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Effect of strain rate on nanocrystalline surface formation in controlled ball impact process in AISI 304 SS
Materials Letters 62 (2008) 4516–4518
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effect of strain rate on nanocrystalline surface formation in controlled ball impact process in AISI 304 SS M. Kodeeswaran, R. Gnanamoorthy ⁎ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e
i n f o
Article history: Received 19 May 2008 Accepted 13 August 2008 Available online 22 August 2008 Keywords: Nanostructure Surface Strain rate Controlled impact Stainless steel
a b s t r a c t Strain rate during severe plastic forming processes affects the grain size formation. Nanocrystalline grain size formation at and near the surface in stainless steel, AISI304, during the controlled ball impact process carried out at a high strain rate are reported. High strain rate exerted by using a small diameter ball resulted in the formation of grains of size ~ 80 nm on the surface. The peening coverage and surface roughness depend on the sample traveling velocity for a given ball diameter and impact velocity. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Performance of machine components experiencing fatigue loads is improved by introducing compressive residual stresses using mechanical surface modification processes such as shot peening, liquid peening and rolling. The grain size refinement at and near the surface is another approach by which the surface strength and fatigue performance can be improved. In recent years, attempts are made to refine the surface grains using severe plastic deformation processes such as ball milling [1–3], high pressure torsion [4], equal channel angular pressing [4], shot peening [5,6], ball drop [7,8] and ultrasonic shot peening [9,10]. Controlled ball impact process [11] is recently developed to create the nanocrystalline surface layer in stainless steel by exerting a high strain rate. Controlled ball impact treatment carried out using a ball of 3.17 mm diameter at the average strain rate of 1970 s− 1at different sample traveling velocities [11] resulted in the formation of nanocrystalline grain size. This paper reports the characteristics of nanocrystalline surface layer and surface roughness of stainless steel, AISI304, samples treated with the controlled ball impact technique at a high strain rate, 2860 s− 1, and at different sample traveling velocities. 2. Test materials and processing Stainless steel, AISI 304, samples of dimension 25 mm× 10 mm× 5 mm were carefully cut from the hot rolled, annealed and pickled plates. The average grain size (~20 µm) of the as-annealed sample was quantified from the metallographically prepared sample. Samples for ⁎ Corresponding author. E-mail address: [email protected] (R. Gnanamoorthy). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.08.021
surface treatment were carefully ground, and polished using emery sheet followed by diamond polishing. The centerline average surface roughness (Ra) of the polished sample is 0.05 µm. Controlled ball impact treatment was carried out using a specially designed system with a high carbon and high chromium hardened steel ball of 2 mm diameter at a ball impact velocity of 0.85 m/s. The strain rate calculated based on the analytical model described in Reference 11, is 2860 s− 1. Controlled impacts were carried out at different sample traveling velocities, 0.51, 0.76 and 1.02 mm/s. These traveling velocities were selected to attain different treatment intensity and coverage on the treated material. The sample was moved perpendicular to the treatment direction (y-axis) in steps of 0.0254 mm after each treatment pass (x-axis) using a computer controlled motorized system. X-ray diffractometer (Shimadzu, XD-D1) with cobalt radiation was used to quantify the grain size and micro-strain accumulated. The wavelength of the X-ray used is 0.1789 nm. Scanning was carried out at 0.02° per second for the angle ranges of 48.5° to 53.5° and 57.5° to 62.5°. The average grain size and micro-strain were calculated from the line broadening of Bragg diffraction peaks using Scherrer method [12]. Surface roughness, both in the treatment direction and perpendicular to the treatment direction, was measured using a diamond tipped Perthometer. 3. Results and discussion The treated surfaces observed using optical micrographs reveal the uniform treatment on the target surface (Fig. 1). The impact coverage is estimated analytically by drawing impacting diameters at regular intervals, which is calculated from the sample traveling velocity. The impact coverage expressed in percentage indicates the intensity of subsequent overlapping impacts occurring in a particular area. The impact coverage at different sample velocities is given in Table 1. Increase in the sample traveling velocity results in a reduced coverage. The sample treated with a ball of 2 mm diameter at 0.51 mm/s gets more impact coverage of 838% compared to 508% impact coverage for
M. Kodeeswaran, R. Gnanamoorthy / Materials Letters 62 (2008) 4516–4518
4517
Fig. 2. X-ray diffraction (XRD) profiles of the as-annealed and treated samples.
Fig. 3. Grain size and micro-strain of the samples treated.
investigation results in the formation of fine grain size. Observation using transmission electron microscope (TEM) earlier reported also revealed the formation nanostructured surface layer with the mean grain size of 10 nm [11]. Fig. 3 shows the variation of grain size and micro-strain (%) of the samples treated at different sample traveling velocities. The sample traveling velocity, which decides the number of overlapping impacts, influences the grain size formation by altering the local heat generation, effective strain and strain rate induced due to subsequent overlapping impacts. Treatment with a sample traveling velocity of 0.51 mm/s results in an increased number of overlaps (~ 14) compared to other samples which experience a less
Fig. 1. Optical micrographs of the samples treated at a sample traveling velocity of (a) 0.51 mm/s, (b) 0.76 mm/s and (c) 1.02 mm/s.
the sample treated at 1.02 mm/s. Similar observations were reported when peened using a ball with a diameter of 3.17 mm [11]. Observations of the treated surface reveal the formation of homogeneous surface (Fig. 1). X-ray diffraction (XRD) profiles of the untreated and treated samples are shown in Fig. 2. The average grain size and micro-strain of surface layer calculated using Scherrer method [12] revealed the formation of fine grains of size ~ 80 nm with a micro-strain of 0.27%. Controlled ball impact at a strain rate of 1970 s− 1 resulted in a high grain size, ~ 210 nm and a micro-strain of 0.16% [11] indicating the high strain rate in the current
Table 1 Coverage due to subsequent impacts for samples treated at different traveling velocities Sample traveling velocity (mm/s)
Coverage(%)
0.51 0.76 1.02
838 631 508
Fig. 4. Centerline average surface roughness (Ra) measured on samples treated at different sample traveling velocities.
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M. Kodeeswaran, R. Gnanamoorthy / Materials Letters 62 (2008) 4516–4518
number of overlaps (less than 8). Increased number of overlapping impacts result in the formation of fine grain size. The micro-strain induced on the sample also depends on the sample traveling velocity. More number of overlapping impacts results in low micro-strain. Liu et al [5] reported that the strain and strain rate increases when the second particle hits on the extruded ridge around the edge of pit formed by first impact. High micro-strain in the treated samples in the current study also confirms the high strain rate (2860 s− 1) experienced compared the previous study (1970 s− 1) [11]. Surface roughness measured on mutually perpendicular directions is nearly the same indicating the treatment is uniform and homogenous. Fig. 4 shows the centerline average surface roughness (Ra) measured on samples treated at different sample traveling velocities. A marginal increase in the roughness was observed with increasing sample-traveling velocity. Low sample traveling velocity is preferred to create a smoother surface. Fine grain size with good surface finish can be obtained by properly selecting the strain rate (which depends on ball diameter, impact velocity of ball and property of the sample) and sample traveling velocity.
4. Conclusions High strain rate controlled ball impact process creates nanocrystalline grains on the surface of AISI304 stainless steel samples. The grain size depends on the sample traveling velocity, which decides the
impact coverage. The surface roughness of the samples treated depends on the strain rate exerted and sample traveling velocity. References [1] Umemoto M, Liu ZG, Masuyama K, Hao XJ, Tsuchiya K. Scr Mater 2001;44:1741–5. [2] Liu ZG, Hao XJ, Masuyama K, Tsuchiya K, Umemoto M, Hao SM. Scr Mater 2001;44:1775–9. [3] Jang JSC, Koch CC. Scripta Metall Mater 1990;24:1559–604. [4] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mat Sci 2000;45(2):103–89. [5] Liu JL, Umemoto M, Todaka Y, Tsuchiya K. J Mater Sci 2007;42:7716–20. [6] Todaka Yoshikazu, Minoru Umemoto, Koichi Tsuchiya. Mater Trans 2004;45 (2):376–9. [7] Umemoto M, Huang B, Tsuchiya K, Suzuki N. Scr Mater 2002;46:383–8. [8] Umemoto M, Todaka K, Tsuchiya K. Mater Sci and Eng A 2004;899:375–7. [9] Tao NR, Sui ML, Lu J, Lu K. Nanostruc Mater 1999;11:433–40. [10] Zhang HW, Hei ZK, Liu G, Lu J, Lu K. Acta Mater 2003;51:1871–81. [11] Kodeeswaran M, Gnanamoorthy R. (Submitted for publication to Surface and Coatings Technology). [12] Cullity BD. Elements of X-ray Diffraction. 2nd ed. Massachusetts: Addison-Wesley; 1978.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Effect of strain rate on nanocrystalline surface formation in controlled ball impact process in AISI 304 SS M. Kodeeswaran, R. Gnanamoorthy ⁎ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e
i n f o
Article history: Received 19 May 2008 Accepted 13 August 2008 Available online 22 August 2008 Keywords: Nanostructure Surface Strain rate Controlled impact Stainless steel
a b s t r a c t Strain rate during severe plastic forming processes affects the grain size formation. Nanocrystalline grain size formation at and near the surface in stainless steel, AISI304, during the controlled ball impact process carried out at a high strain rate are reported. High strain rate exerted by using a small diameter ball resulted in the formation of grains of size ~ 80 nm on the surface. The peening coverage and surface roughness depend on the sample traveling velocity for a given ball diameter and impact velocity. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Performance of machine components experiencing fatigue loads is improved by introducing compressive residual stresses using mechanical surface modification processes such as shot peening, liquid peening and rolling. The grain size refinement at and near the surface is another approach by which the surface strength and fatigue performance can be improved. In recent years, attempts are made to refine the surface grains using severe plastic deformation processes such as ball milling [1–3], high pressure torsion [4], equal channel angular pressing [4], shot peening [5,6], ball drop [7,8] and ultrasonic shot peening [9,10]. Controlled ball impact process [11] is recently developed to create the nanocrystalline surface layer in stainless steel by exerting a high strain rate. Controlled ball impact treatment carried out using a ball of 3.17 mm diameter at the average strain rate of 1970 s− 1at different sample traveling velocities [11] resulted in the formation of nanocrystalline grain size. This paper reports the characteristics of nanocrystalline surface layer and surface roughness of stainless steel, AISI304, samples treated with the controlled ball impact technique at a high strain rate, 2860 s− 1, and at different sample traveling velocities. 2. Test materials and processing Stainless steel, AISI 304, samples of dimension 25 mm× 10 mm× 5 mm were carefully cut from the hot rolled, annealed and pickled plates. The average grain size (~20 µm) of the as-annealed sample was quantified from the metallographically prepared sample. Samples for ⁎ Corresponding author. E-mail address: [email protected] (R. Gnanamoorthy). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.08.021
surface treatment were carefully ground, and polished using emery sheet followed by diamond polishing. The centerline average surface roughness (Ra) of the polished sample is 0.05 µm. Controlled ball impact treatment was carried out using a specially designed system with a high carbon and high chromium hardened steel ball of 2 mm diameter at a ball impact velocity of 0.85 m/s. The strain rate calculated based on the analytical model described in Reference 11, is 2860 s− 1. Controlled impacts were carried out at different sample traveling velocities, 0.51, 0.76 and 1.02 mm/s. These traveling velocities were selected to attain different treatment intensity and coverage on the treated material. The sample was moved perpendicular to the treatment direction (y-axis) in steps of 0.0254 mm after each treatment pass (x-axis) using a computer controlled motorized system. X-ray diffractometer (Shimadzu, XD-D1) with cobalt radiation was used to quantify the grain size and micro-strain accumulated. The wavelength of the X-ray used is 0.1789 nm. Scanning was carried out at 0.02° per second for the angle ranges of 48.5° to 53.5° and 57.5° to 62.5°. The average grain size and micro-strain were calculated from the line broadening of Bragg diffraction peaks using Scherrer method [12]. Surface roughness, both in the treatment direction and perpendicular to the treatment direction, was measured using a diamond tipped Perthometer. 3. Results and discussion The treated surfaces observed using optical micrographs reveal the uniform treatment on the target surface (Fig. 1). The impact coverage is estimated analytically by drawing impacting diameters at regular intervals, which is calculated from the sample traveling velocity. The impact coverage expressed in percentage indicates the intensity of subsequent overlapping impacts occurring in a particular area. The impact coverage at different sample velocities is given in Table 1. Increase in the sample traveling velocity results in a reduced coverage. The sample treated with a ball of 2 mm diameter at 0.51 mm/s gets more impact coverage of 838% compared to 508% impact coverage for
M. Kodeeswaran, R. Gnanamoorthy / Materials Letters 62 (2008) 4516–4518
4517
Fig. 2. X-ray diffraction (XRD) profiles of the as-annealed and treated samples.
Fig. 3. Grain size and micro-strain of the samples treated.
investigation results in the formation of fine grain size. Observation using transmission electron microscope (TEM) earlier reported also revealed the formation nanostructured surface layer with the mean grain size of 10 nm [11]. Fig. 3 shows the variation of grain size and micro-strain (%) of the samples treated at different sample traveling velocities. The sample traveling velocity, which decides the number of overlapping impacts, influences the grain size formation by altering the local heat generation, effective strain and strain rate induced due to subsequent overlapping impacts. Treatment with a sample traveling velocity of 0.51 mm/s results in an increased number of overlaps (~ 14) compared to other samples which experience a less
Fig. 1. Optical micrographs of the samples treated at a sample traveling velocity of (a) 0.51 mm/s, (b) 0.76 mm/s and (c) 1.02 mm/s.
the sample treated at 1.02 mm/s. Similar observations were reported when peened using a ball with a diameter of 3.17 mm [11]. Observations of the treated surface reveal the formation of homogeneous surface (Fig. 1). X-ray diffraction (XRD) profiles of the untreated and treated samples are shown in Fig. 2. The average grain size and micro-strain of surface layer calculated using Scherrer method [12] revealed the formation of fine grains of size ~ 80 nm with a micro-strain of 0.27%. Controlled ball impact at a strain rate of 1970 s− 1 resulted in a high grain size, ~ 210 nm and a micro-strain of 0.16% [11] indicating the high strain rate in the current
Table 1 Coverage due to subsequent impacts for samples treated at different traveling velocities Sample traveling velocity (mm/s)
Coverage(%)
0.51 0.76 1.02
838 631 508
Fig. 4. Centerline average surface roughness (Ra) measured on samples treated at different sample traveling velocities.
4518
M. Kodeeswaran, R. Gnanamoorthy / Materials Letters 62 (2008) 4516–4518
number of overlaps (less than 8). Increased number of overlapping impacts result in the formation of fine grain size. The micro-strain induced on the sample also depends on the sample traveling velocity. More number of overlapping impacts results in low micro-strain. Liu et al [5] reported that the strain and strain rate increases when the second particle hits on the extruded ridge around the edge of pit formed by first impact. High micro-strain in the treated samples in the current study also confirms the high strain rate (2860 s− 1) experienced compared the previous study (1970 s− 1) [11]. Surface roughness measured on mutually perpendicular directions is nearly the same indicating the treatment is uniform and homogenous. Fig. 4 shows the centerline average surface roughness (Ra) measured on samples treated at different sample traveling velocities. A marginal increase in the roughness was observed with increasing sample-traveling velocity. Low sample traveling velocity is preferred to create a smoother surface. Fine grain size with good surface finish can be obtained by properly selecting the strain rate (which depends on ball diameter, impact velocity of ball and property of the sample) and sample traveling velocity.
4. Conclusions High strain rate controlled ball impact process creates nanocrystalline grains on the surface of AISI304 stainless steel samples. The grain size depends on the sample traveling velocity, which decides the
impact coverage. The surface roughness of the samples treated depends on the strain rate exerted and sample traveling velocity. References [1] Umemoto M, Liu ZG, Masuyama K, Hao XJ, Tsuchiya K. Scr Mater 2001;44:1741–5. [2] Liu ZG, Hao XJ, Masuyama K, Tsuchiya K, Umemoto M, Hao SM. Scr Mater 2001;44:1775–9. [3] Jang JSC, Koch CC. Scripta Metall Mater 1990;24:1559–604. [4] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mat Sci 2000;45(2):103–89. [5] Liu JL, Umemoto M, Todaka Y, Tsuchiya K. J Mater Sci 2007;42:7716–20. [6] Todaka Yoshikazu, Minoru Umemoto, Koichi Tsuchiya. Mater Trans 2004;45 (2):376–9. [7] Umemoto M, Huang B, Tsuchiya K, Suzuki N. Scr Mater 2002;46:383–8. [8] Umemoto M, Todaka K, Tsuchiya K. Mater Sci and Eng A 2004;899:375–7. [9] Tao NR, Sui ML, Lu J, Lu K. Nanostruc Mater 1999;11:433–40. [10] Zhang HW, Hei ZK, Liu G, Lu J, Lu K. Acta Mater 2003;51:1871–81. [11] Kodeeswaran M, Gnanamoorthy R. (Submitted for publication to Surface and Coatings Technology). [12] Cullity BD. Elements of X-ray Diffraction. 2nd ed. Massachusetts: Addison-Wesley; 1978.