Machine-Fabricated Carbon and Graphite

11 Machine-Fabricated Carbon and Graphite DONALD E. CORAH and JOHN C. DAVIDSON

Downloaded by FUDAN UNIV on February 14, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch011

Technical Department, Airco Speer Carbon-Graphite, A Division of Airco, Inc., St. Marys, Pa. 15857

Information is presented on the fabrication of carbon and graphite parts with specific properties for use in a variety of mechanical, electrical, and chemical applications. Graphite is manufactured with unique properties, which can be varied by the method of manufacture and types of raw materials. Almost every part is machined to some degree because of the high percent of shrinkage, 3 to 10 percent, during the baking and graphitizing operations. Different types of machining equipment are used to obtain a desired shape and/or finish on graphite parts. Discussion Manufacture and Properties. Figure 1 illustrates the processing cycle for carbon and graphite, requiring up to nine months to manufacture. Carbon is formed by either extruding or molding p r i m a r i l y petroleum coke with a pitch binder and thermally processing (baking and graphitizing) the formed amorphous carbon to polycrystalline graphite. Lampblacks and natural graphites are two alternate raw material f i l l e r s with different electrical properties which are used p r i m a r i l y in manufacturing brushes and electrical contacts. Pitch impregnation is an optional step for densifying graphite. Figure 2 shows a range of properties obtainable with different graphites. Steel furnace electrodes are produced with large carbon particles and a low C T E level for withstanding thermal shock. Nuclear moderator blocks have a high density for reflecting neutrons and are isotropic to increase irradiation stability. Metal melting crucible graphite is fine grained to resist metal penetration and oxidation. Molded high-density graphite is used for rocket nozzles because of its flaw-free 122

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

11.

CORAH AND DAVIDSON

Carbon and Graphite

I

PETROLEUMr COKE I

CALCINING ft*2400f 1

I

COAL TAR h PITCH 1

CRUSHING GRINDING WEIGHING

123

CRUSHING GRINDING WEIGHINGl

MIXING I-

PITCH IMPREGNATION


EXTRUSION OR MOLDING

BAKING TO

1500* F

GRAPHITIZATION TO5000 'F

^IMACHININGI

Figure 1.

Application

Forming Method

Manufacture of graphite

Electrical discharge machining electrodes

Electric steel furnace electrode

Nudeer moderator block

Metal melting crucible

Missile rocket nozzle

Extruded

Extruded

Extruded

Molded

Molded

Maximum Particle Size, in.

0.250

0.066

0.008

0.003

0.0008

Apparent Density, g/cc.

1.64

1.76

1.73

1.77

1.80

Specific Resistance, χ 10* ohm. in.

35

36

40

48

42

Flexural Strength, psi With Grain

1100

2500

4300

3800

8800

900

2400

3700

3000

6600

5

Against Grain CTE, χ KH*C*1

With Grain

1.8

5.3

1.8

3.3

4.4

Against Grain

2.9

6.0

3.0

4.3

4.6

ScJeroscope Hardness Impurities, parts per million

30 8000

Figure 2.

46

50

46

66

500

20

1000

1600

Typical physical properties


PETROLEUM DERIVED CARBONS

124

structure and resistance to erosion. Ultrafine grain, highstrength graphite is required for producing intricate detail in electrical discharge machining electrodes. Machining Graphite. Figure 3 shows typical tolerances and finishes that are obtainable on graphite parts machined with various types of equipment. The RMS surface finish is determined visually, as graphite s inherent porosity does not lend itself to measuring the finish with a profilometer. The tool types, feed rates, and machine speeds generally used in machining graphite on a production basis are shown. These operating conditions are suitable for machining the majority of specialty parts; and the machining methods depend on the configuration, tolerances, and actual application. Graphites are readily machinable with most cutting tools.


1

Fabricated Graphite With Specific Properties F o r Numerous Applications. Figure 4 illustrates typical parts for mechanical applications. Graphite is used as a nozzle in rocket motors because of its ability to withstand temperatures above 5, 000 F. The nozzle inside diameter is contoured on a lathe using a tracer bar to accurately f o r m the desired profile for exiting hot gases during the firing of the rocket nozzle. The nonferrous metal continuous-casting die has a honed inside diameter for producing a surface finish that increases die life by reducing the coefficient of friction. The smooth finish decreases the tendency of molten metal to adhere to the graphite. Graphite's good lubricity enables metal bars to be cast with smooth surfaces, free of tear marks. Graphite crucibles are excellent for melting, drawing, and refining metals because of graphite s chemical inertness to almost everything except very strong oxidizing agents. Some crucibles are produced f r o m high-purity graphite, containing less than 20 parts per m i l l i o n total impurities, for processing pure metals and isotopes. High-purity graphite is machined without lubricants, handled with sanitary protective gloves, and transferred in polyethylene bags to insure high purity. The semiconductor industry uses precision machined fixtures having numerous holes requiring diameter and location tolerances of plus or minus 0.001 inch or less. Graphite's resistance to wetting by glass and its ease of machining to close tolerances make it an excellent material for processing hermetically sealed integrated circuits. Representative graphite parts for electrical applications 1


CORAH AND DAVIDSON

Carbon and Graphite


11.


125


12β

PETROLEUM

DERIVED CARBONS

are shown in Figure 5. Graphite's low specific resistance and strength that increases with temperature are desirable for fabricating resistance furnace heaters. The heater shown illustrates typical lathe and milling operations required in machining graphite parts. Since the specific resistance of graphite is not constant, as it is not a homogeneous material, the heater diameter or the slot width has to be varied in machin ing to manufacture heaters to a desired total resistance. Heaters are generally used in an inert atmosphere or vacuum because graphite w i l l oxidize at temperatures above 750 F. Carbons are used as sliding electrical contacts because of their low wear rates. The pantograph shoe contact represents a very hard carbon m a t e r i a l not readily machinable. Diamond tools are the practical way to machine carbon, generally gasbaked materials, with a Scleroscope hardness over 70. Carbon can be shaped to size in the green state, if liberal tolerances are acceptable, before baking into a hard amorphous carbon. Graphite has electrical and mechanical properties ideal for electrodes used in electrical discharge machining (EDM). Graphite is machined into complex electrode designs, some with cross sections as small as 0.005 inch. Graphite E D M electrodes are used for machining complex mold and die cavities at a very fast metal removal rate and low electrode wear com pared to metal electrodes. Graphite electrodes of much larger size and mass are used for producing steel in electric arc furnaces. Graphite has the ability to withstand thermal stresses over a 5, 000 F. tem perature range. The electrode ends, along with a connecting pin, are machined with precision threads for withstanding severe mechanical stresses and for obtaining a low resistance joint. Figure 6 shows several graphite parts employed in the chemical industry. Hexagonal shaped blocks with a grid of holes are used as a core in nuclear reactors. P r e c i s i o n holes are machined in the core by gun d r i l l s . Graphite is an excellent nuclear core material because of its very low neutron capture cross section. Graphite fluxing tubes are extruded to size on the outside and inside diameters. The tube is suitable for use with a selftapping metal connector for attaching the gas line to the tube. Graphite is inert to fluxing gases that are passed through molten metal and will not contaminate the metal. Fluxing tubes are also produced with a plugged end and radially drilled vent holes for obtaining a better dispersion of fluxing gases. Anodes are used in the electrolytic process for chlorine


CORAH AND DAVIDSON


11.

Carbon and Graphite

127

Figure 4. Graphite for mechanical applications

Figure 5. Graphite for electrical applications

MEAT EXCHANGER

Figure 6. Graphite for chemical applications


PETROLELM DERIVED CARBONS

128

production. Graphite is a preferred material because of its electrical characteristics along with its good resistance to chemical attack. Various slot designs are machined into graphite blocks for producing electrolytic cells. Graphite,with its excellent resistance to corrosive fluids and its high thermal conductivity of typically 100 B T U h r " f t " F " l ft, is used for heat exchangers. Heat exchanger segments, are precision machined and gang drilled so when stacked together they w i l l produce a cylindrically shaped heat exchanger. Two different fluids can be passed through radial and longitudinal holes to effect heat transfer between the fluids without cross contamination.


1

2

Conclusion Graphite is manufactured with properties that make it a versatile material for use in mechanical, electrical, and chemical applications. In general, a l l graphite parts require some machining to obtain the multiplicity of diverse shapes, designs, and tolerances required in industrial applications. In contrast, carbon has much more limited use and is generally made to shape or shaped with diamond-tipped tools.


Machine-Fabricated Carbon and Graphite

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