Patent Application
20130014863
The present disclosure relates generally to the processing of iron-carbon alloys and more specifically to a magnetic field-assisted processing method.
The thermal processing of ferrous alloys to achieve good mechanical properties has been widely studied. Most iron-carbon alloys require a rapid quench to reach a metastable martensite phase that can be transformed by heat treatment (tempered) into a desirable microstructure that exhibits good strength and toughness. During the quench, which may occur at a rate of hundreds of degrees per second, the high temperature austenitic phase rapidly cools and is transformed to martensite. Due to the volume expansion (over 4%) that occurs during the phase transformation, tremendous strains may be generated in the iron-carbon alloy during cooling. The surface of the alloy tends to cool more quickly—and thus transform to martensite more quickly—than the interior. When the more ductile interior finally transforms to martensite and expands in volume, the hard martensitic surface is put into tension, and quench cracks may form. As a result, scientists and engineers have tried a myriad of approaches to control the quench process to minimize cracking, such as altering the cooling rate and employing different media for the quench.
Another approach, referred to as austempering, is an alternative to conventional quenching and tempering that avoids forming the metastable martensitic phase in the first place. In austempering, the quench is halted at a temperature above the martensite start temperature, Ms, and the interior of the alloy is allowed to reach the same temperature as the surface. The temperature is maintained for a time sufficient to transform the entire iron-carbon alloy from austenite to a bainite microstructure, and then the alloy is cooled to room temperature. Consequently, an austempered alloy experiences less distortion and cracking than a conventional quench-and-tempered alloy, and also the energy intensive tempering step is avoided. A downside of austempering, however, is that the transformation from austenite to bainite may take an excessively long time to occur.
Described herein is a magnetic field assisted processing method that may be used to achieve more homogeneous microstructures and improved mechanical properties in ferrous alloys. Using the method, it is possible to produce iron-carbon alloys that include a fine dispersion of iron carbide particles and exhibit a desirable combination of strength and ductility.
The method entails heating an iron-carbon alloy at an austenitizing temperature for a time duration sufficient for the alloy to achieve an austenitic microstructure; cooling the iron-carbon alloy to an intermediate temperature defined by a continuous cooling transformation (CCT) diagram for the iron-carbon alloy at a rate sufficient to avoid phase transformation of the austenitic microstructure, the intermediate temperature being below a bainitic knee of the CCT diagram and above a martensite start temperature; and applying a high field strength magnetic field of at least about 0.2 Tesla to the iron-carbon alloy after reaching the intermediate temperature. The field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix in order to produce a magnetically-processed alloy having improved ductility and strength.
Referring to
A high field strength magnetic field of at least about 0.2 Tesla is applied to the iron-carbon alloy, preferably after the alloy has been cooled to the intermediate temperature, and the field is applied for a time duration sufficient to transform the austenitic microstructure into a fine dispersion of one or more iron carbide phases in a ferrite matrix. A magnetically-processed iron-carbon alloy having improved ductility and strength may thereby be produced. The fine dispersion of carbide phases is a copious population of second phase (or higher order) carbide particles that can be coherent, semi-coherent, or incoherent with the matrix phase of the alloy. The particles may have an average size in the nanometer to submicron length scale. For example, the particles may be from about 1 nm to about 1 micron in average size. The average size may also lie between about 1 nm and about 500 nm, or between about 1 nm and about 100 nm.
The selection of the intermediate temperature is guided by the CCT curve for the iron-carbon alloy as well as by the Curie temperatures of desired iron-carbon phases, as discussed further below. The method may include holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field. The hold time may range from about 1 minute to about 30 minutes. The iron-carbide alloy may be held at the intermediate temperature during the entire time that the magnetic field is applied. Typically, magnetic field is applied for a time duration of between about 1 minute and about 30 minutes.
In some cases, instead of holding the iron-carbon alloy at the intermediate temperature during the application of the high field strength magnetic field, the iron-carbide alloy may be cooled from the intermediate temperature to a lower temperature during the application of the field. The lower temperature may be ambient temperature. The cooling may be carried out by exposing the iron-carbide alloy to a continuous flow of a cooling fluid, which may be an inert gas such as argon or helium, for example, until the lower temperature is reached. The flow of the cooling fluid may be adjusted to achieve a desired cooling rate. Alternatively, the cooling may be achieved by immersing the alloy into an isothermal molten salt or hot oil bath, or fluidized medium bed, or similar approach at the required austempering temperature.
A cooling fluid or one of the alternative cooling methods mentioned above may also be used to cool the iron-carbon alloy to the intermediate temperature from the austenitizing temperature prior to applying the high field strength magnetic field. The iron-carbon alloy may be cooled at a cooling rate that lies between about 1° C./s and 400° C./s, depending on the alloy content and therefore CCT behavior to miss the pearlite/bainite phase transformation knee. The cooling rate during cooling to the intermediate temperature is particularly important; if the rate is not high enough to avoid the knee of the CCT curve, phase transformation of the austenitic microstructure may occur.
The intermediate temperature may be selected to precipitate desired iron and/or other carbide phases in the iron-carbon alloy while avoiding others. If the intermediate temperature is below the Curie temperature of a particular iron carbide phase, then that particular phase may be precipitated during the application of the high field strength magnetic field while other iron carbides having Curie temperatures below the intermediate temperature may be avoided. Table 1 provides a listing of iron carbide phases, including Fe3C, Fe7C3, Fe23C6, Fe5C2, Fe4C, and others, and their Curie temperatures. Other carbide phases that may be precipitated in the iron-carbon alloy can be represented by including M in the compound name to represent one or more solute elements (i.e., alloy additions such as Mo, V, Cr, Nb, Ti, etc.) that may replace part or all of the iron in the compound. For example, a carbide phase in the iron-carbon alloy may be represented by FexM1-xCy, where M is selected from among Mo, V, Cr, Nb, and Ti, and x may lie between 0 and 1.
The carbide phase(s) that form in the iron-carbon alloy during application of the magnetic field depend on the intermediate temperature and the Curie temperatures of the respective phases, and may be influenced by the magnitude and duration of the applied field. For example, if the intermediate temperature is chosen to lie between about 250° C. and 375° C., then the ferrite matrix may include a dispersion of εFe2C or ηFe2C, but θFe3C and χFe5C2 would not be expected to precipitate in the matrix. If the intermediate temperature is chosen to lie between 225° C. and 25° C., then the ferrite matrix may include all of these iron carbide phases. Process parameters may be adjusted to control the size and volume fraction of the precipitated phases in the ferrite matrix since magnetic fields impact nucleation phenomenon significantly.
Magnetic fields increase nucleation in multiple ways. First, the free energy driving force is higher when the precipitating phase is below its Curie compared to the parent phase (that is generally not ferromagnetic) and therefore the nucleation rate would be increased as defined by classical thermodynamic equations. Second, first principles modeling of carbon diffusion in iron is shown to be slowed down and therefore carbon may diffuse a shorter distance in a given amount of time under a high magnetic field, and therefore finer and more copious carbides can be formed under a high magnetic field due to higher free energy driving force. This effect can actually lead to nanoclusters of carbides being formed in an alloy that have never been observed to occur without the presence of a high magnetic field. Also, multiple carbide morphologies can form where normally only one would occur in the absence of a high magnetic field. These effects are shown in
With appropriate control over the type, size and volume fraction of iron carbide phases, it may be possible to produce an iron-carbon alloy having an optimized set of properties, such as simultaneous improvements in both yield strength and toughness, which are normally traded off for optimizing one at the expense of the other.
The high field strength magnetic field may be applied to the iron-carbon alloy only after reaching the intermediate temperature. Alternatively, it may be advantageous to apply the high field strength magnetic field to the iron-carbon alloy during the heating at the austenitizing temperature to increase solubility of alloy additions beyond conventional solubility limits for enhanced solid solution strengthening, as well as after the intermediate temperature is reached. In some cases, the high field strength magnetic field may also be applied to the iron-carbon alloy during the heating to and/or the cooling from the austenitization temperature prior to reaching the intermediate temperature.
The magnetic processing system may be configured to heat the iron-carbon alloy to the austenitizing temperature when the alloy is within the bore of the magnet providing the high field strength magnetic field. Such a magnetic processing system is described for example in U.S. Pat. No. 7,745,765, which issued on Jun. 29, 2010, and is hereby incorporated by reference in its entirety. The austenitizing temperature is generally at least about 25° C. higher than the austenite finish temperature (Af) of the iron-carbon alloy for hypoeutectoid chemistries and between the eutectoid temperature A1, and Acm (or above Acm), which is the boundary between the austenite and austenite plus carbide phase fields for hypereutectoid chemistries. Alternatively, the heating to and/or at the austenitizing temperature may be carried out in a furnace outside the magnet prior to moving the iron-carbon alloy into the bore of the magnet.
The high field strength magnetic field applied during processing may be at least about 0.2 Tesla, or at least about 1 Tesla. In some cases, the field may be about 10 Tesla or greater, about 20 Tesla or greater, or about 30 Tesla or greater, up to a field of about 40 Tesla. For example, the high field strength magnetic field may lie between about 0.2 Tesla and about 40 Tesla or between about 10 Tesla and about 40 Tesla. Lower fields, for example in the range of from about 0.2 Tesla and to less than 1 Tesla (e.g., 0.9 Tesla), may also be suitable. Ferromagnetic materials such as conventional iron alloys can be magnetized to their saturation limits (e.g., on the order of 1 Tesla) by applying a field as low as about 0.2 Tesla.
Many ferrous alloys may benefit from the method described here. Generally speaking, suitable iron-carbon alloys include steels between about 0.1% carbon and about 2.0% carbon and cast irons containing about 2.0% and about 6.7% carbon (since some cast iron alloys are processed to produce austempered ductile iron). The carbon content may also lie between about 2.0% and about 4.3% carbon. The preceding percentages are given in weight percent. The method is further applicable to carburized and carbonitrided alloys where a chemistry gradient in the surface region is employed to achieve tailored surface properties different from the interior bulk chemistry.
The magnetic processing method described above has been applied to low carbon, low alloy steel specimens prepared by Carpenter Technology Corporation (Wyomissing, Pa.). The chemistry of the tested alloy, where the balance is iron, is shown in Table 2.
In a series of experiments carried out using this alloy, the following experimental conditions were employed to compare the microstructure and properties when a high magnetic field was applied during the austempering process versus a reference no-field condition.
For the thermomagnetic processing runs, specimens were processed as follows prior to carrying out tensile and Charpy impact tests:
For the No-Field (NF), 0T processing runs, specimens were processed as follows prior to carrying out tensile and Charpy impact tests:
The micrographs shown in
Referring to
In addition, the mechanical properties of the Carpenter alloy specimens processed with a 9T magnetic field show improvements over the conventionally processed alloys. Table 2 below compiles the 0.2% yield strength, % elongation, % reduction in area, and the Charpy V-notch results of the Carpenter alloys processed with the 9T magnetic field and the Carpenter alloys processed conventionally without a magnetic field. As indicated in the table, the properties show increases ranging from 10% to 22%.
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments included here. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein. Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.
The invention described in this disclosure was made with government support under Prime Contract Number DE-AC05-000R22725 awarded by the Department of Energy. The government has certain rights in this invention.
