Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-08-04
2001-12-25
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S252000
Reexamination Certificate
active
06332903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to materials processing machine components, particularly cylinders used therefor. Still more particularly, the present invention relates to a materials processing cylinder containing titanium carbide.
2. Description of the Prior Art
Materials processing cylinders, such as used for injection and extrusion machines, plastics injection molding and extrusion processes, chemical processes, rubber, elastomer and magnesium processing, and food processing, are subjected to high temperatures, wear and corrosion attack from the materials being processed. Further, screws used for processing work material (as for example plastic) cause galling as the screw rotates within the cylinder. Additionally, such cylinders must also exhibit high strength, on the order of withstanding 30,000 psi at 1,000 degrees F. Still further, such cylinders must be cost effective and yet be highly resistant to failure when exposed to wear, corrosion, high pressure and high temperature.
Since there is no material known which meets all of the above requirements, the art has adopted the following manufacturing techniques.
A first technique is known as a “nitrided cylinder”. As shown at
FIGS. 1 and 1A
, a solid, single piece cylinder
10
is provided, composed of nitrided steel. The inside surface
12
of the cylinder
10
is nitrided and then machine polished to provide a nitrided skin
14
, typically about 0.010 inches thick. Problematically, the nitrided skin
14
is easily spalled off, has poor corrosion resistance, and has high manufacturing cost for larger size cylinders.
A second technique is known as a “bimetallic cylinder”, and is the most popular technique. As shown at
FIG. 2
, a composite cylinder
20
is provided having an outside layer
22
composed of a high strength steel, and an inside layer
24
composed of an inlay alloy metallurgically bonded thereto and typically about 0.065 inches thick, and wherein the outside layer
22
serves as a backing for the inside layer
24
.
The centrifugal casting process used for forming the inside layer
22
is shown at
FIGS. 3A and 3B
. An inlay alloy material in the form of shots, ingots or powder
26
is distributed along the inside surface
28
of the outside layer
22
. Then, the outside layer
22
is slowly rotated and heat Q is applied to achieve a temperature above 2,200 degrees F. The outside layer
22
is then spun at room temperature to provide, for example, a centrifugal force of over 70G's. During this process, the inlay alloy melts and forms a uniform coating, which upon cooling, provides the inner layer
24
. Typical inlay alloys are: Fe—Ni—B, having a hardness of Rc 60-65, and having good wear resistance, but poor corrosion resistance; Co—Ni—B, having a hardness of Rc 48-55, and having good corrosion resistance, but poor resistance to abrasion; and, carbides, mostly with tungsten carbide powder bound by a Ni—Cr—B matrix alloy with a lower melting point, which inlay alloy is superior to the first two inlay alloys, as described in U.S. Pat. No. 3,836,341 and 4,430,389 which are hereby herein incorporated by reference.
Due to an increase in the use of technical plastics containing abrasive and corrosive additives, such as high glass fiber fillings, pigments, UV stabilizers, and flame retardants, the inside surface of processing cylinders are subjected to severe wear and corrosion conditions. The bimetallic cylinders discussed in U.S. Pat. Nos. 3,836,341 and 4,430,389 utilize tungsten carbide to provide corrosion and wear resistance, but nevertheless suffer from the following disadvantages.
1. The centrifugal force of the bimetallic cylinder casting process tends to spin high density tungsten carbide to the outer wall of the inner cylinder. Consequently, there are fewer tungsten carbide particles at the machine finished inside surface of the inner cylinder.
2. Tungsten carbide has a relatively high coefficient of friction. Therefore, frictional wear of the processing screw flight surface occurs where contact is made with the inner surface.
3. Tungsten carbide has a relatively high density and high cost per pound weight.
4. Tungsten carbide does not exhibit good corrosion resistance to hydrochloric and hydrofluoric acid atmosphere attacks, a condition that is presented by most resins.
Accordingly, what is needed is a material for a materials processing cylinder composed of a material which is not subject to the above recounted disadvantages.
SUMMARY OF THE INVENTION
The present invention is a materials processing cylinder having an internal lining composed of a titanium carbide alloy suitable for high pressure (as for example around 30,000 psi (207 MPa)), high temperatures (as for example around 500 to 1,200 degrees F. (260 to 649 degrees C.), extreme wear (as for example from glass fiber fillers and abrasive additives), corrosive atmospheres (as for example hydrochloride and hydrofluoride), and magnesium injection and casting processes.
Titanium carbide has a micro hardness of HV(50 kg) 3,000 as compared to HV(50 kg) 2,200 for tungsten carbide, giving titanium carbide a superior level of wear resistance. The frictional coefficient of titanium carbide is about 35 percent to about 40 percent lower than the coefficient of friction of tungsten carbide, resulting in titanium carbide providing less galling wear against the contacting screw surfaces as compared with tungsten carbide. Titanium carbide is further corrosion resistant in low pH environments, as for example occurring in hydrochloric acid and hydrofluoric acid environments. Still further, titanium carbide is known not to alloy with magnesium, therefore rendering titanium carbide an ideal composition for materials processing cylinders utilized in magnesium injection molding and die casting operations. Lastly, titanium carbide has a lower density (4.91 gm/cm
3
) than the density (15.67 gm/cm
3
) of tungsten carbide. As a result, there is less gravitational segregation at the surface
It is known that titanium carbide is a refractory metal that is very difficult to fuse and melt in an air atmosphere. Accordingly, titanium carbide cannot be processed to form a bimetallic materials processing cylinder using a conventional air melt centrifugal cast bimetallic fabrication process. Therefore, the present invention provides a bimetallic materials processing cylinder formed of a titanium carbide alloy via a vacuum melt process, for example at a pressure of about 10
−4
Torr. The titanium carbide alloy is for example up to about 58 volumetric percent titanium carbide and the remaining volumetric percent is a predetermined bonding alloy.
An example of preferred titanium carbide alloys are given in Table 1.
TABLE 1
Component
Percent Weight
Titanium Carbide
24.80 to 53.60
Nickel
44.30 to 29.20
Chromium
13.50 to 6.00
Silicon
3.60 to 1.75
Boron
2.85 to 1.75
Molybdenum
3.70 to 1.38
Copper
1.50 to 0.46
Iron
3.00 maximum
Carbon
0.45 maximum
Other
2.00 maximum
A hardness test was performed on the titanium carbide alloys of Table 1, indicating a Rockwell C scale of between 62 and 66, which is acceptable for severe wear service. Further, a wear test was performed on the titanium alloys of Table 1 using the ASTM G77 block-on-ring method, modified to a load of 300 pounds and 20,000 cycles duration, which test was similar to tests of two tungsten carbide alloys, as indicated in Table 2.
TABLE 2
Equiv.
Wear Scar Width
Volumetric Loss
Friction
Alloy
(mm)
(mm
3
)
Coefficient
Alloys of Table 1
0.631
0.0076
0.093
Alloy of Patent
0.669 to 0.687
0.0091 to 0.0098
0.143 to .145
4,430,389
(46-57 wt %
WC Grade)
Alloy of Patent
0.698 to 0.992
0.010 to 0.029
over 0.13
3,836,341
(30-45 wt %
WC Grade)
For purposes of evaluation, less scar width and less volumetric loss translates into less wear loss; and lower frictional coefficient translates into less tendency towards galling wear at contacting screw surfaces. The test results indicate that the titanium carbide alloys of the present invention will perform better than tungsten carbid
Keefe Peter D.
Mai Ngoclan
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