Effect of shot peening on the fatigue performance of ductile iron castings

Transcription

1 Effect of shot peening on the fatigue performance of ductile iron castings S. Ji, K. Roberts, Z. Fan Department of Materials Engineering Brunel University Uxbridge, Middlesex, UB8 3PH UK Abstract Ductile iron is a commonly used structural material. However the unsatisfactory fatigue performance has limited its application for some dynamic loads. Shot peening is a mechanical surface modification process, which can extend the fatigue life of materials by introduction of working hardening and compressive layer on surface and removal of the surface irregularities. Results of the influence of shot peening treatment on ductile iron castings with as-cast surface and machined surface are presented. The results showed that the shot peening on ductile iron could double the fatigue life for as-cast surface and increase by 4 times for machined surface. It is believed that shot peening affects fatigue life through the reduction of the density and the length of the cracks formed on the surface of specimens of ductile iron. Keywords: ductile iron, fatigue, surface treatment, shot peening, micro-cracks 1

2 1. Introduction Ductile iron is one of the most commonly used structural materials in the world because of its good combination of low cost, design flexibility, good strength-to-weight ratio, toughness, wear resistance and fatigue performance [1,2]. It offers this good combination of properties with the excellent castability, thus possessing unique production advantages [3]. However, the fatigue performance of ductile iron is not sufficiently good in some applications for dynamic loads. A lot of trials have been undertaken to improve the fatigue life of ductile iron in the last few decades [1], but the industrially applicable processes are still rare because of the limitation of the gains in properties and operation flexibility. However, little is known about its fatigue in response to shot peening surface treatment, especially for the directly shot-peened as-cast surface, even though some results have been found on the austempered ductile iron with other processes [4,5,6,7]. As it involves blasting specimen surface with high velocity steel or glass particles, shot peening puts the specimen s interior in a state of tension while the surface, including a thin layer of sub-surface material, is in compression [8]. This is considered to be an effective life enhancement process because of the compressive residual stress and its effect, which is to delay surface cracking [9, 10]. Therefore, this paper aims to introduce the effect of shot peening on the fatigue life of ductile iron. Four different kinds of surface conditions, as-cast, shot-peened as-cast, machined, machined and shot-peened were used to examine the fatigue life, microstructural and fracture features. 2. Experimental The ductile iron castings were produced in a foundry for the current project. A medium frequency coreless induction furnace was used to melt the alloys and superheat to 1500 o C. The melt was taped into a preheated shank ladle of 20kg capacity, containing the nodulising alloys and inoculation alloys with sandwich techniques. Another kind of inoculating granular additive of ferrosilicon alloys was added as stream inoculation during pouring the treated melt into the mould. The pouring temperature of the melt was controlled at 1450 o C. A plate type casting with a dimension of mm were produced in the moulds manufactured by silica sand with 1% furan resin binder and 0.40% hardener (based on sand). The typical chemical composition of the produced casting was 3.54wt.%C, 2.41wt.%Si, 0.53wt.%Mn, <0.02wt.%P, <0.01wt.%S and 0.03wt.% of retained magnesium. The numbers 2

3 and the nodularity of graphite nodules in casting section were >200mm -2 and >0.90, respectively. The main mechanical properties of the ductile iron include tensile strength of 500MPa, 2% proof stress of 320MPa, elongation of 7% and typical hardness from HB170 to HB230. The produced castings were treated with blasting in a foundry, following the industrial castings. Then the castings were randomly classified into four groups for specimen production. One group of castings was retained without further surface treatment, which was defined as as-cast surface (AC). The specimens, as schematically shown in Fig. 1, were machined from the bottom of the castings. Only one specimen was cut off from one casting. The bottom surface of the specimens was retained for surface treatment and other surfaces were machined to a same quality. Three types of further surface treatments used in this study were shot peened as-cast surface (PC), machined surface (MS), and machined and shot peened surface (MP). For the machined surface, 1.5mm of metal was cut off at the bottom of the casting surface. Shot peening was performed by means of an injector-type system. Peening intensity was measured using an A Almen strip. The main parameters of shot peening are summarised in Table 1. The surface roughness was used to define the surface condition, which was measured by a profilometer. The measured Ra values for different surface conditions were 17.0 µm for AC, 7.3µm for PC, 4.3µm MS and 3.7 for MP, respectively. The specimens were fatigued to failure in a bending machine at a frequency of 25Hz using specimens for S-N curves and the laboratory temperature was 20 o C. The microstructural observations were taken for tested specimens by means of optical microscope and scanning electron microscope (SEM). SEM also observed the fracture characteristics of specimens after failure. 3. Results The relationship between the bending stress and the number of cycles to failure is shown in Fig. 2. It is evident that the shot peening treatment can significantly improve the fatigue life of the ductile iron. The numbers of fatigue life of the specimens with shot-peened surface 3

4 were obviously higher than that of the surface without shot peening treatment, including machined surface and as-cast surface, even though the roughness of shot-peened as-cast surface is higher than that of machined surface. The average increasing range of the fatigue life of the specimens was double between as-cast surface and shot-peened as-cast surface and 4 times between machined surface and shot-peened and machined surface. The fatigue life of the specimens with shot-peened as-cast surface was higher than those with machined surface. At low stresses, the fatigue life of the specimens with shot-peened as-cast surface was close to those with machined surface. Both of them were much higher that the specimens with ascast surface and lower than the specimens with machined and shot-peened surface. Fig. 3 shows the crack developed after bending test on the unetched section of ductile iron with different surface conditions. The results illustrated that shot peening can drastically reduce the number and penetrating depth of the micro-cracks on the casting surface and subsurface. For the specimens with as-cast surface, as shown in Fig. 3a, a lot of deeply penetrated cracks were found on and near the surface. The graphite nodules on and near the casting surface were also apparently distorted, which were prolonged along the crack direction. After shot peening treatment, as shown in Fig. 3b, the numbers of the cracks on the surface of the specimens were considerably reduced. The depth and the length of the cracks penetrating into the specimens were also smaller than those with as-cast surface. Meanwhile, the distortion of graphite nodules near the surface of specimen was pronouncedly reduced. The smaller distortion of the graphite nodules and reduced micro-cracks indicated that the matrix was hardened by shot peening. For the machined surface, as shown in Fig. 3c, small and short cracks were found to distribute randomly on the surface and sub-surface of the specimens and the graphite nodule distortion was quite small. For the machined and shotpeened surface, as shown in Fig. 3d, the numbers of the cracks were further reduced and the distortion of graphite nodules was not pronounced in the micrograph. Fig. 4 shows the microstructures of ductile iron with different surface conditions, which illustrated the variation of the deformation of ferrite and pearlite in the matrix after the bending fatigue life test. Both ferrite and pearlite in the matrix were apparently distorted near the as-cast surface in the specimens, as shown in Fig. 4a. The distortion of the ferrite and pearlite in the matrix was reduced for the specimens with shot-peened as-cast surface (Fig.4b). This could further indicate the existence of the harden layer on the shot-peened surface. The distortion of ferrite and pearlite in the matrix of specimens with machined 4

5 surface (Fig. 4c) and shot-peened and machined surface (Fig. 4d) was not apparent. The results in Fig. 4 revealed that the distortion of the ferrite and pearlite near the shot peened surface is different to those with as-cast surface. Fig. 5 shows the fractographical micrographs of ductile iron with different surface conditions. For specimens with as-cast surface, the fracture surface contains mainly the faceted brittle areas. The cleavage cracks were visible on the fracture surface of the specimens (Fig. 5a). The fracture surface of shot-peened as-cast specimens had less cleavage cracks and more dimpled ductile areas (Fig. 5b). The similar tendency could be found in Fig. 5c and Fig. 5d, corresponding to the machined surface and machined and shot-peened surface, respectively. The faceted brittle fracture was also clearly visible in the shot peened layer near the surface in both Fig. 5b and 5d. 4. Discussion The results illustrated that shot peening on the surface of ductile iron castings improves their fatigue life significantly. The fatigue life of the specimens with shot peening can be doubled for as-cast surface and increased by 4 times for machined surface. The experimental observations also found that the increase of the fatigue life can be attributed the reduction of the cracks on the surface and sub-surface of the specimens. These can be further intercepted by the existence of surface hardening and compressive stresses and the removal of irregularities on the casting surface introduced by shot peening. Numbers of cycles to failure of thin specimens (4mm) are governed by the initiation and growth of small cracks, and residual stresses. The crack density developed in the early stage of fatigue life increases with cycling due to the nucleation of additional cracks. Because of this and the growth of some existing cracks, the cracks spacing is presumed to decrease continuously and approach its steady value when material attains its saturation. Subsequently, a few of the cracks could link up to form a critical crack that in turn can propagate to failure in negligible cycles. As cracks start at the surface, a concern of course is its condition of surface. Being in as-cast state, the surface is rough and usually contains some inclusions. There are many graphite nodules distributed in the matrix. These lead the development of stress concentration and further promote crack nucleation and propagation across the specimen section. As a result, it has a lower fatigue life at all stresses (Fig. 2&3a). The 5

6 machined surface cut off the rough layer, which gives rise to an improved feature. So the fatigue life is longer than those of as-cast surface (Fig.2). But the existence of graphite nodules can be the source of crack nucleation, as shown in Fig. 3 and 4. So the fatigue life is still limited. The improvement of shot peening on the fatigue life can be attributed to three aspects, surface hardening, and the removal of irregularities on the casting surface and the existence of compressive residual stress. Shot peening is a classic method known to bring about working hardening in the surface region of materials, which could considerably improve the fatigue strength [11]. The working hardening can explained by the dislocation mechanism of crystallattice transformation, which responds to the low-temperature phase transformation of ferrite and pearlite to influence the mechanical properties. The significantly increased hardness near the surface region of materials has been found in different materials [12, 13]. It is also conceivable that the removal of irregularities on the casting surface can improve the fatigue life because most cracks start at surface (Fig. 3), especially for the as-cast surface. The shot peening on as-cast surface can drastically reduce its surface roughness Ra from 17.0 µm to 7.3µm. Some irregularities and inclusions on the casting surface are peened off. So the opportunities of crack nucleation from these surface irregularities and inclusions is reduced. The fatigue performance is therefore improved. Similar behaviours have been noted for other alloys during shot peening treatment [14]. Another important factor to improve the fatigue life is the existence of compressive residual stress developed in the ductile iron by shot peening. A high-level compressive residual stress exists up to the depths of µm with a maximum values of 450MPa at the surface layer [15]. When applying a higher level fatigue stress, the compressive residual stress on the surface is rapidly decreased and the tensile stress occurs through the concentration of stress at dents or irregularities on the surface or at graphite nodules existing immediately below the surface. As a result, the cracks can be nucleated and propagate from the surface. In this case, the fatigue life is mainly determined by the value of the stress concentration. The role of the existed compressive residual stress on the surface of ductile iron is thus limited. When applying a lower level fatigue stress, the compressive residual stress at the surface area does not decrease significantly or can last for a longer cycling time, which could prevent the 6

7 nucleation and propagation of the cracks. With the increase of cycling, the cracks can be nucleated from the internal defects such as graphite nodules and inclusions in the matrix and the irregularities on the surface of castings [15,16]. In this case, the compressive residual stress is expected to play an important role to extend the fatigue life. It is difficult to exactly define the role of three aspects, the working hardening on surface, the removal of surface irregularities and the existence of compressive residual stress on surface and sub-surface on the improvement of the fatigue performance of the shot-peened casting surface. 5. Summary Shot peening on the surface of ductile iron castings can significantly extend their fatigue life. Compared to conventional as-cast surface and machined surface of castings, numbers of cycles to failure of thin specimens are doubly increased for shot-peened as-cast surface and increased by 4 times for shot-peened and machined surface, respectively. It is believed that shot peening affects fatigue life through the retardation of crack nucleation and growth because of the introduction of working hardening and compressive stresses on the surface and sub-surface and removal surface irregularities of the ductile iron castings. 7

8 Table 1. The parameters of shot peening treatment for ductile iron castings Pressure Shot size Distance Vertical speed Rotating speed Coverage Intensity 3.0kg/cm mm (S330) 200mm 240 mm/min 30 rpm > 85% 0.30 A 90mm δ=4mm 30mm φ7mm 10mm R32mm Fig. 1. The schematic diagram of the specimen for fatigue life tests. 8

9 Stress (MPa) AC PC MS MP E E E E Number of cycles to failure Fig. 2. S-N curves of ductile iron with as-cast surface (AC), shot-peened as-cast surface (PC), machined surface (MS), and machined and shot-peened surface (MP). 9

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11 Fig. 3. The Un-etched optical micrographs showing the cracks and the distortion of graphite nodules in failed ductile iron specimens at a fatigue stress of 360MPa with different surface qualities. (a) as-cast surface (AC), (b) shot-peened as-cast surface (PC), (c) machined surface (MS), and (d) machined and shot-peened surface (MP). 11

12 12

13 Fig. 4. The optical micrographs etched with 1% nital showing the microstructures of in failed ductile iron specimens at a fatigue stress of 360MPa with different surface qualities. (a) ascast surface (AC), (b) shot-peened as-cast surface (PC), (c) machined surface (MS), and (d) machined and shot-peened surface (MP). 13

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15 Fig.5. The SEM micrographs showing the fracture surface after fatigue failure of ductile iron specimens at a fatigue stress of 360MPa with different surface qualities. (a) as-cast surface (AC), (b) shot-peened as-cast surface (PC), (c) machined surface (MS), and (d) machined and shot-peened surface (MP). 15

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