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The Metallographic Laboratory
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The Metallographic Laboratory
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A metallographic laboratory is a facility which prepares polished specimens capable of revealing the true microstructure of materials submitted for study. It should also be able to perform simple microscopic evaluations, linear measurements, and other related tests. When these services are applied to semi-metals, ceramics, geological materials, glasses, composites and polymers, the facility may be known as a materials laboratory.
To provide the services required, a metallographic laboratory must be knowledgeably organized, adequately equipped, and competently staffed. The extent to which the laboratory is developed depends upon one, or more, of the following considerations:
As an example, a process metallographic laboratory making only spot checks of grain size during production would require minimal space, equipment and staff. On the other hand, an R/D laboratory for a large corporation would require an assortment of equipment to perform specimen preparation and analysis of a variety of materials. Such a facility would likely employ metallurgists, highly trained metallographers, and technicians.
Personnel
The heart of any successful metallographic laboratory is the people who prepare the specimens and perform the subsequent evaluation.
Personnel Qualifications
Standards for qualifying or certifying metallographic personnel are virtually
nonexistent in the United States. Attempts to establish strict job descriptions
and professional standards are usually met with suspicion. This attitude
is usually defended with the argument that metallography is an art, not a
science and, therefore, strict standards are impractical. While there is
an element of truth to this position, it is sometimes used to limit the
development of a professional class of metallographers. Consistent, comparable
performance cannot be expected from laboratory personnel where guidelines
are not established. To meet the increasingly strict demands of certain
authorities (e.g. the military), some means of certifying and developing
qualified workers must be sought. If this is not available through higher
education, it should be provided through alternate means such as technical
schools, seminars, professional societies and well organized on-the-job training.
The following definitions attempt to describe the key functioning personnel in a metallographic laboratory.
Metallurgist--A professional who might be directly associated with
the laboratory. Frequently, he is not an integral part of the laboratory
organization, but relies on its services to provide the information he requires.
He would have likely earned a B.S., M.S. or Ph.D. in metallurgy or metallurgical
engineering. The metallurgist may also be the laboratory manager and possibly
supervise other laboratories as well.
Metallographer--A person capable of preparing, examining and documenting
microstructures without close supervision. His training may include a B.S.
degree but more often would vary with a combination of formal training and
practical experience. In the absence of a supervising metallurgist, the
metallographer may be required to interpret microstructure, perform tests,
and report the results in clear technical prose.
Technician--A person having little or no formal training but who,
through aptitude and experience, is capable of performing acceptable specimen
preparation and other functions. A technician usually requires minimal
supervision and would interpret microstructures to the extent of his recognized
training and proven abilities.
Lab Assistant or Trainee--A person having no formal training, but
able to follow instructions and perform specific laboratory tasks under direct
supervision of one of the previously described persons.
Sample Preparation
The basic principles of abrasive polishing necessitate a sequence of operations which produces a distortion-free, polished surface capable of revealing the true microstructure. Equipment required to prepare an acceptable metallographic surface varies according to the factors previously mentioned--tests required, materials to be processed, work volume, operating philosophy, and other considerations.
A typical preparation sequence and basic equipment for an average laboratory would include:
Sectioning--to produce a manageable size sample using either a
conventional abrasive cutter or low speed saw.
Rough Grinding--to remove surface deposits or level irregular surfaces.
Either belt or disc grinders may be required, but other methods could be
used.
Mounting--to provide a safe means of holding the specimen and protect
its edges from rounding. Compression molding, with a mounting press, is used
for rigid materials and cold mounting for delicate, hard to hold, or larger
than normal size samples.
Fine Grinding--to systematically abrade the specimen with a series of
grits of decreasing coarseness. This operation may be performed on a manual
fine grinder or rotating wheel. In some particularly demanding applications,
the use of lapping with loose abrasive slurries may be necessary. This is
particularly true for the preparation of extremely large specimens or when
large numbers of smaller specimens must be prepared simultaneously.
Rough and Final Polishing--to remove the remaining scratches and produce
the smooth lustrous surface required for microscopic examination.
Etching--to develop the microstructure not normally visible in the
as-polished condition. Inclusion ratings and certain types of defect
identification should be performed prior to application of the etchant.
Microscopy--to realize the ultimate goals of metallographic specimen
preparation in observing, analyzing, and recording the true microstructure
of the material. Other tests, such as microhardness, provide additional
information as required by the authority requesting the services.
The Metallographic Facility
Basic metallographic specimen preparation may be performed in a facility as small as a large walk-in closet or as large as a spacious multi-room corporate facility. The most important consideration is to provide adequate space for equipment and for personnel to perform the required operations efficiently and safely. Ideally, the dirtier and noisier operations should be separated from the cleaner and quieter areas. This is done to prevent cross-contamination of finer abrasive materials by the coarser abrasives. Microscopes and other sensitive instruments must be protected from the damaging effects of vibration and dirt.
The exact room arrangement depends on the individual needs, and common sense, in providing a smooth flow of work. Specific guidelines for planning an efficient metallographic laboratory are described (here).
Operation of the Metallographic Laboratory
Effective organization and operation of a responsible metallographic facility require attention to certain details which might seem unimportant but which are, nevertheless, vital.
Receipt of Work
Specimens, or parts to be sampled, are the responsibility of the laboratory
the moment they are received. They should be promptly logged and accurately
identified from the start. Photographs and macrophotographs should be used
to record the condition of parts at time of receipt. This is particularly
helpful in the case of failure analysis where the appearance (e.g. fracture
face) may be destroyed or damaged in the sampling process. It is advisable
to clearly mark the photographed area of a large part which will be cut up
for samples. Such a sampling map will prevent loss of identity by recording
the exact location of each specimen.
Retention of Identity
Specimen identity must be retained at all stages of preparation. Before mounting,
loose specimens should be bagged or tagged. Completed mounts may be numbered
with a vibrating scriber or other suitable means. A separate log of mounted
specimens is highly recommended. Final polished specimens should be kept
in desiccator jars or drawers in numerical sequence to preserve their finish
and facilitate easy retrieval. Loss of identity means loss of time, effort,
and most important, loss of valuable specimens. Some suggested laboratory
record documents are listed in Table I.
Laboratory Standards and References
Laboratory tests are valid only when performed according to accepted procedures,
using equipment that is functioning correctly and calibrated, if required.
Although metallography has long been considered an art, there is a trend
toward requiring stricter standards of practice. This is particularly important
in critical applications where material failure could result in injury or
loss of life. Committee E-4 of ASTM is responsible for standards regarding
specimen preparation, photomicrography and the addition of the standard for
evaluation of metallographic laboratories. Other standards regarding
microetchants, grain size, inclusion rating and others are periodically revised,
as required. Relevant ASTM (E-4) Standards are listed in
Table II and should be read and understood
by those responsible for metallographic laboratory operation and management.
The contributions of other societies should not be ignored. ASM Metals Handbook, Volume 7, "Atlas of Microstructures" and Volume 8, "Metallography, Structures, and Phase Diagrams" should be on the book shelves of even the smallest laboratory (Webmaster's note: Volume 9, "Metallography and Microstructures" should also be included). Records concerning the calibration and maintenance of laboratory instruments will further add credibility to the laboratory's tests and help fulfill its requirements to the authority.
Additional Services
The metallographic laboratory's main function is to prepare samples for
microscopic evaluation and documentation. However, additional tests, evaluations
or measurements may be required as a natural extension of this primary function.
Microhardness, for example, is a physical test which must be performed on
a final polished metallographic sample. This test requires a sensitive piece
of equipment and a metallographer's careful technique to obtain accurate
readings.
Other tests which may be performed in a metallographic laboratory are:
The need for additional tests will vary from laboratory to laboratory and should be based on the need for the information they produce. Well organized and staffed metallographic laboratories are a natural location for such tests because surface preparation of a specific quality is required. It is also more efficient and cost-effective to perform these tests at or near the sample preparation facility.
Records and Reports
The ultimate product of a metallographic laboratory is the microstructural
data which it gathers. If the authority requires certain reports, the laboratory
has no choice but to fulfill this obligation. Photomicrographs may be required
as evidence that certain microstructures are present in the samples. Records
of magnification and etchants used are vital to a correct interpretation
and should appear on each photograph or photomicrograph. Reports of tests
must be provided in clear technical prose and copies retained by the laboratory.
It is the continuing responsibility of the metallographic facility to maintain
an accurate log of prepared specimens and any permanent photographic negatives
that are produced. These records, and other required documents, are essential
if the laboratory expects to fulfill its purpose and function. For this to
become a reality, however, it is also important that the actual operating
facilities be carefully designed with these objectives in mind.
Metallographic Laboratory Design
The Early Laboratory
Albert Sauveur, in his "Metallurgical Reminiscences", humorously describes the haphazard beginning of the first American metallographic laboratory: "...I installed, in a cozy corner of the balance room, the least dilapidated chair I could find and there I read all the metallurgical books I could get hold of, old and new, day after day, while taking copious notes." ... and later on, "After spending a few months in the laboratory . . . I was given a room to myself, supplied with an old time microscope and instructed to study the structure of steel and the ailments to which his flesh is heir. I may be permitted to say, without being accused of an utter lack of modesty, that this small beginning marked the introduction of metallography in the iron and steel industry in the United States."
It would be simple to dismiss these quotations as irrelevant history, except that it is too often a fitting commentary on the contemporary metallographic laboratory. We must excuse Mr. Sauveur and the Pennsylvania Steel Company for their primitive efforts in metallographic laboratory planning, since they had no precedent to follow. For a modern metal working facility to be so poorly organized seems inexcusable. After nearly one hundred years of history, one might expect metallographic laboratory design, installation and operation to be highly developed and well known. Unfortunately this is frequently not the case. Laboratories are still established with little actual planning beyond finding a vacant area large enough to house some existing, or recently purchased, sample preparation equipment. It is highly doubtful that this method of setting up a metallographic facility is capable of meeting the technical needs of a modern metal processor.
Elements of Good Metallographic
Laboratory Design
Efficient metallographic sample preparation results from attention to various
controlling factors including Work Flow, Equipment Selection, Operating
Procedures, and Functioning Personnel. This METAL DIGEST is Concerned primarily
with the effect of work flow and equipment selection on the physical facilities.
Work Flow describes the pattern of successive sample preparation steps beginning with sectioning and concluding with microscopic analysis and testing. Poor work flow results from interrupted flow, non-linear flow, insufficient free table top space, and inadequate storage, and it creates obstacles to rapid, efficient sample preparation. Laboratory planning ideally addresses not only these situations, but also abrasive contamination control, noise pollution, and other related environmental problems.
To achieve the most efficient laboratory, the work flow should be as smooth and continuous as is practical...regardless of whether the laboratory consists of one room or multiple rooms. In some cases, corporate or building restrictions may influence the final layout, preventing a truly ideal work flow.
The use of furniture designed specifically for metallographic laboratories is important for the creation of smooth work flow. BUEHLER'S TECH-MET Laboratory Furniture is designed specifically for metallographic laboratories and has the necessary depth and sturdiness to handle large, heavy apparatus. Almost unlimited design flexibility is available with the standard TECH-MET Polisher/ Grinder Tables and customized TECH-MET Furniture. Single, double and triple units, high and low styles, upper and lower cabinets, 8 and 12 inch diameter flush mounted polisher/grinders, corner units, drawer units, and plain tables with uninterrupted work surfaces are offered. With this flexibility, there is virtually no limit to the combinations possible, making laboratory design more effective. Specific furniture and equipment layouts will be shown later.
Furniture may be arranged in various ways to suit the general shape of laboratory spaces. Longer, narrower rooms must make the best use of walls where the furniture must be placed. Installations are better utilized when there is a continuous flow of tables without interruption as illustrated in Figure 6 (Webmaster's note: original picture not shown. Please see blueprint 1 for example.). This is only possible if corners are used as a functioning part of the design. Corner units convert potentially "dead spaces" into real assets with five different options:
Larger, more open room spaces may offer the opportunity to employ "peninsula" or an "island" table arrangement as shown in Figure 6 (Webmaster's note: original picture not shown. Please see blueprint 3 items 3 and 6 or blueprint 4 item 5 for example.). With these arrangements, back-to-back tables are separated by an 8" access for plumbing and electrical services. The addition of a reagent shelf, bridging the back-splash tops, creates a functional area and a place for a receptacle box, if required. Filler panels close off the end space between tables. When standard table lengths fail to fully utilize wall space in continuous table arrangements, a special filler unit may be used to produce a completed, panoramic appearance.
Wall mounted cabinets offer storage space directly above the work areas for items such as abrasive powders, extenders, and smaller polishing wheels. Lower cabinets provide generous storage for larger, heavier items such as polishing wheels, abrasive belts and 12" diameter grinding discs. Drawer units offer a convenient storage space for smaller items, polishing cloths, sample holders and supplies.
Equipment Selection is dependent on various factors such as:
Workload is usually defined as the number of samples per work shift that the metallographic laboratory is expected to prepare. This is important to determine, because sample preparation equipment of various work capacities is available. Equipment should be selected that meets the specific needs dictated by the size samples which must be prepared. Figure 8 shows a single position belt grinder and an abrasive cutter that are well suited to the laboratory handling smaller parts (Webmaster's note: picture not shown). The choice of a polisher should be influenced by the workload, i.e., the number and size samples which must be prepared per work shift. The MINIMET Polisher (Webmaster's note: picture not shown), for example, is a single sample polisher, limited to 3 to 7 samples per day of the conventional 1", 1 1/4" and 1 1/2" diameter mounted formats. By comparison, the MAXIMET Sample Preparation System shown in Figure 9 is able to prepare up to 24 mounted samples at one time, or as many as 200, or more, samples per shift (Webmaster's note: picture not shown). It is also capable of preparing a lesser number of very large, unmounted samples up to 11" diameter.
Between these two extremes there are other devices designed for various sample output, depending on the needs of a particular laboratory. It is important to determine the equipment needs before a floor plan is drawn, because the amount of floor space required could vary widely. The amount of space needed to install enough table mounted, semi-automatic devices to equal the output of one MAXIMET would be considerably greater than that required by the MAXIMET: and a storage table.
Economics affect the choice of equipment because table top devices are generally less expensive than table mounted equipment. On the other hand, a well designed furniture installation creates a superior corporate image and is certainly a more efficient and pleasant work facility.
It is important to plan the metallographic laboratory with the future in mind. Selection of devices and furniture barely meeting current requirements could result in extra cost later. If there is a possibility of a significant increase in work-load, a laboratory presently requiring only manually prepared samples may do itself a disservice if the capability of adding semi-automatic devices is ignored. A specific example is a company that purchases the least expensive polisher/ grinders when, for a modest increase in cost, more powerful units capable of accepting semi-automatic devices could be acquired.
Finally, there is a way to achieve a modern, efficient metallographic laboratory on a limited annual budget. With a master plan, selected equipment may be purchased as funds are allocated. If the plan is carefully followed, the final result will be as pleasing and productive as if the entire facility had been installed at one time.
The "Zoned" Principle
In addition to smooth work flow, previously discussed, other ideas remain
to be considered. The "zoned" principle should be followed whenever possible.
Dirtier, noisier operations should be kept as far away from the clean, quiet
operations as possible. Failure to do this may result in cross-contamination
from coarser abrasives falling onto fine abrasive polishing wheels and may
also cause damage to microscopes, hardness testers, and other sensitive test
equipment. Full, floor-to-ceiling walls, with a door for access, should separate
these operations. In a three room facility, the finer polishing steps should
be separated from both the very coarse steps and the microscopy. If it is
impossible to separate the laboratory, dirty and fine operations should be
spaced as far apart as possible, preferably on opposite walls. Dust covers
should be used without fail.
Actual layout must begin with a previously committed area, outlined graph paper, or a proposal for an area more ideally suited to meet the laboratory's needs.
Laboratory Layout
As previously discussed, the realization of a modern productive metallographic
laboratory requires attention to many details, including a facilities survey,
review of equipment and furniture specifications, and development of working
drawings for installation.
Laboratory Facilities Survey
This simple questionnaire asks questions which alert the lab planner to details
essential for successful design; a reduced version appears as
Table III of this METAL DIGEST. For best
results, input should be obtained from managers, laboratory customers and
particularly experienced metallographers and technicians most familiar with
sample preparation.
The survey responses should be tabulated and a consensus obtained. The results should clarify the real needs of the laboratory and help avoid wasteful duplication or serious omissions.
Equipment and Furniture Specifications
These should be examined so that service requirements may be considered in
the design. Plant engineers or building services should be consulted regarding
possible restrictions before final decisions are made.
Voltage and current phase requirements are important details which seriously affect the installation. If service access areas are required at the rear of the tables, space must be allotted.
Drawings
Tools required to produce a simple working drawing include 1/4" increment
cross-ruled graph paper, an easy-to-read 12" scale and sharp pencils. For
maximum accuracy and convenience, the useful BUEHLER Lab Planning Kit should
be used. If additional assistance is required, the help of BUEHLER's Staff
of competent laboratory planning consultants is available on request.
Implementation of a Laboratory Design
Several examples of typical, successful lab designs are shown in this METAL DIGEST. They are the product of years of experience with customers who have trusted BUEHLER for design assistance.
Your design only may be used as a basis for a new facility, or you may choose to seek further help from BUEHLER. Our consultants are prepared to review your survey information, plus any additional input you may choose to communicate, and recommend a laboratory layout.
When the layouts are completed for your specific requirements, you may request a formal quotation or BUEHLER may be requested to determine the capital expenditure required. Our Sales Department will assist you, in every way possible, from our Headquarters in Lake Bluff, and through our local Sales Engineers.
All laboratory furniture is built to the customers specifications; long lead times are not required. We are proud of our prompt delivery service and seldom are there out-of-stock items or back orders.
BUEHLER stands ready to assist you in the design and installation of a modern, efficient metallographic laboratory that will be a credit to your company. We are able to commit ourselves because...
We are BUEHLER...where quality begins.
| A. Work Received | B. Specimen Log |
| 1. Initiator 2. Material Description 3. Work to be Performed 4. Date to be Completed |
1. Specimen Number 2. Work Identity (work received record) 3. Material Description 4. Material Condition (heat treatment, etc.) 5. Location (longitudinal, transverse, etc. a macrophoto may be helpful |
| C. Photo Record | D. Test Reports |
| 1. Negative or Photo Number 2. Specimen Number 3. Magnification 4. Keywords |
1. Inclusion Ratings 2. Microstructure Description 3.Quantitative Analysis 4. Grain Size 5. Macro or Microhardness 6. Disposition |
Table II. ASTM Standards [E-4] Relating Directly to Metallographic Practice (Webmaster's note: This section lists the standards originally listed in the Buehler article. There are many other ASTM standards related to metallographic practice.)
| E-3 Preparation of Metallographic Specimens |
| E-7 Standard Terminology Relating to Metallography |
| E-45 Standard Practice for Determining the Inclusion Content of Steel |
| E-112 Standard Test Methods for Determining Average Grain Size |
| E-340 Macroetching Metals and Alloys |
| E-381 Standard Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings |
| E-384 Test for Microhardness of Materials |
| E-407 Microetching Metals and Alloys |
| E-562 Recommended Practice for Determining Volume Fraction by Systematic Point Count |
| E-588 Recommended Practice for Detection of Large Inclusions in Bearing Quality Steel by the Ultrasonic Method |
| E-768 Standard Practice for Preparing and Evaluating Specimens for Automatic Inclusion Analysis of Steel |
| E-807 Standard Practice for Metallographic Laboratory Evaluation |
| E-883 Standard Guide for Reflected-Light Photomicrography |
| The above standards are available separately, or in the Annual Book of ASTM Standards Volume 3.01 from ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, Phone: (610) 832-9585, Fax: (610) 832-9555 or via the ASTM website. |
Table III. Metallographic Laboratory
Facilities Survey
The purpose of this survey is to identify the actual needs of the proposed
facility. By answering each question, costly mistakes or omissions may be
avoided. The result should be a more efficient, attractive laboratory capable
of meeting your expectations. Interpretation of the completed survey may
be performed by you, or with the assistance of BUEHLER Consultants.
| I. Sample Material | |
| A. The Industry served is: | |
| o Primary Metals o Foundry o Heat Treating o Metal Fabrication o Electronics |
o Geological o Non-Metals o Educational o Other____________ |
| B. Material to be Prepared | |
| o Ferrous and Alloys o Super Alloys o Non-Ferrous and Alloys o Light Metals and Alloys o Exotic Alloys, i.e. Titanium, Zirconium |
o Carbides or other very hard materials o Composites o Noble Metals o Toxic Material o Non-Metals o Other_____________ |
| C. Samples per day maximum | |
| o 0 - 5 o 6 - 15 o 16 - 50 |
o 51 - 200 o 201 - 500 o Over 500 |
| D. Sample Size (as received) | |
| o Suitable for standard format mounts - (1",
1 1/4", or 1 1/2" diameter) o For encapsulation into other format mounts including rectangular shapes having a largest dimension of ____________________ o For unmounted preparation of samples of approximately_____________ |
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| E. As received surface condition: | |
| o Uncut (describe) __________________________ | |
| o Rough cut with: | |
| o Bandsaw o Abrasive wheel o Diamond Wheel o Machined o Ground to ____________ finish o Other (detail): ________________ |
o Wire Saw o Torch o Shear |
II. Operating Philosophy
| A. Primary function: | |
| o Quality Control o Customer Service o Research & development o Other:___________________ |
o Commercial Testing o Educational o Engineering |
| B. Laboratory Access: | |
| o Controlled - access to lab personnel only o Limited access - open to certain qualified non-lab personnel o Unrestricted access |
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| C. Staff - maximum number expected to use facilities at any one time:_______ | |
| D. Services to be performed in lab | |
| o Visual examination o Photomacrography o Photomicrography o Microhardness o Macrohardness o Macroetching o Microetching |
o Grain Size o Linear Measurements o Inclusion counts o Image Analysis o Physical tests o Non-destructive tests o Other:____________ |
| E. If this facility is to be shared with other un-related
activities, explain:_____ ________________________________________________________ |
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III. Sketches should indicate any restrictions or other fixed conditions, such as:
Room dimensions including location and dimension of any obstruction such as pillars or wall mounted heaters.
Door locations with the direction of door swing plus an indication of the nature of adjoining areas such as corridor, offices, factory, outside, etc.
Existing sources of water, drain, electric, air, vacuum, or natural gas service.
| Existing Services o Water o Drain o House Air o House Vacuum o Natural Gas |
Electric o 115-120 V 60 Hz 1 Phase o 208 V 60 Hz 3 Phase o 220-230 V 60 Hz 3 Phase o 440-460 V 60 Hz 3 Phase o 550-575 V 60 Hz 3 Phase |
Indicate whether a Fume Hood (supplied by others) will be included, what size, and if the location is fixed or open to the designer.
Will Office Space be required in lab or provided apart from the plan?
| o Separate | o In Lab |
Is a Dark-Room required within the lab facilities?
| o Negatives only o Negatives, black & white, and color prints |
o Negatives and black and white prints |
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