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

Definitions

• This invention relates to cast iron and more particularly to the improvement in the toughness and abrasive resistance of white cast iron along with a significant increase in tensile strength. More specifically, the present invention relates to a new white cast iron composition and a process for producing such cast iron having improved toughness, ductility and tensile strength while retaining desirable abrasive resistance through modification of the carbide morphology.
• Alloy white cast iron is well known to be a highly wear- resistant material formed with a carbon content generally recognized to be in excess of 1 1/2% and the capability of being alloyed with other metals, usually chromium, to combine with the carbon to form a compound of iron-chromium carbide such as M ⁇ C .
• the inherent abrasive resistance of unalloyed cast iron is adequate to meet its intended use and therefore does not pose a problem to the user.
• the cast iron forming an industrial apparatus is subjected to particular kinds of wear the inherent mechanical properties of cast iron leave much to be desired.
• abrasive particles In another type of wear often referred to as high stress abrasion, abrasive particles, such as may be encountered in a mining operation, are crushed under grinding influence of moving metal surfaces. Stress levels involved in this operative wear process as occur typically in castings used for grinding, crushing rolls or mill liners often exceed the stress capabilities of the conventional cast iron leading to equipment failure.
• the abrasive operation to which the cast iron surfaces of the equipment are subjected are not severe stressful conditions, but yet, require high abrasive resistance.
• the gouging or grooving wear that is associated with a severe shock load requires a toughness that cast iron typically has not characteristically possessed in the past.
• a manganese steel with high plasticity and toughness has been able to meet the severe shock resistant requirements for material subjected to this type of wear.
• the hardness and abrasive resistance is usually found to be inadequate to prevent an extremely high rate of wear in the high stress abrasion operation typical in a wide range of pulverizing processes such as a rotary ball mill.
• chrome molybdenum steel and alloyed white* iron may be used in various types of apparatus depending upon the requirement of toughness and the combination of abrasion resistance required.
• chromium alloyed irons with or without molybdenum or nickel additions may be used with a desirable high martensitic matrix having a carbide embedment.
• This invention also has as a further object, a provision of a cast iron that is tough and wear resistant in which the carbides are of smaller than conventional average size and substantially evenly distributed throughout the matrix.
• the present invention is a unique discovery of an alloy cast iron composition
• an alloy cast iron composition comprising as a base the element iron, with or without .001% to 30% by weight singly or cumulatively vanadium, titanium, niobium, molybdenum, nickel, copper, tantalum or chromium or mixtures thereof, 2.0 to 4.5% by weight carbon forming an alloy composition and introducing .001% to 4.0% by weight boron to improve wear-resistance, toughness and tensile strength properties.
• the alloy has a solidification point between 2200 ⁇ F and 2400 ⁇ F and generally is in a range between 2260°F to 2300 ⁇ F. This solidification point is within 15°F of the eutectic temperature of the cast iron with the selected alloying elements.
• the carbides present in the form of globules that are approaching spherical form and are of a size that average less than 4 microns which is considerably less than the average particle size of carbides in conventional cast iron.
• an alloy white cast iron containing .001% to 30% vanadium, titanium, niobium, molybdenum, nickel, copper, tantalum or chromium or mixtures thereof and 1.8% to 4.5% carbon forming a molten cast iron composition is provided with an entropy increasing additive such as .001% to 4.0% boron then cooling the molten cast iron composition at least 5 ⁇ F below the equilibrium solidification temperature of between 2200 ⁇ F and 2400°F to a super cooled temperature and thereafter solidifying the molten cast iron composition to produce globular shaped carbides having an average size less than the average conventional cast iron or carbide particle and, on the average, less than 4 microns.
• the carbide particles may also take the form of needles but whatever appearance they may have microscopically, their long dimension on the average is still at least 10 microns which increases the propensity for crack initiation under stress which often leads to an ultimate apparatus failure.
• this normal rod or plate geometry of the carbides can be changed into a globular form that approximates a spherical shape producing not only the desired toughness but a significant tensile strength increase.
• This change in the morphology of the carbides of cast iron has altered the non-ductile, brittle, non-deformable cast iron of the past to one that has the capability of plastic deformation, higher tensile strength with retention of the superior wear-resistant characteristics.
• the cast iron of the present invention will bend prior to breaking and the stress level to which it is subjected is significantly higher without fracture as compared to prior known cast irons.
• the cast iron of the present invention is preferably alloyed with chromium but depending upon various additions of vanadium, titanium, niobium, tantalum, nickel, molybdenum or copper from .001% to 30% to substitute for the chromium, the properties of the resultant cast iron vary.
• the cast iron of the present invention has been found to have a tensile strength as high as 151,000 psi compared to the traditional 50,000 to 60,000 psi tensile strength of prior known cast irons.
• Typical cast irons have had a 0% elongation characteristic while the present cast iron has a 3% elongation capability.
• Those skilled in the art would immediately recognize the significant advantages of an increase in elongation or plastic deformation as providing a toughness capability so important in those apparatuses subjected to great wear and shock loading such as, for instance, crushers and pulverizers for the mining industry and also in pumps for the transportion of fluids containing abrasive solids.
• Cast iron is well recognized to be an iron-carbon comp ⁇ osition that may be alloyed. It is generally recognized in the art that the ⁇ dividing line between cast iron and steel is the solubility of carbon in iron in the solid state. At higher levels of carbon, the carbon would be in the form of free graphite unless it was alloyed.
• the alloying element used to form carbides in cast iron and to improve various properties is chromium. However, molybdenum, vana- dium, titanium, copper, nickel, niobium and tantalum in any combination may optionally be added to the chromium or sub ⁇ stitute for the chromium.
• vanadium and niobium may range from .001% to 5%, molybdenum and copper from .001% to 4%, nickel from .001% to 7% and titanium and tantalum range from .001% to 4% with the total in combination with chromium or with chromium alone should be in the range of .001% to 30%.
• the chromium is in the range of 7% to 29% and more preferably in the ranges of 25% to 28% or 14% to 22% or 7% to 12% which ranges of chromium represent the three major groups of commercial alloy white irons.
• the carbon content is preferably not less than 1.8% and no more than about 4.5% and preferably in the range of 1.8% to 3% for cast iron with a content of 25% to 28% chromium and 14% to 22% chromium or 2% to 3.5% for 7% to 12% chromium.
• the typical cast iron compositions outlined above can achieve a changed carbide morphology by the addition of boron generally in the range of .001% to 4% and preferably .01% to 1% and most preferably between .01% to 0.4%. This addition of boron is found to produce globular carbide particles but is more pronounced when the alloyed iron-carbon composition selected is related to the eutectic temperature.
• the solidification point of pure iron is about 2800°F and as carbon is added, the solidification point decreases.
• the solidification temperature varies between 2200 ⁇ F and 2400°F varying primarily in accordance with the amount of chromium present but also varying due to the selection of the particular alloying elements. More desirably it is found that the solidification temperature of the alloyed iron-carbon system should be in the range of 2260°F to 2300°F or approximately 2280 ⁇ F plus or minus 10 to 20°F. Any specific cast iron composition with the selected alloying elements present in amounts in accordance with this invention will solidify within 15 ⁇ F of the eutectic temperature for that system of cast irons formed with those particular alloying elements.
• the alloy cast iron composition of this invention is cooled below the equilibrium solidification temperature into the super cooling range of at least 5 ⁇ F below the equilibrium solidification temperature, when the solidification does occur it is more instantaneous than when super cooling does not take place.
• the super cooling avoids the usual lengthy period of crystal or particle growth that conventionally occurs. Rather, the solidification is more rapid before the growth of the particles can be achieved.
• the minute carbide particles instead of agglomerating into rods or plates as occurs in the conventional cast iron do not have the opportunity to agglomerate with the rapid solidification in the alloy cast iron composition of the present invention nor is there a migration of these particles to agglomerate to form a plate or rod so as to produce non-uniformity in the distribution of the carbides. Rather, the uniformity in the carbide distribution is inherent in the melt phase even during the super cooling phase of the alloy
• a typical cast iron composition containing 27.2% chromium, 2.04% v carbon is an alloy composition with solidification in the range of 2280 ⁇ F which is above the eutectic temperature of about 2263 ⁇ F. With the addition of 0.17% boron the alloy can be super cooled to a temperature of 5 degrees below that equilibrium solidification temperature and to about slightly below 2275 ⁇ F. Between this temperature point and below the equilibrium solidification temperature the melt is super cooled and remains liquid. Further cooling produces carbides having a globular shape that is nearly spherical and of an average particle size of less than 4 microns. The tensile strength of the resulting cast iron is in the range of 151,000 psi with approximately 3% elongation permitted. Such a white cast iron is quite wear-resistant and additionally has improved tensile strength and toughness characteristics that make it particularly useful in high wear and stress operations.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture Of Metal Powder And Suspensions Thereof (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)
• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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• 1983
  • 1983-10-24 CH CH2839/85A patent/CH666908A5/fr not_active IP Right Cessation
  • 1983-10-24 NL NL8320359A patent/NL8320359A/nl not_active Application Discontinuation
  • 1983-10-24 EP EP19830903718 patent/EP0159981A4/en not_active Withdrawn
  • 1983-10-24 GB GB08515282A patent/GB2158462B/en not_active Expired
  • 1983-10-24 JP JP58503752A patent/JPS60501958A/ja active Pending
  • 1983-10-24 WO PCT/US1983/001656 patent/WO1985001962A1/en not_active Ceased
  • 1983-10-24 DE DE19833390548 patent/DE3390548T1/de active Granted

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