Patent Application
20140345756
The present invention is directed to martensitic alloys, articles including martensitic alloys, and processes of forming alloys. More specifically, the present invention is directed to a reduced nickel-chromium martensitic alloy and a process of forming a reduced nickel-chromium martensitic alloy.
Wind turbines are exposed to significant operational stresses from wind, rotational forces and the weight of a plurality of blades. The operational stresses are often amplified by environmental temperatures with extremes depending on geographical location. The materials used for the components of the wind turbines must be able to withstand operating stresses and strains throughout the range of temperatures.
Wind turbines have a main shaft that transmits power from a rotor to a generator. As wind turbines increase their outputs from 1.5 and 2.5 megawatts (MW) to 3, 4, 5, and 6 MW, the size and required properties of the wind turbine drive shaft increases. In addition, the loads from gearbox components, such as planet gear carriers, are typically too high for conventional ductile iron grades (ferritic/pearlitic grades). Forged/hardened steel is the material of choice for gearbox components and drive shafts having sizes greater than 3 tons. This shaft is typically machined out of a steel forging. The material of the shaft is usually quenched-tempered high-strength low-alloy steel with critical fatigue properties. A common alloy currently used for these large wind turbine components is 34CrNiMo6 steel. The nickel and chromium provide a desirable hardenability of the alloy. Although 34CrNiMo6 steel provides desired hardenability and fracture appearance transition temperature (FATT), the nickel and chromium used in 34CrNiMo6 steel is expensive, increasing the price of wind turbines and replacement shafts.
A desired feature for wind turbine components is a FATT of −40° C. (−40° F.). The FATT is the temperature at which the fracture surface of a material is 50% low energy brittle cleavage and 50% high energy ductile fibrous. Both a composition of the material as well as the processes for forming and heat treating the material affect FATT. FATT is important because it represents the temperature above which brittle fracture will not occur. The lower the FATT, the greater the toughness of the material.
Martensitic stainless steels, such as 34 CrNiMo6 steel, having excellent strength, low brittle to ductile transition temperature, and good hardening characteristics in thick sections have long been used as turbine shaft materials. Decreasing the amount of Ni and Cr in the alloy decreases the hardenability, which reduces the amount of martensite that forms in the material. Reducing the amount of martensite has the undesirable consequence of increasing the FATT of the material.
A martensitic alloy and a method of forming an inexpensive martensitic alloy having reduced amounts of nickel and chromium and not suffering from the above drawbacks would be desirable in the art.
In an exemplary embodiment of the present disclosure, a martensitic alloy component includes by weight:
In another embodiment of the present disclosure, a process of forming a reduced nickel-chromium martensitic alloy component includes forging the reduced nickel-chromium alloy component including by weight:
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Provided is an exemplary reduced nickel-chromium alloy component having a plurality of predetermined properties and a process of forming the reduced nickel-chromium alloy component having a plurality of predetermined properties. Embodiments of the present disclosure, in comparison to methods and products not utilizing one or more features disclosed herein, decrease nickel percentage, decrease chromium percentage, increase carbon percentage, increase manganese percentage, decrease material cost, maintain or increase martensite percentage, maintain or decrease fracture appearance transition temperature (FATT), or a combination thereof.
In one embodiment, the disclosure includes a process for producing the main shaft for a wind turbine from a martensitic alloy, though it should be understood that the invention is also well suited for the production of a wide variety of components from martensitic alloy compositions. Other non-limiting examples include automotive components, such as turbine shafts, axles, and various other components used in the energy, automotive, railroad, construction, mining and agricultural industries. Such components are well known in the art and therefore require no further description.
With reference to
The martensitic alloy, according to the present disclosure, includes the composition shown in Table 1.
A component formed from the composition, according to the present disclosure, includes a hardenability corresponding to an ideal diameter of greater than about 10 inches (25.4 cm) or 10 to 18 inches (25.4 to 45.72 cm) or 11 to 12 inches (27.94 to 30.48 cm) or 13 to 14 inches (33.02 to 35.56 cm) or any suitable combination, sub-combination, range, or sub-range within. In one embodiment, the component has a hardenability corresponding to an ideal diameter of about 11.4 inches (28.96 cm). Hardenability corresponding to an ideal diameter, as utilized herein, is the ability of material, component, and heat treatment (e.g., after quench from an austenitizing temperature), to form at least 50% martensite at the center of a solid cylinder. While the above definition of hardenability corresponding to an ideal diameter is based from a solid component, one of ordinary skill in the art would understand that the geometry is not limited to a solid cylinder and may include other geometries and/or hollow components. For example, the hardenability of hollow components corresponds to the corresponding center depth within the material (e.g., the center of the wall) in which at least 50% martensite forms after heat treatment.
The martensitic microstructure has increased material toughness as compared to bainite and ferrite/pearlite microstructures. Increasing the percentage of martensite in the material microstructure will decrease the FATT of the material. Increasing the ideal diameter of a material increases the amount of martensite thus decreasing the FATT of the material in thicker cross sections. A material at a temperature below the FATT will have low fracture toughness and low damage tolerance. To form a damage tolerant component, the operating temperature of the component should be above the FATT.
In one embodiment, a component formed from the composition, according to the present disclosure, includes a FATT at the surface of less than −40° F. (−40° C.) or less than −50° F. (−45.6° C.) or less than −60° F. (−51.1° C.). In addition, the component includes a FATT of less than 86° F. (30° C.) or less than 80° F. (26.7° C.) or less than 75° F. (23.9° C.) at the maximum thickness of the component.
In addition to increasing the ideal diameter, properties of the material that decrease FATT include, but are not limited to, increasing martensite percentage, decreasing grain size, decreasing yield strength, or a combination thereof. In one embodiment, a desired yield strength of the material is 650 MPa or greater or about 650 MPa to about 1000 MPa and tensile strength between about 800 and about 1,000 MPa. In a further embodiment, the average grain size of a material is formed during processing of the material, and is maintained to about 62 μm or less or about 50 μm or less. The FATT of the material having a defined yield strength range and grain size range is adjusted through adjustments in microstructure. In one embodiment, the microstructure is adjusted through increases and/or decreases in concentrations of alloying elements. The alloying elements include, but are not limited to, carbon, silicon, manganese, nickel, chromium, molybdenum, vanadium, sulfur, phosphorus, copper, or a combination thereof. In addition to adjusting microstructure, increases and/or decreases in the concentrations of the alloying elements adjust material strength, toughness, ductility, grain size, or a combination thereof.
In one embodiment, the nickel concentration and the chromium concentration are decreased and a carbon concentration and a manganese concentration are increased. A hardenability of a material is affected by the amount of each element present in the material. The hardenability is the ease at which the material forms a martensitic structure during quenching from an austenitizing temperature. Increasing the carbon concentration and the manganese concentration maintains or increases a hardenability of the material. Increasing the hardenability of the material increases the ideal diameter, which increases martensitic structure formation and decreases the FATT in thick cross sections, thus providing for increased damage tolerance.
An exemplary process for forming the component includes forging of the component. After forging, the component is heat treated through methods including, but not limited to, austenitizing, quenching, tempering, or a combination thereof. Austenitizing is the process of holding the martensitic alloy forging above a critical temperature for a sufficient period of time to ensure that the matrix is fully transformed to austenite. In order to produce a single phase matrix microstructure (austenite) with a uniform carbon distribution, austenitizing includes holding the forging at temperatures greater than about 870° C. (1598° F.) for a time period that is sufficient to fully convert the matrix of the thickest section to austenite. Quenching from the austenitizing temperature forms a martensite microstructure and may be accomplished with any suitable quenching method known in the art. The rate of quench has to be high enough to reduce or eliminate ferrite/pearlite formation. Tempering is provided to increase the toughness and reduce the brittleness of the component. Suitable tempering temperatures include, but are not limited to, between about 550° C. (1022° F.) and about 650° C. (1202° F.), between about 580° C. (1076° F.) and about 620° C. (1048° F.), or about 600° C. (1112° F.), or any combination, sub-combination, range, or sub-range thereof.
Comparative Example 1: The known composition of 34CrNiMo6 steel, a material known for use in wind turbine main shaft manufacture, is shown below:
The nominal composition of Comparative Example 1 corresponds to a hardenability corresponding to an ideal diameter of 7.4 inches.
Comparative Example 2: A steel alloy composition having the following composition:
The nominal composition of Comparative Example 2 has a hardenability corresponding to an ideal diameter of 6.1 inches.
Example 1: A martensitic alloy composition having the following composition:
A component, shown as Example 1, is formed from an exemplary composition according to the present disclosure. The final product includes a hardenability corresponding to an ideal diameter of 13.3 inches (33.78 cm).
Example 2: A martensitic alloy composition having the following composition:
A component, shown as Example 2, is formed from an exemplary composition according to the present disclosure. The final product includes a hardenability corresponding to an ideal diameter of 12.4 inches (31.50 cm).
Example 3: A martensitic alloy composition having the following composition:
A component, shown as Example 3, is formed from an exemplary composition, according to the present disclosure. The final product includes a hardenability corresponding to an ideal diameter of 17.3 inches (43.94 cm).
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
