tions to tool steels, avoiding the risk of overtempering [4].

Through-hardened steels may be relativelyshallow hardened or deep hardened, depend-ing on their chemical composition and methodof hardening. Through-hardened steelsinclude plain carbon and alloy steels with acarbon content ranging from 0.30 to approxi-mately 0.55%.

As a result of their higher carbon content,through-hardened steel gears possess agreater core strength than carburized gears.However, they exhibit lower toughness andductility and moderate fatigue and wear resis-tance compared with the case-hardenedgears, which demonstrate improved surfaceproperties due to the presence of hard com-pound layers and to the development of sur-face compressive stresses.

In general, the more highly alloyed through-hardened steels harden more completelywhen quenched in heavy sections. This greaterhardenability provides higher strength thancan be attained with shallow-hardening steelsquenched in the same size section.

M AT E R I A L S A N D M E T H O D SMaterialsThree critical gear parts from bucket wheelexcavators made by three different manufac-turers were disconnected and submitted for

B I O G R A P H YAthanasios Vazdirvanidisis a mining and metallur-gical engineer from theNational Technical Uni-versity of Athens. Hereceived his MSc fromthe Civil EngineeringDepartment of NTUA in environmentalengineering in 2001. He is now a seniorresearch project leader at the ELKEME R&DCentre of the Viohalco Group of Companies.His main research interests are microstruc-tural characterization and failure analysis.

A B S T R A C THardened steels are used extensively in thegear boxes and bearings of bucket-wheelexcavators in the mining industry. Themicrostructural characteristics and the sur-face and mechanical properties of the mate-rials selected for these parts are critical fac-tors in their reliability under the highlydemanding conditions of use. Appropriateselection of materials will prevent down-time periods caused by unexpected or pre-mature part failures and time-consumingrepairs and replacements. In this study, gearparts of bucket wheel excavators from threedifferent manufacturers were examinedusing light and scanning electron micro-scopy and tested for hardness to evaluatetheir microstructure, microhardness andquality. These metallographical techniqueswere also used to determine the principalstages in manufacturing, to enable futureimprovement in gear lifetimes, and to rec-ommend optimum fabrication procedures.

K E Y W O R D Slight microscopy, scanning electron micro-scopy, microstructure, gear, hardened steel,martensite, tempered bainite, lamellarstructure, dendritic solidification

A C K N O W L E D G E M E N T SThe authors thank Andreas Rikos and Anag-nostis Toulfatzis for their valuable contributionto the completion of the experimental work.

A U T H O R D E TA I L SAthanasios Vazdirvanidis, ELKEME Hellenic Research Centre for MetalsS.A., 252 Pireaus Str., 17778 Athens, GreeceTel: +30 69 74 40 22 36 Email: avazdirvanidis@elkeme.vionet.gr

Microscopy and Analysis 24(1):5-10 (EU),2010

GE A R MI C R O S T R U C T U R E

I N T R O D U C T I O NGear steels can be categorized, as far as theirheat treatment is concerned, as either surface(case)-hardened steels, which provide maxi-mum surface hardness and wear resistance, orthrough-hardened steels, possessing greatercore strength but with less resistance to sur-face compressive stresses and wear than case-hardened gears [1].

The surface-hardened steels are hardened toa relatively thin case depth and include car-burizing and nitriding steels. Surface-hard-ened steels are plain carbon and alloy steelswith a carbon content generally not exceeding0.25%. A case-hardened gear provides maxi-mum surface hardness and wear resistanceand at the same time provides core toughnessfor shock absorbance. In general, case-hard-ened gears can withstand higher loads thanthrough-hardened gears but the latter are lessexpensive because of the simpler heat treat-ment required. Furthermore, surface thermo-chemical treatments performed at lower tem-peratures (such as nitriding and nitrocarburiz-ing) provide near- or net-shape manufactur-ing of the tool steel component, assuring min-imum distortion, dimensional stability andimproved tribological properties [2,3]. Plasmaor ion nitriding in particular are very energy-efficient techniques that work at lower tem-peratures (<350oC) and have many applica-

LM and SEM Metallographical Study ofGear Teeth in Bucket-Wheel Excavators Athanasios Vazdirvanidis,1 George Pantazopoulos 1 and Konstantinos Stamatakis 2

1. ELKEME Hellenic Research Centre for Metals, Athens, Greece. 2. Public Power Corporation, Athens, Greece.

Figure 1: Stereomicroscope image of gear tooth sample 1 (etched).

MICROSCOPY AND ANALYSIS JANUARY 2010 5

material characterization and identification.The chemical composition of the samples wasdetermined with optical emission spectrome-try (OES) and is presented in Table 1.

Specimen PreparationMicrostructural and morphological character-ization was performed on mounted cross-sec-tions. Grinding was done with successive abra-sive SiC papers and fine polishing using succes-sive diamond and silica suspensions. To finish,the samples were rinsed in alcohol and dried inhot air. To reveal the microstructure, immer-sion etching was performed using Nital 2%chemical reagent (solution of 2% v/v HNO3 inethanol).

Light and Electron MicroscopyMetallographical studies were conductedusing a Nikon Epiphot 300 inverted reflectedlight optical microscope using 103 NA 0.3,203 NA 0.46, 503 NA 0.8 and 1003 NA 0.9objectives. Images were taken with a NikonDigital Net DN100 camera with 1.3 megapixelresolution and then processed using Image-Pro Plus image analysis software.

Higher resolution microstructural observa-tions were made using an FEI XL40 Schottkyfield-emission gun scanning electron micro-scope at 20 kV, equipped with secondary andbackscattered electron detectors and an EDAXenergy-dispersive X-ray spectroscopy (EDS) sys-tem for elemental analysis.

Microhardness TestingHardness testing was performed using theVickers microindentation technique under 0.5kg force (4.905 N) applied load (HV0.5) using anInstron-Wolpert Tukon 2100 indentationdevice, according to EN ISO 6507-1 standard.

R E S U LT S A N D D I S C U S S I O NGear Tooth Sample 1 Gear tooth sample 1 had a characteristic den-dritic solidification morphology strongly sug-gesting that it was a cast component (Figure1). Its microstructure consists of temperedupper bainite, a moderately brittle non-equi-librium constituent resulting from a relativelyhigh cooling rate. Bainitic sheaves are nucle-ated at the prior austenite grain boundaries(Figure 2a). Carbides have sufficient time toprecipitate out and form layers between thesheaves. SEM high magnification imagesclearly depicted the bainitic ferrite surroundedby iron carbide particles (Figure 2b). The micro-hardness of the matrix was approximately 280HV0.5 (Figure 3). No case-hardened layer wasdetected. The tempered structure can exhibitslightly lower hardness relative to the non-tempered one but with higher toughness.

Gear Tooth Sample 2 Gear tooth sample 2 (Figure 4) was probablyproduced by thermomechanical processing(hot rolling/forging). Its microstructure con-sisted of a ~1500 µm thick surface-hardenedlayer possessing a maximum microhardness ofup to 740 HV0.5.

The hardened case was composed of variousconstituents:

Figure 5: (a) Light microscope and (b) scanning electron microscope images of the martensitic structure on the surface of gear tooth sample 2 after carburizingheat treatment (etched).

MICROSCOPY AND ANALYSIS JANUARY 20106

Figure 4: Stereomicroscope images of sample 2 showing (a) gear-tooth cross-section and (b) case-hardened layer (etched).

Figure 3: Microhardness profile of gear tooth sample 1.

Figure 2: (a) Light microscope and (b) scanning electron microscope images of the tempered bainite microstructure of gear tooth sample 1. The white-streaksrepresent iron carbide precipitates (cementite).

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MICROSCOPY AND ANALYSIS JANUARY 20108

1. A high-carbon martensite layer of ~300µm thickness (Figure 5). This needle-formstructure resulted from quenching of the car-bon enriched surface layer during the carbur-izing heat treatment. A considerable amountof retained austenite was also anticipated(manifested by the white matrix in Figure 5a).

2. An intermediate zone of ~1200 µm thick-ness (Figure 6) found between the hardmartensite layer and the core. This zone con-sisted of lower bainite and retained austenite.

The core of sample 2 consisted of an upperbainitic structure (Figures 7 & 8) possessing ahardness of up to ~350 HV0.5 (Figure 9).

The microstructural and microhardness vari-ations could be explained by taking into con-sideration the decrease in cooling rate withincreasing distance from the surface and, onthe other hand, by the carbon concentrationgradient established from diffusion processes.

Gear Tooth Sample 3 Sample 3 had a characteristic dendritic/cellularsolidification morphology indicative of a castcomponent (Figure 10). Its microstructure con-sisted of uniform equiaxed ferrite and lamellarpearlite grains with a high pearlite content(Figure 11). The hardness of sample 3 core wasca. 196 HV0.5 (Figure 12). In this sample therewas no case-hardened layer although therewas a hardness gradient (~350–196 HV0.5) fromthe surface to the core due probably to workhardening that occurred mainly on the surfaceduring manufacturing and/or gear operation(Figure 10). In contrast to samples 1 and 2,sample 3 consisted of an annealed normalizedstructure, and there was no evidence of priorhardening or surface treatment.

C O N C L U S I O N SThe findings of the metallurgical investigationof the three gear samples showed that:

1. The received gear parts (gear teeth) 1, 2and 3 approximately matched with three dif-ferent steel grades with the following respec-tive order:

• 43CrMo4 – Werkstoff Nummer (WNr)1.3563 or alternatively 42CrMo4 – WNr 1.7225,structural and constructional steel;

• 18CrNiMo7-6/17CrNiMo6 – WNr 1.6587 oralternatively 15CrNi6 – W.Nr. 1.5919, casehardening steel; and

• C35E/Ck35 – WNr 1.1181 structural andconstructional steel.

2. Gear tooth sample 1 microstructure con-sisted of tempered bainite and had no case-hardened layer while its microhardnessreached up to ~280 HV0.5. Gear tooth sample 1had a characteristic dendritic solidificationmorphology which constituted a typical castcomponent structure.

3. Gear tooth sample 2 had a ~1500 µm thicksurface-hardened case consisting of a needle-form martensitic layer followed by a lower bai-nite intermediate zone. The maximum micro-hardness reached approx. 740 HV0.5. The sam-ple core consisted of an upper bainite struc-ture with an average hardness of ~350 HV0.5.Gear tooth sample 2 had probably been pro-duced by thermomechanical processing (hotrolling/forging) followed by case hardening.

Figure 6: (a,b) Light microscope images of the lower bainitic structure between the martensitic surface and the upper bainitic core of gear tooth sample 2.

Figure 7: (a,b) Light microscope images of upper bainite microstructure of gear tooth sample 2 (core structure).

Figure 8: (a,b) Electron micrographs of upper bainite microstructure of gear tooth sample 2 (core structure). Note the cementite precipitates (white particles) dis-persed uniformly in the upper bainitic ferrite sheaves.

Figure 9: Microhardness profile of gear tooth sample 2.

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MICROSCOPY AND ANALYSIS JANUARY 201010

Table 1: Chemical composition (% wt.) of gear samples (an approximate equivalence is also given according to the relevant standard) [5,6].

Sample 1: Fits approximately to 43CrMo4 -W.Nr. 1.3563 or alternatively to 42CrMo4 -W.Nr. 1.7225, structural and constructional steel.

Figure 11 (below): (a) Light microscope and (b) scanning electron microscope images of the ferritic-pearliticmicrostructure of gear tooth sample 3. Note the fine perlite lamellæ.

Figure 12: Microhardness profile of gear tooth sample 3.

C Mn Si P S Cr Ni Cu Mo As Al

0.4609 0.4609 0.4394 0.0122 0.0149 1.0019 0.1342 0.1391 0.2316 0.0230 0.0598

Co V Ti Nb Sn Pb B N Sb Ca W

0.0163 0.0067 0.0365 0.0027 0.0136 <0.0001 <0.0001 0.0172 0.0037 0.0007 0.0030

C Mn Si P S Cr Ni Cu Mo As Al

0.1499 0.5139 0.2590 0.0098 0.0066 1.6893 1.4797 0.1297 0.2655 0.0107 0.0256

Co V Ti Nb Sn Pb B N Sb Ca W

0.0197 0.0098 0.0043 0.0023 0.0067 <0.0001 <0.0001 0.0118 <0.0010 0.0007 0.0029

C Mn Si P S Cr Ni Cu Mo As Al

0.3808 1.1878 0.4718 0.0120 0.0096 0.6943 0.0743 0.1685 0.0348 0.0230 0.0246

Co V Ti Nb Sn Pb B N Sb Ca W

0.0209 0.0872 0.0024 0.0026 0.0199 <0.0001 <0.0001 0.0167 0.0063 0.0009 0.0024

Sample 2: Fits approximately to 18CrNiMo7-6/17CrNiMo6 -W.Nr. 1.6587 or alternatively to 15CrNi6- W.Nr. 1.5919 case hardening steels.

Sample 3: Fits approximately to C35E/Ck35 – W.Nr. 1.1181, structural and constructional steel.

Figure 10 (left): Stereomicroscope image of sample 3 showing gear-tooth cross-section (etched).

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4. Gear tooth sample 3, which did not showa case-hardened layer, exhibited an annealedferritic-pearlitic microstructure, possessinghigher hardness near the surface due to sur-face plastic shear that took place due to fabri-cation and/or working conditions. The hard-ness varied from ~350 HV0.5 on the surface,reaching ~196 HV0.5 on the core. Gear toothsample 3 demonstrated a characteristic den-dritic-cellular solidification morphology indi-cating that it was a cast component.

R E F E R E N C E S 1. ASM specialty handbook: Carbon and alloy steels. ASM

International, OH, 1996.2. Pantazopoulos, G., Psyllaki, P., Kanakis, D., Antoniou, S.,

Papadimitriou, K., Sideris, J. Tribological properties of aliquid nitrocarburized special purpose cold-work tool steel.Surface & Coatings Technology 200:5889-5895, 2006.

3. Psyllaki, P., Kefalonikas, G., Pantazopoulos, G., Sideris, J.,Antoniou, S. Microstructure and tribological behaviour ofliquid nitrocarburized tool steels. Surface & CoatingsTechnology 162:67-78, 2002.

4. Hutchings, I. M. Tribology: Friction and wear of engineeringmaterials, 1st Edn, Edward Arnold, 1992.

5. Stahlschlüssel (Key to Steel), Verlag Stahlschlüssel WegstGmbH, D-71672 Marbach 2004.

6. MatWeb Online Materials Information Resource -www.matweb.com

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