Effect of Shot Peening on Residual Stress and Microstructure in the Deformed Layer of Inconel 625
2017 Volume 58 Issue 2 Pages 164-166
Details
2017 Volume 58 Issue 2 Pages 164-166
Effect of shot peening on residual stress and microstructure in the deformed layer of Inconel 625 was investigated. Residual stress and microstructure including the domain size, micro strain and dislocation density was characterized by X-Ray diffraction method (XRD). The results showed that shot peening (SP) can significantly improve the compressive residual stress and micro hardness. Moreover, microstructure evaluation revealed that smaller domains were refined and the higher micro strain was induced by shot peening process. Thus a high dislocation density in the near-surface of the deformed layers was induced. Dual shot peening process could optimize the distribution of both compressive residual stress and the microstructure, which will help with the material fatigue and corrosion resistance.
Currently, shot peening process has been widely used to improve micro hardness, stress corrosion and fatigue resistance of materials. During shot peening process, high hardness small balls impact a surface of a material, which introduces high compressive residual stresses and cold work at the surface and subsurface. Usually, as a result, the microstructure is transformed generating smaller domain size and higher micro strain. The depth of the compressive residual stress layer and the finer microstructure can improve the mechanical properties of the material1–5).
Inconel 625 has been well known and widely used in different industries such as: aerospace, chemistry, nuclear power and others, thanks to its superior thermal, fatigue and creep properties6). Many studies were more tuned towards the study of its mechanical properties and few of them relates to the residual stresses with microstructure in the deformed layers after shot peening. Therefore, we especially planned to study the effect of shot peening on residual stress and microstructure in the deformed layer of Inconel 625. Details will be presented in this study.
Inconel 625 samples were cut then polished from a hot rolled slab after 1080℃ solution treatment, which was provided by Shanghai Baosteel group corporation. The dimensions of the samples are 20 mm × 15 mm × 5 mm, and the chemical composition is 60.69Ni, 21.26Cr, 5.61Fe, 0.02Mn, 0.02Si, 0.03C, 3.69Nb, 0.19Al, 0.3Ti, 8.36Mo, 0.002S and 0.01Co (all in mass%).
The samples were SP with 100% coverage by a shot-peening equipment in Shanghai Carthing Machinery corporation. Type A Almen specimen was selected to measure the peening intensity. Cast steel balls (610 HV, 0.6 mm round) and ceramic balls (700 HV, 0.3 mm round) were chosen as shot media to complete both traditional SP and dual SP processes in this work. The peening intensity was a 0.2 mmA (ceramic ball), 0.25 mmA, 0.35 mmA, 0.45 mmA and 0.45 + 0.2 mmA respectively. The difference between traditional SP and dual SP process is that the peened sample (0.45 mmA, cast steel balls) was treated one more time by ceramic balls (0.2 mmA) to strengthen the effect of shot peening.
To characterize the residual stress gradient, microstructure and micro hardness in the deformed layer, each 25 um thin layer was electrolytically polished step by step from the top surface to a depth of 100 um, subsequently a 50 um layer was removed from 100 um to 600 um at ambient temperature.
Residual stresses were measured using a Proto LXRD instrument employing d-sin2ψ technique7). The measurement parameters were chosen as follows: (311) hkl plane, Wavelength, Mn Kalpha for Mn anode, λ = 2.10314 Å.
Subsequently, an X-ray diffractometer (Rigaku Ultima IV, Japan) was used to collect the XRD patterns (Cu Ka target, λ = 1.54056 Å). The speed of scanning is 2°/min, and the step is 0.01°. The measured line profile h(x) can be expressed as8):
| \[h(x) = \int_{-\infty}^{+\infty} g(y)f(x - y)dy\] | (1) |
The integral breadth β is shown as following, Voigt method9):
| \[\beta_C^h = \beta_C^f + \beta_C^g,\quad \beta_G^{h^2} = \beta_G^{f^2} + \beta_G^{g^2}\] | (2) |
In this equation above, G and C means Gaussian and Cauchy integral breadth. The measured line profile, the structural profile and the instrumental profile were presented by h, f and g, respectively.
Accordingly, domain size and micro strain can be calculated by Cauchy and Gaussian integral breadth from f profile. The equation is:
| \[D = \lambda/\beta_C^f \cos (\theta),\quad \varepsilon = \beta_G^f/4\tan (\theta)\] | (3) |
Where $\theta$ is the diffraction angle, and $\lambda$ is the X-ray wavelength. After domain size and micro strain was determined, dislocation density $\rho$ could be calculated via Williamson method9):
| \[\rho = \frac{2\sqrt{3}}{\left| \vec{\mathbf{b}} \right|} \cdot \frac{\left\langle \varepsilon^2 \right\rangle^{1/2}}{D}\] | (4) |
The distribution of micro hardness in the deformed layer was investigated through DHV-1000, a Digital Micro-hardness Tester (2.9 N, 15 s). Each data came from the average of five measurements, which were performed at different locations on the surface after layers removal.
The XRD patterns were scanned on top surface of Inconel 625 samples before and after SP process was applied. On Fig. 1, it shows high level peak breadth associated with high level of plastic deformation and cold-work. The results also indicate that the microstructure was modified in the deformed layers, which is mainly related to the refined coherent domain sizes and micro strain. In addition, the patterns show the typical peaks of austenite phase, which means that no new phase was generated after SP process.
XRD patterns of the Inconel 625 before and after shot peening.
Figure 2 reveals high compressive residual stresses (>700 MPa), which were introduced in the deformed layers. The maximum compressive residual stress (MCRS) appeared in the near-surface layers (<100 um), and MCRS location depends on the different peening intensity levels. After a thin layer removed one by one, the compressive residual stress relaxed sharply from depth of 100 um to 600 um. MCRS of ceramic ball peened sample (0.2 mmA) is almost as high as cast steel ball peened one (0.45 mmA), however, it relaxed much faster. The high MCRS is attributed to the higher micro hardness (700 HV) and better roundness of the ceramic balls. Dual SP process resulted to higher MCRS, deeper MCRS location, and slower relaxation rate in the deformed layers, which was indicated from the comparison of various peened samples. The reason could be the second step SP process (0.2 mmA, ceramic ball) which increased the plastic deformation levels.
Compressive residual stress gradients of shot peened Inconel 625.
The high speed balls impact on the surface of samples causes a drastic increase of plastic deformation in the near-surface layer. When comparing the inner layer, the deformation in the surface layer is more difficult to be recovered. The non-uniform plastic deformation results in the high compressive residual stress gradient.
The distribution of domain size is shown in Fig. 3. It indicates a noticeable upward trend of the domain size evolution, which is directly related on both the depth and shot peening intensities. At the near-surface layer up to <100 um, where the domain size is about 20 nm. At a given depth level, the domain size increases with the decrease of the peening intensity. Typically, the significant changes can be seen from the comparison between the traditional peened samples 0.2 mmA and 0.45 mmA. The domain size is 130 nm and 45 nm at 350 um, respectively. However, bigger domain sizes were found in the deeper layers, which are close to the original size measured in Inconel 625 matrix (140 nm). The effect of dual SP process can also be seen from the smaller domains in the near-surface layers.
Variation of domain size from surface to depth of the deformed layer after shot peening.
The similar distribution of micro strain and dislocation density in the deformed layers were calculated and shown in Fig. 4 and Fig. 5. High micro strains were induced after SP process in the near-surface layers. It dropped down quickly from the top surface layer to 400 um, and then slowly relaxed from 400 um to 600 um. At the same depth, the stronger peening intensity caused higher micro strain. Moreover, dual SP process (0.45 + 0.2 mmA) induced both the smaller domain size and higher micro strain than traditional SP (0.45 mmA) measured in the deformed layers. According to the previous research10,11), micro strain mainly related to the dislocations density in the material. The calculated distribution of dislocation density showed a similar trend of micro strain in the deformed layers.
Variation of micro strain from surface to depth of the deformed layer after shot peening.
Variation of dislocation density from surface to depth of the deformed layer after shot peening.
Figure 6 indicates a significant improvement of micro hardness in the deformed layers after shot peening. The maximum micro hardness is about 600 HV on the top surface layer. However, the levels are decreasing to about 200 HV in the subsurface layers. The trend of relaxation is similar to micro strain and dislocation density. The reason could be related to the microstructure modification during the shot peening.
Variation of micro hardness size from surface to depth of the deformed layer after shot peening.
The effect of shot peening including residual stress and microstructure improved the mechanical properties in the deformed layers of Inconel 625. High compressive residual stresses increased the fatigue property because it delays or stops the initiation and propagation of micro-cracks12,13). On the other hand, fine domain size and high dislocation density slowed down the dislocation gliding, to lead to an improvement on yield strength14,15), which was mentioned above.
Effect of shot peening on residual stress and microstructure in the deformed layer of Inconel 625 was investigated. It is revealed that high compressive residual stresses were introduced at the deformed layers. MCRS appeared in the near-surface layer then relaxed at depth. Higher shot peening intensity caused higher compressive residual stresses, deeper location of MCRS, and the slower stress relaxation rate. In terms of microstructure, the refined coherent domains and high micro strain increased the dislocation density level and slowed down the dislocation gliding, which caused structure reinforcement in the deformed layers after SP process. Dual SP can improve the compressive residual stress gradient and enhance the effect of microstructure reinforcement, which caused the further increment of micro hardness. Results indicate dual SP process is an essential way to effectively improve the surface properties of Inconel 625.
We thank Baosteel, Carthing, and Proto to support this work.
