ASM Metals Hand Book Volume 9 - Metallography and Microstructures, Notas de estudo de Engenharia Mecânica

Baixe ASM Metals Hand Book Volume 9 - Metallography and Microstructures e outras Notas de estudo em PDF para Engenharia Mecânica, somente na Docsity!

ASM

INTERNATIONAL ®

The Materials Information Company

Publication Information and Contributors

Metallography and Microstructures was published in 1985 as Volume 9 of the 9th Edition Metals Handbook. With the fifth printing (1992), the series title was changed to ASM Handbook. The Volume was prepared under the direction of the ASM Handbook Committee.

Fig. 1 As-Drawn hafnium crystal bar. Changes in grain orientation produce different colors when viewed under polarized light. Some twinning is also evident. Specimen was attack polished and heat tinted at ~425 °C ( °F). 180×. Courtesy of Paul E. Danielson, Teledyne Wah Chang Albany. Additional color micrographs can be found in the article "Color Metallography." in this Volume.

Authors and Reviewers

• Rafael Menezes Nunes UFRGS
• Hubert I. Aaronson Carnegie-Mellon University
• John K. Abraham LTV-Republic Steel Research Center
• N.R. Adsit Rohr Industries, Inc.
• Samuel M. Allen Massachusetts Institute of Technology
• P. Ambalal Lawrence Livermore National Laboratory
• R.J. Barnhurst Noranda, Inc. (Canada)
• Edmund F. Baroch Consultant
• Charles S. Barrett University of Denver
• Charles E. Bates Southern Research Institute
• R. Batich Brush Wellman Inc.
• Alan M. Bayer Teledyne VASCO
• Arlan O. Benscoter Bethlehem Steel Corporation
• Michael L. Bess Eastern Alloys, Inc.
• Michael B. Bever Massachusetts Institute of Technology
• C.R. Bird Stainless Foundry & Engineering, Inc.
• George A. Blann Buehler Ltd.
• Arne Boe Struers, Inc.
• William J. Boettinger National Bureau of Standards
• T.F. Bower Chase Brass & Copper Company
• Rodney R. Boyer Boeing Commercial Airplane Company

• Richard B. Gundlach Amax Research & Development Center
• Martin N. Haller Kennametal, Inc.
• William B. Hampshire Tin Research Institute, Inc.
• John Harkness Brush Wellman Inc.
• E. Harper Systems Research Laboratories
• Walter T. Haswell Colt Industries
• R.M. Hemphill Carpenter Technology Corporation
• John A. Hendrickson Wyman-Gordon Company
• Helen Henson Oak Ridge National Laboratory
• Tommy Henson Oak Ridge National Laboratory
• Dennis W. Hetzner The Timken Company
• James Hoag Abex Corporation
• William F. Hosford University of Michigan
• Helmut Hoven Institut für Reaktorwerkstoffe (West Germany)
• Norman S. Hoyer Westinghouse Electric Corporation
• Hsun Hu University of Pittsburgh
• James Lee Hubbard Georgia Institute of Technology
• Paul L. Huber Seco/Warwick Corporation
• Glenn S. Huppi Colorado School of Mines
• K.A. Jackson AT&T Bell Laboratories
• Mitchell A. Jacobs Taussig Associates, Inc.
• Hughston M. James Carpenter Technology Corporation
• N.C. Jessen Martin Marietta Energy Systems
• Wilbur Johns Rockwell International
• Mark J. Johnson Allegheny Ludlum Steel Corporation
• E.A. Jonas Consulting Metallurgical Engineer
• John J. Jonas McGill University (Canada)
• Jerald E. Jones Colorado School of Mines
• Frederick W. Kern U.S Steel Corporation
• Jon A. Kish Rhenium Alloys, Inc.
• Michael Kim Rhenium Alloys, Inc.
• Roger W. Koch Ladish Company
• Karl Koizlik Institut für Reaktorwerkstoffe (West Germany)
• T. Kosa Carpenter Technology Corporation
• J.A. Kowalik Lehigh University
• R. Wayne Kraft Lehigh University
• George Krauss Colorado School of Mines
• John B. Lambert Fansteel
• John A. Larson Ingersoll-Rand Company
• David E. Laughlin Carnegie-Mellon University
• James L. Laverick The Timken Company
• Harvie H. Lee Inland Steel Company
• Peter W. Lee The Timken Company
• Franklin D. Lemkey United Technologies Research Center/Dartmouth College
• William C. Leslie University of Michigan
• Jochen Linke Institut für Reaktorwerkstoffe (West Germany)
• Stephen Liu Pennsylvania State University
• Ken Lloyd D.A.B. Industries, Inc.
• Richard F. Lynch Zinc Institute, Inc.
• William L. Mankins Huntington Alloys International
• M.J. Marcinkowski University of Maryland
• A.R. Marder Bethlehem Steel Corporation
• James M. Marder Brush Wellman Inc.
• T.B. Massalski Carnegie-Mellon University

• M.S. Masteller Carpenter Technology Corporation
• John E. Masters American Cyanamid Company
• Daniel J. Maykuth Tin Research Institute, Inc.
• James L. McCall Battelle Columbus Laboratories
• George McClary H. Cross Company
• E.J. Minarcik Lead Industries Association, Inc.
• T.E. Mitchell Case Western Reserve University
• L. Mondolfo Rensselaer Polytechnic Institute
• L. Mongeon Noranda, Inc. (Canada)
• Jeremy P. Morse Huntington Alloys International
• William M. Mueller Colorado School of Mines
• Michael S. Nagorka Colorado School of Mines
• James A. Nelson Buehler Ltd.
• Hubertus Nickel Institut für Reaktorwerkstoffe (West Germany)
• B. Oliver University of Tennessee
• Oliver E. Olsen Lead Industries Association, Inc.
• T. Palomaki Honeywell Inc.
• W.B. Pearson University of Waterloo (Canada)
• Leander F. Pease III Powder-Tech Associates, Inc.
• John H. Perepezko University of Wisconsin at Madison
• A. Jeffrey Perkins Naval Postgraduate School
• Robert N. Peterson Enduro Stainless, Inc.
• G. Petzow Max-Planck-Institut für Metallforschung (West Germany)
• Mark Podob Abar Ipsen Industries
• Larry E. Pope Sandia National Laboratories
• C.E. Price Oklahoma State University
• S.M. Purdy National Steel Corporation
• Dennis T. Quinto Kennametal, Inc.
• M.R. Randlett Chase Brass & Copper Company
• W.P. Rehrer Carpenter Technology Corporation
• R. Ricksecker Chase Brass & Copper Company
• N. Ridley University of Manchester (England)
• H.C. Rogers Drexel University
• Kempton Roll Metal Powder Industries Federation
• Alton D. Romig, Jr. Sandia National Laboratories
• Charles R. Roper, Jr. Lukens Steel Company
• H.W. Rosenberg Alta Group
• M. Rühle Max-Planck-Institut für Metallforschung (West Germany)
• Moy Ryvola Alcan International, Ltd. (Canada)
• N. Saenz Battelle Pacific Northwest Laboratories
• Anant V. Samudra LTV Steel Company
• L.E. Samuels Samuels Consulting (Australia)
• Ernest A. Schoefer Technical Consultant
• J. Schruers Westinghouse Electric Corporation
• D.D. Schwemmer Rockwell International
• Brian Scott International Tin Research Institute (England)
• J. Self Colorado School of Mines
• Jerome F. Smith Lead Industries Association, Inc.
• William A. Soffa University of Pittsburgh
• Peter D. Southwick Inland Steel Company
• R.E. Spear Aluminum Company of America
• G.R. Speich Illinois Institute of Technology
• D.L. Sponseller Amax Research & Development Center
• E.E. Stansbury University of Tennessee

• J.E. Costa Carnegie-Mellon University
• S.L. Couling Battelle Columbus Laboratories
• A. Datta University of Pittsburgh
• L.W. Davis NETCO
• L. Delaey Katholieke Universiteit (Belgium)
• K. Detert Vacuumschmelze Siemens (West Germany)
• J. Dibee Chase Brass & Copper Company, Inc.
• J.E. Gatehouse Bethlehem Steel Corporation
• J.J. Gilman Allied Chemical Corporation
• R.C. Glenn U.S. Steel Corporation
• S.R. Goodman U.S. Steel Corporation
• F.E. Goodwin International Lead Zinc Research Organization
• N. Grant Massachusetts Institute of Technology
• G. Grosse Chase Brass & Copper Company, Inc.
• N. Hansen Riso National Laboratory (Denmark)
• W.C. Harrigan DWA Composite Specialties
• M. Hatherly University of New South Wales (Australia)
• M. Henry General Electric Research & Development
• D. Hull University of Liverpool (England)
• J. Humphries University of Oxford (England)
• M.S. Hunter Alcoa Research Laboratories
• F.I. Hurwitz NASA Lewis Research Center
• G. Ibe Vacuumschmelze Siemens (West Germany)
• S. Jin AT&T Bell Laboratories
• A.R. Jones Riso National Laboratory (Denmark)
• Anwar-ul Karim Engineering University (Bangladesh)
• R.S. Karz University of Illinois
• T.J. Kelly International Nickel Company, Inc.
• J.R. Kilpatrick Bethlehem Steel Corporation
• M. Kitada Hitachi Ltd. (Japan)
• J.W. Koger Martin Marietta
• M.M. Lappin Sandia National Laboratories
• P.K. Lattari Texas Instruments, Inc.
• M. Lee San Jose State University
• P.R. Lee NASA Ames Research Center
• I. Lefever Katholieke Universiteit (Belgium)
• D.S. Lieberman University of Illinois
• J.D. Livingston General Electric Research & Development
• A.C. Lon Phillips Petroleum Company
• T. Long Boeing Commercial Airplane Company
• D.M. Maher AT&T Bell Laboratories
• A.S. Malin University of New South Wales (Australia)
• J.J. Manganello Chrysler Corporation
• M.E. McAllaster Sandia National Laboratories
• H. McQueen Sir George Williams University (Canada)
• D. Metzler University of Pittsburgh
• J.T. Michalak U.S. Steel Corporation
• M.K. Miller Oak Ridge National Laboratory
• P.N. Mincer Battelle Columbus Laboratories
• L.R. Morris Alcan Kingston Laboratories (Canada)
• R. Moss Ford Aerospace and Communications Corporation
• A.W. Mullendore Sandia Corporation
• G. Müller Struers GmbH (West Germany)
• A. Needleman Brown University

• J.R. Patel AT&T Bell Laboratories
• N.E. Paton North American Rockwell Corporation
• H.W. Paxton U.S. Steel Corporation
• J.F. Peck Massachusetts Institute of Technology
• L. Penn Midwest Research Institute
• R.L. Perry Bethlehem Steel Corporation
• W.G. Pfann AT&T Bell Laboratories
• V.A. Phillips General Electric Company
• K.M. Prewo United Technologies Research Center
• S.V. Ramani NASA Ames Research Center
• B.B. Rath U.S. Steel Corporation
• T. Redden General Electric Company
• W. Reinsch Timet
• W.H. Rowley, Jr. The Stackpole Corporation
• M.A. Scherling University of Illinois
• C. Scholl Wyman-Gordon Company
• M. Scott Bethlehem Steel Corporation
• G. Shaw Midwest Research Institute
• D. Shechtman Technion, Israel Institute of Technology
• M.J. Shemanski AT&T Bell Laboratories
• H.M. Shih NASA Ames Research Center
• J.W. Shilling Allegheny Ludlum Steel Corporation
• V.L. Shultes Boeing Vertol Company
• J.R. Sims Square D Company
• D.P. Skinner Princeton Gamma-Tech, Inc.
• E. Snell Lawrence Livermore National Laboratory
• R.L. Snyder Bendix Aircraft Brake and Strut Division
• C.N. Su The Aerospace Corporation
• D.A. Thomas Massachusetts Institute of Technology
• G. Thomas University of California--Berkeley
• D. Tyler Olin Corporation Metals Research Laboratories
• J.L. Uvira Steel Company of Canada, Ltd.
• J.M. Van Orden Lockheed Corporation
• G.B. Wadsworth Boeing Vertol Company
• E. Walden Lockheed Corporation
• H. Warlimont Max-Planck-Institut für Metallforschung (West Germany)
• B. Weinberger Struers, Inc.
• J. Williams North American Rockwell Corporation
• J.C. Williams Carnegie-Mellon University
• D.J. Willis Broken Hill Proprietary Company, Ltd. (Australia)
• P. Wingert GTE Products Corporation
• W.N. Wise NLO Inc.
• G.J. Wiskow Falk Corporation
• D.A. Witmer University of Denver
• W.A. Wong McGill University (Canada)
• J.H. Wood General Electric Company
• S.A. Wright Bethlehem Steel Corporation
• P. Yaffe Chase Brass & Copper Company, Inc.
• K.P. Young ITT Engineered Metal Processes
• A. Zeltser University of Pittsburgh
• J.E. Zimmer Acurex Corporation, Aerotherm Division

information on the characteristics and constituents of various alloy systems, and a series of representative micrographs are presented in each article. Also included in this Section is an in-depth discussion of the metallography of metal-matrix and resin-matrix fiber composite materials.

The science of metallography lies in the interpretation of structures, which is thoroughly reviewed in the final Section, "Structures." Following an introductory overview of the subject, 18 articles deal with the principles underlying metallographic structures. Among the microstructural features of metals discussed are:

• Solidification structures, including those of pure metals, solid solutions, eutectic alloys, steels, aluminum alloy ingots, and copper alloy ingots
• Transformation structures, including structures resulting from precipitation from solid solution, spinodal structures, massive transformation structures, eutectoid structures, bainitic structures, martensitic structures, peritectic structures, and ordered structures
• Deformation and annealing structures, including structures resulting from plastic deformation, from plastic deformation at elevated temperature, and from recovery, recrystallization, and grain growth
• Textured structures
• Crystal structures

By virtue of its comprehensive coverage of metallographic techniques and the representation and interpretation of microstructures, metallurgical engineers and technicians should find this Volume a valuable reference work. Undergraduate and graduate students involved in physical metallurgy and/or microscopy coursework should also find it useful.

ASM is grateful to the many authors and reviewers who gave freely of their time and knowledge and to the dozens of engineers and metallographers who contributed the thousands of micrographs published in this Volume. Special thanks are due to Robert J. Gray, George F. Vander Voort, and Paul E. Danielson for their extraordinary efforts and assistance throughout this project. Publication if this Volume would not have been possible without the valuable contributions of all these individuals.

The Editors

General Information

Officers and Trustees of the American Society for Metals (1984-1985)

Officers

• John W. Pridgeon President and Trustee Consultant
• Raymond F. Decker Vice President and Trustee Michigan Technological University
• M. Brian Ives Immediate Past President and Trustee McMaster University
• Frank J. Waldeck Treasurer Lindberg Corporation

Trustees

• Herbert S. Kalish Adamas Carbide Corporation
• William P. Koster Metcut Research Associates, Inc.
• Robert E. Luetje Armco, Inc.
• Richard K. Pitler Allegheny Ludlum Steel Corporation
• Wayne A. Reinsch Timet
• C. Sheldon Roberts Consultant Materials and Processes
• Gerald M. Slaughter Oak Ridge National Laboratory
• William G. Wood Technology Materials
• Klaus M. Zwilsky National Materials Advisory Board National Academy of Sciences

• Edward L. Langer Managing Director
• Allan Ray Putnam Senior Managing Director

Members of the ASM Handbook Committee (1984-1985)

• Thomas D. Cooper (Chairman 1984- ; Member 1981-) Air Force Wright Aeronautical Laboratories
• Roger J. Austin (1984-) Materials Engineering Consultant
• Deane I. Biehler (1984-) Caterpillar Tractor Company
• Rodney R. Boyer (1982-) Boeing Commercial Airplane Company
• Wilson G. Dobson (1982-) Binary Engineering Associates
• Jess F. Helsel (1982-) Helsel Metallurgical, Inc.
• John D. Hubbard (1984-) HinderTec, Inc.
• Dennis D. Huffman (1983-) The Timken Company
• Conrad Mitchell (1983-) United States Steel Corporation
• David LeRoy Olson (1982-) Colorado School of Mines
• Ronald J. Ries (1983-) The Timken Company
• Derek E. Tyler (1983-) Olin Corporation
• Leonard A. Weston (1982-) Lehigh Testing Laboratories, Inc.

Previous Chairmen of the ASM Handbook Committee

Previous Chairmen of the ASM Handbook Committee

• R.S. Archer (1940-1942) (Member, 1937-1942)
• L.B. Case (1931-1933) (Member, 1927-1933)
• E.O. Dixon (1952-1954) (Member, 1947-1955)
• R.L. Dowdell (1938-1939) (Member, 1935-1939)
• J.P. Gill (1937) (Member, 1934-1937)
• J.D. Graham (1966-1968) (Member, 1961-1970)
• J.F. Harper (1923-1926) (Member, 1923-1926)
• C.H. Herty, Jr. (1934-1936) (Member, 1930-1936)
• J.B. Johnson (1948-1951) (Member, 1944-1951)
• L.J. Korb (1983) (Member, 1978-1983)
• R.W.E. Leiter (1962-1963) (Member, 1955-1958, 1960-1964)
• G.V. Luerssen (1943-1947) (Member, 1942-1947)
• Gunvant N. Maniar (1979-1980) (Member, 1974-1980)
• James L. McCall (1982) (Member, 1977-1982)
• W.J. Merten (1927-1930) (Member, 1923-1933)
• N.E. Promisel (1955-1961) (Member, 1954-1963)
• G.J. Shubat (1973-1975) (Member, 1966-1975)
• W.A. Stadtler (1969-1972) (Member, 1962-1972)
• Raymond Ward (1976-1978) (Member, 1972-1978)
• Martin G.H. Wells (1981) (Member, 1976-1981)
• D.J. Wright (1964-1965) (Member, 1959-1967)

Staff

ASM International staff who contributed to the development of the Volume included Kathleen Mills, Manager of Editorial Operations; Joseph R. Davis, Senior Technical Editor; James D. Destefani, Technical Editor; Deborah A. Dieterich, Production Editor; George M. Crankovic, Assistant Editor; Heather J. Frissell, Assistant Editor; and Diane M. Jenkins, Word Processing Specialist. Editorial Assistance was provided by Robert T. Kiepura and Bonnie R. Sanders. The Volume was prepared under the direction of William H. Cubberly, Director of Publications, and Robert L. Stedfeld, Assistant Director of Publications.

SAN 204-

Printed in the United States of America

Sectioning

Introduction

SECTIONING, the removal of a conveniently sized, representative specimen from a larger sample, is one of five major operations in the preparation of metallographic specimens. The other operations are mounting (optional), grinding, polishing, and etching. In many ways, sectioning is the most important step in preparing specimens for physical or microscopic analysis.

Incorrect preparation techniques may alter the true microstructure and lead to erroneous conclusions. Because the microstructure should not be altered, conditions that may cause microstructural changes ideally should be avoided. However, hot and cold working accompany most sectioning methods.

The damage to the specimen during sectioning depends on the material being sectioned, the nature of the cutting device used, the cutting speed and feed rate, and the amount and type of coolant used. On some specimens, surface damage is inconsequential and can be removed during subsequent grinding and polishing. The depth of damage varies with material and sectioning method (Fig. 1).

Fig. 1 Depth of deformation in different metals due to cutting method. (Ref 1)

Sectioning methods discussed in this article include fracturing, shearing, sawing (using hacksaws, band saws, and wire saws), abrasive cutting, and electric discharge machining. Additional information can be found in Ref 1, 2, 3, 4.

Sectioning methods discussed in this article include fracturing, shearing, sawing (using hacksaws, band saws, and wire saws), abrasive cutting, and electric discharge machining. Additional information can be found in Ref 1, 2, 3, 4.

Fracturing

Fracture surfaces can be obtained by breaking specimens with blows of a hammer or by steadily applying pressure. Controlled fractures can be produced by impact or tension testing, and the location of the fracture can be controlled by nicking or notching the material. Less brittle materials can be cooled in liquid nitrogen before breaking to obtain a flatter surface. Fracturing has also been used on other brittle materials, such as carbides and ceramics.

Fracturing is not recommended, because it seldom follows desired directions, unless the sample is prenotched. Also, the fracture surface is the one usually prepared, and lengthy coarse grinding may be required to obtain a flat surface. Moreover, damage from fracturing can mask inherent features, obscuring the outside surface from microscopic examination.

Shearing (Ref 1)

Low-carbon sheet steel and other thin, reasonably soft materials can be cut to size by shearing, a fast, simple, effective sectioning technique. Although little heat is generated, shearing produces substantial deformation and is not recommended for materials sensitive to mechanical twin formation. The area affected by shearing must be removed by grinding.

Sawing

Sawing, perhaps the oldest sectioning method, can be performed using a hand-held hacksaw, a band saw, or an oscillating power hacksaw. Hand-held hacksaws or band saws, either vertical or horizontal, generally do not generate enough frictional heat to alter the microstructure; however, frictional heat can temper the blades enough to eliminate their cutting ability.

Power hacksaws are not appropriate in the metallographic laboratory. This type of sectioning equipment can irreparably damage a material, particularly if it is prone to deformation. A power hacksaw should be used only to cut a larger piece down so that a smaller piece can be subsequently sectioned by some other means. Saw-cut surfaces are rough, and coarse grinding is required to obtain a flat surface prior to fine grinding.

Although coolants should be used in any type of sectioning, band saw cutting can be performed without a coolant; the speed is slow enough that frictional heat is not detrimental to the material. In the case of power hacksaws, with their thicker and coarser blades, a coolant must be used, because the depth of deformation introduced by this severe method of sectioning can be quite deep.

Abrasive Cutting (Ref 2)

Abrasive cutting is the most widely used method of sectioning materials for microscopic examination and other material investigations. Conventional abrasive cutting using consumable wheels is the most popular method for routine metallographic sectioning, because it is fast, accurate, and economical.

The quality of the cut surface obtained is often superior to that obtained by other means, and fewer subsequent steps may be required. Metal-matrix diamond blades handle such specialized applications as ceramics, rocks, very hard metallics, and printed circuit boards. Methods of abrasive cutting offer various cutting characteristics useful for most material sectioning situations. Figure 2 illustrates a typical abrasive cutting machine.

Resin-bonded wheels, which have very high cutting rates, are generally used for dry cutting and find application in plant production cutting. Wet cutting wheels require a rubber or rubber-resin bond and are used in metallographic laboratories.

The rate of wheel deterioration depends on the type of bond used. Resin- and resinoid-bonded wheels generally break down more rapidly than rubber-bonded wheels. The rubber bond retains abrasive particles more tenaciously, resulting in slower wheel wear and more cuts per wheel. In addition, the rubber forms a solid bond; that is, there are no pores. However, resin used as a bond sets up in a polymerization process and there are extremely small pores throughout the wheel that may or may not be near abrasive grains. Therefore, resin-bonded wheels wear away faster, but always present a fresh cutting surface, because each abrasive grain is ejected before it becomes dull. The abrasive used is more important than the bond. Selection of bond is usually based on objections to the odor of burning rubber as the wheel degrades.

Two terms used in selecting abrasive cutoff wheels are "hard" and "soft." These terms do not refer to the hardness of the abrasive grains but to how the wheel breaks down. Silicon carbide (approximately 9.4 on the Mohs scale) and Al 2 O 3 (approximately 9.0) differ only slightly in hardness. A hard wheel (one made with hard bonding material) is usually best for cutting soft stock, but a soft wheel is preferred for cutting hard materials. A good general-purpose cutoff wheel is a medium-hard silicon carbide abrasive wheel.

In rubber-resin wheels, the amount of bonding material and the percentage of free space determine the hardness or wheel grade. A more porous, less dense (softer) wheel breaks down faster because the abrasive particles are held more loosely. Softer wheel's are used because fresh, sharp abrasive grains are more frequently exposed. Less porous, more dense wheels are harder, break down slower, and are better for softer materials.

Coolants. Water alone should not be used as a coolant for wet sectioning. A coolant should contain a water-soluble oil with a rust-inhibitor additive, which protects the moving parts of the cutoff machine, minimizes the possibility of burning, and produces better cuts. Some foaming of the coolant is desirable.

The preferred cooling condition is submerged sectioning, in which the entire piece is under water. Submerged sectioning is recommended for heat-sensitive materials that undergo microstructural changes at low temperatures. For example, as- quenched alloy steels with an untempered martensitic microstructure can readily transform to tempered martensite with the frictional heat developed. The quality of a submerged cut is excellent, and the specimens produced will not require extensive grinding. Section size, material, and hardness dictate whether submerged cutting can be employed. Submerged cutting will tend to make a wheel bond act harder.

Wheel speed must be carefully considered in the design of a cutter and the selection of wheels for a given cutter. In the interest of safety, maximum operating speeds printed on the specific blade or wheel should never be exceeded. Also, increased wheel speed may introduce frictional heat, which damages the microstructure.

Wheel edge wear may be used to determine whether the correct wheel has been selected. Abrasive wheels that show little or no wear are not performing satisfactorily. Controlled wheel loss indicates that the wheel bond is breaking down, exposing fresh abrasive grains for faster, more effective, and cooler cutting. Wheels that do not deteriorate fast enough may become glazed with specimen material, resulting in poor cutting and excessive specimen heating. Exerting additional pressure will most likely cause over-heating.

The acceptable rate of wheel loss is:

LR M W

=

where LR is wheel life ratio, M is area of material cut, and W is area of abrasive wheel consumed. In plant production cutting, resin-bonded wheels are commonly used without a coolant. Rate of cutting is the main concern, because this step probably precedes any heat treating. In this application, an M / W ratio of 1.5:1 is acceptable. In other words, 1.5 times more material should be cut as wheel area consumed.

Shelf Life. Rubber-bonded wheels have a definite shelf life, which ranges from 12 to 18 months, depending on storage and climatic conditions. The rubber has a tendency to harden and become brittle. Storing abrasive wheels in an extremely warm area hastens the degradation of the rubber, further reducing shelf life. Abrasive wheels should be removed from their shipping containers and laid flat on a rigid surface in a relatively dry environment; they should never be hung on a

wall or stored on edge, because warpage can occur. Resin-bonded wheels should be stored in the same manner as rubber- bonded wheels; a dry atmosphere is particularly important. Storage in a high-humidity area can lead to early disintegration of the resin bond, because resin can absorb moisture, which eventually weakens the bond.

Surface Damage. Abrasive-wheel sectioning can produce damage to a depth of 1 mm (0.04 in.). However, control of cutting speed, wheel pressure, and coolant application minimizes damage.

Nonconsumable Abrasive Cutting

The exceptional hardness and resistance to fracturing of diamond make it an ideal choice as an abrasive for cutting. Because of its high cost, however, diamond must be used in nonconsumable wheels. Diamond bort (imperfectly crystallized diamond material unsuitable for gems) that has been crushed, graded, chemically cleaned, and properly sized is attached to a metal wheel using resin, vitreous, or metal bonding in a rimlock or a continuous-rim configuration.

Metal-bonded rimlock wheels consist of metal disks with hundreds of small notches uniformly cut into the periphery. Each notch contains many diamond particles, which are held in place with a metal bond. The sides of the wheel rim are serrated and are considerably thicker than the core itself, a construction that does not lend itself to delicate cutting. When cutting more ductile materials, the blades will require more frequent dressing.

Rimlock blades are recommended for the bulk cutting of rocks and ceramics where considerable material loss may be tolerated. Kerosene or mineral spirits are used as the coolant/lubricant, and a constant cutting pressure or feed must be maintained to avoid damaging the rim.

Continuous-rim resin-bonded wheels consist of diamond particles attached by resin bonding to the rim of a metal core. These blades are suitable for cutting very hard metallics, such as tungsten carbide, and nonmetals, such as high- alumina ceramics, dense-fired refractories, and metal-ceramic composites. Water-base coolants are used.

Wafering Blades. For precision cutting of metallographic specimens or thin-foil specimens for transmission electron microscopy, very thin, small-diameter wafering blades are used. These blades are usually constructed of diamond, metal powders, and fillers that are pressed, sintered, and bonded to a metal core. Wafering blades are available in high and low diamond concentrations. Lower concentrations are better for harder materials, particularly the nonmetals; higher concentrations are preferred for softer materials.

Wafering blades may be used with diamond saws. Unlike some other methods of sectioning, the diamond saw uses relatively low speeds (300 rpm maximum) and a thin, continuous-rim diamond-impregnated blade to accomplish true cutting of nearly all solid materials. Applications include cutting of hard and soft materials, brittle and ductile metals, composites, cermets, laminates, miniature devices, and honeycombs. The as-cut surface is generally free of damage and distortion and is ready for microscopic examination with minimum polishing or other preparation. Figure 3 illustrates a typical low-speed diamond saw.

Fig. 3 Typical low-speed diamond saw. (Leco Corp.)

Wire Saws (Ref 3)

The need to produce damage-free, single-crystal semiconductor surfaces for the electronics industry has generated interest in using the wire saw in the metallographic laboratory. Applications include:

Fig. 7 Amorphous iron (Metglas) cut with a wire saw. Each laminate is 0.1 mm (0.004 in.) thick. (Laser Technology, Inc.)

In principle, a fine wire is continuously drawn over the sample at a controlled force. Cutting is accomplished using an abrasive slurry applied to the wire, a chemical solution (generally acidic) dripped onto the wire, or electrolytic action. Although cutting rates are much lower than those of abrasive cutoff wheels, hacksaws, or band saws, the deformation produced is negligible, and subsequent grinding and polishing is often not necessary.

Wire saws are available in a variety of designs. Some move the specimen into the wire, some move the wire into the specimen, some run horizontal, and some run vertical. A saw in which the wire runs vertical is advantageous if a specimen is to be removed from bulk material. In this case, the material is attached to an x-y table and is moved into the saw.

Various methods have been devised for drawing the wire across the specimen. The endless-wire saw consists of a loop of wire fastened together at its ends and driven in one direction (Fig. 8). The oscillating wire saw passes a wire back and forth across the sample, usually with a short stroke. A variation of this technique employs a 30-m (100-ft) length of wire that is fed from a capstan across the workpiece and back onto the capstan. The direction of the capstan is reversed at the end of each stroke. The capstan is further shuttled back and forth to maintain the alignment of the wire regarding the pulleys.

Abrasives. Any crystalline material can be used as an abrasive in wire sawing if the abrasive is harder than the specimen to be cut. Although natural abrasives, such as emery and garnet, have been used extensively, the best overall abrasive currently available is synthetic diamond. There are two methods for applying abrasives to the wire. Loose abrasive can be mixed with a liquid vehicle as a slurry to be applied at the kerf behind the wire, or the abrasive can be bonded to a stainless steel wire core.

In the first method, part of the abrasive remains with the specimen and erodes the wire. Furthermore, much of the abrasive is wasted, which precludes using diamond in a slurry. In the second method, all the abrasive moves with the wire to cut the specimen. Therefore, only a fixed quantity of abrasive is employed; diamond then becomes economically feasible. Figure 9 illustrates typical diamond-impregnated wires.

Fig. 8 Wire saw with an endless loop. (South Bay Technology, Inc.)

Lubricants. Water is used in wire sawing with diamond- impregnated wire. This is not used to lubricate the cut, nor is it used to prevent heat buildup. The amount of heat generated is negligible, and lubrication of the wire is unnecessary. Water is used to wash out the debris that would accumulate above the wire and prevent the easy exit of the wire when the cut is complete.

Force. As force is increased between the wire and the specimen, the bow in the wire increases, even though the wire is under maximum tension. Little is gained in cutting time by increasing the force. When the force is increased excessively, the bow becomes so great that the wire has a tendency to wander, which increases the kerf. When wandering occurs, more material is being cut away, and cutting time increases. This also shortens wire life. Therefore, high force with the resulting wider kerf is a poor alternative to lighter force with a straighter wire and a more accurate cut. Lighter force also yields a better finish. If the cut is to be flat at the bottom, the saw should be allowed to dwell for a short time with no force.

The force between the wire and the specimen ranges from 10 to 500 gf. As an example, for a specimen that is in limited supply, fragile, high priced, and/or delicate, a 0.08- mm (0.003-in.) diam wire impregnated with 8-μm diamonds would be selected. The force between the wire and the crystal would range from 10 to 35 gf. The tension on the wire would be 500 to 750 gf, and the wire would travel 20 to 30 m/min (60 to 100 ft/min).

When a firm, hard, tough specimen is to be cut and when surface damage poses little or no problem, the fastest and most economical method of cutting usually is best. For example, a 0.4-mm (0.015-in.) diam wire impregnated with 60-μm diamonds would be chosen. The tension on the wire would be approximately 6000 to 8000 gf. The machine would operate at 60 m/min (200 ft/min). The force between the wire and the specimen would range from 200 to 500 gf.

Electric Discharge Machining (Ref 4)

Electric discharge machining (EDM), or spark machining, is a process that uses sparks in a controlled manner to remove material from a conducting workpiece in a dielectric fluid (usually kerosene or transformer oil). A spark gap is generated between the tool and the sample, and the material is removed from the sample in the form of microscopic craters. The material produced by the disintegration of the tool and workpiece as well as by the decomposition of the dielectric is called "swarf." Sparking is done while the sample and tool are immersed in the dielectric.

The dielectric must be kept clean to achieve the full accuracy capability of the instrument, and this is routinely accomplished by using a pump and filter attachment. Depending on the polarity of discharge, type of generator, and particularly the relative hardness of the sample and tool, material can be removed effectively and accurately. No contact is required between the tool and workpiece.

Wire size Kerf size

mm in.

Diamond size, μm

mm in.

0.08 0.003 8 0.08 0.

0.13 0.005 20 0.14 0.

0.2 0.008 45 0.23 0.

0.25 0.010 60 0.29 0.

0.3 0.012 60 0.34 0.

Fig. 9 Diamond-impregnated wires