Metallographic Preparation of Cast Iron Application Notes

Home , Ductile iron, Ledeburite, Malleable iron

Metallographic preparation of Cast iron Application Notes Iron is one of the most diverse metals 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 in the automotive and engineering in- train stations, market halls, and green dustry. In addition, specific cast irons houses of botanical gardens, bridges are the material of choice for sea water and the Eiffel Tower still document the pump housings, rolling mill rolls and substantial application of cast iron dur- parts for earth moving equipment. ing that time. As the morphology of graphite has a major influence on the mechanical The term cast iron refers to those iron- properties of cast iron, metallographic carbon-silicon alloys which contain quality control of grey iron is an in-

2.5% - 4% carbon and usually 1-3% tegral part of its production process. Austempered ductile silicon. Cast iron is an important engi- Using standard reference comparison iron, Beraha color etch, neering material with a number of ad- charts and/or image analysis tech- DIC, 500x vantages, mainly good castability and niques, the morphology, size and dis- machinability and moderate mechani- tribution of the graphite is determined cal properties. on an unetched, polished sample. Depending on the specification, the Because of its economical advantages sample is then etched to check the cast iron is used for many applications structure of the matrix.

Difficulties during metallographic preparation Cutting: White cast iron is very hard and size can be difficult. The matrix of and therefore difficult to cut. ferritic and/or austenitic cast irons is Grinding and polishing: Graphite is prone to deformation and scratching. soft and retaining it in its true shape Solution - Cubic boron nitride cut-off wheel - Thorough diamond polishing on hard polishing cloths and final oxide polishing.

Fig.1: Grey iron with flake graphite, 200x Fig. 2: Same as Fig.1, showing correct polish 200x insufficient polish

28.02.2019 R05 / 62140306 Fig. 6: Exhaust manifold, Production and compacted graphite iron application of cast irons

Production Austempered ductile iron (ADI) is a ductile Cast irons are melted in a cupola- or induc- iron austenitized at 840-950°C and then tion furnace charged generally with pig quenched to 250-400°C where it is held un- iron, cast iron scrap, steel scrap and vari- til the matrix is changed to ausferrite. This is ous additions. The alloy composition and a mixture of needle-like ferrite and a carbon the cooling rate will influence whether the saturated retained austenite, which gives the iron will solidify grey or white. ADI iron a high strength and ductility. The A fast cooling rate results in a white solidi- microstructure looks like bainite but has no fication and the formation of iron carbide carbides.

(Fe3C or cementite). At the eutectoid trans- High-strength ADI irons are mainly used formation a fast cooling rate promotes the Fig. 4: Grey iron with flake graphite in 200x for wear resistant parts for heavy trucks, formation of pearlite, whereas a slow cool- pearlitic matrix farm and earth moving equipment. Applica- ing rate promotes the formation of graphite tions of ductile ADI irons are for parts with and ferrite. evenly dispersed graphite flakes in a pearl- dynamic stress such as axle journals, gear The microstructure of grey cast irons can itic matrix (see Figs. 3 and 4). drives, crankshafts, pull hooks and wheel have either a pearlitic and/or ferritic matrix Grey iron has a high damping capacity, hubs. with free graphite in the shape of flakes, excellent sliding properties and thermal nodules or temper carbon respectively. conductivity, which makes it suitable for For making Compacted graphite iron (CG) Through alloying and heat treatment the machine bases, damping plates for pianos, the same raw material is used as for mak- properties of cast iron can be adjusted for engine blocks, flywheels, piston rings, ing ductile iron. By carefully controlling the certain applications, for instance, alloying brake discs and drums. amount of magnesium added to the melt with molybdenum and nickel improves for nodulizing approx. 80% of graphite is their heat and corrosion resistance. Ductile iron with spheroidal graphite formed as compacted graphite, the rest as In the following the individual cast irons (SG), also called nodular or spheroidal nodules. will be briefly described and their major iron, is made from the same raw material The quality control of compacted iron is fields of application mentioned. as grey iron but requires higher purity. very important as the formation of graphite The melt should be free of Pb, As, Sb, Ti, is critical. A slightly higher percentage of and Al and have very little phosphorus nodules can be tolerated, but the formation and sulphur. By adding trace amounts of of flakes has to be avoided as they would magnesium to the melt before casting, the lower or even eliminate the beneficial prop- graphite forms in a spherical shape instead erties of the compacted iron. of flakes. Compacted graphite iron has better Ductile iron has greater strength and duc- strength, ductility, alternating stress fatigue tility than grey iron of similar composition. strength and higher resistance to oxidation Ductile iron has good machining qualities than grey iron; and it is better to cast, easier and is used for heavy duty gears, pistons, to machine, has better damping qualities rolls for rolling mills, gear cases (Fig.10), Fig. 3: Grey iron with fine flake graphite, 100x unetched valves, tubes and door hinges. Pearlitic ductile iron is the initial material for cam- Grey iron with flake graphite (FG) has and crankshafts which are surface hard- between 2.5-4% carbon, 1-3% silicon and ened for wear resistance (Fig. 8). 0.2-1% manganese. Carbon and silicon promote the formation of graphite flakes and ferrite. Phosphorus in small amounts increases the fluidity of grey iron. It also forms a ternary phosphorus eutectic called “steadit”, which constitutes a web like structure increasing the wear resistance. In the flake form, graphite provides notches within the metallic matrix and consequently lowers the tensile strength, especially when the flakes are very large. In unalloyed grey iron the best mechanical properties can be achieved with fine and Fig. 5: Filter head of ADI cast iron for the Fig.7: Part of a wheel cassette of austempered hydraulic system of a pressure die casting ductile iron machine for plastics Fig. 8: Crankshaft, ductile iron

Fig. 9: White cast iron, pearlite with ledeburite 200x Austenitic cast iron, etched with 3% Nital 200x + modified Beraha’s reagent and thermal conductivity and retains the good retention of strength and hardness is easier to cast and therefore suitable for shape better under temperature changes at elevated temperatures. Due to its large precision casting of complicated shaped than ductile iron. masses of carbides, especially when al- parts with a narrow wall thickness. Applications: cylinder heads for high turn- loyed, white cast iron has an excellent The main properties of austenitic cast irons ing diesel motors, axle- and gear cases, resistance against wear and abrasion. It is are: corrosion resistance against sea water exhaust manifolds (Fig. 6), housings of used for shot-blasting nozzles, rolling mill and alkaline media and high strength and turbo chargers. rolls, crushers, pulverizers and ball mill scale resistance at high temperatures. liners. They are used specifically for applications White cast iron contains 1.8-3.6% carbon, in the maritime environment, for instance 0.5-1.9% silicon and 1-2% manganese. By chilling grey or ductile iron on the out- for large pump housings and other parts of A fast cooling rate prevents the precipita- side and letting it cool slowly inside, it is desalination plants, or bushings and linings tion of carbon as graphite. Instead the car- possible to produce parts with a hard sur- in chemical plants, compressors for ag- bon, which is in solution in the melt, forms face of white cast iron with a ductile core gressive gases, housings for gas turbines iron carbide (Fe3C, also called cementite). (chilled cast). and turbo chargers. The structure of white cast iron consists of pearlite and ledeburite (Fig. 9), a eutectic Malleable iron with tempered of pearlite, converted from austenite, and graphite (TG) cementite. Ni-hard alloys (8-9% Cr, 5-6% Malleable iron is made by heat treating Ni) have a martensitic matrix with chro- white cast iron. Through a two stage, long mium carbides. time heat treatment (tempering) white cast iron is converted to ferritic or pearlitic mal- White cast iron has a high compressive leable iron. The carbon of the iron carbide strength and alloyed versions have a first goes into solution, and through slow cooling then precipitates in irregular nodules called temper carbon. Pearlitic malleable iron can be hardened. Increasingly malleable iron is replaced by nodular iron for economical reasons, especially since the fields of application are very similar.

Austenitic cast iron Cast 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 economic alternative to stainless steel as it Ferritic malleable iron 200x Fig.10: Differential housing of ductile iron Difficulties in the preparation of cast iron

Alloyed white cast irons are very hard (HV Time constraints often make it difficult to 600) and can be difficult to cut, especially maintain consistent preparation results large sections. It is important to point out, using manual methods and often, due to that despite this hardness diamond cut-off the geometry of the test piece, automatic wheels are not suitable for cutting white preparation is not a suitable alternative. cast iron. However, as the design of the test pieces The main problem when preparing sam- is usually arbitrary, their dimension and ples of cast iron is to retain the graphite form can be changed in order to fit into an in its original shape and size. Although in automatic system (Fig.13). This has been the microscope the image of the graphite successfully carried out by some manu- Fig.11: Insufficient polish leaves graphite 200x is viewed as 2-dimensional, it should be nodules covered with smeared metal facturers who where then able to make the remembered that it is actually 3-dimen- preparation more efficient and improve the sional. This means that during grinding and evaluation of the graphite. polishing the appearance of graphite can Most of the standard microscopic checks slightly change, and that a certain percent- of cast irons are done with a magnification age of graphite is cut very shallow with of 100x, which makes the graphite appear only a weak hold in the matrix. Therefore black. Only with higher magnifications can there is always a possibility that the graph- it be verified if the carbon is completely ite can not be completely retained. Espe- retained. Well polished graphite is grey cially very large flakes or agglomerations (Fig.14). of flakes have the tendency to loose the graphite. Therefore graphite nodules can Fig.12: Correct polish shows shape and size of 200x Note: cast irons with graphite are not not always be retained or polished well. graphite nodules suitable for evaluation suitable for electrolytic polishing as the In malleable cast irons graphite exists in graphite is washed away by the electrolyte. the form of rosettes or temper carbon. A common preparation error is the insuffi- However, if only a quick identification of This is a friable form of graphite and can cient removal of smeared matrix metal after the microstructure of the matrix is required be particularly difficult to retain during the grinding, which can obscure the true shape electrolytic polishing and etching can be preparation. and size of graphite (compare Figs.11 and used (Fig.15). 12). This is particularly prevalent in ferritic or austenitic cast irons that are prone to deformation and scratching. For these ma- terials a thorough diamond and final polish is especially important.

The difficulties associated with the preparation of cast irons with graphite can be compounded in situations where metallography is an integral part of the casting line quality system.

SEM image of grey iron with flake graphite

Fig.14: Well polished graphite flakes 500x

SEM image of ductile iron with graphite nodules Fig.13: Sample holder for semi-automatic polishing of Fig.15: Ductile iron electrolytical polished and etched quality control samples in cast line shows the pearlitic matrix and ferrite surrounding graphite. Graphite is washed away Grinding Grinding

Step PG FG Step PG FG

Surface MD-Piano 220 MD-Allegro Surface Foil/Paper MD-Largo*

Type Diamond Diamond Type SiC Diamond Abrasive Abrasive Size #220 9 μm Size #220 9 μm

Suspension/ Water DiaPro Suspension/ Water DiaPro Lubricant Allegro/Largo 9 Lubricant Allegro/Largo 9

rpm 300 150 rpm 300 150

Force [N]/ 30 30 Force [N]/ 30 30 specimen specimen

Time (min) Until plane 5 Time (min) Until plane 5

Polishing Polishing

Step DP 1 DP 2 Step PG FG OP**

Surface MD-Dac MD-Nap Surface MD-Dac MD-Nap OP-Chem

Type Diamond Diamond Type Diamond Diamond Collodial Silica Abrasive Abrasive Size 3 µm 1 μm Size 3 µm 1 μm 0.04 μm

Suspension/ DiaPro DiaPro Suspension/ DiaPro DiaPro OP-U Lubricant Dac 3 Nap B 1 Lubricant Dac 3 Nap B 1 NonDry

rpm 150 150 rpm 150 150 150

Force [N]/ 40 30 Force [N]/ 30 20 10 specimen specimen

Time (min) 4 1-2 Time (min) 4 1-2 1

Table 1: Alternatively DiaPro diamond suspension can be Table 2: *In cases where retention of graphite is very difficult, MD-Plan cloth can be tried for Preparation method replaced by DP-Suspension, P, 9 µm, 3 µm and Preparation method fine grinding. for white cast irons 1 µm respectively, applied with blue lubricant. for cast irons with **This step is optional graphite Alternatively DiaPro diamond suspension can be replaced by DP-Suspension P, 9 µm, 3 µm and 1 µm respectively, applied with blue lubricant

Recommendations for the Grinding and polishing: preparation of cast iron Traditionally cast irons with graphite have been ground with silicon carbide Foil/Paper. Cutting: For sectioning hard, white cast In recent years diamond grinding has irons a cubic boron nitride wheel is recom- replaced silicon carbide for fine grinding mended. For large sections automatic cut- most cast irons as it keeps the samples ting is more efficient than very flat and doesn’t leave the graphite in manual cutting. For cutting relief (compare Fig.16 and 17). cast irons with graphite it is Hard white cast irons and ADI irons can recommended to select an be plane ground with diamond (MD-Piano aluminium oxide wheel 220) and also fine ground with diamond Fig.16: Grey iron prepared with fine grinding on silicon according to the hardness (MD-Allegro, see table 1). Soft and medi- carbide Foil/Paper, still shows scratches of the cast iron to be cut. um hard cast irons with a ferritic, austenitic or pearlitic matrix are plane ground with Mounting: Quality control samples are silicon carbide Foil/Paper and fine ground usually prepared unmounted. For failure with diamond on MD-Largo, see table 2). analyses samples it is recommended to For cast irons that tend to corrode dur- use hot compression mounting. For soft ing polishing it is recommended to use to medium hard cast irons a phenolic resin water free diamond suspension, A, and (MulitFast) is recommended, for harder yellow lubricant. The preparation data are types of cast irons a reinforced resin for 6 samples, 30 mm dia., mounted and (DuroFast) is more suitable. clamped into a specimen holder. Fig.17: Same as Fig.16, prepared with fine grinding with diamond on MD-Largo, showing good edge retention Struers ApS Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected] www.struers.com Cleaning: As many cast irons tend to cor- AUSTRALIA & NEW ZEALAND NETHERLANDS rode easily the cleaning of samples has to Struers Australia Struers GmbH Nederland 27 Mayneview Street Zomerdijk 34 A be fast and should always be carried out Milton QLD 4064 3143 CT Maassluis with cold water. Under no circumstances Australia Telefoon +31 (10) 599 7209 Phone +61 7 3512 9600 Fax +31 (10) 5997201 should the samples be left in contact with Fax +61 7 3369 8200 [email protected] [email protected] water. Thorough rinsing with ethanol and NORWAY BELGIUM (Wallonie) Struers ApS, Norge fast drying with a strong stream of warm Struers S.A.S. Sjøskogenveien 44C 370, rue du Marché Rollay 1407 Vinterbro air is recommended. If corrosion still oc- F- 94507 Champigny Telefon +47 970 94 285 sur Marne Cedex [email protected] curs cleaning and rinsing with water free Téléphone +33 1 5509 1430 alcohol only is recommended. Télécopie +33 1 5509 1449 AUSTRIA Austempered ductile iron, etched with 200x [email protected] Struers GmbH Zweigniederlassung Österreich 3% Nital, pol. light BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 Etching: Initially, the cast iron samples Struers GmbH Nederland 5412 Puch are microscopically examined unetched Note: do not use diamond cut-off wheels! Zomerdijk 34 A Telefon +43 6245 70567 3143 CT Maassluis Fax +43 6245 70567-78 to evaluate shape, size and distribution of Plane grinding, fine grinding and polishing Telefoon +31 (10) 599 7209 [email protected] Fax +31 (10) 5997201 graphite and possible cast porosity. After are carried out with diamond. [email protected] POLAND Struers Sp. z o.o. this initial evaluation the sample is etched CANADA Oddział w Polsce Struers Ltd. ul. Jasnogórska 44 for microstructure with 1 - 3% Nital. Integrated into online casting, semi-auto- 7275 West Credit Avenue 31-358 Kraków Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 The following Beraha reagent can be used matic preparation equipment can achieve Phone +1 905-814-8855 Fax +48 12 626 01 46 Fax +1 905-814-1440 [email protected] for colour etching and can be modified better results for a reliable and reprodu- [email protected] according to the alloy: cible graphite evaluation than manual ROMANIA CHINA Struers GmbH, Sucursala preparation. Struers Ltd. Bucuresti 1000 ml water No. 1696 Zhang Heng Road Str. Preciziei nr. 6R Zhang Jiang Hi-Tech Park 062203 sector 6, Bucuresti 200 ml hydrochloric acid Shanghai 201203, P.R. China Phone +40 (31) 101 9548 24 g ammonium difluoride Author Phone +86 (21) 6035 3900 Fax +40 (31) 101 9549 Fax +86 (21) 6035 3999 [email protected] To 100 ml of this stock solution add 1 g Elisabeth Weidmann, Anne Guesnier, [email protected] Struers A/S, Copenhagen, Denmark SWITZERLAND potassium metabisulfite. CZECH REPUBLIC & SLOVAKIA Struers GmbH Acknowledgements Struers GmbH Organizační složka Zweigniederlassung Schweiz Note: When working with chemicals the vědeckotechnický park Weissenbrunnenstraße 41 We wish to thank CLAAS GUSS GmbH, Bielefeld, Přílepská 1920, CH-8903 Birmensdorf standard safety precautions have to be CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 Germany, for supplying sample material and giving Phone +420 233 312 625 Fax +41 44 777 63 09 observed! permission for the reproduction of the foundry photo on Fax +420 233 312 640 [email protected] page 1 and Figs. 5 and 7. Our special thanks go to Dr. [email protected] Christine Bartels for her generous support and also to [email protected] SINGAPORE Summary Struers Singapore Cast irons are ferrous alloys with mostly Ute Böhm. GERMANY 627A Aljunied Road, We thank GF Eisenguss GmbH, Herzogenburg, Austria, Struers GmbH #07-08 BizTech Centre 2.5%-4% carbon and 1-3% silicon. The Carl-Friedrich-Benz-Straße 5 Singapore 389842 for the permission to reproduce Figs. 6, 8 and 10. D- 47877 Willich Phone +65 6299 2268 carbon is either present as graphite in grey Telefon +49 (0) 2154 486-0 Fax +65 6299 2661 We thank Zentrale für Gussverwendung, Düsseldorf, Fax +49 (0) 2154 486-222 [email protected] irons or in form of iron carbide and alloy for the permission to reproduce the two SEM photos [email protected] on page 4. SPAIN carbides in white cast iron. The difficulty in FRANCE Struers España Bibliography Struers S.A.S. Camino Cerro de los Gamos 1 the metallographic preparation is to retain 370, rue du Marché Rollay Building 1 - Pozuelo de Alarcón the true shape and size of the graphite in Literature from Zentrale für Gussverwendung, F-94507 Champigny CP 28224 Madrid Düsseldorf sur Marne Cedex Teléfono +34 917 901 204 its flake, nodular or tempered form. Dur- Téléphone +33 1 5509 1430 Fax +34 917 901 112 Vera Knoll, Gusseisen, 2003 Télécopie +33 1 5509 1449 [email protected] ing grinding the matrix is smeared over [email protected] Metals Handbook, Desk Edition, ASM, Metals Park, FINLAND the graphite and unless it is followed by a Ohio, 44073, 1997 HUNGARY Struers ApS, Suomi Struers GmbH Hietalahdenranta 13 very thorough diamond polish, the graphite ASM Handbook Vol. 9, Metallography and Magyarországi Fióktelepe 00180 Helsinki Microstructures, ASM, 2004 2040 Budaörs Puhelin +358 (0)207 919 430 is not shown in its true form. Especially Szabadság utca 117 Faksi +358 (0)207 919 431 cast irons with a soft ferritic matrix tend Schumann, VEB Deutscher Verlag für Phone +36 2380 6090 [email protected] Grundstoffindustrie, Leipzig, 1968 Fax +36 2380 6091 Email: [email protected] SWEDEN to smear and are prone to deformation Werkstoffkunde und Werkstoffprüfung, W. Domke, Struers Sverige and scratching. Plane grinding with silicon Verlag W. Giradet, Essen, 1977 IRELAND Box 20038 Struers Ltd. 161 02 Bromma carbide Foil/Paper is recommended, fol- Unit 11 Evolution@ AMP Telefon +46 (0)8 447 53 90 Whittle Way, Catcliffe Telefax +46 (0)8 447 53 99 lowed by fine grinding and polishing with Rotherham S60 5BL [email protected] Tel. +44 0845 604 6664 diamond. A brief final polish with colloidal Fax +44 0845 604 6651 UNITED KINGDOM [email protected] Struers Ltd. silica is optional. Unit 11 Evolution @ AMP ITALY Whittle Way, Catcliffe Struers Italia Rotherham S60 5BL White cast irons are very hard and a cubic Via Monte Grappa 80/4 Tel. +44 0845 604 6664 20020 Arese (MI) Fax +44 0845 604 6651 boron nitride cut-off wheel is recommend- Tel. +39-02/38236281 [email protected] Fax +39-02/38236274 ed for sectioning. [email protected] USA Struers Inc. JAPAN 24766 Detroit Road Marumoto Struers K.K Westlake, OH 44145-1598 Takanawa Muse Bldg. 1F Phone +1 440 871 0071 3-14-13 Higashi-Gotanda, Fax +1 440 871 8188 Shinagawa [email protected] Tokyo 141-0022 Japan Phone +81 3 5488 6207 Fax +81 3 5488 6237 [email protected] 28.02.2019 R05 / 62140306