Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2002-04-24
2004-05-11
Mai, Ngoclan T. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S567000, C075S568000, C075S314000, C075S315000, C075S252000
Reexamination Certificate
active
06733565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to production of irons and steels, and additives used in such production.
2. Description of the Related Art
Irons with compacted graphite microstructures can provide numerous economies in many industrial applications. They can provide the strength characteristics of ductile irons along with the thin section casting capability of gray irons. Castings made from irons containing graphite with “a compacted shape” can be made much thinner than normal gray cast irons and approach the weight savings offered by aluminum castings. At the same time, such castings have a higher modulus, higher strengths, excellent dampening properties and wear resistance. Difficulties in controlling the microstructure has prevented wide spread conversion to these irons.
Cast Irons
It is generally recognized that there are three distinct classes of cast irons. The first class of cast irons is gray iron. The usual microstructure of gray iron is a matrix of ferrite and pearlite with graphite flakes dispersed throughout. It is called gray iron because when it fractures, the color of the fracture is gray. When gray iron, which is quite brittle, is broken, the fracture propagates through an interconnected network of graphite flakes, hence giving rise to a grayish colored fracture. This network of interconnecting graphite flakes imparts some unique characteristics to gray cast irons; the flake structure provides excellent damping properties, a high level of thermal conductivity and excellent machining capabilities.
A second class of cast irons is ductile irons. In ductile irons, the usual microstructure is a matrix of ferrite and pearlite with the graphite now in the form of graphite nodules or spheroids dispersed throughout the structure. Since the graphite is now in the shape of individual graphite spheres, without the interconnecting and weakening effect of flake graphite, tensile strengths are double to triple that of gray cast irons. The irons have significant improved ductility and impact properties. Ductile irons are used in many applications requiring high strength and ductility.
Compacted graphite irons (CG) are a relative newcomer to the family of cast irons. These irons exhibit tensile strengths almost comparable to ductile iron while exhibiting the castability of gray iron. The structure is characterized by graphite particles intermediate in shape between the flake graphite of gray iron and the spheroidal form of graphite in ductile iron. However, the unique combination of properties in CG irons give these irons a number of significant advantages in a variety of applications over both gray and ductile iron.
The CG shape has been known for some time and has also been called quasi flake, semi-nodular and vermicular graphite. Its production is similar to that of ductile iron in requiring close metallurgical control, but it is far more difficult to produce than ductile iron and requires extremely close metallurgical control. It is extremely important to minimize or eliminate formation of spherulitic graphite forms. The physical and mechanical properties of CG irons are to a large extent related to the interconnected graphite phase. While individual properties are generally intermediate to those of gray and ductile cast iron, some of the better properties of both gray and ductile iron are combined in CG irons.
Strength properties of CG irons can be adjusted by using the same alloys that are commonly used in ductile iron. Tensile strengths of CG irons are equal to or greater than those of alloyed high strength gray cast irons, and tensile and yield strengths approach those of ductile cast irons. Tensile strengths of 50,000 to 75,000 psi and yield strengths of 35,000 to 60,000 psi have been reported for as cast CG irons. Elongation values vary from 1% to 6% for the higher and lower strength CG irons, respectively.
The thermal conductivity of CG is intermediate between gray and ductile cast iron. The thermal conductivity and camping capacity of CG irons of near eutectic compositions are comparable, however, to the thermal conductivity and damping capacity of lower carbon equivalent high strength gray cast irons. Impact properties of CG irons are substantially better than gray cast irons although lower than ductile iron.
Because the graphite in CG is interconnected, the machinability of CG irons is appreciably better than the machinability of ductile cast irons. Because CG iron castings can be poured from higher carbon equivalent irons, they are less susceptible to chill and carbide formation than are high strength gray irons.
Production of CG Irons
Early in the identification of the structure of CG irons, process control difficulties have made the commercial production of these irons impractical if not impossible for some foundries. Thus, CG irons have not realized their true potential.
Early research in developing a procedure for the commercial production of CG irons showed that its manufacture was not a situation where a producer under-treated molten ductile irons by employing reduced magnesium levels. This under-treatment method targeted a residual magnesium level of 0.017 to 0.021%. Magnesium variations of as little as 0.005% could mean the difference between containing CG iron and failure. Great difficulties were encountered in achieving consistently good structures because it is extremely difficult to control the magnesium reaction in molten cast irons (magnesium boils at just above 1,994° F., which is far below the processing temperatures used in making cast irons). Hence, it was difficult to operate within this narrow window of magnesium concentration needed for CG iron formation. Nevertheless, this method is still used by some producers of nodular graphite cast iron.
Other treatment methods incorporating rare earths have not met with success because of the tendency for rare earth treated irons to be susceptible to carbide or chill formation.
A substantial amount of the total tonnage of CG irons is produced using magnesium ferrosilicon master alloys containing titanium and rare earths or magnesium ferrosilicon master alloys with small additions of titanium. See, e.g., U.S. Pat. No. 3,421, 886. This method for producing CG irons widens the magnesium window for CG formation, and utilizes a 5% magnesium ferrosilicon master alloy containing 8.5 to 10.5% titanium, 4.0 to 5.5% calcium, 1.0 to 1.5% aluminum, 0.20 to 0.35% cerium, 48.0 to 52.0% silicon, the balance being iron. The treatment of the liquid iron with this master alloy is done in a similar way to the treatment of regular ductile iron with 5% magnesium ferrosilicon. This means that sandwich, plunging, or open ladle methods are applicable. As with nodular cast iron, a ladle inoculation is necessary. The composition of the melt should be near eutectic, and the sulfur content should not exceed 0.035%. The compositions utilized may vary according to the treatment method, type of ladle, sulfur-content of the base iron, and treatment temperature.
Although this method of using magnesium ferrosilicon master alloys containing titanium as ladle additions has been used for almost 25 years, in actual practice, there is considerable concern about titanium contamination stemming from residual titanium in casting gates and risers that are subsequently used for re-melting. Since many potential CG iron foundries also pour ductile iron, the presence of unwanted titanium from titanium containing master alloys can be the source of considerable scrap ductile iron castings due to titanium contamination. Titanium carryover from CG production will ruin the properties of ductile iron. Hence, these concerns have prevented large-scale conversions to CG irons.
To eliminate the need for titanium bearing master alloys, with recent developments in computer aided thermal analysis of cooling molten iron, it was now possible to achieve improved control over magnesium levels. Although the use of these relatively sophisticated thermal analysis techniques has made it somewhat easier to control re
Mai Ngoclan T.
Renner , Otto, Boisselle & Sklar, LLP
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