Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C37/00—Cast-iron alloys
  • C22C37/10—Cast-iron alloys containing aluminium or silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D5/00—Heat treatments of cast-iron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/08—Making cast-iron alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C37/00—Cast-iron alloys
  • C22C37/04—Cast-iron alloys containing spheroidal graphite

Definitions

• the present invention relates to cast iron that exhibits improved strength and high temperature properties. More specifically, the present invention relates to cast iron alloys which contain certain amounts of carbide formers selected from the group including tungsten, vanadium and niobium. Other carbide formers such as molybdenum, and/or chromium may be employed in addition to at least one of tungsten, vanadium and niobium. Other alloy additions of silicon and aluminum for oxidation resistance are also disclosed.
• the cast iron may include a graphite morphology that is primarily nodular, vermicular or a combination referred to herein as hybrid or duplex.
• SiMo nodular or compacted graphite irons such as those presented in the comparative example of Table I herein are currently employed in the manufacture of exhaust manifolds of the high volume production engines because they often have advantages in terms of cost and durability.
• SiMo alloys exhibit improved high temperature strength and thermal fatigue resistance over many other known ductile cast irons, as well as improved high temperature oxidation resistance.
• the high oxidation rate at high temperatures remains a problem in parts such as exhaust manifolds and turbocharger turbine housings, where the in-use temperatures can reach 850 0 C and higher.
• cast irons in these applications are also subject to thermal fatigue cracking. This is due at least in part to the thermal cycling during heating and cooling.
• the part in use the part is cycled up to temperatures associated with engine operation and then back down to approximately room temperature.
• the part undergoes the thermal expansion upon heating and contraction upon cooling. This continued thermal cycling and associated thermal expansion/contraction is said to contribute to thermal fatigue in the part which, in time, leads to cracking.
• Nobuaki cast iron alloys containing silicon, molybdenum, manganese and vanadium have higher heat resistance than the conventional ductile irons.
• Nobuaki further indicates that alloys containing vanadium and manganese (Mn) improve the elevated temperature physical properties of a nodular graphite iron.
• Mn + V in the range of 0.3-2.0% by weight (preferably 0.4-1.8% by weight) while Mo content is 0.3-1.0% by weight (preferably 0.3-0.7% by weight).
• compositions may address one or more of the perceived issues with conventional SiMo cast irons, still further improvements are demonstrated under the present invention.
• the iron alloys of the present invention including one or more of tungsten, vanadium and niobium give rise to alloys exhibiting the combined properties of high mechanical strength and ductility. Further parts cast from such alloys are readily machined, abrasively cleaned at room temperature, can withstand oxidation at high in-use temperatures and can withstand thermal-mechanical fatigue cracking during cycling.
• the high silicon iron composition of the present invention contains up to 1.5 wt.% tungsten, up to 0.8 wt.% vanadium, and up to 1.2 wt.% niobium, preferably in combination with at least one of molybdenum and chromium.
• the cast iron alloys of the present invention yield high strength and good ductility over a wide temperature range, compared to conventional SiMo iron having nodular, compacted graphite iron or other graphite morphologies.
• the addition of higher silicon and aluminum offers improved hot oxidation resistance, compared to conventional SiMo iron having nodular, compacted graphite iron or other graphite morphologies.
• the iron alloy of the present invention contains, from about 0.02 to 0.8% vanadium, from about 0.03% to about 1.5% tungsten, from 0.02% to about 1.2% niobium, from about 2.8 to about 5% silicon, from 2.8% to about 3.8% carbon, less than 0.06% magnesium, and less than 0.02% cerium, the balances being at least 60.0t% iron and impurities, with all percentages based on the total weight of the composition.
• the compositions may further contain up to 1.5% molybdenum, up to 1.0% chromium, up to 5.0% nickel and between 0.2 and 3.0% aluminum.
• Articles cast from the compositions of the invention are ductile and can withstand thermal cycling without failure. 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, hot end components such as catalytic converter housings and fuel cell components.
• the cast iron compositions of the invention may be used in any application calling for nodular or compacted graphite iron, Ni-Resist ductile iron, chrome molybdenum steel, or a low grade stainless steel.
• compositions of the invention provide cast iron articles having desired combinations of elevated temperature strength, ductility, high oxidation resistance, and thermal fatigue resistance.
• the cast iron compositions of the present compositions are considered to be a viable alternative to the conventional SiMo nodular and compacted graphite irons. They are useful generally in any iron application, particularly high temperature cast iron applications.
• Figure 1 is a graph setting forth a comparison of the influence of tungsten on strength at 800 0 C to that of molybdenum;
• Figure 2 is a graph setting forth a comparison of the influence of tungsten on strength at room temperature to that of molybdenum;
• Figure 3 is a graph setting forth weight change rate versus exposure time at the temperature of 820° C for different materials measured by daily cyclic oxidation testing.
• Figure 4 is a graph setting forth average depth of oxide scales measured after testing in Figure 3.
• compositions of the present invention are alternative materials to the conventional SiMo irons used in high temperature applications, the composition of the invention can be referred to as "high silicon iron alloys" or "modified high temperature SiMo" alloys which include more than an impurity level of molybdenum.
• high silicon iron alloys or "modified high temperature SiMo” alloys which include more than an impurity level of molybdenum.
• modified high temperature SiMo and “modified SiMo” will be used interchangeably to refer to the cast iron compositions and molded articles of the present invention containing molybdenum.
• Cast iron articles of the invention are prepared by pouring a molten composition into a mold.
• the molten composition is a cast iron composition containing, in addition to at least about 60% by weight iron, tungsten at levels up to about 1.5% by weight, vanadium at levels up to about 0.8% by weight, and niobium at levels up to about 1.2% by weight.
• the cast iron composition includes at least 80 wt.% iron.
• Vanadium at the appropriate levels is believed to increase the high temperature strength of the cast iron articles, but too high vanadium would result in too much vanadium carbide thus decreasing ductility significantly.
• Tungsten at the appropriate levels is believed to increase the elevated temperature strength of cast irons. More particularly, tungsten is believed to improve high temperature creep and fatigue resistance.
• Tungsten appears to have comparable strengthening characteristics as molybdenum, and both form very fine tungsten or molybdenum carbide precipitates.
• higher tungsten content is generally associated with higher carbide content. This makes the cast articles tend to be more brittle with some risk of cracking during thermal cycling, as for example, in normal automotive engine use, or during simulative or accelerated engine dynamometer durability tests.
• the upper limit of tungsten should be no more than about 1.5% by weight.
• the preferred amount of tungsten is from about 0.03% by weight to 0.8% by weight.
• Niobium at the appropriate levels of between about 0.02% and 1.2% are believed to increase the ductility at room and elevated temperatures and improve high temperature properties.
• the iron compositions may further comprise silicon and carbon.
• Silicon is generally present in an amount of from about 2.8% to about 5.0% by weight. In a preferred embodiment, silicon is present at a level of from about 3.9% to 4.6% by weight. Carbon is generally present in an amount such that the weight percent carbon plus 1/3 the weight percent silicon is numerically equal to a value up to about 4.9%, preferably up to about 4.7%.
• compositions of the invention contain less than 0.02% sulfur. Higher sulfur levels tend to lead to a requirement for additional magnesium additions and cause more rapid fading of magnesium during the treatment step to control production of either compacted (vermicular), nodular graphite structures or other graphite morphologies. 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 0.04%.
• the desirable properties of ductility and machinability exhibited by the compositions of the invention are believed to derive from the microstructure of the modified SiMo alloys.
• the graphite present in the molded articles is predominantly present in either nodular or vermicular form.
• the compositions are generally referred to as ductile irons.
• the nodularity is greater than about 85% for ductile irons.
• the compositions are referred to as compacted or vermicular graphite iron.
• nodularity is generally about 50% or less, with the remainder of the graphite predominantly present in vermicular form. High levels of flake graphite are undesirable.
• the nodularity is between 50-80% a structure referred to as hybrid or duplex graphite exists. It is an iron containing significant fractions of both nodular graphite and compacted or vermicular graphite.
• the hybrid or duplex graphite iron has a nodularity of from 60% to 75% (i.e. 60-75% of the carbon is present as graphite nodules); the remaining is in the form of compacted or vermicular graphite.
• Examples 1-3 are irons in which vanadium and/or tungsten is used instead of molybdenum.
• the tensile testing for the comparative example (the conventional SiMo ductile iron) and the example 1 containing 0.3% vanadium and 0.5% tungsten is given in Tables 2-4 from room temperature to 900 0 C. It can be seen that Example 1 has the mechanical properties which are similar to or even better than the conventional SiMo iron of the comparative example. This indicates that 0.8% molybdenum in the conventional SiMo ductile irons can be completely substituted by vanadium and tungsten while the tensile properties are maintained or may even be better.
• Example 1 The composition of the invention as demonstrated in Example 1 has up to 18% elongation at room temperature, so the material of the invention shows ductility indicating the machinability may be similar to the comparative example.
• Examples 2 and 3 have 0.1-0.3% vanadium and 0.4 -0.6% tungsten and both have comparable tensile properties to the comparative example (the conventional SiMo iron).
• Examples 4-6 are improved SiMo irons and have 0.2-0.3% vanadium added into SiMo ductile iron containing 0.5-0.6% molybdenum. It can be seen from Tables 2-4 that the addition of vanadium and molybdenum may increase the high temperature strength such as at 800 0 C, while the ductility at room temperature is reasonable, i.e. there is about 10% elongation for room temperature and more than 25% for 80O 0 C.
• Examples 7-10 showed that the compositions of the invention containing tungsten and molybdenum have mechanical properties comparable to the conventional SiMo iron after some of molybdenum in conventional SiMo irons is at least partially replaced by tungsten.
• Examples 11 and 12 use tungsten, vanadium, niobium, and molybdenum in the ductile iron containing about 4.2% silicon. It is shown from the Tables 2-4 that the strength at high temperature is significantly increased, compared to conventional SiMo cast iron as set forth in the comparative example. It is also important to note that the ductility at room temperature is more than 6%.
• Example 13 uses niobium in the ductile iron containing about
• the tungsten As the atomic weight of tungsten is twice as much as molybdenum, one would expect the tungsten to have 50% the effect of molybdenum (i.e. 1% W is equivalent to 0.5% Mo in terms of strengthening). This is seen in the steels mentioned above, but is not seen in the alloys of the present invention. In the compositions of the present invention, the tungsten has 80-100% the effect of molybdenum (i.e. 1% tungsten is equivalent to 0.8 -1 % molybdenum) both at room temperature and 800 0 C, which was surprising.
• a manifold for a 6.0 liter engine was cast from an iron composition containing 3.35% carbon, 3.99% silicon, 0.3% vanadium, 0.51% tungsten, with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• the microstructure displayed good nodularity (about 95%), nodule count of about 400 nodules/mm 2 , no pearlite and about 3% carbide.
• the carbide is blocky vanadium carbide and some tungsten-rich precipitate which is similar to the molybdenum-rich precipitate in the Silvio irons.
• the manifold was evaluated in an engine exhaust simulation test.
• the test consisted of 1810 thermal cycles before failure.
• the test included heat shields applied with an exhaust gas temperature of 1616°F (88O 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off.
• the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 76O 0 C to around 780 0 C.
• the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C.
• the manifold showed good stability and heat resistance in the engine exhaust simulation test.
• a manifold for a 6.0 liter engine was cast from an iron composition containing 3.45% carbon, 4.15% silicon, 0.43% tungsten, 0.41% molybdenum with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• the microstructure displayed good nodularity (approximately 94%), nodule count (approximately 350 nodules/mm 2 ), 6 to 10% molybdenum-rich and tungsten-rich precipitates, very low pearlite levels (below 5%) and carbide (approximately 1%) levels.
• the manifold was evaluated in an engine exhaust simulation test.
• the test consisted of 1790 thermal cycles prior to failure. This test included heat shields applied with an exhaust gas temperature of 1616°F (880 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 76O 0 C to around 78O 0 C. After the burners are turned off, the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C. The manifold showed good stability and heat resistance in the engine exhaust simulation test. These results were comparable to tests run with SiMo chemistry of the comparative example.
• a manifold for a 6.0 liter engine was cast from an iron composition exhibiting a hybrid/duplex graphite microstructure containing 3.15% C, 4.45% Si, and 0.85% Mo with additions of Mg, Ce, rare earths, and the remainder being iron plus impurities.
• This test included heat shields applied with an exhaust gas temperature of 1616°F (880 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-00 0 C and portions of the surface of the manifold reached temperatures varying from 760 0 C to around 780 0 C. After the burners are turned off, the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C. The test consisted of 2012 thermal cycles prior to failure.
• a manifold for a 6.0 liter engine was cast from an iron composition exhibiting a nodular graphite microstructure containing 3.35% carbon, 4% silicon, 0.3% vanadium and 0.51% tungsten with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• This test included heat shields applied with an exhaust gas temperature of 1616°F (880°C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 760°C to around 78O 0 C. After the burners are turned off, the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C. The test consisted of 1977 thermal cycles prior to failure.
• a manifold for a 6.0 liter engine was cast from an iron composition exhibiting a nodular graphite microstructure containing 3.15% carbon, 4.46% silicon, 0.4% aluminum and 0.51% molybdenum with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• This test included heat shields applied with an exhaust gas temperature of 1616 0 F (880 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 760 0 C to around 780 0 C. After the burners are turned off, the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C.
• the test consisted of 1515 thermal cycles prior to failure.
• a manifold for a 6.0 liter engine was cast from an iron composition exhibiting a nodular graphite microstructure containing 3.41% carbon, 4.47% silicon, 0.4% aluminum and 0.59% molybdenum with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• This test included heat shields applied with an exhaust gas temperature of 1616°F (880 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 760 0 C to around 78O 0 C.
• the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C.
• the test was stopped at 1565 thermal cycles because a fastener was sheared off during testing and the engine head actually failed in tensile due to distortion of the manifold. So the test was incomplete.
• a manifold for a 6.0 liter engine was cast from an iron composition exhibiting a nodular graphite microstructure containing 3.1% carbon, 4.4% silicon and 0.65% tungsten with additions of Mg, Ce, rare earths and the remainder being iron plus impurities, all percentages being presented as percentages by weight.
• This test included heat shields applied with an exhaust gas temperature of 1616°F (880 0 C).
• a thermal cycle consisted of a 6 minute heating portion with burners on followed by a 6 minute cooling period with burners off. During heating, the exhaust gas had a temperature of about 860-900 0 C and portions of the surface of the manifold reached temperatures varying from 760°C to around 780 0 C.
• the exhaust gas and manifold cool down within a period of 4 or 5 minutes to a uniform temperature of about 70 0 C.
• the test was stopped at 1321 thermal cycles because a fastener failed during testing and the engine head actually failed in tensile due to distortion of the manifold. Thus, the test was incomplete.
• Oxidation resistance was also tested in accordance with the below described evaluation.
• Oxidation resistance is improved when the Si content is increased from 4.0% to 4.4%.
• the resistance consists of weight gain, depth of oxide scales, and oxide adhesion. There is little change in oxidation resistance when molybdenum is increased from 0 to 0.6%.
• the non-AI containing samples the difference is evident between the as-cast and machined surfaces in the oxidation behavior.
• With the addition of 0.35% Al alloyed specimens significantly improved oxidation resistance (weight change, depth, and especially oxide adhesion). In contrast to non-AI specimens, there is much less difference between as-cast and machined surfaces for the 0.35% Al alloyed materials.

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