Inoculation filter

Details Inoculation filter Inoculation filter

2002-01-10

2004-09-21

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Solid treating composition for liquid metal or charge

C075S316000, C075S407000, C075S526000, C420S029000, C420S030000, C420S031000, C420S032000, C420S033000, C266S216000

Reexamination Certificate

active

06793707

ABSTRACT:

TECHNICAL FIELD
The present invention is related to an improved method for inoculating cast iron late in the casting process and to an inoculant which affords more consistency in the inoculation of iron being cast. The inventive casting process, referred to as in the mold inoculation incorporates filtration and inoculation combining the advantages of both techniques for the manufacture of parts for which it is desired to obtain a structure free of iron carbides.
BACKGROUND
Cast iron is an extremely versatile engineered material comprising iron-carbon-silicon alloys that have been used in many commercial application manufacture of mechanical parts. The versatility of cast iron has led to the utilization of this material in many structural applications where the homogeneity and consistency of the iron will have a critical impact on the components performance. The casting of clean homogenous iron, specifically grey or ductile, is an essential step in the production of high quality engineered castings. Due to the importance of these cast items it is imperative that iron, specifically gray or ductile, be consistently cast with uniform morphology, with minimum included impurities and with properties that are reproducible.
Cast iron has an unusual metallurgical structure. Most metals form a single metallic crystalline structure during solidification. Cast iron, however, has a far more complex morphology during solidification. The crystalline phases that form during solidification of cast iron are dependent on the rate of solidification. Most engineered castings desire the formation of crystalline graphite within the iron matrix during solidification. If the cast iron solidifies too rapidly primary iron carbides can crystallize within the casting. Primary iron carbide is a hard brittle phase that makes the iron very difficult to machine and changes the physical and mechanical properties of the primary cast iron. Primary iron carbides are commonly referred to as “chill”. Carbon contained as iron carbides is generally considered to be detrimental in most iron castings whereas carbon present as graphite improves the physical and mechanical properties of cast iron. Carbon can crystallize as either iron carbide or graphite during solidification. The formation of either phase is driven by the rate of solidification and the degree of nucleation contained within the liquid iron. The rate of solidification is constrained by the geometry of the casting, the rate of heat extraction of the mold material and the amount of superheat of the iron contained when the metal entered the mold. The degree of nucleation is constrained by the metallurgical history of the molten iron. Carbon present as graphite is an advantageous form and persuading carbon to crystallize as graphite is an ongoing goal of standard foundry operations. Graphite can be present in several morphological forms including spherical, as is the case with ductile iron, and flake-like, which is the case with gray iron.
Standard foundry metallurgical practice includes inoculation wherein the nucleation and growth of graphite is encouraged at the expense of iron carbide formation. Preferential nucleation greatly enhances the mechanical and physical properties of the finished casting. Inoculation is typically done by addition of an inoculating agent to either the pouring ladle, the metal stream or within the mold. The inoculating agent is typically added to the pouring ladle by pouring the granulated inoculating agent into the ladle when the ladle is filled with liquid iron, whereas the inoculant is added to the metal stream by injecting or spraying a finely divided powder of the inoculating agent in the molten metal stream as the molten metal enters the mold. It is typically desirable to add the inoculating agent to the molten metal as late as possible to minimize fading. Insufficient or improper inoculation is constantly at the forefront of losses due to poor quality in a foundry operation.
It may be preferred for the formed graphite to be spheroidal, if a spheroidal graphite cast iron called “SG” or “ductile” iron is required. Alternatively, a lamellar graphite cast iron is required for “LG” or “grey” iron. The essential prior condition to be met is to prevent the formation of primary iron carbide.
To this end the liquid cast iron is subject before casting to an inoculation treatment, which will, as it cools, favour the appearance of graphite rather than that of primary iron carbide.
The inoculation treatment is therefore very important. It is in fact well known that inoculation, whatever the inoculants used, has on the liquid cast iron an effectiveness which reduces with time and which, generally, has already reduced by 50% after a few minutes. To obtain maximum effectiveness, one skilled in the art generally practises progressive inoculation, applying to this end several additions of inoculants at different stages of the development of the cast iron. The final addition is made in the mold as the molds are fed or even in the feed conduits of the molds by placing in the path of the liquid cast iron inserts constituted by an inoculant material. These inserts are generally used associated with a filter; in this case they generally have a perfectly defined shape in order to be able to be fixed in the filter, most often in an adapted cavity. These inserts of defined shape are known as “pellets” or “slugs”. We will denote by the name “filter inoculant package” the unit constituted by the pellet and the filter.
There are two types of pellets. “Molded” pellets are obtained by molding the molten inoculant. “Agglomerated” pellets are obtained from a pressed powder with generally very little binding agent, or even without binding agent.
Commercial inoculants create nucleation sites by seeding the liquid iron with highly reactive elements. The reactive elements combine with oxygen and sulfur dissolved within the liquid iron and the resultant reaction products precipitate out of solution to form nucleation sites for graphite during solidification. These nucleation products continue to grow within the melt until the metal has completely solidified. These particulates must be within a narrow size range in order to nucleate graphite crystal growth. Thus seeding the metal with the reactive elements as close to solidification as possible increases the probability that the precipitated particles remain within the narrow size window necessary for nucleation of graphite crystals. Formation of crystalline graphite is contrary to the kinetically favored products. The critical parameters which affect inoculation are not understood and are still the subject of academic debate. The ability of a skilled artisan to predict, and therefore improve, inoculation efficiency is very much desired in the art.
Pellet inoculation, wherein the molten metal is exposed to a pellet just prior to a filter, is known wherein a base material comprising minor amounts of calcium, aluminum, and rare earths are used. As the casting proceeds the inoculation efficiency changes with time due to the kinetics associated with dissolution of the inoculating agent from the pellet. Further complicating the problems of inoculation is the realization that various pour volumes and times are desired for manufacturing different parts with different sizes. If long pour times are utilized, the method of ladle inoculation is undesirable due to fading of the inoculant in the ladle. If short pour times are utilized, the time may by insufficient to allow for the onset of inoculation by pellet inoculation. The properties which allow for effective inoculation in the metal stream are not well understood and typically a suitable working range is developed by experimentation at great cost and loss of material.
The Daussan patent FR 2,692,654 describes a filter inoculant package wherein the pellet is obtained by agglomeration of powder at 0.5 to 2 mm preferentially. The efficiency of this filter inoculant package is quite limited.
The Foseco Patent EP 0 234 825 describes a filter inoculant package wherein the i

Affiliated with

Also associated with

Rating

Inoculation filter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Inoculation filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inoculation filter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3272166