Application Notes Metallographic preparatio n of Cast iron Iron is one of the most diverse met- als and alloyed with carbon and other elements it offers an enormous variety of cast iron and steel alloys. Cast iron has been produced in China as early as 600BC and in Europe it was first known in the 14th century. With the development of coal fired blast fur- naces the properties of iron improved and a better castability opened new fields of applications for products in every day life. With the industrialisa- tion cast iron became an important construction material as buildings from the 19th century show: the domes of train stations, market halls, and green houses of botanical gardens, bridges and the Eiffel Tower still document the substantial application of cast iron dur- ing that time. The term cast iron refers to those iron- carbon-silicon alloys which contain 2.5% - 4% carbon and usually 1-3% silicon. Cast iron is an important engi- neering material with a number of ad- vantages, mainly good castability and machinability and moderate mechani- cal properties. Because of its economical advantages cast iron is used for many applications Cutting: White cast iron is very hard and therefore difficult to cut. Grinding and polishing: Graphite is soft and retaining it in its true shape in the automotive and engineering in- dustry. In addition, specific cast irons are the material of choice for sea water pump housings, rolling mill rolls and parts for earth moving equipment. As the morphology of graphite has a major influence on the mechanical properties of cast iron, metallographic quality control of grey iron is an in- tegral part of its production process. Using standard reference comparison charts and/or image analysis tech- niques, the morphology, size and dis- tribution of the graphite is determined on an unetched, polished sample. Depending on the specification, the sample is then etched to check the structure of the matrix. Solution Difficulties during metallographic preparation - Cubic boron nitride cut-off wheel - Thorough diamond polishing on hard polishing cloths and final oxide polishing. and size can be difficult. The matrix of ferritic and/or austenitic cast irons is prone to deformation and scratching. Fig.1: Grey iron with flake graphite, 200xinsufficient polishFig. 2: Same as Fig.1, showing correct polish 200xAustempered ductileiron, Beraha color etch, DIC, 500x
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8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
Iron is one of the most diverse met-als and alloyed with carbon and otherelements it offers an enormous varietyof cast iron and steel alloys. Cast ironhas been produced in China as earlyas 600BC and in Europe it was firstknown in the 14th century. With thedevelopment of coal fired blast fur-naces the properties of iron improvedand a better castability opened newfields of applications for products inevery day life. With the industrialisa-tion cast iron became an importantconstruction material as buildings fromthe 19th century show: the domes oftrain stations, market halls, and greenhouses of botanical gardens, bridgesand the Eiffel Tower still document thesubstantial application of cast iron dur-ing that time.
The term cast iron refers to those iron-carbon-silicon alloys which contain2.5% - 4% carbon and usually 1-3%silicon. Cast iron is an important engi-neering material with a number of ad-vantages, mainly good castability andmachinability and moderate mechani-cal properties.
Because of its economical advantagescast iron is used for many applications
Cutting: White cast iron is very hardand therefore difficult to cut.Grinding and polishing: Graphite issoft and retaining it in its true shape
in the automotive and engineering in-dustry. In addition, specific cast ironsare the material of choice for sea waterpump housings, rolling mill rolls andparts for earth moving equipment.As the morphology of graphite hasa major influence on the mechanicalproperties of cast iron, metallographicquality control of grey iron is an in-tegral part of its production process.Using standard reference comparisoncharts and/or image analysis tech-niques, the morphology, size and dis-tribution of the graphite is determinedon an unetched, polished sample.Depending on the specification, thesample is then etched to check thestructure of the matrix.
Solution
Difficulties during metallographic preparation
- Cubic boron nitride cut-off wheel- Thorough diamond polishing on hard
polishing cloths and final oxidepolishing.
and size can be difficult. The matrix offerritic and/or austenitic cast irons isprone to deformation and scratching.
Fig.1: Grey iron with flake graphite, 200x
insufficient polish
Fig. 2: Same as Fig.1, showing correct polish 200x
Austempered ductile
iron, Beraha color etch,
DIC, 500x
8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
ProductionCast irons are melted in a cupola- or induc-tion furnace charged generally with pigiron, cast iron scrap, steel scrap and vari-ous additions. The alloy composition andthe cooling rate will influence whether theiron will solidify grey or white.A fast cooling rate results in a white solidi-fication and the formation of iron carbide(Fe3C or cementite). At the eutectoid trans-formation a fast cooling rate promotes theformation of pearlite, whereas a slow cool-ing rate promotes the formation of graphiteand ferrite.The microstructure of grey cast irons canhave either a pearlitic and/or ferritic matrixwith free graphite in the shape of flakes,nodules or temper carbon respectively.Through alloying and heat treatment theproperties of cast iron can be adjusted forcertain applications, for instance, alloyingwith molybdenum and nickel improvestheir heat and corrosion resistance.In the following the individual cast ironswill be briefly described and their majorfields of application mentioned.
Production andapplication ofcast irons
Grey iron with flake graphite (FG) hasbetween 2.5-4% carbon, 1-3% silicon and0.2-1% manganese. Carbon and siliconpromote the formation of graphite flakesand ferrite. Phosphorus in small amountsincreases the fluidity of grey iron. It alsoforms a ternary phosphorus eutectic called“steadit”, which constitutes a web likestructure increasing the wear resistance.In the flake form, graphite providesnotches within the metallic matrix andconsequently lowers the tensile strength,
especially when the flakes are very large.In unalloyed grey iron the best mechanicalproperties can be achieved with fine and
Fig. 3: Grey iron with fine flake graphite, 100x
unetched
evenly dispersed graphite flakes in a pearl-itic matrix (see Figs. 3 and 4).Grey iron has a high damping capacity,excellent sliding properties and thermalconductivity, which makes it suitable formachine bases, damping plates for pianos,engine blocks, flywheels, piston rings,brake discs and drums.
Ductile iron with spheroidal graphite(SG), also called nodular or spheroidaliron, is made from the same raw materialas grey iron but requires higher purity.The melt should be free of Pb, As, Sb, Ti,and Al and have very little phosphorusand sulphur. By adding trace amounts ofmagnesium to the melt before casting, thegraphite forms in a spherical shape insteadof flakes.Ductile iron has greater strength and duc-tility than grey iron of similar composition.Ductile iron has good machining qualitiesand is used for heavy duty gears, pistons,rolls for rolling mills, gear cases (Fig.10),valves, tubes and door hinges. Pearliticductile iron is the initial material for cam-and crankshafts which are surface hard-ened for wear resistance (Fig. 8).
Austempered ductile iron (ADI) is a ductileiron austenitized at 840-950°C and thenquenched to 250-400°C where it is held un-til the matrix is changed to ausferrite. This isa mixture of needle-like ferrite and a carbonsaturated retained austenite, which gives theADI iron a high strength and ductility. Themicrostructure looks like bainite but has nocarbides.High-strength ADI irons are mainly usedfor wear resistant parts for heavy trucks,farm and earth moving equipment. Applica-tions of ductile ADI irons are for parts withdynamic stress such as axle journals, geardrives, crankshafts, pull hooks and wheelhubs.
For making Compacted graphite iron (CG)the same raw material is used as for mak-ing ductile iron. By carefully controlling theamount of magnesium added to the meltfor nodulizing approx. 80% of graphite isformed as compacted graphite, the rest asnodules.The quality control of compacted iron isvery important as the formation of graphiteis critical. A slightly higher percentage ofnodules can be tolerated, but the formationof flakes has to be avoided as they wouldlower or even eliminate the beneficial prop-erties of the compacted iron.Compacted graphite iron has betterstrength, ductility, alternating stress fatiguestrength and higher resistance to oxidationthan grey iron; and it is better to cast, easierto machine, has better damping qualities
Fig. 4: Grey iron with flake graphite in 200x
pearlitic matrix
Fig. 6: Exhaust manifold,
compacted graphite iron
Fig. 5: Filter head of ADI cast iron for the
hydraulic system of a pressure die casting
machine for plastics
Fig.7: Part of a wheel cassette of austempered
ductile iron
8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
and thermal conductivity and retains theshape better under temperature changesthan ductile iron.Applications: cylinder heads for high turn-ing diesel motors, axle- and gear cases,exhaust manifolds (Fig. 6), housings ofturbo chargers.
White cast iron contains 1.8-3.6% carbon,0.5-1.9% silicon and 1-2% manganese.A fast cooling rate prevents the precipita-tion of carbon as graphite. Instead the car-bon, which is in solution in the melt, formsiron carbide (Fe3C, also called cementite).The structure of white cast iron consists ofpearlite and ledeburite (Fig. 9), a eutecticof pearlite, converted from austenite, andcementite. Ni-hard alloys (8-9% Cr, 5-6%Ni) have a martensitic matrix with chro-mium carbides.
White cast iron has a high compressivestrength and alloyed versions have a
Fig. 9: White cast iron, pearlite with ledeburite 200x
good retention of strength and hardnessat elevated temperatures. Due to its largemasses of carbides, especially when al-loyed, white cast iron has an excellentresistance against wear and abrasion. It isused for shot-blasting nozzles, rolling millrolls, crushers, pulverizers and ball millliners.
By chilling grey or ductile iron on the out-side and letting it cool slowly inside, it ispossible to produce parts with a hard sur-face of white cast iron with a ductile core(chilled cast).
Malleable iron with temperedgraphite (TG)Malleable iron is made by heat treatingwhite cast iron. Through a two stage, longtime heat treatment (tempering) white castiron is converted to ferritic or pearlitic mal-
leable iron. The carbon of the iron carbidefirst goes into solution, and through slowcooling then precipitates in irregular nod-ules called temper carbon. Pearlitic malle-able iron can be hardened.Increasingly malleable iron is replacedby nodular iron for economical reasons,especially since the fields of application arevery similar.
Austenitic cast ironCast irons with at least 20% nickel and 1-5.5% chromium have an austenitic matrix
with graphite in form of flakes or nodules.Austenitic cast iron can be an economicalternative to stainless steel as it is easier
Ferritic malleable iron 200x
to cast and therefore suitable for precisioncasting of complicated shaped parts with anarrow wall thickness.The main properties of austenitic cast ironsare: corrosion resistance against sea waterand alkaline media and high strength andscale resistance at high temperatures.They are used specifically for applications
in the maritime environment, for instancefor large pump housings and other parts ofdesalination plants, or bushings and liningsin chemical plants, compressors for ag-gressive gases, housings for gas turbinesand turbo chargers.
Fig. 8:
Crankshaft, ductile iron
Fig.10: Differential housing of ductile iron
Austenitic cast iron, etched with 3% Nital 200x
+ modified Beraha’s reagent
8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
Alloyed white cast irons are very hard (HV600) and can be difficult to cut, especiallylarge sections. It is important to point out,that despite this hardness diamond cut-offwheels are not suitable for cutting whitecast iron.The main problem when preparing sam-ples of cast iron is to retain the graphitein its original shape and size. Although inthe microscope the image of the graphiteis viewed as 2-dimensional, it should beremembered that it is actually 3-dimen-sional. This means that during grinding andpolishing the appearance of graphite canslightly change, and that a certain percent-age of graphite is cut very shallow withonly a weak hold in the matrix. Thereforethere is always a possibility that the graph-ite can not be completely retained. Espe-cially very large flakes or agglomerationsof flakes have the tendency to loose thegraphite. Therefore graphite nodules cannot always be retained or polished well.In malleable cast irons graphite exists inthe form of rosettes or temper carbon.This is a friable form of graphite and canbe particularly difficult to retain during thepreparation.
A common preparation error is the insuffi-cient removal of smeared matrix metal aftergrinding, which can obscure the true shapeand size of graphite (compare Figs.11 and12). This is particularly prevalent in ferriticor austenitic cast irons that are prone todeformation and scratching. For these ma-terials a thorough diamond and final polishis especially important.
The difficulties associated with the prepa-ration of cast irons with graphite can becompounded in situations where metallog-raphy is an integral part of the casting linequality system.
Fig.13: Sample holder for semi-automatic polishing of
quality control samples in cast line
Fig.15: Ductile iron electrolytical polished and etched
shows the pearlitic matrix and ferrite surrounding
graphite. Graphite is washed away
SEM image of grey iron with flake graphite
Difficulties in thepreparation ofcast iron
SEM image of ductile iron with graphite nodules
Fig.14: Well polished graphite flakes 500x
Time constraints often make it difficult tomaintain consistent preparation resultsusing manual methods and often, due tothe geometry of the test piece, automaticpreparation is not a suitable alternative.However, as the design of the test piecesis usually arbitrary, their dimension andform can be changed in order to fit into anautomatic system (Fig.13). This has beensuccessfully carried out by some manu-facturers who where then able to make thepreparation more efficient and improve theevaluation of the graphite.Most of the standard microscopic checksof cast irons are done with a magnificationof 100x, which makes the graphite appearblack. Only with higher magnifications canit be verified if the carbon is completelyretained. Well polished graphite is grey(Fig.14).
Note: cast irons with graphite are notsuitable for electrolytic polishing as thegraphite is washed away by the electrolyte.However, if only a quick identification ofthe microstructure of the matrix is requiredelectrolytic polishing and etching can beused (Fig.15).
Fig.11: Insufficient polish leaves graphite 200x
nodules covered with smeared metal
Fig.12: Correct polish shows shape and size of 200x
graphite nodules suitable for evaluation
8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
Alternatively DiaPro diamond suspension can be replaced by
DP-Suspension, P, 9 µm, 3 µm and
1 µm respectively, applied with blue
lubricant.
Table 1: Preparation method
for white cast irons
Step PG FG
Surface SiC-paper 220# MD-Largo*
Suspension DiaProAllegro/Largo
Force [N] 180 180
Time Until plane 5 min.
Grinding
Lubricant Water
rpm 300 150
Step DP 1 DP 2 OP**
Surface MD-Dac MD-Nap OP-Chem
Suspension DiaPro Dac DiaPro Nap B OP-U
Force [N] 180 120 60
Time 4 min. 1-2 min. 1 min.
Polishing
rpm 150 150 150
*In cases where retention of graphite is very difficult,MD-Plan cloth can be tried for fine grinding.
**This step is optional
Alternatively DiaPro diamond suspension can be
replaced by DP-Suspension P, 9 µm, 3 µm and 1 µm
respectively, applied with blue lubricant
Table 2: Preparation method
for cast irons with graphite
Fig.16: Grey iron prepared with fine grinding on silicon
carbide paper, still shows scratches
Fig.17: Same as Fig.16, prepared with fine grinding with
diamond on MD-Largo, showing good edge retention
Grinding and polishing: Traditionally cast irons with graphite havebeen ground with silicon carbide paper.In recent years diamond grinding hasreplaced silicon carbide for fine grindingmost cast irons as it keeps the samplesvery flat and doesn’t leave the graphite inrelief (compare Fig.16 and 17).Hard white cast irons and ADI irons canbe plane ground with diamond (MD-Piano220) and also fine ground with diamond(MD-Allegro, see table 1). Soft and medi-um hard cast irons with a ferritic, austeniticor pearlitic matrix are plane ground withsilicon carbide paper and fine ground withdiamond on MD-Largo, see table 2).For cast irons that tend to corrode duringpolishing it is recommended to use waterfree diamond suspension, A, and yellow
lubricant. The preparation data are for 6samples, 30 mm, mounted and clampedinto a specimen holder.
Recommendations for thepreparation of cast iron
Cutting: For sectioning hard, white castirons a cubic boron nitride wheel is recom-
mended. For large sections auto-
matic cutting is more efficient thanmanual cutting.For cutting cast irons with graph-ite it is recommended to select
an aluminium oxide wheel according to thehardness of the cast iron to be cut.
Mounting: Quality control samples areusually prepared unmounted. For failureanalyses samples it is recommended touse hot compression mounting. For softto medium hard cast irons a phenolic resin(MulitFast) is recommended, for harder
types of cast irons a reinforced resin (Iso-Fast, DuroFast) is more suitable.
8/8/2019 Metal Lo Graphic Application Notes Cast Iron English
Elisabeth Weidmann, Anne Guesnier,Struers A/S, Copenhagen, Denmark
AcknowledgementsWe wish to thank CLAAS GUSS GmbH, Bielefeld,Germany, for supplying sample material and givingpermission for the reproduction of the foundry photo onpage 1 and Figs. 5 and 7. Our special thanks go to Dr.Christine Bartels for her generous support and also toUte Böhm.
We thank GF Eisenguss GmbH, Herzogenburg, Austria,for the permission to reproduce Figs. 6, 8 and 10.
We thank Zentrale für Gussverwendung, Düsseldorf,for the permission to reproduce the two SEM photoson page 4.
Bibliography
Literature from Zentrale für Gussverwendung,Düsseldorf
Schumann, VEB Deutscher Verlag fürGrundstoffindustrie, Leipzig, 1968
Werkstoffkunde und Werkstoffprüfung, W. Domke,Verlag W. Giradet, Essen, 1977
05.06 / 62140306. Printed in Denmark by Richard Larsen Grafisk - 42
Cleaning: As many cast irons tend to cor-rode easily the cleaning of samples has tobe fast and should always be carried outwith cold water. Under no circumstancesshould the samples be left in contact withwater. Thorough rinsing with ethanol andfast drying with a strong stream of warmair is recommended. If corrosion still oc-curs cleaning and rinsing with water freealcohol only is recommended.
Etching: Initially, the cast iron samples
are microscopically examined unetchedto evaluate shape, size and distribution ofgraphite and possible cast porosity. Afterthis initial evaluation the sample is etchedfor microstructure with 1 - 3% Nital.The following Beraha reagent can be usedfor colour etching and can be modifiedaccording to the alloy:
100 ml water200 ml hydrochloric acid24 g ammonium difluorideTo 100 ml of this stock solution add 1 gpotassium metabisulfite.Note: When working with chemicals the standard safety precautions have to be
observed!
SummaryCast irons are ferrous alloys with mostly2.5%-4% carbon and 1-3% silicon. Thecarbon is either present as graphite in greyirons or in form of iron carbide and alloycarbides in white cast iron. The difficulty inthe metallographic preparation is to retainthe true shape and size of the graphite inits flake, nodular or tempered form. Dur-
ing grinding the matrix is smeared overthe graphite and unless it is followed by avery thorough diamond polish, the graphiteis not shown in its true form. Especiallycast irons with a soft ferritic matrix tend tosmear and are prone to deformation andscratching. Plane grinding with silicon car-bide paper is recommended, followed byfine grinding and polishing with diamond.A brief final polish with colloidal silica isoptional.
White cast irons are very hard and a cubic
boron nitride cut-off wheel is recommend-ed for sectioning.
Austempered ductile iron, etched with 200x
3% Nital, pol. light
Note: do not use diamond cut-off wheels!Plane grinding, fine grinding and polishingare carried out with diamond.
Integrated into online casting, semi-auto-matic preparation equipment can achievebetter results for a reliable and reprodu-cible graphite evaluation than manualpreparation.
USA and CANADAStruers Inc.24766 Detroit RoadWestlake, OH 44145-1598Phone +1 440 871 0071Fax +1 440 871 [email protected]
