STRUCTURE AND FATIGUE FAILURE ANALYSIS OF...

METAL 2006 23.-25.5.2006, Hradec nad Moravicí___________________________________________________________________________

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STRUCTURE AND FATIGUE FAILURE ANALYSIS

OF NITRIDED NODULAR CAST IRON

Radomila Konečnáa

Gianni Nicolettob

Viera Majerováa

a Department of Materials Engineering, University of Žilina, Univerzitná 1, 01026 ŽilinaSlovakia, [email protected]

b Department of Industrial Engineering, University of Parma, Viale G.P. Usberti, 181/A,43100 Parma, Italy, [email protected]

Abstract

This paper presents and discusses the influence of a nitriding treatment by the patentedNitreg Controlled Potential process on the case material structure, the fatigue response andthe fatigue fracture mechanisms of the ferritic nodular cast iron EN - GJS 400. The structuralanalysis of the nitrided layer was performed and microhardness was measured on thesespecimens at different distances starting from the surface down to the basic material. Thehardness profile characterizes the effectiveness of the nitriding treatment and is used to definean effective nitrided depth. A standard procedure for fatigue curve and fatigue limitdetermination at 50 % probability of survival using a limited number of specimens wasadopted. The nitriding treatment is demonstrated to give a very significant improvement ofthe fatigue response, confirming the range of improvement determined by previous tests onsteels. The high fatigue strength is not due exclusively to the formation of the hardenedsurface layer, because favorable compressive residual stresses are also produced in the surfacelayers by nitriding. Residual stresses are generated by the distortion of the crystalline latticedue to diffusion of carbon and nitrogen from the surface and a change in the specific volumefollowing a phase change.

1. INTRODUCTION

Nodular cast irons (NCI) are construction materials with a wide range of applications inengineering practice [1]. For their optimal application it is important to know their basicmechanical properties and the available methods to improve them. Since fatigue resistance issensitive to surface conditions, any phenomenon that changes the surface characteristics of amaterial will greatly affect its fatigue performance. Three main surface modification factorsare identified: i) control of surface roughness, ii) changes in the mechanical properties of themetal surface, and iii) changes in the residual stress condition near the surface.

Thermo chemical surface treatments, such as nitriding, are frequently applied to ferrousmaterials because they promote the formation of a hard and strong surface layer and of asystem of compressive residual stress that simultaneously improve the fatigue endurance andthe wear resistance, [1, 2]. Since the properties of the nitrided layers depend on the materialand its original pre-process hardness, the part must be given proper preliminary heat treatmentto develop the right kind of structure of the base material. The relative low treatmenttemperatures involved in nitriding (i.e. 510 – 590°C) eliminates the need for a final finishingoperation because no phase transformation and part distortion occur upon cooling. Onlyproper allowance for material growth should be accounted for.

In nitriding, residual stresses are generated by the distortion of the crystalline lattice due todiffusion of carbon and nitrogen from the surface and by the specific volume changefollowing a phase change [2, 3]. Atomic nitrogen is diffused into the metal surface and such


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diffusion continues as long as the temperature is high enough (usually between 500 °C and590 °C [4]), and there is a fresh supply of nascent nitrogen. The diffused nitrogen interactswith the substitutional alloying elements to form an interstitial solid solution. The thinnitrided layer that forms is usually subdivided into a compound (or white) layer near thesurface (i.e. ε phase: Fe2-3N) usually less than 25 µm thick and a diffusion zone beneath thecompound layer (i.e. phase γ´: Fe4N) [4].

In traditional nitriding, the atmosphere flow rate and the resultant dissociation rate of NH3

are controlled but none of these parameters is directly related to the properties of the nitridedlayer. Recently, a controlled gas nitriding, i.e. patented Nitreg Controlled Potential processby Nitrex, USA, has been developed based on constant monitoring and maintenance of thenitriding potential by a computer controlled and fully automated gas nitriding system,according to the modified Lehrer phase diagram (i.e. plot of nitriding potential vs.temperature) [4]. The nitriding potential corresponds to the equilibrium concentration ofnitrogen in ferrite for a given temperature and gives the nitriding capacity of the atmosphere.The nitrided surface, particularly the white layer, obtained with this process is more resilientwith a higher toughness than the traditional process.

This paper presents and discusses the influence of the Nitreg treatment on the hardenedlayer structure, the fatigue response and the fatigue fracture mechanisms of a ferritic nodularcast iron EN - GJS 400.

2. MATERIAL AND EXPERIMENTAL PROCEDURES

The test material was a ferritic nodular cast iron EN - GJS 400 with chemical compositionpresented according to the norm EN 1564 in Table1.

Chemical composition of EN - GJS 400 Table 1

Material Chemical composition [%]

C Si Mn S P Mg Cr Cu Sn Ni MoGJS 400

3.609 2.684 0.131 0.009 0.033 0.047 0.034 0.037 0.013 0.021 0.016

The following basic mechanical properties of untreated ferritic NCI EN - GJS 400 wereobtained: Rm = 450 MPa, A = 19 %, E = 168 GPa. Two sets of smooth fatigue specimenswere prepared by machining from castings. Then, one set of specimens was subjected to anitriding treatment by the patented Nitreg Controlled Potential process (Nitrex, USA). The structural analysis was performed on polished and etched specimen cross-sections inthe optical metallographic microscope according to the norm and by quantitativemetallography [5]. The nitrided layer was analyzed using methods of color etching because ahigh chemical heterogeneity characterized this region [6]. The analysis of carbides found inthe structure and the distribution of nitrogen in nitrided layer were performed by EDS analysisat TU of Brno.

Microhardness (HV 0.2) was measured on metallographic specimens at different distancesstarting from the surface to the base material at TU of Brno. The hardness profilecharacterizes the effectiveness of the nitriding treatment and is used to define an effectivenitrided depth. A second method of hardness profile characterization consisted in directsurface microhardness measurement followed by sequential layer removal until the untreatedsubstrate is reached. In this laborious method a greater volume of material is available underthe indenter and a more reliable hardness indication is obtained, especially near the surface. The fatigue S/N curves for the untreated and the nitrided NCI were obtained using smooth6-mm-dia specimens on a rotating-bending testing machine operating at 50 Hz (i.e. load ratio


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R = -1). A standard procedure for fatigue curve and fatigue limit determination at 50 %probability of survival using a limited number of specimens was adopted [7]. Tests wereinterrupted at 107 cycles if the specimen did not fail. The fatigue limit σc was determinedaccording to a reduced staircase method [7]. Special attention was finally given to the investigation of the fatigue fracture surfacesusing the SEM on selected specimens. The fatigue initiation location and the mechanisms ofstable crack propagation were sought. Nitrided specimens tested at the same stress level andshowing very different fatigue lives were selected to identify possible sources of weakness.

3. RESULTS AND DISCUSSION

The objectives of this paper are: i) the presentation of the improvement in fatigue responseof NCI obtained by the nitriding treatment and ii) the search for a structural explanation ofthis improvement and of the observed scatter in the nitrided test data. The hardness profileprovides a traditional means to assess the effect of the treatment (peak hardness and effectivehardened depth). The structural analysis of selected fatigue specimens and the fracture surfaceinvestigation provides further insight into the explanation of the observed behavior.

3.1 Fatigue propertiesThe S/N curves of the untreated and nitrided NCI are shown in Fig.1. The fatigue limit is

σc = 169 MPa in the untreated NCI and σc = 381 MPa for the nitrided NCI. Therefore, thenitriding treatment is demonstrated to give a very significant improvement of the fatigueresponse, confirming the range of improvement determined by previous tests on steels [1].

Fig.1. Fatigue data and fatigue curves of NCI at 50 % probability of failure

Inspection of Fig. 1, however, shows also that the scatter in fatigue life is much less in theuntreated condition than in the nitrided condition. For example specimens denoted as number3 and 4 were subjected to the same applied stress amplitude but their fatigue lives differed bymore than two orders of magnitude. The following part of this contribution will seek astructural explanation of this different behavior.

3.2 Hardness profiles

Vickers microhardness profiles of the nitrided layer for two fatigue specimens (No. 3 and4 in Fig. 1) tested at the same stress level are presented in Fig. 2. The hardness decreases with


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distance from the surface due to decreasing nitrogen penetration but with some differencesbetween specimens. The peak value for both specimens is about 600 HV 0.2 and it is found at0.05 mm from the surface. A microhardness of 178 HV 0.2 (measured in distance 0.6 mmfrom surface) corresponds with the hardness of the untreated ferritic NCI.

Fig. 2 shows that the trend of the measurements on the cross sections of differentspecimens is compatible with reference data obtained by the layer removal technique. Thepeak value is slightly lower (HV 0.2 = 555) than the cross-section measurements and thehardness gradient is steeper than in the case of cross sectional measurements.

Fig. 2. Vickers microhardness measurementsacross the surface hardened layer

Fig. 3. Nitrogen content across the surfacehardened layer

The origin of the local differences in microhardness profiles of the different specimens ofFig. 2 was investigated by determining the nitrogen distribution in the diffusion zone by EDSanalysis. Fig. 3 shows the content of N in the nitrided layer of the specimens 3 and 4, wherethe highest content was measured in the thin white layer. With increasing distance fromsurface the content of N decreases more rapidly in specimen 3 compared to the specimen 4.On the interface of sub-diffused zone and basic material (from 130 to 200 µm) highernitrogen content was also found in specimen 3.

3.3 Structural characterizationTherefore, the next step was a structural characterization of the base material and of the

nitrided layer of the two NCI specimens. A metallographic analysis provided insight into thecharacteristics of the local microstructural inhomogeneities of NCI in base and nitridedconditions. The base microstructure is characterized by a ferritic matrix with a regulardistribution of graphite spherical nodules, see Fig. 4a with size ranging from 15 to 30 µm anda nodule count of N = 197 nodules/mm2. A discontinuous network of carbides on theboundaries of eutectic cells in the ferrite matrix was also observed, see Fig. 4b. Carbides inthe structure are not eliminated by the surface treatment process and could be responsible offatigue scatter.

The carbides were analyzed in the SEM using EDS analysis as shown in Fig. 5. Manydifferent types of carbides were found depending of the other compositional elements in thematerial.

b)


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Fig. 4. EN GJS 400 - a) microstructure, etched with 3 % nitric acid, b) discontinuous networkof carbides - specimen 4, SEM

If the basic core microstructure of specimen 3 and 4 (i.e. defined in Fig. 1) is compared, itis found that the carbide network of specimen 3 (i.e. long fatigue life) was less visiblecompared to specimen 4. Significant micro shrinkages were also identified on the boundariesof eutectic cells and predominately in carbides areas (Fig. 4, 5a).

The EDS analysis confirmed an unusual high content of Mg, which, togetherwith elements like Ce, La and Nd, favors the development of microshrinkage cavities. The presence of Mg in the micro shrinkages may be due to inappropriatetemperature of modification.

Fig. 5. Carbides in the matrix of NCI - a) detail of Fig.4b, SEM; b) EDS analysis

a) b)

ab


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Inspection of the nitrided layers (see Fig. 6a) showed a thin white (compound) layer (A) onthe surface, a diffusion zone (B) and a sub - diffusion zone (C) [1]. The average ferrite grainsize (dm = 17.6 µm) in nitrided region was not influenced by the nitriding process. The whitelayer was continuous with variable thickness from 10 to 28 µm and with local presence ofgraphite particles. A local porosity in the white layer of specimen 4 (i.e. short fatigue life) wasfound. Thicker nitrided layer and diffusion zone were identified in areas where graphiteparticles presence was observed. The carbide localities were observed in diffusion zone (i.e.Fig 6b). The nitrided layer in specimen 3 was without presence of cracks. In the white layer ofpecimen 4, short cracks initiated on the surface of specimen were observed and a long crackinitiated in the white layer propagated along the boundaries of ferrite grains in diffusion zone.

Fig. 6. Structure of the nitrided layer – a) etched with 5 % molybdenum acid, b) nitrides onthe boundaries of ferrite grains, specimen 4, etched with Klemm I

Therefore, from these local structural and chemical analyses it appears that the specimen 4,in comparison with specimen 3, has a non-uniform distribution of N with a high nitrideconcentration on the ferrite grain boundaries (see Fig. 6b) that result in a negative influenceon fatigue crack initiation and a short fatigue life.

3.4 Microfractographic analysis

In all fatigue fracture surfaces in untreated and nitrided specimens, two regions werefound: i) the fatigue region and ii) region of final static failure. Stable propagation of fatiguecrack occurs in the first region, while the second correspond to unstable crack propagation atfailure. In the case of untreated specimens, the fatigue cracks were initiated in the places ofmicro shrinkages located in the subsurface areas. From these places the fatigue cracksprogressively propagated generating striations in the ferrite matrix.

A

C

B

ab

carbides


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Fig. 7. Fracture surface of nitrided NCI - a) nitrided layer - micro shrinkage shown by arrow,plastic deformation of ferrite around graphite nodule in square, specimen 3;

b) intercrystalline cleavage in diffusion zone, specimen 4, SEM

The study of fatigue fracture of nitrided specimens started with analysis of places of crackinitiation. Multiple sites of fatigue crack initiation were confirmed by the presence of radialstairs on the fracture surface. The cracks initiated at casting defects (micro shrinkages) foundbelow the white layer (see Fig.7a). A detailed study of fracture surface of nitrided layer basedon metallographic study and EDS analyses (see Fig. 5 and 6) showed the presence of carbidesin white layer and in diffusion zone, see Fig. 8. In these places the cracks can easily initiatebecause the carbides are responsible of crack creation.

Fig. 8. Fracture surface of a) nitrided layer, b) diffusion zone with carbides, SEM

The initiated cracks then propagated in two directions: the growth through the white layerto the surface of specimen is characterized by transcrystalline cleavage (see Fig. 7a) and thefatigue cracks propagation into the material (from micro shrinkage at the interface of whitelayer and diffusion zone) was characterized initially by local plastic deformation of ferritearound graphite nodule (see square detail in Fig. 7a). In diffusion and sub - diffusion zonecracks continued predominately by intercrystalline cleavage along grain boundaries of ferrite

ab

ab


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grains (see Fig. 7b) and partly by formation of fine striations with more multiple localizationin sub – diffusion zone (see Fig. 9).

Fig. 9. Fatigue region with striations innitrided specimen 3, SEM

Fig. 10. Final static fracture with dimplemorphology in ferrite, SEM

The presence of striations supports plastic deformation mechanisms in ferrite. The fatiguecracks propagated mainly through the ferrite grains and only rarely along the ferrite/graphiteinterface by transcrystalline cleavage. In the region of final static fracture of both untreatedand nitrided specimens, the crack propagated by transcrystalline ductile fracture of ferrite withdimple morphology (see Fig .10).

4. CONCLUSIONS

This study has investigated the influence of a nitriding treatment by the patented NitregControlled Potential process (Nitrex, USA) on the fatigue response of GJS 400 nodular castiron with special attention to the origin of scatter in specimen life. The following conclusionsare reached:• Tests on smooth specimens of nodular cast iron demonstrated a very significant increase

(more than 100 %) in the fatigue limit after nitriding due to the formation of a hardenedsurface layer and a residual stress system.

• Fatigue fracture mechanisms were investigated in the SEM demonstrating subsurfacefatigue initiation in the nitrided specimens at micro shrinkages located below the whitelayer.

• A structural and local elemental analysis suggests that the reduction in fatigue life ofnitrided nodular cast iron is due to a high concentration of nitrides at ferrite grainboundaries favoring early crack initiation.

Acknowledgments

This work was done as a part of the SK/IT project No10/NT and SK/CZ project No144and of the Scientific Grant Agency of Ministry of Education of Slovak Republic and SlovakAcademy of Sciences grant No.1/3194/06. It is also consistent with the objectives ofMATMEC, one of Emilia-Romagna newly established regional net-laboratories(http://www.diem.ing.unibo.it/matmec/). The authors acknowledge the contribution of Ing.Drahomíra Janová (TU of Brno) to the EDS analysis.


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3. PLUHAŘ, J., KORITTA, J. Strojírenské materiály. 2. přepracované vyd., SNTL - ALFA,Praha 1977.

4. SINHA, A. Physical metallurgy handbook, 2003, McGraw-Hill, NY, ISBN 0-07-05 7986-5.

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