-
Metallography andMicrostructures of Cast IronJanina M.
Radzikowska, The Foundry Research Institute, Krakow, Poland
CAST IRON is an iron-carbon cast alloy withother elements that
is made by remelting pigiron, scrap, and other additions. For
differentia-tion from steel and cast steel, cast iron is denedas a
cast alloy with a carbon content (min 2.03%)that ensures the
solidication of the nal phasewith a eutectic transformation.
Depending onchemical specications, cast irons can be non-alloyed or
alloyed. Table 1 lists the range ofcompositions for nonalloyed cast
irons (Ref 1).The range of alloyed irons is much wider, andthey
contain either higher amounts of commoncomponents, such as silicon
and manganese, orspecial additions, such as nickel, chromium,
alu-minum, molybdenum, tungsten, copper, vana-dium, titanium, plus
others.
Free graphite is a characteristic constituent ofnonalloyed and
low-alloyed cast irons. Precipi-tation of graphite directly from
the liquid occurswhen solidication takes place in the range
be-tween the temperatures of stable transformation(Tst) and
metastable transformation (Tmst), whichare, respectively, 1153 C
(2107 F) and 1147 C(2097 F), according to the iron-carbon
diagram.In this case, the permissible undercooling degreeis DTmax
Tst Tmst. In the case of a higherundercooling degree, that is, in
the temperaturesbelow Tmst, primary solidication and
eutecticsolidication can both take place completely orpartially in
the metastable system, with precipi-tation of primary cementite or
ledeburite.Graphitization can also take place in the rangeof
critical temperatures during solid-state trans-formations. The
equilibrium of phases Fec
Fig. 1 Spheroidal graphite in as-cast ductile iron
(Fe-3.7%C-2.4%Si-0.59%Mn-0.025%P-0.01%S-
0.095%Mo-1.4%Cu) close to the edge of the specimen,which was 30
mm (1.2 in.) in diameter. The specimen wasembedded. As-polished.
100
Table 1 Range of chemical compositions for typical nonalloyed
and low-alloyed castirons
Type of iron
Composition, %C Si Mn P S
Gray (FG) 2.54.0 1.03.0 0.21.0 0.0021.0 0.020.025Compacted
graphite (CG) 2.54.0 1.03.0 0.21.0 0.010.1 0.010.03Ductile (SG)
3.04.0 1.82.8 0.11.0 0.010.1 0.010.03White 1.83.6 0.51.9 0.250.8
0.060.2 0.060.2Malleable (TG) 2.22.9 0.91.9 0.151.2 0.020.2
0.020.2FG, ake graphite; SG, spheroidal graphite; TG, tempered
graphite. Source: Ref 1
Fe Fe3C occurs only at the temperature 723 2 C (1333 4 F), while
equilibrium ofphases Fec Fe Cgr occurs at the tempera-ture 738 3 C
(1360 5 F). So, in the rangeof temperatures 738 to 723 C (1360 to
1333 F),the austenite can decompose only into a mixtureof ferrite
with graphite instead of with cementite(Ref 2).
The previous considerations regard only pureiron-carbon alloys.
In cast iron, which is a mul-ticomponent alloy, these temperatures
can bechanged by different factors: chemical compo-sition, ability
of cast iron for nucleation, andcooling rate. Silicon and
phosphorus bothstrongly affect the carbon content of the
eutectic.That dependence was dened as a carbon equiv-alent (Ce)
value that is the total carbon contentplus one-third the sum of the
silicon and phos-phorus content (Ref 2). Cast iron, with a
com-position equivalent of approximately 4.3, solid-ies as a
eutectic. If the Ce is 4.3, it ishypereutectic; if it is 4.3, cast
iron is hypoeu-tectic (Ref 3).
Eutectic cells are the elementary units forgraphite nucleation.
The cells solidify from theseparate nuclei, which are basically
graphite butalso nonmetallic inclusions such as oxides andsuldes as
well as defects and material discon-tinuities. Cell size depends on
the nucleation ratein the cast iron. When the cooling rate and
thedegree of undercooling increase, the number ofeutectic cells
also increases, and their micro-structure changes, promoting
radial-sphericalshape (Ref 2).
Preparation for Microexamination
Preparation of cast iron specimens for micro-structural
examination is difcult due to the needto properly retain the very
soft graphite phase,when present, that is embedded in a harder
ma-trix. Also, in the case of gray irons with a softferritic
matrix, grinding scratches can be difcultto remove in the polishing
process. When shrink-age cavities are present, which is common,
thecavities must not be enlarged or smeared over.
Retention of graphite in cast iron is a commonpolishing problem
that has received considerable
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566 / Metallography and Microstructures of Ferrous Alloys
Fig. 3 Flake graphite in as-cast gray iron
(Fe-3.5%C-2.95%Si-0.40%Mn-0.08%P-0.01%S-0.13%Ni-
0.15%Cu) close to the edge of the unembedded specimen,which was
30 mm (1.2 in.) in diameter. As-polished.100
Fig. 2 Same as-cast ductile iron as in Fig. 1, but thespecimen
was not embedded. The arrows show
the pulled-out graphite. As-polished. 100Fig. 4 Same as in Fig.
3 but close to the center of the
specimen. As-polished. 100
attention. Coarse grinding is a critical stage, so,if the soft
graphite is lost during coarse grinding,it cannot be recovered in
subsequent steps and
will be seen as an open or collapsed cavity. Sil-icon carbide
(SiC) grinding papers are preferredto emery, because SiC cuts
efciently, while em-ery paper does not, and SiC produces less
dam-
age. Fresh paper should always be used; nevergrind with worn
paper. White iron, by contrast,contains extremely hard iron
carbides that resistabrasion and tend to remain in relief above
thesofter matrix after polishing (Ref 4).
Quality-control studies, based on image anal-ysis measurements
of the amount of phases andthe graphite shape and size, also need
perfectlyprepared specimens with fully retained graphitephase and
with microstructural constituents cor-rectly revealed by
etching.
Specimen Preparation. The metallographicspecimen preparation
process formicrostructuralinvestigations of cast iron specimens
usuallyconsists of ve stages: sampling, cold or hotmounting,
grinding, polishing, and etching witha suitable etchant to reveal
the microstructure.Each stage presents particular problems in
thecase of cast iron. Of course, the graphite phaseis studied after
polishing and before etching.
Sampling is the rst stepselecting the testlocation or locations
to be evaluated metallo-graphically. Usually, cast iron castings
have aconsiderable variation in microstructure betweensurface and
core. Selection of the test location isvery important to obtain
representative resultsfrom the microstructural examination.
Samplescan be obtained by cutting them out from eithera large or
small casting or from standard testbars, such as microslugs, ears,
or keel bars; how-ever, the microstructure of these pieces may
notbe representative for the actual casting due tosubstantial
differences in the solidication rates.Production saws, such as
large, abrasive cutoffsaws, band saws, or power hacksaws, can
beused for dividing medium-sized casting intosmaller samples. In
the case of very large cast-ings, ame cutting may be used. Next,
the pieces
Fig. 5 Temper graphite in malleable iron
(Fe-2.9%C-1.5%Si-0.53%Mn-0.06%P-0.22%S-0.08%Ni-
0.1%Cu-0.09%Cr-0.003%Bi) after grinding on P1000 SiCwaterproof
paper. The casting was annealed at 950 C(1740 F), held 10 h,
furnace cooled to 720 C (1330 F),held 16 h, and air cooled. The
arrows show the pulled-outgraphite. As-polished. 400
Fig. 6 Same as in Fig. 5 but after polishing with 9 lmdiamond
suspension. The arrows show the
pulled-out graphite. As-polished. 400
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Metallography and Microstructures of Cast Iron / 567
can be reduced to the desired size for metallo-graphic specimens
by using a laboratory abra-sive cutoff saw or a band saw. If the
casting was
sectioned by ame cutting, the specimen mustbe removed well away
from the heat-affectedzone. The pieces cut out for metallographic
ex-amination may be ground prior to mounting (thismay be done to
round off sharp cut edges or toreduce the roughness of band-saw-cut
surfaces)and subsequent preparation. Overheating isavoided by
proper selection of the speed of cut-off saws, the use of the
correct wheel, and ade-quate water cooling. Overheating during
grind-ing is avoided by using fresh abrasive paper andproper
cooling. When metallographic specimensare cut out from the standard
cast bars, they aresometimes prepared using standard machineshop
equipment, such as turning in a lathe ormilling. These devices can
deform the testpiecesurfaces to a considerable depth, so care must
beexercised to remove any damage from theseoperations before
starting specimen preparation.
Mounting. Specimens can be mounted in apolymeric material using
either cold or hotmounting procedures. The mounting resin ischosen
depending on the cast iron hardness (softor hard) and the need to
enhance edge retention.Use of an incorrect resin, or ignoring the
mount-ing process, can make it very difcult to obtainproperly
polished graphite in the area close tothe specimen edge. Figures 1
and 2 show themicrostructure of spheroidal graphite in ductileiron
close to the edge of the specimens, whichwere cut off from a 30 mm
(1.2 in.) diameter barand polished with and without embedding in
apolymer resin, respectively. In the specimen pre-pared without
embedding in a resin, the graphitewas pulled out, while in the
specimen that wasembedded in a resin and prepared, the graphite
nodules were perfectly retained. Figures 3 and 4show that the
uniform grinding of nonmountedspecimens is more difcult, and the
ake graph-ite in gray iron close to the edge of such a spec-imen is
not polished perfectly, in comparison towell-polished graphite in
the mounted specimen.
Grinding and Polishing. To ensure propergraphite retention, the
use of an automatedgrinding-polishing machine is recommendedover
manual preparation. The automated equip-ment makes it possible, in
comparison to manualspecimen preparation, to properly control
theorientation of the specimen surface relative tothe grinding or
polishing surface, to maintainconstantly the desired load on the
specimens, touniformly rotate the specimens relative to thework
surface, and to control the time for eachpreparation step. Proper
control of these factorsinuences graphite retention, although other
fac-tors are also important.
A good, general principle is to minimize thenumber of grinding
and polishing stages. Also,the load on each specimen, or on all
specimensin the holder, must be chosen to obtain a cor-rectly
polished surface in the shortest possibletime. This precludes the
risk of pulling out thegraphite phase and ensures that the graphite
pre-cipitates will be perfectly at with sharp bound-aries.
The recommended procedure for automatedpreparation of the
specimens of nonalloyed andlow-alloyed cast iron with graphite
specimens isto grind with a high-quality, waterproof 220-
or240-grit (or equivalent) SiC paper until plane,with a load of 100
N for six specimens mountedin the sample holder, with central
loading. Pol-
Fig. 8 Same as in Fig. 7 but after nal polishing withthe 1 lm
diamond suspension applied on a
napped cloth. The arrows show the pulled-out
graphite.As-polished. 400
Fig. 9 White high-chromium iron
(Fe-3.2%C-4.65%Cr-2.9%Mn-0.51%Si-0.050%P-0.024%S). Eutectic
and secondary carbides in the matrix. Specimen was pre-pared
correctly. The casting was austenitized at 1000 C(1830 F), held 1
h, furnace cooled to 400 C (750 F) for2 h, taken to salt bath at
400 C (750 F), held for 4 h, andair cooled. Etched with glyceregia.
500
Fig. 10 White high-chromium iron
(Fe-3.16%C-8.86%Cr-0.50%Si-3.04%Mn-0.051%P-
0.018%S). Eutectic and secondary carbides in the matrix.Specimen
was prepared incorrectly. The casting was aus-tenitized at 1000 C
(1830 F), held 1 h, furnace cooled to700 C (1290 F) for 2 h, taken
to salt bath at 700 C (1290F), held 4 h, and air cooled. Etched
with glyceregia. 500
Fig. 7 Same as in Fig. 6 but after nal polishing withthe 1 lm
diamond paste applied on a napless
cloth. Graphite is free of any visible pullouts.
As-polished.400
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568 / Metallography and Microstructures of Ferrous Alloys
ishing is carried out in four steps with a differentgrain size
diamond paste:
StepDiamond pastegrain size, lm
Load,N Duration
Recommendedpolishing cloth
1 9 120(a) 5 min(d) Napless woven2 3 120(a) 3 min(d) Napless
woven3 1 120(a) 2 min(d) Napless woven4 (b) 25(c) 4560 s(e) Napless
synthetic
polyurethane
(a) Load per six specimens. (b) Aqueous 0.05 lm alumina
suspension.(c) Load per single specimen (switching to individual
force to makespecimen cleaning easier). (d) Comp direction. (e)
Contra
Figure 5 shows temper graphite in malleableiron after the last
step of grinding, which wascarried out in three steps using,
consecutively,SiC grit papers P220, P500, and P1000. Figure6 shows
the same specimen after grinding on
P220 SiC paper and then polishing with 9 lmdiamond paste
according to the procedure givenpreviously. In both cases, there is
some pulled-out graphite after these steps. Each
specimenwasprepared further. The specimen ground withthree SiC
steps was polished with 3 lm diamondsuspension on a napless cloth
and then with 1lm diamond suspension on a napped cloth.
Thepulled-out graphite was still visible. However,the specimen
ground with P220 SiC and pol-ished with 9 lm diamond paste, when
nishedwith the recommended practice given previ-ously, was free of
any visible pullouts, as shownin Fig. 7. By using a napped cloth
and an aque-ous 1 lm diamond suspension for the nal dia-mond
polishing step, it was impossible to obtainperfectly retained
graphite, as shown in Fig. 8.Napped cloths should not be used with
diamondabrasive, either in paste, suspension, or aerosolform.
Graphite retention appears to be slightly
better using diamond paste and the preferred lu-bricant than
with an aqueous suspension, al-though more work needs to be
conducted to de-termine if this difference is important.
Finalpolishing with an alumina suspension, such asMasterprep
alumina (Buehler, Ltd.), makes thegraphite boundaries sharper by
removing thematrix, which was smeared over the edge of thegraphite
during grinding and was not removedby the diamond polishing
steps.
Alloyed chromium iron is much harder, and adifferent preparation
procedure must be used.The grinding process is carried out in three
steps,and polishing is carried out in three steps, al-though only
two polishing steps are needed formost routine work. Grinding of
the specimens,mounted in the sample holder, used central load-ing
(150 N/six specimens), with high-qualitySiC waterproof paper (water
cooled) with thefollowing grit sizes:
Table 2 Etchants
No. of etchant Name of etchant Composition Comments Ref
1 Nital 9698 mL ethanol24 mL nitric acid (HNO3)
Most common etchant for iron, carbon, alloyed steels, and cast
iron. Revealsalpha grain boundaries and constituents. The 2 or 4%
solution is commonlyused. Use by immersion of sample for up to 60
s.
4, p 648
2 Picral 4 g picric acid ((NO2)3C6H2OH)100 mL ethanol
Recommended for structures consisting of ferrite and carbides.
Does not revealferrite grain boundaries and martensite as-quenched.
Addition of approximately0.51% zephiran chloride improves etch rate
and uniformity.
4, p 648
3 Glyceregia (modied) 3 parts glycerine2 parts hydrochloric acid
(HCl)1 part nitric acid (HNO3)
For austenitic stainless steels and cast irons. Reveals grain
structure; outlinessigma and carbides. Mix fresh; do not store. Use
by swabbing. Heat up to 50C (120 F) when etching time at 20 C (70
F) does not bring results.
4, p 634
4 Alkaline sodium picrate(ASP)
2 g picric acid ((NO2)3C6H2OH))25 g sodium hydroxide (NaOH)100
mL distilled water
Immerse sample in solution at 6070 C (140160 F) for 13 min.
Colorscementite (Fe3C) dark brown to black, depending on etching
time.
4, p 646
5 Klemm I 50 mL sat. aq. sodium thiosulfate(Na2S2O35H2O)
1 g potassium metabisulte (K2S2O5)
Immerse sample for 40100 s. Reveals phosphorus segregation
(white); colorsferrite blue or red; martensite brown; cementite and
austenite are unaffected
4, p 642
6 Beraha CdS 240 g aq. sodium thiosulfate(Na2S2O35H2O)
30 g citric acid (C6H8O7H2O)2025 g cadmium chloride
(CdCl22.5H2O)100 mL distilled water
Tint etch for iron, steel, cast irons, and ferritic and
martensitic stainless steel.Dissolve in order shown. Allow each to
dissolve before adding next. Allow toage 24 h at 20 C (70 F) in a
dark bottle. Before use, lter 100 mL of solutionto remove
precipitates. Preetch with a general-purpose reagent. Etch 2090
s;good for 4 h. For steels and cast irons, after 2040 s only
ferrite is colored, redor violet. Longer times color all
constituents: ferrite is colored yellow or lightblue; phosphide,
brown; carbide, violet or blue. For stainless steels, immersesample
6090 s; carbides are colored red or violet-blue; matrix, yellow;
colorsof ferrite vary. Suldes red-brown after 90 s
4, p 644
7 . . . 28 g sodium hydroxide (NaOH)4 g picric acid
((NO2)3C6H2OH))1 g potassium metabisulte (K2S2O5)(a)100 mL
distilled water
Immerse sample in hot solution (close to boiling temperature)
for 3060 min. Thisreagent reveals silicon segregation in ductile
iron. The colors of microstructurechange themselves from green
through red, yellow, blue, and dark brown tolight brown as the
silicon content is reduced from the graphite nodule to
cellboundaries. The regions with lowest silicon content at the cell
boundariesremain colorless. Before etching, ferritization of the
specimen is recommendedto enhance the visibility of the colors.
6
8 Murakami reagent 10 g potassium ferricyanide (K3Fe(CN)6)10 g
potassium hydroxide (KOH) or
sodium hydroxide (NaOH)100 mL distilled water
Use fresh, cold or hot. Cold, at 20 C (70 F) for up to 1.5 min,
tints chromiumcarbides; Fe3C unattacked or barely attacked. Hot, at
50 C (120 F) for 3 min,tints iron phosphide. The higher temperature
or etching time also tintscementite into yellow color.
4, p 646
9 Beraha reagent with selenicacid
2 mL hydrochloric acid (HCl)0.5 mL selenic acid (H2SeO4)100 mL
ethanol
For differentiation of the constituents in steadite in cast
iron, immerse sample for710 min; iron phosphide colored blue or
green, cementite colored red, andferrite is bright (unaffected).
Preetching with nital is recommended.
4, p 643
10 Beraha-Martensite (B-M) 2 g ammonium biuoride (NH4FHF)2 g
potassium metabisulte (K2S2O5)100 mL stock solution: 1:5, HCl
to
distilled water
Immerse sample for 13 s. Coarse martensite is blue or yellow; ne
martensiteand bainite are brown. Use fresh reagent, and wet sample
with tap water beforeetching.
6, p 26
11 10% sodium metabisulte(SMB)(b)
10 g sodium pyrosulte (Na2S2O5)100 mL distilled water
Tints as-quenched martensite into brown; bainite into blue;
carbides, phosphides,and residual austenite, unaffected. Immerse
sample in etchant solutionapproximately 20 s. Preetching sample
with nital is recommended.
4, p 642
12 Lichtenegger and Bloech I(LB I)
20 g ammonium biuoride (NH4FHF)0.5 g potassium metabisulte
(K2S2O5)100 mL hot distilled water
Dissolve in given order. In austenitic Cr-Ni alloys, it tints
austenite and revealsdendritic segregation. Ferrite and carbides
remain unaffected. Wet-etch for 15min immediately after
polishing.
7, p 51
13 . . . 50% aq. hydrochloric acid (HCl) Immerse sample for 3090
min. Every 1520 min, wash sample with distilledwater, quickly etch
in hydrouoric acid (conc.), and wash in tap water. Whenthe etching
process is nished, immerse sample in 5% aq. KOH or NaOH for1020
min, wash with distilled water in an ultrasonic washer, then in
ethanol,and dry with blowing hot air.
8, p 97
(a) Potassium metabisulte and potassium pyrosulte are both
synonomous with K2 S2 O5. (b) Sodium metabisulte and sodium
pyrosulte are synonomous with Na2S2O5.
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Metallography and Microstructures of Cast Iron / 569
First step: P220 grit until plane. Second step: P500 grit for 3
min. Third step: P1000 grit for 3 minPolishing is done in three
steps, with differentgrain size diamond in paste for the rst two
steps:
First step: 3 lm diamond, 120 N load/sixspecimens for 3 min with
a napless cloth.
Second step: 1lm diamond, 100 N load/sixspecimens for 3 min with
a napless cloth
The last polishing step is carried out with a col-loidal silica
suspension on a napless synthetic
polyurethane pad but in a single specimenholder with an
individual load of 30 N for 1.5min.
Figure 9 shows the microstructure of a heattreated chromium iron
after this preparation. Thecarbides are perfectly at, with very
sharp edgesand boundaries etched uniformly. Figure 10shows the
primary eutectic carbides in the mi-crostructure of a high-chromium
iron. They ap-pear to be sticking out from the matrix, and
theirboundaries are not outlined uniformly. This re-sult occurs if
the load is too low or the nal pol-ishing time on the silica
suspension is too long.Both problems will result in too much
removalof the softer matrix that was surrounding the pri-mary
carbides.
During grinding, the paper must be moistenedwith owing tap
water, and the specimens shouldbe washed with water after each
step. Also, dur-ing the rst planning step, the sheet of papershould
be changed every 1 min. Used grit paperis not effective and will
introduce heat and dam-age, impairing specimen atness. During
polish-ing with diamond paste from a tube, the cloth ismoistened
with the recommended lubricant forthe paste. If a water-based
diamond suspensionis applied on the cloth, the use of an
additionallubricant is not required.
The speed of the grinding-polishing head was150 rpm, and it was
constant. The speed of theplaten during grinding was always 300
rpm, andduring polishing was always 150 rpm. After eachgrinding
step, the specimens were washed withrunning tap water and dried
with compressed air,while after each polishing step, they
werewashed with alcohol and dried with hot air froma hair
dryer.
Fig. 11 Ductile iron (Fe-3.8%C-2.4%Si-0.28%Mn-1.0%Ni-0.05%Mg)
after annealing. Ferrite and
approximately 5% pearlite. Etched with 2% nital. 100.Courtesy of
G.F. Vander Voort, Buehler Ltd.
Fig. 12 As-cast gray iron
(Fe-2.8%C-0.8%Si-0.4%Mn-0.1%S-0.35%P-0.3%Cr). Pearlite. Etched
with
4% nital. Arrows show the white areas with weakly etchedor
nonetched pearlite. 500
Fig. 13 Same as in Fig. 12 but after etching with 4%picral.
Pearlite was etched uniformly. 500
Fig. 14 As-cast high-chromium white iron
(Fe-1.57%C-18.64%Cr-2.86%Mn-0.53%Si-
0.036%P-0.013%S). Eutectic chromium carbides typeM7C3 in
austenitic matrix. Etched with glyceregia. 500
Fig. 15 Same as in Fig. 14 but after etching with 4%nital.
500
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570 / Metallography and Microstructures of Ferrous Alloys
Microexamination Methods
Chemical Etching. The examination of theiron microstructure with
a light optical micro-scope is always the rst step for phase
identi-
cation and morphology. One should always be-gin microstructural
investigations by examiningthe as-polished specimen before etching.
This isa necessity, of course, for cast iron specimens, ifone is to
properly examine the graphite phase.
Standard Etchants. To see the microstruc-tural details,
specimens must be etched. Etchingmethods based on chemical
corrosive processeshave been used by metallographers for manyyears
to reveal structures for black-and-whiteimaging.
Specimens of nonalloyed and low-alloyedirons containing ferrite,
pearlite, the phosphoruseutectic (steadite), cementite, martensite,
andbainite can be etched successfully with nital atroom temperature
to reveal all of these micro-structural constituents. Usually, this
is a 2 to 4%alcohol solution of nitric acid (HNO3) (Table 2,etch
No. 1). Figure 11 shows a nearly ferriticannealed ductile iron with
uniformly etchedgrain boundaries of ferrite and a small amountof
pearlite. Nital is very sensitive to the crystal-lographic
orientation of pearlite grains, so, in thecase of a fully pearlitic
structure, it is recom-mended to use picral, which is an alcohol
solu-tion of 4% picric acid (Table 2, etch No. 2). Fig-ures 12 and
13 show the differences in revealingthe microstructure of pearlite
with nital or picral.Picral does not etch the ferrite grain
boundaries,or as-quenched martensite, but it etches the pear-litic
structure more uniformly, while nital leaveswhite, unetched areas,
especially in the casewhere pearlite is very ne.
When the austempering heat treatment is veryshort, the
microstructure of austempered ductileiron (ADI) consists of
martensite and a smallamount of acicular ferrite. After etching in
4%nital, martensite as well as acicular ferrite areboth etched
intensively, which makes it very dif-
Fig. 16 As-cast ductile iron
(Fe-3.07%C-0.06%Mn-2.89%Si-0.006%P-0.015%S-0.029%Mg). C,
cementite; L, ledeburite; F, ferrite; and P, pearlite.
Etchedwith 4% nital. 650 (microscopic magnication 500)
Fig. 17 Same as in Fig. 16 but after etching with hotalkaline
sodium picrate. C, eutectic cementite;
L, ledeburite; F, ferrite; and P, pearlite with slightly
etchedcementite. 650 (microscopic magnication 500)
Fig. 18 As-cast gray iron
(Fe-3.24%C-2.32%Si-0.54%Mn-0.71%P-0.1%S). E, phosphorous
ternary eutectic. Etched with 4% nital. 100
Fig. 19 Same as in Fig. 18 but after etchingwithKlemmI reagent.
E, phosphorous ternary eutectic.
100
Fig. 20 Austempered ductile iron
(Fe-3.6%C-2.5%Si-0.06%P-1.5%Ni-0.7%Cu). CB, cell bound-
aries; H, ferritic halo around the graphite nodules. Etchedwith
Klemm I reagent. 200
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Metallography and Microstructures of Cast Iron / 571
cult to distinctly see the needles of acicular fer-rite. Picral
reveals this phase very well; marten-site is barely etched due to
the very shortaustempering heat treatment of the specimen,
which was 2 min, because the martensite was as-quenched. The
needles of acicular ferrite aredark and very sharp (Fig. 32, 33).
In this case,picral is very convenient for estimating theamount and
morphology of the acicular ferrite inthe ADI microstructure. Picral
is safer to be
stored in the lab than nital, which can be an ex-plosive mixture
under certain conditions when itis stored in a tightly closed
bottle.
Glyceregia (Table 2, etch No. 3), which is amixture of
glycerine, hydrochloric acid (HCl),and nitric acid (HNO3), is
recommended for re-vealing the microstructure of high-chromiumand
chromium-nickel-molybdenum irons. Fig-ure 14 shows the
microstructure of a high-chro-mium cast iron after etching with
glyceregia (seealso Fig. 43, 49, and 96). Nital can be also usedfor
revealing the carbide morphology in the mi-crostructure of chromium
or chromium-nickelirons when the carbon and chromium
contentpromotes solidication of eutectic carbides.When the
microstructure of a high-chromiumwhite iron contains columnar
primary carbides,glyceregia is recommended.
Figure 15 shows the microstructure of thesame cast iron as Fig.
14 but after etching with4% nital (see also Fig. 40, 41). Both
etchantsreveal carbide boundaries sharply and uniformly.
Selective Color Etching. If the black-and-white etchants are
inadequate for positive iden-tication of the iron microstructures,
other pro-cedures must be used, such as selective coloretching. The
reagents referred to as tint etchantsare usually acidic solutions
with either water oralcohol as a solvent. They are chemically
bal-anced to deposit a thin (40 to 500 nm), trans-parent lm of
oxide, sulde, complex molyb-date, elemental selenium, or chromate
on thespecimen surface. Coloration is developed by in-terference
between light rays reected at the in-Fig. 21 Same as in Fig. 20 but
after etching with Ber-
ahas CdS reagent. H, ferritic halo; CB, cellboundaries. 250
Fig. 22 Nodular iron
(Fe-3.9%C-2.9%Si-0.32%Mn-0.06%P-0.037%Mg-1.5%Ni-0.57%Cu). Sili-
con microsegregation was revealed. The casting was an-nealed.
Etched with hot aqueous solution of sodiumhydroxide, picric acid,
and potassium pyrosulte (Table 2,etchant No. 7). 500
Fig. 23 As-cast gray iron
(Fe-3.33%C-1.64%Si-0.31%Mn-1.37%P-0.107%S). Ternary phos-
phorus eutectic. Etched with 4% nital. 1300 (micro-scopic
magnication 1000)
Fig. 24 Same as in Fig. 23 but after etching with hotalkaline
sodium picrate. C, cementite; F, ferrite
(unaffected); IP, iron phosphide ferrite; and TiN,
titaniumnitride. 1300 (microscopic magnication 1000)
Fig. 25 Same as in Fig. 23 but after etchingwithKlemmI reagent.
F, ferrite; C, cementite; and C IP,
cementite iron phosphide inside the precipitate of phos-phorous
eutectic. 1300 (microscopic magnication1000)
-
572 / Metallography and Microstructures of Ferrous Alloys
ner and outer lm surfaces. Crystallographic ori-entation, local
chemical composition, andetching time affect lm thickness and
controlcolor production. The use of selective etchants
is invaluable for quantitative metallography, aeld of growing
importance (Ref 5).
When ferritic-pearlitic microstructures of castiron contain
large amounts of cementite and led-
eburite, the differentiation of the white structuralconstituents
is difcult after etching a specimenwith nital. In such cases, hot
alkaline sodium pic-rate (ASP) is recommended (Table 2, etch No.4),
which reveals cementite, tinted a browncolor, while ferrite remains
unaffected. This etchis a mixture of sodium hydroxide (NaOH),
picricacid, and distilled water. Figure 16 shows themicrostructure
of a thin-walled, chilled ductileiron casting after etching with
nital, while Fig.17 shows the microstructure of the same speci-men
after etching with ASP; the brown-coloredcementite and ledeburite
are clearly visible in thepearlitic-ferritic matrix (the cementite
in pearlitewas also slightly tinted). It allows one to esti-mate
the amount of cementite that should be re-moved from the casting
microstructure in the an-nealing process.
Segregation of silicon and phosphorus in ironis very strong and
can be revealed with selectivecolor etching methods. Klemms I
reagent (Table2, etch No. 5), which tints ferrite while
austeniteand carbides remain colorless, consists ofpotassium
metabisulte (K2S2O5) and a cold-sat-urated water solution of sodium
thiosulfate(Na2S2O35H2O) and is one of the etchants thatcan be used
to reveal phosphorus and silicon seg-regation.
Usually, the distribution of the phosphorus eu-tectic, which
solidies in gray iron on the cellboundaries, is revealed by etching
up to 4 minin 4% nital. Figure 18 shows the microstructureof
as-cast gray iron with the ternary phosphorouseutectic. The cell
boundaries, lled with steadite,
Fig. 28 Same as in Fig. 25 but after etching with Ber-ahas
reagent with selenic acid. IP, iron phos-
phide; C, cementite; and F, ferrite. The dark points in
pearl-ite, which look like artifacts, can be iron
phosphideprecipitates or ne, nonmetallic inclusions. 1300
(micro-scopic magnication 1000)
Fig. 29 As-cast gray iron
(Fe-3.62%C-2.03%Si-1.13%P-0.61%Mn-0.137%S-0.113%Cr-
0.478%Ni-0.004%Al). E, binary phosphorous eutectic; F,ferrite at
the graphite precipitate; and P, pearlite. Etchedwith 4% nital.
500
Fig. 26 Same as in Fig. 23 but after etching with hotMurakamis
reagent. IP, iron phosphide; C F,
cementite ferrite inside the precipitate of phosphorouseutectic.
1300 (microscopic magnication 1000)
Fig. 30 Same as in Fig. 29 but after etching with hotalkaline
sodium picrate and 4% nital. Pearlitic
matrix is revealed; phosphorous eutectic is unaffected.500
Fig. 27 Same as in Fig. 26 but after overetching withhot
Murakamis reagent. IP, iron phosphide; C,
cementite; and F, ferrite. 1300 (microscopic magnica-tion
1000)
-
Metallography and Microstructures of Cast Iron / 573
are white, while their interiors are almost blackdue to
overetching the pearlitic-ferritic matrix.Figure 19 shows the same
microstructure afteretching with Klemms I reagent. The
microre-gions inside the eutectic cells with a lower phos-phorus
content are tinted a blue color, while the
areas with the ternary eutectic, situated at theboundaries of
the eutectic cells, are almost col-orless. In both cases, the
cementite and iron
phosphide in steadite are not etched, and the net-work is
clearly visible.
Figure 20 shows silicon segregation in an ADImicrostructure
after etching with Klemms I re-agent. The regions outlining the
cell boundaries,low in silicon, are tinted a blue color, while
thevery thin halos around the graphite nodules,where the silicon
content is highest, remain col-orless. Acicular ferrite is orange,
and austenite isnot tinted. Figure 21 shows the same
microstruc-ture after etching with Berahas CdS reagent (Ta-ble 2,
etch No. 6), an aqueous solution of sodiumthiosulfate
(Na2S2O35H2O), citric acid(C6H8O7H2O), and cadmium
chloride(CdCl22.5H2O). The silicon segregation is re-vealed the
same way as after using Klemms Ireagent.
To reveal silicon segregation in nonalloyedductile cast iron
inside eutectic cells, the hotaqueous solution of sodium hydroxide,
picricacid, and potassium pyrosulte (K2S2O5) can beused (Table 2,
etch No. 7). Figure 22 shows thedifferent colors of the
microstructure, whichchange from green through red, yellow, blue,
anddark brown to light brown as the silicon contentis changing from
the graphite nodule to the cellboundaries. The regions with the
lowest siliconcontent at the cell boundaries remain
colorless.Before etching, ferritization of the specimen wascarried
out to enhance the visibility of the mi-crostructural colors.
The revealing and differentiation of all con-stituents in
steadite is invaluable for the deter-mination of the type of
eutectic as well as the
Fig. 33 Same as in Fig. 32 but after etching with 4%picral. AF,
acicular ferrite; PM, plate marten-
site. 1000
Fig. 32 Austempered ductile iron
(Fe-3.6%C-2.5%Si-0.056%P-0.052%Mg-0.7%Cu). Martensite
and acicular ferrite. The casting was austempered at 900C (1650
F), held 2 h, taken to salt bath at 360 C (680 F),held 2 min, and
air cooled. Etched with 4% nital. 1000
Fig. 31 Same as in Fig. 29 but after etching with hotMurakami
reagent. Only brown-tinted iron
phosphide was revealed. 500
Fig. 34 Same as in Fig. 32 but after etching with
Ber-aha-Martensite reagent. PM, blue-yellow plate
martensite; FM, brown ne martensite; and AF A, darkneedles of
acicular ferrite surrounded with colorless aus-tenite. 1000
Fig. 35 Same as in Fig. 32 but after etching with 10%sodium
metabisulte. PM, plate martensite;
FM, ne martensite; and AF A, acicular ferrite and aus-tenite.
1000
-
574 / Metallography and Microstructures of Ferrous Alloys
amount of each constituent. In the case of theternary
phosphorous eutectic, which consists offerrite, cementite, and iron
phosphide (Fe3P), ni-tal does not help in the identication of the
con-
stituents, nor does it provide enough informationabout
distribution of the eutectic constituents.Figure 23 shows the
microstructure of the ter-nary phosphorous eutectic in gray iron
after etch-
ing in 4% nital. The white areas that surroundthe ternary
eutectic and are also visible insidethe eutectic can be either
ferrite or cementite.Figure 24 shows the microstructure of the
samespecimen after selective color etching with hotASP (Table 2,
etchant No. 4). Cementite in theeutectic is tinted brown and blue
colors (also inpearlite), while ferrite and iron phosphide are
nottinted. To reveal the ferrite, the same specimenwas etched with
Klemms I reagent. Figure 25shows the eutectic regions with
precipitates ofbrown ferrite (also in pearlite), while cementiteand
iron phosphide are not tinted.
Murakamis reagent (Table 2, etch No. 8),used at 50 C (120 F) for
3 min and containingpotassium hydroxide (KOH), potassium
ferri-cyanide (K3Fe(CN)6), and distilled water, can beused for
revealing and estimating the amount ofiron phosphide in steadite.
Figure 26 shows thisconstituent of the ternary phosphorous
eutecticmicrostructure, which was tinted a light-browncolor, while
cementite and ferrite remained col-orless. The microstructure of
the same specimenafter overetching (5 min) with the same reagentis
shown in Fig. 27. This time, cementite wasalso revealed and was
tinted a yellow color,while ferrite remained white. The color of
theiron phosphide changed to a dark-brown andgray-blue color.
Nevertheless, extending theetching time beyond 3 min is not
recommended,because this can falsify the true results of
themicrostructural examination.
Good differentiation of all constituents in theternary
phosphorus eutectic can be obtained withBerahas reagent (Table 2,
etch No. 9), a mixtureof hydrochloric acid (HCl), selenic acid
Fig. 36 Austempered ductile iron
(Fe-3.6%C-2.5%Si-0.052%Mg-0.7%Cu). AF, acicular ferrite; A,
austenite; and M, martensite. The casting was austemperedat 900
C (1650 F), held 2 h, taken to salt bath at 360 C(680 F), held 30
min, and air cooled. Etched with 4%nital.1000
Fig. 37 Same as in Fig. 36 but after etching with
Ber-aha-Martensite reagent. AF, acicular ferrite; A,
austenite; and M, martensite. 1000
Fig. 38 Same as in Fig. 36 but after etching with 10%sodium
metabisulte. AF, acicular ferrite; A,
austenite; and M, martensite. 1000
Fig. 39 White alloyed cast iron
(Fe-3.4%C-0.92%Mn-1.89%Si-9.52%Cr-6.27%Ni). Etched with Ber-
aha-Martensite. PM, plate martensite; FM, ne martensite;EC,
eutectic carbides type M7C3; SC, secondary carbides;and MS,
manganese sulte. The casting was heat treated:austenitized at 750 C
(1380 F), held 2 h, and air cooled;tempered at 250 C (480 F), held
4 h, and air cooled.1300 (microscopic magnication 1000)
Fig. 40 Same white iron as in Fig. 39 but after etchingwith 4%
nital. M, martensite; EC, eutectic car-
bides; and SC, secondary carbides. 1300 (microscopicmagnication
1000)
-
Metallography and Microstructures of Cast Iron / 575
(H2SeO4), and ethanol. According to Beraha,this etchant tints
iron phosphide a dark-bluecolor, cementite a violet or dark red,
and ferriteremains white. Figure 28 shows the microstruc-
ture of the ternary eutectic, with cementite tinteda light-pink
color, while the rest of the constit-uents were colored
properly.
Figure 29 shows the microstructure of thepseudobinary
phosphorous eutectic, which con-sists of iron phosphide and
ferrite, after etchingwith 4% nital. The same specimen was
etchedwith hot ASP (Table 2, etchant No. 4). This didnot tint the
iron phosphide or the ferrite. Becausecementite is not present in
the eutectic, the onlyetching was of cementite in the pearlite,
whichshowed up very lightly. To reveal the microstruc-ture of the
eutectic, the specimen was nextetched with 4% nital. Figure 30
shows the resultsafter using hot ASP and then nital. Hot Murak-amis
reagent perfectly tinted the iron phosphidein the binary
phosphorous eutectic a browncolor, while pearlite was colorless, as
shown inFig. 31.
Beraha-Martensite (B-M) (Table 2, etch No.10) and aqueous 10%
sodium metabisulte(SMB) (Table 2, etch No. 11) reagents for
selec-tive color etching are very useful in cases
wheremicrostructural details are very ne and barelyvisible after
etching with nital. They reveal allof the constituents, tinting
them to expected col-ors that are useful for verifying that the
heattreatment process was carried out correctly.
The B-M etchant is a mixture of stock solution(1:5, HCl to
water), potassium metabisulte(K2S2O5), and ammonium acid
uoride(NH4FHF). According to Ref 6, B-M tints mar-tensite a blue
color and bainite a brown color,while the residual austenite and
carbides remainunaffected.
The B-M etchant can be used for identicationof the constituents
after heat treatment of castiron by tinting phases to different
colors. It also
Fig. 42 Same as in Fig. 41 but after etching with Lich-tenegger
and Bloech I. Austenite is dark brown,
and dendritic segregation is visible around unaffected
car-bides. 1000
Fig. 43 As-cast high-chromium white iron
(Fe-4.52%C-0.4%Si-2.86%Mn-35.0%Cr-0.06%P-
0.012%S). PC, primary carbides; EC, eutectic carbides,both M7C3
type. Etched with glyceregia. 500
Fig. 41 Same white iron as in Fig. 39 and 40 but as-cast.
Eutectic carbides in austenitic matrix.
Etched with glyceregia. 500
Fig. 44 Same as in Fig. 43 but after etching with Mu-rakamis
reagent (at room temperature). PC, or-
ange primary carbides; EC, orange and gray eutectic car-bides.
400
Fig. 45 Same as in Fig. 43 but as-polished and exam-ined in
differential interference contrast. Pri-
mary and eutectic carbides are sticking up from the
softeraustenitic matrix. 400
-
576 / Metallography and Microstructures of Ferrous Alloys
improves microstructural contrast, enhancingvisibility and
permitting estimation of evensmall amounts of the residual
austenite (althoughx-ray diffraction results are always more
than10% greater than by light microscopy) and necarbides. Figure 32
shows the black-white mi-
crostructure of ADI after etching with 4% nital,while Fig. 33
shows the same microstructure af-ter etching with 4% picral.
Figures 34 and 35
show the microstructure of the same specimenafter color etching,
respectively, with B-M andwith aqueous 10% SMB etchants. The SMB
tintsmartensite a brown color and bainite a blue color,while
austenite and carbides are colorless. Bothetching time in B-M and
the different crystallo-graphic orientations affected the color of
thecoarse, high-carbon martensitic plates, whichvary from blue to
yellow. The brown areas in themicrostructure (Fig. 34) are the
patches of nemartensite. This color differentiation of
micro-structure occurs due to the change in size of theplate
martensite as transformation progresses.However, this is not the
only factor, becausesome of the larger plates are also brown.
Thereis only a very small amount of austenite, whichsurrounds the
acicular ferrite at the graphite nod-ules and in the matrix. The
SMB etchant is evenmore useful than the B-M etchant in the
casewhere the dominating phase in the ADI micro-structure is
martensite, and acicular ferrite isweakly visible. In ADI, the
martensite of bothtypes is tinted a brown color, the acicular
ferriteis colored the same as bainite, that is, blue color,while
austenite is colorless, which was shown inFig. 35 (see the section
Ductile Iron in thisarticle).
Figure 36 shows the microstructure of ADIafter etching with 4%
nital, while Fig. 37 and 38show the same microstructure after
etching withB-M and 10% SMB, respectively. Nital etchedthe acicular
ferrite, while the austenite is white.Some areas that were darkened
may be marten-site, but there is no clear distinction
betweenmartensite and acicular ferrite with nital. Selec-Fig. 46
Same white iron as in Fig. 39 but after slightetching with 4% nital
and examined in bright-
eld illumination. EC, eutectic carbides type M7C3; SC,secondary
carbides; and M, martensite. 1000
Fig. 47 Same as in Fig. 46 but examined in dark-eldillumination.
EC, eutectic carbides; SC, sec-
ondary carbides; and M, martensite. 1000
Fig. 48 Same as Fig. 46 but examined in differentialinterference
contrast. EC, eutectic carbides;
SC, secondary carbides; and M, martensite. 1000
Fig. 49 Same white iron as in Fig. 14 and 15 but cast-ing was
heat treated at 1000 C (1830 F), held
1 h, furnace cooled to 400 C (750 F) for 2 h, taken to saltbath
at 400 C (750 F), held 4 h, and air cooled. Examinedin bright-eld
illumination. EC, eutectic carbides typeM7C3; SC, secondary
carbides. Etched with glyceregia.1000
Fig. 50 Same as in Fig. 49 but examined in dark-eldillumination.
EC, eutectic carbides; SC, sec-
ondary carbides. 1000
-
Metallography and Microstructures of Cast Iron / 577
tive color etching with the two previously men-tioned reagents
clearly revealed small patches ofmartensite, which were blue after
etching withB-M and brown after etching with 10% SMB;acicular
ferrite was colored blue (darker with
B-M than with SMB), and austenite remainedcolorless.
The same results were achieved with the useof B-M reagent to
reveal the microstructure of
alloyed cast iron after heat treatment. The mi-crostructure,
which is shown in Fig. 39, consistsof brown patches of ne
martensite (which mayhave transformed from austenite during or
aftertempering), while the blue needles are high-car-bon plate
martensite. Figure 40 shows the mi-crostructure of the same
specimen after etchingwith 4% nital; in this case, the recognition
ofmartensite is not straightforward.
Figures 41 and 42 show the microstructure ofthe same iron but in
the as-cast condition afteretching with glyceregia and with
Lichteneggerand Bloech I (LBI), respectively (Table 2, etch-ant No.
3 and 12). The LBI is an aqueous solu-tion of ammonium biuoride
(NH4FHF) andpotassium metabisulte (K2S2O5) (Table 2, etchNo. 12).
In chromium-nickel alloys, LB I tintsaustenite, while carbides and
ferrite (if present)remain unaffected (white). Glyceregia
outlinesonly the eutectic carbides, while the LB I etchantalso
reveals microsegregation. The microstruc-ture shown in Fig. 42
consists of austenite, tintedbrown and blue color, and white
(noncolored)carbides. The blue austenitic areas surrounding
Fig. 51 Graphite nodule examined in bright-eld illu-mination.
As-polished. 1000
Fig. 52 Same as in Fig. 51, but graphite nodule wasexamined in
crossed polarized light. 1000
Fig. 53 Flake graphite in as-cast gray iron examined incrossed
polarized light. As-polished. 200
Fig. 54 Same as in Fig. 33 but microstructure was ex-amined in
crossed polarized light. Acicular fer-
rite is shining brightly; plate martensite is slightly
gray-blue.1000
Fig. 55 Flake graphite in as-cast gray iron examinedwith SEM.
Sample was deeply etched with
50% HCl. 500
Fig. 56 Nodular graphite in as-cast ductile iron exam-ined with
SEM. Sample was deeply etchedwith
50% HCl. 1000
-
578 / Metallography and Microstructures of Ferrous Alloys
the eutectic carbides indicate regions with alower concentration
of carbide-forming ele-ments, such as carbon and chromium, and
per-haps a higher concentration of elements that arenot present in
the carbides, such as silicon andnickel.
Figure 43 shows the microstructure of a high-chromium cast iron
after etching with glycere-gia, revealing columnar primary carbides
and eu-tectic carbides, both (FeCr)7C3 type, uniformlydistributed
in the matrix. Figure 44 shows themicrostructure of the same
specimen after etch-ing with the standard Murakamis etchant atroom
temperature. The carbides are tinted an or-ange color, while the
matrix is not colored. X-ray diffraction determined that the matrix
wasaustenitic-ferritic; the matrix was not colored us-
ing the LB I tint etchant due to missing nickelin that iron.
The basic etchants mentioned previously,which are suitable for
revealing microstructures
as well as for phase identication in irons, arelisted in Table
2.
Dark-Field Illumination and DifferentialInterference Contrast.
Dark-eld illumination(DFI) technique for microstructural
examinationproduces a very strong image contrast that makesit
possible to see features in the microstructurethat are invisible or
weakly visible in bright-eldillumination (BFI) when the surface of
the spec-imen is normal to the optical axis of the micro-scope and
white light is used. This image con-trast is a result of reversing
the image, which isbright in BFI, into a dark one when DFI is
used.
Table 3 Constituents commonly found incast iron microstructures,
and their generaleffect on physical properties
Constituent Characteristics and effects
Graphite(hexagonalcrystalstructure)
Free carbon; soft; improvesmachinability and dampingproperties;
reduces shrinkage andmay reduce strength severely,depending on
shape
Austenite (c-iron) Face-centered cubic crystal structure.The
character of the primary phase,which solidies from theoversaturated
liquid alloy indendrite form, is maintained untilroom temperature.
Austenite ismetastable or stable equilibriumphase (depending upon
alloycomposition). Usually transformsinto other phases. Seen only
incertain alloys. Soft and ductile,approximately 200 HB
Ferrite (-iron) Body-centered cubic crystal structure.Iron with
elements in solidsolution, which is a stableequilibrium,
low-temperaturephase. Soft, 8090 HB; contributesductility but
little strength
Cementite (Fe3C) Complex orthorhombic crystalstructure. Hard,
intermetallic phase,8001400 HV depending uponsubstitution of
elements for Fe;imparts wear resistance; reducesmachinability
Pearlite A metastable lamellar aggregate offerrite and cementite
due toeutectoidal transformation ofaustenite above the bainite
region.Contributes strength withoutbrittleness; has good
machinability,approximately 230 HB
Martensite Generic term for microstructures thatform by
diffusionlesstransformation, where the parentand product phases
have a speciccrystallographic relationship. Hardmetastable
phase
Steadite(phosphorouseutectic)
A pseudobinary or ternary eutectic offerrite and iron phosphide
orferrite, iron phosphide, andcementite, respectively. It can
formin gray iron or in mottled iron witha phosphorous content
0.06%.Hard and brittle; solidies from theliquid on cooling at the
cellboundaries as a last constituent ofthe microstructure
Ledeburite Massive eutectic phase composed ofcementite and
austenite; austenitetransforms to cementite andpearlite on cooling.
Produces highhardness and wear resistance;virtually
unmachinable
Source: Ref 3, 10
Fig. 57 Compacted graphite examinedwith SEM. Sam-ple was deeply
etched with 50% HCl. 1500
Fig. 58 Hypoeutectic as-cast gray iron
(Fe-2.8%C-1.85%Si-0.5%Mn-0.04%P-0.025%S). Flake
graphite type A. As-polished. 100
Fig. 59 Hypoeutectic as-cast gray iron
(Fe-2.1%C-2.8%Si-0.38%Mn-0.06%P-0.03%S). Flake
graphite type D. As-polished. 100
Fig. 60 Hypereutectic as-cast gray iron
(Fe-3.5%C-2.95%Si-0.4%Mn-0.08%P-0.02%S-0.13%Ni-
0.15%Cu). Flake graphite type A. As-polished. 100
-
Metallography and Microstructures of Cast Iron / 579
Features that are perpendicular to the optic axisin BFI appear
white, while in DFI they are black.Likewise, features that are at
an angle to the sur-face, such as grain boundaries and phase
bound-aries, appear black in BFI but are white (self-luminous in
appearance) in DFI.
When crossed polarized light is used alongwith a double-quartz
prism (Wollaston prism)placed between the objective and the
vertical il-luminator, two light beams are produced that ex-
hibit coherent interference in the image plane.This leads to two
slightly displaced (laterally)images differing in phase (k/2),
which produceshigher contrast for nonplanar detail. This iscalled
differential interference contrast (DIC),and the most common system
is the one devel-oped by Nomarski. The image reveals topo-graphic
detail somewhat similar to that producedby oblique illumination but
without the loss ofresolution. Images can be viewed with
naturalcolors similar to those observed in bright eld,or articial
coloring can be introduced by addinga sensitive tint plate (Ref
4).
The microstructure of Fe-Cr-Mn cast irons canbe examined in DIC
after polishing. Figure 45shows the microstructure of the
high-chromiumcast iron (shown previously in Fig. 43 and 44)but
as-polished (unetched) and examined in DIC.The chromium carbides,
which are much harderthan the matrix, stand above it in relief and
canbe seen very well.
The DFI and DIC methods also can be usedfor revealing the
details of microstructures in al-loyed irons, for example, a
chromium-nickeliron after heat treatment, which were barely
visi-ble after etching with nital. Figures 46 to 48show the
microstructure of the Fe-Cr-Ni alloyediron after heat treatment and
after etching with4% nital, but for a shorter time than usual.
Thethree micrographs show the results for the sameeld after
examination with BFI, DFI, and DIC,respectively. The BFI image
reveals the marten-site poorly, and the secondary carbides are
barelyvisible. However, both the DFI and DIC imagesreveal the
martensite, although not strongly,Fig. 61
Hypereutectic as-cast gray iron
(Fe-2.18%C-2.49%Si-0.7%Mn-0.06%P-0.05%S). Flake
graphite type E. As-polished. 100. Courtesy of G.F. Van-der
Voort, Buehler Ltd.
Fig. 63 Hypereutectic as-cast gray iron
(Fe-3.4%C-3.4%Si-0.07%Mn-0.04%P-0.03%Cr-
0.47%Cu). Pointlike ake graphite type D. As-polished.100
Fig. 64 Hypereutectic as-cast gray iron
(Fe-4.3%C-1.5%Si-0.5%Mn-0.12%P-0.08%S). Flake
graphite type C (kish graphite). As-polished. 100Fig. 65 As-cast
gray iron. Flake graphite type V (star-
like graphite). As-polished. 100
Fig. 62 Hypereutectic as-cast gray iron
(Fe-3.3%C-2.75%Si-0.88%Mn-0.42%P-0.086%S). Flake
graphite type B. As-polished. 90 (microscopic examina-tion
100)
-
580 / Metallography and Microstructures of Ferrous Alloys
while the secondary carbides are very distinct(see also Fig. 39,
40).
The microstructure of an Fe-C-Cr cast iron af-ter heat
treatment, revealed with glyceregia and
examined with BFI and DFI, is shown in Fig. 49and 50,
respectively. The details of the micro-structure were revealed much
more strongly withDFI than with BFI.
Polarized Light Response. Polarized light isobtained by placing
a polarizer (usually a Polar-oid lter, Polaroid Corp.) in front of
the con-denser lens of the microscope and placing an an-alyzer
(another Polaroid lter) before theeyepiece. The polarizer produces
plane-polar-ized light that strikes the surface and is
reectedthrough the analyzer to the eyepieces. If an an-isotropic
metal is examined with the analyzer set90 to the polarizer, the
grain structure will bevisible. However, viewing of an isotropic
metal(cubic metals) under such conditions will pro-duce dark,
extinguished conditions (completedarkness is not possible using
Polaroid lters).Polarized light is particularly useful in
metallog-raphy for revealing grain structures and twinningin
anisotropic metals and alloys as well as foridentifying anisotropic
phases and inclusions.This method also improves the contrast of
mi-crostructures, providing more structural details(Ref. 4).
Figure 51 shows the microstructure of agraphite nodule in BFI,
while Fig. 52 shows thesame nodule in crossed polarized light.
Polarizedlight reveals much better than BFI the radialstructure of
the graphite nodule that grows fromthe central nucleus. Also, the
cross, which ischaracteristic of the perfectly formed
graphitenodule, can be made visible only with the use ofthis
technique. Figure 53 shows the anisotropiclayered structure of
graphite akes, which alsorespond to polarized light. In both cases,
the col-ors of graphite vary due to the anisotropy ofgraphite.
Figure 54 shows the microstructure ofADI, consisting of martensite
and a small
Fig. 66 As-cast ductile iron
(Fe-3.45%C-2.25%Si-0.30%Mn-0.04%P-0.01%S-0.8%Ni-
0.07%Mg-0.55%Cu). Nodular graphite size is 20 lm. As-polished.
100
Fig. 67 As-cast ductile iron
(Fe-3.6%C-2.9%Si-0.14%Mn-0.04%P-0.02%S-0.16%Ni-
0.06%Mg). Nodular graphite size is 40 lm. As-polished.100
Fig. 68 As-cast ductile iron. Nodular graphite size is100 lm.
As-polished. 100
Fig. 69 As-cast ductile iron. Imperfectly formed graph-ite
nodules. As-polished. 100
Fig. 70 As-cast ductile iron. Exploded graphite.As-pol-ished.
100
-
Metallography and Microstructures of Cast Iron / 581
amount of acicular ferrite. The specimen wasetched with 4%
picral to reveal the acicular fer-rite in the background of the
almost invisiblemartensite (see Fig. 33). When examination
wascarried out with crossed polarized light plus a
sensitive tint lter, shown in Fig. 54, the ferriticneedles can
be seen by shining with a whitecolor. Also, many of the coarsest
martensiticplates (body-centered tetragonal crystal struc-ture)
with the proper lattice orientation were visi-ble in polarized
light but barely visible in BFI.
Microstructures
Table 3 lists the characteristics of typical con-stituents of
cast iron microstructures and theireffect on mechanical properties
(Ref 3, 10).
Microstructure of Graphite
The eutectic graphite cell has a continuousgraphite skeleton,
but on the metallographiccross section, the three-dimensional
nature is notobvious. Figures 55 to 57 show scanning elec-tron
microscopy (SEM) micrographs of ake,nodular, and compacted
graphite, respectively.Contrary to ake graphite, where the akes
so-lidify as an aggregate and each such agglomer-ation is one
eutectic cell, both spheroidal andcompacted graphite solidify as
separate precipi-tates. For SEM examination, the specimens
weredeeply etched (Table 2, etch No. 13).
Characteristic properties of graphite precipi-tations are shape,
size, and distribution. There isa correlation between different
graphite mor-phologies, the chemical composition, and thecooling
rate.
Flake Graphite in Gray Iron. Flake (lamel-lar) graphite is
characteristic of gray iron, andcomponents such as aluminum,
carbon, and sil-icon promote its formation. When the coolingrate
increases, the akes get ner, and their dis-tribution tends to be
interdendritic. Figure 58shows a hypoeutectic gray iron with a
uniformdistribution of randomly oriented graphite akeswith a
maximum length of 320 lm (type A akegraphite in the ASTM A 247
classications).Figure 59 shows a ne, type D ake graphitewith a
maximum length of 40 lm, also in a hy-poeutectic alloy, but it
solidied at a higher cool-ing rate, which promoted the
interdendritic dis-tribution. Type A graphite in a
hypereutecticgray iron is shown in Fig. 60. It has the mostdesired
shape and distribution, and it is verycharacteristic of gray iron
casts with high mach-inability. The maximum length of graphite
akesis 160 lm. Figures 61 and 62 show type E andtype B graphite,
respectively, which occur in hy-pereutectic gray iron at high
cooling rates. Notein Fig. 62 that each rosette group of
graphiterepresents one eutectic cell. That type of graphiteis
characteristic of thin-walled castings.
Fig. 71 Austempered ductile iron
(Fe-3.6%C-2.5%Si-0.052%Mg-0.7%Cu). Chunky (CH) and spiky
(SP) types of graphite. As-polished. 100
Fig. 72 As-cast iron with compacted graphite
(Fe-3.7%C-2.3%Si-0.21%Mn-0.03%P-0.01%S-
0.82%Ni-0.02%Mg). Graphite size is 80 to 160 lm.As pol-ished.
100
Fig. 73 Malleable iron
(Fe-2.65%C-1.2%Si-0.53%Mn-0.06%P-0.21%S-0.08%Cr-0.10%Cu-
0.07%Ni-0.01%Al). Temper graphite type III with maxi-mum size of
80 lm. As-polished. 100
Fig. 74 The diagram of correlation between a type of matrix in
nonalloyed cast irons and silicon and phosphoruscontent as well as
the thickness, R, of the casting wall, which relates to the cooling
rate. Kg C (Si logR)
is a coefcient of graphitization, and CE C 13Si 13P is the
coefcient of saturation (carbon equivalent). Region Iis white cast
iron (pearlite, cementite, no graphite); Region IIa is mottled cast
iron (pearlite, graphite, cementite); RegionIIb is
ferritic-pearlitic cast iron; Region III is ferritic cast iron.
Source: Ref 13
-
582 / Metallography and Microstructures of Ferrous Alloys
A high degree of undercooling of hypereutec-tic gray iron can
promote the solidication ofvery ne, pointlike type D graphite with
an in-terdendritic distribution, as shown in Fig. 63. Inthe other
direction, undertreatment of the graph-
itizing inoculants, such as ferrosilicon, producesother ake
graphite types in gray iron. For ex-ample, Fig. 64 shows a
hypereutectic gray ironwith graphite type C, where very coarse,
needle-
like akes (kish graphite) form before the eutec-tic, which is
very ne. Kish graphite, which isshown in Fig. 64, can be changed
into a starlikegraphite, shown in Fig. 65, under higher
coolingrates, which is referred to as type V (plate I ofASTM A
247). The carbide-forming alloy ele-ments, for example, chromium,
manganese, andvanadium, and the low-melting-point metals,
forexample, bismuth, lead, and sulfur, also affectgraphite
morphology.
Nodular Graphite in Ductile Iron. The ad-dition of magnesium in
the inoculation processdesulfurizes the iron and makes graphite
precip-itate as nodules rather than akes. Moreover,me-chanical
properties are greatly improved overgray iron; hence, nodular iron
is widely knownas ductile iron. Nodule size and shape perfectioncan
vary, depending on composition and coolingrate. Figure 66 shows ne
nodules with a max-imum diameter of 20 lm in a chill-cast thin
sec-tion, while Fig. 67 and 68 show coarser nodules,with maximum
diameters of 40 and 100 lm, re-spectively. Note that the number of
nodules perunit area is different and changes from approxi-mately
350 to 125 to 22/mm2, respectively, forFig. 66 to 68.
Certain factors can cause weak nodularity.Figure 69 shows an
irregular graphite shape dueto poor inoculation or excessive fading
of inoc-ulant. Exploded graphite, shown in Fig. 70, mayoccur due to
excessive rare earth additions. Nor-mally, it is found in
thick-section castings or athigher-carbon equivalents (Ref 11).
Figure 71shows chunky and spiky types of graphite. Therst one is
caused by high-purity charge mate-Fig. 75 As-cast gray iron
(Fe-2.8%C-1.85%Si-
1.05%Mn-0.04%P-0.025%S). Pearlitic matrix.Etched with 4% nital.
100
Fig. 76 Same as in Fig. 75. Fine pearlite. 500
Fig. 77 As-cast gray iron. Pearlitic-ferritic matrix
withphosphorous eutectic (E). Etchedwith 4%nital.
100
Fig. 78 Same as in Fig. 77. E, ternary phosphorous eu-tectic; P,
pearlite; and F, ferrite. 500
Fig. 79 As-cast gray iron
(Fe-3.4%C-3.4%Si-0.07%Mn-0.04%P-0.03%Cr-0.47%Cu). Ferri-
tic-pearlitic matrix. Etched with 4% nital. 100
-
Metallography and Microstructures of Cast Iron / 583
rials and excess rare earth additions in high-car-bon-equivalent
compositions, while the secondone is caused by small amounts of
tramp ele-ments, for example, lead, bismuth, tin, and tita-nium, in
the absence of cerium (Ref 12).
Compacted Graphite. Methods that producecompacted graphite cast
iron include the treat-ment of molten iron with rare earth
inoculants,adding a lower amount of magnesium than isrequired to
obtain nodular graphite, or adding
elements such as titanium and aluminum thatmake it difcult to
spheroidize graphite. Thistype of iron is a more recent
development, madein an effort to improve the mechanical
propertiesof ake gray iron. Figure 72 shows an example
where the longest compacted graphite precipi-tations are in the
80 to 160 lm range.
Temper Graphite in Malleable Iron. Tem-per graphite is formed by
annealing white castiron castings to convert carbon in the form
ofcementite to graphite, called temper-carbon nod-ules. Figure 73
shows the type III graphite pre-cipitation with 80 lm maximum
size.
Microstructure of Matrix
The matrix of gray, nodular, compacted, andmalleable cast irons
can be pearlitic, pearlitic-ferritic, ferritic-pearlitic, or
ferritic. The samematrix constituents can be present in white
castiron, but cementite precipitates from the melt,rather than
graphite, due to crystallization in ametastable system.
The matrix microstructure depends on chem-ical composition as
well as on the temperatureof the eutectoidal transformation. Figure
74shows the diagram of the correlation between thetype of matrix in
nonalloyed cast irons and thesilicon and phosphorus content as well
as thethickness, R, of the casting wall, which relatesto the
cooling rate. Kg C (Si logR) givesthe coefcient of graphitization,
and CE C13Si 13P is the coefcient of saturation (car-bon
equivalent). A low value of Kg promotes so-lidication of white cast
iron, with cementite andpearlite as the microstructure, regardless
of thetotal carbon content, C. When the Kg coefcientand the silicon
content increase, the microstruc-
Fig. 80 As-cast gray iron
(Fe-3.4%C-3.2%Si-0.09%Mn-0.04%P-0.01%S-0.86%Cu-
0.01%Mg). Ferritic matrix. Etched in 4% nital. 100
Fig. 81 The Fe-C-P equilibrium diagram. The x-axis isthe carbon
content, and the y-axis is the phos-
phorus content. Source: Ref 13
Fig. 82 As-cast ductile iron
(Fe-3.7%C-1.25%Si-0.03%Mn-0.02%P-0.02%S-0.24%Ni-
0.06%Mg). Pearlite matrix with ferritic halos aroundgraph-ite
nodules. Etched with 4% nital. 100
Fig. 83 As-cast ductile iron. Ferritic-pearlitic matrix.Etched
with 4% nital. 100
Fig. 84 Ductile iron. Ferritic matrix. The casting wasannealed
at 900 C (1650 F), held 2 h, quick
furnace cooled to 730 C (1345 F), slow furnace cooled to600 C
(1110 F), and air cooled. Etched with 4% nital.100
-
584 / Metallography and Microstructures of Ferrous Alloys
ture of the cast iron matrix tends to be pearliticthrough
pearlitic-ferritic to ferritic, and it de-pends on the CE value
(Ref 13).
In pearlitic-ferritic cast irons, the regions withferrite always
occur within the eutectic cells and
in the neighborhood of graphite precipitates dueto
microsegregation. The microregions of solid-ication, like the axis
of dendrites and the inte-riors of eutectic cells, contain more
silicon,which promotes ferrite formation. Slow cooling,as well as
higher silicon contents, usually pro-duces ferrite, while a very
fast cooling rate canproduce free cementite. Ferritic
microstructuresalso can be obtained by annealing of pearliticcast
irons or in thick-walled castings (Ref 13).
Gray iron is classied according to minimumtensile strength of
the test bars. The matrix ispredominantly pearlitic (Fig. 75, 76)
but also canbe pearlitic-ferritic (Fig. 77, 78),
ferritic-pearlitic(Fig. 79), or ferritic (Fig. 80).
A common characteristic constituent of grayiron microstructures
is the phosphorus eutecticknown as steadite. Figure 81 shows the
Fe-C-Pequilibrium diagram, while Table 4 shows thetransformations
that occur in this system during
solidication as well as in the solid state. Thecharacteristic
property of this system is a largearea of the ternary phosphorous
eutectic due tothe strong tendency for phosphorus to segregate.This
type of eutectic appears in the microstruc-ture of cast irons
already at 0.07% P (Ref 13).
The form of the phosphorus eutectic dependson the chemical
composition of the gray iron. Inirons with an average tendency to
graphitizationand a phosphorus content of approximately0.4%, the
ne-grain ternary eutectic solidiesfrom the liquid and consists of
ferrite Fe3PFe3C. In gray iron with a strong tendency forcementite
solidication and with carbide-form-ing elements, the ternary
eutectic may also con-tain large columnar precipitates of
cementite.
Increasing the amount of strong graphitizingelements, such as
silicon, promotes solidicationof the binary phosphorous eutectic,
ferrite Fe3P Cgr, instead of the ternary one. The bi-
Table 4 Transformations in the range of solidication and in the
solid state according tothe Fe-C-P diagram (Fig. 81)
Symbol on theFe-C-P diagram Transformation(a) Type of
transformation Temperature Composition of phases(b)TJ L c Fe3P
Peritectic 1005 C (1840 F) L 0.8 C, 9.2% P
0.3% C, 2.2% Pc 0.5% C, 2.0% P
E L c Fe3P Fe3C Eutectic 950 C (1740 F) L 2.4% C, 6.89% Pc 1.2%
C, 1.1% P
MN c Fe3P Fe3C Peritectoidal 745 C (1375 F) c 0.8% C, 1.0% P
0.1% C, 1.5% P
(a) L, melt (liquid metal); , ferrite (-iron); c, austenite
(c-iron); Fe3P, iron phosphide; Fe3C, cementite. (b) Maximum
solubility of chemicalcomponents in the particular phases. Source:
Ref 13
Fig. 85 As-cast ductile iron
(Fe-3.35%C-2.05%Si-0.08%Mn-0.04%P-0.02%Cr-0.02%Ni-
0.045%Mg). C, cementite; L, ledeburite; and P, pearlite
andferrite around graphite nodules. Etched with 4% nital.125
(microscopic magnication is 100)
Fig. 86 Same as in Fig. 85. C, cementite; L, ledeburite;F,
ferrite; and P, pearlite. 500
Fig. 87 As-cast austenitic ductile iron
(Fe-2.7%C-2.85%Si-1.15%Mn-0.03%P-0.01%S-2.8%Cr-
20.0%Ni-0.1%Mg). Austenite and eutectic carbides typeM7C3.
Etched with glyceregia. 500
Fig. 88 Austempered ductile iron (the same ductileiron as in
Fig. 32). Acicular ferrite and austen-
ite. The casting was austempered: 900 C (1650 F), held 2h, taken
to salt bath at 360 C (680 F), held 180 min, andair cooled. Etched
with 4% nital. 500
-
Metallography and Microstructures of Cast Iron / 585
nary euetectic is called a pseudobinary eutectic,because carbon
is removed from the eutectic dur-ing diffusion in the solid state
(Ref 13).
Ductile iron microstructures normally con-sist of pearlite or
pearlite and ferrite with graph-ite nodules surrounded with
ferrite, which looklike a white halo, although the more commonname
of this structure is the bulls-eye structure.Figures 82 and 83 show
this type of microstruc-ture, while Fig. 84 shows a fully
ferritic-matrixmicrostructure that was achieved after annealinga
chilled casting with a microstructure contain-ing cementite,
ferrite, and pearlite (see alsoFig.11). In some cases, usually at a
high coolingrate, cementite occurs as a separate constituentin the
matrix.
The precipitates of cementite are situated inthe exterior
regions of the eutectic cells or in theinterdendritic spaces of the
transformed austen-ite due to the microsegregation of
carbide-form-ing elements, such as manganese, chromium, orvanadium.
Cementite appears very frequently inchill castings or in
thin-walled sand castings (Ref13). Figures 85 and 86 show the
microstructureof pearlitic ductile iron with cementite and
led-eburite. In this case, cementite was a desired con-stituent of
the microstructure to improve thewear resistance of cast iron. It
was achieved byfeeding the melted metal with an iron-magne-sium
foundry alloy, without inoculation. Thegraphite nodules do not have
a perfect shape.Figure 87 shows the microstructure of a heat-and
wear-resistant ductile iron containing chro-mium and nickel, which
consists of chromiumcarbides in an austenitic matrix.
The austempering heat treatment is used toachieve better
mechanical properties in ductile
iron. The casting is heated to a temperature rangeof 840 to 950
C (1545 to 1740 F) and held atthis temperature until the entire
matrix is trans-formed to austenite saturated with carbon.
Thecasting is then quenched rapidly to austemperingtemperature,
between 230 and 400 C (445 and750 F), and held at this temperature
for the re-quired time (Ref 14). When this cycle is
properlyperformed, the nal structure of the matrix,which is called
ausferrite, consists of acicularferrite and austenite, and the iron
is called aus-tempered ductile iron (ADI). The morphologyand amount
of ferrite depends on the time andtemperature of the austempering
process. Fig-ures 88 and 89 show an ADI microstructure
afteraustempering heat treatment, when the casting inboth cases was
held in the furnace for 180 min,and the annealing temperatures were
different.The use of a higher temperature changed themorphology of
the ferrite and increased theamount of austenite. Figure 90 shows
the ADImicrostructure after 2 min austempering heattreatment, when
the transformation of austeniteto acicular ferrite was just
started, so martensiteis the dominant phase, and there is only a
smallamount of acicular ferrite, mostly surroundingthe graphite
nodules (see also Fig. 31 to 37).
Compacted graphite-iron matrix micro-structures consist of
ferrite and pearlite; theamount of each constituent depends on the
cool-ing rate and the chemical composition (elementsthat promote
either ferrite or pearlite solidica-Fig. 89 Austempered ductile
iron (the same composi-
tion as in Fig. 32). Acicular ferrite and austen-ite. The
casting was austempered: 900 C (1650 F), held 2h, taken to salt
bath at 380 C (715 F), held 180 min, andair cooled. Etched with 4%
nital. 500
Fig. 90 Austempered ductile iron (the same composi-tion as in
Fig. 32). Martensite and small amount
of acicular ferrite (AF). The casting was austempered: 900C
(1650 F), held 2 h, taken to salt bath at 300 C (570 F),held 2 min,
and air cooled. Etched with 4% nital. 1000
Fig. 91 As-cast iron with compacted graphite
(Fe-2.8%C-1.9%Si-0.55%Mn-0.04%P-0.2%S-
0.018%Mg). Ferritic-pearlitic matrix. Etched with 4%
nital.100
Fig. 92 Malleable iron
(Fe-2.95%C-1.2%Si-0.53%Mn-0.06%P-0.21%S-0.08%Cr-0.10%Cu-
0.07%Ni-0.01%Al). The casting was annealed: at 950 C(1740 F),
held 10 h, furnace cooled to 720 C (1330 F),held 16 h, and air
cooled. Pearlitic-ferritic matrix. Etchedwith 4% nital. 125
(microscopic magnication 100)
-
586 / Metallography and Microstructures of Ferrous Alloys
tion). Figure 91 shows a ferritic-pearlitic micro-structure with
a small amount of nodular graph-ite.
Malleable iron matrix microstructures aremostly ferritic,
ferritic-pearlitic, or pearlitic, de-pending on the cast-making
process used. Thetype of matrix is a result of transformation in
thesolid state during the annealing of white castiron. Figures 92
and 93 show the microstructure
of the pearlitic-ferritic and ferritic malleableiron,
respectively. The annealing process was thesame one in both cases,
but the nal microstruc-tures were achieved by the use of different
foun-dry processes. In the case of the
pearlitic-ferriticmicrostructure, the inoculation and
deoxidationprocesses were not carried out. When these twoprocesses
were used, that is, deoxidation withaluminum and inoculation with
iron-silicon andpure bismuth, the result was a
ferriticmicrostruc-ture after annealing.
Unalloyed white iron microstructures arecreated when the
castings solidify in the meta-stable system and the structure is
free of graph-ite. The matrix may consist of cementite andpearlite,
as shown in Fig. 94, or, after heat treat-ment, of cementite and
martensite, as shown inFig. 95. This type of white iron belongs to
theclass of abrasion-resistant cast irons and usuallyis produced as
chill castings.
High-chromium white iron is a type ofhighly wear-resistant iron
with a microstructurethat consists of primary and eutectic
carbides, forexample, (FeCr)3C, (FeCr)7C3, and (FeCr)23C6,depending
on the chemical composition andcooling rate during solidication.
The primarycarbides have a typical shape, which in cross sec-tion
is similar to the letter L, and have thechemical formula M3C at
higher cooling rate andlow chromium content. When the cooling
rateslows down and the chromium content increasesto, for example,
30% and higher, the primarychromium carbides have a hexagonal shape
witha characteristic hole in the center. The eutecticcarbides are
always type M7C3, independent ofchromium content and cooling rate
(Ref 15).
The type M23C6 carbides solidify when thechromium content is
approximately 50 to 60%(Ref 16). The same factors that affect the
type ofcarbides also inuence the type of matrix, whichcan be
austenitic but also ferritic or pearlitic.When the cooling rate is
respectively slow, sec-ondary precipitations can occur around the
aus-tenitic dendrites (Ref 17).
It has been proven that the annealing heattreatment destabilizes
the matrix, which willtransform into either bainite or martensite
withsmall secondary carbide precipitates. The result-ing
transformation product is a function of thecooling rate from
annealing as well as the chem-ical composition; for example, when
Cr/C 5,the matrix microstructure was bainitic, while ex-ceeding
this value produced martensitic trans-formation (Ref 18). Figure 96
shows the micro-structure of high-chromium white cast iron
afterisothermal heat treatment; the secondary, necarbides
precipitated from the austenitic matrix,which transformed into
martensite (see also Fig.43, 44, and 45).
ACKNOWLEDGMENTS
For samples and examination results, manythanks to: Colleagues
at the Foundry Research Institute,
Krakow, Poland: Adam Kowalski, W. Wierz-Fig. 94 As-cast white
iron (Fe-3.0%C-2.7%Si-
0.45%Mn-0.07%P-0.025%S). C, cementite; P,pearlite. Etched with
4% nital. 400
Fig. 93 Malleable iron
(Fe-2.9%C-1.5%Si-0.53%Mn-0.06%P-0.22%S-0.09%Cr-0.10%Cu-
0.08%Ni-0.02%Al). The castingwas annealed as in Fig. 92.Ferrite.
Etched with 4% nital. 125 (microscopic magni-cation 100)
Fig. 95 White cast iron after heat treatment. A networkof
massive cementite and temperedmartensite.
Etched with 4% picral. 140. Courtesy of G.F. VanderVoort,
Buehler Ltd.
Fig. 96 White high-chromium cast iron (see Fig. 43).PC, primary
carbides; EC, eutectic carbides in
martensitic matrix with ne, globular secondary carbides.The
casting was heat treated at 1000 C (1830 F), held 1h, furnace
cooled to 550 C (1020 F), held 4 h in a 400 C(750 F) salt bath, and
air cooled. Etched with glyceregia.1000
-
Metallography and Microstructures of Cast Iron / 587
chowski, J. Turzynski, W. Madej, K.Gownia, A. Pytel, and J.
Bryniarska
M. Sorochtej of The Krakow Technical Uni-versity
Also, many thanks to M. Warmuzek at theFoundry Research
Institute for making the SEMmicrographs of different types of
graphite.
Special thanks to G.F. Vander Voort fromBuehler Ltd. for his
advice and help in preparingthis article.
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