SUMMER INSTITUTE ON LABOUR ECONOMICS AND INDUSTRIAL RELATIONS ...

SUMMER INSTITUTE ON LABOUR ECONOMICS AND INDUSTRIAL RELATIONS ...

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1SUMMER INSTITUTE ON LABOUR ECONOMICS AND INDUSTRIAL RELATIONS, DEPARTMENT OF LABOUR & SOCIAL WELFARE, PATNA UNIVERSITY, PATNA-5 THE STRUCTURE OF WAGES BY Dr. G.P. Sinha, Ph.D. (Cornell), University Professor and Head, Department of Labour & Social Welfare, Patna University, Patna Introduction There are two sources of income in society : property and labour. Labour here is used in its widest connotation ranging from the labour of the unskilled to the labour of engineers or other most highly skilled and talented groups of people.
  • objective terms
  • terms of a fixed percentage
  • early days of the evolution of the job evaluation methods
  • economic forces
  • relative position
  • wage structure
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ASM Handbook, Volume 9: Metallography and Microstructures Copyright © 2004 ASM International®
G.F. Vander Voort, editor, p493–512 All rights reserved.
DOI: 10.1361/asmhba0003752 www.asminternational.org
Color Metallography
George F. Vander Voort, Buehler Ltd.
THE USE OF COLOR in metallography has opment of wavelength-dispersive spectrometers etchants that produced color either by immer-
a long history, with color micrographs published and energy-dispersive spectrometers used on sion, sometimes in boiling solutions, or electro-
lytically. Historical information on these etch-over the past eighty-some years. A number of electron microprobe analyzers and scanning
general articles (Ref 1–15) have been published microscopes, the color of inclusions us- ants can be found in Ref 16.
reviewing methods and applications. ing different illumination modes was part of the Tint etchants may color either the anodic (ma-
Natural color is of use in only a few classic identification schemes used. However, natural trix) or cathodic constituents. There are also
metallographic applications. Prior to the devel- color has limited applicability. electrolytic reagents known as anodizing solu-
Color can be created by optical methods, such tions. They have been used most commonly with
as with polarized light and differential interfer- aluminum and its alloys. These solutions may
ence contrast illumination. Polarized light ex- produce a thin film on the surface, with a degree
amination is extremely useful for studying the of roughness. Examination in bright field reveals
structure of certain metals, without etching, that little, but polarized light reveals the structure
have noncubic crystal structures, such as beryl- clearly.
lium, hafnium, -titanium, uranium, and zirco- There are other procedures to create interfer-
nium. In many cases, polarized light can be used ence films using heat (heat tinting), vapor de-
with etched specimens, regardless of their crystal position, or by reactive sputtering. Color can be
structure, to produce color. Differential interfer- observed with bright-field illumination but often
ence contrast reveals height differences between can be enhanced using polarized light.
constituents and the matrix, but in most cases,
the color is of esthetic value only.
Optical Methods forColor etching methods are widely used, al-
though they are not universal. Color etchants Producing Color
have been developed for a limited number of
metals and alloys, and they are not always easy There are few instances where naturally oc-
to use, nor are they fully reliable. Color etchants curring color differences are observed in metallic
Microstructure of a porous high-carbon steel are used by immersion or electrolytically. AFig. 1 systems. Specimens plated with copper or gold
powder metallurgy specimen infiltrated with complete listing of all color etchants is beyond are a common example. There are two main op-
copper showing the natural color of the copper, which is the scope of this article, but good compilations tical methods for producing coloring: polarizedeasier to see when the steel has been tint etched (revealing
are available (Ref 7, 10–15). Aside from the im-coarse plate martensite and retained austenite)
mersion tint etchants, there are a number of older
Microstructure of as-cast Au-22%Al showing Cuprous oxide in tough pitch arsenical copper Microstructure of walnut (plane perpendicular toFig. 2 Fig. 3 Fig. 4
the “purple plague,” AuAl intermetallic (red- (hot extruded and cold drawn) viewed in dark the trunk axis) showing the cells and pores re-2
dish), surrounded by the Al-AuAl eutectic after polishing field, revealing the classic ruby-red color. Magnification vealed using dark-field illumination. Magnification bar is2
to a 1 lm finish. Magnification bar is 50 lm long. bar is 10 lm long. 100 lm long.494 / Metallographic Techniques
light and differential interference contrast. In the natural color of copper is clearly seen against nium, uranium, zinc, and zirconium. Figure 5
both cases, color per se is of minimal value be- the steel matrix. It may be easier to see the cop- shows the grain structure near the surface of an
yond simple esthetics. The color of inclusion electron-beam-melted crystal bar of high-purityper color when the steel matrix is etched. Figure
phases in bright field, dark field, and polarized zirconium (not etched) that was hot rolled, an-1 shows an example of porous high-carbon steel
light has been used for identification purposes nealed, and cold drawn. The deformation processthat was partially infiltrated with liquid copper,
for many years. produces mechanical twins that are quite numer-where the natural color of the copper can be eas-
Natural color is not a common occurrence in ous at the surface but nearly absent in the inte-ily observed. There is a substantial difference in
metallic systems; many metals have a similar rior. Other examples of color developed with po-the reflectivity of iron and copper. Etching of the
white color. When polished, only a few metals larized light on as-polished specimens of metalshigh-carbon martensitic/pearlitic matrix in-
exhibit a color other than white; for example, with noncubic crystal structures are shown increases the image contrast difference, making it
gold and copper appear yellow when polished. Fig. 6, hafnium; Fig. 7, ruthenium; and Fig. 8,easier to see the copper color. The so-called
Platings of these metals can be easily recognized Cd-20% Bi.“purple plague,” the intermetallic phase Al Au2
by their color. A classic example of natural color Unfortunately, not all noncubic phases orthat can occur in brazing of integrated circuits,
differences is the detection of liquid metal em- metals respond well to polarized light. In somehas a natural purple or red-violet color, as illus-
brittlement in steels due to copper. In this case, cases, a well-prepared specimen responds to po-trated in Fig. 2. Nitrides and some inclusions ex-
larized light, revealing the microstructure quitehibit specific colors when examined with bright-
clearly but without appreciable color. The con-field illumination, but overall, natural color is
trast produced, and the color intensity, may beuncommon with metals and alloys.
a function of both the degree of anisotropy ofDark-Field Illumination. Inclusions in met-
the metal or alloy and the quality of specimen
als have been identified using known colors
when viewed with bright field, dark field, or po-
larized light (Ref 17). Cuprous oxide, Cu O, in2
tough pitch copper, for example, is easily rec-
ognized because it glows ruby red in dark-field
illumination (Fig. 3) but appears bluish-gray in
bright field. Cuprous sulfide, Cu S, has a similar2
color in bright field but remains dark and dull in
dark field (Ref 18). Wood also exhibits natural
color in dark field, as shown in Fig. 4.
Polarized Light. There are purely optical
methods for generating color images employing
polarized light and differential interference con-
trast illumination. Polarized light examination is
useful with phases or anisotropic metals that
Extensive mechanical twinning was observed inFig. 5 have noncubic crystallographic structures (Ref
high-purity, electron-beam-melted zirconium af-
2), such as antimony, beryllium, cadmium, co-ter hot working and cold drawing. Viewed in polarized
light. Magnification bar is 100 lm long. balt, magnesium, scandium, tellurium, tin, tita-
Microstructure of as-cast pure ruthenium, as-pol-Fig. 7
ished and viewed in polarized light plus sensitive
tint, revealing a mixture of equiaxed and columnar hex-
agonal close-packed grains and some small shrinkage cav-
ities (black). The magnification bar is 200 lm long.
Microstructure of Cd-20%Bi in the as-cast con-Fig. 8
dition, unetched and viewed with polarized light
(slightly off the crossed position) plus sensitive tint, reveal-
ing cadmium dendrites of various orientation. The inter-
Microstructure of wrought pure hafnium, with an as-polished specimen viewed in polarized light plus sensitive dendritic constituent is a eutectic of cadmium and bismuthFig. 6
tint, revealing an equiaxed alpha hexagonal close-packed grain structure. A few mechanical twins can be seen but is too fine to resolve at this magnification. Magnifica-
at the surface (arrows). The magnification bar is 100 lm long. tion bar is 200 lm long.Color Metallography / 495
preparation. The quality of the surface appears etchants for magnesium did not provide this im- with a specific reagent that produces etch pits or
to be the key factor, but the quality of the op- provement). furrows within the grains that respond to polar-
tical system is also very important. In some In some cases, isotropic metals and alloys may ized light. Figure 11 shows an example of an alu-
cases, the color response in polarized light can respond to polarized light after being etched with minum brass (Cu-22%Zn-2%Al) that was cold
be markedly improved after etching specimens a particular reagent that either produces an aniso- worked and annealed at 750 C (1380 F). The
having noncubic crystal structures with some tropic film on the surface or roughens the surface. specimen was etched with the classic potassium
specific reagent. Figure 9 shows the grain struc- In some cases, anodizing solutions may produce dichromate reagent, which produces a black-and-
ture of hot-rolled hexagonal close-packed (hcp) an optically anisotropic film on the surface that white grain contrast image in bright field that
Zn-0.1%Ti-0.1%Cu in the as-polished condition produces color by interference. Color tint etch- yields excellent color contrast in polarized light.
and after etching, which improved polarized ants form a film on the surface of certain metals The etch furrows are aligned crystallographically,
light response and color formation. Note that that produces interference colors. If such a film is and this produces grain-orientation coloring in
the fine precipitates between the elongated formed, color will be observed in bright field. In crossed polarized light aided by a sensitive tint
grains are much easier to see in polarized light many cases, color contrast can be further en- plate (also called a lambda plate, a full wave plate,
after etching. Figure 10 shows pure hcp mag- hanced when viewed with polarized light, due to or a first-order red plate). Fine lamellar structures
nesium containing mechanical twins that were the birefringence of these films. Anodizing and will respond to polarized light regardless of the
brought out vividly in color only after etching tint etching are discussed subsequently. The sur- etchant used, producing strong coloration but of-
with the acetic-picral reagent (other standard faces of many isotropic metals can be etched ten without any benefit except esthetics.
Microstructure of wrought 99.98% Mg etchedFig. 10
Microstructure of Zn-0.1%Ti-0.1%Cu hot rolled to 6 mm (0.24 in.) thickness. (a) The as-polished condition,Fig. 9 with acetic-picral reagent and viewed with
using polarized light, revealed elongated hexagonal close-packed grains containing mechanical twins. Some crossed polarized light plus a sensitive tint filter. The mag-
fine precipitates are present in the grain boundaries but are not clearly revealed. (b) The structure after etching with nification bar is 200 lm long.
Palmerton reagent and viewing with polarized light plus sensitive tint better reveals both the precipitates and grain
structure. Magnification bars are 50 lm long.
Microstructure of a shape memory alloy (Cu-Fig. 12
26%Zn-5%Al) showingb martensite in a face-1
Wrought aluminum brass (Cu-22%Zn-2%Al) annealed at 750C (1380F), producing equiaxed alpha grains centered cubic alpha matrix, using Nomarski differentialFig. 11
containing annealing twins, and etched with potassium dichromate. Images in (a) bright field and (b) crossed interference contrast without etching. The magnification
polarized light plus sensitive tint. The magnification bars are 50 lm long. bar is 25 lm long.496 / Metallographic Techniques
Differential interference contrast illumi- the color was produced by the use of the sensi- specimens after anodizing with Barker’s reagent
nation (DIC) (Ref 19) can be used to enhance tive tint filter with the Wollaston prism. Without should reveal color in bright-field illumination,
height differences between constituent and ma- the sensitive tint filter, the images would exhibit if an anodic film is produced, but color is not
trix on a prepared surface. Introducing a small, gray tones. observed. Instead, the surface looks etch-pitted
controlled amount of relief in final polishing can when examined with the scanning electron mi-
enhance these height differences. Color is intro- croscope at high magnification. Figure 16 shows
Film Formation andduced using a sensitive tint plate. In most cases, the surface of 1100 aluminum foil after anodiz-
the color is of no real value, but in some Interference Techniques ing with Barkers’s reagent. The bright-field im-
it has more value. Figure 12 shows an example age (Fig. 16a) simply shows the intermetallic
where Nomarski DIC was highly effective in re- particles that have been slightly attacked by theColor can be produced by a number of tech-
vealing b martensite in a Cu-26%Zn-5%Al solution. If Barker’s had produced an anodic1 niques that rely on film formation and interfer-
shape memory alloy. A more complex example film, color should be observed. A classic exper-ence effects. These films can be formed ther-
of a shape memory alloy is given in Fig. 13. This iment regarding this problem is discussed in themally, as in heat tinting, or by chemical
shows the structure of Spangold, a jewelry alloy next section. Figure 16(b) shows the specimendeposition, as in tint etching, or by vapor depo-
(Au-19%Cu-5%Al), where some martensite was viewed in polarized light; note that the grains aresition, as in the Pepperhoff interference film
formed during hot mounting (it could be seen in revealed in gray-level contrast. Figure 16(c)method. These methods tend to be selective in
polarized light). Then, the polished specimen shows the same area viewed in polarized lightnature, in that the films either color a specific
was heated in boiling water and quenched, form- with the addition of a sensitive tint filter; thisphase, but not others, or color all constituents
ing new martensite. The new martensite crosses yields the grains in color contrast. Figure 16(d)differently. In practice, the color produced is not
the original martensite in some places (these are shows the microstructure of as-continuously casta reliable means of phase identification com-
the areas with two crossing sets of parallel col- 1100 aluminum after anodizing with Barker’s re-pared to what is, or is not, colored.
ored bands), referred to as antispangle by the al- agent and viewing with polarized light plus sen-
loy inventors. sitive tint. Dendrites with the same orientation
AnodizingFigure 14 shows a rather interesting use of have been colored uniformly. However, Barker’s
DIC. A metallographically prepared specimen of usually does not reveal the segregation within (Ref 20–29) is an electrolytic pro-
High-Expansion 22-3 alloy (Fe-22%Ni-3%Cr) the dendrites. (Compare this result to that using
cedure for depositing an anodic film on alumi-
was cooled to73C(100F), which caused Weck’s color tint etch for aluminum, shown in
num and certain other metals, for example, nio-
martensite to form in areas where the austenite Fig. 52.) Anodizing with Barker’s, or other so-
bium, tantalum, titanium, uranium, and
stability was low. When martensite forms, it does lutions, is the most universal procedure for re-
zirconium. Lacombe and Beaujard (Ref 20) first
so by a shear transformation that produces sur- vealing grain structures in cast and wrought alu-
described the method in 1945. In was initially
face movement at a free surface. The specimen minum alloys. Anodizing solutions have been
thought that this film varied in thickness from
was brought back to room temperature, cleaned developed for a number of metals and alloys, and
grain to grain, according to their crystallographic
off, dried, and viewed with Nomarski DIC, pro- some of these do deposit anodic films that pro-
orientation, and that the birefringent properties
ducing an excellent rendering of the martensite duce color by interference effects, but Barker’s
of the oxide film varied the ellipticity produced
without etching. In some cases, DIC can be used does not.
by reflection of the beam. However, experiments
effectively to study the structure of materials
have shown that a film is not formed on alumi-
with significant variations in hardness and pol- Chemical Etchingnum when anodized by reagents such as Bar-
ishing rates. Figure 15 shows the microstructure
ker’s. Instead, the coloration effects in polarized
of high-density polyethylene containing a filler There are many cases where a chemical etch-light are due to double reflection from a fur-
material, viewed with DIC. In these examples, ant, when used on an isotropic metal, results inrowed surface produced by the anodizing solu-
grain-orientation coloration when viewed withtion, similar to certain chemical etchants dis-
polarized light and sensitive tint. Mott andcussed previously. Examination of aluminum
Haines (Ref 30) and Gifkins (Ref 31) have de-
scribed suitable preparation and etching proce-
Microstructure of Spangold (Au-19%Cu-Fig. 13
5%Al), a new jewelry alloy, using martensite
formation to create ripples (“spangles”) on the surface. The
specimen was polished, heated to 100 C (212 F) for 2
min, and quenched in water to form martensite, which pro- Martensite formed on the free polished surfaceFig. 14
duces shear at the free surface. This roughness can be seen of High-Expansion 22-3 alloy after refrigeration
using Nomarski differential interference contrast without to73C(100F) to convert any unstable austenite to Microstructure of high-density polyethyleneFig. 15
etching. The crisscrossed pattern is produced by forming martensite. The specimen was brought back to room tem- containing a filler revealed using a polished
martensite, polishing, and then forming new martensite. perature, cleaned, and viewed with Nomarski differential specimen and Nomarski differential interference contrast.
The magnification bar is 50 lm long. interference contrast illumination without etching. The magnification bar is 100 lm long.Color Metallography / 497
dures for a number of isotropic metals and alloys Perryman and Lack (Ref 33) performed a clas- polarized light, and all yielded good colored mi-
for producing color with polarized light. These sic study to determine if polarization response crostructures. Then, the surfaces were coated by
procedures have been known and reported since was due to surface roughness or to the presence vapor deposition of a thin (80 nm) film of silver.
at least the 1920s. of an anisotropic surface film. The work used Silver has a fcc crystal structure and is isotropic.
Woodard (Ref 32) studied the deformation of four specimens that respond to polarized light. Hence, if the polarization effect is due to optical
face-centered cubic (fcc) Monel using a grain The first two, electrolytically polished zinc and anisotropy, then the coated surface will no longer
contrast etchant (3 g chromic acid, 10 mL nitric cadmium, are anisotropic metals with hcp crystal respond to polarized light. If, however, the po-
acid, 5 g ammonium chloride, and 90 mL water) structures that respond to polarized light when larization response is due to surface roughness,
that produced an intensity contrast pattern with properly prepared (without need for etching). the silver film should not alter polarized light
polarized light that he attributed to variations in The second two specimens were isotropic metals response. After deposition of the silver film, the
crystal orientation. Woodard proposed that an with fcc crystal structures that were etched to anisotropic zinc and cadmium specimens did not
anisotropic surface film, as in anodizing, pro- respond to polarized light. They were electrolyt- respond to polarized light, but the anodized alu-
duced the grain contrast effect. This is probably ically polished and anodized aluminum and Mo- minum and the etched Monel did respond to po-
not the case, as suggested by the study of Per- nel treated using Woodard’s method (Ref 32). larized light. Thus, surface roughness is respon-
ryman and Lack (Ref 33). The surfaces were prepared and examined with sible for the polarized light response from
Grain structure of wrought 1100-grade aluminum foil after electrolytic polishing and anodizing with Barker’s reagent (20 V direct current, 2 min). (a) Viewed with bright-Fig. 16
field illumination, revealing only the intermetallic precipitates. If anodizing had produced an interference film, colored grains should be visible. (b) Viewed with polarized
light and (c) with polarized light plus a sensitive tint filter. The magnification bars in (b) and (c) are 100lm long. (d) As-cast (concast) 1100 aluminum (99% Al) anodized with Barker’s
reagent (30 V direct current, 2 min), revealing a dendritic solidification structure. Viewed with crossed polarized light plus sensitive tint498 / Metallographic Techniques
anodizing or from etching with these specific re- that was solution annealed and aged to peak Murakami’s reagent has been used to color cer-
agents. This roughness was observed when these hardness was etched with equal parts ammonium tain alloy carbides (room-temperature immer-
surfaces are examined by electron optical meth- hydroxide and hydrogen peroxide (3% concen- sion) or delta ferrite and sigma in stainless steels
ods. Reed-Hill et al. (Ref 34) examined the sur- tration) (Fig. 17a) and with Klemm’s I tint etch (immersion while boiling). Figure 20 illustrates
faces of four fcc alloys (Ni 200, Ni 270, Monel (Fig. 17b). Results with the standard etch are the use of two modified versions of Murakami’s
400, and Cu-10 Zn), etched to produce polarized spectacular and come from the fine surface to color delta ferrite and sigma in stainless steel
light response, and confirmed the grooved sur- roughness created by etching a surface contain- welds. Groesbeck’s reagent is used less fre-
face roughness responsible for the response. ing submicroscopic precipitates and their sur- quently but is also useful for coloring alloy car-
In general, any fine lamellar structure, etched rounding coherency strain fields. Klemm’s I, like bides, as shown in Fig. 21.
with any general-purpose reagent, will exhibit most tint etchants, does not do significant etch- Color etching became a more useful and pop-
color when viewed with polarized light plus sen- ing of the surface but deposits a film epitaxially ular tool with the development of reagents by
sitive tint. Also, any etchant that yields a grain with the underlying microstructure. Conse- Klemm (Ref 50, 51) and Beraha (Ref 7, 52–64).
contrast gray-scale image will exhibit color quently, only a hint of the strain fields is seen. These works were aided by developments by
when viewed with polarized light plus sensitive Benscoter, Kilpatrick, and Marder (Ref 65–68),
tint, as shown in Fig. 11. In some cases, pre- Lichtenegger and Blo ¨ch (Ref 69), Weck (RefTint Etching
cipitation-hardened specimens can exhibit dra- 14), and others. The books by Beraha and Shpi-
matic coloration after a standard etching reagent gler (Ref 7) and by Weck and Leistner (Ref 12–Tint etching, also called stain etching or color
has been used, but mediocre coloration when a 14) have helped metallographers learn these use-etching, can be performed by using simple
ful techniques.tint etchant is used. Figure 17 shows a classic chemical immersion etchants, by electrolytic
example of this effect, where beryllium-copper There are a number of processes, besides me-etching (such as, but not limited to, anodizing),
tallographic etching, that deposit thin films ofand by potentiostatic etching. Immersion etching
various compositions on metals, but not all willis the simplest; potentiostatic etching is the most
reveal the microstructure. Film thickness is im-complex. Deposition of color films on precipi-
portant; coloration due to interference effects istates or matrix phases has been known for many
a function of film thickness. Passivation treat-years, because alkaline sodium picrate (Ref 35,
ments, used on aluminum and stainless steels,36), Murakami’s reagent (Ref 37, 38), Groes-
produce thin, transparent films that do not revealbeck’s reagent (Ref 39, 40), and Malette’s re-
the microstructure. Oxides produced by high-agent (Ref 41) have been used for many years.
temperature exposure are usually quite thick andFrench metallographers (Ref 42–48) were very
also do not reveal the microstructure. Betweenactive in the 1950s developing color etchants
these extremes, films of oxides, sulfides, and mo-based on aqueous solutions containing sodium
lybdates produce interference effects, revealingbichromate, sodium nitrate, sodium nitrite, and
the structure in color as a function of thickness.sodium bisulfite. Vilella and Kindle (Ref 49) at
The classic historical example of a process thatU.S. Steel tried the sodium bisulfite tint etch and
yields oxide films of the correct thickness forfound it useful for steels. However, these etch-
interference-generated colors is heat tinting. Cer-ants are used infrequently today. Electrolytic
tain metals, when heated to temperatures thatetching with strong basic solutions also produces
yield thin oxides, produce a visible color on thecolor films and is widely used with stainless
surface known as temper colors. At some lowsteels to color delta ferrite or sigma phase (Fig.
temperature, the film becomes thick enough to18). Alkaline sodium picrate is widely used to
produce a straw-yellow color. As the tempera-color cementite in steels, as shown in Fig. 19.
ture is increased, the film grows and the color
changes to green, then red, violet, and blue. This
Wrought, solution-annealed, and aged beryl-Fig. 17
lium-copper (Cu-1.8%Be-0.3%Co) in the heat
treated condition: 790C (1455F), held 1 h, oil quenched,
and aged at 315 C (600 F) for 2 h (380 HV). (a) Swab Microstructure of wrought 7-Mo duplex stain-Fig. 18
etched with equal parts ammonium hydroxide and hydro- less steel (Fe-0.1%C-27.5%Cr-4.5%Ni-
gen peroxide (3% conc.). Polarized light and sensitive tint 1.5%Mo) solution annealed and then aged 48 h at 816C
bring out the diffuse crisscross markings due to the sub- (1500F) to form sigma. Electrolytic etching with aqueous
microscopic c precipitates and coherency strain fields. 20% NaOH (3 V direct current, 10 s) revealed the ferrite High-carbon tool steel etched with boiling al-Fig. 19
The magnification bar is 50 lm long. (b) Tint etching with as tan and the sigma as orange, while the austenite was not kaline sodium picrate to color the cementite.
Klemm’s I did not reveal the structure as well, although the colored. The arrows point to austenite that formed during Note the lighter-colored carbides in the segregation streak.
grain size is revealed. Tint etchants produce very little etch the conversion of ferrite to sigma. Magnification bar is 10 These probably contain a small amount of molybdenum,
attack. lm in length. present in this steel.Color Metallography / 499
same sequence is obtained when films are grown them in this article. Instead, some of the more can look quite different after drying than when
on a polished surface during tint etching. It may useful and widely used color etchants are dis- immersed. If the solution contains ammonium
be difficult to grow a thick enough film to pro- cussed. The films are the product of a controlled bifluoride, NH FHF, it is best to use a plastic4
duce good color in bright field for some alloy chemical reaction between the specimen surface beaker and plastic tongs. Getting the specimen
compositions. In such a case, coloration can usu- and the reagent. The electrochemical potential to form a film at the extreme edges can be dif-
ally be improved, sometimes extensively, by on the surface of a polished specimen varies. For ficult. This can be improved by wet etching, that
viewing the specimen with polarized light plus example, the potential at a grain boundary is dif- is, squirting a small amount of distilled water on
sensitive tint. Figure 22 demonstrates this, where ferent than the grain interior, while the potential the surface before immersing it in the beaker.
Monel 400 was color etched with Beraha’s se- of a second-phase particle may be greater than Then, agitate the specimen strongly for a few
lenic acid reagent, producing a weak color image the matrix. In this case, which is quite common, seconds. If the surface is not properly cleaned
(Fig. 22a). However, polarized light plus sensi- the matrix is anodic while the particles are cath- before etching, the results will be poor. Speci-
tive tint yielded a very good color image of the odic, that is, more noble. It is far easier to grow men preparation must be performed properly,
grain structure (Fig. 22b). When a good film can an interference film on the anodic matrix phase with all preparation-induced damage removed.
be produced, as illustrated in Fig. 23(a) colora- than on the cathodic second-phase particles. An- Reagents that Deposit Sulfide Films. These
tion is excellent in bright field. Using polarized odic tint etchants are quite sensitive to crystal- are the best-known tint etches and usually the
light plus sensitive tint merely changes the color lographic orientation, with the film thickness and easiest to use. Klemm (Ref 50, 51) and Beraha
scheme (Fig. 23b) without any improvement in the color being a function of crystal orientation. (Ref 53, 54, 57, 58) have developed the most
image quality. This is not the case for cathodic tint etches,
There are a great many tint etchants, and it is which invariably color the noble phase uni-
not possible to list, describe, and illustrate all of formly, regardless of their crystallographic ori-
entation. A few reagents will color both anodic
and cathodic constituents and are referred to as
complex reagents. In metallographic work, par-
ticularly for phase identification or for selective
etching before performing quantitative measure-
ments, anodic and cathodic etchants are gener-
ally more useful than complex reagents. Re-
agents that deposit sulfide films are usually
anodic, while reagents that deposit selenium or
molybdate films are usually cathodic.
Tint etching is always done by immersion, be-
cause swabbing would prevent formation of the
interference film. Beraha often recommends
lightly pre-etching the specimen with a general-
purpose reagent before tint etching. This is not
always necessary, and the author rarely does it.
The author first etches specimens with a general-
purpose reagent to see what the structure is. This
is also useful because it may help determine
what the best tint etchant may be, or at least
which to try first. Immerse the specimen in the
beaker, and watch the surface for coloration.
This may be difficult, because the surface color
Use of modified versions of Murakami’s re-Fig. 20
agent to color delta ferrite and sigma phase in The microstructure of hot-worked, annealed,Fig. 22
stainless steel welds. (a) Delta colored blue and and cold-drawn Monel 400 (Ni-32%Cu-
brown in an austenitic matrix in type 312 stainless steel 0.3%C-2%Mn-0.5%Si) revealed using Beraha’s se-
weld metal (as-welded) using modified Murakami’s reagent lenic acid etch for copper (longitudinal axis is horizontal).
(30 g sodium hydroxide, 30 g potassium ferricyanide, 100 Monel alloys are very difficult to color etch, especially
mL water, at 100C, or 212F, for 10 s). The arrow points wrought alloys (as-cast alloys are easier). Bright field (a)
to a slag inclusion in the weld nugget. (b) Sigma phase revealed a weak image, because the interference film pro-
formed in a type 312 stainless steel weld (from the delta duced is thin (inclusions, arrows, can be seen). When this
ferrite phase) by aging at 816C (1500F) for 160 h. Sigma Alloyed white cast iron (Fe-2.2%C-0.9%Mn- occurs, polarized light (b) will often enhance the imageFig. 21
was colored green and orange by etching with Murakami’s 0.5%Si-12.7%Cr-0.4%Mo-0.1%V) with a mar- quality dramatically (the sensitive tint filter enhances col-
reagent (10 g sodium hydroxide, 10 g potassium ferricya- tensitic matrix and a network of eutectic alloy carbides oration), as shown. Note the deformed, twinned face-cen-
nide, 100 mL water) for 60 s at 80 C (175 F). The mag- (colored). Etched with Groesbeck’s reagent. (80C, or 175 tered cubic alpha grain structure. The magnification bars
nification bars are 20 lm in length. F, for 30 s) to color the alloy carbides are 50 lm long.500 / Metallographic Techniques
widely used sulfide-base tint etchants using so- does not color cementite (neither does nital, but montage of the microstructure of a weld in a low-
dium thiosulfate, Na S O , and potassium me- the contrast is too weak). Figure 27 shows an carbon steel after etching with 2% nital. While2 2 3
tabisulfite, K S O . Klemm’s I, II, III and one of example of how Klemm’s I colors ferrite in a the structure is visible, the grain boundaries are2 2 5
Beraha’s reagents use both ingredients, while wrought iron historic artifact. This is a section poorly revealed in the heat-affected zone and the
Beraha recommends a range of HCl concentra- of a musket barrel that was hammer forged from base metal (center and right side). Figure 29(b)
tions used with potassium metabisulfite for etch- wrought iron at the Henry gun factory in Naza- shows a montage of the specimen after etching
ing a variety of iron-base alloys. To make reth, Pennsylvania, in the 19th century. Across with Klemm’s I. It revealed the grain structure
Klemm’s reagents, prepare a stock solution of the top is a layer of iron oxide made magenta in with exceptional clarity.
cold water saturated with sodium thiosulfate. color by the sensitive tint filter. At the surface, Beraha Color Etching with Sulfide Films. Ber-
The compositions of Klemm’s three reagents are the grains are coarse and columnar in shape. The aha has a somewhat similar composition (Ref
given in Table 1. central region is fine grained and equiaxed, and 57) that works much like Klemm’s I. It contains
Klemm Color Etchants. To illustrate the use uniform colors are seen within the grains. At the 10gNa S O ,3g K S O , and 100 mL water.2 2 3 2 2 5
2of the Klemm color etchants, Fig. 24(a) shows bottom of the image, the grains are larger and In these reagents, the metabisulfite ion (S O )25
the microstructure of annealed cartridge brass more irregular in shape, and the grain coloration decomposes in an aqueous solution in contact
etched with equal parts ammonium hydroxide is not uniform. This difference in the grain struc- with a metallic surface, yielding SO ,H S, and2 2
and hydrogen peroxide (3%), which produced a ture must be due to differences in the amount of H . The SO depassivates surfaces, particularly2 2
weak grain contrast etch. Klemm’s I is a bit weak deformation these two regions experienced. The stainless steel surfaces, promoting film forma-
2mottled grain color suggests that the composi-to etch cartridge brass in a reasonable amount of tion. The H S provides S ions to form the sul-2
tion is more variable in these grains.time. After 3 min, weak coloration was obtained fide film when ions of iron, nickel, or cobalt are
in bright field, but results were better in polarized Figure 28 shows the microstructure of a pow- present. Figure 46(c) discussed later in the text,
light plus sensitive tint (Fig. 24b). Klemm’s II is der-made gear that was not fully consolidated shows the microstructure of as-continuously cast
stronger, and after 2 min immersion, bright field (note the dark voids). The structure is tempered low-carbon, high-strength, low-alloy steel grade
produced a better image (Fig. 24c), while polar- martensite, and Klemm’s I revealed the structure etched with Beraha’s 10/3 version of Klemm’s I.
ized light and sensitive tint yielded a much better of the lath martensite. Prior-particle shapes are In general, this reagent performs much like
image (Fig. 24d). Klemm’s II often produces easily seen. Klemm’s I but with slightly less aggressive col-
crystallographic line etching within many grains. Figure 29 shows a dramatic example of the oring of ferrite.
This can be more easily seen in Fig. 24(c). value of color etching. Figure 29(a) shows a Beraha also developed sulfide-film-forming
Klemm’s III is an excellent tint etch for copper reagents using a mineral acid (HCl) to permit
alloys and worked best for the cartridge brass tinting of stainless steels and nickel- and cobalt-
(Fig. 24e). Results were very good in bright field base heat-resisting alloys (Ref 53, 56, 58). Ber-
(Fig. 24f), although the color range was limited, aha promoted these etches with a wide range of
and even better in polarized light plus sensitive acid content to accommodate variations in cor-
tint (Fig. 24e). Tint etchants produce noticeably rosion resistance and, with possible additions to
different results on specimens that can be age the composition, to enhance coloration. These
hardened. Figure 25 shows a series of specimens etchants include the BI, BII, and BIII reagents
of Kunial brass (Cu-20.34%Zn-5.87%Ni- promoted by Weck and Leistner (Ref 13). The
1.39%Al) that were tint etched with Klemm’s III. basic compositions recommended by Beraha are
Figure 25(a) shows the grain structure of the al- given in Table 2. The HCl-base reagents are
loy after solution annealing (73 HV hardness), mainly useful for the austenitic stainless steels
revealing a multitude of colors in the grains and and nickel- and cobalt-base alloys. The author
twins. Results were the same with aging at 300 has not had success with them for ferritic stain-
C (570F), which produced only a slight hard- less steels, but they can be used to color high-
ness increase (8 HV units). However, aging at alloy steels, such as tool steels, and martensitic
400 C (750 F), which increased the hardness and precipitation-hardenable stainless steels.
to 143 HV, yielded a markedly different color While they can color duplex stainless steels, they
response (Fig. 25b). Aging at the peak tempera- are far more difficult to use than aqueous 20%
ture, 500C (930F), increased the hardness to
192 HV, and the coloration within the grains was
no longer uniform (Fig. 25c). The grain bound-
Table 1 Klemm’s reagents
aries also appear to be wide. Overaging at 700
Reagent Composition(a) UseC (1300 F) reduced the hardness to 127 HV
Klemm I 50 mL stock Immerse up to 3 min.and produced a mottled-color appearance, pre-
solution Colors ferrite andcipitate in the grain boundaries, and denuding
1gK S O martensite in cast iron,2 2 5adjacent to the grain boundaries. carbon and low-alloy
Klemm’s I has been used to color ferrite and steels; reveals
segregation. Colors b-martensite in carbon and low-alloy steels. Figure
phase in brass (-phase26(a) shows the microstructure of an as-rolled
can be colored, but very
1.31% C water-hardened tool steel etched with slowly). Colors zinc and
4% picral. The structure is fine pearlite, and there alloys
Klemm II 50 mL stock Immerse up to 8 min.is a grain-boundary carbide film present, but this
solution Colors-phase in coppercannot be easily seen with nital, even at 500
5gK S O brass, tin, and manganese2 2 5magnification (2% nital was slightly poorer for steels
This carbon steel weld developed an excellentrevealing the cementite films). Figure 26(b) Fig. 23 Klemm III 5 mL stock Immerse up to 8 min.
interference film when tint etched with solution Colors bronzes andshows the specimen after color etching with
Klemm’s I. Consequently, the bright-field image (a) reveals 45 mL water MonelKlemm’s I and viewed with polarized light plus the grain structure very well, and the use of polarized light 20gK S O2 2 5sensitive tint. Note that the grain-boundary ce- and sensitive tint (b) merely alters the color scheme without
(a) Stock solution: aqueous cold-saturated Na S O solution2 2 3improving the image.mentite film is clearly visible, because Klemm’sColor Metallography / 501
NaOH electrolytically or Murakami’s reagents lution-annealed and double-aged condition. The was solution annealed and tint etched with
(Fig. 18, 20, respectively). specimen was tint etched with Beraha’s BIV, Beraha’s sulfamic acid reagent 3. Figure 37
Figure 30 shows a portion of a weld made with an addition of ferric chloride. Figure 34 shows the fcc grain structure in an Fe-39%Ni
from Nitronic 50 and the heat-affected zone and shows the microstructure of Elgiloy, a cobalt- magnetic alloy color etched with Beraha’s sul-
base metal of 7-Mo PLUS duplex stainless steel base alloy used for watch springs. The strip was famic reagent 3. Figure 38 shows the decarbu-
color etched with Beraha’s BI reagent. Note the hot rolled and then solution annealed at 1040C rized surface of quenched and tempered 420
coarseness of the heat-affected zone compared (1900F), not high enough for complete recrys- martensitic stainless steel tint etched with sul-
to the base metal and the acicular structure of the tallization. The specimen was tint etched with famic reagent 4. Note that ferrite grains are pres-
weld metal. Ferrite was colored, while the aus- Beraha’s BIV plus an addition of ferric chloride. ent at the surface.
tenite was not. Figure 31 shows the austenitics etchants, based on sulfamic acid, a Beraha has also developed two rather special-
grain structure of Custom Flo 302 HQ stainless weak organic acid, have not been used much, ized tint etches that deposit cadmium sulfide
steel in the solution-annealed condition. Ber- although they are quite useful, reliable, and easy (CdS) or lead sulfide (PbS) films on the surfaces
aha’s BI was used to color the grain structure. to employ (Ref 63). The sulfamic-acid-based re- of steels and copper-base alloys (Ref 61, 62).
Figure 32 shows the austenitic grain structure of agents (Table 3) are applicable to iron, low-car- These two etchants are quite useful. The CdS
316L stainless steel that was cold reduced 30% bon and alloy steels, tool steels, and martensitic reagent is useful for carbon and alloy steels, tool
in thickness and then solution annealed from stainless steels. The author finds them to be steels, and ferritic, martensitic, and precipitation-
1150 C (2100 F). It was color etched with highly reliable and simple to use. hardenable stainless steels, while the PbS re-
Beraha’s BII reagent. The streaks indicate alloy The sulfamic acid reagents are very useful for agent does an excellent job on copper-base al-
color metallography of iron-base alloys. Fur- loys and can be used to color sulfides in steelssegregation, because they are parallel to the de-
formation axis. Color etchants are excellent for thermore, they are easy to use and quite reliable. white (the specimen is pre-etched with nital, and
revealing segregation, and numerous studies However, they do not seem to be used much. the etch colors the darkened matrix, so that the
have demonstrated that microprobe determina- Figure 35 shows lath martensite in quenched and white sulfides are visible). Table 4 lists these two
tions of compositions can be made on an etched tempered 4118 alloy steel (the core of a carbu- reagents.
surface without impairment of the chemical anal- rized specimen) tint etched with Beraha’s sul- Figure 39 shows the microstructure of
ysis results. Figure 33 shows the microstructure famic reagent 1. Figure 36 shows the microstruc- quenched and tempered 416 stainless steel, a
of Waspaloy, a nickel-base superalloy, in the so- ture of a Hadfield manganese steel specimen that grade designed for improved machinability. The
Wrought cartridge brass (Cu-30%Zn) cold reduced 50% and annealed at 704C (1300F) for 30 min. Fully recrystallized and grown, equiaxed face-centered cubic grainsFig. 24
with annealing twins. (a) Etched with equal parts ammonium hydroxide and hydrogen peroxide (3%). (b) The specimen was tint etched with Klemm’s I reagent for 3 min,
producing a lightly colored image in bright field. The structure was imaged with polarized light and sensitive tint, which dramatically improved the color contrast. The magnification
bar is 200 lm long. (c) Etching with Klemm’s II reagent for 2 min produced line etching within certain twins and grains. The lines are parallel to specific crystal planes. The specimen
was viewed in bright field. The magnification bar is 50 lm long. (d) Tint etched with Klemm’s II reagent and viewed with polarized light plus sensitive tint. This version line-etches
many of the alpha grains. (e) Tint etched with Klemm’s III reagent and viewed with polarized light and sensitive tint. (f) Viewed with bright-field illumination. Magnification bar is 200
lm long.502 / Metallographic Techniques
CdS reagent has colored the martensitic matrix specimen previously shown in Fig. 24. The col- viewed with bright field (Fig. 44a) for the 60 s
blue and brown but has not colored the delta fer- oring is even more dramatic with the PbS reagent hold and with polarized light plus sensitive tint
rite. The sulfide inclusions (gray) were not at- than with Klemm’s III. Figure 43 shows the mi- (Fig. 44b) for the 45 min hold. After 60 s, only
tacked by this reagent. Figure 40 shows the mi- crostructure of aluminum brass (Cu-22%Zn- a small amount of upper bainite (colored white
crostructure of austempered ductile iron after 2%Al) that was cold drawn and then annealed at and blue—the blue areas are where the carbide
isothermal heat treatment. The CdS reagent col- 750C (1380F). Tint etching with Beraha’s PbS has precipitated) has formed before the remain-
ored the ausferrite yellow, brown, and blue, reagent gave a good rendering of the grain struc- ing austenite was quenched, forming martensite
while the retained austenite was not colored (it ture of the alloy. (colored light brown). However, after 45 min,
is tinted slightly by the sensitive tint filter). The Sodium metabisulfite (Ref 66, 67, 70) has more upper bainite has formed, and the remain-
nodule structure is visible in color due to the use been used in a number of concentrations, from ing austenite transformed to very fine pearlite
of polarized light and sensitive tint. Figure 41 approximately 1 to 20 g per 100 mL water, as a (colored violet, green, orange, and dark blue). It
shows an as-cast specimen of Ni-Hard alloyed safe, reliable, and useful color etch for irons and is hard to see the bainitic carbide regions, which
cast iron that was tint etched with Beraha’s CdS steels. It is not as strong a coloring etch as the were colored blue, against the slightly darker
reagent. Because retained austenite is the domi- others listed previously, and better results are blues in the pearlite. Concentrations of 10 to
nant matrix phase, the CdS reagent (it often acts usually obtained by viewing with polarized light 20% Na S O have been used to color etch Had-2 2 5
as a complex reagent) colored the retained aus- and sensitive tint; but, this is not always a prob- field manganese steels. Figure 45 shows marten-
tenite light brown. The massive cementite par- lem and sometimes can be an advantage. Figure site formed in the decarburized (0.5% C) sur-
ticles are uncolored by the reagent but are tinted 44 shows the microstructure of 5160 alloy steel face region of a wrought Hadfield manganese
slightly by the sensitive tint filter. The plate mar- that was austenitized at 830 C (1525 F) and steel specimen etched with 10% sodium meta-
tensite is colored light blue, dark blue, and then isothermally held at 538 C (1000 F) for bisulfite and viewed with polarized light plus
shades of violet. 60 s (Fig. 44a) and 45 min (Fig. 44b) and then sensitive tint.
Figure 42 shows Beraha’s PbS reagent used to water quenched. The specimens were etched Comparison of Sulfide-Film-Forming Tint
color the grain structure of the cartridge brass with aqueous 10% sodium metabisulfite and Etchants for Steels. As a comparison of these
various sulfide-film-forming tint etchants for
steels, Fig. 46(a) shows the microstructure of a
Microstructure of as-rolled Fe-1.31%C-Fig. 26
Microstructure of Kunial brass (Cu-20.3%Zn-5.9%Ni-1.4%Al) that was (a) hot worked and solution annealed 0.35%Mn-0.25%Si high-carbon water-harden-Fig. 25
at 800C (1470F) (73 HV) and then tint etched with Klemm’s III. (b) Solution annealing and aging at 400C able tool steel. (a) Etching with picral revealed the Wid-
(750F) (143 HV) and then tint etching with Klemm’s III produced less color differences in a specimen with a finer grain mansta ¨tten intragranular cementite that precipitated as
size. The color difference may only be due to growth of a thinner interference film. Both specimens were viewed with proeutectoid cementite before the eutectoid reaction, but
polarized light plus sensitive tint. (c) The same alloy was hot worked, solution annealed at 800C (1470F), aged at 500 the intergranular is not visible. Etching with nital
C (930F) (192 HV, peak aged), and then tint etched with Klemm’s III, which produced mottled grain coloring and some was not as good as picral. (b) Color etching of the specimen
elongated features within grains. (d) Solution annealing and aging at 700C (1300F) (127 HV, overaged) and then tint with Klemm’s I clearly revealed the intergranular and intra-
etching with Klemm’s III produced a narrower range of grain colors, and the strengthening precipitates are now visible granular cementite films (viewed with polarized light and
with the light microscope. sensitive tint).