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Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings
Accepted Manuscript Title: Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings Author: Yanbing Guo Kai Feng Fenggui Lu Ke Zhang Zhuguo Li Seyed Reza Elmi Hosseini Min Wang PII: DOI: Reference:
S0169-4332(15)01945-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.132 APSUSC 31079
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-4-2015 6-7-2015 16-8-2015
Please cite this article as: Y. Guo, K. Feng, F. Lu, K. Zhang, Z. Li, S.R.E. Hosseini, M. Wang, Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
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Nanobainitic coatings under 200, 250 and 300 °C heat treatments are fabricated. The size of bainite sheaves increased with the isothermal temperature increasing. Textured and chaotic distribution are observed in 200 and 300 °C microstructures. The evolution model of nanobainite morphology is established and analyzed. The bainitic ferrite of 200 °C heat treatment has a true thickness of 45 nm.
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Effects of isothermal heat treatment on nanostructured bainite
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morphology and microstructures in laser cladded coatings Yanbing Guo a, Kai Feng, Fenggui Lu, Ke Zhang, Zhuguo Li a,*, Seyed Reza Elmi Hosseini a, Min Wang a, b
Shanghai Key Lab of Materials Laser Processing and Modification, School of Materials Science
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a
State Key Laboratory of Metal Matrix Composite, School of Materials Science and Engineering,
Ac ce pt e
b
d
and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Shanghai Jiao Tong University, Shanghai 200240, China
Abstract:
Laser cladding and subsequent isothermal heat treatments have been used to fabricate nanostructured bainitic coatings. XRD has been used to determine the kinetics of bainitic transformation process. OM, SEM and TEM have been used to characterize the morphology and microstructures at different stages of transformation. The results showed that at the initial stage of bainitic transformation, the bainite sheaves are short and thin at a relatively low transformation
*
Corresponding author. Tel.: +86 21 54745878; fax: +86 21 34203024; E-mail address: [email protected] (Zhuguo Li).
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temperature. The fully transformed bainitic microstructure obtained at a relatively high temperature present a textured morphology. The chaotic growth orientations of the sheaves and the island like of the retained austenite have been observed at the low transformation temperature. A simple model has
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been established to describe the microstructures and the bainite sheaves growth evolutions during the isothermal holding at the different transformed temperatures. The morphology and distribution of the
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bainite in the coatings were analyzed by using the nucleation and growth rate of bainitic
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transformation theories, which is consisted with the experiment results.
Key words: Laser cladding; Nanostructured bainite; Microstructure; Morphology; Transformation;
1. Introduction
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Isothermal
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Recently, nanoengineering of steels has been taken a significant awareness by researchers and
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engineers for the potential applications in the modern steel industry [1, 2]. Several nanostructured steels have been developed using some special mechanic and annealing processes by the researchers in the last decade [3, 4]. Nanostructured bainitic steel is one of these nanocrystalline steels which is based on the pioneering research by Bhadeshia and co-workers at Cambridge University [5]. The new series of nanostructured steels present a ultrahigh strength of 2.3 GPa, toughness of 30 MPa·m1/2, ductility of 30% and high strain rate sensitivity [6]. High silicon content in the steels prevents the carbide precipitation, and high carbon and other austenite stabilizing elements (Mn, Cr, Mo, etc.) significantly retard the bainitic transformation process and cause the ultra-fine structures [6-8]. The morphology, scale and distribution of the nanoscale bainitic ferrite and interval film austenite microstructure are the crucial factors for the outstanding mechanical properties [9, 10].
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The nucleation and growth of the bainite sheaf are determinants for kinetics of bainitic transformation, and the morphology of the bainite is also controlled by the two factors [11]. In the past decades, many researchers have studied the kinetics of the bainitic transformation through the
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calculation and experiments. Garcia and Bhadeshia have investigated the theory describing the nucleation of bainite in the high carbon steels [12].The growth rate of the bainite has been calculated
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by Quidort and Brechet with the Trivedi equation [13]. In addition, Hu et al. have measured the
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growth rate of bainite plates in Fe-C-Mn-Si steel by in situ observation [14]. Almost all of the recent studies on kinetics of bainitic transformation focused on the overall transformation process, however,
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the correlation between the kinetic and bainite morphology is seldom reported. All the researches on
transformations.
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the kinetics of the bainitic transformation has inspired the acceleration of the nanobaintic
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Since the nanostructured bainitic transformation in the steels is always very slow at the low
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temperature [15], in the present study, laser cladding is used to fabricate fine prior austenite because of its high cooling rate [16], and aluminum and cobalt are added as the alloying elements to enhance the driving force of the austenite-ferrite transformation, which items cause to accelerate the transformation process [17]. In the present work, the morphology and distributions of the microstructures during the isothermal bainitic transformation in the laser cladded coatings are studied. The nucleation and growth theories are applied to analyze the formation process of the bainite sheaves and growth morphologies. A simple transformation processing model is established to describe and reveal the nanobainitic microstructures evolution process under different isothermal heat treatments.
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2. Materials and Experimental Procedure The coating materials were deposited on the mild steel substrate with dimensions of 160 mm × 15 mm × 12 mm. The substrate surface was polished to remove any stains or contaminants, then
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cleaned with acetone and alcohol prior to laser cladding. The coating material was Fe-based alloyed powder prepared by gas atomization method, and the diameter of the powder was in the range of
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50–150 µm. The nominal chemical composition (wt. %) of the substrate material and alloy powders
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used in this study are listed in Table 1.
Si
Mn
Mo
Co
0.81
1.51
1.96 1.13 0.32
1.59
Substrate 0.14
0.22
0.58 -
-
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C
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-
Al
P
S
Fe
0.93 0.008
0.008
Bal.
-
0.02
Bal.
0.02
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Powder
Cr
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Table 1 Chemical composition of the powder and substrate (wt. %).
Fig. 1. 3.5 kW diode laser system.
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Fig. 2. Image of the laser cladded coating.
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The 3.5 kW diode laser system equipped with powder coaxial feeding system (as shown in Fig.
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1) was employed to fabricate the coatings (as shown in Fig. 2), and the laser beam size at the focal length (165 mm) was a rectangular spot with a dimensions of 2.5 mm × 3.5 mm. High-purity argon
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gas was used as shielding gas through the coaxial nozzle with a flow rate of 5 L/min to prevent the
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melted pool from heavy oxidation. The power of the laser beam was selected with variant value from 2.3 to 2.5 kW for considering the constant dilution at variant preheating temperatures of the
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substrates. The cladding materials were deposited on the preheated substrates, then the specimens
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were transferred into the muffle furnace for isothermal heat treatment. Moreover a digital thermometer was applied to monitor the surface temperature of the specimen to insure its temperature did not decrease from the temperature which set for isothermal heat treatment during the transferring. The specimens were directly quenched in the water at the ambient temperature after the isothermal holding. The whole process of the experiments are demonstrated in Fig. 3.
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Fig. 3. The temperature-time curves for laser cladding and heat treatment process.
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The time of isothermal heat treatment at the different temperatures was based on the calculated
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time-temperature-transformation diagram, and all of the parameters about laser processing and isothermal heat treatment are given in Table 2.
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Table 2 Processing parameters for laser cladding and isothermal transformation. Laser cladding
Isothermal transformation
Preheating
Scanning velocity
Powder
Transformation
Transformation
(W)
temperature (°C)
(mm/s)
Feeding rate
times
(g/min)
(°C)
(h)
2400 2300
temperature
220
10
30
200
0.5/1/2/4/8/12/24/48/72/120/180/240
270
10
30
250
0.25/0.5/1/2/3/4/8/12/16/20/24
320
10
30
300
0.25/0.5/0.75/1/1.5/2/3/4/6/8/16
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2500
d
Laser power
The phases of the coatings were determined by X-ray diffraction with an Ultima IV diffractometer. The radiation source was Cu Kα, operated at 40 kV and 30 mA with a scanning rate of 1 °/min. The specimens were sectioned, polished and finally etched using 4% nital for metallographic examination. The microstructure and morphology of the coatings were characterized using a Zeiss Axioplan 2 optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM JEOL JEM-7600F) equipped with Inca Energy energy-dispersive spectrometer (EDS). The TEM film foils
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were polished to 80 µm, and machined to 3 mm diameter rods, then electropolished with a twin-jet electropolisher at -30 °C in a solution of 5% perchloric acid and 95% methanol and an operating voltage at 50 V. A transmission electron microscopy (TEM JEOL JEM-2100F) operated at 200 kV
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was applied to study these thin film foils. 3. Results and discussion
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3.1 Transformation kinetics
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An assessment of the isothermal transformation kinetic is performed using the JMatPro software which is developed by Sente Software Corporation. The calculated TTT diagram is illustrated in Fig.
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4, which shows the experimental results of the time taken to initiate (open circles) and cease
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transformation (filled circles) under each isothermal temperature. The measured value for the degree of transformation are reasonable agreement with those calculated (by using the JMat Pro software).
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The average grain size number of the prior austenite during the calculation was set as 10 (about 10
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µm), which was determined by the liner intercept method under the standard ASTM: E112-13.
Fig. 4. Calculated TTT diagram plotted by the experimental results of the bainitic transformation. The transformation process is investigated through the measurement of the volume fraction of the retained austenite (RA), and the volume fractions of RA are measured using the XRD results (Fig. 5). The kinetic process of the bainitic transformation is illustrated using the evolutions of the RA
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volume fraction. The volume fraction of the RA is increased and then decreased with the increasing of the isothermal holding time, it inferred that after cladding and shorter time isothermal holding, the initial microstructure in the coatings is under cooling austenite, then most of the austenite transforms
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to martensite after quenching, and a small volume austenite remains. The fraction of bainitic ferrite is increasing with the isothermal time increased, then the stability of the adjacent austenite is increased
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by the carbon diffusion from bainitic ferrite, and more austenite remains after quenching. After a
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specific holding time, most undercooling austenite has been changed to bainite and all of the remaining austenite have high stability to maintain the austenite after quenching, and no martensite is
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forming. The volume of austenite remaining is decreasing with isothermal holding time increasing
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when most under cooling austenite continually transforms to bainitic ferrite. Finally, the volume of the retained austenite is no longer changing after the transformation is ceased, the microstructure
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consists of RA and bainitic ferrite. The microstructure is considered as the full transformed bainitic
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microstructure when the changing of the determined volume fraction of the retained austenite ceases (determined by using XRD). For the full transformed microstructure, the final fraction of the retained austenite is decreased as the decreasing isothermal temperature. Since this condition is met at ever increasing carbon concentrations when the transformation temperature is reduced, more bainite can form with greater undercoolings below BS [18].
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treatment.
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3.2 Microstructural evolution
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Fig. 5. Volume fraction of retained austenite as a function of the holding time in the isothermal heat
Fig. 6 illustrates the optical microstructure obtained at the initiate, medium and ceased stages of
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the bainitic transformation at three different temperatures. Fig. 6 (a) shows the initial microstructure
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of the coating after 1 hour holding at 200 °C and then quenching. The grey martensite distributes at
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the inner dendritic areas and the white austenite distributes at the prior austenite grain boundaries. Only few short and thin black bainite plates can be observed, which indicate that this stage is just at the end of the pre-bainitic (incubation) period [19]. Then martensite and austenite is observed beside bainite in the coatings after 4 hours isothermal holding (Fig. 6 (b)). When austenite continues to transform into bainitic ferrite, some branches outspread from the side of the sheaf trunk, and the growth direction of the branches tilt an angle with the prior sheaf orientation. Fully bainitic coating is obtained after the transformation ceases (Fig. 6 (c)), and only a small amount of retained austenite distributes at the prior austenite grain boundaries [20]. Fig. 6 (d) shows the microstructure of the initial stage of the bainite transformation obtained at 250 °C for 1 hour. The microstructure consists of a mixture of martensite, bainitic ferrite and retained austenite. The amount of bainite (black needle phase) increases with increasing isothermal holding time (Fig. 6 (e)), and the transformation ceases
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at about 8 hours isothermal treatment (Fig. 6 (f)). Similar regularity is found in the samples heat treated at the 300 °C. The bainitic transformation incubates before 1 hour and finished at about 4 hours. More retained austenite distributed at the prior austenite grain boundaries at a relatively higher
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transformation temperatures for the fully transformed microstructures, which is consistent with the
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XRD result in Fig. 5.
Fig. 6. Optical micrographs of bainite obtained at 200 °C (a) 1 h, (b) 4 h, (c) 48 h; 250 °C (d) 1 h, (e) 4 h, (f) 8 h; 300 °C (g) 1 h, (h) 1.5 h, (i) 4 h.
It can be found that the incubation period is prolonged with the bainitic transformation temperature decreasing compared the microstructures evolution under the three temperatures. Because the incubation period is a formation process of the carbon depleted region [21] and [22], high carbon diffusion coefficient at high isothermal temperature shorten the period. The morphology and the densities of the bainite sheaves are also different at various transformation temperatures. The amount of the bainite sheaves are in inverse relation to the transformation temperature. The length and thickness of the bainite sheaves increased with the increasing of the isothermal holding
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temperature, as can be seen from the Fig. 6 (b), (d) and (g). The average length and the average thickness of the sheaves increase from about 18 µm and 0.4 µm obtained at 200 °C to about 32 µm and 6.2 µm obtained at 300 °C, respectively. It also extracted form Fig. 7 that the increase of the
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average length is relatively lower than the thickness, caused by the different nucleation and growth
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rate under different transformation temperatures.
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Fig. 7. The average length and thickness of the bainite sheaf as a function of transformation
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temperature.
3.3 Morphology of the nanostructured bainite Fig. 8 shows SEM micrographs of the initial and ceased stages of bainitic transformation under three heat-treatment cycles. Bainite nucleates at sites near the austenite grain boundaries, then the bainite sheaves or lathes propagate through the repeated formation of the limited size sub-units [23]. The sheaves are composed of two or more paralleled bainitic ferrite laths which are separated by the thin film retained austenite, as shown in Fig. 8. The sheaves grow toward the grain interior until the growth are constricted by the grain boundary or the other sheaves. The length of the primary sheaves are usually very long, even some of them prolong across the whole dendrite, especially in the relatively higher temperatures. Some of the shorter sheaves form from the side of the prior sheaves at the relative lower temperature, as shown in Fig. 8 (a). The thickness of the sheaves formed at the
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lower temperature is thinner than that formed at higher temperatures, because the sheaves at higher
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temperature consists more bainite lathes.
Fig. 8. SEM micrographs of bainite obtained at 200 °C (a) 4 h, (b) 48 h; 250 °C (c) 1 h, (d) 8 h; 300 °C (e) 1 h, (f) 4 h.
The microstructures consist of several bainite sheaves with different growth directions when the transformation ceased, and the blocky retained austenite distributed at the prior austenite interdendritic areas. The disorder orientation of sheaves growth at the low temperature makes the microstructures exhibit chaotic features. The feature of the sheaves with independent growth directions is becoming uniform when transformed at a relatively higher temperature (Fig. 8 (d) and (f)). The sheaves usually grow through the whole prior austenite grains at a relatively higher transformed temperature.
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The distribution of the blocky retained austenite is discontinuous at the lower transformed temperatures (Fig. 8 (b)), and it presents more continuous at higher temperatures (Fig. 8 (d) and (f)). Apart from the thin film austenite between the bainitic ferrite laths and blocky retained austenite,
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there are also some islands like austenite distribution in the inner dendrite areas (dark arrowed in the Fig. 8 (b) and (d)). These retained austenite are surrounded by several sheaves which have different
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growth directions, and there are more island like retained austenite distributed between the bainite
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sheaves formed at lower temperatures.
Fig. 9. Model of microstructures evolution in laser cladded bainitic coatings.
The bainite nucleus firstly forms at the defects sites near the grain boundaries with fluctuation of the free energies and structures. The nucleus grows to subunit of bainitic ferrite through a shearing mechanism, and then grows to a lath or sheaf of bainite [24]. Thus, isolated lathes emanate from nucleation sites near the grain boundaries, and the sheaves are therefore free to increase until restricted by grain boundaries or other sheaves growing from other sides of the grain boundaries, as shown in Fig. 9. The growth of the sheaves is confined by the other platelets growing from the other
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sides of the grain boundary to make the length of these sheaves be shorter especially when the transformation temperature decreases. Some bainite nucleate at the surface of the existing or newly formed bainite sheaves when these sites satisfied the nucleation as the transformation continues. This
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phenomenon is called autocatalysis, which is related to the sympathetic nucleation [25]. The new sub-units nucleate at both sides of the prior bainite lathes, and then grow to a paralleled or angled
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lath with the prior ones. The sheaves angled with the prior bainite sheaves are more obviously in
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microstructures transformed at relatively lower temperatures, because the autocatalysis nuclei are always formed near the tips of the subunits of the prior sheaves, and the dislocations in this area
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cause deflection of the growth direction. The dislocation density in this area is higher at the lower
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isothermal temperature [26], which make the sheaves have more chance to branch off. The phenomena of bainite sheaves branching off and the chaotic microstructure are controlled
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by the kinetics of the bainitic transformation process. Tszeng has used Eq. (1) to describe the overall
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transformation kinetics of the bainite [25], and the autocatalysis on the surface of the sheaves is taking into consideration. The equation is given by: (1)
where f is the actual volume fraction of the bainitic ferrite, η and a are the coefficients related to the bainite subunits scale, I0 is the volumetric nucleation rate of bainitic ferrite at the austenite grain boundaries, IS is the nucleation rate at the austenite/bainite interface and t is the transformation time. The nucleation rate of bainitic ferrite decreases with decreasing temperature [27], which is in consistent with displacive interpretation of the nucleation rate. The scale of the subunit and the temperature dependence are controlled by the carbon diffusion, not by driving force, so the scale of subunit decreases with temperature decreasing [28]. When the temperature increases, the values of I0
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and IS increase, and η is the aspect ratio can be considered a constant, a1 < a2. If the volume fraction of the bainitic ferrite under the temperature of T1 and T2 are identical at the initial stage of transformation, f1 = f2, the reaction time at T1 is longer than that at T2, t1>t2. The I0 of the prior
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austenite with small grain size is higher than that of the larger grain size, I0s > I0l, that is the reason why the bainitic transformation in the laser cladded coatings are accelerated. The growth velocity of
(2)
is the weighted average diffusion
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where v is the growth rate of the bainitic ferrite,
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bainitic ferrite according to the Quidort’s model [13] is given by
is the
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coefficient of carbon in austenite. σ is the ferrite/austenite interface energy per unit area,
molar volume of ferrite, Ω* is an algebraic combination value about the supersaturation, and the
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three parameters can be regarded as constants in the temperature range of bainitic transformation.
[13].
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∆Gm is the driving force of the bainitic transformation, which is also not changed as obviously as the is the main factor which controls the growth rate of bainitic ferrite. According to Eq.
(2), the growth rate of the bainitic ferrite increased with the increasing transformation temperature. Since the growth of the sheaves is soon restricted by the grain boundaries or other sheaves, the growth on the direction of the thickness is continued by repeat nucleation of the paralleled bainite plates. That is why the thickness increasing is higher than the length with the increasing transformation temperature.
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Fig. 10. (a) The images of the austenite island in nanostructured bainite and (b) the schematic of the
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formation of the island austenite wireframe in (a).
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The growth rate of ferrite is low and the dimensions of the bainite sheaves are short at a relatively lower transformation temperature and long transformation time, and there are more
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bainite/austenite interface areas for the new autocatalysis nucleus, and the ratio for the branch off
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growth is high. The growth orientations exert more directions and present a chaotic morphology.
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More areas interior of the grain are isolated by the prior bainite sheaves, as shown in the Fig. 10 (a). The stability of small areas of austenite isolated by the bainite sheaf is increased at the final stage of the isothermal bainitic transformation, as a result of the carbon diffuses from the surrounded bainite sheaves (Fig. 10 (b)). The free energy difference (ΔGγα = Gα - Gγ) at the austenite/ferrite interface is identical or less than zero when the carbon concentration is higher than T0ʹ , the bainitic transformation ceased. The retained austenite remains as the island like morphology in the inner area of the bainite microstructures.
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3.4 Thickness of the nanostructured bainite
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Fig. 11. TEM morphology of the bainite plate obtained under different isothermal temperatures, (a) 200 °C, (b) 250 °C, (c) 300 °C, (d) nanoscale twins in retained austenite after 200 °C transformation. The TEM was conducted to analysis the microstructures of the bainite plates and retained austenite (Fig. 11). There are some similar features among the microstructures formed at different temperatures. The microstructures consist of parallel bainitic ferrite plates and retained austenite between them, some twinnings are observed in the retained austenite which is caused by the plastic deformation during the bainitic transformation [28]. Beside similarities, the scales of the bainite plates under different transformation temperatures are obviously different. The bainite plate scales are crucial factors for the properties of the coatings, and the thickness of the true plate are determined through the measurement of the mean linear intercept in the normal direction to the plate length. The
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measurements were made for approximately 30 times for each specimens. The true-thickness of the ferrite plates are calculated as follows [29] (3) is the mean linear intercept normal to the length of the plates, t is the true thickness
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where
of the plates. The measured plate thickness t has been plotted as a function of the transformation
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temperature in Fig. 12.
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Fig. 12. Stereology corrected thickness of bainite plates as a function of the transformation temperature.
The maximum true thicknesses of the bainitic ferrite is lower than 100 nm, which is in the scale of the nanostructured bainite. The thickness of bainitic ferrite is thinner at a relatively lower transformation temperature. As the theory of the diffusionless transformation, if the shape deformation during the transformation is elastically accommodated and the bainite plates are allowed to grow free, the thickness of the lath should be thicker at lower temperatures [30]. The experiment result is obviously contradict with the theory, because the main determinant of the bainitic ferrite thickness is the strength adjacent retained austenite. During the thickening process of the bainitic ferrite, plastic relaxation of the adjacent austenite generate dislocation debris will resist the movement of the ferrite/austenite interface, high yield strength of the austenite at low temperature is
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expected caused thinner bainitic ferrite plates [23]. The effect of free energy change accompanying transformation is smaller than the strength of the adjacent austenite. The most important parameter that can increase the strength of nanostructured bainite is to obtain the ultra-fine size of bainitic
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ferrite [17]. 4. Conclusions
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Laser cladding and subsequent isothermal holding were used to fabricate Fe-based coatings with
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nanostructured bainite. The kinetics process of the isothermal bainitic transformation in the laser cladded coatings was accelerated compared with the barely isothermal heat treatment, as a result of
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the small prior austenite grain size after laser cladding. Textured morphology was clearly observed at
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relatively higher transformation temperatures as a result of the less orientations and high growth rate of the bainite sheaves. On the contrary, the branch off nucleation of the bainite sheaves and low
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growth rate caused a chaotic microstructures obtained at a relatively lower temperatures. Island like
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retained austenite surrounded by bainite sheaves were observed in the grain interior when the transformation is ceased at the lower temperatures. The carbon diffusion from the surrounded bainite sheaves to the austenite increases the stability of the retained austenite. The thickness of the bainitic ferrite at three transformation temperatures is in the scale of the nanostructures, which decreased with the decreasing of the temperature. Acknowledgment
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No.51171116) and the Ministry of Science and Technology of the People's Republic of China (Grant No. 2009DFB50350).
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[13] D. Quidort, Y.J.M. Brechet, Isothermal growth kinetics of bainite in 0.5% C steels, Acta Mate. 49 (2001) 4161-4170. [14] Z.W. Hu, G. Xu, H.J. Hu, L. Wang, Z.L. Xue, In situ measured growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel, Int. J. Miner. Metall. Mater. 21 (2014) 371-378. [15] P. Zhang, F.C. Zhang, T.S. Wang, Preparation and microstructure characteristics of low-temperature bainite in surface layer of low carbon gear steel, Appl. Surf. Sci. 257 (2011) 7609-7614. [16] M.X. Li, Y.Z. He, X.M. Yuan, Effect of nano-Y2O3 on microstructure of laser cladding cobalt-based alloy coatings, Appl. Surf. Sci. 252 (2006) 2882-2887.
[17] H.K.D.H. Bhadeshia, F. García Caballero, C. García Mateo, Acceleration of Low-temperature Bainite, ISIJ inter. 43 (2003) 1821-1825.
[18] H.K.D.H. Bhadeshia, New bainitic steels by design, Model. Simul. Mater. Design. (1998) 227-232. [19] Z.G. Yang, H.S. Fang, An overview on bainite formation in steels, Curr. Opin. Solid State Mater. Sci. 9 (2005) 277-286. [20] Y.B. Guo, Z.g. Li, C.W. Yao, K. Zhang, F.G. Lu, K. Feng, J. Huang, M. Wang, Y.X. Wu, Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding, Mater. Design 63 (2014) 100-108. [21] M.K. Kang, Y.Q. Yang, Q.M. Wei, Q.M. Yang, X.K. Meng, On the prebainitic phenomenon in some alloys, Metall. Mater. Trans. A 25 (1994) 1941-1946. [22] R. Zhou, Y.H. Jiang, D.H. Lu, R.F. Zhou, Z.H. Li, Development and characterization of a wear resistant bainite/martensite ductile iron by combination of alloying and a controlled cooling heat-treatment. Wear. 250 (2001) 529-534. [23] H.K.D.H. Bhadeshia, Bainite in Steels, The Institute of Materials, London, 2001.
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[24] H. Matsuda, H.K.D.H. Bhadeshia, Kinetics of the bainite transformation, 2004. [25] T.C. Tszeng, Autocatalysis in bainite transformations, Mater. Sci. Eng. A 293 (2000) 185-190. [26] C. Garcia-Mateo, F.G. Caballero, C. Capdevila, C.G.d. Andres, Estimation of dislocation density in bainitic microstructures using high-resolution dilatometry, Scripta Mater. 61 (2009) 855-858. [27] D. Quidort, Y.J.M. Brechet, A Model of Isothermal and Non Isothermal Transformation Kinetics of Bainite in 0.5% C Steels, ISIJ Inter. 42 (2002) 1010-1017. [28] L.C. Chang, H.K.D.H. Bhadeshia, Metallographic observations of bainite transformation mechanism, Mater. Sci. Technol. 11 (1995) 105-108.
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[29] L.C. Chang, H.K.D.H. Bhadeshia, Austenite films in bainitic microstructures, Mater. Sci. Technol. 11 (1995) 874-882. [30] S.B. Singh, H.K.D.H. Bhadeshia, Estimation of bainite plate-thickness in low-alloy steels, Mater. Sci. Eng. A 245 (1998)
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S0169-4332(15)01945-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.132 APSUSC 31079
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-4-2015 6-7-2015 16-8-2015
Please cite this article as: Y. Guo, K. Feng, F. Lu, K. Zhang, Z. Li, S.R.E. Hosseini, M. Wang, Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
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Nanobainitic coatings under 200, 250 and 300 °C heat treatments are fabricated. The size of bainite sheaves increased with the isothermal temperature increasing. Textured and chaotic distribution are observed in 200 and 300 °C microstructures. The evolution model of nanobainite morphology is established and analyzed. The bainitic ferrite of 200 °C heat treatment has a true thickness of 45 nm.
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Effects of isothermal heat treatment on nanostructured bainite
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morphology and microstructures in laser cladded coatings Yanbing Guo a, Kai Feng, Fenggui Lu, Ke Zhang, Zhuguo Li a,*, Seyed Reza Elmi Hosseini a, Min Wang a, b
Shanghai Key Lab of Materials Laser Processing and Modification, School of Materials Science
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a
State Key Laboratory of Metal Matrix Composite, School of Materials Science and Engineering,
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b
d
and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Shanghai Jiao Tong University, Shanghai 200240, China
Abstract:
Laser cladding and subsequent isothermal heat treatments have been used to fabricate nanostructured bainitic coatings. XRD has been used to determine the kinetics of bainitic transformation process. OM, SEM and TEM have been used to characterize the morphology and microstructures at different stages of transformation. The results showed that at the initial stage of bainitic transformation, the bainite sheaves are short and thin at a relatively low transformation
*
Corresponding author. Tel.: +86 21 54745878; fax: +86 21 34203024; E-mail address: [email protected] (Zhuguo Li).
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temperature. The fully transformed bainitic microstructure obtained at a relatively high temperature present a textured morphology. The chaotic growth orientations of the sheaves and the island like of the retained austenite have been observed at the low transformation temperature. A simple model has
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been established to describe the microstructures and the bainite sheaves growth evolutions during the isothermal holding at the different transformed temperatures. The morphology and distribution of the
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bainite in the coatings were analyzed by using the nucleation and growth rate of bainitic
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transformation theories, which is consisted with the experiment results.
Key words: Laser cladding; Nanostructured bainite; Microstructure; Morphology; Transformation;
1. Introduction
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Isothermal
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Recently, nanoengineering of steels has been taken a significant awareness by researchers and
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engineers for the potential applications in the modern steel industry [1, 2]. Several nanostructured steels have been developed using some special mechanic and annealing processes by the researchers in the last decade [3, 4]. Nanostructured bainitic steel is one of these nanocrystalline steels which is based on the pioneering research by Bhadeshia and co-workers at Cambridge University [5]. The new series of nanostructured steels present a ultrahigh strength of 2.3 GPa, toughness of 30 MPa·m1/2, ductility of 30% and high strain rate sensitivity [6]. High silicon content in the steels prevents the carbide precipitation, and high carbon and other austenite stabilizing elements (Mn, Cr, Mo, etc.) significantly retard the bainitic transformation process and cause the ultra-fine structures [6-8]. The morphology, scale and distribution of the nanoscale bainitic ferrite and interval film austenite microstructure are the crucial factors for the outstanding mechanical properties [9, 10].
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The nucleation and growth of the bainite sheaf are determinants for kinetics of bainitic transformation, and the morphology of the bainite is also controlled by the two factors [11]. In the past decades, many researchers have studied the kinetics of the bainitic transformation through the
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calculation and experiments. Garcia and Bhadeshia have investigated the theory describing the nucleation of bainite in the high carbon steels [12].The growth rate of the bainite has been calculated
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by Quidort and Brechet with the Trivedi equation [13]. In addition, Hu et al. have measured the
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growth rate of bainite plates in Fe-C-Mn-Si steel by in situ observation [14]. Almost all of the recent studies on kinetics of bainitic transformation focused on the overall transformation process, however,
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the correlation between the kinetic and bainite morphology is seldom reported. All the researches on
transformations.
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the kinetics of the bainitic transformation has inspired the acceleration of the nanobaintic
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Since the nanostructured bainitic transformation in the steels is always very slow at the low
Ac ce pt e
temperature [15], in the present study, laser cladding is used to fabricate fine prior austenite because of its high cooling rate [16], and aluminum and cobalt are added as the alloying elements to enhance the driving force of the austenite-ferrite transformation, which items cause to accelerate the transformation process [17]. In the present work, the morphology and distributions of the microstructures during the isothermal bainitic transformation in the laser cladded coatings are studied. The nucleation and growth theories are applied to analyze the formation process of the bainite sheaves and growth morphologies. A simple transformation processing model is established to describe and reveal the nanobainitic microstructures evolution process under different isothermal heat treatments.
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2. Materials and Experimental Procedure The coating materials were deposited on the mild steel substrate with dimensions of 160 mm × 15 mm × 12 mm. The substrate surface was polished to remove any stains or contaminants, then
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cleaned with acetone and alcohol prior to laser cladding. The coating material was Fe-based alloyed powder prepared by gas atomization method, and the diameter of the powder was in the range of
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50–150 µm. The nominal chemical composition (wt. %) of the substrate material and alloy powders
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used in this study are listed in Table 1.
Si
Mn
Mo
Co
0.81
1.51
1.96 1.13 0.32
1.59
Substrate 0.14
0.22
0.58 -
-
M
C
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-
Al
P
S
Fe
0.93 0.008
0.008
Bal.
-
0.02
Bal.
0.02
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Powder
Cr
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Table 1 Chemical composition of the powder and substrate (wt. %).
Fig. 1. 3.5 kW diode laser system.
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Fig. 2. Image of the laser cladded coating.
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The 3.5 kW diode laser system equipped with powder coaxial feeding system (as shown in Fig.
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1) was employed to fabricate the coatings (as shown in Fig. 2), and the laser beam size at the focal length (165 mm) was a rectangular spot with a dimensions of 2.5 mm × 3.5 mm. High-purity argon
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gas was used as shielding gas through the coaxial nozzle with a flow rate of 5 L/min to prevent the
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melted pool from heavy oxidation. The power of the laser beam was selected with variant value from 2.3 to 2.5 kW for considering the constant dilution at variant preheating temperatures of the
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substrates. The cladding materials were deposited on the preheated substrates, then the specimens
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were transferred into the muffle furnace for isothermal heat treatment. Moreover a digital thermometer was applied to monitor the surface temperature of the specimen to insure its temperature did not decrease from the temperature which set for isothermal heat treatment during the transferring. The specimens were directly quenched in the water at the ambient temperature after the isothermal holding. The whole process of the experiments are demonstrated in Fig. 3.
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Fig. 3. The temperature-time curves for laser cladding and heat treatment process.
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The time of isothermal heat treatment at the different temperatures was based on the calculated
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time-temperature-transformation diagram, and all of the parameters about laser processing and isothermal heat treatment are given in Table 2.
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Table 2 Processing parameters for laser cladding and isothermal transformation. Laser cladding
Isothermal transformation
Preheating
Scanning velocity
Powder
Transformation
Transformation
(W)
temperature (°C)
(mm/s)
Feeding rate
times
(g/min)
(°C)
(h)
2400 2300
temperature
220
10
30
200
0.5/1/2/4/8/12/24/48/72/120/180/240
270
10
30
250
0.25/0.5/1/2/3/4/8/12/16/20/24
320
10
30
300
0.25/0.5/0.75/1/1.5/2/3/4/6/8/16
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2500
d
Laser power
The phases of the coatings were determined by X-ray diffraction with an Ultima IV diffractometer. The radiation source was Cu Kα, operated at 40 kV and 30 mA with a scanning rate of 1 °/min. The specimens were sectioned, polished and finally etched using 4% nital for metallographic examination. The microstructure and morphology of the coatings were characterized using a Zeiss Axioplan 2 optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM JEOL JEM-7600F) equipped with Inca Energy energy-dispersive spectrometer (EDS). The TEM film foils
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were polished to 80 µm, and machined to 3 mm diameter rods, then electropolished with a twin-jet electropolisher at -30 °C in a solution of 5% perchloric acid and 95% methanol and an operating voltage at 50 V. A transmission electron microscopy (TEM JEOL JEM-2100F) operated at 200 kV
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was applied to study these thin film foils. 3. Results and discussion
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3.1 Transformation kinetics
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An assessment of the isothermal transformation kinetic is performed using the JMatPro software which is developed by Sente Software Corporation. The calculated TTT diagram is illustrated in Fig.
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4, which shows the experimental results of the time taken to initiate (open circles) and cease
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transformation (filled circles) under each isothermal temperature. The measured value for the degree of transformation are reasonable agreement with those calculated (by using the JMat Pro software).
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The average grain size number of the prior austenite during the calculation was set as 10 (about 10
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µm), which was determined by the liner intercept method under the standard ASTM: E112-13.
Fig. 4. Calculated TTT diagram plotted by the experimental results of the bainitic transformation. The transformation process is investigated through the measurement of the volume fraction of the retained austenite (RA), and the volume fractions of RA are measured using the XRD results (Fig. 5). The kinetic process of the bainitic transformation is illustrated using the evolutions of the RA
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volume fraction. The volume fraction of the RA is increased and then decreased with the increasing of the isothermal holding time, it inferred that after cladding and shorter time isothermal holding, the initial microstructure in the coatings is under cooling austenite, then most of the austenite transforms
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to martensite after quenching, and a small volume austenite remains. The fraction of bainitic ferrite is increasing with the isothermal time increased, then the stability of the adjacent austenite is increased
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by the carbon diffusion from bainitic ferrite, and more austenite remains after quenching. After a
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specific holding time, most undercooling austenite has been changed to bainite and all of the remaining austenite have high stability to maintain the austenite after quenching, and no martensite is
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forming. The volume of austenite remaining is decreasing with isothermal holding time increasing
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when most under cooling austenite continually transforms to bainitic ferrite. Finally, the volume of the retained austenite is no longer changing after the transformation is ceased, the microstructure
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consists of RA and bainitic ferrite. The microstructure is considered as the full transformed bainitic
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microstructure when the changing of the determined volume fraction of the retained austenite ceases (determined by using XRD). For the full transformed microstructure, the final fraction of the retained austenite is decreased as the decreasing isothermal temperature. Since this condition is met at ever increasing carbon concentrations when the transformation temperature is reduced, more bainite can form with greater undercoolings below BS [18].
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treatment.
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3.2 Microstructural evolution
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Fig. 5. Volume fraction of retained austenite as a function of the holding time in the isothermal heat
Fig. 6 illustrates the optical microstructure obtained at the initiate, medium and ceased stages of
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the bainitic transformation at three different temperatures. Fig. 6 (a) shows the initial microstructure
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of the coating after 1 hour holding at 200 °C and then quenching. The grey martensite distributes at
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the inner dendritic areas and the white austenite distributes at the prior austenite grain boundaries. Only few short and thin black bainite plates can be observed, which indicate that this stage is just at the end of the pre-bainitic (incubation) period [19]. Then martensite and austenite is observed beside bainite in the coatings after 4 hours isothermal holding (Fig. 6 (b)). When austenite continues to transform into bainitic ferrite, some branches outspread from the side of the sheaf trunk, and the growth direction of the branches tilt an angle with the prior sheaf orientation. Fully bainitic coating is obtained after the transformation ceases (Fig. 6 (c)), and only a small amount of retained austenite distributes at the prior austenite grain boundaries [20]. Fig. 6 (d) shows the microstructure of the initial stage of the bainite transformation obtained at 250 °C for 1 hour. The microstructure consists of a mixture of martensite, bainitic ferrite and retained austenite. The amount of bainite (black needle phase) increases with increasing isothermal holding time (Fig. 6 (e)), and the transformation ceases
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at about 8 hours isothermal treatment (Fig. 6 (f)). Similar regularity is found in the samples heat treated at the 300 °C. The bainitic transformation incubates before 1 hour and finished at about 4 hours. More retained austenite distributed at the prior austenite grain boundaries at a relatively higher
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transformation temperatures for the fully transformed microstructures, which is consistent with the
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XRD result in Fig. 5.
Fig. 6. Optical micrographs of bainite obtained at 200 °C (a) 1 h, (b) 4 h, (c) 48 h; 250 °C (d) 1 h, (e) 4 h, (f) 8 h; 300 °C (g) 1 h, (h) 1.5 h, (i) 4 h.
It can be found that the incubation period is prolonged with the bainitic transformation temperature decreasing compared the microstructures evolution under the three temperatures. Because the incubation period is a formation process of the carbon depleted region [21] and [22], high carbon diffusion coefficient at high isothermal temperature shorten the period. The morphology and the densities of the bainite sheaves are also different at various transformation temperatures. The amount of the bainite sheaves are in inverse relation to the transformation temperature. The length and thickness of the bainite sheaves increased with the increasing of the isothermal holding
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temperature, as can be seen from the Fig. 6 (b), (d) and (g). The average length and the average thickness of the sheaves increase from about 18 µm and 0.4 µm obtained at 200 °C to about 32 µm and 6.2 µm obtained at 300 °C, respectively. It also extracted form Fig. 7 that the increase of the
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average length is relatively lower than the thickness, caused by the different nucleation and growth
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rate under different transformation temperatures.
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Fig. 7. The average length and thickness of the bainite sheaf as a function of transformation
Ac ce pt e
temperature.
3.3 Morphology of the nanostructured bainite Fig. 8 shows SEM micrographs of the initial and ceased stages of bainitic transformation under three heat-treatment cycles. Bainite nucleates at sites near the austenite grain boundaries, then the bainite sheaves or lathes propagate through the repeated formation of the limited size sub-units [23]. The sheaves are composed of two or more paralleled bainitic ferrite laths which are separated by the thin film retained austenite, as shown in Fig. 8. The sheaves grow toward the grain interior until the growth are constricted by the grain boundary or the other sheaves. The length of the primary sheaves are usually very long, even some of them prolong across the whole dendrite, especially in the relatively higher temperatures. Some of the shorter sheaves form from the side of the prior sheaves at the relative lower temperature, as shown in Fig. 8 (a). The thickness of the sheaves formed at the
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lower temperature is thinner than that formed at higher temperatures, because the sheaves at higher
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temperature consists more bainite lathes.
Fig. 8. SEM micrographs of bainite obtained at 200 °C (a) 4 h, (b) 48 h; 250 °C (c) 1 h, (d) 8 h; 300 °C (e) 1 h, (f) 4 h.
The microstructures consist of several bainite sheaves with different growth directions when the transformation ceased, and the blocky retained austenite distributed at the prior austenite interdendritic areas. The disorder orientation of sheaves growth at the low temperature makes the microstructures exhibit chaotic features. The feature of the sheaves with independent growth directions is becoming uniform when transformed at a relatively higher temperature (Fig. 8 (d) and (f)). The sheaves usually grow through the whole prior austenite grains at a relatively higher transformed temperature.
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The distribution of the blocky retained austenite is discontinuous at the lower transformed temperatures (Fig. 8 (b)), and it presents more continuous at higher temperatures (Fig. 8 (d) and (f)). Apart from the thin film austenite between the bainitic ferrite laths and blocky retained austenite,
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there are also some islands like austenite distribution in the inner dendrite areas (dark arrowed in the Fig. 8 (b) and (d)). These retained austenite are surrounded by several sheaves which have different
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growth directions, and there are more island like retained austenite distributed between the bainite
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sheaves formed at lower temperatures.
Fig. 9. Model of microstructures evolution in laser cladded bainitic coatings.
The bainite nucleus firstly forms at the defects sites near the grain boundaries with fluctuation of the free energies and structures. The nucleus grows to subunit of bainitic ferrite through a shearing mechanism, and then grows to a lath or sheaf of bainite [24]. Thus, isolated lathes emanate from nucleation sites near the grain boundaries, and the sheaves are therefore free to increase until restricted by grain boundaries or other sheaves growing from other sides of the grain boundaries, as shown in Fig. 9. The growth of the sheaves is confined by the other platelets growing from the other
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sides of the grain boundary to make the length of these sheaves be shorter especially when the transformation temperature decreases. Some bainite nucleate at the surface of the existing or newly formed bainite sheaves when these sites satisfied the nucleation as the transformation continues. This
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phenomenon is called autocatalysis, which is related to the sympathetic nucleation [25]. The new sub-units nucleate at both sides of the prior bainite lathes, and then grow to a paralleled or angled
cr
lath with the prior ones. The sheaves angled with the prior bainite sheaves are more obviously in
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microstructures transformed at relatively lower temperatures, because the autocatalysis nuclei are always formed near the tips of the subunits of the prior sheaves, and the dislocations in this area
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cause deflection of the growth direction. The dislocation density in this area is higher at the lower
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isothermal temperature [26], which make the sheaves have more chance to branch off. The phenomena of bainite sheaves branching off and the chaotic microstructure are controlled
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by the kinetics of the bainitic transformation process. Tszeng has used Eq. (1) to describe the overall
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transformation kinetics of the bainite [25], and the autocatalysis on the surface of the sheaves is taking into consideration. The equation is given by: (1)
where f is the actual volume fraction of the bainitic ferrite, η and a are the coefficients related to the bainite subunits scale, I0 is the volumetric nucleation rate of bainitic ferrite at the austenite grain boundaries, IS is the nucleation rate at the austenite/bainite interface and t is the transformation time. The nucleation rate of bainitic ferrite decreases with decreasing temperature [27], which is in consistent with displacive interpretation of the nucleation rate. The scale of the subunit and the temperature dependence are controlled by the carbon diffusion, not by driving force, so the scale of subunit decreases with temperature decreasing [28]. When the temperature increases, the values of I0
Page 14 of 21
and IS increase, and η is the aspect ratio can be considered a constant, a1 < a2. If the volume fraction of the bainitic ferrite under the temperature of T1 and T2 are identical at the initial stage of transformation, f1 = f2, the reaction time at T1 is longer than that at T2, t1>t2. The I0 of the prior
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austenite with small grain size is higher than that of the larger grain size, I0s > I0l, that is the reason why the bainitic transformation in the laser cladded coatings are accelerated. The growth velocity of
(2)
is the weighted average diffusion
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where v is the growth rate of the bainitic ferrite,
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cr
bainitic ferrite according to the Quidort’s model [13] is given by
is the
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coefficient of carbon in austenite. σ is the ferrite/austenite interface energy per unit area,
molar volume of ferrite, Ω* is an algebraic combination value about the supersaturation, and the
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three parameters can be regarded as constants in the temperature range of bainitic transformation.
[13].
Ac ce pt e
∆Gm is the driving force of the bainitic transformation, which is also not changed as obviously as the is the main factor which controls the growth rate of bainitic ferrite. According to Eq.
(2), the growth rate of the bainitic ferrite increased with the increasing transformation temperature. Since the growth of the sheaves is soon restricted by the grain boundaries or other sheaves, the growth on the direction of the thickness is continued by repeat nucleation of the paralleled bainite plates. That is why the thickness increasing is higher than the length with the increasing transformation temperature.
Page 15 of 21
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Fig. 10. (a) The images of the austenite island in nanostructured bainite and (b) the schematic of the
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formation of the island austenite wireframe in (a).
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The growth rate of ferrite is low and the dimensions of the bainite sheaves are short at a relatively lower transformation temperature and long transformation time, and there are more
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bainite/austenite interface areas for the new autocatalysis nucleus, and the ratio for the branch off
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growth is high. The growth orientations exert more directions and present a chaotic morphology.
Ac ce pt e
More areas interior of the grain are isolated by the prior bainite sheaves, as shown in the Fig. 10 (a). The stability of small areas of austenite isolated by the bainite sheaf is increased at the final stage of the isothermal bainitic transformation, as a result of the carbon diffuses from the surrounded bainite sheaves (Fig. 10 (b)). The free energy difference (ΔGγα = Gα - Gγ) at the austenite/ferrite interface is identical or less than zero when the carbon concentration is higher than T0ʹ , the bainitic transformation ceased. The retained austenite remains as the island like morphology in the inner area of the bainite microstructures.
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3.4 Thickness of the nanostructured bainite
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Fig. 11. TEM morphology of the bainite plate obtained under different isothermal temperatures, (a) 200 °C, (b) 250 °C, (c) 300 °C, (d) nanoscale twins in retained austenite after 200 °C transformation. The TEM was conducted to analysis the microstructures of the bainite plates and retained austenite (Fig. 11). There are some similar features among the microstructures formed at different temperatures. The microstructures consist of parallel bainitic ferrite plates and retained austenite between them, some twinnings are observed in the retained austenite which is caused by the plastic deformation during the bainitic transformation [28]. Beside similarities, the scales of the bainite plates under different transformation temperatures are obviously different. The bainite plate scales are crucial factors for the properties of the coatings, and the thickness of the true plate are determined through the measurement of the mean linear intercept in the normal direction to the plate length. The
Page 17 of 21
measurements were made for approximately 30 times for each specimens. The true-thickness of the ferrite plates are calculated as follows [29] (3) is the mean linear intercept normal to the length of the plates, t is the true thickness
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where
of the plates. The measured plate thickness t has been plotted as a function of the transformation
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temperature in Fig. 12.
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Fig. 12. Stereology corrected thickness of bainite plates as a function of the transformation temperature.
The maximum true thicknesses of the bainitic ferrite is lower than 100 nm, which is in the scale of the nanostructured bainite. The thickness of bainitic ferrite is thinner at a relatively lower transformation temperature. As the theory of the diffusionless transformation, if the shape deformation during the transformation is elastically accommodated and the bainite plates are allowed to grow free, the thickness of the lath should be thicker at lower temperatures [30]. The experiment result is obviously contradict with the theory, because the main determinant of the bainitic ferrite thickness is the strength adjacent retained austenite. During the thickening process of the bainitic ferrite, plastic relaxation of the adjacent austenite generate dislocation debris will resist the movement of the ferrite/austenite interface, high yield strength of the austenite at low temperature is
Page 18 of 21
expected caused thinner bainitic ferrite plates [23]. The effect of free energy change accompanying transformation is smaller than the strength of the adjacent austenite. The most important parameter that can increase the strength of nanostructured bainite is to obtain the ultra-fine size of bainitic
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ferrite [17]. 4. Conclusions
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Laser cladding and subsequent isothermal holding were used to fabricate Fe-based coatings with
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nanostructured bainite. The kinetics process of the isothermal bainitic transformation in the laser cladded coatings was accelerated compared with the barely isothermal heat treatment, as a result of
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the small prior austenite grain size after laser cladding. Textured morphology was clearly observed at
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relatively higher transformation temperatures as a result of the less orientations and high growth rate of the bainite sheaves. On the contrary, the branch off nucleation of the bainite sheaves and low
d
growth rate caused a chaotic microstructures obtained at a relatively lower temperatures. Island like
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retained austenite surrounded by bainite sheaves were observed in the grain interior when the transformation is ceased at the lower temperatures. The carbon diffusion from the surrounded bainite sheaves to the austenite increases the stability of the retained austenite. The thickness of the bainitic ferrite at three transformation temperatures is in the scale of the nanostructures, which decreased with the decreasing of the temperature. Acknowledgment
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No.51171116) and the Ministry of Science and Technology of the People's Republic of China (Grant No. 2009DFB50350).
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