Research Article, Res Rep Metals Vol: 1 Issue: 3
Surface Nanostructuring of AISI 1017 by Severe Shot Peening
Received: May 27, 2017 Accepted: June 24, 2017 Published: June 29, 2017
Citation: Unal O, Varol R (2017) Surface Nanostructuring of AISI 1017 by Severe Shot Peening. Res Rep Metals 1:3.
In this study, AISI 1017 low carbon steel was subjected to severe shot peening (SSP) by using 35mmA Almen intensity. SSP led to surface severe plastic deformation by the impingement of the shot media with very high air pressure. 40-50 µm highly deformed nanograin layer was formed right below the surface by means of sub grain creation with dislocation interaction and non-homogenous strain. Nanoindentation tests were performed to detect the hardness and reduced modulus of fine grained layer and the results reveal the hardness increased up to almost 1,5 times with compared to core. The reduced modulus is also influenced from very hard nanocrystalline layer.
Keywords: Defects; Deformation and fracture; Severe shot peening; Severe plastic deformation; SMAT; Nanostructured layer
Nanostructured materials have been assessed as crucial for metallurgical, industrial and biological environments [1-3]. These materials have been manufactured with two methods, one is bottomup and the other is top-down. The top-down method called “severe plastic deformation (SPD)” methods principle is to create large deformations on the coarse grain bulk materials and convert them into bulk nanostructured materials . The great interest have been shown to these materials due to having very high hardness and strength . Also electrical, magnetic and superplastic behaviors can be improved via nanograin formation mechanisms . Besides, the application of severe plastic deformation to create bulk nanostructured materials have some limitations. Higher force requirements, non-uniform strain exposion, unexpected material failures and also limited sample sizes are given as the examples .
Most failures of metallic materials such as fatigue, fretting fatigue, corrosion and wear are directly related with surface characteristics [8,9]. Therefore, refining the surface grain layers by using surface severe plastic deformation (S2PD) applications could be beneficial for overcoming such type of these failures [10,11]. S2PD methods have been applied to induce nanograin layer on and just below the surface and cover the most common applications such as surface mechanical attrition treatment (SMAT) , surface nanocrystallization and hardening (SNH) , ultrasonic nanocrystal surface modification (UNSM) [14-17]. In recent years, severe shot peening (SSP) has been applied for exposing high plastic deformation to surfaces besides increasing fatigue strength by inducing compressive residual stresses [18-21]. The studies reveal SSP influence with Almen intensity higher than the conventional ones behaves like the methods mentioned above [22-24]. Application of SSP is much more easier, only changing the initial parameters of shot peening equipment will be adequate. Also type of the application provides convenience for complex and intrinsic samples regardless of size and dimensions [4,25-27].
In this study, AISI 1017 low carbon steel was applied to the SSP with 35mmA Almen intensity. Then the surface was investigated by using optical microscopy, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Nanoindentation tests were performed to show the hardness change and compare the results with the microstructural observations.
AISI 1017 low carbon steel specimens with the following chemical compositions in mass (%) C 0,15-0,20, Mn 0,30–0,60, P 0,04(max), S 0,05(max) and balance Fe. The specimens were ground with 180-1200 grit papers then mechanically polished. Annealing treatment has been applied at 8100C for 30 minutes and then cooled in the furnace to room temperature. Air blast severe shot peening was performed via 2000S Peenmatic shot peening equipment. 35 mmA Almen intensity was selected to compensate severe plastic deformation conditions. Shot peening parameters used for the treatment are shown in Table 1.
|Almen intensity||Shot size||Coverage (%)||Time (s)||Pressure (psi)||Arc height (mmA)|
Table 1: The air blast severe shot peening initial parameters.
The specimens were analyzed via optical microscopy (Zeiss AXIO A1 optical microscope), SEM (Tescan MAIA3 XMU) and HRTEM (JEOL JEM 2100). The specimens were ion polished as mechanically thinning to a thickness of 80 μm by disc grinding and to about 5 μm by dimple grinding from metal side. Hardness distribution from surface to interior was determined by using Schimadzu DUH-W201S ultra micro-hardness tester. The applied load was 50 mN with a duration of 10 s.
Results and Discussion
SSP applied with 35mmA Almen intensity leads to form highly deformed, oriented layer with a thickness of approximately 40-50 μm. The peened specimens microstructure can be seen from Figure 1. The deformed layer shows SSP is an effective way to expose severe plastic deformation at least as much as SMAT and etc [28,29].
In SEM microstructure (Figure 2) reveals the deformed layer in detail. Homogenous ferrite-pearlite structure in the core structure vanishes in the deformed layer. Bagherifard and Guagliano  show the distinction of conventional shot peening (CSP) and SSP effect on low alloy steel and the heavily deformed layer is much thicker than CSP however the surface peened with SSP is much more deteriorated which is similar to this study. Besides creation of ultra fine grain layer, the treatments leads to surface deterioration. Higher pressure application for SSP can not prevent hazordous effects on the  surface. Thus, surface quality and integrity should be assessed simultaneously with microstructural improvement during SSP.
Figure 3a shows typical HRTEM observations of highly deformed surface layer. During deformation, cementite phases are broken where subgrain formations under 100 nm are observed in Figure 3b particularly inside the subgrains. Since, Moering et al. presents white layers in HRTEM investigations prove the fragmentation of Fe3C compounds due to supersaturation of carbon into nanocrystalline alpha-iron . The grain refinement is formed as non-uniform and non-homogenous. Since the grain sizes could be assessed within a broad range (40-400 nm). Also SMAT application to ultra low carbon steel forms the grain size distribution with is between 10 nm and 100 nm . However, the microstructure has high density dislocations and interactions (Figure 3c) and stacking faults near the grain boundary (Figure 3a). Also in Figure 3b, ultrafine subgrains are distinguished via grain boundaries. In severe plastic deformation treatments the deformation intensifies near the grain boundaries  and subgrain is formed so this is consistent with the study HRTEM investigations. Typical BCC metals for instance low alloy steels have high stacking fault energy . Exposion of severe plastic deformation leads to dislocation movement, tangles and dense walls (Figure 3c). This dislocation behavior can be accepted as the first step of subgrain formation . The actual subgrains and its boundaries are created by high dislocation interactions regardless of active or inactive type. Subgrains are formed in the original grains inside original grain boundaries (Figure 3b) .
According to the nanoindentation measurements, surface have been exactly influenced from severe plastic deformation. Thus, the nanohardness values are much higher than the core. The hardness alteration is compatible with optical microscope and SEM observations. The topmost layer (approximately 40 μm) mostly influenced from severe plastic deformation could be easily realized according to the orientation of grains. The plastic deformation effect has been substantially vanished after 150 μm (Figure 4). Also the reduced elatic modulus has been investigated by means of indentation-depth curves and Oliver & Pharr mathematical approach . Nanocrystalline layer with high hardness leads to increase reduced modulus through the interior of 40 μm.
In this study, AISI 1017 low carbon steel was exposed to SSP with an Almen intensity of 35 mmA. The effect severe plastic deformation created by SSP were analyzed via optical microscopy, SEM, HRTEM and nanoindentation tests. Following results can be drawn according to the practical approaches.
Optical microscope and SEM observations presents the deformed and oriented layer which has so much distinctions from the core structure on and right below the surface. The thickness of the layer can be identified approximately 40 μm.
According to the HRTEM results, SSP forms nanocrystalline layer with the grain size of 40 nm to 400 nm. The distribution of the grains and also grain sizes can be assessed as non-uniform and non-homogenous. The HRTEM approach reveals the formation of nanocrystalline layer is directly related with dislocation movement, interactions and then sub-grain formation. In the figures highly dislocation densed grains and subgrains are distinguished.
Nanocrystalline layer makes the structure so hard with compared to interior. Moreover, reduced modulus on the surface is also improved. The change in elastic modulus and hardness could pave the way for particularly tensile strength.
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