Metal treatment – Stock – Copper base
Reexamination Certificate
1995-11-03
2004-04-06
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Copper base
C148S553000, C148S686000, C420S473000
Reexamination Certificate
active
06716292
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to continuous cast Cu—Ni—Sn spinodal alloys, and more particularly to a method for producing a continuous cast Cu—Ni—Sn spinodal alloy wherein it is unnecessary to subject the billet or rod to wrought processing prior to the spinodal heat treatment. The Cu—Ni—Sn spinodal alloy is characterized by a substantial absence of discontinuous &ggr;′ phase precipitate at its grain boundaries. The symbol &ggr;′ corresponds to the metastable coherent discontinuous precipitate referred to in the prior art having elevated percentages of Ni and Sn. The superscript distinguishes &ggr;′ from &ggr; which is also an elevated percentage Ni and Sn precipitate which is both stable and incoherent and differs from &ggr;′ in that it does not cause embrittlement while adding strength.
It has been known to be beneficial to all casting and metal working schemes to have the grain boundaries be as thin and grain size as small as possible. For this reason, it was considered desirable to develop an arrangement which would readily facilitate obtaining such fine grain structures in continuous cast copper alloy rods and tubes. Such rods and tubes would then satisfactorily accommodate subsequent cold drawing or working or would exhibit better properties than other unwrought materials. Thus, U.S. Pat. No. 4,315,538 disclosed a method and apparatus to effect a fine grain size in continuous cast metals. This method involved the use of a continuous casting die totally submerged in a reservoir of liquid alloy material and the use of feed openings in the die arranged so that the liquid metal entering the die would impart a generally cyclonic motion at the interface zone between the liquid and solid alloy material. This cyclonic motion caused shearing of primary dendrites in the alloy material from adjacent the internal side wall of the die and distributed those dendrites across the interface zone to provide nuclei for equiaxed crystals, thereby preventing the formation of thermal gradients in the alloy material of a sufficient magnitude to produce gross directional solidification at the interface zone.
The subject of U.S. Pat. No. 5,279,353 was a die construction for use with the same type of continuous casting apparatus, but with an improved ability to produce a fine grain structure in tubes with wall thicknesses greater than 0.5 inch, as well as in other cast shapes, such as round shaped rods, billets, or non-round rods and billets. We estimate the grain size of the resultant cast shape to be greater than 20 &mgr;m, possibly as large as 40 &mgr;m, but still substantially smaller than shapes cast by other means.
We have subsequently discovered that an additional benefit for producing copper metallic alloys according to the continuous cast method set out in our U.S. Pat. No. 5,279,353, said metallic alloys composed of small, equiaxed crystals, relates to the production of copper alloys requiring spinodal decomposition type phase transformation to achieve desired physical properties.
Spinodal decomposition type phase transformation in a multicomponent alloy system is described in U.S. Pat. No. 3,806,336 issued Apr. 23, 1974; U.S. Pat. No. 3,954,519 issued May 4, 1976; and U.S. Pat. No. 4,171,978 issued Oct. 23, 1979. As described in those patents, a certain binary and other metallic has, in its composition diagram, a “limit of metastability” or “spinodal” which is thermodynamically defined as the locus of disappearance of the second derivative of the chemical free energy with respect to composition of the system. When a high-temperature composition, which is of homogeneous single-phase structure, of the alloy is brought within the spinodal in a low temperature range, it is transformed into a separated two-phase structure, the phase separation being called spinodal decomposition. The decomposed alloy has a periodic microstructure generally in the order of hundred of angstroms and which consists of composition modulated two isomorphous phases in which one phase is in the form of a fine precipitate uniformly distributed in another phase which forms the matrix.
It is known that an alloy requiring a spinodal transformation must have a homogeneous composition throughout the entire alloy. Within the homogeneous volume, it is possible, by thermal treatment, to cause a shift in atomic concentration of certain of the solute metals comprising the alloy. Such a change, spinodal decomposition, imparts new physical properties to the alloy.
One skilled in the art is aware that microsegregation of solute elements results in areas exhibiting various responses to spinodal heat treatments. Typically, continuous cast billets or other castings always exhibit gross inverse segregation as well as “coring” or microsegregation within dendritic cells. To render such billets or castings fit for spinodal treatment, the metal parts have in the past been subjected to wrought processing to reduce microsegregation by “kneading” the material to mechanically reduce the secondary inter-dendritic distances. The wrought processing typically involves rolling, drawing, or pilgering to reduce the cross-sectional area dimensions by 40-90%. When very large degrees of cold working are employed to effect a 40-90% reduction in the cross-sectional area, however, it is very costly or even impossible to produce alloy parts large enough to function in many applications.
Further wrought processing cannot overcome the gross inverse segregation exhibited by alloys that have wide freezing ranges. Because these alloys have wide freezing ranges, concentration fluctuations of solute elements over a given distance within the body of the alloy are too large to effectively eliminate by solution heat treatment; consequently, these alloys will not spinodally decompose; furthermore, they are susceptible to forming other embrittling metastable phases via discontinuous precipitation.
By use of our continuous cast process to form billets, rods, or tubes, in combination with our method for manufacturing the Cu—Ni—Sn spinodal alloy, it is now possible to avoid all of the aforementioned difficulties of the processes disclosed in the past, particularly the need to subject the metal parts to wrought processing prior to spinodal decomposition. A rod or tube manufactured according to our continuous cast process advantageously has the following characteristics: (1) uniform solute distribution from surface to center and (2) a greatly reduced secondary inter-dendritic arm spacing, which cannot be achieved by conventional means. The secondary inter-dendritic arm spacing of the crystals manufactured in accordance with our process is only one-tenth or less than that for other materials. Consequently, fluctuations in the solute element concentration in the homogenized alloy are minimized, thereby permitting the spinodal transformation of a greater volume of the alloy while simultaneously avoiding precipitation of other undesirable metastable phases which have an adverse effect on ductility and toughness. Further, by use of our method disclosed for manufacturing the Cu—Ni—Sn spinodal alloy, the resultant spinodal alloy is characterized by a substantial absence of discontinuous &ggr;′ phase precipitate at its grain boundaries. In this regard, it is now possible to produce rods having a cross-section greater than ⅜ inches, which can subsequently be thermally processed in an unwrought condition to develop high strength and ductility and are therefore suitable in the production of various components which include, among other things, journal bearings, wear plates, mold plates, and gravure printing rolls.
INCORPORATION BY REFERENCE
U.S. Pat. No. 5,279,353 is incorporated by reference herein as background information with respect to the present invention.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a Cu—Ni—Sn spinodal alloy which is continuous cast in such a manner as to effect small, equiaxed crystals, and subjected to various heat and aging treatments to effect spinodal decomposition typ
Nielsen, Jr. William D.
Nielsen, Sr. William D.
Castech, Inc.
Fay Sharpe Fagan Minnich & McKee LLP
Ip Sikyin
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