Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-06-07
2003-10-21
Dunn, Tom (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S116000, C164S117000, C164S286000, C164S289000, C164S418000, C164S459000, C148S555000, C427S228000
Reexamination Certificate
active
06634413
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for making metallic alloys such as nickel base superalloys into hollow tubes, cylinders, pipes, rings and similar tubular products by melting the alloys in a vacuum or under a low partial pressure of inert gas and subsequently centrifugally casting the melt under vacuum or under a low pressure of inert gas in molds machined from fine grained high density, high strength isotropic graphite revolving around its own axis. The method also relates to a centrifugal casting mold apparatus that includes an isotropic graphite mold.
BACKGROUND OF THE INVENTION
Nickel base superalloys fabricated in shapes such as seamless rings, hollow tubes and pipes find many engineering applications in jet engines, oil and chemical industries and other high performance components. Complex highly alloyed nickel base superalloys are produced in seamless ring configurations for demanding applications in jet engines such as turbine casings, seals and rings.
FIG. 1
shows a diagram of turbine casing
10
and a compressor casing
20
. The turbine casing
10
is made of high temperature nickel base superalloys. Attached
FIG. 2
also shows a diagram of a turbine casing
30
made of high temperature nickel base superalloys. Seamless rings can be flat (like a washer), or they can feature higher vertical walls (approximating a hollow cylindrical section). Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing.
The two primary processes for forging rings differ not only in equipment, but also in quantities produced. Also called ring forging, saddle-mandrel forging on a press is particularly applicable to heavy cross-sections and small quantities. Essentially, an upset and punched ring blank is positioned over a mandrel, supported at its ends by saddles. As the ring is rotated between each stroke, the press ram or upper die deforms the metal ring against the expanding mandrel, reducing the wall thickness and increasing the ring diameter.
In continuous ring rolling, seamless rings are produced by reducing the thickness of a pierced blank between a driven roll and an idling roll in specially designed equipment. Additional rolls (radial and axial) control the height and impart special contours to the cross-section. Ring rollers are well suited for, but not limited to, production of larger quantities, as well as contoured rings. In practice, ring rollers produce seamless rolled rings to closer tolerances or closer to finish dimensions.
FIGS. 3A-3G
show schematically the various steps of seamless rolled ring forging process operations.
FIG. 4
shows a ring rolling machine in operation.
FIGS. 3A-3G
show an embodiment of a seamless rolled ring forging process operation to make a ring
40
.
FIG. 3A
shows the ring rolling process typically begins with upsetting of the starting stock
42
on flat dies
44
at its plastic deformation temperature—in the case of grade 1020 steel, approximately 2200 degrees Fahrenheit to make a relatively flatter stock
43
.
FIG. 3B
shows that piercing the relatively flatter stock
43
involves forcing a punch
45
into the hot upset stock causing metal to be displaced radially, as shown by the illustration.
FIG. 3C
shows a subsequent operation, namely shearing with a shear punch
46
, serves to remove a small punchout
43
A to produce an annular stock
47
.
FIG. 3D
shows that removing the small punchout
43
A produces a completed hole through the annular stock
47
, which is now ready for the ring rolling operation itself. At this point the annular stock
47
is called a preform
47
.
FIG. 3E
shows the doughnut-shaped preform
47
is slipped over the ID (inner diameter) roll
48
shown from an “above” view.
FIG. 3F
shows a side view of the ring mill and preform
47
workpiece, which squeezes it against the OD (outer diameter) roll
49
that imparts rotary action.
FIG. 3G
shows that this rotary action results in a thinning of the section and corresponding increase in the diameter of the ring
40
. Once off the ring mill, the ring
40
is then ready for secondary operations such as close tolerance sizing, parting, heat treatment and test/inspection.
FIG. 4
shows a photograph of a ring
40
roll forging machine in operation.
Even though basic shapes with rectangular cross-sections are common, rings featuring complex, functional cross-sections are produced by machining or forging from simple rings to meet virtually any design requirements. Aptly named, these “contoured” rolled rings can be produced in many different shapes with contours on the inside and/or outside diameters.
Production of superalloy rings from forging billets requires multiple steps by ring rolling. These alloys are difficult to hot work and can be hot deformed with small percentage of deformation in each step of ring roll forging. After each deformation operation, the outside and inside diameters of the stretched ring need to be ground to remove oxidized layers and forging cracks before reheating the ring for the next cycle of hot forging. Because of the extensive fabrication steps involved, the production costs are very high and yields are low. Typically, a 60 inch diameter ring weighing 250 lbs. suitable for application as a large jet engine casing is produce by ring roll forging of a starting billet weighing 2000 lbs. The high loss of expensive materials during fabrication steps results in high cost of the finished products.
The conventional route of tube making typically includes argon-oxygen decarburization (AOD) melting, continuous casting, hot rolling, boring, and extrusion. This route is mainly used for the high volume production of tubes up to 250 mm diameter. However, complex nickel base superalloys that are prone to macrosegregation are difficult or impossible to hot work.
Centrifugal casting complements the conventional tube making process and also offers considerable flexibility in terms of tube diameter and wall thickness. The mechanical properties of centrifugally cast tubes are often equivalent to conventionally cast and hot-worked material. The uniformity and density of centrifugal castings approaches that of wrought material, with the added advantage that the mechanical properties are nearly equal in all directions. Although many engineering ferrous and non-ferrous alloys which are amenable to processing by air melting and casting can be conveniently processed in tubes by centrifugal casting in air. However, complex nickel base superalloys require melting and casting in vacuum. Furthermore, during high speed rotation of the centrifugal mold lined with high purity ceramics, the highly reactive nickel base superalloy melts are likely to cause cracking and spalling of the ceramic liner leading to formation of very rough, outside surface of the cast tube. The ceramic liners spalling off the mold are likely to get trapped inside the solidified superalloy tube as detrimental inclusions that will significantly lower fracture toughness properties of the finished products.
There is a need for an improved cost effective process for making highly alloyed complex such as nickel based superalloys as tubes and seamless rings with simple or contoured cross sections which can be inexpensively machined into final shapes suitable for jet engine and other high performance engineering applications.
The term superalloy is used in this application in conventional sense and describes the class of alloys developed for use in high temperature environments and typically having a yield strength in excess of 100 ksi at 1000 degrees F. Nickel base superalloys are widely used in gas turbine engines and have evolved greatly over the last 50 years. As used herein the term superalloy will mean a nickel base superalloy containing a substantial amount of the &ggr;′ (Ni
3
Al) strengthening phase, preferably from about 30 to about 50 volume percent of the &ggr;&pri
Ray Ranjan
Scott Donald W.
Dunn Tom
Lin I.-H.
Santoku America, Inc.
Stevens Davis Miller & Mosher LLP
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