than 3.0%, and preferably not more than 2.5%. In order to more reliably achieve the above effect of Ta, a lower limit of the Ta content is preferably 0.3%. Considering a balance with γ′ generating elements, such as Ti and Nb, and a matrix, if Ta is not intentionally added (i.e. it is inevitable impurity), the lower limit of Ta is 0%.
Iron (Fe) is an optional element which may be used instead of expensive Ni or Co and is effective in reducing cost of the alloy. In order to achieve the effect, it is preferable to determine whether to add Fe considering a combination with other elements. However, an excessive amount of Fe results in formation of a brittle phase such as a σ (sigma) phase to deteriorate strength and hot workability during the raw material production. Therefore, an upper limit of the Fe content is, for example, 10.0%. The upper limit of the Fe content is preferably 9.0%, and more preferably 8.0%. On the other hand, considering a balance with γ′ generating elements and a Ni matrix, if Fe is not intentionally added (i.e. it is inevitable impurity), a lower limit of Fe is 0%.
Vanadium (V) is an optical element which is effective for solid solution strengthening of a matrix and generation of carbide to increase grain boundary strength. However, an excessive amount of vanadium results in formation of such a phase that is unstable at a high temperature during a manufacturing process, and adversely affects the productivity and high-temperature dynamic performance. Therefore, an upper limit of the vanadium content is, for example, 1.2%. The upper limit of the vanadium content is preferably 1.0%, and more preferably 0.8%. In order to more reliably achieve the above effect of vanadium, a lower limit of the vanadium content is preferably 0.5%. Considering a balance with other γ′ generating elements in the alloy, if vanadium is not intentionally added (i.e. it is inevitable impurity), the lower limit of vanadium is 0%.
Hafnium (Hf) is an optional element which is effective for improvement of oxidation resistance of the alloy and generation of carbide to increase grain boundary strength. However, an excessive amount of Hf results in formation of oxide, and such a phase that is unstable at a high temperature during a manufacturing process, and adversely affects the productivity and high-temperature dynamic performance. Therefore, an upper limit of the Hf content is, for example, 1.0%. In order to more reliably achieve the above effect of Hf, a lower limit of the Hf content is preferably 0.1%. Considering a balance with other γ′ generating elements in the alloy, if Hf is not intentionally added (i.e. it is inevitable impurity), the lower limit of Hf is 0%.
Boron (B) increases grain boundary strength and improves creep strength and ductility. However, boron has an effect of lowering a melting point. Furthermore, when boron forms coarse boride, hot workability during the raw material preparation is deteriorated. Therefore, the boron content is preferably controlled not to exceed 0.300% for example. An upper limit of the boron content is preferably 0.200%, more preferably 0.100%, still more preferably 0.050%, and particularly preferably 0.020%. In order to achieve the above effect of boron, the boron content is preferably 0.001% at minimum. The lower limit of the boron content is more preferably 0.003%, still more preferably 0.005%, and particularly preferably 0.010%. Considering a balance with other γ′ generating elements in the alloy, if boron is not intentionally added (i.e. it is inevitable impurity), the lower limit of boron is 0%.
Zirconium (Zr) has an effect of increasing grain boundary strength, similarly to boron. However, an excessive amount of Zr also lowers a melting point to deteriorate high-temperature strength and hot workability during raw material preparation. Therefore, an upper limit of the Zr content is, for example, 0.300%. The upper limit of the Zr content is preferably 0.250%, more preferably 0.200%, still more preferably 0.100%, and particularly preferably 0.050%. In order to achieve the above effect of Zr, the Zr content is preferably 0.001% at minimum. The lower limit of the Zr content is more preferably 0.005%, and still more preferably 0.010%. Considering a balance with other γ′ generating elements in the alloy, if Zr is not intentionally added (i.e. it is inevitable impurity), the lower limit of Zr is 0%.
The balance of the Ni-based alloy other than the elements described above is Ni, while inevitable impurities may be included.
The super heat resistant Ni-based alloy of the present invention has a cross-sectional structure including “nano-grains” having a maximum grain size of not more than 75 nm. Thereby, cold plastic workability is dramatically improved. This mechanism has not yet been sufficiently clear. However, as described above, the phase interface between the y phase and the γ′ phase seems to contribute to the generation of nano-grains. As a plastic working rate is increased, the number of generated nano-grains is increased and boundary slip and crystal rotation of the nano-grains occur, which enable plastic deformation of the Ni-based alloy. Thus, there is a possibility that the deformation mechanism differs from conventional plastic deformation which is caused by slip in crystals due to occurrence and increase of dislocation. The inventors have observed a fact suggesting the possibility of this hypothesis in cold plastic working (described later) of the Ni-based alloy. Once nano-grains generate, the number of nano-grains is increased as the alloy is further plastically worked (i.e. as increasing a plastic working rate). However, regardless of the increase in the plastic working rate (including a case where the plastic working rate is slightly increased), a hardness of the alloy is “approximately constant” (e.g., not less than 500 HV for the Ni-based alloy having a γ′ mol % of not less than 35 mol %). This phenomenon suggests that the plastic working does not cause increase in dislocation density.
Thus, it is the nano-grains having the “maximum grain size of not more than 75 nm” in the cross sectional structure that contribute to improvement of plastic workability of the Ni-based alloy. The size is different from a size of conventional grains observed produced in a typical process. In a case of a wire of the Ni-based alloy for example, the cross sectional structure may be taken from a cross section of the halved surface in a longitudinal direction of the wire (i.e., a cross section including a central axis of the wire). The cross-sectional structure can be taken from each portion, for example, at a position near a surface of the wire, a position at ¼D distant from the surface toward a central axis (D indicates a wire diameter), and a position at the central axis. Then, it is sufficient to confirm that the nano-grains are present in one or more of the cross-sectional structures at each position.
When the Ni-based alloy is in a form other than a wire, a cross section of the material halved in a longitudinal direction of the material may be observed similarly to the above case.
As described above, the Ni-based alloy of the present invention includes nano-grains having a “maximum grain size of not more than 75 nm” in the cross-sectional structure. It is preferable that not less than 5 nano-grains having the maximum grain size of not more than 75 nm are present per 1 μm2 of the cross-sectional structure. As the number of the nano-grains is increased, a medium contributing the plastic deformation is increased and it further improves plastic workability. The number of nano-grains per 1 μm2 of the cross-sectional structure is more preferably not less than 10, still more preferably not less than 50, still more preferably not less than 100, still more preferably not less than 200, and further still more preferably not less than 300. A number density of the nano-grains may be obtained by calculating an average value of a total number of nano-grains observed in the cross-sectional structures divided by a total area of the observed field of view.
It is not necessary to particularly define a lower limit of the maximum grain size of the nano-grains in the cross-sectional structure. The presence and the number of the nano-grains in the cross-sectional structure can be observed by using e.g. EBSD images as below. The nano-grains having a maximum grain size of not more than 75 nm may be recognized and its number may be counted from grains observed under conditions of the EBSD measurement of a scan step: 0.01 μm when a grain boundary is defined as having an orientation difference of not less than 15°. For example, the presence and the number of the nano-grains having a maximum grain size of approximately not less than 25 nm can be observed.
The Ni-based alloy preferably has a hardness of not less than 500 HV.
As the above, the Ni-based alloy of the present invention has excellent cold plastic workability, and thus may be used “for cold plastic working”.
Furthermore, the Ni-based alloy of the present invention may be in a form of a “wire material” which is an intermediate product to be subjected to cold plastic working. For example, the wire material has a wire diameter of not more than 10 mm, not more than 8 mm, or not more than 6 mm. Alternatively, the wire material has a small wire diameter of not more than 5 mm, not more than 4 mm, not more than 3 mm, or not more than 2 mm. For example, the wire material has a large length of not less than 10 times, not less than 50 times, or not less than 100 times the above wire diameter of the wire material.
Alternatively, the Ni-based alloy of the present invention may be in a form of a “wire product” which is an end product form produced through the cold plastic working. For example, the wire product has a wire diameter of not more than 5 mm, not more than 4 mm, or not more than 3 mm. Alternatively, the wire product has a smaller wire diameter of not more than 2 mm or not more than 1 mm. For example, the wire product has a larger length of not less than 50 times, not less than 100 times, or not less than 300 times the above wire diameter of the wire product.
Next, a method of manufacturing the Ni-based alloy of the present invention which has the above composition and includes grains having a maximum grain size of not more than 75 nm in the cross-sectional structure will be described. The manufacturing method includes: a preparation step of preparing a raw material of the super heat resistant Ni-based alloy having the above composition; and a working step of performing plastic working multiple times the raw material at a temperature of not higher than 500° C. such that a cumulative working rate is not less than 30%. The inventors have found that the “nano-grains” can be generated in the structure of the Ni-based s alloy material by the high “working rate” of compression processing.
The manufacturing method will be described.
A raw material of the Ni-based alloy may be produced by a melting method in which a molten metal is poured into a mold to produce an ingot. The ingot may be produced by combining, for example, vacuum melting with a usual method such as vacuum arc remelting or electroslag remelting. Alternatively, the raw material may be produced by a powder metallurgy method. If necessary, the ingot or an alloy ingot produced by the powder metallurgy method may be hot worked or machined to form a raw material having a predetermined form, such as in a form of a billet or a bar material.
Next, the raw material is subjected to plastic working with a cumulative working rate of not less than 30% at a temperature of not higher than 500° C. Unlike conventional “hot” plastically worked material, the Ni-based alloy according to the present invention can have the nano-grains generate in its structure through “cold” plastic working and has excellent plastic workability. In order to achieve it, the cold plastic working needs to be conducted in a low temperature range in which no recovery or no recrystallization can occur during the plastic working. Accordingly, it is preferable not to perform heat treatment during the plastic working. The heat treatment indicates a heat treatment in a high temperature range in which recovery or recrystallization occurs, such as a heat treatment at a temperature of higher than 500° C.
It is important according to the invention that the temperature at which the plastic working is conducted is “not higher than 500° C.”. The temperature is preferably not higher than 300° C., more preferably not higher than 100° C., and still more preferably not higher than 50° C. (e.g., at a room temperature).
Apparently the above manufacturing of the Ni-based alloy is applicable to a produce a wire material, a sheet material, a strip material, or the like. Furthermore, it is also apparent that the Ni-based alloy of the present invention may be in a form of an intermediate product such as a wire material, a sheet material, or a strip material, as well as it may be in a form of an end product such as a wire product, a sheet product, or a strip product. As for the sheet material (sheet product) and the strip material (strip product), the above wire diameter of the wire material (wire product) can be replaced with a plate thickness or a strip thickness.
In particular, when the raw material of the Ni-based alloy is in a form of a bar material, the bar material can be compressed to reduce its cross-sectional area in order to form the above nano-grains. In this case, the “bar material” of the Ni-based alloy is used as a starting material and is subjected to a process in which a pressure can be uniformly applied from an entire periphery of the bar material as the plastic working, i.e. “working to compress an area of a cross section perpendicular to a longitudinal direction of the bar material”. The raw bar material is worked so that a cross-sectional area (bar diameter) is plastically compressed to extend a length of the bar material. In particular for producing a wire material, it is efficient to prepare a “bar material” having a larger cross-sectional area (diameter) than the wire material by plastic working. The bar material is plastically worked by compressing the cross-sectional area from a peripheral surface toward an axis of the material, at a cumulative working rate of not less than 30%, at a temperature of not higher than 500° C. The working include swaging, cassette roller dice wire drawing, hole dice wire drawing, and the like.
On the other hand, a sheet material, a strip material, or the like of the Ni-based alloy may be produced through rolling process.
In the present invention, the cumulative working rate of the plastic working is set to a high value of “not less than 30%” in order to form the nano-grains. The cumulative working rate is preferably not less than 40%. Such a cumulative working rate is preferable, for example, for generating not less than 10 nano-grains per 1 μm2 of the cross-sectional structure.
The cumulative working rate is more preferably not less than 60%. Such a cumulative working rate is preferable, for example, for generating not less than 50 nano-grains. The cumulative working rate is more preferably not less than 70%, and still more preferably not less than 80%. Such cumulative working rates are preferable, for example, for generating not less than 100 nano-grains. The cumulative working rate is further still more preferably not less than 90%, and particularly preferably not less than 97%. These cumulative working rates are preferable, for example, for respectively generating not less than 200 nano-grains and not less than 300 nano-grains.
Here, the “working rate” is explained. In a case where the bar material is subjected to swaging or dice wire drawing, the working rate is indicated by an area reduction rate. The area reduction rate is calculated by the following formula, as a relationship between a cross-sectional area A0 of a bar material before plastic working and a cross-sectional area A1 of a wire material or a wire product after plastic working.
[(A0−A1)/A0]×100 (%) (1)
On the other hand, in a case of rolling process, the working rate is indicated by a rolling reduction rate. The rolling reduction rate is calculated by the following formula, where t0 represents a thickness of a raw material before plastic working, and t1 represents a thickness of a sheet material, a strip material, a sheet product, or a strip product after plastic working.
[(t0−t1)/t0]×100 (%) (2)
The cumulative working rate indicates a working rate of a final workpiece in relation to a raw material through multiple times or passes of the plastic working.
Although a mechanism of the generation of the nano-grains has not yet been fully known, the inventors have experimentally found that the working rate needs to be approximately 30% at minimum (see Examples) in order to generate sufficient nano-grains. It was observed that the nano-grain is first generated preferentially at a phase interface between the γ phase and the γ′ phase when the bar material of the Ni-based alloy is plastically cold-worked and a cumulative working rate of the working has reached approximately 30%. Once the nano-grains are generated and the alloy (e.g., bar material or wire material) is further plastically cold-worked, the number of nano-grains is increased, and the increase in nano-grains further improves plastic workability of the Ni-based alloy (e.g., bar material or wire material). The inventors have observed a phenomenon as if “room temperature superplastic” working. That is, repeat of plastic working (i.e. increasing a cumulative working rate) increasingly improved plastic workability of the Ni-based alloy (e.g., bar material or wire material), and cold plastic working at a cumulative working rate of not less than 97% is achieved without performing heat treatment during the working.
Preferably, the plastic working at a working rate of “not less than 30%” may be completed, not by the working once, but by multiple times workings in order to prevent occurrence of cracks, flaw, or the like in the alloy until the nano-grains are generated in the structure. Since a “large strain” caused by a working rate of not less than 30% is applied to the raw material in several stages of the multiple times workings, the strain is moderately dispersed in the raw material. This is effective to uniformly cause the boundary slip and crystal rotation of the nano-grains. As a result, it is possible to generate the nano-grains uniformly and evenly in the raw material and to prevent occurrence of cracks, flaws, or the like during the working. No heat treatment is necessary between the multiple times of the plastic working. There is no need to define an upper limit of the working rate of not less than 30% and it may be set according to an intermediate product form, an end product form, or the like. In a case where an alloy material as described later is prepared, for example, a value such as 50%, 45%, 40%, or 35% may be set according to a configuration or the like of the alloy material.
When the plastic working is performed multiple times, it is possible to set a working rate (area reduction rate) of any plastic working (pass) to be higher than a working rate (area reduction rate) of the previous plastic working (pass) to increase working efficiency. The working rate (area reduction rate) may be increased each time the plastic working (pass) is performed.
With regard to the “pass” described in the present invention, “one pass” indicates one plastic working performed by a single dice or roll (or a pair of dices or rolls) in a case of working such as swaging, dice wire drawing, or roll described above.
In a case where the raw material of the Ni-based alloy is a bar material in particular, it is presumed to be important to apply pressure uniformly and evenly from the entire periphery of the bar material during the plastic working in order to generate the nano-grains. It is effective to compress a cross-sectional area of the bar material from a peripheral surface toward an axis of the bar material. At this time, a plastic working method is not limited. However, such process is advantageous which applies pressure evenly to an entire circumference of the bar material. Specific examples of such a process include swaging processing. In the swaging processing, a peripheral surface of a bar material is forged while rotating a plurality of dice surrounding an entire circumference of the bar material. Thus, the swaging processing is preferable for generating the nano-grains. Alternatively, other types of plastic working such as cassette roller dice wire drawing and hole dice wire drawing are also applicable.
In the present invention, a raw material (e.g., bar material) before the plastic working may be heat-treated, in which the material is heated and held at a temperature Th of not lower than a γ′ solid solution temperature (solvus temperature) Ts and then cooled. By the heat treatment, a γ′ phase can be re-precipitated uniformly in a structure of the raw material. Thus, the nano-grains are more likely to be generated in the structure after being subjected to the plastic working. This is presumably because the phase interface between the γ phase and the γ′ phase of the Ni-based alloy becomes uniform, thereby the formation of nano-grains is facilitated.
The heating and holding temperature Th is preferably higher by not less than 10° C. above the solvus temperature Ts (i.e. Th≥Ts+10° C.). There is no need to determine an upper limit of the temperature Th. In principal, the Th should be lower than a temperature (solidus temperature) at which the raw material starts melting. Furthermore, a holding time of the bar material from a time when the material reached the temperature Th is preferably not shorter than 2 hours. A practical holding time is not longer than 10 hours. The holding time is preferably not longer than 7 hours, and more preferably not longer than 4 hours. The heat treatment also effect of making the composition homogeneous (soaking effect).
According to another embodiment of the present invention, provided is a method of manufacturing a super heat resistant Ni-based alloy having the above composition. The method includes: a preparation step of preparing an alloy material that has a hardness of not less than 500 HV and has a cross-sectional structure including grains having a maximum grain size of not more than 75 nm; and a working step of plastically working the raw material at a temperature of not higher than 500° C., thereby producing an alloy having a hardness of not less than 500 HV. The alloy material which is a starting material to be worked is the above-described super heat resistant Ni-based alloy according to the present invention, and for example, in a form of a wire material, a sheet material, or a strip material described above. The inventors have found that when plastic working at a temperature of not higher than 500° C. is repeated for the Ni-based alloy which has a hardness of not less than 500 HV and has the cross-sectional structure including grains having a maximum grain size of not more than 75 nm, nano-grains are increased (continuously formed) each time the plastic working is performed, and plastic workability is maintained. At that time, the hardness of the alloy is maintained to be not less than 500 HV, or is slightly increased. Thus, the Ni-based alloy including the “nano-grains in the cross-sectional structure” of the present invention has excellent plastic workability not only in an early stage, but also the excellent plastic workability is maintained during the subsequent working. After the entire plastic working process, the Ni-based alloy also includes the nano-grains in the cross-sectional structure, and may be processed in a form of an end product such as a wire product, a sheet product, or a strip product.
The worked Ni-based alloy has a banded structure in which the γ phase and the γ′ phase extend in a worked direction. However, when the Ni-based alloy having the worked predetermined size and form is supplied as an end product, the alloy may be heat-treated according to necessity to have a desired equiaxed structure. For example, the Ni-based alloy may be heat-treated to have a hardness of less than 500 HV, thereby the end product can be bended or cut to into a form appropriate to transportation or applications.
According to the above manufacturing method, it is possible to provide an Ni-based alloy, for example only through cold plastic working, in various forms including an intermediate product form such as a wire material, a sheet material, or a strip material and an end product form such as a wire product, a sheet product, or a strip product as described above.
A molten metal produced by vacuum melting was cast to produce a cylindrical ingot of a super heat resistant Ni-based alloy “A” with a diameter of 100 mm and a weight of 10 kg. Table 1 shows a composition (by mass %) of the Ni-based alloy “A”. Table 1 also shows a “γ′ mol %” and a “γ′ solid solution temperature (solvus temperature) Ts” of the ingot, which were calculated with use of commercially available thermodynamic equilibrium calculation software “JMatPro (Version 8.0.1, manufactured by Sente Software Ltd.)”. The content of each element in Table 1 were input into the software to obtain the “γ′ mol %” and the “γ′ solid solution temperature Ts”. The ingot was heat treated at a holding temperature Th of 1200° C. for a holding time of 8 hours, and then cooled in a furnace. Then, a bar material was cut out from the ingot along a longitudinal direction of the ingot. The bar material has a diameter of 6.0 mm and a length of 60 mm, and was used as a raw material for plastic working. The bar material had a hardness of 320 HV. The bar material was subjected to “swaging processing 1” (shown in a column for alloy 1-2 in Table 2) at a working rate of 31% at a room temperature (25° C.) to produce a wire material (with a wire diameter of 5.0 mm) of the Ni-based alloy according to the present invention of Example 1. The produced wire material of Example 1 maintained its good surface condition. The wire material had a hardness of 595 HV. The working rate was calculated by the above-described formula (1).
On the other hand, an alloy No. 1-1 was subjected to swaging processing at a room temperature (25° C.) to have a wire diameter of 5.5 mm. A working rate (area reduction rate) was 16.0%. When a cross-sectional microstructure of the alloy No. 1-1 was observed in a manner similar to that of the alloy No. 1-2 as shown in
The wire material of the alloy No. 1-2 according to the present invention was sequentially and cumulatively subjected to “swaging processings 3 to 10” at the respective working rates shown in Table 2 at a room temperature (25° C.). Thus, wire materials from an alloy No. 1-3 to an alloy No. 1-10 were produced, in which a cumulative working rate was increased as the alloy number increases. No heat treatment was performed between the swaging processings. The produced wire materials of the alloy Nos. 1-3 to 1-10 all maintained their good surface condition. The wire materials also had nano-grains having a maximum grain size of not more than 75 nm in their cross-sectional structures (shown as black grains in the drawings).
The results in Table 2 show that once nano-grains had been generated in the Ni-based alloy, further cold plastic working increased the number of the nano-grains. Although the number of nano-grains was increased, the hardness of the Ni-based alloy was approximately constant regardless of the increase of the plastic working rate. Thus, the Ni-based alloy was able to be plastically cold-worked through swaging processing to produce the wire material example No. 1-10 according to the present invention having a wire diameter of 1.0 mm. When the wire material of the alloy No. 1-2 was used as a starting material (i.e., an alloy material having a hardness of not less than 500 HV and including grains having a maximum grain size of not more than 75 nm in a cross-sectional structure), cold plastic working was able to be performed so that a cumulative working rate from the wire material was 96% and that a cumulative working rate from the original bar material was 97%. Furthermore, even after the plastic working at the large cumulative working rate, the wire material example of the alloy No. 1-10 according to the present invention was able to be further plastically cold-worked. Since the hardness of the worked alloy example according to the present invention was approximately constant (595 HV to 605 HV) regardless of the working rate, an alloy material in which once grains having a maximum grain size of not more than 75 nm have been formed and which has a hardness of not less than 500 HV can be subjected to further cold working.
A molten metal produced by vacuum melting was cast to produce a cylindrical ingot of a super heat resistant Ni-based alloy “B” with a diameter of 100 mm and a weight of 10 kg. Table 3 shows a composition (by mass %) of the Ni-based alloy “B”. The “γ′ mol %” and the “γ′ solid solution temperature Ts” in Table 3 were also calculated with use of commercially available thermodynamic equilibrium calculation software “JMatPro (Version 8.0.1, manufactured by Sente Software Ltd.)”. The ingot was heat treated at a holding temperature Th of 1250° C. for a holding time of 8 hours, and then cooled in a furnace. Then, a bar material having a diameter of 6.0 mm and a length of 60 mm was cut out from the ingot along a longitudinal direction of the ingot, and was used as a raw material for plastic working. The bar material had a hardness of 381 HV. Similarly to Example 1, the bar material was sequentially subjected to swaging processing to produce wire materials of alloys No. 2-1 to No. 2-6.
As shown in Table 4, the wire material of the alloy No. 2-1 after the swaging processing had a wire diameter of 5.5 mm, and a working rate (area reduction rate) was 16.0%. In a cross-sectional microstructure of the wire material of the alloy No. 2-1, no nano-grains having a maximum grain size of not more than 75 nm were observed. Furthermore, the wire material had a hardness of 494 HV.
For the wire materials of the alloys No. 2-2 to No. 2-6, a working rate (area reduction rate) was not less than 30%. In a cross-sectional structure of each of the wire materials of the alloys No. 2-2 to No. 2-6, nano-grains having a maximum grain size of not more than 75 nm were observed. As the working rate was increased, a number density of the nano-grains was increased. These alloys each had a hardness of not less than 500 HV. Unlike the results of Example 1, however, as the working rate was increased, the hardness tended to be slightly increased. The wire materials further subjected to the working had a hardness of not less than 600 HV.
A molten metal produced by vacuum melting and cast to produce a cylindrical ingot of a super heat resistant Ni-based alloy “C” with a diameter of 100 mm and a weight of 10 kg. Table 5 shows a composition (by mass %) of the Ni-based alloy C. The “γ′ mol %” and the “γ′ solid solution temperature Ts” in Table 5 were also calculated with use of commercially available thermodynamic equilibrium calculation software “JMatPro (Version 8.0.1, manufactured by Sente Software Ltd.)”. The ingot of the Ni-based alloy “C” was heat-treated at a holding temperature Th of 1200° C. for a holding time of 8 hours, and then cooled in a furnace. Then, a bar material having a diameter of 6.0 mm and a length of 60 mm was cut out from the ingot along a longitudinal direction of the ingot, and was used as a raw material for plastic working. The bar material had a hardness of 389 HV. Similarly to Example 1, the bar material was sequentially subjected to swaging processing to produce wire materials of alloys No. 3-1 to No. 3-10.
As shown in Table 6, the wire material of the alloy No. 3-1 after the swaging processing had a wire diameter of 5.5 mm, and a working rate (area reduction rate) was 16.0%. In a cross-sectional microstructure of the wire material of the alloy No. 3-1, no nano-grains having a maximum grain size of not more than 75 nm were observed. Furthermore, the wire material had a hardness of 468 HV.
In the wire materials of the alloys No. 3-2 to No. 3-10, a working rate (area reduction rate) was not less than 30%. In a cross-sectional structure of each of the wire materials of the alloys No. 3-2 to No. 3-10, nano-grains having a maximum grain size of not more than 75 nm were observed. As the working rate was increased, a number density of the nano-grains was increased. These alloys each had a hardness of not less than 500 HV, which was approximately constant (524 HV to 542 HV) regardless of the working rate, similarly to Example 1.
The wire material (wire diameter: 1.5 mm) of the alloy No. 1-9 of Example 1 was used as a starting material, and the wire material was subjected to hole dice wire drawing processing performed in 4 passes at a room temperature (25° C.). The wire material of the alloy No. 1-9 was processed into a wire material of an alloy No. 4-1 (wire diameter: 1.35 mm), a wire material of an alloy No. 4-2 (wire diameter: 1.20 mm), and a wire material of an alloy No. 4-3 (wire diameter: 1.05 mm), and finally a wire material of an alloy No. 4-4 (wire diameter: 0.95 mm) was produced. The wire material of the alloy No. 1-9 was able to be successfully processed into the wire material having a diameter of less than 1 mm. No heat treatment was performed between the passes. A working rate was obtained by the above-described formula (1).
During the 4 passes, the alloys No. 4-1, 4-2, and 4-3 respectively had a hardness of 593 HV, 602 HV and 598 HV. In a cross-sectional structure of each of the wire materials, nano-grains having a maximum grain size of not more than 75 nm were observed. As the working rate was increased, a number density of the nano-grains was increased. As shown in Table 7, in a cross-sectional structure of the wire material of the alloy No. 4-4 obtained by completing the dice wire drawing processing in 4 passes, 620 nano-grains having a maximum grain size of not more than 75 nm were observed per 1 μm2 of the cross-sectional structure, and the wire material had a hardness of 593 HV. The wire materials of the alloys No. 4-1 to 4-4 each had a hardness of not less than 500 HV, which was approximately constant (593 HV to 602 HV) regardless of the working rate, similarly to Example 1.
As the above, it was observed that the super heat resistant Ni-based alloys in the Examples had excellent plastic workability and that the Ni-based alloys can be processed into a wire material having any wire diameter by plastically cold-working the Ni-based alloys of the present invention examples. While the wire materials were manufactured in the Examples, the wire material may be, of course, a wire product which is an end product form. Since the Ni-based alloy of the present invention has excellent plastic workability, it is apparent to those skilled in the art that the Ni-based alloy of the present invention can also be subjected to plastic working into forms other than the wire material or the wire product.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-029974 | Feb 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/006046 | 2/20/2018 | WO | 00 |
