STRUCTURE AND FATIGUE FAILURE ANALYSIS OF NITRIDED NODULAR CAST IRON Radomila Konečná a Gianni Nicoletto b Viera Majerová a a Department of Materials Engineering, University of Žilina, Univerzitná 1, 01026 Žilina Slovakia, radomila.konecna@fstroj.utc.sk b Department of Industrial Engineering, University of Parma, Viale G.P. Usberti, 181/A, 43100 Parma, Italy, gianni.nicoletto@unipr.it Abstract This paper presents and discusses the influence of a nitriding treatment by the patented Nitreg Controlled Potential process on the case material structure, the fatigue response and the fatigue fracture mechanisms of the ferritic nodular cast iron EN - GJS 400. The structural analysis of the nitrided layer was performed and microhardness was measured on these specimens at different distances starting from the surface down to the basic material. The hardness profile characterizes the effectiveness of the nitriding treatment and is used to define an effective nitrided depth. A standard procedure for fatigue curve and fatigue limit determination at 50 % probability of survival using a limited number of specimens was adopted. The nitriding treatment is demonstrated to give a very significant improvement of the fatigue response, confirming the range of improvement determined by previous tests on steels. The high fatigue strength is not due exclusively to the formation of the hardened surface layer, because favorable compressive residual stresses are also produced in the surface layers by nitriding. Residual stresses are generated by the distortion of the crystalline lattice due to diffusion of carbon and nitrogen from the surface and a change in the specific volume following a phase change. 1. INTRODUCTION Nodular cast irons (NCI) are construction materials with a wide range of applications in engineering practice [1]. For their optimal application it is important to know their basic mechanical properties and the available methods to improve them. Since fatigue resistance is sensitive to surface conditions, any phenomenon that changes the surface characteristics of a material will greatly affect its fatigue performance. Three main surface modification factors are identified: i) control of surface roughness, ii) changes in the mechanical properties of the metal surface, and iii) changes in the residual stress condition near the surface. Thermo chemical surface treatments, such as nitriding, are frequently applied to ferrous materials because they promote the formation of a hard and strong surface layer and of a system of compressive residual stress that simultaneously improve the fatigue endurance and the wear resistance, [1, 2]. Since the properties of the nitrided layers depend on the material and its original pre-process hardness, the part must be given proper preliminary heat treatment to develop the right kind of structure of the base material. The relative low treatment temperatures involved in nitriding (i.e. 510 590 C) eliminates the need for a final finishing operation because no phase transformation and part distortion occur upon cooling. Only proper allowance for material growth should be accounted for. In nitriding, residual stresses are generated by the distortion of the crystalline lattice due to diffusion of carbon and nitrogen from the surface and by the specific volume change following a phase change [2, 3]. Atomic nitrogen is diffused into the metal surface and such 1
diffusion continues as long as the temperature is high enough (usually between 500 C and 590 C [4]), and there is a fresh supply of nascent nitrogen. The diffused nitrogen interacts with the substitutional alloying elements to form an interstitial solid solution. The thin nitrided layer that forms is usually subdivided into a compound (or white) layer near the surface (i.e. ε phase: Fe 2-3 N) usually less than 25 µm thick and a diffusion zone beneath the compound layer (i.e. phase γ : Fe 4 N) [4]. In traditional nitriding, the atmosphere flow rate and the resultant dissociation rate of NH 3 are controlled but none of these parameters is directly related to the properties of the nitrided layer. Recently, a controlled gas nitriding, i.e. patented Nitreg Controlled Potential process by Nitrex, USA, has been developed based on constant monitoring and maintenance of the nitriding 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 of nitrogen 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 resilient with a higher toughness than the traditional process. This paper presents and discusses the influence of the Nitreg treatment on the hardened layer structure, the fatigue response and the fatigue fracture mechanisms of a ferritic nodular cast iron EN - GJS 400. 2. MATERIAL AND EXPERIMENTAL PROCEDURES The test material was a ferritic nodular cast iron EN - GJS 400 with chemical composition presented according to the norm EN 1564 in Table1. Chemical composition of EN - GJS 400 Table 1 Material Chemical composition [%] GJS 400 C Si Mn S P Mg Cr Cu Sn Ni Mo 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 were obtained: R m = 450 MPa, A = 19 %, E = 168 GPa. Two sets of smooth fatigue specimens were prepared by machining from castings. Then, one set of specimens was subjected to a nitriding treatment by the patented Nitreg Controlled Potential process (Nitrex, USA). The structural analysis was performed on polished and etched specimen cross-sections in the optical metallographic microscope according to the norm and by quantitative metallography [5]. The nitrided layer was analyzed using methods of color etching because a high chemical heterogeneity characterized this region [6]. The analysis of carbides found in the structure and the distribution of nitrogen in nitrided layer were performed by EDS analysis at TU of Brno. Microhardness (HV 0.2) was measured on metallographic specimens at different distances starting from the surface to the base material at TU of Brno. The hardness profile characterizes the effectiveness of the nitriding treatment and is used to define an effective nitrided depth. A second method of hardness profile characterization consisted in direct surface microhardness measurement followed by sequential layer removal until the untreated substrate is reached. In this laborious method a greater volume of material is available under the 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 smooth 6-mm-dia specimens on a rotating-bending testing machine operating at 50 Hz (i.e. load ratio 2
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 were interrupted at 10 7 cycles if the specimen did not fail. The fatigue limit σ c was determined according to a reduced staircase method [7]. Special attention was finally given to the investigation of the fatigue fracture surfaces using the SEM on selected specimens. The fatigue initiation location and the mechanisms of stable crack propagation were sought. Nitrided specimens tested at the same stress level and showing 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 response of NCI obtained by the nitriding treatment and ii) the search for a structural explanation of this improvement and of the observed scatter in the nitrided test data. The hardness profile provides a traditional means to assess the effect of the treatment (peak hardness and effective hardened depth). The structural analysis of selected fatigue specimens and the fracture surface investigation provides further insight into the explanation of the observed behavior. 3.1 Fatigue properties The S/N curves of the untreated and nitrided NCI are shown in Fig.1. The fatigue limit is σ c = 169 MPa in the untreated NCI and σ c = 381 MPa for the nitrided NCI. Therefore, the nitriding treatment is demonstrated to give a very significant improvement of the fatigue response, 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 the untreated condition than in the nitrided condition. For example specimens denoted as number 3 and 4 were subjected to the same applied stress amplitude but their fatigue lives differed by more than two orders of magnitude. The following part of this contribution will seek a structural explanation of this different behavior. 3.2 Hardness profiles Vickers microhardness profiles of the nitrided layer for two fatigue specimens (No. 3 and 4 in Fig. 1) tested at the same stress level are presented in Fig. 2. The hardness decreases with 3
distance from the surface due to decreasing nitrogen penetration but with some differences between specimens. The peak value for both specimens is about 600 HV 0.2 and it is found at 0.05 mm from the surface. A microhardness of 178 HV 0.2 (measured in distance 0.6 mm from surface) corresponds with the hardness of the untreated ferritic NCI. Fig. 2 shows that the trend of the measurements on the cross sections of different specimens is compatible with reference data obtained by the layer removal technique. The peak value is slightly lower (HV 0.2 = 555) than the cross-section measurements and the hardness gradient is steeper than in the case of cross sectional measurements. b) Fig. 2. Vickers microhardness measurements across the surface hardened layer Fig. 3. Nitrogen content across the surface hardened layer The origin of the local differences in microhardness profiles of the different specimens of Fig. 2 was investigated by determining the nitrogen distribution in the diffusion zone by EDS analysis. Fig. 3 shows the content of N in the nitrided layer of the specimens 3 and 4, where the highest content was measured in the thin white layer. With increasing distance from surface 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) higher nitrogen content was also found in specimen 3. 3.3 Structural characterization Therefore, 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 the characteristics of the local microstructural inhomogeneities of NCI in base and nitrided conditions. The base microstructure is characterized by a ferritic matrix with a regular distribution of graphite spherical nodules, see Fig. 4a with size ranging from 15 to 30 µm and a nodule count of N = 197 nodules/mm 2. A discontinuous network of carbides on the boundaries of eutectic cells in the ferrite matrix was also observed, see Fig. 4b. Carbides in the structure are not eliminated by the surface treatment process and could be responsible of fatigue scatter. The carbides were analyzed in the SEM using EDS analysis as shown in Fig. 5. Many different types of carbides were found depending of the other compositional elements in the material. 4
ab Fig. 4. EN GJS 400 - a) microstructure, etched with 3 % nitric acid, b) discontinuous network of carbides - specimen 4, SEM If the basic core microstructure of specimen 3 and 4 (i.e. defined in Fig. 1) is compared, it is found that the carbide network of specimen 3 (i.e. long fatigue life) was less visible compared to specimen 4. Significant micro shrinkages were also identified on the boundaries of eutectic cells and predominately in carbides areas (Fig. 4, 5a). The EDS analysis confirmed an unusual high content of Mg, which, together with elements like Ce, La and Nd, favors the development of micro shrinkage cavities. The presence of Mg in the micro shrinkages may be due to inappropriate temperature of modification. a) b) Fig. 5. Carbides in the matrix of NCI - a) detail of Fig.4b, SEM; b) EDS analysis 5
Inspection of the nitrided layers (see Fig. 6a) showed a thin white (compound) layer (A) on the surface, a diffusion zone (B) and a sub - diffusion zone (C) [1]. The average ferrite grain size (d m = 17.6 µm) in nitrided region was not influenced by the nitriding process. The white layer was continuous with variable thickness from 10 to 28 µm and with local presence of graphite particles. A local porosity in the white layer of specimen 4 (i.e. short fatigue life) was found. Thicker nitrided layer and diffusion zone were identified in areas where graphite particles 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 of pecimen 4, short cracks initiated on the surface of specimen were observed and a long crack initiated in the white layer propagated along the boundaries of ferrite grains in diffusion zone. a b A B carbidesc Fig. 6. Structure of the nitrided layer a) etched with 5 % molybdenum acid, b) nitrides on the 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 nitride concentration on the ferrite grain boundaries (see Fig. 6b) that result in a negative influence on fatigue crack initiation and a short fatigue life. 3.4 Microfractographic analysis In all fatigue fracture surfaces in untreated and nitrided specimens, two regions were found: i) the fatigue region and ii) region of final static failure. Stable propagation of fatigue crack occurs in the first region, while the second correspond to unstable crack propagation at failure. In the case of untreated specimens, the fatigue cracks were initiated in the places of micro shrinkages located in the subsurface areas. From these places the fatigue cracks progressively propagated generating striations in the ferrite matrix. 6
ab 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 crack initiation. Multiple sites of fatigue crack initiation were confirmed by the presence of radial stairs on the fracture surface. The cracks initiated at casting defects (micro shrinkages) found below the white layer (see Fig.7a). A detailed study of fracture surface of nitrided layer based on metallographic study and EDS analyses (see Fig. 5 and 6) showed the presence of carbides in white layer and in diffusion zone, see Fig. 8. In these places the cracks can easily initiate because the carbides are responsible of crack creation. ab 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 layer to the surface of specimen is characterized by transcrystalline cleavage (see Fig. 7a) and the fatigue cracks propagation into the material (from micro shrinkage at the interface of white layer and diffusion zone) was characterized initially by local plastic deformation of ferrite around graphite nodule (see square detail in Fig. 7a). In diffusion and sub - diffusion zone cracks continued predominately by intercrystalline cleavage along grain boundaries of ferrite 7
grains (see Fig. 7b) and partly by formation of fine striations with more multiple localization in sub diffusion zone (see Fig. 9). Fig. 9. Fatigue region with striations in nitrided specimen 3, SEM Fig. 10. Final static fracture with dimple morphology in ferrite, SEM The presence of striations supports plastic deformation mechanisms in ferrite. The fatigue cracks propagated mainly through the ferrite grains and only rarely along the ferrite/graphite interface by transcrystalline cleavage. In the region of final static fracture of both untreated and nitrided specimens, the crack propagated by transcrystalline ductile fracture of ferrite with dimple morphology (see Fig.10). 4. CONCLUSIONS This study has investigated the influence of a nitriding treatment by the patented Nitreg Controlled Potential process (Nitrex, USA) on the fatigue response of GJS 400 nodular cast iron with special attention to the origin of scatter in specimen life. The following conclusions are 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 hardened surface layer and a residual stress system. Fatigue fracture mechanisms were investigated in the SEM demonstrating subsurface fatigue initiation in the nitrided specimens at micro shrinkages located below the white layer. A structural and local elemental analysis suggests that the reduction in fatigue life of nitrided nodular cast iron is due to a high concentration of nitrides at ferrite grain boundaries favoring early crack initiation. Acknowledgments This work was done as a part of the SK/IT project No10/NT and SK/CZ project No144 and of the Scientific Grant Agency of Ministry of Education of Slovak Republic and Slovak Academy of Sciences grant No.1/3194/06. It is also consistent with the objectives of MATMEC, 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. 8
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