ROLE OF SURFACE CONDITION ON THE PLANE BENDING FATIGUE BEHAVIOR OF NODULAR CAST IRON

Transcription

1 Acta Metall. Slovaca Conf. 128 ROLE OF SURFACE CONDITION ON THE PLANE BENDING FATIGUE BEHAVIOR OF NODULAR CAST IRON Marián Kokavec 1)*, Radomila Konečná 1), Gianni Nicoletto 2) 1) University of Zilina, Faculty of Mechanical Engineering, Žilina, Slovakia 2) Università degli Studi di Parma, Facolta di Ingegneria, Parma, Italy Received: Accepted: * Corresponding author: marian.kokavec@fstroj.uniza.sk, Tel.: , Department of Materials Engineering, Faculty of Mechanical Engineering, University of Žilina, Univerzitná 1, Žilina, Slovakia, Abstract Nodular cast irons are technologically important materials and are used extensively in automotive industry. Defects produced during casting process often play a dominant role in limiting mechanical properties and fatigue life under cyclic loading in cast alloy components. Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. In this study the effects of surface condition on the fatigue life of nodular cast iron were evaluated under cyclic plane bending with maximum stress reached at the surface of interest. The results show differences in fatigue strength, which are associated with the surface conditions. The characteristics of the surface layers of the test specimens with different surface condition were examined by metallography. Selected fracture surfaces were investigated to determine the initiation location of fatigue cracks. Keywords: casting, fatigue test, iron, defects, surface structure 1 Introduction Machine parts are increasing designed under simultaneous requirements, such as heat resistance, abrasion resistance and structural strength, to improve overall performance and efficiency of machinery. Nodular cast iron (NCI) is largely used as a key structural material because it is superior to other materials (steels, aluminum alloys) in its abrasion resistance, damping property, low temperature shock property, cost and workability. It is extensively used in automotive applications such cylinder block, cylinder head, connecting rod, piston rings and brake parts [1]. The fatigue strength of cast metals does not depend only on chemical composition and microstructure. A critical role is frequently played by the growth of cracks initiated from inclusions, nodules or other casting defects, such as microshrinkages or dross defects [2, 3]. Casting defects are inherent in the foundry process and can be found in different cast alloys: aluminum alloys [4, 5], steels [6, 7] or cast irons [8]. Size, type and distribution of casting defect population depend on the alloy type and casting process [9]. Since defects make also NCI susceptible of accelerated nucleation of fatigue cracks due to microscopic stress concentration, previous studies have examined NCI with respect to the role of cast defects on fatigue crack initiation and propagation [8, 10-12]. Many approaches to fatigue resistance evaluation of defect-containing materials consider that the crack initiation stage is negligible and therefore fatigue crack propagation composes the whole fatigue process [4, 5].

2 Acta Metall. Slovaca Conf. 129 Part geometry through its dimensions and associated notch stress gradients and surface finishing also contribute to the fatigue strength and should be assessed at the design stage, [13]. Surface modification with various surface treatment techniques has become one of the most active fields and a preferred solution for cost-effective part strengthening. Nitriding and carbonitriding are among the most widely used surface hardening techniques for ferrous materials because that they provide improved combination of mechanical properties [14]. In this work the authors present how fatigue strength of a nodular cast iron is influenced by different surface conditions. The material is subjected to cyclic plane bending with maximum stress amplitude reached at the surface of interest. The characteristics of the surface layers in the different test specimens are examined by metallography. The fracture surfaces are inspected in the SEM to explain the differences in fatigue lives and the causes of fatigue cracks initiation. 2 Experimental material and methods The experimental NCI was prepared as synthetic melt from 2000 kg of pig iron, 300 kg of steel scrap, and 1500 kg of cast iron scrap. The melting was performed using an arched alkaline furnace with basic lining and 35 kg of FeSi was added to the melted metal as an additive to increase the content of Si, [15, 16]. Chemical composition of the NCI is given in Table 1. The cast material was supplied in the form of 140 x 100 x 20 mm plates. Table 1 Chemical composition of pearlite/ferritic NCI (in wt. %) C Si Mn S P Mg Cr Cu Ni 3,68 2,62 0,51 0,005 0,05 0,034 0,2 0,02 0,01 No annealing treatment was performed before machining of the specimens used for tensile and fatigue testing. Tensile tests performed according to the ASTM E8 standard yielded a tensile strength R m = 576 MPa and an elongation to the rupture A = 6 %. Fatigue specimens, see geometry in Fig. 1, were prepared by machining from cast plates. Fig.1 Shape and dimension of fatigue specimens The role of different surface conditions was investigated. Therefore, a first set of specimens had the as-cast test surface, Fig. 2, (the opposite face was in all cases fine ground). A second set of specimens had the as-cast surface sand blasted, Fig. 2, by compressed air (i.e. 1MPa) and a SiC sand abrasive (i.e. grain size µm). A third set of specimens had the test surface fine ground, Fig. 2, with a smooth finish achieved by removing the casting layer and transition layers on a vertical milling machine and finishing by soft grinding on a disk sander. The final soft grinding was conducted by ceramic aluminum oxide wheel under the conditions of the grinding speed of 30 m/s, down feed mm/pass. Other two specimen sets with fine ground surfaces were subjected to thermo-chemical treatment, either nitriding or carbonitriding, to investigate the role of surface hardening on the fatigue behavior.

3 Acta Metall. Slovaca Conf. 130 Fig.2 Surface condition of specimens (from left as-cast, sand blasted, fine ground) Fig.3 Characteristic microstructure of experimental nodular cast iron, etched with 3% Nital The structural analysis of NCI was performed on polished and etched specimens taken from cast plates. Structure details were analyzed in the light microscope Neophot 32 according to the standard EN STN and by the methods of quantitative metallography, [16]. Fig. 3 shows typical microstructure of experimental material. The matrix was pearlite/ferritic with ferrite surrounded the graphite nodules. Effective ferrite (volume of ferrite in the specimen) calculated using image analysis software NIS Elements 3.0 was 36 %. The number of graphite particles for studied nodular cast iron was in range from 80 to 123 mm -2. The graphite nodules were observed in fully (VI) and partly not fully globular (V) shape. Size was predominately ranging from 30 to 60 µm (6) and with a small number of nodules ranging in the size from 15 to 30 µm (7). The nitrided and carbonitrided layers were analyzed using methods of color etching to distinguish structural characteristics after thermo-chemical treatment. Fatigue tests were performed on specimens using a fatigue test machine for cyclic plane bending with loading ratio R = 0 and 25 Hz frequency. Tests were interrupted at cycles if the specimen did not fail. The ratio R = 0 allowed to apply a cyclic tensile loading (the most critical in fatigue) to the surface of interest, either as-cast, sand blasted, fine grounded, nitrided and carbonitrided. The initial stress range was associated to a fixed displacement range. A load-cell monitoring the specimen stress during the test allowed the determination of the evolution of specimen compliance. It was observed that the fixed initial stress range remained constant for a substantial part of the test followed by a continuous stress reduction in the final part because of fatigue crack initiation and propagation. 3 Results and discussion The results of fatigue tests for all specimens are presented in Fig. 4. The S/N dependence for specimens with different surface conditions was identified. Specimens with as-cast surface achieved low results of fatigue behavior and the fatigue strength was determined 70 MPa at cycles. The scatter of results for as-cast surface specimens was very large. The large scatter of results can be explained by the presence of many casting defects in the cast surface layer, from which the fatigue cracks can initiate very quickly [17].The fatigue strength of sand blasted specimens was 96 MPa at cycles and 125 MPa at cycles for fine ground specimens. After removing the cast layer by sand blasting the fatigue strength increased approx. 40 % going from the as-cast to the sand blasted surface and of approx. 70 % going from the

4 Acta Metall. Slovaca Conf. 131 as-cast to the fine ground surface. The thermo-chemical surface treatments showed the best fatigue behavior compared to the non-treated materials and their fatigue lives showed quite similar results. At cycles, the fatigue strength shows an increase of approx. 60 % going from a fine ground to nitrided or carbonitrided surfaces. Fig.4 S/N fatigue data after cyclic plane bending The surface and subsurface characteristics were metallographically investigated on cross sections perpendicular to the fracture surface and are discussed with reference to Fig. 5. Typically, the as-cast surface, Fig. 5a, is covered by a thin cast brittle layer containing pores and cavities which negatively affect fatigue life because of easy fatigue crack initiations. Just below the surface layer, a pearlitic layer with lamellar graphite, formed due to the rapid solidification and cooling rate at the surface, was found. Below these two layers the structure corresponds to the characteristic NCI structure, see Fig. 3. Thickness of cast layer was approx. 36 µm and pearlitic layer showed variable thickness in the range from 80 to 110 µm. The sand blasting treatment removed only the thin cast layer and locally deformed the surface of specimens. Fig. 5b shows that in the sand blasted specimens the pearlite layer contains lamellar graphite, which gradually turns into vermicular and finally nodular shape going from the surface to the core of material. The combination of lower strength of pearlite matrix because of presence of the lamellar graphite is expected to negatively affect the surface layer strength in case of fatigue loading because it results in early crack initiation compared to the presence of nodular graphite. Only fine grinding reduced significantly the surface roughness (i.e. by one order of magnitude) by complete elimination of the surface layers (formed by cast layer and layers with transition structures). The average surface roughness of fine grounded specimens was R a = 2.3 µm. The investigation of the thermo-chemically treated NCI specimens showed that both nitriding and carbonitriding produced layers formed by a thin white layer (WL) on the specimen surface and diffused zone (DZ) below, Fig. 6. The WL was continuous with thickness about 21 µm for nitrided surface and about 15 µm for carbonitrided surface and with the local presence of graphite nodules in both cases. Thicker nitrided and carbonitrided layer and diffused zone were identified in areas where graphite particles presence was observed and WL was found below the graphite nodules.

5 Acta Metall. Slovaca Conf. 132 a) b) Fig.5 Typical structures of a) as-cast surface, b) sand blasted surface, etched 3% Nital a) b) Fig.6 Structure of a) nitrided layer, b) carbonitrided layer, etched with Klemm I Fatigue life of castings strongly depends on the surface condition. Only a few studies, [5,6,18,19,20], have been conducted on NCI castings with as-cast surfaces. In the presented case fatigue fracture initiation in as-cast and sand blasted specimens was observed with a scanning electron microscope (SEM) and big casting defects (observed during metallographic analysis on/or near the surface) were found as fatigue crack initiation places, Fig. 7. In these cases the fatigue crack initiation places were lustrous carbon films, Fig. 7a, b, and slag inclusions, Fig. 7c, d. The fatigue life was also influenced by the presence of transitional structural surface layers, mainly pearlitic layer with lamellar graphite. In the case of fine ground specimens, Fig. 8, and thermo-chemical treated specimens, Fig. 9, the fatigue cracks were initiated in the places of defect (microshrinkage) located near the surface, Fig. 8b, on microshrinkage near the surface, Fig. 9a, in the interface graphite particles/matrix or graphite particles/white layer, Fig. 9b. In the case of thermo-chemical treated specimens the fatigue cracks in the diffusion zone continued predominately by intercrystalline cleavage along ferrite grain boundaries, Fig. 9c, and locally propagated generating striations in the ferrite around graphite particles, Fig. 9d. 4 Conclusions The influence of five different surface conditions on the fatigue behavior of pearlite/ferritic NCI has been investigated. From this study, the following conclusions can be drawn: The as-cast and sand-blasted surface conditions result in considerably lower fatigue strength of NCI specimens (70 MPa at cycles for as-cast surface and 108 MPa at

6 Acta Metall. Slovaca Conf cycles for sand blasted surface) compared to the fine-ground surface condition (134 MPa at cycles). a) lustrous carbon film, SEM c) slag inclusion, SEM 100µm 500µm b) lustrous carbon film, etched 3% Nital d) slag inclusion, etched 3% Nital Fig.7 Fatigue crack initiation places and casting defects specimens in as-cast and sand blasted a) macroview of fracture surface b) defect in initiation place Fig.8 Fatigue crack initiation place in fine ground specimens, SEM

7 Acta Metall. Slovaca Conf. 134 a) microshrinkages below surface b) interface of graphite particle/white layer c) intercrystalline fracture of ferrite in d) ferrite striation in the fatigue region difusion zone Fig.9 Fatigue crack initiation places in the thermo-chemical treated specimens, SEM Thermo-chemical treatment of smooth NCI specimens increases further the fatigue strength because of the hardened surface layer and the residual stress system. Nitriding and carbonitriding treatments achieve similar and considerable fatigue strength improvements (i.e. 60 %) compared to the fine grounded surface (201 MPa at cycles for nitriding and MPa at cycles for carbonitriding). Fatigue fracture origins of NCI with as-cast and sand-blasted surface are largely attributed to the surface roughness and defects existing on the surface and sub-surface. The as-cast surface layers are characterized by presence of many big casting defects and a brittle surface structure due to the presence of lamellar graphite, which is very different from the base pearlite/ferritic microstructure with nodular graphite. Defects (lustrous carbon film, slag inclusions, shrinkages) are fatigue initiation places of as-cast and sand-blasted specimens. In the case of fine ground specimens, fatigue cracks were initiated in the places of microshrinkages located near the surface. Finally, thermo-chemical treated specimens failed because cracks initiated at microshrinkages located near white layer or on the interface graphite particles/white layer.

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