CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to United States provisional application serial no. 62/084,872, filed on November 26, 2014, the contents of which are herein incorporated by reference in their entirety.

TECHNICAL FIELD

[0002] The present disclosure generally relates to ferritic alloys that exhibit oxidation resistance at elevated temperatures and improved ductility. More particularly, the present disclosure relates to silicon molybdenum aluminum (SiMoAl) cast iron alloys used for casting applications, such as turbine and turbocharger housings, exhaust manifolds, and combustion chambers.

BACKGROUND

[0003] During operation, automotive or aircraft turbocharger housings are subjected to elevated operating temperatures. These housings must be able to contain a turbine wheel generating very high rotational speeds. Exhaust gas from the automotive or aircraft engine initially contacts the turbocharger in metal sections, such as the gas inlet area of the turbocharger, at elevated temperatures. As high-speed performance improves through exhaust temperature increase, there have been attempts to gradually raise the exhaust temperature of the engine. Due to these high temperatures, the thermal load on the parts such as the exhaust manifold and the turbine housing becomes very great.

[0004] Various problems have been encountered by these increased exhaust gas temperatures contacting metal sections of the turbocharger. For example, one problem caused by the exhaust temperature rise is the problem of thermal deformation of the material, wherein the turbine housing and exhaust manifold, which alternates between regions of high temperature and low temperature is accompanied by thermal expansion and thermal shrinkage depending on the situation, which can cause surface oxidation wrinkles by such thermal deformation, and which can progress and develop into a penetration crack.

[0005] Various iron alloys have been proposed in the prior art for use in such high temperature applications. In addition to exhibiting high temperature capabilities, iron alloys generally must be easy to handle, machine, and abrasively clean at room temperature. Further, alloys are desired that are less brittle at room temperature and at the temperature of use.

[0006] Cast iron alloys that are currently known generally have a relatively high mechanical strength but tend to have a relatively low ductility, i.e., the components manufactured therefrom tend to be somewhat brittle. It is desirable to increase the mechanical strength of the alloy at elevated temperatures, while maintaining or improving the ductility at both room and elevated temperatures.

[0007] In an attempt to address the above described concerns of high strength at elevated temperature, good thermal fatigue resistance, good oxidation resistance, and reasonable ductility, prior efforts have resulted in the development ductile cast irons with high silicon and high molybdenum content, commonly known as SiMo alloys. The SiMo alloys exhibit improved high temperature strength and thermal fatigue resistance over other ductile cast irons, as well as improved high temperature oxidation resistance. However, these property improvements are accompanied with a somewhat lower ductility at ambient temperatures and reduced machinability than other ductile cast irons. Despite these improvements in high temperature strength and thermal fatigue resistance and oxidation resistance, further enhancement of these properties would be desirable, in an alloy with acceptable ductility. The high oxidation rate at the high temperatures is especially a problem in parts such as exhaust manifolds and turbocharger turbine housings, wherein the in-use temperatures can reach 1000 °C and higher.

[0008] In addition, cast irons in these applications are subject to thermal fatigue cracking. This is due at least in part to the fact that the ferrite-austenite phase change temperature of these alloys is typically below the temperature of use. Therefore, in use, the part is cycled up to temperatures associated with engine operation and then back down to room temperature. The part undergoes the ferrite austenite phase change upon heating and again upon cooling. This continued thermal cycling and associated phase transformation is said to contribute to thermal fatigue in the part which, in time, leads to cracking.

[0009] In addition to SiMo alloys, iron alloys that contain aluminum are also known. Such high aluminum iron alloys tend to have better high temperature oxidation resistance than conventional iron alloys. Furthermore, the aluminum content of the iron shifts the ferrite-austenite phase change temperature to higher temperatures, with the shift being greater as more aluminum is added. This is desirable for applications such as exhaust manifolds because it may be possible to formulate an alloy with a phase change temperature above the use temperature. Components formed from such alloys would not be subject to fatigue cracking associated with phase changes developed through thermal cycling. As higher aluminum contents push the phase change temperature to higher levels, higher use temperatures may be reached. It has been observed that as the alloys go to higher aluminum content, the iron alloys become harder and more brittle at room temperature. Parts cast from the brittle iron alloys are more difficult to machine or to abrasively clean because of their hardness, and they are subject to fracture during subsequent processing and handling because of their brittleness.

[0010] Therefore, it would be desirable to provide an iron alloy with a high level of aluminum that combines the properties of high mechanical strength and high ductility. It would also be desirable to provide parts cast from such an alloy that could be readily machined and abrasively cleaned at low temperature, and that could also withstand oxidation at high in-use temperatures. Because the ferrite-austenite phase transformation temperature would be above the in-use temperature in many engine applications, such parts used under these conditions would be more resistant to fatigue cracking associated with any transformation-induced thermal cycling. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter. BRIEF SUMMARY

[0011] Disclosed is silicon molybdenum aluminum ferritic iron alloy that includes, in addition to greater than about 50.0 wt % iron, the following amounts: from about 2.0 to about 4.0 wt % aluminum and from about 2.0 to about 4.3 wt % silicon, with the proviso that an equivalent silicon amount be greater than about 4.7 wt%. A ratio of the amount of silicon to the amount of aluminum is from about 1.1 :0.9 to about 0.9: 1.1. The iron alloy further includes greater than about 1.5 wt% carbon, with the proviso that an equivalent carbon amount be less than about 5.2 wt%, and from about 0.2 to about 1.1 wt% molybdenum. Articles of manufacture that may be fabricated using alloys according to the foregoing specification include, for example, turbocharger housings, exhaust manifolds, and combustion chambers.

[0012] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

DETAILED DESCRIPTION

[0013] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word "exemplary" means "serving as an example, instance, or illustration." Thus, any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Furthermore, as used herein, numerical ordinals such as "first," "second," "third," etc., such as first, second, and third components, simply denote different singles of a plurality unless specifically defined by language in the appended claims. Still further, all numeric values are herein assumed to be modified by the term "about", whether or not explicitly indicated. The term "about" generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value, i.e., having the same function or result. An example of this would be +/- 5% of the stated value. All of the embodiments and implementations of the ferritic alloys, turbocharger turbine housings, and methods for the manufacture thereof described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention, which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.

[0014] Unless specifically stated or obvious from context, as used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. "About" can be understood as within 10%, 5%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term "about."

[0015] The compositions and methods of the invention provide cast iron articles having desired combinations of ductility, high oxidation resistance, and thermal fatigue resistance. They are useful generally in any iron application, particularly high temperature ductile iron applications. Examples include, but are not limited to, automotive exhaust components including exhaust manifolds and catalytic converter cans, turbocharger components including turbine housings and center housings, and engine components such as blocks, heads, cylinder liners, cam shafts, crank shafts, dampers, bed plates, valve train components, pistons, piston inserts, bearing caps, and pump housings. The cast articles of the present disclosure also find use as transmission components including cases, carriers, housings, barring caps, fly wheels, and pump housings. Other applications will be appreciated by those having ordinary skill in the art.

[0016] Broadly speaking, the cast iron articles of the present are prepared by pouring a molten composition into a mold. The molten composition is a cast iron composition that includes, in addition to greater than about 50% by weight iron, aluminum at levels, independently, from about 2.0% by weight to about 4.0% by weight, such as from about 2.5% by weight to about 3.5% by weight, for example about 3.0% by weight; silicon at levels, independently, from about 2.0% by weight to about 4.3% by weight, such as from about 2.5% by weight to about 3.8% by weight, for example about 3.2% by weight; and molybdenum at levels, independently, from about 0.2% by weight to about 1.1% by weight, such as from about 0.4% by weight to about 0.9% by weight, for example about 0.6% by weight.

[0017] Molybdenum generally increases the tensile strength, hardness and hardenability of cast irons. It improves high temperature creep and fatigue resistance, and increases carbide formation as found in the final structure on grain boundaries, often in pearlitic regions. Molybdenum levels on the higher side of the range may be used to increase the high temperature strength of the cast articles. As noted above, higher molybdenum content is generally associated with a higher carbide content. Because of the higher carbide content, the cast articles will tend to be more brittle with some risk of cracking during thermal cycling, as for example, in normal automotive engine use, or under simulative or accelerated engine dynamometer durability tests.

[0018] The iron compositions may further include carbon. Carbon is present in an amount by weight of at least about 1.5%, such as at least about 2.0%, for example at least about 2.5%. Carbon may be present as graphite. The graphite present in the molded articles in predominantly present in either nodular or vermicular form. When greater than 50% of the carbon is present as graphite nodules, the compositions are generally referred to as ductile irons. When the nodularity is less than about 50% (i.e., when less than about 50% of the carbon is present as graphite nodules), the compositions are referred to as compacted graphite iron if the remainder of the carbon is present as vermicular graphite. Generally, high levels of flake graphite are less desirable. In compacted graphite irons of the present disclosure, nodularity is generally about 15% or greater, with the remainder of the graphite predominantly present in vermicular form. In ductile irons of the present disclosure, the nodularity is greater than 50%, preferably greater than 70%, and the balance of the carbon is predominantly in vermicular form. In a preferred embodiment, the nodularity is greater than about 90%.

[0019] The iron composition specification may further be provided with regard to "equivalent silicon content" and "equivalent carbon content." As is known in the art, the equivalent silicon content is defined as the actual silicon content plus 0.8 times the aluminum content. In an embodiment, the equivalent silicon content may be at least about 4.5% by weight, such as at least about 5.0% by weight, for example at least about 5.2% by weight. As is further known in the art, equivalent carbon content is defined as the actual carbon content plus one third of the combined amount of silicon and phosphorous present. In an embodiment, independently, the equivalent carbon content may be less than about 5.2 % by weight, for example less than about 4.7% by weight, such as less than about 4.2% by weight. [0020] The iron composition specification may further be provided with regard to a ratio between the amount of silicon and the amount of aluminum present (Si:Al ratio). In one embodiment, the Si:Al ratio may be from about 1.1 :0.9 to about 0.9: 1.1, such as about 1.0: 1.0. It has been surprisingly discovered that the foregoing Si:Al ratio results in an iron composition that exhibits a high degree of ductility while also exhibiting a high degree of oxidation resistance.

[0021] It is generally preferred that the compositions of the present disclosure contain less than about 0.03% by weight sulfur, such as less than about 0.02% by weight sulfur. Higher sulfur levels tend to lead to a requirement for additional magnesium additions and cause more rapid chemical fade during the pretreatment step to control production of either compacted (vermicular) or nodular graphite structures. For similar reasons, it is preferred to keep the oxygen content of the compositions low, typically less than about 0.005% (50 ppm). Phosphorus should also be kept to minimum, preferably below about 0.10% by weight, such as below about 0.05% by weight. Manganese and nickel may also be present in low amounts. For example, in one embodiment, manganese is present in an amount of less than about 0.30% by weight, such as less than about 0.20% by weight. Independently, nickel is present in an amount of less than about 0.50% by weight, for example less than about 0.20% by weight, such as less than about 0.10% by weight.

[0022] Because of the molybdenum, silicon, and aluminum content of the composition, as well as its desirable high temperature properties, the composition of the present disclosure can be referred to as a high temperature SiMoAl ferritic iron. The desirable properties of ductility and machinability exhibited by the compositions of the present disclosure are believed to derive from the microstructure of the high temperature SiMoAl ferritic iron.

[0023] In general, though complete elimination is impossible, carbides in the compositions of the present disclosure may be substantially avoided. At room temperature, they may cause the iron to be brittle. Iron carbide is a hard and brittle phase called "cementite" or "massive carbide". The presence of too much carbide will make a part difficult to handle. The part will be more susceptible to crack propagation under stress or load, and it may crack during tumble cleaning, or in service due to mechanical stresses. Heat treatment will tend to decompose the iron carbide and the amount of pearlite (which is a lamellar mix of ferrite and cementite). Molybdenum carbides are present in SiMoAl irons, and are often on grain boundaries and associated with pearlite. At levels of only a few percent, the molybdenum carbide is not detrimental to the mechanical properties. Fine pearlite up to about 25% is permitted in these SiMoAl ductile irons.

[0024] The cast iron articles of the present disclosure are in general readily machinable. A material with good machinability requires easy formation of machining chips, easy break away of the chips, a reasonable tool wear rate, and reasonable fixturing. The parts may be machined dry or with a machining coolant or lubricant. Good machinability is thought to be in general due to the good ductility of the materials.

[0025] Ductile materials such as those of the present disclosure are materials which produce a measurable elongation at room temperature in tensile tests such as ASTM E-8. Generally, the materials exhibit ductility such that a tensile test results in an elongation greater than zero. Typically, the materials exhibit an elongation of about 1% or greater. Generally, an elongation result of less than about 1% is considered fairly brittle.

[0026] Molded articles of the present disclosure may, in one exemplary embodiment, be prepared by pouring a molten composition into a mold. The molten composition is prepared by a process containing a number of steps generally familiar to those of skill in the art of making cast iron articles. In a first step, a number of material streams are provided that contain sources of the elements iron, silicon, molybdenum, aluminum, and carbon. The material streams are melted together to form a first melt. The first melt generally contains at least the elements iron, silicon, molybdenum, aluminum, and carbon. The first melt is treated with a nodularizer containing magnesium or other sulfide/oxysulfide/silicate former to form a second melt containing graphite nucleation sites. In this manner, magnesium may be present in the composition in an amount by weight of less than about 0.10%, such as less than about 0.08%. Next, the second melt is inoculated with an inoculant to provide nucleation sites in addition to those generated by treatment with the nodualizer. The inoculant may be a rare earth metal, such as cerium or lanthanum. In this manner, a rare earth metal may be present in the composition in an amount by weight of less than about 0.15% by weight, such as less than about 0.10% by weight. Also at this point, additional aluminum bearing materials may be added to the molten composition to bring the final aluminum content within the range as stated above. More generally, the process of the present disclosure provides for aluminum-bearing materials to be added possibly at several points during the process. In a preferred embodiment, the aluminum additions are made after the inoculant treatment. Whenever the aluminum addition is made, the aluminum bearing materials may be made of pure aluminum or a number of other materials containing minor impurities. The aluminum-bearing materials may be scrap, returns, alloys, pure aluminum, wire, and may be in a molten or solid state. The returns may be from articles cast from compositions of the invention. The aluminum alloys preferably contain limited levels of carbide-forming elements. The aluminum can be added as a block, for example in a plunging bell or cage. In a preferred embodiment, the aluminum is introduced into the molten iron composition at a location below the melt surface. The aluminum may also be in the form of clean pellets, chunks, or shavings. It is also possible to add the aluminum as a wire feed.

[0027] In other embodiments, the molded articles may be prepared in other manners, such as those conventionally known in the art.

[0028] A variety of material streams may be used to provide the elements of iron, silicon, carbon, aluminum, and molybdenum for the first melt. Pig iron may be used as a source of iron. Other iron sources may also be conveniently used, including steel scrap and other ductile or nodular or compacted graphite iron scrap and returns. Carbon is generally added as graphite but could also be added as silicon carbide. Other additions to the first melt can be ferrosilicon alloys and ferromolybdenum alloys. Ferrosilicon alloys, for example, may typically contain 65 to 75% silicon by weight, balance iron with small amounts of other elements. Ferromolybdenum may typically contain about 60% molybdenum, balance iron and small amounts of other elements. Generally, a variety of such streams are first charged into a melting furnace. After the streams are melted together, it is preferred to check the elemental content. If it is outside specifications, other materials can be added to bring them within specifications. In this way, the process provides a convenient way of reusing scrap and recycling returns from the pouring process.

[0029] While at least one exemplary embodiment has been presented in the foregoing detailed description of the inventive subject matter, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the inventive subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the inventive subject matter. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the inventive subject matter as set forth in the appended claims.