Patent Grant
7497915
1. Field of the Invention
The subject invention generally relates to a machinable austempered cast iron article having improved machinability, fatigue performance, and resistance to environmental cracking, a method of making the machinable austempered cast iron article, and a machinable austempered cast iron composition. More specifically, the subject invention relates to a machinable austempered cast iron article having a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite that exhibits improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking.
2. Description of the Related Art
Regular ductile iron (RDI) articles and regular austempered ductile iron (ADI) articles are well known in the art, as are methods for making these articles. RDI articles are used extensively in automotive applications and ADI articles are used in limited vehicular applications, including crankshaft and chassis components. RDI articles are generally made by casting a ductile iron composition without subjecting the ductile iron composition to a post-casting heat treatment process. The ductile iron composition can vary in percentage of components, but must include carbon and sufficient alloying elements to form a microstructure of well-formed graphite nodules in a matrix of ferrite and pearlite in the regular ductile iron articles.
Typical RDI articles that require a good combination of strength and toughness have the ferritic and pearlitic microstructure, not a substantially pearlitic microstructure. The ferritic and pearlitic microstructure has superior physical properties to the substantially pearlitic microstructure when the ductile iron composition is cast without subjecting the ductile iron composition to post-casting heat treatment. Alternatively, RDI articles can be heat treated by normalizing or quenching and tempering. However, the typical ductile iron compositions do not respond to a heat treatment process of the present invention, forming unwanted pearlite during rapid cooling. Thus, typical ductile iron compositions are not suitable for use with the heat treatment process of the subject invention.
ADI articles are made by subjecting the ductile iron composition to a post-casting heat treatment process. A microstructure of the ductile iron composition prior to the heat treatment process is not a factor and is overlooked, with emphasis on the heat treatment process itself for producing the ADI article. ADI articles are generally produced by austenitizing followed by austempering.
Another method for producing ADI articles is step austenitizing, which is disclosed in “Improving The Properties of Austempered Ductile Iron” to Gundlach (the DIS publication), but remains an experimental method that has not been applied to and optimized for production. Step austenitizing is a process by which the ductile iron composition is heated to and held at an initial austenitizing temperature. The step austenitizing proceeds by quenching the ductile iron composition to sequentially lower temperatures and holding the ductile iron composition at each temperature for a short amount of time. The process ends by quenching the ductile iron composition to produce the ADI article. The ADI article produced through step austenitizing typically has an ausferritic microstructure. The ausferritic microstructure generally provides higher strength than the regular ductile iron articles, but is also less ductile and less machinable than the regular ductile iron articles.
Austenitizing followed by austempering is performed by first austenitizing a ferritic and pearlitic microstructure at an austenitizing temperature, typically in a range of from 1550° F. to 1650° F., although austenitizing temperatures as low as 1450° F., which may be in an intercritical temperature range, have been documented. The ductile iron composition is then austempered at a significantly lower temperature, typically between 350° F. and 725° F., to produce the regular austempered ductile iron article. Austenitizing and austempering temperatures are varied to achieve desired physical properties in the regular austempered ductile iron articles. The resulting regular austempered ductile iron articles have an ausferrite microstructure, i.e., acicular ferrite plus austenite. Time at temperature for the austenitizing and austempering process is also crucial. For articles with the ferritic and pearlitic starting microstructure, the carbon must diffuse into the austenitic matrix from graphite nodules interdispersed throughout the ductile iron composition to form a high carbon austenite before quenching to the austempering temperature. As a result, an austenitizing time of 90 minutes is typical to achieve production of the high carbon austenite.
ADI articles that are austenitized at lower temperatures exhibit better machinability than ADI articles austenitized at higher temperatures. However, the acicular ferritic plus austenitic microstructure (ausferrite) that is produced through austenitizing at the lower temperatures does not have sufficient strength for many applications in which ADI articles are used.
Spanish Patent No. ES 8104423 to Muhlberger discloses a method for producing another austempered ductile iron article having a microstructure of austenite mixed with bainite and spherical graphite. The Muhlberger austempered ductile iron (ADI) article is produced by heat treating a ductile iron composition as shown in Table 1.
The heat treating is performed by austenitizing the ductile iron composition in a temperature range of from 1472° F. to 1580° F. for between 10 and 60 minutes. The ductile iron composition is then quenched over a period of less than 2 minutes to a temperature range of between 662° F. and 752° F. The ductile iron composition is maintained within the temperature range of between 662° F. and 752° F. for a period of between 5 and 60 minutes to produce the Muhlberger ADI article having a microstructure of austenite mixed with bainite and spherical graphite, which is a regular austempered ductile iron structure. The Muhlberger ADI article is insufficient for applications of the subject invention. The molybdenum composition is too high, resulting in the iron article having a Brinell Hardness that is too high, and the composition requires manganese. Furthermore, the resulting microstructure of the austempered ductile iron composition is austenite mixed with bainite and spherical graphite, and does not contain equiaxed ferrite with islands of austenite because the method does not begin with a substantially pearlitic microstructure prior to austenitizing. In addition, the combination of chemistry and austenitizing temperature are not suitable for the subject invention. Referring to
RDI articles and ADI articles have physical properties that are suitable for many applications, however, RDI articles and ADI articles are often not suitable for the same applications. Referring to
On the other hand, ADI articles, as shown in
Thus, there remains an opportunity for a machinable austempered cast iron (MADI) article and a method of producing the MADI article having a unique combination of improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking that has not been achieved by the prior art.
The subject invention provides a machinable austempered cast iron article, a machinable austempered cast iron composition, and a method of making the machinable austempered cast iron article. The machinable austempered cast iron article is made from an iron composition that has a substantially pearlitic microstructure. The substantially pearlitic microstructure includes carbon, silicon, nickel, copper, and molybdenum.
The method of making the machinable austempered cast iron article includes austenitizing the substantially pearlitic microstructure in an intercritical temperature range of from 1380° F. to 1500° F. for a period of at least 10 minutes. This produces a ferritic plus austenitic microstructure. Having a substantially pearlitic microstructure prior to austenitizing allows for improved time to complete austenitizing that is not possible with other microstructures. The method proceeds by quenching the ferritic plus austenitic microstructure at a rate sufficient to prevent formation of pearlite. Next is austempering the ferritic plus austenitic microstructure in an austempering temperature range of from 575° F. to 750° F. for a period of at least 8 minutes to produce a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite. The microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is then cooled to ambient temperature to produce the machinable austempered cast iron article.
The machinable austempered cast iron article of the subject invention has improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking. The improved strength and machinability makes the machinable austempered cast iron article ideal for crankshaft and chassis components that currently sacrifice strength for machilability, or machinability for strength. The improved strength also provides for an improvement in weight for the machinable austempered cast iron article, and thus a decrease in cost. Furthermore, the method of the subject invention is capable of reducing the time required to make the iron article, which can also result in lower costs.
Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
The subject invention provides a machinable austempered cast iron article and a method of making the machinable austempered cast iron article from an iron composition. The machinable austempered cast iron article has improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking. Improved machinability makes the machinable austempered cast iron article ideal for many applications in the automotive industry. Furthermore, improved strength provides for an improvement in weight and cost of the machinable austempered cast iron article.
The iron composition includes carbon, silicon, nickel, copper, molybdenum, and iron. Optimum ranges for the iron composition of the present invention are disclosed in Table 2.
An amount of each element is varied within the ranges to ensure sufficient formation of a desired microstructure within the iron composition during production of the machinable austempered cast iron article. For example, formation of the desired microstructure is determined primarily by two factors: cooling rate and chemistry of the iron composition. Cooling rate is controlled by a number of factors that vary according to aspects of each particular production line, such as geometry of a particular article, composition of material used in making a mold for producing the article, i.e. sand or metal, and cooling time for the article in the mold before being removed. The cooling time can be slightly adjusted by controlling a speed of the production line, but only to a limited extent. Thus, microstructure is, in large part, controlled through variation in the amount of each element in the iron composition.
The carbon included in the iron composition is a necessary component of various microstructures formed at different stages in the production of the machinable austempered cast iron article. Silicon, nickel, copper, and molybdenum in the iron composition are alloying agents. The alloying agents are necessary to promote a substantially pearlitic microstructure and to suppress the formation of pearlite during the production of the machinable austempered cast iron article. It is to be understood that the microstructure is substantially pearlitic in the “as cast” condition. By substantially pearlitic microstructure it is meant that the microstructure includes greater than 50% pearlite. Preferably, the substantially pearlitic microstructure includes at least 80% pearlite. Additional alloying agents can also be used, such as manganese, chromium, tin, arsenic, and antimony, but are not specifically required for the subject invention. The remainder of the iron composition is iron. The most preferred iron composition includes:
The iron composition is a ductile iron composition and has improved castability and production economies over other types of iron compositions. In other embodiments, the iron composition is a gray iron composition, a compacted graphite iron composition, or a carbidic ductile iron composition, depending on physical property and production requirements for a particular application.
The method includes austenitizing the substantially pearlitic microstructure in an intercritical temperature range of from 1380° F. to 1500° F. Preferably the substantially pearlitic microstructure is austenitized in the intercritical temperature range of from 1380° F. to 1472° F., more preferably in the intercritical temperature range of from 1380° F. to 1449° F. The substantially pearlitic microstructure is maintained in the intercritical temperature range for a period of at least 10 minutes, preferably for a period of from 10 to 360 minutes. The austenitizing step produces a ferritic plus austenitic microstructure. The substantially pearlitic microstructure is an essential element to the austenitizing step of the subject invention. Carbon necessary for the formation of an austenitic portion of the ferritic plus austenitic microstructure is derived from the substantially pearlitic microstructure. The substantially pearlitic microstructure allows the austenitic portion of the ferritic plus austenitic microstructure to form in as little as 10 minutes. This allows for improved production speed for the overall process.
In a quenching step, the ferritic plus austenitic microstructure is quenched from the austenitizing temperature into an austempering temperature range of from 575° F. to 750° F. Preferably, the quenching step occurs in a salt bath with water injection.
In an austempering step, the ferritic plus austenitic microstructure is held in the austempering temperature range for at least 8 minutes. The austempering step prevents formation of a martensitic or pearlitic microstructure, which have undesirable physical properties for intended applications of the subject invention. The austempering step produces a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite. The microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is produced because only a portion of the substantially pearlitic microstructure is transformed into the austenitic portion of the ferritic plus austenitic microstructure during the austenitizing step. During austempering, the ferritic portion of the ferritic plus austenitic microstructure remains as ferritic microstructure, as opposed to acicular or bainitic ferrite. The austenitic portion of the ferritic plus austenific microstructure is stabilized.
An amount of ferrite in the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite depends on the intercritical temperature at which the substantially pearlitic microstructure is austenitized. At higher temperatures within the intercritical temperature range, more austenite forms, with a remainder of the pearlitic microstructure forming ferrite. The austenite is maintained and stabilized in the austempering step. Therefore, the amount of austenite in the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is limited by the amount of the austenite formed prior to the austempering step.
In a cooling step, the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is cooled to ambient temperature to retain the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite produced in the austempering step. The microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is cooled to ambient temperature by air cooling or water quenching.
More specifically, the method includes casting the iron composition at a temperature of greater than 2200° F., at which temperature the iron composition is molten. Next is a cooling step, in which the iron composition is cooled to a temperature of from 1000° F. to 1340° F. The iron composition is held at the temperature of from 1000° F. to 1340° F. for at least 8 seconds to form the substantially pearlitic microstructure.
In the austenitizing step, the substantially pearlitic microstructure is heated in the intercritical temperature range of from 1380° F. to 1500° F. Preferably the substantially pearlitic microstructure is austenitized in a temperature range of from 1380° F. to 1472° F., more preferably in a temperature range of from 1380° F. to 1449° F. The substantially pearlitic microstructure is maintained in the intercritical temperature range for the period of at least 10 minutes, preferably for the period of from 10 to 360 minutes. The austenitizing step produces the ferritic plus austenitic microstructure.
In a quenching step, the ferritic plus austenitic microstructure is quenched into the austempering temperature range of from 575° F. to 750° F. to stabilize the ferritic plus austenitic microstructure produced in the austenitizing step. Preferably, the ferritic plus austenitic microstructure is quenched into the austempering temperature range within a period of from 5 to 180 seconds. Preferably, the quenching step is performed in the salt bath with water injection. The salt bath contains liquid including at least one of nitrate salts, nitrite salts, and combinations of nitrate salts and nitrite salts for rapidly cooling the ferritic plus austenitic microstructure. More specifically, the salt bath contains liquid including Park Metallurgical low temperature draw salt manufactured by Heatbath Corporation. Alternatively, the second quenching step may be performed in a fluidized bed. The machinable austempered cast iron article is preferably a crankshaft or a chassis component, but the method is not limited to production of such components.
In the austempering step, the ferritic plus austenitic microstructure is held in the austempering temperature range of from 575° F. to 750° F. to stabilize the austenite and prevent the formation of a martensitic or pearlitic microstructure. The ferritic plus austenitic microstructure is maintained in the austempering temperature range for the period of at least 8 minutes, preferably for a period of from 8 to 1440 minutes, and more preferably for a period of from 60 to 180 minutes. The austempering step produces the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite. Finally, the machinable austempered cast iron is cooled to ambient temperature.
As shown in Table 4, a range in average ultimate tensile strength (UTS), yield strength (YS), elongation (% El.), and Brinell hardness (BHN) of machinable austempered cast iron articles having the iron composition as set forth in Table 3, austenitized at different intercritical temperatures within the intercritical temperature range, correlates to the intercriticl temperature at which the machinable austempered cast iron articles are austenitized, and thus to a ratio of ferrite to austenite in the machinable austempered cast iron articles.
Therefore, the intercritical temperature at which the machinable austempered cast iron articles are austenitized is adjusted to achieve desired properties of the machinable austempered cast iron article for particular applications.
Likewise, austempering temperature has an effect on the BHN of the machinable austempered cast iron article. Referring to
Time at intercritical temperature also has an effect on the BHN of the machinable austempered cast iron article, albeit not as significant as intercritical or austempering temperature. As a result of only minor differences in BHN, production line and cost strategies generally dictate the time at temperature for the machinable austempered cast iron article during the austenitizing and austempering steps, as long as the time at temperature is at least 10 minutes for the austenitizing step and at least 8 minutes for the austempering step.
The macninable austempered cast iron article has improved strength and ductility, as measured through a number of standard testing procedures to be set forth below. Generally, strength refers to UTS and YS, and ductility refers to the % El. The improved strength and ductility is attributable to the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite of the machinable austempered cast iron article.
Preferably, the machinable austempered cast iron article has a BHN of between 180 and 340 BHN, as measured through standard procedures known to those skilled in the art. As shown above in Table 4, the BHN of the machinable austempered cast iron article has a direct correlation to the intercritical temperature at which the iron composition is austenitized. Referring to
Preferably, as shown in
Preferably, as generally shown in
In one embodiment, as shown at 10 in
The B10 life, median life, and the range of the results from the lowest to the highest fatigue life show that the lower control arm 10, and thus the MADI of the subject invention, heat treated through the method of the subject invention, has improved flaw tolerance over RDI and ADI of the prior art. Therefore, machinable austempered cast iron articles produced according to the method of the subject invention are ideal for lower control arm applications in which the lower control arm 10 is subjected to repeated loading cycles, i.e., fatigue loading.
UTS testing was also performed on the same lower control arm testing apparatus. A straight end fixture was substituted for the angled-end fixture used in the fatigue life test. The lower control arm testing apparatus was programmed to apply a ramped load at a rate of 0.01 Hz up to a maximum load of 94,000 N. The 94,000 N maximum load represents a threshold that the lower control arm 10 must exceed to be suitable for vehicular applications. The lower control arm 10 having a BHN of 243 BHN did not succumb to the maximum load of 94,000 N. Therefore, the lower control arm 10 has sufficient UTS for vehicular applications using the lower control arm 10.
In another embodiment, the machinable austempered cast iron article is a torsion bar adjuster, shown generally at 14 in
The B10 life, median life, and the range of the results from the lowest to the highest fatigue life show that the torsion bar adjuster 14 comprised of the MADI of the subject invention heat treated through the method of the subject invention has improved fatigue performance and flaw tolerance over torsion bar adjusters 14 comprised of RDI and ADI of the prior art. Therefore, the machinable austempered cast iron articles of the subject invention are ideal for torsion bar adjuster applications in which the torsion bar adjuster 14 is subjected to repeated loading cycles, i.e., fatigue loading.
Milling and drilling tests were performed to verify the machinability of the machinable austempered cast iron articles in light of the combination of BHN, UTS, YS and % El. measured in prior tests. Milling and drilling are two principal methods that will be employed in machining the machinable austempered cast iron articles. The machinable austempered cast iron articles will often be subjected to extensive milling and drilling. Thus, the machinable austempered cast iron articles must be conducive to milling and drilling to be economically and mechanically feasible for use in automotive applications, which generally require mass production of the machinable austempered cast iron articles. Machinability of machinable austempered cast iron articles, in general, is not accurately predictable based only upon % El. and BHN, although % El. and BHN of the machinable austempered cast iron article generally suggest a relative machinability for the machinable austempered cast iron article. Only actual testing can reliably measure machinability.
The milling test was performed on regular ductile iron articles (RDI) of the prior art having a BHN of 277 BHN, regular austempered ductile iron articles (ADI) of the prior art having a BHN of 311 BHN, and the machinable austempered cast iron articles (MADI) of the subject invention having a BHN of 302 BHN. A Kennametal KSSR 3.94-SE4-45-5 right hand cutter with a diameter of 100 mm was used. Kennametal KC520M inserts with a 25 micron hone edge preparation were used in the right hand cutter. The right hand cutter had a 45° lead angle, a radial rake of −5°, and an axial rake of 20°. A depth of cut was held constant at 2.3 mm. Cutting using a single Kennametal KC520M insert was performed to increase the wear rate and exclude the effects of non-uniform cutting. The results of the milling tests are shown in Table 7.
The milling tests show unique and unexpected machinability for the MADI of the subject invention. Machining forces were found to first increase and then decrease as feed rate increased. The machining forces have been verified through multiple tests. The decrease in machining forces may be due to work hardening behavior of the MADI. Nevertheless, the machinability of the MADI can be exploited to increase production rate and decrease wear on milling tools used on the MADI.
The drilling test was performed on RDI of the prior art having a BHN of 277 BHN, ADI of the prior art having a BHN of 311 BHN, and MADI of the subject invention having a BHN of 302 BHN. Kennametal grade KC 7210 drills with a TiAIN coating, a point angle of 130°, a helix angle of 30°, a rake angle of 60°, and a web thickness of 1.97 mm were used. A Kistler 9272A machining dynamometer was used to measure torque and thrust at various feed rates and drill speeds. Piezoelectric signals from the machining dynamometer were amplified and Labview software was used to gather the data. A 2000 Hz sampling rate was used. Results of the tests are shown in Table 8.
The drilling tests show thrust values for the MADI of the subject invention increased or increased then leveled off as drill speed increased and feed rate remained constant. Torque values increased and then decreased as drill speed increased and feed rate remained constant. The MADI performed best at higher speeds, indicating suitability for applications requiring drilling. The milling and drilling test results for the MADI show an improvement in machinability of the MADI over the RDI and ADI of the prior art.
Environmental cracking tests were performed to test resistance of the machinable austempered cast iron articles to cracking when exposed to environmental conditions. The machinable austempered cast iron articles are often subjected to harsh environmental conditions. For example, torsion bar adjusters and lower control arms, and engine parts are frequently subjected to moisture, oil from leaks, and fuel from spillage. When subjected to various strain rates, there may be a heightened rate of cracking as opposed to when the machinable austempered cast iron articles are dry. Thus, the machinable austempered cast iron articles must be sufficiently resistant to environmental cracking under such conditions to be economically and mechanically feasible for use in automotive applications.
The tests for resistance to environmental cracking were performed on samples of machinable austempered cast iron articles (MADI) of the subject invention having a BHN of 243 BHN, regular ductile iron (RDI) of the prior art, and regular austempered ductile iron (ADI) of the prior art. The samples were subjected to various types of fluid, including water, fresh motor oil, used motor oil, and diesel fuel. The samples were then subjected to various strain rates. Measurements of % El, UTS, and YS were made on the samples treated under the various conditions to determine how well the samples retained their properties. Results of the tests are shown in Table 9.
The results of the tests for resistance to environmental cracking show that the machinable austempered cast iron performed better than regular austempered ductile iron in terms of retaining % El and UTS. The machinable austempered cast iron performed better than regular ductile iron in terms of retaining UTS and YS. Furthermore, the machinable austempered cast iron did not show significant loss of % El, UTS, or YS under any of the tests. Thus, the machinable austempered cast iron exhibits improved resistance to environmental cracking over both regular ductile iron and regular austempered ductile iron.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.
The present application is a divisional of Ser. No. 10/655,237 filed on Sep. 4, 2003, which claims priority to and all advantages of the United States Provisional Patent Application No. 60/408,147filed on Sep. 4, 2002.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3920490 | Koppenaal et al. | Nov 1975 | A |
| 4032368 | Grange | Jun 1977 | A |
| 4222793 | Grindahl | Sep 1980 | A |
| 4432812 | Rice | Feb 1984 | A |
| 4484953 | Kovacs et al. | Nov 1984 | A |
| 4541878 | Muhlberger et al. | Sep 1985 | A |
| 4596606 | Kovacs et al. | Jun 1986 | A |
| 4737199 | Kovacs | Apr 1988 | A |
| 4838956 | Satou et al. | Jun 1989 | A |
| 4880477 | Hayes et al. | Nov 1989 | A |
| 5028281 | Hayes et al. | Jul 1991 | A |
| 5139579 | Kovacs | Aug 1992 | A |
| 5180450 | Rao | Jan 1993 | A |
| 5268044 | Hemphill et al. | Dec 1993 | A |
| 5352304 | DeArdo et al. | Oct 1994 | A |
| 5837069 | Deards et al. | Nov 1998 | A |
| 5849114 | Keough et al. | Dec 1998 | A |
| 5853504 | Nishimura | Dec 1998 | A |
| 6258180 | Wilde et al. | Jul 2001 | B1 |
| 6277216 | Leap et al. | Aug 2001 | B1 |
| 6406565 | Hashimura et al. | Jun 2002 | B1 |
| 20010024621 | Leppanen | Sep 2001 | A1 |
| Number | Date | Country |
|---|---|---|
| 0018445 | Nov 1980 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20060157160 A1 | Jul 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60408174 | Sep 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10655237 | Sep 2003 | US |
| Child | 11375327 | US |
