US20110256017A1

US20110256017A1 - High temperature cast iron with niobium and having compacted graphite structures

Info

Publication number: US20110256017A1
Application number: US13/077,365
Authority: US
Inventors: David M. Decker
Original assignee: BorgWarner Inc
Current assignee: BorgWarner Inc
Priority date: 2010-04-19
Filing date: 2011-03-31
Publication date: 2011-10-20

Abstract

A product including an iron casting alloy including iron, niobium and compacted graphite structures.

Description

This application claims the benefit of U.S. Provisional Application No. 61/325,551 filed Apr. 19, 2010.

TECHNICAL FIELD

The field to which the disclosure generally relates includes cast iron alloys and structures including compacted graphite and methods of making and using the same.

BACKGROUND

FIG. 1 is a scanning electron microscope (SEM) view illustrating graphite in gray iron. In the production of gray iron, when the composition of molten iron and its cooling rate are appropriate, the carbon (C) in the iron separates during solidification and forms separate graphite flakes that are interconnected within each eutectic cell. The graphite grows edgewise into the liquid and forms the characteristic flake shape. When gray iron is broken, most of the fracture occurs along the graphite, thereby accounting for the characteristic gray color of the fractured surface. Because the large majority of the iron castings produced are of gray iron, the generic term, cast iron, is often improperly used to mean gray iron specifically.
The properties of gray iron are influenced by the size, amount and distribution of the graphite flakes, and by the relative hardness of the matrix metal around the graphite. These factors are controlled mainly by the C and silicon (Si) contents of the metal and the cooling rate of the casting. Slower cooling and higher C and Si contents tend to produce more and larger graphite flakes, a softer matrix structure and lower strength. The flake graphite provides gray iron with unique properties such as excellent machinability at hardness levels that produce superior wear-resisting characteristics, the ability to resist galling and excellent vibration damping.
FIG. 2 is an SEM illustrating graphite in ductile iron. Ductile iron (also known as nodular iron) compositions typically include 3.2-4.1% carbon, 1.7-2.8% silicon, 0.45-0.8% manganese, 0.1-0.14% phosphorus, 0.05-0.13% sulfur by weight. In nodular iron, the magnesium may be used as a treatment element and is retained in the final casting in an amount of about 0.04% and sulfur is reduced to about 0.002%. However, in compacted graphite irons (described in greater detail hereafter), the magnesium is typically retained in an amount of about 0.01-0.035% or up to 0.4% by weight if titanium is added.
Castable iron-base ductile alloys can be formulated for high temperature strength applications. Such applications include casted hot side engine parts. Such parts include turbochargers, center housings, back plates, exhaust manifolds and integrated turbo-manifold components used in the automobile and truck manufacturing industries. One known chemistry for such a castable iron-based high temperature ductile alloy includes carbon in an amount of 3.0-3.4% by weight, silicon in an amount of 3.75-4.25% by weight, molybdenum in an amount of 0.5-0.7% by weight, magnesium in an amount of 0.6% by weight or less, sulfur in an amount of 0.7% by weight or less, phosphorus in an amount of 0.02% by weight or less, nickel in an amount of 0.5% by weight or less, magnesium in an amount of 0.08% by weight or less, and iron and impurities.
FIG. 3 is an SEM illustrating graphite in malleable iron. Malleable iron is characterized by having the majority of its C content occur in the microstructure as irregularly shaped nodules of graphite. This form of graphite is called temper carbon because it is formed in the solid state during heat treatment. The iron is cast as a white iron of a suitable chemical composition. After the castings are removed from the mold, they are given an extended heat treatment starting at a temperature above 1650° F. (900° C.). This causes the iron carbide to dissociate and the free carbon precipitates in the solid iron as graphite. The rapid solidification rate that is necessary to form the white iron limits the metal thickness in the casting that is practical for the malleable iron process.
A wide range of mechanical properties can be obtained in malleable iron by controlling the matrix structure around the graphite. Pearlitic and martensitic matrices are obtained both by rapid cooling through the critical temperature and with alloy additions. Malleable irons containing some combined carbon in the matrix often are referred to as pearlitic malleable, although the microstructure may be martensitic or a spheroidized pearlite.
FIG. 4 is an SEM illustrating graphite in compacted graphite iron. In compacted graphite iron, the graphite occurs as blunt flakes that are interconnected with each cell. This compacted graphite structure and resulting properties of the iron are intermediate between gray and ductile irons. The compacted graphite shape is also called quasiflake, aggregated flake, seminodular and vermicular graphite (vermiculite). As will be appreciated from FIGS. 1, 2 and 4, compacted graphite iron has a structure intermediate between those of gray flake iron and nodular ductile iron. Known methods of making compacted graphite iron include the Bruhl Oxycast route or the SinterCast method. Compact graphite iron was developed primarily to provide an iron that did not need extensive alloying but would be stronger than gray iron while being easier to machine than nodular iron. Compacted graphite iron typically has a 35% greater stiffness and a 75% higher tensile strength than gray iron and a higher fatigue strength than aluminum at automotive engine operating temperatures. Tensile strengths between 250 and 450 MPa, are commercially available (yield strengths 175 to 315 MPa). One known use for such compacted graphite iron alloys is in the production of cylinder blocks of diesel engines. Using compacted graphite iron allows for the wall thicknesses of such blocks to be half those required by flake cast iron. The compacted graphite iron block will have a weight of only about 60 kilograms, but be stronger and stiffer than a flake cast iron block.
Compacted graphite irons exhibit a graphite shape intermediate between that of stringy, interconnected flakes in gray iron and the dispersed, disconnected spheroids in ductile iron. As a result, the better properties of both gray and nodular iron are combined in compacted graphite irons. The yield strength approaches that of ductile iron while the material retains the machining properties and castability of gray irons.
The chemistry of compacted graphite iron is essentially that of nodular iron except that, in processing, the nodularizing agent, such as magnesium (or cerium), is either added in small proportions or is allowed to fade prior to casting. Alternatively titanium is added, so that the graphite formation is changed to a compacted configuration as opposed to a spheroid. See Kovacs, et al. U.S. Pat. No. 4,596,606.

SUMMARY OF SELECT EXAMPLES OF EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a product comprising an iron casting alloy comprising iron, niobium and compacted graphite structures.
Another embodiment of the invention includes a product comprising an iron casting alloy comprising iron in an amount of about 88-91% by weight, carbon in an amount of about 3.0-3.6% by weight, silicon in an amount of about 4.0-4.60% by weight, niobium in an amount of about 0.40-0.7% by weight, and wherein at least the portion of a carbon is present in the alloy as compacted graphite.
Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Select examples of embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is an SEM illustrating graphite in gray iron.

FIG. 2 is an SEM illustrating graphite in ductile iron.

FIG. 3 is an SEM illustrating graphite in malleable iron.

FIG. 4 is an SEM illustrating graphite in compacted graphite iron according to one embodiment of the invention.

FIG. 5 is a graph of creep test data for a variety of alloy including a Niobium containing alloy according to one embodiment of the invention.

DETAILED DESCRIPTION OF SELECT EXAMPLES OF EMBODIMENTS

The following description of the embodiments is merely illustrative in nature and are in no way intended to limit the invention, its application, or uses.
One embodiment of the invention includes a product including an iron casting alloy including iron, niobium and compacted graphite structures.
Another embodiment of the invention includes a product including an iron casting alloy including iron (Fe) present in an amount of about 88-91% by weight, carbon (C) present in an amount of about 3.0-3.6% by weight, silicon (Si) present in an amount of about 4.0-4.6% by weight, niobium (Nb) present in an amount of about 0.40-0.70% by weight, and wherein at least a portion of the carbon is present in the alloy as compacted graphite structures.
Another embodiment of the invention includes a product including an iron casting alloy including iron (Fe) present in an amount of about 88-91% by weight, carbon (C) present in an amount of about 3.0-3.6% by weight, silicon (Si) present in an amount of about 4.0-4.6% by weight, niobium (Nb) present in an amount of about 0.40-0.70% by weight, manganese (Mn) present in an amount 0.3% by weight or less, sulfur (S) present in an amount of about 0.02% by weight or less, phosphorus (P) present in an amount of about 0.07% by weight or less, nickel (Ni) present in an amount of about 0.6% by weight or less, titanium (Ti) present in an amount of 0.20% by weight or less, magnesium (Mg) present in an amount of about 0.05% by weight or less, and wherein at least a portion of the carbon is present in the alloy as compacted graphite structures. Optionally, the alloy may also include molybdenum (Mo) as a substitute for a portion of the niobium (Nb). However, due to cost niobium is preferred. As such, another embodiment of the invention is free of molybdenum.
In one embodiment of the invention compacted graphite iron cast alloys may be produced using graphite modifiers in the form of magnesium or cerium, the latter being made as additions in very small, regulated amounts to the melt prior to solidification. When the magnesium or cerium content of the solidified structure is about 0.025%, nodular graphite usually precipitates. Flake graphite is formed at magnesium concentrations below about 0.015% by weight. Thus, with magnesium or cerium concentrations in the range of 0.015-0.025% by weight, compacted graphite (otherwise also referred to as vermiculite)) will precipitate. The addition of titanium to magnesium or cerium treated irons makes it possible to produce compacted iron in both medium and heavy castings at higher magnesium and cerium concentrations. The presence of titanium reduces the amount of control required on the magnesium concentration and is considered beneficial in compacted graphite formation. Thus, with a magnesium addition containing titanium, compacted graphite, will form with magnesium and/or cerium concentrations in the range of 0.015-0.04% by weight. In another embodiment, the magnesium and/or cerium is present in a range of 0.015-0.035% by weight.
According to another embodiment of the invention a compacted graphite iron cast alloy may be produced by a process including melting performed in a furnace heated to a temperature of about 2800-2850° F., and then teamed into a ladle at a temperature of about 2750° F. Alloying elements are added to the treating ladle along with graphite modifiers in the form of magnesium or cerium, with the optional addition of titanium. Commercial graphite modifying agents may include (a) rare earth elements added to a desulfurized iron, or (b) magnesium and titanium added prior to post-inoculation (slightly higher base sulfur can be used). Magnesium or cerium may be used in an amount of about 0.15-0.04% by weight in a casting when titanium is used in an amount of about 0.08-15% by weight. The treated melt is then poured into one or more pouring ladles, and at each of the pouring ladles a post-inoculant in the form of ferro-silicon or ferro-silicon with aluminum and calcium is added. The melt is then poured into molds at a temperature in the range of 2500-2600° F. and the mold cooled without any special cooling treatment. Graphite modifying agents may be added in a commercially available form which typically has a composition of 52% silicon, 10% titanium, about 0.9% calcium, 5% magnesium, 0.25% cerium, with the modifier being added in an amount of about 0.5% of the total melt. The post-inoculant added to the pouring ladle comprises ferro-silicon or titanium bearing ferro-silicon added in an amount of about 0.5% by weight. Copper may be added to the melt in an amount of about 0.4-1.9% by weight to maintain the carbon in the matrix of the casting microstructure. The casting may be thereafter heat-treated by any of a variety of methods known to those skilled in the art.
In one embodiment of the invention at least 60% of graphite formation may be present conforming to graphite type III with graphite size 5-8 in accordance with ISO 945. The balance is conforming to graphite type VI with the graphite size 5-8 in accordance with ISO 945. In one embodiment, reticulate formation of vermicular graphite and chunky graphite are prohibited. In one embodiment of the invention the microstructure is free of cementit (Fe₃C). The overall matrix microstructure may include a minimum of 90% ferrite; the balance consists of pearlite and carbides.
A variety of tests are performed by those skilled in the art to provide critical design information regarding the strength of alloys, including those according to various embodiments of the invention. For example, the high temperature progressive deformation of a material at constant stress is called “creep.” in a creep test, a constant load is applied to a tensile specimen maintained at constant temperature, for example, at room temperature. Strain is then measured over a period of time. The data collected is plotted in a curve of the strain rate or creep of the material. Stress rupture testing is similar to creep testing however, the stresses used are higher than the creep test and concludes when the material fans. FIG. 5 is a graph of creep test data for a variety of alloy including data illustrate by line HSNb5.1 for a compacted graphite cast iron alloy wherein molybdenum has been replaced by niobium according to one embodiment of the invention, Although the creep data is comparable, with other alloy, alloys according to embodiment of the invention have a higher thermal conductivity relative to ductile iron, and thus thermally induced stresses are much less. The elevated temperature strength of alloys according to embodiments of the invention is less that a comparable grade ductile iron, but the thermal conductivity difference overcomes the deficit in strength.
Ductility of an alloy is used to indicate the extent to which the alloy can be deformed without fracture. One way of measuring the ductility is to determine the strain at which fracture occurs, which is usually called the “elongation,” This measurement is obtained after fracture by putting the specimen back together and taking the elongation measurement. Because an appreciable fraction of the deformation will be concentrated in a “neck” region of the tensile specimen, the value of percentage elongation will depend upon the length over which the measurement is taken. If an alloy were prepared according to one of the embodiments of the invention described above, it is believed that it would have a percent elongation of 10% or greater.
Hardness is also a measurement that characterizes an alloy. The HBW Number expresses the hardness of an alloy as a ratio of the pressure applied to a ball forced into the surface of the alloy to the surface area of the resulting indentation. If an alloy according to one of the embodiments of the invention described above were produced, it is believed that it would have a BHW Number ranging from 200-250 HBW.
One embodiment of the invention may include an iron casting alloy including iron, niobium and compacted graphite structures, and wherein the alloy has the mechanical properties set forth in Table 1.

TABLE 1

Mechanical Properties

		At room temperature	At 780° C.²⁾

Tensile strength	Rm [Mpa]¹⁾	Min. 550	Min. 70
Yield strength	R_p0,2[Mpa]¹⁾	Min. 470	Min. 60
(offset 0,2%)
Elongation	A [%]	Min. 2	Min. 35
Hardness	HBW 10/3000	200-250	n.a.
	HBW 2,5/187,5

¹⁾1Mpa = N/mm²
²⁾values only informative

The mechanical properties (see Table 1) shall be verified in accordance with EN 10002-1/ASTM E 8M with a round specimen of length L₀=50 mm (2 Inches). The hardness shall be verified in accordance with ISO 6506-1/ASTM E 10 (polished tungsten carbide ball with diameter D=10 mm and test force F=29.42 kN (3000 kgf) or polished tungsten carbide ball with diameter D=2.5 mm and test force F=1.839 kN (187.5 kgf)).
The following description of select variants is only illustrative of embodiments considered within the scope of the invention and is not in any way intended to limit such scope by what is specifically disclosed or not expressly set forth.
Embodiments may include Variant 1 which may include a product comprising: an iron casting alloy comprising iron, niobium, and compacted graphite structures.
Embodiments may include Variant 2 which may include a product as set forth in Variant 1 wherein the niobium is present in an amount ranging from 0.4-0.7% by weight of the alloy.
Embodiments may include Variant 3 which may include a product as set forth in one or more of Variants 1-2 wherein the alloy further comprises molybdenum.
Embodiments may include Variant 4 which may include a product as set forth in one or more of Variants 1-3 wherein the iron is present in an amount of about 88-91% by weight of the alloy.
Embodiments may include Variant 5 which may include a product as set forth in one or more of Variants 1-4 wherein the compacted graphite structures comprise carbon present in an amount of 3.0-3.6% by weight.
Embodiments may include Variant 6 which may include a product as set forth in one or more of Variants 1-5 further comprising silicon present in an amount of 4.0-4.6% by weight of the alloy.
Embodiments may include Variant 7 which may include a product as set forth in one or more of Variants 1-6 further comprising manganese present in an amount up to 0.3% by weight.
Embodiments may include Variant 8 which may include a product as set forth in one or more of Variants 1-7 further comprising sulfur present in an amount up to 0.02% by weight of the alloy.
Embodiments may include Variant 9 which may include a product as set forth in one or more of Variants 1-8 further comprising phosphorus present in an amount up to 0.07% by weight of the alloy.
Embodiments may include Variant 10 which may include a product as set forth in one or more of Variants 1-9 further comprising nickel present in an amount up to 0.6% by weight of the alloy.
Embodiments may include Variant 11 which may include a product as set forth in one or more of Variants 1-10 further comprising titanium present in an amount up to 0.2% by weight of the alloy.
Embodiments may include Variant 12 which may include a product as set forth in one or more of Variants 1-11 further comprising a graphite modifier present in an amount ranging from 0.015-0.04 weight percent of the alloy, the graphite modifier comprising at least one of magnesium or cerium.
Embodiments may include Variant 13 which may include a product as set forth in one or more of Variants 1-12 further comprising titanium.
Embodiments may include Variant 14 which may include a product as set forth in one or more of Variants 1-13 wherein the graphite modifier is present in an amount ranging from 0.015-0.025 weight percent of the alloy.
Embodiments may include Variant 15 which may include a product as set forth in one or more of Variants 1-14 wherein the alloy has a microstructure that is free of Fe₃C.
Embodiments may include Variant 16 which may include a product as set forth in one or more of Variants 1-15 wherein at least 60 percent of the graphite structures conform to graphite type III with graphite size 5-8 according to the standard ISO 945.
Embodiments may include Variant 17 which may include a product as set forth in one or more of Variants 1-16 wherein balance of the graphite structures conform to graphite type IV with graphite size 5-8 according to the standard ISO 945.
Embodiments may include Variant 18 which may include a product comprising an iron casting alloy comprising iron present in an amount of about 88-91% by weight, carbon present in an amount of about 3.0-3.6% by weight, silicon present in an amount of about 4.0-4.6% by weight, niobium present in an amount of about 0.4-0.7% by weight, manganese in an amount of 0.05% or less by weight, sulfur in an amount of about 0.02% by weight or less, phosphorus in an amount of about 0.07% by weight or less, nickel in an amount of about 0.6% by weight or less, wherein the carbon is present in a form comprising compacted graphite structures.
Embodiments may include Variant 19 which may include a product as set forth in Variant 18 further comprising titanium in an amount ranging from about 0.08-0.2% by weight of the alloy.
Embodiments may include Variant 20 which may include a product as set forth in one or more of Variants 18-19 wherein at least 60 percent of the graphite structures conform to graphite type III with graphite size 5-8 according to the standard ISO 945.
Embodiments may include Variant 21 which may include a product as set forth in one or more of Variants 18-20 wherein balance of the graphite structures conform to graphite type IV with graphite size 5-8 according to the standard ISO 945.
Embodiment may include Variant 22 which may include a product as set forth in one or more of Variants 18-21 wherein the alloy has a microstructure free of Fe₃C.
Embodiments may include Variant 23 which may include a product as set forth in one or more of Variants 18-22 further comprising molybdenum.
Embodiments may include Variant 24 which may include a method of making compacted graphite iron, comprising: forming a ferrous alloy melt comprising iron present in an amount of about 88-91% by weight, carbon present in an amount of about 3.0-3.6% by weight, silicon present in an amount of about 4.0-4.6% by weight, niobium present in an amount of about 0.4-0.7% by weight, and causing the melt to form compacted graphite particles upon solidification; solidifying said melt to form a compacted graphite iron casting.
Embodiments may include Variant 25 which may include a method as set forth in Variant 24 wherein the forming comprises heating the elements to a temperature of 2800-2850° F. prior to the solidifying.
Embodiments may include Variant 26 which may include a method as set forth in one or more of Variants 24-15 wherein the melt further comprises a graphite modifying agent is present in an amount ranging from 0.015-0.035% by weight, and wherein the graphite modifying agent comprises at least one of magnesium and cerium.
Embodiments may include Variant 27 which may include a method as set forth in one or more of Variants 24-26 wherein the melt is free of molybdenum.
Embodiments may include Variant 28 which may include a product as set forth in one or more of Variants 14-27 wherein the casting is free of Fe₃C.
Embodiments may include Variant 29 which may include a product as set forth in one or more of Variants 24-28 wherein at least 60 percent of the graphite structures conform to graphite type III with graphite size 5-8 according to the standard ISO 945.
Embodiments may include Variant 30 which may include a product as set forth in one or more of Variants 24-29 wherein balance of the graphite structures conform to graphite type IV with graphite size 5-8 according to the standard ISO 945.
The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.

Claims

1. A product comprising:

an iron casting alloy comprising iron, niobium, and compacted graphite structures.

2. A product as set forth in claim 1 wherein the niobium is present in an amount ranging from 0.4-0.7% by weight of the alloy.

3. A product as set forth in claim 2 wherein the alloy further comprises molybdenum.

4. A product as set forth in claim 1 wherein the iron is present in an amount of about 88-91% by weight of the alloy.

5. A product as set forth in claim 1 wherein the compacted graphite structures comprise carbon present in an amount of 3.0-3.6% by weight.

6. A product as set forth in claim 5 further comprising silicon present in an amount of 4.0-4.6% by weight of the alloy.

7. A product as set forth in claim 5 further comprising manganese present in an amount up to 0.3% by weight.

8. A product as set forth in claim 5 further comprising sulfur present in an amount up to 0.02% by weight of the alloy.

9. A product as set forth in claim 5 further comprising phosphorus present in an amount up to 0.07% by weight of the alloy.

10. A product as set forth in claim 5 further comprising nickel present in an amount up to 0.6% by weight of the alloy.

11. A product as set forth in claim 5 further comprising titanium present in an amount up to 0.2% by weight of the alloy.

12. A product as set forth in claim 1 further comprising a graphite modifier present in an amount ranging from 0.015-0.04 weight percent of the alloy, the graphite modifier comprising at least one of magnesium or cerium.

13. A product as set forth in claim 12 further comprising titanium.

14. A product comprising an iron casting alloy comprising iron present in an amount of about 88-91% by weight, carbon present in an amount of about 3.0-3.6% by weight, silicon present in an amount of about 4.0-4.6% by weight, niobium present in an amount of about 0.4-0.7% by weight, manganese in an amount of 0.05% or less by weight, sulfur in an amount of about 0.02% by weight or less, phosphorus in an amount of about 0.07% by weight or less, nickel in an amount of about 0.6% by weight or less, wherein the carbon is present in a form comprising compacted graphite structures.

15. A method of making compacted graphite iron, comprising:

forming a ferrous alloy melt comprising iron present in an amount of about 88-91% by weight, carbon present in an amount of about 3.0-3.6% by weight, silicon present in an amount of about 4.0-4.6% by weight, niobium present in an amount of about 0.4-0.7% by weight, and causing the melt to form compacted graphite particles upon solidification; solidifying said melt to form a compacted graphite iron casting.