- Email: [email protected]
Dynamic recrystallization in friction surfaced austenitic stainless steel coatings
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Dynamic recrystallization in friction surfaced austenitic stainless steel coatings Ramesh Puli⁎, G.D. Janaki Ram Materials Joining Laboratory, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
AR TIC LE D ATA
ABSTR ACT
Article history:
Friction surfacing involves complex thermo-mechanical phenomena. In this study, the
Received 14 July 2012
nature of dynamic recrystallization in friction surfaced austenitic stainless steel AISI 316L
Received in revised form
coatings was investigated using electron backscattered diffraction and transmission
29 August 2012
electron microscopy. The results show that the alloy 316L undergoes discontinuous
Accepted 1 September 2012
dynamic recrystallization under conditions of moderate Zener–Hollomon parameter during friction surfacing.
Keywords:
© 2012 Elsevier Inc. All rights reserved.
Friction surfacing Austenitic stainless steel Dynamic recrystallization EBSD Electron microscopy
1.
Introduction
Friction surfacing is a solid-state process with a strong industrial potential for depositing corrosion and wear resistant coatings [1]. In this process, the material to be deposited is taken in the form of a rod, which gets consumed during the process. The substrate is firmly clamped to an anvil. The consumable rod (mechtrode) is rotated, under constant axial force, against the substrate. Friction between the mechtrode and the substrate results in intense localized heating, causing significant softening and plastic deformation of the mechtrode material. After a short dwell time, the substrate is made to move. As this happens, hot plasticized metal from the mechtrode gets deposited on the substrate, resulting in a metallurgically bonded coating along the line of traverse. Being a solid-state coating process, friction surfacing offers several advantages (such as zero dilution, absence of solidification cracking, and lesser opportunity for
⁎ Corresponding author. Tel.: + 91 44 22574780; fax: +91 44 22576780. E-mail address: [email protected] (R. Puli). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.09.001
brittle intermetallic formation) over conventional weld overlay processes [1]. There have been quite a few recent studies on friction surfacing. Most of these investigations deal with optimization of process parameters [2–4] and evaluation of coating microstructures and properties [5–7]. The thermo-mechanical phenomena involved in the process have been investigated in some detail [8–12] and attempts have also been made to model the process [13,14]. Friction surfaced coatings invariably show very fine equiaxed grains, notwithstanding the coarse grain size of the consumable rod material. For example, Rafi et al. [15] reported a grain size of less than 10 μm in AISI H13 friction surfaced coatings. While the formation of fine equiaxed grains in friction surfaced coatings is often attributed to dynamic recrystallization, detailed studies in this regard are not available. In the present work, electron backscattered diffraction (EBSD) and transmission electron
50
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
microscopy (TEM) studies were carried out on friction surfaced AISI 316L stainless steel coatings to gain greater insights into the mechanisms of hot restoration during friction surfacing.
2.
Experimental Details
Austenitic stainless steel AISI 316L rods (chemical composition in wt.%: Fe–0.015C–18Cr–10Ni–2.2Mo–1.6Mn–0.35Si–0.03S–0.03P, melting temperature range: 1375 °C to 1400 °C) of 18 mm diameter in hot rolled and annealed condition were used for friction surfacing. A mild steel (Fe–0.12C–0.4Mn–0.01S–0.02P, melting temperature range: 1325 °C to 1400 °C) plate of dimensions 200×150×10 mm was used as the substrate. Friction surfaced coatings of 1.2 mm thickness and 16 mm wide were produced using a commercially available friction surfacing machine. The process parameters used were: 39 MPa axial pressure, 1200 rpm spindle rotation speed and 3 mm/s substrate traverse speed. The dwell time used was 30 s. Infrared thermography was used to determine the peak temperature at the interface between the rotating consumable rod and the substrate (during the initial dwell period). An infrared camera (a CEDIP JADE mercury cadmium telluride camera commercially available from Flir System, Croissy-Beaubourg, France) with a mean noise equivalent temperature difference of 20 mK was used for this purpose. The camera operates in the long wave infrared band (7.9 to 9.7 μm) with a focal plane array of 320×240 detectors (each detector is a pixel of 25 μm size) and a pixel pitch of 30 μm. The infrared camera was calibrated as per standard practice. Transverse sections from friction surfaced coatings were prepared for electron backscattered diffraction studies by electropolishing at 12 V in a solution consisting of 20% perchloric acid and 80% methanol, maintained at −10 °C. Electron backscattered diffraction studies were conducted using an FEI Quanta-200 scanning electron microscope. The scan area was 200 ×200 μm at a step size 0.5 μm. TSL-OIM software was used for electron backscattered diffraction analysis. The grain area (diameter) method was used for grain size determination. These studies were also performed on 316L consumable rods for comparison. For transmission electron microscopy studies, 3 mm disks (0.5 mm thick) were obtained from the middle of the coating thickness. These disks were first mechanically polished to about 100 μm and were then further thinned by electrolytic jet polishing on a Struers Tenupol-5 twin jet polisher at 12 V, and −30 °C in a 20% perchloric acid solution. Transmission electron microscopy analysis was carried out using a Philips CM-12 microscope operated at 120 kV.
3.
Results and Discussion
The inverse pole figure (IPF) map of the AISI 316L consumable rod in as-received condition is shown in Fig. 1a. The average grain size of the consumable rod was approximately 61± 22 μm. As can be seen, the consumable rod contained quite a few annealing twins and the grain boundaries were mainly high angle in character. There were a few low angle grain boundaries, but the consumable rod material contained no subgrain boundaries (Fig. 1b).
Fig. 1 – EBSD results of AISI 316L consumable rod: (a) IPF map, (b) grain boundary misorientation distribution plot. In the case of friction surfaced coatings, EBSD scans were conducted at three different locations on the coating cross-section: near the coating top surface, at the middle of the coating thickness and near the coating/substrate interface. Fig. 2 shows the IPF maps corresponding to these three locations. While the grains were equiaxed over the entire coating thickness, EBSD studies revealed some differences in the grain size across the coating thickness — the average grain size increased from 4.8 ± 1.5 μm near the coating/substrate interface to 6.4 ± 2.3 μm at the middle of the coating thickness to 9.4 ± 3.2 μm near the coating top surface. However, the grain size distribution was more or less homogeneous within a given scan region. More importantly, however, the grain size over the entire coating thickness is much finer compared to the grain size of the consumable rod. Fig. 3 shows the grain boundary misorientation map, grain boundary misorientation plot and IPF texture plot obtained from an EBSD scan done near the coating top surface. In these respects, EBSD scans conducted near the coating/substrate interface and at the middle of the coating thickness yielded qualitatively similar information. The coating consisted of
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
Fig. 2 – IPF maps of the 316L friction surfaced coating: (a) near the coating top surface, (b) at the middle of the coating thickness, and (c) near the coating/substrate interface.
51
Fig. 3 – Grain boundary misorientation map (a), grain boundary misorientation plot (b), and IPF texture plot (c) of the 316L friction surfaced coating.
52
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
mainly high angle grain boundaries along with some low angle grain boundaries (Fig. 3a and b). Many of the grains showed a necklace of much smaller subgrains along their boundaries. This can be clearly seen in Fig. 3a inset. The grains in the coating were randomly oriented and the coating did not develop any texture, as can be seen from Fig. 3c. Thin foil samples prepared from 316L friction surfaced coatings were examined under TEM. The microstructures observed in different regions of the sample are shown in Fig. 4. Fig. 4a shows a well-developed cell, while Fig. 4b shows a developing cell boundary, cutting across the stacking faults of a recrystallizing grain. Fig. 4c shows a few subgrains necklacing a prior grain boundary. Similarly, the dislocation density was observed to be low in some grains, while it was high in other grains (Fig. 4d). These observations reflect the spatial and temporal heterogeneity of microstructural evolution during dynamic restoration. EBSD and TEM observations clearly show that the coating material has undergone dynamic recrystallization. It is well known that the nature of restoration during hot deformation depends on the stacking fault energy of the material. In low stacking fault energy materials, such as the austenitic stainless steel, dynamic recrystallization is the most important hot restoration mechanism [16]. In such materials, the process of dynamic recrystallization, often referred to as discontinuous dynamic recrystallization (DDR), involves distinct nucleation and growth stages. Upon reaching a critical deformation condition, new grains originate at the prior grain boundaries and they begin to grow. These grains, however, quickly cease to grow due to increased dislocation density with continued deformation (causing a reduction in the driving force for further growth). Subsequently, more new grains nucleate at the migrating boundaries. Overall, as these processes continue to
occur successively, there will be a gradual thickening of the band of recrystallized grains, eventually leading to complete recrystallization. The process of DDR usually leaves the following signatures [16]: (i) developing cells/subgrains, and (ii) necklacing of subgrains on prior grain boundaries. These features were clearly seen in the current study, as shown in Figs. 3 and 4. Further, as can be seen from Fig. 3, the coating microstructure showed no loose high angle grain boundaries inside the grains and contained very few low angle grain boundaries with misorientation in the range of 10–15°. Therefore, it can be concluded that continuous dynamic recrystallization (of the type involving progressive lattice rotation) is not significant in alloy 316L friction surfaced coatings [17]. Friction surfacing involves severe plastic deformation at high temperatures. The thermo-mechanical phenomena involved in the process are yet to be fully understood. However, the process can be expected to involve, as in the case of friction stir welding/ processing, transients and gradients in strain, strain rate and temperature [18]. The strain rates involved in friction surfacing can be very high and are difficult to experimentally determine. A reasonable estimate of the strain rate involved in friction surfacing can be obtained from the following equation, which was utilized in earlier investigations for estimating the strain rates in friction stir welding/processing [19] and high pressure torsion processes [20]: ε_ ¼
Rm :2πre Le
ð1Þ
where, ε_ is the strain rate, Rm is the average material flow rate (half of the mechtrode rotational speed in revolutions per second (rps) in the present case), re is the effective radius of dynamically recrystallized zone (half of the coating width in the present case), and Le is the effective depth of dynamically recrystallized zone
Fig. 4 – TEM micrographs of friction surfaced coatings showing (a) a well-developed cell, (b) a developing cell boundary, running across the stacking faults, (c) necklace formation (arrows show DRX grains at a prior grain boundary), and (d) grains with different dislocation densities (annotations 1, 2 and 3 mark grains with low, medium and high dislocation densities, respectively).
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
53
relationship is not available in published literature for alloy 316L, it is unlikely to be very different. On this assumption, one can arrive at an estimated average dynamically recrystallized grain size of 3.2 μm in friction surfaced 316L coatings. This is reasonably close to the observed grain size in the coating close to the coating/substrate interface (4.8 ± 1.5 μm). The increase in grain size towards the coating top surface is expected to be a result of reduced cooling rate from the coating/substrate interface to the coating top surface (cooling is mainly due to heat conduction through the substrate). It should be noted that the initial grain size can influence the final dynamically recrystallized grain size. This should be investigated in future work using consumable rods of different grain sizes.
4. Fig. 5 – Temperature close to the rotating mechtrode/ substrate interface as a function of time during the initial dwell period of friction surfacing.
(thickness of the coating in the present case). Taking the following values – Rm =10 rps, re =8 mm, and Le =1.2 mm – a strain rate of 418 s−1 can be obtained in friction surfacing. The temperature of deformation in friction surfacing is another parameter that needs to be considered. Earlier studies have shown that the peak temperatures involved in friction surfacing are above the 0.8 homologous temperature of the mechtrode material, although there has been considerable variation in the reported peak temperatures by various investigators [10,12,21,22]. Finite difference modeling studies by Liu et al. [10] show that the temperatures along the mechtrode radius would be more or less the same once the process attains steady state. In the current study, a peak temperature of 1200 °C (1473 K) was measured using infrared thermography at the rubbing end of the rotating mechtrode during the initial dwell period (Fig. 5). The temperature at the rub interface is expected to be more or less the same during the entire deposition process [12]. The strain rate and the temperature of deformation can be integrated into a single parameter, known as the Zener– Hollomon parameter (Z). It relates the combined effect of strain rate and temperature to the microstructure development during hot deformation and is defined as: Q Z ¼ ε_ exp RT
ð2Þ
where, Q is the activation energy for hot deformation (460 kJ/mol for alloy 316L for a mean characterizing temperature of 1522 K [23]), R is the universal gas constant (8.314 J/K/mol) and T is the temperature of deformation in absolute scale. Taking the following values – ε_ =418 s−1 and T=1473 K – the Z value is 8.6×1018 s−1. Discontinuous dynamic recrystallization under conditions of moderate (which is the present case) or high Z values is expected to result in a homogeneous grain size distribution. Based on the Z value, the dynamically recrystallized grain size (dDRX) can be obtained as [24]: dDRX ðin μmÞ ¼ 5:2 103 ðZÞ−0:17 :
ð3Þ
Note that the above relationship was originally proposed for the austenitic stainless steel AISI 304. While such a direct
Conclusions
The current study shows that the austenitic stainless steel AISI 316L undergoes discontinuous dynamic recrystallization under conditions of moderate Zener–Hollomon parameter during friction surfacing. The process involves homologous deformation temperatures in the range of 0.8 to 0.9 and strain rates in excess of 400 s−1. In the current study, estimated dynamically recrystallized grain size based on these matched well with the experimental observations. Friction surfaced coatings in alloy 316L show some increase in grain size from the coating/ substrate interface to the coating top surface, which is expected to be a result of decreasing cooling rate from the coating/ substrate interface to the coating top surface.
REFERENCES [1] Grainger S, Blunt J. Engineering coatings — design and application. 2nd ed.Cambridge: Abington; 1998. [2] Chandrasekaran M, Batchelor AW, Jana S. Study of the interfacial phenomena during friction surfacing of mild steel with tool steel and inconel. J Mater Sci 1998;3:2709-17. [3] Rafi HK, Janaki Ram GD, Phanikumar G, Prasad Rao K. Friction surfaced tool steel (H13) coatings on low carbon steel: a study on the effects of process parameters on coating characteristics and integrity. Surf Coat Technol 2010;205:232-42. [4] Goverdhan D, Kumar ACS, Murti KGK, Madhusudhan Reddy G. Characterization of austenitic stainless steel friction surfaced deposit over low carbon steel. Mater Des 2012;36:206-14. [5] Chandrasekaran M, Batchelor AW, Jana S. Friction surfacing of metal coatings on steel and aluminum substrate. J Mater Process Technol 1997;72:446-52. [6] Rafi HK, Janaki Ram GD, Phanikumar G, Prasad Rao K. Microstructure and properties of friction surfaced stainless steel and tool steel coatings. Mater Sci Forum 2010;638:864-9. [7] Puli R, Nandha Kumar E, Janaki Ram GD. Characterization of friction surfaced martensitic stainless steel (AISI 410) coatings. Trans Indian Inst Met 2011;64:41-5. [8] Bedford GM, Vitanov VI, Voutchkov II. On the thermo-mechanical events during friction surfacing of high speed steels. Surf Coat Technol 2001;141:34-9. [9] Liu XM, Zou ZD, Zhang YH, Qu SY, Wang XH. Transferring mechanism of the coating rod in friction surfacing. Surf Coat Technol 2008;202:1889-94. [10] Liu X, Yao J, Wang X, Zou Z, Qu S. Finite difference modeling on the temperature field of consumable-rod in friction surfacing. J Mater Process Technol 2009;209:1392-9.
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[11] Rafi HK, Phanikumar G, Prasad Rao K. Material flow visualization during friction surfacing. Metal Mater Trans A 2011;42:937-9. [12] Rafi HK, Balasubramanian K, Phanikumar G, Prasad Rao K. Thermal profiling using infrared thermography in friction surfacing. Metal Mater Trans A 2011;42:3425-9. [13] Vitanov VI, Voutchkov II. Process parameters selection for friction surfacing applications using intelligent decision support. J Mater Process Technol 2005;159:27-32. [14] Vitanov VI, Javaid N, Stephenson DJ. Application of response surface methodology for the optimisation of micro friction surfacing process. Surf Coat Technol 2010;204:3501-8. [15] Rafi HK, Janaki Ram GD, Phanikumar G, Prasad Rao K. Microstructural evolution during friction surfacing of tool steel H13. Mater Des 2011;32:82-7. [16] Humphreys FJ, Hatherly M. Recrystallization and related annealing phenomena. 2nd ed.Oxford: Elsevier; 1996. [17] Mirzadeh H, Cabrera JM, Najafizadeh A, Calvillo PR. EBSD study of hot deformed austenitic stainless steel. Mater Sci Eng A 2012;538:236-45. [18] McNelley TR, Swaminathan S, Su JQ. Recrystallization mechanisms during friction stir welding/processing of aluminum alloys. Scr Mater 2008;58:349-54.
[19] Chang CI, Lee CJ, Huang JC. Relationship between grain size and Zener–Hollomon parameter during friction stir processing in AZ31 Mg alloys. Scr Mater 2004;51:509-14. [20] Zhilyaev AP, Langdon TG. Using high-pressure torsion for metal processing: fundamentals and applications. Prog Mater Sci 2008;53:893-979. [21] Vitanov VI, Javaid N. Investigation of the thermal field in micro friction surfacing. Surf Coat Technol 2010;204:2624-31. [22] Prasad Rao K, Veera Sreenu A, Rafi HK, Libin MN, Balasubramanian K. Tool steel and copper coatings by friction surfacing — a thermography study. J Mater Process Technol 2012;212:402-7. [23] McQueen HJ, Ryan ND. Constitutive analysis in hot working. Mater Sci Eng A 2002;322:43-63. [24] Manshadi AD, Barnett MR, Hodgson PD. Hot deformation and recrystallization of austenitic stainless steel: part I. dynamic recrystallization. Metal Mater Trans A 2008;39:1359-70.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Dynamic recrystallization in friction surfaced austenitic stainless steel coatings Ramesh Puli⁎, G.D. Janaki Ram Materials Joining Laboratory, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
AR TIC LE D ATA
ABSTR ACT
Article history:
Friction surfacing involves complex thermo-mechanical phenomena. In this study, the
Received 14 July 2012
nature of dynamic recrystallization in friction surfaced austenitic stainless steel AISI 316L
Received in revised form
coatings was investigated using electron backscattered diffraction and transmission
29 August 2012
electron microscopy. The results show that the alloy 316L undergoes discontinuous
Accepted 1 September 2012
dynamic recrystallization under conditions of moderate Zener–Hollomon parameter during friction surfacing.
Keywords:
© 2012 Elsevier Inc. All rights reserved.
Friction surfacing Austenitic stainless steel Dynamic recrystallization EBSD Electron microscopy
1.
Introduction
Friction surfacing is a solid-state process with a strong industrial potential for depositing corrosion and wear resistant coatings [1]. In this process, the material to be deposited is taken in the form of a rod, which gets consumed during the process. The substrate is firmly clamped to an anvil. The consumable rod (mechtrode) is rotated, under constant axial force, against the substrate. Friction between the mechtrode and the substrate results in intense localized heating, causing significant softening and plastic deformation of the mechtrode material. After a short dwell time, the substrate is made to move. As this happens, hot plasticized metal from the mechtrode gets deposited on the substrate, resulting in a metallurgically bonded coating along the line of traverse. Being a solid-state coating process, friction surfacing offers several advantages (such as zero dilution, absence of solidification cracking, and lesser opportunity for
⁎ Corresponding author. Tel.: + 91 44 22574780; fax: +91 44 22576780. E-mail address: [email protected] (R. Puli). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.09.001
brittle intermetallic formation) over conventional weld overlay processes [1]. There have been quite a few recent studies on friction surfacing. Most of these investigations deal with optimization of process parameters [2–4] and evaluation of coating microstructures and properties [5–7]. The thermo-mechanical phenomena involved in the process have been investigated in some detail [8–12] and attempts have also been made to model the process [13,14]. Friction surfaced coatings invariably show very fine equiaxed grains, notwithstanding the coarse grain size of the consumable rod material. For example, Rafi et al. [15] reported a grain size of less than 10 μm in AISI H13 friction surfaced coatings. While the formation of fine equiaxed grains in friction surfaced coatings is often attributed to dynamic recrystallization, detailed studies in this regard are not available. In the present work, electron backscattered diffraction (EBSD) and transmission electron
50
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
microscopy (TEM) studies were carried out on friction surfaced AISI 316L stainless steel coatings to gain greater insights into the mechanisms of hot restoration during friction surfacing.
2.
Experimental Details
Austenitic stainless steel AISI 316L rods (chemical composition in wt.%: Fe–0.015C–18Cr–10Ni–2.2Mo–1.6Mn–0.35Si–0.03S–0.03P, melting temperature range: 1375 °C to 1400 °C) of 18 mm diameter in hot rolled and annealed condition were used for friction surfacing. A mild steel (Fe–0.12C–0.4Mn–0.01S–0.02P, melting temperature range: 1325 °C to 1400 °C) plate of dimensions 200×150×10 mm was used as the substrate. Friction surfaced coatings of 1.2 mm thickness and 16 mm wide were produced using a commercially available friction surfacing machine. The process parameters used were: 39 MPa axial pressure, 1200 rpm spindle rotation speed and 3 mm/s substrate traverse speed. The dwell time used was 30 s. Infrared thermography was used to determine the peak temperature at the interface between the rotating consumable rod and the substrate (during the initial dwell period). An infrared camera (a CEDIP JADE mercury cadmium telluride camera commercially available from Flir System, Croissy-Beaubourg, France) with a mean noise equivalent temperature difference of 20 mK was used for this purpose. The camera operates in the long wave infrared band (7.9 to 9.7 μm) with a focal plane array of 320×240 detectors (each detector is a pixel of 25 μm size) and a pixel pitch of 30 μm. The infrared camera was calibrated as per standard practice. Transverse sections from friction surfaced coatings were prepared for electron backscattered diffraction studies by electropolishing at 12 V in a solution consisting of 20% perchloric acid and 80% methanol, maintained at −10 °C. Electron backscattered diffraction studies were conducted using an FEI Quanta-200 scanning electron microscope. The scan area was 200 ×200 μm at a step size 0.5 μm. TSL-OIM software was used for electron backscattered diffraction analysis. The grain area (diameter) method was used for grain size determination. These studies were also performed on 316L consumable rods for comparison. For transmission electron microscopy studies, 3 mm disks (0.5 mm thick) were obtained from the middle of the coating thickness. These disks were first mechanically polished to about 100 μm and were then further thinned by electrolytic jet polishing on a Struers Tenupol-5 twin jet polisher at 12 V, and −30 °C in a 20% perchloric acid solution. Transmission electron microscopy analysis was carried out using a Philips CM-12 microscope operated at 120 kV.
3.
Results and Discussion
The inverse pole figure (IPF) map of the AISI 316L consumable rod in as-received condition is shown in Fig. 1a. The average grain size of the consumable rod was approximately 61± 22 μm. As can be seen, the consumable rod contained quite a few annealing twins and the grain boundaries were mainly high angle in character. There were a few low angle grain boundaries, but the consumable rod material contained no subgrain boundaries (Fig. 1b).
Fig. 1 – EBSD results of AISI 316L consumable rod: (a) IPF map, (b) grain boundary misorientation distribution plot. In the case of friction surfaced coatings, EBSD scans were conducted at three different locations on the coating cross-section: near the coating top surface, at the middle of the coating thickness and near the coating/substrate interface. Fig. 2 shows the IPF maps corresponding to these three locations. While the grains were equiaxed over the entire coating thickness, EBSD studies revealed some differences in the grain size across the coating thickness — the average grain size increased from 4.8 ± 1.5 μm near the coating/substrate interface to 6.4 ± 2.3 μm at the middle of the coating thickness to 9.4 ± 3.2 μm near the coating top surface. However, the grain size distribution was more or less homogeneous within a given scan region. More importantly, however, the grain size over the entire coating thickness is much finer compared to the grain size of the consumable rod. Fig. 3 shows the grain boundary misorientation map, grain boundary misorientation plot and IPF texture plot obtained from an EBSD scan done near the coating top surface. In these respects, EBSD scans conducted near the coating/substrate interface and at the middle of the coating thickness yielded qualitatively similar information. The coating consisted of
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
Fig. 2 – IPF maps of the 316L friction surfaced coating: (a) near the coating top surface, (b) at the middle of the coating thickness, and (c) near the coating/substrate interface.
51
Fig. 3 – Grain boundary misorientation map (a), grain boundary misorientation plot (b), and IPF texture plot (c) of the 316L friction surfaced coating.
52
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
mainly high angle grain boundaries along with some low angle grain boundaries (Fig. 3a and b). Many of the grains showed a necklace of much smaller subgrains along their boundaries. This can be clearly seen in Fig. 3a inset. The grains in the coating were randomly oriented and the coating did not develop any texture, as can be seen from Fig. 3c. Thin foil samples prepared from 316L friction surfaced coatings were examined under TEM. The microstructures observed in different regions of the sample are shown in Fig. 4. Fig. 4a shows a well-developed cell, while Fig. 4b shows a developing cell boundary, cutting across the stacking faults of a recrystallizing grain. Fig. 4c shows a few subgrains necklacing a prior grain boundary. Similarly, the dislocation density was observed to be low in some grains, while it was high in other grains (Fig. 4d). These observations reflect the spatial and temporal heterogeneity of microstructural evolution during dynamic restoration. EBSD and TEM observations clearly show that the coating material has undergone dynamic recrystallization. It is well known that the nature of restoration during hot deformation depends on the stacking fault energy of the material. In low stacking fault energy materials, such as the austenitic stainless steel, dynamic recrystallization is the most important hot restoration mechanism [16]. In such materials, the process of dynamic recrystallization, often referred to as discontinuous dynamic recrystallization (DDR), involves distinct nucleation and growth stages. Upon reaching a critical deformation condition, new grains originate at the prior grain boundaries and they begin to grow. These grains, however, quickly cease to grow due to increased dislocation density with continued deformation (causing a reduction in the driving force for further growth). Subsequently, more new grains nucleate at the migrating boundaries. Overall, as these processes continue to
occur successively, there will be a gradual thickening of the band of recrystallized grains, eventually leading to complete recrystallization. The process of DDR usually leaves the following signatures [16]: (i) developing cells/subgrains, and (ii) necklacing of subgrains on prior grain boundaries. These features were clearly seen in the current study, as shown in Figs. 3 and 4. Further, as can be seen from Fig. 3, the coating microstructure showed no loose high angle grain boundaries inside the grains and contained very few low angle grain boundaries with misorientation in the range of 10–15°. Therefore, it can be concluded that continuous dynamic recrystallization (of the type involving progressive lattice rotation) is not significant in alloy 316L friction surfaced coatings [17]. Friction surfacing involves severe plastic deformation at high temperatures. The thermo-mechanical phenomena involved in the process are yet to be fully understood. However, the process can be expected to involve, as in the case of friction stir welding/ processing, transients and gradients in strain, strain rate and temperature [18]. The strain rates involved in friction surfacing can be very high and are difficult to experimentally determine. A reasonable estimate of the strain rate involved in friction surfacing can be obtained from the following equation, which was utilized in earlier investigations for estimating the strain rates in friction stir welding/processing [19] and high pressure torsion processes [20]: ε_ ¼
Rm :2πre Le
ð1Þ
where, ε_ is the strain rate, Rm is the average material flow rate (half of the mechtrode rotational speed in revolutions per second (rps) in the present case), re is the effective radius of dynamically recrystallized zone (half of the coating width in the present case), and Le is the effective depth of dynamically recrystallized zone
Fig. 4 – TEM micrographs of friction surfaced coatings showing (a) a well-developed cell, (b) a developing cell boundary, running across the stacking faults, (c) necklace formation (arrows show DRX grains at a prior grain boundary), and (d) grains with different dislocation densities (annotations 1, 2 and 3 mark grains with low, medium and high dislocation densities, respectively).
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 4 (2 0 1 2) 4 9–5 4
53
relationship is not available in published literature for alloy 316L, it is unlikely to be very different. On this assumption, one can arrive at an estimated average dynamically recrystallized grain size of 3.2 μm in friction surfaced 316L coatings. This is reasonably close to the observed grain size in the coating close to the coating/substrate interface (4.8 ± 1.5 μm). The increase in grain size towards the coating top surface is expected to be a result of reduced cooling rate from the coating/substrate interface to the coating top surface (cooling is mainly due to heat conduction through the substrate). It should be noted that the initial grain size can influence the final dynamically recrystallized grain size. This should be investigated in future work using consumable rods of different grain sizes.
4. Fig. 5 – Temperature close to the rotating mechtrode/ substrate interface as a function of time during the initial dwell period of friction surfacing.
(thickness of the coating in the present case). Taking the following values – Rm =10 rps, re =8 mm, and Le =1.2 mm – a strain rate of 418 s−1 can be obtained in friction surfacing. The temperature of deformation in friction surfacing is another parameter that needs to be considered. Earlier studies have shown that the peak temperatures involved in friction surfacing are above the 0.8 homologous temperature of the mechtrode material, although there has been considerable variation in the reported peak temperatures by various investigators [10,12,21,22]. Finite difference modeling studies by Liu et al. [10] show that the temperatures along the mechtrode radius would be more or less the same once the process attains steady state. In the current study, a peak temperature of 1200 °C (1473 K) was measured using infrared thermography at the rubbing end of the rotating mechtrode during the initial dwell period (Fig. 5). The temperature at the rub interface is expected to be more or less the same during the entire deposition process [12]. The strain rate and the temperature of deformation can be integrated into a single parameter, known as the Zener– Hollomon parameter (Z). It relates the combined effect of strain rate and temperature to the microstructure development during hot deformation and is defined as: Q Z ¼ ε_ exp RT
ð2Þ
where, Q is the activation energy for hot deformation (460 kJ/mol for alloy 316L for a mean characterizing temperature of 1522 K [23]), R is the universal gas constant (8.314 J/K/mol) and T is the temperature of deformation in absolute scale. Taking the following values – ε_ =418 s−1 and T=1473 K – the Z value is 8.6×1018 s−1. Discontinuous dynamic recrystallization under conditions of moderate (which is the present case) or high Z values is expected to result in a homogeneous grain size distribution. Based on the Z value, the dynamically recrystallized grain size (dDRX) can be obtained as [24]: dDRX ðin μmÞ ¼ 5:2 103 ðZÞ−0:17 :
ð3Þ
Note that the above relationship was originally proposed for the austenitic stainless steel AISI 304. While such a direct
Conclusions
The current study shows that the austenitic stainless steel AISI 316L undergoes discontinuous dynamic recrystallization under conditions of moderate Zener–Hollomon parameter during friction surfacing. The process involves homologous deformation temperatures in the range of 0.8 to 0.9 and strain rates in excess of 400 s−1. In the current study, estimated dynamically recrystallized grain size based on these matched well with the experimental observations. Friction surfaced coatings in alloy 316L show some increase in grain size from the coating/ substrate interface to the coating top surface, which is expected to be a result of decreasing cooling rate from the coating/ substrate interface to the coating top surface.
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