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A number in parentheses indicates the year of last reapproval. This standard has been approved for use by agencies of the U. Department of Defense. The values given in parentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.
It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. For specificwarning statements, see 6. Referenced Documents2. Significance and Use3. Metallographic specimens and chemicalanalyses will provide the necessary detailed information aboutspecific localities but they cannot give data about variationfrom one place to another unless an inordinate number ofspecimens are taken.
The information provided about varia-tions in chemical composition is strictly qualitative but thelocation of extremes in segregation will be shown. Chemicalanalyses or other means of determining the chemical compo-sition would have to be performed to determine the extent ofvariation.
Macroetching will also show the presence of discon-tinuities and voids, such as seams, laps, porosity, flakes, bursts,extrusion rupture, cracks, and so forth. It is also used in the heat-treating shop todetermine location of hard or soft spots, tong marks, quenchingcracks, case depth in shallow-hardening steels, case depth incarburization of dies, effectiveness of stop-off coatings incarburization, and so forth. In the machine shop, it can be usedfor the determination of grinding cracks in tools and dies.
Forge shops,in addition, use macroetching to reveal flow lines in setting upthe best forging practice, die design, and metal flow. For anexample of the use of macroetching in the steel forgingindustry see Method E Forging shops and foundries alsouse macroetching to determine the presence of internal faultsand surface defects.
The copper industry uses macroetching forcontrol of surface porosity in wire bar. In the aluminumindustry, macroetching is used to evaluate extrusions as well asthe other products such as forgings, sheets, and so forth. Defects such as coring, cracks, and porthole die welds areidentified. When macroetching is used to solve a problem, theproblem itself largely dictates the source of the sample as to thelocation on the work piece and the stage of manufacture; forexample, when looking for pipe, the sample should representthe top of the ingot, or when looking for bursts or flakes, thesample should be taken as soon after hot working as possible.
Current edition approved June 1, Published July, Originallyapproved in Last previous edition approved in as E — United States However, the sample should not be taken soearly that further working can introduce serious defects. In thesteel industry, for example, the sample is usually taken afteringot breakdown and after most chances of bursts or flakesoccurring have passed.
Billets or blooms going into small sizesare sampled after initial breakdown. Material going intoforging billets or die blocks is sampled near finish size. Sampling may be done systematically or on a random basis. The use of torch cutting or hot cuttingshould be used only when necessary to cut a sample from alarge piece.
The sample then is sectioned well away from thehot-cut surface. An example of permissible use of torch cuttingis the excising of a piece from a large plate and then cutting asample for macroetching 4 to 5 in. Samples cut tooclose to the end, however, may have false structures because offish-tailing. Disks from large blooms are sometimes cut intosmaller pieces for ease in handling.
Forgingsmay also be cut parallel to the long dimension to show flowlines. In complicated forgings, some thought will have to begiven to the proper method of cutting so as to show flow lines.
Macroetching of an unprepared specimen will show surfacedefects such as shuts, flats, seams, and so forth. In extrusions,coring and coarse grain are more commonly found in the backend of the extrusion. An ideal lengthwould be the circumference of the last roll, but this may beinconveniently long.
Several samples totaling some givenfraction of the circumference can be used; however, there isalways a chance then that a defect arising from faulty rollswould not be detected.
When seeking information onlaminations, a transverse section is used. In many cases,however, to reduce the size of the specimen, only a section outof the center of the plate may be taken. Careful preparation is usually rewardedwith highly detailed structures giving a large amount ofinformation. Welds involving dissimilar metals will produceproblems in etching.
The best method is to etch the leastcorrosion-resistant portion first and the more resistant portionafterwards. Occasionally an intermediary etchant may berequired. The boundaries between etched and unetched portionwill give an idea of weld penetration and dilution. Because the machined or ground part is often thefinished part, it may be undesirable to immerse the part in acid.
In this case, other methods such as dye penetrant methods maybe more desirable. Any methodof presenting a smooth surface with a minimum amount of coldwork will be satisfactory. Disks may be faced on a lathe or ashaper. The usual procedure is to take a roughing cut, then afinish cut. This will generate a smooth surface and remove coldwork from prior operations. Sharp tools are necessary toproduce a good specimen. Grinding is usually conducted in thesame manner, using free-cutting wheels and light finishingcuts.
When fine detail is required, the specimen should beground down through the series of metallographic papers seeGuide E3. Where necessary, details are given in the tabulationof procedures. Any grease, oil, or other residuewill produce uneven attack. Once cleaned, care should be takennot to touch the sample surface or contaminate it in any way.
In most cases a good grade ofreagent should be used but need not be chemically pure or ofanalytical quality. The so-called technical grades are usuallysatisfactory. The solution should be clean and clear, free ofsuspended particles, scum, and so forth. Many of theetchants are strong acids. In all cases, the various chemicalsshould be added slowly to the water or solvent while stirring. In the cases where hydrofluoric acid is used, the solutionshould be mixed and used in polyethylene vessels.
Warning—Hydrofluoric acid must not be allowed to contactthe skin since it can cause painful serious ulcers if not washedoff immediately. Etching should be done in awell-ventilated room, preferably under a fume hood. Thesolution should be mixed and placed in a corrosion resistanttray or dish and brought to the operating temperature. Thespecimen or specimens should be placed in a tray of stainlesssteel screen or on some non-reactive support. Glass rods oftenare placed on the bottom of the acid container and thespecimens laid directly on the rods.
When etching iscompleted, remove the specimens from the dish taking greatcare not to touch the etched surface. When desmutting isrequired, dip the specimen into a second solution. After rinsingthe specimen with hot water, blow dry with clean compressedair.
An effort should be made to wet the entire surface assoon as possible. After the initial wetting, keep the swabsaturated with solution and frequently sweep over the surfaceof the specimen to renew the solution. When the structure hasbeen suitably developed, rinse the specimen, either with a swabsaturated with water, or better still, by pouring water over thespecimen. After rinsing with hot water, blow the specimen drywith compressed air.
Details of the procedure not discussedhere are covered in the sections for the various metals and theiralloys. In fact, the progress ofetching should be closely watched and etching stopped whenthe preferred structural details have been revealed. Specimensshould be etched to develop structure. Generally, a light etch isbetter than a heavy etch; overetching can often lead tomisinterpretation. The actual time to develop a structureproperly may be quite different from the one suggested.
Specific Preparation Procedures and RecommendedSolutions8. All these methods will cause cold work at the surface andwill generate heat. The temperature rise can be enough to causechanges in structure.
For these reasons sharp tools and gener-ous lubrication are necessary for sectioning. Again sharp tools and copious lubricationare required. If fine detail is required, the machined surfaceshould be ground using silicon carbide paper lubricated withwater or kerosine.
Inthese cases the specimen is periodically removed from thesolution, cooled in running water, and reimmersed in theetchant. This procedure is repeated until the desired degree ofetching is obtained. Warning—Before starting any workinvolving beryllium, a review of hazards and plans for han-dling should be made. Anumber of references on beryllium areavailable. First, beryllium is a rather brittle metal and sectioning can bedifficult. Cut-off wheels with the designation C46FR70 havebeen the most successful.
Secondly, beryllium does not grindeasily; hence, specimens should be as small as possible tominimize grinding time. Grinding has been most successfulwith the entire sequence of wet silicon carbide papers. Rinse in water, and removesmut in strong HNO3solution.
Significance and Use 3. Metallographic specimens and chemical analyses will provide the necessary detailed information about specific localities but they cannot give data about variation from one place to another unless an inordinate number of specimens are taken. The information provided about variations in chemical composition is strictly qualitative but the location of extremes in segregation will be shown. Chemical analyses or other means of determining the chemical composition would have to be performed to determine the extent of variation. Macroetching will also show the presence of discontinuities and voids, such as seams, laps, porosity, flakes, bursts, extrusion rupture, cracks, and so forth.
ASTM E340 – 金属と合金をマクロエッチングするための標準プラクティス