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Microstructural manifestations in color: Some applications for steels
ELSEVIER
Microstructural Manifestations in Color: Some Applications for Steels Amitava Ray and Sanjay K. Dhua Physical Metallurgy Group, Research and Development lndia Limited, Ranchi 834002, India The interpretational
advantages
white images is recognized
Centrefor
of colored microstructures
Iron and Steel, Steel Authority of
over conventional
black-and-
in view of the natural response of the human eye to color varia-
tions. This applies well for steels, where ambiguities
in phase and feature discrimination
can often arise from intrinsic lack of contrast or otherwise subtle grey-level differences of the observed microstructural constituents. Although staining techniques are advantageous from this standpoint, their use, even today, is rather limited in routine metallography Textbook instructions and standard reagent formulations alone cannot guarantee optimum results unless sample preparation is meticulous and the etching technique is perfected. Moreover, the etch response of different steel chemistries being unique to each grade demands experimentation as a prerequisite for obtaining optimum results. This article provides a pictorial insight into the fascinating world of microstructures obtained in a gamut of plaincarbon, dual-phase, low-alloy, stainless, and high-alloy tool steels investigated in our laboratory. Issues concerning the revelation of anodic matrix and second phases and crystallographic orientation aspects of microstructural features, such as grains, twins, and colonies, are also elucidated and discussed. 0 Elsevier Science Inc., 1996
INTRODUCTION
ages can be brought about by “optical staining” techniques or by interference film deposition methods. Optical staining techniques, such as polarization, sensitive-tint, and differential interference contrast (DIC), generate color contrast as a consequence of microstructural anisotropy or variations in surface topography. Metals with cubic crystal structures, such as iron, aluminium, and copper, are generally not responsive to polarized light in the as-polished state. Polarization contrast in weakly anisotropic materials can be enhanced by interposing a sensitive-tint plate between the polarizer and the analyzer whereby double refraction or birefringence is accentuated. The DIC technique can be used for obtaining color contrast in microstructures [2] and is particularly suitable for as-polished specimens which contain phases of widely vary-
Conventional metallography of steels involving etching with common reagents, such as, nital, picral, and Vilella’s, generally reveals microstructures with a lightand-dark contrast. Although many microstructural constituents can normally be revealed in images possessing adequate contrast, discrimination of phases with subtle grey-level differences can be both difficult and misleading. It is in this perspective that color discrimination of microstructures presents exciting possibilities of revealing much more meaningful and reliable information. This interpretation advantage arises primarily because the human eye can discriminate a vast array of colors more easily than a few shades of grey [l]. Color contrasts in light-microscopic imMATERIALS CHARACTERIZATION 37:1-S 1996 0 Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
3 1044.5803/96/$15.00 PI1 SlO44-5803(96)00022-S
A. Ray and S. K. Dhua
ing hardness levels [3]. In etched microstructures, the DIC technique can be used to highlight subgrain structures and mechanical twins. Chemical color staining, on the other hand, involves deposition of a thin interference film, 40-500nm thick [4], on the polished surface of a metallographic specimen by etching. The microstructural color contrast is a consequence of interference of light reflected from the film and metal surfaces and is controlled by the film thickness in the usual interference order: yellow, red, violet, blue, and green [5]. The characteristics of the base metal and the mode of illumination employed also govern the coloration obtained. Stain etchants can be used to tint either anodic or cathodic microconstituents. Reagents based on metabisulfite and thiosulfate systems [6] for coloring anodic matrix phases, such as austenite and ferrite, and those employing sodium molybdate Na2Mo04.2H20) for preferentially etching cathodic microconstituents [7] have been developed. Nevertheless, it is worthwhile to study the etch response of different types of steels for a comprehensive understanding of their microstructures. Most metallographic investigations in structure-property correlation studies involve quantitative determination of phase volume fractions, grain size, and particle size distributions. The accuracy of such measurements is largely governed by the clarity of phase and feature delineation of the microstructure. The application of selective color-staining techniques can, therefore, obviate interpretational ambiguities concerning primary matrix phases, minor second-phase constituents, grain orientation, and grain/twin delineation. This article discusses these microstructural issues in some varieties of plain-carbon, dual-phase, free-cutting, stainless, and alloy tool steels on the basis of our laboratory investigations.
METALLOGRAPHIC
PROCEDURES
Although conventional metallographic procedures per se are adequate for the prepa-
ration of steel samples subjected to normal etching, the execution of sample preparation steps must be absolutely perfect for satisfactory color staining. The most critical step is specimen cutting, which should be carefully exercised to prevent specimen damage and impair subsequent polishing quality. To obtain a scratch-free finish, the final polishing operation was carried out in two steps, using suspensions of 0.3 km-size a-alumina and 0.05 pm-size y-alumina on Selvyt cloth. Moderate hand pressure and low polishing wheel speeds (120-150rpm) are recommended to prevent generation of artifacts or excessive relief on polished surfaces. Careful control needs to be exercised at all stages of preparing stainless and lowcarbon ferritic sheet-steel samples, which are particularly scratch prone. Moreover, since the austenitic grades can work harden to produce stress-induced martensite, precautions should be taken to minimize deformation during cutting and grinding operations. Color etching, nevertheless, is far easier for ferritic sheet steel than for austenitic (or ferritic) stainless steels. Meticulous preparation of samples to a scratch-free and blemish-free finish is a precondition for satisfactory revelation of colored microstructures. The problem arises from the fact that very fine scratches, apparently innocuous in conventional blackand-white microstructures, tend to be grossly accentuated as prominent imperfections after stain etching. To obviate such possibilities, polished samples should be examined with dark-field illumination to detect such “ghost” scratches and repolished, if necessary. Prior to stain etching, polished samples should be thoroughly cleaned from polishing debris, as any adherent dirt or film can inhibit film formation during etching. Ultrasonic cleaning, followed by rinsing in ethyl alcohol and drying, has been found to yield excellent results. As regards etching, immersion is required (never swab) and the etch time is controlled by the visual appearance of the color stain on the polished surface. For revelation of true microstructural colors, examination should be carried out with
Steel Structures in Color
3
bright-field illumination without the aid of filters. In special cases, however, polarization or sensitive-tint modes can be used in conjunction to enhance color contrast. Microphotography can then be performed on standard color negative or positive film by adjusting the color temperature of the quartz-iodine halogen lamp to 3200K.
MICROSTRUCTURAL PLAIN-CARBON
REVELATIONS STEEL
Nital-etched low-carbon steel with an essentially ferritic matrix is not responsive to optical staining methods. Examination under crossed-polarized light results in complete extinction of the image of the ferrite grains. Further, no manifestations of grainto-grain orientation variations can be revealed in the sensitive-tint mode, because all ferrite grains appear magenta, even with microscope stage rotation. Stain etching with a sulfide film-forming reagent can tint ferrite grains in different colors. The stain-etched microstructure of the transverse section of a hot-rolled, lowcarbon “rimming quality” steel-wire rod at 100X is shown in Fig. 1. The polished sample was pre-etched with 4% picral and subsequently tint etched with Beraha’s reagent, an aqueous solution of 3% K&Os and 10% Na2S203, by immersion. Note that although the ferrite grains below the peripheral rim have been preferentially tinted in many hues, the low-impurity and more noble rim layer (practically pure iron) is unaffected. This differential ferrite-rim color contrast enables a more precise microscopic measurement of rim thickness than would be possible for a conventional nital-etched microstructure where the rim and the ferritic matrix would both appear white. The multiple colorations of the ferrite grains are a consequence of different film thicknesses formed on the underlying grains and the resulting light interference. Sulfide filmforming etchants have been reported to be orientation sensitive [8]. The different colorations of the ferrite grains, in spite of sim-
ilar etching time, are therefore indicative of orientation differences among them. Upon prolonged etching with the same Beraha’s reagent, a line-etch pattern (Fig. 2) can be developed in the ferrite grains. Microscopic observations at high magnifications indicate that these line patterns are manifestations of film microcracking. It is worthwhile noting that the line patterns are clearly discernible on the green-tinted ferrite grains. The green interference is a manifestation of a greater film thickness, which is naturally expected after prolonged etching. The occurrence of such striations along preferred directions is unique to each ferrite grain and is, therefore, possibly related to their respective orientations. Unlike low-carbon steel, the pearlitic regions in nital-etched high-carbon steel are found responsive to crossed-polarized light. The bright-field image of a 0.6wt.% carbon steel at 500 X magnification is shown in Fig. 3(a). Under crossed-polarized light, comof the isotropic grain plete extinction boundary ferrite occurs. Upon insertion of a sensitive-tint plate between the polarizer and the analyzer, the ferrite coloration changes to magenta, while the pearlitic regions display an array of fascinating colors as shown in Fig. 3(b). The different colorations in the pearlitic regions are indicative of their anisotropic behavior under polarized light. Chemical etching, carried out to generate an anisotropic film or an uneven surface, has been reported [9] to induce polarization effects even in isotropic metals. Rotation of the specimen stage does not change the magenta coloration of the grain boundary ferrite; the pearlitic regions, however, are found to transform from one set of colors to another with each 45” rotation of the stage. This behavior is presumably a manifestation of etch-induced roughness in the pearlitic regions. Stain etching of the sample in Beraha’s aqueous 1% NazMo04 reagent (acidified with nitric acid to a pH between 2.5 and 3), containing a pinch of NHhFHF, preferentially tints the cathodic cementite phase blue and the pearlitic regions blue-brown. The anodic grain boundary ferrite is tinted
FIG. 1. Stain-etched microstructure of the transverse section of low-carbon “rimming quality” steel wire showing multicolored ferrite grains and untinted peripheral rim.
FIG. 3. Nital-etched 0.6wt.% carbon steel showing (a) pearlitic regions of varying resolution and ferrite precipitation along boundaries under bright-field illumination and, (b) sensitive-tint image showing multicolored pearlitic regions of varying orientation and magenta-colored isotropic ferrite. FIG. 2. Line-etch patterns showing orientation differences among ferrite grams of low-carbon “rimming quality” steel after prolonged stain etching.
FIG. 4. High-carbon steel which is stain etched with sodium-molybdate reagent showing cathodic cementite regions tinted blue and brown and ferrite precipitates colored yellow.
FIG. 5. Stain-etched microstructure of hardened and tempered EnlA-grade free-cutting steel showing silvery white sulfide stringers highlighted against green and purple matrix of tempered martensite.
FIG. 6. Dual-phase steel tint etched to reveal dispersed islands of martensite in brilliant white against brown ferrite matrix.
FIG. 7. Annealed AISI-316-grade stainless steel exhibiting austenite grains and twins in differential color contrast after stain etching.
FIG. 8. Stain-etched microstructure of duplex stainless-steel sheet showing light etching austenite islands in a matrix of brown ferrite.
FIG. 9. Hardened and tempered 6Cr:lMo:lC steel: (a) Vilella-etched microstructure showing both alloy carbides and retained austenite in white against dark martensite matrix, and (b) violet chromium-rich alloy carbides and orange retained austenite against browngreen martensite following sodium-molybdate color etch.
steel exhibiting orange tungsten-rich carbides in a matrix of greenish tempered martensite following stain etching in sodium-molybdate reagent.
6
A. Ray and S. K. Dhua
yellow by this reagent, as shown in Fig. 4. Because the ferrite is colored yellow, this etch is a complex tint etch rather than a cathodic etch. The complex stain etch, however, does not reveal orientation differences among pearlitic colonies as is exhibited by ferrite which is stain etched with a sulfide film-forming reagent. FREE-CUTTING
STEEL
In nital-etched, hardened, and tempered free-cutting steel, the dove-grey MnS stringers are camouflaged in the dark tempered martensite matrix and, hence, are difficult to discern. Optical staining methods employing polarized light are futile in selective discrimination of the MnS owing to its extinction. To highlight inclusionmatrix contrast, stain etching can be carried out in an aqueous solution containing 3% K&Os, 1% NHJFHF, and 1% sulfamic acid [lo] until the polished surface turns bluish. The stain-etched micrograph of a hardened and tempered EnlA-grade free-cutting steel containing 0.2wt% S is shown in Fig. 5. The revelation of the MnS inclusions as silvery white stringers against the purplegreen matrix of tempered martensite is a consequence of preferential staining of the anodic tempered martensite. The multiple colorations of the tempered martensite (anodic phase) may be attributed to the orientation sensitivity of the sulfide film etch on different martensite packets. The highlighting of MnS inclusions, in preference to other inclusion species such as silicate stringers, is thus conducive for their easier identification from etched microstructures. DUAL-PHASE
STEEL
Dual-phase steels are characterized by a microstructure of fine-grained polygonal ferrite with a dispersion of small islands of martensite [ll]. The properties of this steel are largely governed by the relative volume fractions of the individual phases and their distribution. Consequently, estimation of the phase contents by automatic image analysis is often necessary. In nital-etched as dual-phase steel, ferrite is revealed
bright polygonal grains with well-defined boundaries. The minor constituent, martensite, being selectively attacked, appears darker. However, image analysis of such a microstructure can pose problems of greylevel selection, since regions of the ferrite grain boundaries are often picked up while adjusting the discrimination level for the dark martensite phase. In overcoming this difficulty, stain etching with Le Pera’s reagent [12], containing equal volumes of 4% picral and 2% aqueous Na&.Os, is found helpful. The micrograph of a dual-phase steel sheet (O.OSwt.% C, l.OOwt.% Mn, 1.20wt.O~ Si, 1.40wt.% Cr, 0.026% S, and O.O27wt.% I’) stain etched with this reagent is shown in Fig. 6 at 1000X magnification. The dispersed martensite islands in this microstructure are revealed in brilliant white while the selectively attacked ferrite matrix is tinted brown. Incidentally, since the ferrite grain boundaries are not prominently defined after this stain etch, the striking white contrast of the martensite enables easier discrimination and estimation by image analysis techniques. STAINLESS
STEEL
Unlike carbon and tool steels, the delineation of microstructures in stainless steels by either electrolytic or conventional chemical etching is comparatively difficult. Although in single-phase austenitic stainless Kalling’s reagent (5g steels, “Waterless” CuC12,40ml HCl, 30ml water) can produce excellent grain boundary delineation, the annealing twins are nevertheless revealed in the same contrast. This situation is particularly problematic for reliable assessment of austenite grain size by chart and automatic image analysis methods, since twins cannot be ignored selectively. An electrolytic etch with 60% HNOs, 40% water, has been reported [13] to reveal twinfree austenite successfully. In this case, proper control of voltage and use of stainless-steel cathodes can yield satisfactory results. Although a more noble cathode is useful, it is not necessary.
Steel Structures in Color
The microstructure of a solution-annealed AISI 316 stainless steel, stain etched with Beraha’s reagent (aqueous solution of 20% HCl, 2% NHJFHF, and 1% KzS20s) is shown in Fig. 7. The tinted austenite grains show varying shades of green, purple, and orange. The annealing twins within the grains appear in red and orange hues. Actually, the structure within the twin is austenite and is similar to that just outside the twin. In some austenite grains, wide twins are visible whereas, in others, single twin boundaries can be observed. As a matter of fact, there is no real difference, except for the stacking sequence in the austenite on either side of these boundaries. Thus, it is the twin boundary itself which is important. Although the austenite grain size in this tint-etched microstructure may be more difficult to rate by conventional chart methods, as compared with conventional swabetched stainless steel, the structure is nevertheless fascinating and enables quick visual detection of twins. The color variations in the microstructure are a consequence of interference effects from regions of different film thicknesses and can be attributed to the orientation sensitivity of sulfide films. This differential twin-grain color contrast is a manifestation of their crystallographic orientation differences. Most wrought duplex stainless steels in the annealed state contain about 40-50% austenite in a matrix of ferrite 1141. For understanding structure-property correlations in this steel, quantitative assessment of austenite and ferrite contents is indispensable. With conventional reagents, such as glyceregia (5ml HN03, lOm1 glycerol, and 15ml HCl), the primary matrix phases, ferrite and austenite, are not completely delineated. Although prolonged (about 30 minutes) etching in ethanolic 15% HCl (15ml HCl in 100ml ethyl alcohol) can render excellent microstructural delineation of the ferrite-austenite boundaries 1151, the identity of the matrix phases is rather ambiguous, since both ferrite and austenite are revealed in the same contrast. Figure 8 shows the stain-etched microstructure of a duplex stainless-steel sample, etched with a
7
reagent containing aqueous 20% HCl with 1% K&Os. The ferrite matrix is preferentially attacked and is tinted brown while the untinted austenite appears as dispersed islands. The sharp delineation of ferrite and austenite phases in differential color contrast thus enables authentic phases identification and reliable quantitative estimation. TOOL STEELS Microstructural characterization of hardened and tempered tool steels not only involves delineation of alloy carbides, tempered martensite, and retained austenite, but their estimations as well. With conventional etchants such as nital, picral, or Vilella’s tempered martensite is selectively etched in preference to both alloy carbides and retained austenite. Consequently, under bright-field illumination, discrete white particles of alloy carbides are revealed in a matrix of dark-etching tempered martensite. Retained austenite, if present, is optically discernible as bright regions interspersed in the martensite. Figure 9(a) shows the bright-field monochrome image of a hardened and tempered 6Cr:lMo:lC wear-resistant steel etched with Vilella’s reagent (lg picric acid, 5ml HCl, and 100ml ethanol). The black-and-white microstructure shows large white skeletal-like eutectic carbide and numerous small globular carbides in a matrix of dark tempered martensite. The microstructure also shows some amount of retained austenite which, owing to similar color contrast as that of the carbides, is difficult to discern optically. The lack of adequate optical contrast between the retained austenite and the alloy carbides makes quantitative estimation of their volume fractions by image analysis difficult. To bring about a differential color contrast between the alloy carbides and the retained austenite, stain etching is preferable for selectively tinting the cathodic carbides. The microstructure of the same steel, stain etched with Beraha’s sodium-molybdate reagent (lg NazMoOd, 100ml water, 0.4ml HNOs, and 300mg NHJFHF) after a pre-
A. Ray and S. K. Dhua
8 etch in 2% nital, is shown in Fig. 9(b). The etchant selectively colors the chromiumrich carbide particles violet while the retained austenite appears orange and the martensitic regions are dark brown and green. When the sodium-molybdate etch is applied to hardened and tempered highspeed steel (18wt.% W, 4wt.% Cr, lwt.% V) after an initial nital pre-etch, the brightfield microstructure (Fig. 10) reveals orange carbide particles dispersed in a matrix of greenish tempered martensite. Note that while the sodium-molybdate reagent colors the chromium-rich alloy carbides in 6Cr: 1Mo:lC steel violet, the tungsten-rich alloy carbides in 18:4:1 high-speed steel are colored orange. The cathodic stain etch can, therefore, provide qualitative indications of carbide chemistry in complex alloy steels on the basis of their interference colors.
CONCLUSIONS
Although the vast majority of steels are unaffected by optical staining techniques owing to their intrinsic cubic crystal structures, they are responsive to stain etching. Compared with usual monochrome images, stain-etched microstructures of steels can display a world of information which is not simply fascinating, but informative and interpretation friendly as well. The delineation of matrix and second phases, revelation of grain and grain-twin orientation differences in vivid color, and selective tinting of carbides as a function of their chemistry and their discrimination from retained austenite can take the guesswork out of ferrous metallography. The authors are grafeftll to Dr. S. Banerjee, Director, Dr. S. K. Bhaffachayya, GM (Steel), and MY. Sudhaker Jha, GM (Products), Research and Development Cenfre for Iron and Steel, Steel Authority of India Limited, for their encouragement and support. They also express their deep appreciation to MY. John Guria, who painstakingly helped in sample preparation,
which ultimately microstructures.
rendered
such fascinating
References 1. R. J. Gray, Color metallography-introduction, in Metals Handbook, 9th ed. vol. 9, K. Mills, J. R. Davis, J. D. Destefani, D. A. Dietrich, G. M. Crankovic and H. J. Frissel, eds., American Society for Metals, Metals Park, OH, pp. 135-139 (1985). 2. H. Grahm and F. Jeglitsch, Colour methods and their application in metallography, in Microstructuru2 Science, vol. 9, G. Petzow, R. Paris, E. D. Albrecht, and J. L. McCall, eds., Elsevier North-Holland, New York, pp. 65-80 (1981). 3. C. E. Price, Differential interference contrast, in Metals Handbook, 9th ed., vol. 9, K. Mills, J. R. Davis, J. D. Destefani, G. M. Crankovic, and H. J. Frissel, eds., American Society for Metals, Metals Park, OH. pp. 150-151 (1985). 4. G. F. Vander Voort, Tint etching, Metal Prog. 127: 3141 (1985). 5. R. C. Cochrane, Optical microscopy, in Microstructuru2 Characterisation E. Metcalfe, ed. Institute of Metals, London, pp. 43-93 (1988). 6. E. Beraha, Metallographic reagents based on sulfide films, Pruc. Metall. 7242-248 (1970). 7. E. Beraha, Metallographic reagents based on molybdate solutions, Pruct. Metnll. 11:271-275 (1974). 8. J. R. Kilpatric, A. 0. Benscoter, and A. R. Marder, Tint etching improves resolution and contrast of microstructures, Metal Prog. 100:79-81 (1971). 9. R. C. G&ens, The polarizing microscope, in Optical Microscopy ofMetals,Sir Issac Pitman and Sons Limited, Melbourne, p. 103-123 (1970). 10. G. F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, New York pp. 632-655 (1984). 11. I’. E. Repas, Physical metallurgy of dual-phase steels, Iron Steel Maker 7~12-17 (1980). 12. F. S. Le Pera, Improved etching techniques for determination of percent martensite in HSLA dualphase Steel, Metullogrupky 12:263-268 (1979). 13. F. C. Bell and D. E. Sonon, Improved metallographic etching techniques for stainless steel and for stainless steel to carbon steel weldments, Metallography 9:91-107 (1976). 14. T. A. DeBold, Duplex stainless steel - microstructures and properties, Journal of Metals 41:12-15 (1989). 15. G. F. Vander Voort, The metallography less steels, J. Met& 41:6-11 (1989). Received July 1995, accepted March 1996.
of stain-
Microstructural Manifestations in Color: Some Applications for Steels Amitava Ray and Sanjay K. Dhua Physical Metallurgy Group, Research and Development lndia Limited, Ranchi 834002, India The interpretational
advantages
white images is recognized
Centrefor
of colored microstructures
Iron and Steel, Steel Authority of
over conventional
black-and-
in view of the natural response of the human eye to color varia-
tions. This applies well for steels, where ambiguities
in phase and feature discrimination
can often arise from intrinsic lack of contrast or otherwise subtle grey-level differences of the observed microstructural constituents. Although staining techniques are advantageous from this standpoint, their use, even today, is rather limited in routine metallography Textbook instructions and standard reagent formulations alone cannot guarantee optimum results unless sample preparation is meticulous and the etching technique is perfected. Moreover, the etch response of different steel chemistries being unique to each grade demands experimentation as a prerequisite for obtaining optimum results. This article provides a pictorial insight into the fascinating world of microstructures obtained in a gamut of plaincarbon, dual-phase, low-alloy, stainless, and high-alloy tool steels investigated in our laboratory. Issues concerning the revelation of anodic matrix and second phases and crystallographic orientation aspects of microstructural features, such as grains, twins, and colonies, are also elucidated and discussed. 0 Elsevier Science Inc., 1996
INTRODUCTION
ages can be brought about by “optical staining” techniques or by interference film deposition methods. Optical staining techniques, such as polarization, sensitive-tint, and differential interference contrast (DIC), generate color contrast as a consequence of microstructural anisotropy or variations in surface topography. Metals with cubic crystal structures, such as iron, aluminium, and copper, are generally not responsive to polarized light in the as-polished state. Polarization contrast in weakly anisotropic materials can be enhanced by interposing a sensitive-tint plate between the polarizer and the analyzer whereby double refraction or birefringence is accentuated. The DIC technique can be used for obtaining color contrast in microstructures [2] and is particularly suitable for as-polished specimens which contain phases of widely vary-
Conventional metallography of steels involving etching with common reagents, such as, nital, picral, and Vilella’s, generally reveals microstructures with a lightand-dark contrast. Although many microstructural constituents can normally be revealed in images possessing adequate contrast, discrimination of phases with subtle grey-level differences can be both difficult and misleading. It is in this perspective that color discrimination of microstructures presents exciting possibilities of revealing much more meaningful and reliable information. This interpretation advantage arises primarily because the human eye can discriminate a vast array of colors more easily than a few shades of grey [l]. Color contrasts in light-microscopic imMATERIALS CHARACTERIZATION 37:1-S 1996 0 Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
3 1044.5803/96/$15.00 PI1 SlO44-5803(96)00022-S
A. Ray and S. K. Dhua
ing hardness levels [3]. In etched microstructures, the DIC technique can be used to highlight subgrain structures and mechanical twins. Chemical color staining, on the other hand, involves deposition of a thin interference film, 40-500nm thick [4], on the polished surface of a metallographic specimen by etching. The microstructural color contrast is a consequence of interference of light reflected from the film and metal surfaces and is controlled by the film thickness in the usual interference order: yellow, red, violet, blue, and green [5]. The characteristics of the base metal and the mode of illumination employed also govern the coloration obtained. Stain etchants can be used to tint either anodic or cathodic microconstituents. Reagents based on metabisulfite and thiosulfate systems [6] for coloring anodic matrix phases, such as austenite and ferrite, and those employing sodium molybdate Na2Mo04.2H20) for preferentially etching cathodic microconstituents [7] have been developed. Nevertheless, it is worthwhile to study the etch response of different types of steels for a comprehensive understanding of their microstructures. Most metallographic investigations in structure-property correlation studies involve quantitative determination of phase volume fractions, grain size, and particle size distributions. The accuracy of such measurements is largely governed by the clarity of phase and feature delineation of the microstructure. The application of selective color-staining techniques can, therefore, obviate interpretational ambiguities concerning primary matrix phases, minor second-phase constituents, grain orientation, and grain/twin delineation. This article discusses these microstructural issues in some varieties of plain-carbon, dual-phase, free-cutting, stainless, and alloy tool steels on the basis of our laboratory investigations.
METALLOGRAPHIC
PROCEDURES
Although conventional metallographic procedures per se are adequate for the prepa-
ration of steel samples subjected to normal etching, the execution of sample preparation steps must be absolutely perfect for satisfactory color staining. The most critical step is specimen cutting, which should be carefully exercised to prevent specimen damage and impair subsequent polishing quality. To obtain a scratch-free finish, the final polishing operation was carried out in two steps, using suspensions of 0.3 km-size a-alumina and 0.05 pm-size y-alumina on Selvyt cloth. Moderate hand pressure and low polishing wheel speeds (120-150rpm) are recommended to prevent generation of artifacts or excessive relief on polished surfaces. Careful control needs to be exercised at all stages of preparing stainless and lowcarbon ferritic sheet-steel samples, which are particularly scratch prone. Moreover, since the austenitic grades can work harden to produce stress-induced martensite, precautions should be taken to minimize deformation during cutting and grinding operations. Color etching, nevertheless, is far easier for ferritic sheet steel than for austenitic (or ferritic) stainless steels. Meticulous preparation of samples to a scratch-free and blemish-free finish is a precondition for satisfactory revelation of colored microstructures. The problem arises from the fact that very fine scratches, apparently innocuous in conventional blackand-white microstructures, tend to be grossly accentuated as prominent imperfections after stain etching. To obviate such possibilities, polished samples should be examined with dark-field illumination to detect such “ghost” scratches and repolished, if necessary. Prior to stain etching, polished samples should be thoroughly cleaned from polishing debris, as any adherent dirt or film can inhibit film formation during etching. Ultrasonic cleaning, followed by rinsing in ethyl alcohol and drying, has been found to yield excellent results. As regards etching, immersion is required (never swab) and the etch time is controlled by the visual appearance of the color stain on the polished surface. For revelation of true microstructural colors, examination should be carried out with
Steel Structures in Color
3
bright-field illumination without the aid of filters. In special cases, however, polarization or sensitive-tint modes can be used in conjunction to enhance color contrast. Microphotography can then be performed on standard color negative or positive film by adjusting the color temperature of the quartz-iodine halogen lamp to 3200K.
MICROSTRUCTURAL PLAIN-CARBON
REVELATIONS STEEL
Nital-etched low-carbon steel with an essentially ferritic matrix is not responsive to optical staining methods. Examination under crossed-polarized light results in complete extinction of the image of the ferrite grains. Further, no manifestations of grainto-grain orientation variations can be revealed in the sensitive-tint mode, because all ferrite grains appear magenta, even with microscope stage rotation. Stain etching with a sulfide film-forming reagent can tint ferrite grains in different colors. The stain-etched microstructure of the transverse section of a hot-rolled, lowcarbon “rimming quality” steel-wire rod at 100X is shown in Fig. 1. The polished sample was pre-etched with 4% picral and subsequently tint etched with Beraha’s reagent, an aqueous solution of 3% K&Os and 10% Na2S203, by immersion. Note that although the ferrite grains below the peripheral rim have been preferentially tinted in many hues, the low-impurity and more noble rim layer (practically pure iron) is unaffected. This differential ferrite-rim color contrast enables a more precise microscopic measurement of rim thickness than would be possible for a conventional nital-etched microstructure where the rim and the ferritic matrix would both appear white. The multiple colorations of the ferrite grains are a consequence of different film thicknesses formed on the underlying grains and the resulting light interference. Sulfide filmforming etchants have been reported to be orientation sensitive [8]. The different colorations of the ferrite grains, in spite of sim-
ilar etching time, are therefore indicative of orientation differences among them. Upon prolonged etching with the same Beraha’s reagent, a line-etch pattern (Fig. 2) can be developed in the ferrite grains. Microscopic observations at high magnifications indicate that these line patterns are manifestations of film microcracking. It is worthwhile noting that the line patterns are clearly discernible on the green-tinted ferrite grains. The green interference is a manifestation of a greater film thickness, which is naturally expected after prolonged etching. The occurrence of such striations along preferred directions is unique to each ferrite grain and is, therefore, possibly related to their respective orientations. Unlike low-carbon steel, the pearlitic regions in nital-etched high-carbon steel are found responsive to crossed-polarized light. The bright-field image of a 0.6wt.% carbon steel at 500 X magnification is shown in Fig. 3(a). Under crossed-polarized light, comof the isotropic grain plete extinction boundary ferrite occurs. Upon insertion of a sensitive-tint plate between the polarizer and the analyzer, the ferrite coloration changes to magenta, while the pearlitic regions display an array of fascinating colors as shown in Fig. 3(b). The different colorations in the pearlitic regions are indicative of their anisotropic behavior under polarized light. Chemical etching, carried out to generate an anisotropic film or an uneven surface, has been reported [9] to induce polarization effects even in isotropic metals. Rotation of the specimen stage does not change the magenta coloration of the grain boundary ferrite; the pearlitic regions, however, are found to transform from one set of colors to another with each 45” rotation of the stage. This behavior is presumably a manifestation of etch-induced roughness in the pearlitic regions. Stain etching of the sample in Beraha’s aqueous 1% NazMo04 reagent (acidified with nitric acid to a pH between 2.5 and 3), containing a pinch of NHhFHF, preferentially tints the cathodic cementite phase blue and the pearlitic regions blue-brown. The anodic grain boundary ferrite is tinted
FIG. 1. Stain-etched microstructure of the transverse section of low-carbon “rimming quality” steel wire showing multicolored ferrite grains and untinted peripheral rim.
FIG. 3. Nital-etched 0.6wt.% carbon steel showing (a) pearlitic regions of varying resolution and ferrite precipitation along boundaries under bright-field illumination and, (b) sensitive-tint image showing multicolored pearlitic regions of varying orientation and magenta-colored isotropic ferrite. FIG. 2. Line-etch patterns showing orientation differences among ferrite grams of low-carbon “rimming quality” steel after prolonged stain etching.
FIG. 4. High-carbon steel which is stain etched with sodium-molybdate reagent showing cathodic cementite regions tinted blue and brown and ferrite precipitates colored yellow.
FIG. 5. Stain-etched microstructure of hardened and tempered EnlA-grade free-cutting steel showing silvery white sulfide stringers highlighted against green and purple matrix of tempered martensite.
FIG. 6. Dual-phase steel tint etched to reveal dispersed islands of martensite in brilliant white against brown ferrite matrix.
FIG. 7. Annealed AISI-316-grade stainless steel exhibiting austenite grains and twins in differential color contrast after stain etching.
FIG. 8. Stain-etched microstructure of duplex stainless-steel sheet showing light etching austenite islands in a matrix of brown ferrite.
FIG. 9. Hardened and tempered 6Cr:lMo:lC steel: (a) Vilella-etched microstructure showing both alloy carbides and retained austenite in white against dark martensite matrix, and (b) violet chromium-rich alloy carbides and orange retained austenite against browngreen martensite following sodium-molybdate color etch.
steel exhibiting orange tungsten-rich carbides in a matrix of greenish tempered martensite following stain etching in sodium-molybdate reagent.
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A. Ray and S. K. Dhua
yellow by this reagent, as shown in Fig. 4. Because the ferrite is colored yellow, this etch is a complex tint etch rather than a cathodic etch. The complex stain etch, however, does not reveal orientation differences among pearlitic colonies as is exhibited by ferrite which is stain etched with a sulfide film-forming reagent. FREE-CUTTING
STEEL
In nital-etched, hardened, and tempered free-cutting steel, the dove-grey MnS stringers are camouflaged in the dark tempered martensite matrix and, hence, are difficult to discern. Optical staining methods employing polarized light are futile in selective discrimination of the MnS owing to its extinction. To highlight inclusionmatrix contrast, stain etching can be carried out in an aqueous solution containing 3% K&Os, 1% NHJFHF, and 1% sulfamic acid [lo] until the polished surface turns bluish. The stain-etched micrograph of a hardened and tempered EnlA-grade free-cutting steel containing 0.2wt% S is shown in Fig. 5. The revelation of the MnS inclusions as silvery white stringers against the purplegreen matrix of tempered martensite is a consequence of preferential staining of the anodic tempered martensite. The multiple colorations of the tempered martensite (anodic phase) may be attributed to the orientation sensitivity of the sulfide film etch on different martensite packets. The highlighting of MnS inclusions, in preference to other inclusion species such as silicate stringers, is thus conducive for their easier identification from etched microstructures. DUAL-PHASE
STEEL
Dual-phase steels are characterized by a microstructure of fine-grained polygonal ferrite with a dispersion of small islands of martensite [ll]. The properties of this steel are largely governed by the relative volume fractions of the individual phases and their distribution. Consequently, estimation of the phase contents by automatic image analysis is often necessary. In nital-etched as dual-phase steel, ferrite is revealed
bright polygonal grains with well-defined boundaries. The minor constituent, martensite, being selectively attacked, appears darker. However, image analysis of such a microstructure can pose problems of greylevel selection, since regions of the ferrite grain boundaries are often picked up while adjusting the discrimination level for the dark martensite phase. In overcoming this difficulty, stain etching with Le Pera’s reagent [12], containing equal volumes of 4% picral and 2% aqueous Na&.Os, is found helpful. The micrograph of a dual-phase steel sheet (O.OSwt.% C, l.OOwt.% Mn, 1.20wt.O~ Si, 1.40wt.% Cr, 0.026% S, and O.O27wt.% I’) stain etched with this reagent is shown in Fig. 6 at 1000X magnification. The dispersed martensite islands in this microstructure are revealed in brilliant white while the selectively attacked ferrite matrix is tinted brown. Incidentally, since the ferrite grain boundaries are not prominently defined after this stain etch, the striking white contrast of the martensite enables easier discrimination and estimation by image analysis techniques. STAINLESS
STEEL
Unlike carbon and tool steels, the delineation of microstructures in stainless steels by either electrolytic or conventional chemical etching is comparatively difficult. Although in single-phase austenitic stainless Kalling’s reagent (5g steels, “Waterless” CuC12,40ml HCl, 30ml water) can produce excellent grain boundary delineation, the annealing twins are nevertheless revealed in the same contrast. This situation is particularly problematic for reliable assessment of austenite grain size by chart and automatic image analysis methods, since twins cannot be ignored selectively. An electrolytic etch with 60% HNOs, 40% water, has been reported [13] to reveal twinfree austenite successfully. In this case, proper control of voltage and use of stainless-steel cathodes can yield satisfactory results. Although a more noble cathode is useful, it is not necessary.
Steel Structures in Color
The microstructure of a solution-annealed AISI 316 stainless steel, stain etched with Beraha’s reagent (aqueous solution of 20% HCl, 2% NHJFHF, and 1% KzS20s) is shown in Fig. 7. The tinted austenite grains show varying shades of green, purple, and orange. The annealing twins within the grains appear in red and orange hues. Actually, the structure within the twin is austenite and is similar to that just outside the twin. In some austenite grains, wide twins are visible whereas, in others, single twin boundaries can be observed. As a matter of fact, there is no real difference, except for the stacking sequence in the austenite on either side of these boundaries. Thus, it is the twin boundary itself which is important. Although the austenite grain size in this tint-etched microstructure may be more difficult to rate by conventional chart methods, as compared with conventional swabetched stainless steel, the structure is nevertheless fascinating and enables quick visual detection of twins. The color variations in the microstructure are a consequence of interference effects from regions of different film thicknesses and can be attributed to the orientation sensitivity of sulfide films. This differential twin-grain color contrast is a manifestation of their crystallographic orientation differences. Most wrought duplex stainless steels in the annealed state contain about 40-50% austenite in a matrix of ferrite 1141. For understanding structure-property correlations in this steel, quantitative assessment of austenite and ferrite contents is indispensable. With conventional reagents, such as glyceregia (5ml HN03, lOm1 glycerol, and 15ml HCl), the primary matrix phases, ferrite and austenite, are not completely delineated. Although prolonged (about 30 minutes) etching in ethanolic 15% HCl (15ml HCl in 100ml ethyl alcohol) can render excellent microstructural delineation of the ferrite-austenite boundaries 1151, the identity of the matrix phases is rather ambiguous, since both ferrite and austenite are revealed in the same contrast. Figure 8 shows the stain-etched microstructure of a duplex stainless-steel sample, etched with a
7
reagent containing aqueous 20% HCl with 1% K&Os. The ferrite matrix is preferentially attacked and is tinted brown while the untinted austenite appears as dispersed islands. The sharp delineation of ferrite and austenite phases in differential color contrast thus enables authentic phases identification and reliable quantitative estimation. TOOL STEELS Microstructural characterization of hardened and tempered tool steels not only involves delineation of alloy carbides, tempered martensite, and retained austenite, but their estimations as well. With conventional etchants such as nital, picral, or Vilella’s tempered martensite is selectively etched in preference to both alloy carbides and retained austenite. Consequently, under bright-field illumination, discrete white particles of alloy carbides are revealed in a matrix of dark-etching tempered martensite. Retained austenite, if present, is optically discernible as bright regions interspersed in the martensite. Figure 9(a) shows the bright-field monochrome image of a hardened and tempered 6Cr:lMo:lC wear-resistant steel etched with Vilella’s reagent (lg picric acid, 5ml HCl, and 100ml ethanol). The black-and-white microstructure shows large white skeletal-like eutectic carbide and numerous small globular carbides in a matrix of dark tempered martensite. The microstructure also shows some amount of retained austenite which, owing to similar color contrast as that of the carbides, is difficult to discern optically. The lack of adequate optical contrast between the retained austenite and the alloy carbides makes quantitative estimation of their volume fractions by image analysis difficult. To bring about a differential color contrast between the alloy carbides and the retained austenite, stain etching is preferable for selectively tinting the cathodic carbides. The microstructure of the same steel, stain etched with Beraha’s sodium-molybdate reagent (lg NazMoOd, 100ml water, 0.4ml HNOs, and 300mg NHJFHF) after a pre-
A. Ray and S. K. Dhua
8 etch in 2% nital, is shown in Fig. 9(b). The etchant selectively colors the chromiumrich carbide particles violet while the retained austenite appears orange and the martensitic regions are dark brown and green. When the sodium-molybdate etch is applied to hardened and tempered highspeed steel (18wt.% W, 4wt.% Cr, lwt.% V) after an initial nital pre-etch, the brightfield microstructure (Fig. 10) reveals orange carbide particles dispersed in a matrix of greenish tempered martensite. Note that while the sodium-molybdate reagent colors the chromium-rich alloy carbides in 6Cr: 1Mo:lC steel violet, the tungsten-rich alloy carbides in 18:4:1 high-speed steel are colored orange. The cathodic stain etch can, therefore, provide qualitative indications of carbide chemistry in complex alloy steels on the basis of their interference colors.
CONCLUSIONS
Although the vast majority of steels are unaffected by optical staining techniques owing to their intrinsic cubic crystal structures, they are responsive to stain etching. Compared with usual monochrome images, stain-etched microstructures of steels can display a world of information which is not simply fascinating, but informative and interpretation friendly as well. The delineation of matrix and second phases, revelation of grain and grain-twin orientation differences in vivid color, and selective tinting of carbides as a function of their chemistry and their discrimination from retained austenite can take the guesswork out of ferrous metallography. The authors are grafeftll to Dr. S. Banerjee, Director, Dr. S. K. Bhaffachayya, GM (Steel), and MY. Sudhaker Jha, GM (Products), Research and Development Cenfre for Iron and Steel, Steel Authority of India Limited, for their encouragement and support. They also express their deep appreciation to MY. John Guria, who painstakingly helped in sample preparation,
which ultimately microstructures.
rendered
such fascinating
References 1. R. J. Gray, Color metallography-introduction, in Metals Handbook, 9th ed. vol. 9, K. Mills, J. R. Davis, J. D. Destefani, D. A. Dietrich, G. M. Crankovic and H. J. Frissel, eds., American Society for Metals, Metals Park, OH, pp. 135-139 (1985). 2. H. Grahm and F. Jeglitsch, Colour methods and their application in metallography, in Microstructuru2 Science, vol. 9, G. Petzow, R. Paris, E. D. Albrecht, and J. L. McCall, eds., Elsevier North-Holland, New York, pp. 65-80 (1981). 3. C. E. Price, Differential interference contrast, in Metals Handbook, 9th ed., vol. 9, K. Mills, J. R. Davis, J. D. Destefani, G. M. Crankovic, and H. J. Frissel, eds., American Society for Metals, Metals Park, OH. pp. 150-151 (1985). 4. G. F. Vander Voort, Tint etching, Metal Prog. 127: 3141 (1985). 5. R. C. Cochrane, Optical microscopy, in Microstructuru2 Characterisation E. Metcalfe, ed. Institute of Metals, London, pp. 43-93 (1988). 6. E. Beraha, Metallographic reagents based on sulfide films, Pruc. Metall. 7242-248 (1970). 7. E. Beraha, Metallographic reagents based on molybdate solutions, Pruct. Metnll. 11:271-275 (1974). 8. J. R. Kilpatric, A. 0. Benscoter, and A. R. Marder, Tint etching improves resolution and contrast of microstructures, Metal Prog. 100:79-81 (1971). 9. R. C. G&ens, The polarizing microscope, in Optical Microscopy ofMetals,Sir Issac Pitman and Sons Limited, Melbourne, p. 103-123 (1970). 10. G. F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, New York pp. 632-655 (1984). 11. I’. E. Repas, Physical metallurgy of dual-phase steels, Iron Steel Maker 7~12-17 (1980). 12. F. S. Le Pera, Improved etching techniques for determination of percent martensite in HSLA dualphase Steel, Metullogrupky 12:263-268 (1979). 13. F. C. Bell and D. E. Sonon, Improved metallographic etching techniques for stainless steel and for stainless steel to carbon steel weldments, Metallography 9:91-107 (1976). 14. T. A. DeBold, Duplex stainless steel - microstructures and properties, Journal of Metals 41:12-15 (1989). 15. G. F. Vander Voort, The metallography less steels, J. Met& 41:6-11 (1989). Received July 1995, accepted March 1996.
of stain-