Materials Science & Engineering A 724 (2018) 260–268
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Microstructure and mechanical behavior of an AISI 304 austenitic stainless steel prepared by cold- or cryogenic-rolling and annealing
T
⁎
Chengsi Zhenga,b, , Chunjiao Liua, Minghao Rena, Heng Jianga, Longfei Lic a
National Safety Engineering Technology Research Center for Pressure Vessels and Pipelines, Hefei General Machinery Research Institute Co. Ltd., Hefei 230031, China Hefei General Machinery Research Institute Special Equipment Inspection Station Co. Ltd., Hefei 230031, China c State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metastable austenite Cold- or cryogenic-rolling Annealing Microstructure Mechanical properties
An AISI 304 austenitic stainless steel (ASS) with various ultrafine- or fine-grained structures was fabricated by cold- or cryogenic-rolling and annealing. The microstructure and mechanical properties of the ultrafine- or finegrained ASS were investigated based on statistical data and physical metallurgy. The results showed that much more volume fraction of α′-martensite can be obtained by cryogenic-rolling in comparison with cold-rolling under a similar rolling strain, and ε-martensite was a medium to transform into α′-martensite finally during cryogenic-rolling. The deformed ASS with larger volume fraction of α′-martensite was beneficial to obtaining finer structure with a narrow distribution of grain sizes after the similar annealing process. The cycle annealing was a feasible method to make reverse transformation completely and to inhibit the structural coarsening simultaneously for the cold- or cryogenic-rolling ASS. The yield strength was enhanced by cryogenic-rolling and cycle annealing to be approximately 2.7 times higher than that of solution-treatment state. The tensile strength was not changed evidently, and the uniform strain was apparently decreased with reducing grain size. There is no significant relevance between the mechanical stability of austenite and average grain size for the ultrafine- or fine-grained ASS; however, their mechanical stability was less than that of solution-treatment state.
1. Introduction Austenitic stainless steels (ASSs) have a wide range of industrial applications due to their good corrosion resistance, good comprehensive mechanical properties, good malleability and weldability [1–3]. A feature of ASSs is low yield strength of approximately 100 MPa resulted from coarse γ-austenite (face-centered cubic, fcc) phase [4], which is detrimental to the lightweight design of ASSs equipment. The yield strength of ASSs can be improved via pre-straining due to the strengthening effect of crystal defects produced by lower strain and strain-induced martensite (SIM) caused by higher strain [5,6]. The SIM consists of strain-induced α′-martensite (SIMα′, body-centered cubic, bcc) and strain-induced ε-martensite (SIMε, close-packed hexagonal, hcp) [7]. However, the presence of crystal defects and SIM are harmful to the corrosion resistance [8–10] and toughness [11,12], and, namely, the strain-strengthening is limited. It has been well known that the yield strength and the toughness of single-phase metal can be significantly improved by grain-refinement because of the Hall-Petch effect [13]. Meanwhile, unlike the ultrafine-grained (0.1–2 µm) ferritic steel and aluminum alloys, etc., the ability of work-hardening and uniform
plastic deformation of ASSs will be not deteriorated obviously when the average gain size is refined to less than 2 µm, since the density of crystal defects can be effectively stored by SIM and strain-induced twin (SIT) [14,15]. Additionally, there are some experimental results to show that the corrosion resistance of ASSs can be also enhanced by grain refinement [16,17]. Summarily, the ultrafine- or fine-grained strengthening is a hopeful pathway to improve the yield strength, toughness and corrosion resistance of ASSs simultaneously without obviously deteriorating ductility and work-hardening ability. Recently, the grain-refinement of ASSs is usually realized via severe plastic deformation (SPD) and subsequently reversion annealing [18–20]. The SPD technique imposes large accumulated plastic strains to obtain a large number of SIMα′ and crystal defects, which typically includes cold-rolling (CR), equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), etc. [21,22]. The volume fraction of SIMα′ mainly depends on the chemical composition, accumulated plastic strain and deformation temperature, etc. [23–25]. The main purpose of reversion annealing is to revert the SIMα′ back to nano- or ultrafinegrained austenite under specific conditions. The reversion transformation of SIMα′, recovery and recrystallization or discontinuous
⁎ Corresponding author at: National Safety Engineering Technology Research Center for Pressure Vessels and Pipelines, Hefei General Machinery Research Institute Co. Ltd., Hefei 230031, China. E-mail address:
[email protected] (C. Zheng).
https://doi.org/10.1016/j.msea.2018.03.105 Received 5 February 2018; Received in revised form 25 March 2018; Accepted 26 March 2018 Available online 27 March 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved.
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fα′. The calculating method for fα′ has been reported in Ref. [32] in detail and the corresponding uncertainties are believed to be less than 3%. The amount of ferromagnetic α′-martensite (F) was evaluated by using a Feritscope® (Helmut Fischer GmbH, model MP3E-S), and the Feritscope was calibrated with standard samples before measurement. The microstructure observations were performed by using a scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) in a Zeiss SUPRA55 field-emission SEM with a detector of backscattered electrons and an orientation-imaging microscope (OIM). The specimens were etched to reveal the α′-martensite phase by using the standard method with two solutions in 1:1 ratio, i.e., 0.20 g sodiummetabisulfate in 100 ml distilled water and 10 ml hydrochloric acid in 100 ml distilled water [33]. The α′-martensite phase was showed in grey after etching a period of time in the range of 1–6 min depending on its volume fraction. The grain boundaries of the ASS were etched by using the standard method with aqua regia consisting of nitric acid and hydrochloric acid in a volume ratio of 1:3. The average grain size and the distribution of grain sizes were measured from the SEM images by using linear intercept method. Tensile specimens were cut from the cold- or cryogenic-rolled samples with a gage section of 30 × 6 × 1.4 mm3, as shown in Fig. 1, and the tensile axis of specimens were machined along the rolling direction. The tensile specimens were annealed at 800 °C for 18 min or cycle annealing to fabricate various ultrafine- or fine-grained structures mainly consisting of single-phase austenite. The cycle annealing process was to heat the sample at 800 °C for 2.5 min with two cycles. The surfaces of tensile specimens after heat treatment were mechanically polished sequentially using wet sandpapers of 120–1000 grit at room temperature. Room-temperature (RT) tensile tests were conducted by using a custom built mechanical test system at a strain rate of 6 × 10−4 s−1. A contact extensometer was used to measure strain in the sample gage upon loading. Feritscope testing were carried out in the gage section to estimate the content of α′-martensite before and after tensile testing.
recrystallization are probably to occur simultaneously or sequentially observing different kinetics during the process of reversion annealing. As a result, ASSs prepared by SPD and reversion annealing are often mischcrystal structure, showing a clear bimodal or multimodal distribution of grain sizes [19,26]. To date, there are some works on the cold-rolling and annealing to obtain a mostly single-phase austenite structure with an average grain size less than 2 µm for the ASSs with lower carbon content [14,27,28]. It is relatively difficult to obtain the ultrafine-grained structure consisting of mostly single-phase austenite through cold-rolling and annealing for the ASS with higher carbon content, i.e., higher mechanical stability of austenite, which probably results from a lower volume fraction of SIMα′ in the cold-rolling condition [23] and discontinuous recrystallization during the subsequently annealing [29]. It is noteworthy that Kumar et al. [30] recently has been fabricated a single-phase ultrafine-grained 304L ASS by using cold-rolling to a total reduction of 90% and 775 °C annealing for 60 s with four cycles, which is mainly ascribed to the inhibitory effect of cycle annealing on the discontinuous recrystallization. Moreover, it can be obtained much more volume fraction of SIMα′ under cryogenic deformation than that under room-temperature at the same strain, which has been clearly elucidated by Olson-Cohen model [31]. In this paper, cryogenic-rolling and cycle annealing are used to obtain a large number of SIMα′ under a lower plastic strain and to inhibit the discontinuous recrystallization, respectively, in an attempt to form an ultrafine- or fine-grained structure consisting mainly of single-phase austenite for an AISI 304 ASS. Furthermore, the microstructure evolution, mechanical properties and work-hardening behavior of the ultrafine- or fine-grained ASS are discussed in comparison with that of coarse-grained ASS prepared by solution-treatment. As compared to previous studies that main focus on the microstructure evolution and mechanical tests for ASSs prepared by cryogenic-rolling and annealing, this paper also pays attention to the effect of grain size and pre-existing dislocations on the mechanical behavior through tailoring the mechanical stability of austenite. 2. Experimental
3. Results and discussion
The material studied was a commercial AISI 304 ASS with a chemical composition of 0.045 C, 1.11 Mn, 18.13 Cr, 8.25 Ni, 0.44 Si, 0.053 N, 0.0076 S and 0.034 P (mass%). The AISI 304 ASS was solutiontreated at 1100 °C for 30 min, and the grain size after solution-treatment was approximately 51 µm. Subsequently, solution-treated plates were placed in liquid nitrogen for 20 min to ensure the temperature of sample around − 196 °C before each pass rolling, and the samples were cryogenic-rolled from 8 mm to 5.3–6.7 mm by multi-pass unidirectional rolling, i.e., total true stain (ε) of approximately 0.15–0.42. For comparison, the samples were also cold-rolled from 16 mm to 8 mm (ε ≈ 0.69) by the similar process but without cryogenic-treatment in each pass rolling. The amount of α′-martensite was measured by using X-ray diffraction (XRD) analysis due to different crystal structures between austenite and α′-martensite and magnetic technique based on ferromagnetism of α′-martensite. The XRD measurement was performed on SmartLab® diffractometer (Rigaku Corporation). The volume fraction of α′-martensite can be calculated by using the following equation:
3.1. Microstructure after rolling The SEM microstructure of the AISI 304 ASS after cold-rolling to ε ≈ 0.69 is shown in Fig. 2a, and the counterparts after cryogenicrolling to ε ≈ 0.15–0.42 are displayed in Fig. 2b-d. Obviously, the original coarse-grained austenite was divided into a large number of pieces, and the XRD analysis (Fig. 3) reveal that the transformation of partial austenite to SIM was occurred by cold- or cryogenic-rolling. These results suggested that the austenite was cut apart into patches due to the formation of SIM and the residual austenite should store
n
fα′ =
1/ n ∑i = 1 Iαi ′/ Rαi ′ n
m
1/ n ∑i = 1 Iαi ′/ Rαi ′ + 1/ m ∑i = 1 Iγi / R γi
(1)
where fα′ is the volume fraction of α′-martensite; Iαi ′ and Iγi are the integrated intensity of any diffraction peak from α′-martensite and austenite, respectively; Rαi ′ and R γi are the scattering factors of any diffraction peak from α′-martensite and austenite, respectively; n and m are the examined number of peaks from α′-martensite and austenite, respectively. The austenite peaks (111), (200), (220) and (311), and α′martensite peaks (110), (200) and (211) were selected to determine the
Fig. 1. Schematic of the sample dimensions for uniaxial tensile test. 261
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Fig. 2. SEM images of the cold- or cryogenic-rolling (CR or CRR) AISI 304 ASS at various total true stains: (a) CR to ε ≈ 0.69; (b) CRR to ε ≈ 0.15; (c) CRR to ε ≈ 0.19; and (d) CRR to ε ≈ 0.42.
than that of cold-rolling to ε ≈ 0.69 (fα′ ≈ 43%). Namely, the cryogenic-rolling can obtain much more volume fraction of α′-martensite than that of cold-rolling under a similar rolling strain, and the kinetics of SIMα′ was accelerated by reducing deformation temperature for the AISI 304 ASS. The slower kinetics of SIMα′ for the cold-rolling was mainly attributed to the temperature rise during deformation, which has been well confirmed by works of Hecker et al. [34]. The temperature rise can be suppressed by the cryogenic-treatment before rolling, resulting in a speedy kinetics of SIMα′ for the cryogenic-rolling in line with Olson-Cohen model [31]. Additionally, there is SIMε peak (indicated by arrows in Fig. 3) to appear in XRD pattern after cryogenicrolling to ε ≈ 0.15 and disappear after cryogenic-rolling to ε ≈ 0.42 (Fig. 3), accompanying with an increase in fα′ from ≈ 77% to ≈ 98%. Since the intersection of ε-martensite is one of the nucleation sites for the formation of SIMα′ [31], it was indicated that ε-martensite was a medium to transform into α′-martensite finally [35]. 3.2. Microstructure after annealing
Fig. 3. XRD patterns of the cold- or cryogenic-rolling (CR or CRR) AISI 304 ASS at various total true stains.
The tensile specimens were cut from the cold- or cryogenic-rolling (ε ≈ 0.69 or 0.42) ASS plate along rolling direction. Subsequently, the specimens were annealed at 800 ℃ for 18 min or 2.5 min with two cycles, and Figs. 4 and 5 show the corresponding SEM microstructures and their grain size distributions, respectively. Moreover, the fα′ in annealed specimens were determined by Feritscope reading (F) and a conversion coefficient of 1.42, i.e., fα′ = 1.42 F (Appendix A). The coldrolled sample annealed at 800 ℃ for 18 min was nearly fully made up of austenite (fα′ ≈ .23%) with large amounts of ultrafine-grains and a few fine-grains (Fig. 4a), i.e., mischcrystal structure. Moreover, an observed maximum grain size in Fig. 4a is ≈ 11 µm and the average grain size is ≈ 2.8 µm (Fig. 5a), indicating the selective growth of some grains. The corresponding cryogenic-rolled sample also shows a practically singlephase ultrafine-grained austenite structure (fα′ ≈ 1.2%) with an average
amounts of crystal defects without SIM. It should be noted that amounts of austenite are curved after cold-rolling to ε ≈ 0.69 (indicated by arrows in Fig. 2a), and, however, the appearance of austenite after cryogenic-rolling to ε ≈ 0.15–0.42 shows straightness, as shown in Fig. 2a-d. These phenomena implied that much more strain energy (SE) was consumed to occur SIM for cryogenic-rolling but part of SE was used to coordinate deformation between austenite and α′-martensite for cold-rolling. The XRD results indicated that the cryogenic-rolling ASS with ε ≈ 0.15 and 0.19 contained fα′ ≈ 77% and 84%, respectively. Moreover, almost all the austenite has been transformed into SIMα′ (fα′ ≈ 98%) by the cryogenic-rolling to ε ≈ 0.42, which was much higher 262
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Fig. 4. SEM images of the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenic-rolling (CR or CRR) and annealing: (a) CR to ε ≈ 0.69 + 800 ℃ for 18 min; (b) CRR to ε ≈ 0.42 + 800 ℃ for 18 min; (c) CR to ε ≈ 0.69 + 800 ℃ for 2.5 min with two cycles; and (d) CRR to ε ≈ 0.42 + 800 ℃ for 2.5 min with two cycles.
Fig. 5. Size distributions of grains within the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenic-rolling (CR or CRR) and annealing: (a) CR to ε ≈ 0.69 + 800 ℃ for 18 min; (b) CRR to ε ≈ 0.42 + 800 ℃ for 18 min; (c) CR to ε ≈ 0.69 + 800 ℃ for 2.5 min with two cycles; and (d) CRR to ε ≈ 0.42 + 800 ℃ for 2.5 min with two cycles. 263
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grain size of ≈ 1.8 µm (Fig. 4b), however, exhibiting a more uniform distribution of ultrafine- and fine-grained sizes (Figs. 4b and 5b). This indicated that the microstructure of cryogenic-rolled sample was not easy to coarsen during the annealing at 800 ℃ for 18 min in comparison with the cold-rolled counterpart. Moreover, it can be inferred that the ultrafine-grains in the Fig. 4a were probably to be derived from the reverse transformation of SIMα′. Namely, the nucleation-growth rate of reverted austenite was less than the recrystallization-growth rate of deformed austenite, which resulted in a mischcrystal structure for the cold-rolled sample annealing at 800 ℃ for 18 min. Fig. 4c displays a nearly single-phase structure of the cold-rolled sample after annealing at 800 ℃ for 2.5 min with two cycles, mainly consisting of ultrafine- and fine-grained austenite (fα′ ≈ 2%) with an average grain size of ≈ 1.2 µm (Fig. 5c). However, the corresponding cryogenic-rolled counterpart shows a duplex-structure composed of ultrafine-grained austenite matrix embedded with unreversed α′-martensite (fα′ ≈ 7.3%) with an average size of ≈ 1.0 µm (Figs. 4d and 5d). Although the distribution of grain sizes in the cold-rolled sample after cycle annealing is more uniform than that after isothermal annealing (Fig. 5a and c), its distribution of grain sizes is not well-distributed as the cryogenic-rolled sample after cycle annealing (Fig. 5d). Hence, there was probably an overlapping among recrystallization, reversed transformation and grain growth for the cold-rolled sample during cycle annealing [30]; however, the discontinuous recrystallization was inhibited by cycle annealing. Fig. 4d exhibits a nearly uniform ultrafinegrained structure embedded with few fine-grains, which suggests that the simultaneous occurrence of nucleation and growth in all reverted austenite are the main process for the cryogenic-rolled sample during cycle annealing. In summary, the difference in deformed structure between cold- and cryogenic-rolled sample, i.e., SIMα′ + deformed-austenite and SIMα′, respectively, led to the different kinetics of recrystallization, nucleation and growth during annealing, resulting in different final microstructures. Specifically, the cryogenic-rolling was beneficial to obtaining finer structure with a narrow distribution of grain sizes after annealing, which was opposite to the case of cold-rolling. Moreover, the cycle annealing at 800 ℃ for several minutes with some cycles could be a feasible way to occur reverse transformation completely and to inhibit the structural coarsening simultaneously for cold- and cryogenic-rolled sample.
Fig. 6. Engineering stress-strain curves (a) and true stress-strain curves (b) of the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenicrolling and annealing.
3.3. Mechanical behavior Fig. 6 shows the engineering stress-strain curves and the true stressstrain curves of the ultrafine- or fine-grained ASS prepared by cold- or cryogenic-rolling and annealing, and the corresponding microstructural parameters and mechanical properties are summarized in Fig. 7 and Table 1. In addition, the counterpart of coarse-grained ASS prepared by solution-treatment is also displayed in Figs. 6 and 7 and Table 1 for comparison. The yield strength of the ultrafine- or fine-grained ASS are higher than that of the coarse-grained ASS, but their tensile strength and uniform strain (εu) are less than that of coarse-grained ASS (Table 1 and Fig. 7). Although the uniform strain decreases with refining grains, it still can be maintained in approximately 0.2 even if the average grain size is refined to 1 µm for the ultrafine-grained ASS by cryogenic-rolling and cycle annealing. Moreover, the corresponding yield strength can be enhanced to approximately 2.7 times higher than that of coarse-grained ASS through cryogenic-rolling and cycle annealing, i.e., 697 MPa vs. 251 MPa. Nevertheless, in integrated view, the strength-ductility synergy of the ultrafine-grained ASS prepared by cold-rolling and cycle annealing is better than that by cryogenic-rolling and cycle annealing, i.e., yield strength of 611 MPa and uniform strain of 0.28. In brief, the suitable grain-refinement can significantly increase the yield strength without the dramatic decline of ductility for the ASS, which is a feasible method to realize the lightweight of equipment which designs according to the yield strength of materials. Moreover, the enhancement
Fig. 7. Statistical charts of the true yield strength (σy), the true tensile strength (σu) and the true uniform strain (εu) as a function of average grain size (d) for the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenicrolling and annealing.
of yield strength via refining grains can be mainly attributed to the HallPetch effect besides the strengthening caused by residual SIMα′ [22], and the characters of tensile strength and uniform strain changing with grain size are related to the work-hardening behavior [36]. Fig. 8 exhibits the work-hardening rate as a function of true strain for the ASSs. The work-hardening behavior of the ultrafine-grained ASS prepared by cryogenic-rolling and cycle annealing is distinctly different from other ASSs, and, namely, its work-hardening rate decrease continuously to necking but other ASSs demonstrate a typical three stage work-hardening behavior (the inset in Fig. 8) [25]. The fα′ of gage part 264
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Table 1 Microstructural parameters and mechanical properties of the AISI 304 ASS prepared by cold- or cryogenic-rolling and annealing. Process
d (μm)
fα′0 (%)
fα′s (%)
CR0.69 + 800-18 CRR0.42 + 800-18 CR0.69 + 800-2.5 × 2 CRR0.42 + 800-2.5 × 2 Solution treatment
2.8 ± 1.7 1.8 ± 1.0 1.2 ± 0.5 1.0 ± 0.4 51 ± 2.7
1.2 0.6 1.8 7.3 0.9
74.0 61.4 55.6 48.1 67.8
± ± ± ± ±
0.5 0.2 0.4 0.6 0.2
± ± ± ± ±
1.6 5.1 12.4 8.2 0.7
σy (MPa)
σu (MPa)
σy/σu
εu
Δfα′/εu (%)
311 391 611 697 251
1165 1131 1219 1138 1352
0.27 0.35 0.50 0.61 0.19
0.32 0.32 0.28 0.18 0.44
228 190 192 227 152
Note: d is average grain size; fα′0 is the volume fraction before tensile testing; fα′s is the volume fraction after tensile testing; σy is the true yield strength; σu is the true tensile strength; Δfα′ is the increase in volume fraction of SIMα′, fα′s−fα′0; εu is the true uniform strain.
fine-grained ASS. On the one hand, these results showed that the transformation of SIMα′ has been triggered during tensile straining for the ultrafine- or fine-grained ASS, and the formation of SIMα′ can enhance the work-hardening ability of the ASSs due to the accumulation of dislocations [14]. On the other hand, the dislocations density cannot be effectively stored during tensile straining for an ultrafine-grained single-phase metal caused by the severe annihilation near grain boundaries, and, as a result, the work-hardening ability was deteriorated, which was common to ultrafine-grained ferritic steel and aluminum alloy, etc. [37–39]. Therefore, the ultrafine-grained ASS prepared by cryogenic-rolling and cycle annealing does not show a peak of work-hardening rate as other ASSs (Fig. 8), which probably resulted from that the formation of SIMα′ to promote hardening was weakened by the ultrafine-grained austenite to cause softening. According to the Considére criterion [40], σ ≤ dσ/dε, where σ, ε and dσ/dε, are the true stress, the true strain and the work-hardening rate, respectively, the suitable work-hardening behavior is the critical factor to improve the uniform strain for the ultrafine-grained ASS, which was mainly affected by the mechanical stability of austenite [14,26].
Fig. 8. Work-hardening rate as a function of true strain for the ultrafine- or finegrained AISI 304 ASS prepared by the cold- or cryogenic-rolling and annealing.
before and after tensile testing (fα′0 and fα′s) determined by fα′ = 1.42 F (Appendix A) are summarized in Table 1 and the SIMα′ after tensile testing detected by EBSD are displayed in Fig. 9 for the ultrafine- or
Fig. 9. EBSD images of the gage part after tensile testing for the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenic-rolling (CR or CRR) and annealing: (a) CR to ε ≈ 0.69 + 800 ℃ for 18 min; (b) CRR to ε ≈ 0.42 + 800 ℃ for 18 min; (c) CR to ε ≈ 0.69 + 800 ℃ for 2.5 min with two cycles; and (d) CRR to ε ≈ 0.42 + 800 ℃ for 2.5 min with two cycles. Note: Grain boundaries (> 15°) are shown in black, austenite in yellow, ε-martensite in blue and α′-martensite in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 265
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coarse-grained ASS (Table 1 and Fig. 10), suggesting that the inhibition effect of grain-refinement on SIMα′ was overwhelmed by the promotion effect of pre-existing dislocations on SIMα′. The characteristic of mechanical stability for austenite, i.e., the kinetics of SIMα′, is the main factor to control the work-hardening behavior, which probably can be tailored by the grain size and original dislocations density based on the above analysis. Hence, it is extremely important to tailor the austenite not to complete the SIMα′ at small strains for obtaining a higher uniform strain, since the early completion of SIMα′ is disadvantage to play the role of transformation-induced plasticity (TRIP), e.g., the ultrafine-grained ASS prepared by cryogenicrolling and cycle annealing. Summarily, both effects of grain-refinement to inhibit the SIMα′ and the pre-existing dislocations to promote SIMα′ coexist in the ultrafine- or fine-grained ASS during tensile testing, and these are macroscopically embodied in the effect on work-hardening behavior. Furthermore, the mechanical stability of austenite in the ultrafine-grained ASS could be controlled by accommodating the pre-existing dislocations density through an additional annealing process or pre-straining, and then a better strength-ductility synergy may be obtained, which will be explored in depth in a later work.
Fig. 10. The volume fraction of SIMα′ (fα′) in gage part before and after tensile testing (fα′0 and fα′s), the increase in fα′ (Δfα′ = fα′s - fα′0) and the ratio of Δfα′ to uniform strain (εu) as a function of average grain sizes (d) for the ultrafine- or fine-grained AISI 304 ASS prepared by the cold- or cryogenic-rolling and annealing.
4. Conclusions 3.4. Mechanical stability of austenite
An AISI 304 ASS with various ultrafine- or fine-grained structures was prepared by combining cold- or cryogenic-rolling with subsequently annealing, and the corresponding microstructures and mechanical properties were investigated based on quantitative characterization, uniaxial tensile tests and physical metallurgy. The main results and conclusions are the following:
There are some experimental results [15,27,41] to show that the mechanical stability of austenite was strongly dependent on its grain size. The effect of grain size on the mechanical stability can be evaluated by the increase in elastic strain energy required for the transformation of austenite to martensite, which can be expressed as [26]:
ΔEV = 1276.1(x / d )2 + 562.6(x / d )
(2)
1. The kinetics of SIMα′ of the ASS during cryogenic-rolling was much higher than that during cold-rolling, and the phase transformation procedure of γ-austenite → ε-martensite → α′-martensite was probably occurred during cryogenic-rolling. 2. The finer structure with a narrow distribution of grain sizes can be fabricated by cryogenic-rolling and annealing in comparison with the counterpart of cold-rolling, which was attributed to its deformed structure nearly completely consisting of SIMα′. 3. The cycle annealing at 800 ℃ for several minutes was an effective pathway to reverse α′-martensite into austenite completely and to inhibit the structural coarsening simultaneously for the cold- or cryogenic-rolling ASS. 4. The yield strength of the ultrafine-grained ASS prepared by cryogenic-rolling and cycle annealing can be up to 2.7 times higher than that of solution-treated ASS, i.e., 697 MPa vs. 251 MPa, but the corresponding tensile strength and uniform strain were lower than that of solution-treated ASS. 5. The mechanical stability of austenite in the ultrafine- or fine-grained ASS was less than that in the solution-treated ASS, which was probably resulted from the residual dislocations after cryogenicrolling and annealing process.
where ΔEV is the increase in elastic strain energy; x is the thickness of martensite plate, which is approximately of 0.2 µm [42]; and d is the grain size of austenite. According to the results of Challa et al. [15] and Matsuoka et al. [26], the value of ΔEV increases abruptly when the average grain size of austenite is refined to 0.1–1 µm, and, meanwhile, the transformation of austenite to SIMα′ is suppressed. In other words, the probability to nucleate SIMα′ decreases with reducing grain size of austenite, especially when the grain size is refined to a comparable size to martensitic lath [15,41]. The Δfα′/εu, Δfα′ = fα′s − fα′0, is the average growth rate of SIMα′ during uniform straining, which can be used as a characteristic parameter to evaluate the mechanical stability of austenite. As shown in Fig. 10, although the Δfα′ decreases with reducing average grain size for the ultrafine- or fine-grained ASS, there is no significant relevance between Δfα′/εu and average grain size. Additionally, it should be noted that the strength-ductility synergy of the ultrafine-grained ASS prepared by cold-rolling and cycle annealing is better than that prepared by cryogenic-rolling and cycle annealing, accompanying with a similar average grain size and a better mechanical stability of austenite (Fig. 10). These implied that there was another factor to affect the mechanical stability of austenite besides the average grain size [15], and the original dislocations density was probably the other factor for metastable ASSs [29]. Kisko et al. [29] has proved that there are still many dislocations to remain in austenite grains when a cold-rolled ASS was annealed at 700–1000 ℃ for 1–200 s, accompanying with a similar relationship between mechanical stability of austenite and average grain size to this study. The pre-existing dislocations are incline to form stacking faults rapidly during tensile straining in line with the work of Spencer et al. [43], and it has confirmed that the intersections of stacking fault are effective sites for the nucleation of SIMα′ [31]. Hence, it is safely to infer that the pre-existing dislocations can promote the formation of SIMα′ and accelerate the kinetics of SIMα′. Furthermore, all the Δfα′/εu of ultrafine- or fine-grained ASS are higher than that of
Acknowledgments Financial support from the National Natural Science Foundation of China (51601052) and from the Anhui Provincial Natural Science Foundation (1708085QE95) is gratefully acknowledged. We would like to express our sincere thanks to Prof. Yandong Wang, Prof. Zuqing Sun and Prof. Wangyue Yang from the University of Science and Technology for discussing the effect of microstructure evolution on mechanical behavior. We would also like to express our sincere thanks to Ph.D. Jinghui Xu, Ph.D. Jintao Shi, M.S. Qizhe Ye and M.S. Jian Luo from the University of Science and Technology for the help of cold- or cryogenicrolling and microstructural characterization.
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Appendix A Feritscope measurement is non-destructive, takes only a few seconds and can be performed on a small sample in comparison with XRD analysis. However, Feritscope taking the determination of the magnetic permeability as the operational principle, which causes an error in the measured values due to the magnetic permeability of α′-martensite dependent upon strain [44]. The fα′ obtained by XRD analysis is probably close to the real content, since it has an evidence in Fig. 2d exhibiting almost no austenite and the fα′ obtained by XRD analysis is approximately 98%. Although the value of Feritscope testing (F) deviates from the fα′ determined by XRD analysis, the results of Feritscope testing can be converted to α′-martensite contents of X-ray analysis by using a calibration curve, which is beneficial to conveniently obtain credible α′-martensite contents. Hence, the coldand cryogenic-rolled samples, some cold-rolled samples with various annealing processes were used to establish the relationship between fα′ and F. The cold- and cryogenic-rolled samples were cut into the cuboid specimens with size of 18 × 5 × 5 mm3, and some cold-rolled samples were annealed at 700–950 °C for 5–30 min or cycle annealing to fabricate various duplex structure consisting of austenite matrix embedded with α′martensite. The cycle annealing process was to heat the sample at 900 °C for 5 min with two cycles and then at 850 °C for 5 min with two cycles. The fα′ quantified by using XRD analysis (Eq. (1), Fig. 3 and Fig. A1) and Feritscope testing are exhibited in Fig. A2, indicating a clearly difference between XRD analysis and Feritscope reading, and the conversion coefficient between fα′ determined by XRD and Feritscope reading is approximately 1.42, i.e., fα′ = 1.42 F, which is similar to the result of Talonen et al. [44].
Fig. A1. XRD patterns of the ultrafine- or fine-grained AISI 304 ASS prepared by the cold-rolling and various annealing processes.
Fig. A2. The volume fraction of α′-martensite determined by XRD analysis (fα′) as a function of Feritscope testing (F) for the cold- or cryogenic-rolling AISI 304 ASS and the ultrafine- or fine-grained AISI 304 ASS prepared by combining cold-rolling with various annealing processes.
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