FERRITIC HIGH-SILICON CAST IRONS

FIELD OF THE INVENTION

[0001] The present invention relates to ferritic high-silicon cast irons that exhibit improved microstructure stability, hot oxidation resistance, ductility at both room temperature (RT) and middle temperatures, and elevated-temperature strength, thereby an improved thermal durability. More specifically, the present invention provides a new understanding of the interrelationship among the cast iron chemistry, graphite morphologies, matrix structure, and material properties, which leads to the following disclosed high-silicon cast iron formulae and processes for their production: (1 ) a broad range of graphite nodularity in microstructure, dependent on the casting geometry and service conditions, (2) no greater than 3.00% by mass of the total contents of Groups 5 and 6 transition metals for the desirable balance between the strength and ductility, and (3) controlling the minor elements of magnesium, rare earth metals, and phosphorus for the high elongation at both RT and middle temperatures.

BACKGROUND OF THE INVENTION

[0002] It is well known that graphite cast irons with 4 to 6% Si provide good service and low cost in many elevated-temperature applications. The critical temperature Ad of the onset of phase transformation from ferrite to austenite upon heating at a defined heating rate and hot oxidation resistance is increased by increasing the Si content. However, Si has little effect in improving high-temperature strength and even decreases the strength slightly when the tensile testing is carried out at temperature above approximately 700 ⁰ C.

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[0003] Transition metals of Groups 5 and 6 (V, Cr, Nb, Mo, Ta, and W) with body-centered cubic structure in the Periodic Table are added into high-Si cast irons to improve high-temperature strength to a certain extent. For example, SiMo irons are currently used to produce exhaust manifolds, turbocharger housings, turbo housing-integrated exhaust manifolds, turbo outlet pipes, and catalytic converter housings for automobiles. Furthermore, numerous attempts have been made in the published documents to improve heat-resistant ferritic cast irons, basically in the three directions: (1 ) enhancing Ad and hot oxidation resistance, (2) adding carbide formers mainly from Groups 5 and 6 to increase the hot tensile strength, and (3) alleviating the middle temperature brittleness (MTB), especially on spheroidal graphite (SG) cast irons. The previous art related to the three aspects is reviewed below.

[0004] US Patent 5236660, US Patent 6508981 B1 , and US Patent

7156929, each of which is hereby incorporated by references, deal with different amounts of aluminum (Al) additions into ferritic high-Si cast irons. There is no doubt that Ad temperature and hot oxidation resistance of Al-alloyed cast iron is significantly improved over no-AI added compositions. However, similar to Si, Al was found to have virtually no effect in improving high-temperature strength. Furthermore, Al-alloyed iron can cause casting defects and foundry process issues. It is difficult to make a large-scale production of the Al-alloyed cast irons, even though they have attractive resistance to hot oxidation and thermal growth, as commented by the Iron Castings Engineering Handbook.

[0005] Other published documents relating to ferritic cast iron strengthening by adding a variety of alloying elements are known. For example, US or WO Patents or Applications (20030024608, 6939414, 2004/0091383 A1 ,

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20040223866, WO2005085488, WO 2006/121826 A2, 20060266447, and WO2007/040464 A1), Japan Patents (2002-339033 and 7118790), and European Patent (EP 0829551 A3) all relate generally to alloying ferritic cast irons. As commonly reported in these documents, the elevated-temperature tensile strength is increased to a certain extent by increasing the contents of certain alloying elements. For example, austenite-stabilizing elements such as nickel and manganese have been used to strengthen ferrite-base high-Si irons, but at the expense of Ad , stability of the ferrite matrix, and hot oxidation resistance. As should be understood by those skilled in the art, the strengthening effects on ferritic cast irons will be leveled off when the total contents of the alloying elements approach a critical value. Above this critical value, the material ductility will be significantly reduced because of an abrupt increase in the percentage of pearlite and other carbide phases in microstructures.

[0006] Middle temperature brittleness (MTB) has long been known as a deleterious phenomenon in ferritic ductile irons for elevated temperature applications. MTB - also referred to as ETB or ITB (elevated or intermediate temperature brittleness) - is a concern for exhaust components that are thermally cycled through the middle temperature range. It has been found that the MTB of SG cast irons can occur in a temperature range, approximately from 300 ⁰ C to 500 ⁰ C, mainly dependent on the chemistry and the testing strain rate. The temperature 425° C is used to represent the middle temperature for most testing.

[0007] There are two opposite approaches of improving the middle temperature ductility, thereby reducing the middle temperature brittleness: (1 ) reducing residual levels of magnesium (Mg) and sulfur (S), for example, (Mg + 4.5S) is no greater than 0.070%, as expressed in the European Patent Application (EP

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0241812 A2); and (2) adding a small amount of phosphorus (P) or arsenic (As), as described in Japan Patent (07-018367) and Japan Patent (10-195587), each of these documents being incorporated by references herein. The Mg/P ratio between 0.5 and 1.5 or an arsenic content between 0.03 and 0.20% in cast iron was proposed in the second approach. On the downside, arsenic is toxic, and it has proven difficult to control the Mg/P ratio or the values of (Mg + 4.5S) in cast iron production.

[0008] Apart from the iron compositions, the graphite morphologies exert a significant impact on cast iron properties. Prior art focused on spheroidal graphite (SG) iron with nodularity typically higher than 80% or 90%, and compacted graphite (CG) iron with nodularity typically lower than 30% or 50%. There does not appear to be any prior art on high-Si cast irons with a mixed graphite (MG) structure defining an intermediate nodularity, from 30 to 90% (preferably 40 to 80%). MG is also referred to hybrid graphite or duplex graphite structure. This type of structure is believed to exhibit certain unique properties making it a viable candidate for high- temperature use including exhaust system components. To a certain extent, there is a crossover in microstructure between CG and MG, and between MG and SG cast irons, respectively.

[0009] When cast irons undergo thermal and thermomechanical fatigue conditions, microstructural and mechanical degradations can occur in one or more than one forms of: (1 ) coarsening of precipitates, (2) grain-boundary damage, (3) phase transformation, (4) accumulated strains, (5) enhanced mismatch between graphite and iron matrix, and (6) ductility exhaustion. Therefore, when designing a ferritic high-Si cast iron component for isothermal or cyclic thermal applications, the following important material properties must be taken into consideration: (1) high Ad temperature for the ferrite-based structure stability, (2) hot oxidation resistance, (3)

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low thermal growth rate, (4) elevated-temperature strength, and (5) ductility from room temperature (RT) to middle temperatures.

[0010] There is a continuing need to provide ferritic high-Si iron alloys having desirable compositions and microstructures that yield the best balance among the ductility at RT and middle temperatures, elevated-temperature strength, Ad temperature, and oxidation and deformation resistance in order to improve thermal or thermomechanical fatigue durability of such ferritic high-Si iron alloys.

SUMMARY OF THE INVENTION

[0011] The present invention includes a microstructure of ferritic high-Si iron with mixed graphite (MG), namely the graphite nodularity between 30 to 90% (preferably 40 to 80%), or spheroidal graphite (SG), namely the graphite nodularity higher than 90%. The compositions of the two major elements are 2.80 to 3.80% C and 3.80 to 4.80% Si, which are connected by the carbon equivalent CE = C + Si/3. The CE range is from 4.35 to 4.95 for superior castability and uniform microstructures. This is a base composition for alloying or doping of other elements as described below.

[0012] To achieve the mixed graphite microstructure, lower residual contents of magnesium and REM (rare earth metals) are used than those required to achieve a spheroidal graphite structure; one result is that decent ductility at both RT and middle temperatures is achieved. EES (engine exhaust simulator) testing shows improved thermal durability of cast exhaust manifolds with the mixed graphite (MG) structure. The production cost of MG iron is generally lower than both CG and SG cast irons, which is advantageous.

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[0013] To produce spheroidal graphite (SG) iron to achieve high elongation at room temperature, higher residual contents of Mg, REM, or combined Mg and REM at a certain ratio are required, which however may bring about middle temperature brittleness (MTB). Therefore, a small amount of P is introduced into SG cast iron in order to suppress the residual magnesium side effects and avoid MTB. In the prior art, the Mg/P ratio and the sum of Mg and 4.5S (Mg + 4.5xS) were employed to prevent the MTB. The empirical formulae were fitted from the prior art, but they may not hold valid for other cases. For example, assuming Mg/P = 1.0, the cast iron of 0.04% Mg and 0.04% P will significantly differ from that of 0.010% Mg and 0.01 % P in the mechanical properties. Similarly, for the second formula, (Mg + 4.5xS), it is not always case that the lower S in the iron composition, the better properties of cast irons. A minute amount of sulfur in iron melts acts as the potent nuclei of graphite.

[0014] In the present invention, we unexpectedly found that by controlling the Mg, S, and P contents separately dramatic reductions in middle temperature brittleness were achieved. When the Mg contents are below a certain level with the mixed graphite shapes, no phosphorus doping is required. When the spheroidal graphite cast iron is made with higher Mg and REM contents, a small amount of phosphorus may be added to prevent the MTB. P-bearing spheroidizer agents and P-bearing inoculants are developed to realize accurate P-doping levels. There are various types of spheroidizers and inoculants. But no agents with phosphorus additions have been commercially available. Practical P-doped levels were determined to achieve elongation of higher than 10.0% at both RT and middle temperatures. The thermal fatigue testing results also show the cycles to failure were decidedly increased for the P-doped SG cast iron samples.

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[0015] One of alloying elements Cr increases Ad temperature, hot oxidation resistance, elevated-temperature strength, with low material cost. A series of Cr-containing formulae has been identified through the new classification diagram of ferritic high-Si irons (Figs. 1 and 2), including the above-described baseline high- Si compositions alloyed with 0.40 to 1.00% Cr. The preferable range 0.4 to 2.0% is used for the multiple element additions. However, Cr additions significantly increase the amount of lamellar pearlite and carbides in the microstructures. Therefore, annealing heat treatment is often used to reduce the amount of pearlite and carbides and break up them into dispersed structures. It was found in the present invention that the P-doped Cr-containing cast irons exhibit high elongation at the middle temperature even under as-cast conditions. Accordingly, heat treatment may or may not be utilized for the Cr-alloyed high-Si cast irons.

[0016] In various embodiments, other transition metals in Groups 5 and

6 are used to strengthen ferritic high-Si irons to a certain extent, including Nb, Mo, Ta, V, and W. The selection principles put forward here include (1 ) controlling the total content of the six metals to no greater than 3.00% by mass (balancing up to 100% of iron and other elements) and (2) controlling the resultant pearlite plus other carbide phases to no greater than 35.0%. In terms of the chemistry-microstructure relationship, the sum of pearlite and carbide phases is directly correlated to the total contents of the carbide former elements such as the six elements mentioned-above. The preferred total amount of the six transition metals is 0.40 to 2.00% and the preferred sum of pearlite and other carbide phases is 4.0 to 25.0%.

[0017] By combining the microstructure classifications with alloying classifications illustrated in Figs. 1 and 2, hundreds of new cast iron formulae come

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out, such as MG SiMo, MG SiMoCr, SG SiMo P-doped, and so on. This is a new way of defining cast irons, by combining the microstructure with the chemistry.

[0018] In various embodiments, elements such as Ni, Mn, Cu, Co, and

Al are not recommended in the present invention for exhaust components made of ferritic high-Si cast irons, and should only be present as inevitable impurities. Ni, Mn, Cu, Co tend to reduce Ad and oxidation resistance for ferritic cast irons, and Al may cause casting defects and foundry process issues, and thus intentional inclusion of any of these should be limited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 is a diagram showing new classifications of ferritic high-Si cast irons. The graphite shapes are divided into CG, MG, and SG in terms of the nodularity. However, there should be no distinct demarcation between CG and MG, and between MG and SG structures in this classification. There will be hundreds of new cast iron formulae when combining graphite shape classifications with alloying classifications, such as MG SiMoCr, SG SiCr P-doped, MG SiCrW, and so on. The way of defining cast irons is novel, by combining the microstructure with the chemistry. Only novel formulae are listed above.

[0020] Fig. 2 is a tabulated graph illustrating possible chemistry

combinations C? = ^' — of six transition metals with high-Si iron where n = 6

^ ^r r!(n - r)! stands for the six transition metals of Groups 5 and 6 in the Periodic Table , and r (from 0 to 6) for the number of the six metals taken at a time. There are 64 combinations in total.

[0021] Fig. 3 is the carbon equivalent (CE) parallelogram diagram showing the ranges of 3.80 to 4.80% Si, 4.35 to 4.95 CE, and the dependent variable

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carbon to achieve uniform solidification mode and microstructures. The inside smaller parallelogram is the preferred compositional zone of which the CE ranges from 4.55 to 4.75 and Si from 4.10 to 4.50% for high-Si castings with the critical thickness of up to 25 mm. The target CE is altered with the casting critical thickness. Generally speaking, the target CE should be decreased with increasing the thickness of castings, because of a decrease in the solidification rates.

[0022] Fig. 4 is a graph showing the effects of cerium (Ce) additions on graphite nodularity and on tensile elongation at room temperature and 425° C. The baseline compositions are: 4.40% Si, 3.20% C, 0.50% Mo, and 0.018% residual magnesium. Similar to magnesium, cerium additions increased the nodularity and thus elongation at RT, but still showed the MTB trend.

[0023] Fig. 5 illustrates the effects of phosphorus on the tensile testing results at RT and 425° C for SG cast iron with 3.20% C, 4.40% Si, and 0.50% Mo: (a) elongation and (b) 0.20% offset yield strength. The elongation at 425° C was significantly increased for the P-doped SG SiMo samples.

[0024] Fig. 6 illustrates the effects of phosphorus (P) on the tensile testing elongation at RT and 425° C for SG SiCr iron (3.20% C, 4.40% Si, and 0.75% Cr, and Mo< 0.10%). The samples underwent annealing heat treatment prior to tensile testing. The elongation at 425° C was significantly increased for P-doped SG SiCr samples.

[0025] Fig. 7 is the SEM (Scanning Electron Microscopy) micrographs showing the fracture surface of tensile samples tested at 425° C for SG SiCr iron (4.40% Si and 0.75% Cr): (a) A mixed-mode fracture with some intergranular fracture was observed for regular samples, and (b) no intergranular fracture was observed for P-doped samples.

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[0026] Fig. 8 shows the hot oxidation curves: the weight change rate as a function of the exposure time at the temperature 827° C for four compositions: 4.0%Si SiMo stands for 4.00% Si, 3.40% C, and 0.60% Mo; 4.4%Si SiMo for 4.40% Si, 3.20% C, and 0.45% Mo; 4.4%Si-0.85Cr for 4.40% Si, 0.85% Cr, 3.20% C, and 0.45% Mo; and D5S denotes a grade of Ni-resist cast iron with 35.0% Ni.

[0027] Fig. 9 is a graph showing the percentage of pearlite plus Mo-rich phases measured by using a light microscopy image analysis method as a function of Mo contents for SiMo iron samples.

[0028] Fig. 10 shows the 0.20% offset yield strength from tensile testing at 700° C as a function of Cr contents. HT stands for an annealing heat treatment, and As-cast for as-cast condition, prior to tensile testing. As expected, annealing heat treatment reduced the strength of cast irons.

[0029] Fig. 1 1 is a graph showing tensile elongation at RT as a function of Cr additions. HT stands for an annealing heat treatment, and As-cast for as-cast condition, prior to tensile testing. As expected, annealing heat treatment increased the elongation at RT for Cr-alloyed cast irons.

[0030] Table 1 summarizes the process, main chemistry, microstructures, and tensile elongation at 425 ⁰ C for the three examples. HT ^* stands for the heat treatment, A ¹ for annealing at 800° to 960° C and then furnace cooling, and C ² for the comparison between heat treatment and as-cast conditions. FeP ³ stands for a mixture of 0.4% Fe-Si (75% Si grade) inoculant out of the total melt charges and 0.10% FeP (25% P) out of the total melt charges, which is used as post inoculation.

[0031] Table 2 summarizes high temperature oxidation characteristics of ferritic high-Si irons with different graphite shapes (CG, MG and SG).

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[0032] Table 3 lists tensile testing results at RT and 425° C of high-Si iron with different graphite nodularity (There were no titanium additions for the samples). The main compositions are 3.20% C, 4.40% Si, and 0.45% Mo.

[0033] Table 4 compares average CTE (coefficient of thermal expansion) in μm m ^'1 C ^'1 , and Ad temperature of P-doped and baseline SiMo with 4.32% Si: 0.042% P for samples #1 and #2, and 0.016% P for samples #3 and #4.

[0034] Table 5 contains tensile testing results of SG SiCr irons (3.20%

C-4.40% Si-0.80% Cr) at RT and 425° C for the comparison of as-cast versus annealed conditions, and baseline P content versus P-doped samples. Annealing heat treatment did improve the elongation at RT. However, no beneficial effects of heat treatment were observed on the middle temperature ductility. Again, the P- doped samples showed significantly higher elongation at 425° C for both annealed and as-cast conditions.

[0035] Table 6 illustrates the tensile elongation measured at multiple temperatures in the middle temperature range for SG SiCr iron with 3.20% C, 4.40% Si, 0.75% Cr, 0.030% Mg, and 0.030% P, indicating the elimination of MTB across the middle temperature range instead of a certain temperature for the P-doped samples. When the testing temperature exceeds 500° C, the elongation of higher than 10% is typically observed regardless of the sample conditions tested in the invention.

[0036] Table 7 lists the cycles to failure of SG ferritic high-Si irons with different compositions, measured by using high-frequency induction heating thermal fatigue testing facilities. The test specimen has a cylindrical geometry with approximately 100 mm long, 12 mm outer diameter, and 5 mm inner diameter. The cycling temperatures were from 150° to 820° C. The sample was heated to 820° C

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in 3 minutes, held for 1 minute, and naturally cooled to 150° C in 5 minutes. It is seen from Table 7 that (1 ) the thermal cycles to failure were increased by using high- Si low-Mo, as comparing samples 02 to 01 , and (2) the thermal fatigue life of P- doped samples were superior to the baseline samples, as comparing samples 03 to 02, and 05 to 04.

[0037] Table 8 summarizes the cycles to failure of EES (engine exhaust simulator) testing by using different exhaust manifold designs and thermal cycling profiles. EGT stands for the exhaust gas temperature and PMT for the peak metal temperature. The testing temperature profiles include 3 to 10 minutes for heating-holding segments and 4 to 10 minutes for cooling segments, dependent on the product programs. Types 1 , 2, and 3 manifolds were made of SiMo irons while type 4 parts contained 0.40% to 1.00% Cr. Type 1 manifold geometry was for turbo charger applications, and types 2, 3, and 4 parts were for natural aspirations. It is seen that (1 ) the cycles to failure of SiMoCr parts were higher than those of SiMo in terms of alloying element effects, and (2) the cycles to failure of the MG parts were higher than those of SG parts in terms of graphite shape effects.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The present invention relates to ferritic high-Si cast irons and processes for their production. The iron alloys in accordance with the present invention are particularly suitable for high-temperature use such as automotive exhaust system parts, which will be explained in detail below.

[0039] In one embodiment, a high silicon cast iron composition is provided that exhibits advantageous middle temperature brittleness properties. The composition, which in various embodiments also takes the form of cast articles made

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from cooled melts of the iron composition, comprises a carbon equivalent of 4.35 to 4.95, wherein the carbon equivalent is the sum of carbon plus 1/3 silicon; transition metal or metals selected from the group consisting of V, Nb, Ta, Cr, Mo, W, and mixtures thereof at a level of 3% by weight or less; essentially no aluminum; essentially no Mn, Ni, Co, or Cu; magnesium and rare earth metals at levels sufficient to provide the cast iron microstructures upon cooling with 30 to 90% nodularity; and iron. The cast iron compositions are characterized by a microstructure with a nodularity of 30 to 90%. In particular embodiments, the compositions comprise up to 1.0% or up to 0.8% by weight Mo, up to 0.8% Cr, or up to 1 % Cr. In various embodiments, the nodularity is 30 to 80%, 40 to 80%, 30 to 50%, or 30 to 70%. In some embodiments, the transition metal or metals are present at a level of 0.4 to 2% by weight and/or the carbon equivalent is from 4.5 to 4.75.

[0040] In another embodiment, a high silicon cast iron composition has a nodularity of 90% or higher and comprises a carbon equivalent of from 4.35 to 4.95; transition metal or metals selected from the group consisting of V, Nb, Ta, Cr, Mo, W, and mixtures thereof at a level of 3% or less by weight; essentially no aluminum; essentially no Mn, Ni, Co, or Cu; 0.012 to 0.050% phosphorous; iron; and magnesium and rare earth metals at sufficient levels to provide a nodularity in the composition upon cooling of greater than 90%. In particular embodiments, the iron composition comprises up to 0.8% molybdenum, up to 1 % molybdenum, up to 0.8% chromium, or up to 1 % chromium. In other embodiments, the composition is characterized by a carbon equivalent is 4.5 to 4.75, and/or comprises 0.4% to 2% by weight of the transition metals.

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[0041] In other embodiments, a high-Si cast iron article is characterized by ferrite above 70%, a graphite nodularity of 35% to 85%, and a carbon equivalent of 4.35 to 4.95, preferably 4.55 to 4.75, while the elongation measured at 425 ⁰ C is 4% or higher. In particular embodiments, the elongation is from 5% to 8%. The iron can comprise up to 3% by weight of transition metals selected from V, Nb, Ta, Cr, Mo, W, and combinations thereof, and preferably 0.4% to 2%.

[0042] In other embodiments, a high-silicon cast iron article is characterized by ferrite above 70%, a graphite nodularity of 90% or greater, and an elongation measured at 425 ⁰ C of 10% or greater, and in some embodiments of 12%- 20%. In preferred embodiments the iron has a phosphorus content of 0.012% to 0.05%, and a carbon equivalent of 4.55 to 4.75. In various embodiments, the iron composition of the article comprises up to 3% by weight, and preferably from 0.4% to 2% by weight of transition metals selected from V, Nb, Ta, Cr, Mo, W, and combinations thereof.

[0043] In one aspect, cast iron compositions and cast articles made from them are provided that have a unique set of physical and material properties. Some embodiments exhibit combinations of nodularity, ferritic structure, and ductility at middle temperatures (as illustrated for example by elongation at 425 ⁰ C).

[0044] In other aspects, the compositions and article have a unique chemical structure as well as material and physical properties. Thus in one embodiment, a ferritic cast iron composition consists essentially of Fe, C, Si, Mg, rare earth metal, and Group 5 or 6 transition metals, wherein the carbon equivalent is 4.35 to 4.95 or 4.55 to 4.75 and the elongation measured at 425 ⁰ C is more than 4%. The composition is typically marked by a nodularity in the "mixed graphite" range from 35% to 85%, and the elongation is 5-8%. In particular embodiments, the

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compositions and articles comprise 0.4-2% by weight of any combination of V, Nb, Ta, Cr, Mo, and W.

[0045] In a further embodiment, a ferritic cast iron composition consists essentially of Fe, P, C, Si, Mg, rare earth metal, and Group 5 or 6 transition metals, the carbon equivalent is 4.35 to 4.95 or 4.55 to 4.75 and the elongation measured at 425 ⁰ C is more than 10%. In this P-doped embodiment, the composition is typically marked by a nodularity in the "spheroidal graphite" range of 90% or more, and the elongation is typically in the range of 12-20% at the middle temperature value of 425 ⁰ C.

[0046] Fig. 1 shows a classification scheme of ferritic high-Si cast irons.

The effects of elements and selective rules for their application are given below. Fig. 2 shows the possible 64 types of combinations by alloying with Groups 5 and 6 elements of body-centered cubic structure. The main elements of cast irons are C, Si and Fe, whose individual effects are well understood. The Si contents are selected according to Ad , oxidation resistance, and ductility requirements. The carbon equivalent, CE relates to C and Si. There will be a higher tendency of shrinkage and forming other casting defects when the CE is either too low or too high. The carbon contents are selected according to the target CE and desired microstructure including graphite shape, dimension, volume percentage and the required distance between graphite inclusions. Thus, the composition parallelogram consisting of 4.35 to 4.95 CE and 3.80 to 4.80% Si was developed to optimize the solidification path and the resultant microstructures, as shown in Fig. 3. The inside parallelogram zone shows the preferred values of CE from 4.55 to 4.75, Si from 4.10 to 4.50%, and the dependent C content, for castings with the critical thickness of up to 25 mm. Moreover, the target CE is considered as a function of the critical

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thickness of castings. Generally speaking, the target CE should be decreased with increasing the thickness of castings, because of a decrease in cooling rates.

[0047] As used herein, elongation is the percent elongation at break and is measured by standard tensile testing such as ASTM E8 or ASTM E8M. A test specimen is subjected to tension until it fractures from the applied force. Elongation is determined by removing the fractured specimen from the grips, fitting the broken ends together, and measuring the distance between gage marks as L _z . The percent elongation at break is calculated by:

Elongation at break (%) = 100 ^* (L ₂ -L _o )/L _o , where L ₀ is the original gage length

[0048] In various embodiments, the following selection rules are to be followed for the respective elements of the cast iron composition: C and Si

[0049] The elements C and Si are related by carbon equivalent CE =

C+1/3 Si to influence castability and structures. Si increases Ad (50 to 60° C per 1.0%) and heat resistance. The ductility tends to decrease as Si is increased above about 4.8%. In embodiments, the carbon equivalent is from 4.35 to 4.95. This is shown graphically in the parallelogram map of Fig. 3. The carbon equivalent is controlled so as to achieve uniform solidification mode and microstructure of castings. The target CE is altered with the critical thickness of castings. Cr. Mo. Nb. Ta. V. and W

[0050] These transition metals of Groups 5 and 6 with body-centered cubic structure are carbide formers and ferrite promoters, and increase hot strength to a certain extent. Cr increases Ad (30 to 40 C per 1.0% Cr) and heat resistance

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significantly. Fig. 2 shows a possible 64 combinations of the six elements. In preferred embodiments, the total content of the six is held to no greater than 3.0%, and illustratively from about 0.4% to about 2%. The pearlite and carbide of the resulting compositions are preferably less than about 35%. Mq and rare earth metals (REM)

[0051] Mg and REM (rare earth metals) are graphite nodulizers. As inclusion and segregation formers, Mg and REM may contribute to MTB and intergranular fracture. The total residual of Mg and REM is to be varied to achieve a desired nodularity, either mixed graphite or spheroidal microstructures as described herein. In various embodiments, total residual Mg and REM is less than 0.08% Phosphorus

[0052] In general, phosphorus has been considered a detrimental element in ferritic ductile irons. But it has been found that doping a small amount of P may provide beneficial effects to alleviate the MTB of spheroidal graphite microstructures described herein. In various embodiments, the level of phosphorus is controlled from about 0.012% to about 0.050% to balance the ductility at RT and MTB. P-bearing agents and process are developed for P-doping levels.

Mn. Ni. Co. and Cu

[0053] As austenite stabilizers, they reduce the ferrite stability, Ad and oxidation resistance of ferritic high-Si cast irons. These elements are to be avoided in the ferritic microstructures described herein, except as trace levels or as inevitable impurities.

Aluminum

[0054] Aluminum is to be avoided except for inevitable impurities.

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Microstructures

[0055] In addition to the composition of the alloying elements, the nodular structure of the irons described herein contributes to their desirable properties. In various embodiments, cast irons having a graphite nodularity between 30 and 90% (preferably 40 to 80%) are provided, which can be referred to as a mixed graphite (MG) structure. The microstructure is controlled by the process used for making the cast irons. The microstructure responds most strongly to the amount of magnesium (Mg) and rare earth metal used as spheroidizers adding before the final pour. The level of Mg and REM can be varied along with other process parameters (time of addition, temperature of addition, rate of addition, and so on, all of which tend to vary from furnace to furnace and foundry to foundry) to achieve a desired nodularity. A number of benefits of the MG cast irons have been identified in the present invention: (1 ) better castability by reducing the amounts of spheroidizer and inoculant, elimination of anticompactizing elements such as Ti and Al, (2) modified oxidation resistance characteristics, as seen in Table 2, (3) improvements in MTB, as shown for example in Table 3, and (4) lower manufacturing cost. Moreover, as compared to the SG irons, the MG irons may exhibit a lower CTE (coefficient of thermal expansion) and Young's modulus, thus generating lower thermal stress during thermal cycling.

[0056] It has been reported that the MTB is due to the magnesium- assisted sulfur segregation at the grain boundaries. Therefore it could have been posited that reducing magnesium would reduce segregation and thus might improve the MTB. But too low a magnesium level will cause the graphite nodularity to be too low and even result in flake graphite, thus leading to poor ductility at room and middle temperatures, and a reduced strength as well. In one aspect, the mixed

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graphite iron has been designed to use the combined effects of a certain level of residual magnesium and moderate graphite nodularity to achieve adequate ductility at both RT and middle temperatures, as shown in Table 3. A new understanding of the interrelationship among the residual magnesium contents, nodularity, and MTB has been provided.

[0057] As mentioned above, ductility at middle temperatures is controlled by two factors; nodularity and magnesium contents. Moreover the nodularity and the magnesium content are related. More specifically, a certain level of nodularity is achieved by adding a certain amount of magnesium. However, too much magnesium causes segregations, thus giving rise to the MTB.

[0058] Spheroidal graphite tends to be formed by increasing magnesium or/and cerium content. This results in alloys with high elongation and strength at room temperature, but in the middle temperature brittleness range of approximately 300° C to 500° C, a low tensile elongation is often observed. In various embodiments, a small amount of P is added to counter this tendency. In the present invention, two types of SG iron compositions are chosen to reveal the P- doping effects as examples: (1 ) high-Si SG SiMo and (2) SG SiCr. Fig. 5 illustrates the P effects on the tensile testing results of high-Si SiMo iron at RT and 425° C. The elongation at RT was not changed significantly when the P content was increased from 0.016% to 0.048%. However, the ductility at 425° C was substantially enhanced when doping with 0.025% to 0.048% P. The yield strength at room temperature and at 425° C was also found to slightly increase with increasing P contents.

[0059] The effect of P on SG SiCr iron (3.20% C, 4.40% Si, and 0.75%

Cr) was akin to high-Si SiMo iron in terms of MTB. Phosphorous has the same

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effects on the MTB for all magnesium/REM-treated SG cast irons. As shown in Fig. 6, the elongation at 425° C abruptly increased when the P content exceeded 0.025%. But there was a different impact on the elongation at RT. The tensile elongation at RT started to decrease when the P content was higher than 0.04%. This may indicate an interaction between Cr and P when the elongation at RT is used as a response, namely that Cr may facilitate the formation of phosphide at the eutectic boundaries. Fig. 7 shows the SEM fracture surfaces of two tensile samples tested at 425 C. For sample (a) without P doping, the elongation was only 3.0% and the micrograph revealed approximately 20 to 30% of intergranular fracture surface. However, for sample (b) with doped 0.029% P, the fracture surface was characteristically ductile, with no cleavage fracture surface found, and the elongation of 18.0% was achieved.

[0060] Returning to the classification diagram in Fig. 1 , the influences of graphite shapes and MTB were described above. Now strengthening by the six transition metals is discussed, as completely laid out in Fig. 2. The six transition metals with body-center cubic structure in Groups 5 and 6 of the Periodic Table have been recognized to strengthen ferritic high-Si cast irons to some extent. They are carbide formers and ferrite stabilizers. Nb and Ta are often associated. As depicted in Fig. 2, single or multiple metals may be used, leading to 64 possible combinations. Furthermore, the amounts of additions can vary in each combination. The selective rules determined in present invention are composed of (1 ) controlling the total content no greater than about 3.00% as to form with less than 35.0% of pearlite and other carbide phases, and balance up to 100% of iron and other elements, in contrast to prior art specifying each alloying element content, (2) different strengthening effectiveness of elements, and (3) the cost. The preferred total

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amount of the six transition metals is 0.40 to 2.00%, thus maintaining the sum of pearlite and other carbide phases between about 4% and about 25%.

[0061] Taking a single combination SiMo as example, different designations from A to D (Fig. 1 ) can be proposed according to the Mo contents. It is well known that there is a continuous increase in the strength with increasing Mo. But the most significant response to increased Mo contents is realized over the range of 0.40% to 0.80% Mo additions. At additions above 1.0% Mo, the strengthening effects will level out. Furthermore, high Mo contents tend to generate more primary interdendritic carbides and a high percentage of pearlite and Mo-rich phases as shown in Fig. 9, thereby reducing the ductility of cast irons. Furthermore, hundreds of new cast iron formulae can be developed when combining the microstructure classifications with alloying classifications as proposed in Figs. 1 and 2, such as MG SiMoCr, MG SiVMo, MG SiMo, SG SiCr P-doped, and so on. In the present invention, a combination of 4.10 to 4.50% Si with 0.30 to 0.60% Mo with MG microstructure was found to yield an optimization of ductility, strength, oxidation and thermal cracking resistance, for SiMo iron. For SiMo irons, the contents of one alloying element Mo are optimized to be 0.30 to 0.60%. When the multiple alloying elements are used, the total amounts are preferably from 0.4 to 2.0%, namely that any combinations are displayed in Fig. 2.

[0062] From Fig. 2 with 64 combinations, another simple composition formula with low cost and attractive properties has been identified in the present invention: SiCr. Cr influences ferritic high-Si iron in these aspects: (1 ) increasing Ad by 30 to 40° C per 1% Cr addition, (2) enhancing hot oxidation resistance, (3) increasing tensile strength, for example, for the yield strength at 700° C as shown in Fig. 10, and (3) decreasing the elongation as shown in Fig. 1 1. Combining Si with a

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certain amount of Cr in cast irons will significantly increase Ad and hot oxidation resistance without causing severe brittleness at RT which may occur if continuing to increase the Si contents alone. Cr is a strong former of pearlite and carbides, and heat treatment (annealing) is often utilized to break up the networking of pearlite and carbides into a globular structure. As expected, annealing heat treatment increased the elongation at RT but decreased the strength. However, the middle temperature brittleness (MTB) is not reduced or removed by heat treatment. It was found in the present invention that the tensile elongation of up to 8.1% at RT and 17.2% at 425° C have been achieved for P-doped SG SiCr specimens under as-cast conditions, as shown in Table 5. This finding surprisingly dictates that heat treatment may not be required for P-doped Cr-alloyed cast irons with SG microstructure, or at least the heat treatment cycle may be significantly shortened from standard industry practices, thereby holding temperature much lower to decompose the as-cast pearlitic structures. Therefore, the Cr-alloyed ferritic high-Si cast irons claimed in the present invention may consist of (1 ) different graphite shapes (MG and SG), (2) with or without P-doping, (3) with (such as SiMoCr, SiWCr) or without (straight SiCr) other alloying elements such as Mo, W ₁ V, and Nb, and (4) annealed heat treated and as- cast conditions.

[0063] In contrast to prior art, the present invention significantly reduces the elements such as Ni ₁ Mn, Cu, Co, and Al. As noted above, first four elements are austenite promoters and reduce Ad of ferritic cast irons while Al causes casting defects and foundry process issues.

[0064] The invention has been described above with respect to various aspects and embodiments. Further non-limiting disclosure is given in the Examples.

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EXAMPLES

[0065] Much work has been carried out to better understand the interrelationship among the cast iron chemistry, graphite morphologies, and material properties, and to prepare the ferritic high-Si irons for high-temperature use in light of the new classification scheme in Fig. 1 , namely from the following five aspects: (1) graphite shapes in microstructures, (2) MTB, (3) strengthening by the transition metals of Groups 5 and 6, (4) Ad temperature, and (5) hot oxidation resistance. The present invention will be described more specifically by four examples.

[0066] The chemistry and process conditions for the compositions discussed in Examples 1 -3 are given in Table 1. Individual rows are provided for Examples 1 a-1 b, Examples 2a - 2f, and Examples 3a - 3b. The "main chemistry" columns give the carbon equivalent and levels of Si, Mo, and Cr in the compositions. Under "Process", the amount and nature of inoculant is indicated, and a range of spheroidizers addition is given. The amount of spheroidizer is varied within the range to achieve the noted values for nodularity and ferrite in the "Microstructure" column. Generally, components containing the listed elements of the Main Chemistry (e.g. graphite, iron, ferrosilicon, ferromolybdenum, and ferrochromium) are melted together in a furnace. For pouring, the temperature is maintained in the range 1385 ⁰ C to 147O ⁰ C as indicated. The melt is tapped into a ladle containing the inoculant and spheroidizers, and the resulting composition is poured to make cast articles. For the SG irons of the Examples, the inoculant FeP ³ is used, which is a mixture of Fe-Si inoculant and 0.10% FeP (25% P) used as a post inoculation. The mixture inoculant FeP ³ is made as follows. Approximately 0.40% (out of the total melt charges) Fe-Si inoculants (75% Si grade and commercially available) were mixed with approximately 0.10% (out of the total melt charges) Ferrous Phosphorus, FeP

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(25% P, commercially available). The mixed inoculants were added into ladle. The final phosphorus doped levels are attributed to the FeP additions and the inevitable P contents in the baseline irons.

[0067] Heat treatment of the cast articles is carried out according to the

"HT" column of Table 1. Low Cr examples are not heat treated. Heat treatment A ¹ indicates the part is annealed at 800° to 96O ⁰ C and then furnace cooled. C ² indicates that a comparison is made between a heat treated article and a non-heat treated article, the latter also being referred to as "as-cast."

[0068] For convenience, Table 1 also includes a cross reference to the

Figure and/or Table in which the respective example is illustrated

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Table 1 - Summar of the exam les

25

Example 1 - High-Si Based Cast Iron with Mixed Graphite (MG

[0069] Heats of irons weighing approximately 100 kg were made using an induction furnace operating at 350 kW and 1 kHz. Different types and addition levels of Fe-Si-Mg based spheroidizers and Fe-Si based inoculants were used, such as 0.50% to 1.20% ReMag ^® or Elmag ^® supplied by Elkem (ReMag ^® and Elmag ^® are the registered trademark owned by Elkem AS). Pour temperatures varied from 1385° to 1470° C to cast test Y-blocks and exhaust products. Solidification curves of cast irons were measured using a thermal analysis apparatus. Optical microscopy and digital image analysis software were used in microstructural analysis of the trial samples. A lower amount of spheroidizers and inoculants were added to achieve the mixed graphite structures, as compared to making SG cast irons.

[0070] Tables 2, 3, 8, and Fig. 4 illustrate the influences of graphite nodularity on oxidation, tensile testing data at RT and 425 C, and EES durability results. The MG iron with 30 to 90% nodularity (without flake graphite) exhibited the combined oxidation features of CG and SG irons. Table 2 briefly summarizes the hot oxidation characteristics of high-Si CG, MG, and SG irons, through a number of tests primarily following ASTM Designation G54-84, performed by the inventors.

[0071] Since lower residuals of magnesium and REM were needed to produce the MG iron, the elongation at 425° C was much higher than that of SG iron. Meanwhile, the elongation at RT of the MG iron was higher than that of conventional CG irons with containing Ti additions and less graphite nodularity.

[0072] Experiments were conducted to achieve high graphite nodularity by partially replacing magnesium with cerium (Ce). The results have shown that the graphite nodularity and elongation at RT were increased with Ce additions, as expected. Unfortunately, the elongation at 425° C was still decreased with

increasing Ce additions, as shown in Fig. 4. This implies that both magnesium and REM (rare earth metals) Ce may contribute to the MTB. Therefore, it is the total residual level of magnesium and REM which must be considered in optimizing composition with respect to reduction of the MTB.

[0073] Engine Exhaust Simulator (EES) testing was utilized to assess the thermal durability of exhaust manifolds. The thermal cycling profiles in EES testing are characterized by heating/cooling rates, peak/valley temperatures, and holding time at the maximum or minimum temperatures. The cycles to failure determined from EES testing were expressed as the manifold durability, and were in turn influenced by a variety of factors such as manifold geometry, test temperature profiles, constraint ratio, cast iron compositions, and microstructures. The main chemistry elements for manifolds 1 , 2, and 3 were made of SiMo iron, while manifolds 4 contained 0.40 to 1.00% Cr, namely SiMoCr cast irons with different graphite morphologies. Under conditions of the same compositions, Table 8 shows the influences of graphite shapes on the EES performance. The MG cast iron has shown equal or higher EES cycles to failure than CG and SG irons. It also should be pointed out that (1 ) there is a different application scope for CG, MG, and SG irons in terms of product geometries and engine characteristic, and (2) there is a microstructure overlap between CG and MG, and between MG and SG boundaries, respectively.

Example 2 - Ferritic Spheroidal Graphite Cast Iron Immune to the Middle Temperature Brittleness (MTB)

[0074] Heats of irons weighing approximately 100 kg were made using an induction furnace operating at 350 kW and 1 kHz. Different types and addition

levels of Fe-Si-Mg based spheroidizers and Fe-Si based inoculants were used for making SG irons, such as 0.8 to 1.8% ReMag® or Elmag® supplied by Elkem (ReMag® and Elmag® are the registered trademark owned by Elkem AS). Pour temperatures varied from 1385° to 1470° C to cast test Y-blocks and products. Solidification curves of cast irons were measured using a thermal analysis apparatus. Optical microscopy and digital image analysis software were used in microstructural analysis of the trial samples. A small amount of ferrous-phosphorus was added into ladle with a mixture inoculant for making the SG cast irons with P-doped.

[0075] Iron melts are treated with an adequate amount of spheroidizers and inoculants to make SG cast irons with higher graphite nodularity and elongation at RT than MG irons. The total residual content of magnesium and REM is high of up to 0.080% which usually induces the MTB.

[0076] An effective approach to avoid the MTB is to add a small amount of dopant, phosphorus (P) into SG iron. Tables 4, 5, 6, 7, Figs. 5, 6, and 7 separately demonstrate the effects of phosphorus on CTE (coefficient of thermal expansion), Ad , tensile testing data at RT and middle temperatures, and thermal fatigue life.

[0077] The results consistently demonstrate that the ductility at middle temperatures is substantially increased when the P content exceeded 0.025%, by taking SiMo and SiCr irons as examples. The testing results presented in Table 6 at different temperatures 400° C, 425° C, and 450° C, show that the MTB was indeed eliminated across the middle temperature range in the P-doped samples, and not merely shifted to another temperature. Regular samples with 0.016% P showed the presence of brittle intergranular fracture while no brittle cleavage fracture was found for the P-doped samples tested at 425° C, as shown in Fig. 7. The preferred P-

doped level is 0.025 to 0.040% to attain high elongation at both RT and middle temperatures and to avoid any P-induced shrinkage during solidification, for different ferritic high-Si SG cast irons. Table 4 presents the average CTE from 200° to 800° C, and Ad of regular P content (0.016%) and high P level (0.042%) samples. From the dilatometer tests, no significant difference in CTE and Ad was detected between regular and P-doped samples, even though it has been reported that P additions may increase the Ad temperature.

[0078] During the uniaxial thermal fatigue testing, the temperature was cycled from 150° to 820 ⁰ C, and each cycle took approximately 9 minutes. Table 7 lists the cycles to test failure from the testing. The thermal fatigue life of the P-doped samples was significantly enhanced over the base iron materials. After completing thermal fatigue testing, the fracture surfaces of the P-doped specimens showed more ductile features than non P-doped samples. It was also observed that the high-Si low-Mo samples showed the higher cycles than regular SiMo irons as comparing samples 02 to 01. The Cr-alloyed P-doped specimen exhibited the highest cycles to failure, as described more details in Example 3.

Example 3 - Cr-Alloyed Cast Iron with Improved Properties and Low Cost

[0079] Heats of irons weighing approximately 100 kg were made using an induction furnace operating at 350 kW and 1 kHz. Different types and addition levels of Fe-Si-Mg based spheroidizers and Fe-Si based inoculants were used, such as ReMag® or Elmag® supplied by Elkem (ReMag® and Elmag® are the registered trademark owned by Elkem AS). Pour temperatures varied from 1385° to 1470° C to cast test Y-blocks and exhaust products. Solidification curves of cast irons were measured using a thermal analysis apparatus. Optical microscopy and digital image

analysis software were used in microstructural analysis of the trial samples. Lower amounts of spheroidizers and inoculants are used for the MG irons than SG irons. A small amount of ferrous-phosphorus was added into ladle with a mixture inoculant for making the SG Cr-alloyed cast irons with P-doped. For the heat treatment of Cr- alloyed cast irons, annealing treatment is employed. The holding temperatures ranged from 800° C to 96O ⁰ C. The dwell time ranged from 2 to 3 hours. The cooling rate ranged from 0.5 to 3.5 C/min.

[0080] Through the classification diagrams (Figs. 1 and 2) Cr has been noted as an effective element to increase Ac1 , oxidation resistance, and high- temperature strength, with a relatively low cost. From Fig. 8, the hot oxidation resistance of 0.85% Cr samples was significantly improved over the straight high-Si SiMo samples in terms of the weight change measured in oxidation testing and oxide de-scaling carried out via tumblasting operations. The oxidation testing samples typically have the rectangular geometry with a dimension approximately 12x12x45 mm. Needless to say, the material oxidation behaviors will be different between the free growth and the constraint conditions. The yield strength was increased and the elongation at RT was decreased with increasing Cr, as shown in Fig. 10 and 11. In order to break-up the pearlite and carbide microstructures, annealing heat treatment was often used. The experimental results of heat treatment were added in the graphs for comparison. In contrast to the effects of increasing Cr contents, annealing decreased the strength and increased elongation at RT. From Table 5, it is seen that there is no or little impact of the heat treatment on the elongation at 425 C. Therefore, there may be a possibility of removing annealing heat treatment for Cr-alloyed cast irons. The cycles to failure of exhaust manifolds from EES testing were observed to increase for Cr-alloyed cast iron, especially for the mixed graphite

Cr-alloyed cast iron, as shown in Table 8. The cycles to failure of the test bars from the constrained thermal fatigue testing were increased, especially for the Cr-alloyed P-doped SG samples, as presented in Table 7. In the present invention, the Cr- alloyed high-Si cast irons can consist of (1) different graphite shapes (MG and SG), (2) with or without P-doping, (3) with or without other alloying elements such as Mo, V, W, and Nb, and (4) annealed or as-cast conditions.

Example 4 - Products Using the Improved Ferritic High-Si Cast Irons

[0081] Articles cast from the compositions and microstructures of the present invention can withstand static and cyclic thermomechanical stresses and chemical attack. Such articles find use in a variety of automotive transportation and industrial applications. Such applications include, but are not limited to, exhaust components such as exhaust manifolds, turbocharger housings, back plates, turbo housing-integrated exhaust manifolds, turbo outlet pipes, hot end components such as catalytic converter housings, and fuel cell components.

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8