metals Article
Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates Cheng-Yi Chen, Fei-Yi Hung *, Truan-Sheng Lui and Li-Hui Chen Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan;
[email protected] (C.-Y.C.);
[email protected] (T.-S.L.);
[email protected] (L.-H.C.) * Correspondence:
[email protected]; Tel.: +886-6-2757575 (ext. 62950); Fax: +886-6-2346290 Academic Editor: Hugo F. Lopez Received: 30 November 2015; Accepted: 30 January 2016; Published: 15 February 2016
Abstract: SUS440 is a high-carbon stainless steel, and its martensite matrix has high heat resistance, high corrosion resistance, and high pressure resistance. It has been widely used in mechanical parts and critical materials. However, the SUS440 martempered matrix has reliability problems in thin plate applications and thus research uses different austempering heat treatments (tempering temperature: 200 ˝ C–400 ˝ C) to obtain a matrix containing bainite, retained austenite, martensite, and the M7C3 phase to investigate the relationships between the resulting microstructure and tensile mechanical properties. Experimental data showed that the austempering conditions of the specimen affected the volume fraction of phases and distribution of carbides. After austenitizing heat treatment (1080 ˝ C for 30 min), the austempering of the SUS440 thin plates was carried out at a salt-bath temperature 300 ˝ C for 120 min and water quenching was then used to obtain the bainite matrix with fine carbides, with the resulting material having a higher tensile fracture strength and average hardness (HRA 76) makes it suitable for use as a high-strength thin plate for industrial applications. Keywords: stainless steel; austempering; bainite; mechanical properties
1. Introduction SUS440 is a high-chrome and carbon martensite stainless steel with high strength and corrosion resistance properties and thus been widely used in machine parts, screws, and knives [1,2]. In general, martensite stainless steel almost always undergoes austenitizing heat treatment, followed by quenching and tempering. However, if this process is carried out improperly, then this will adversely affect the mechanical properties of the resulting materials [3,4]. This is because the quenching rate and temper-brittleness, and especially quench-tempering, can lead to reliability problems with regard to thin plates [5,6]. In the current study, SUS440 plate is made into a double-loop type thin plate specimen by a punch-shear process, in order to highlight the stress concentration and study the effects on brittleness. Austempering heat treatment can produce excellent mechanical properties in iron-based materials [7,8], although the characteristics of austempered SUS440 have still not been studied. Notably, the salt that was used in the current study can also meet the demand for a more environmentally-friendly process [9]. Therefore, this research controlled the heat treatment conditions (Ms temperature is about ´40 ˝ C [10]) to obtain SUS440 with a bainite matrix, retained austenite, and stable MC carbides, and then investigated the tensile strength and hardness of this material in order to obtain data for use in SUS440 thin plate applications [11]. 2. Experimental Procedure The chemical composition of SUS440 is given in Table 1, the carbon content is 0.75% by mass, the chrome content is 18% by mass, and it contains other alloying elements, such as Si, Mn, and V.
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punch‐shear process on brittleness. Figure 1 shows the dimensions of the specimen. Figure 2 shows This research uses a double loop-type thin plate (t = 0.78 mm) specimen to examine the effects of the punch‐shear process on brittleness. Figure 1 shows the dimensions of the specimen. Figure 2 shows a diagram of the austempering process. The heat treatment condition of the specimen was 1080 °C punch-shear process on brittleness. Figure 1 shows the dimensions of the specimen. Figure 2 shows a diagram of the austempering process. The heat treatment condition of the specimen was 1080 °C (vacuum) held for 30 min for austenitic treatment (the austenite conditions of all specimens are the a diagram of the austempering process. The heat treatment condition of the specimen was 1080 ˝ C (vacuum) held for 30 min for austenitic treatment (the austenite conditions of all specimens are the same), and then the specimen was immediately moved into a salt‐bath furnace for austempering. (vacuum) held for 30specimen min for austenitic treatmentmoved (the austenite conditions of all for specimens are the same), and then the was immediately into a salt‐bath furnace austempering. There were two different conditions for the salt‐bath phase and thus two sets of specimens were same), and then the specimen was immediately moved into a salt-bath furnace for austempering. There were two different conditions for the salt‐bath phase and thus two sets of specimens were prepared. The first set underwent the same salt‐bath time (120 min) and of different constant There wereThe twofirst different conditions for salt-bath phase and(120 thusmin) two and sets specimens were prepared. set underwent the the same salt‐bath time different constant temperatures (200, 300, and 400 °C), and were then quenched in the water. The second set underwent prepared. The first set underwent the same salt-bath time (120 min) and different constant temperatures temperatures (200, 300, and 400 °C), and were then quenched in the water. The second set underwent the same salt‐bath temperature (300 °C) and different time intervals (30, 60, 90, and 120 min), and (200, 300, and 400 ˝ C), and were then quenched in the water. The second set underwent the same the same salt‐bath temperature (300 °C) and different time intervals (30, 60, 90, and 120 min), and were then quenched in the water. Each austempering condition was called x °C‐y min, based on the salt-bath temperature (300 ˝ C) and different time intervals (30, 60, 90, and 120 min), and were then were then quenched in the water. Each austempering condition was called x °C‐y min, based on the salt‐bath condition, such as 200 °C‐120 min [7,8]. quenched in the water. Each austempering condition was called x ˝ C-y min, based on the salt-bath salt‐bath condition, such as 200 °C‐120 min [7,8]. condition, such as 200 ˝ C-120 min [7,8]. Table 1. The chemical composition of the SUS440 (mass %). Table 1. The chemical composition of the SUS440 (mass %). Table 1. The chemical composition of the SUS440 (mass %).
Element Element Element Content Content Content
C C C 0.75 0.75
0.75
Mn Si P S V Cr Mo Fe Mn Si P S V Cr Mo Fe Mn Si P S V Cr Mo Bal. Fe 1.00 1.00 0.04 0.04 0.15 18.20 0.30 1.00 1.00 0.04 0.04 0.15 18.20 0.30 Bal.
1.00
1.00
0.04
0.04
0.15
18.20
0.30
Bal.
Figure 1. Configuration of a double-loop thin plate specimen. Figure 1. Configuration of a double‐loop thin plate specimen. Figure 1. Configuration of a double‐loop thin plate specimen.
Figure 2. The tempering process with different salt‐bath temperatures. Figure 2. The tempering process with different salt-bath temperatures. Figure 2. The tempering process with different salt‐bath temperatures.
The characteristics of each austempered specimen were determined quantitatively by SEM The austempered specimen determined quantitatively The characteristics characteristics of of each each austempered specimen were were determined quantitatively by by SEM SEM (Hitachi SU8000, Hitachi, Tokyo, Japan) and an image analyzer (Oxford Instrument, Abingdon, UK). (Hitachi SU8000, Hitachi, Tokyo, Japan) and an image analyzer (Oxford Instrument, Abingdon, UK). (Hitachi SU8000, Hitachi, Tokyo, Japan) and an image analyzer (Oxford Instrument, Abingdon, UK). −1) of each specimen were The hardness (HRA) and the tensile properties (tensile rate: 1 mm∙min´−1 The hardness (HRA) (HRA) and and the the tensile tensile properties properties (tensile (tensile rate: rate: 11 mm¨ mm∙min The hardness min 1) ) of of each each specimen specimen were were evaluated. The structural phases were identified by XRD (Bruker AXS, Karlsruhe, Germany). The evaluated. The structural structural phases phases were identified XRD (Bruker Karlsruhe, Germany). The evaluated. The were identified by by XRD (Bruker AXS,AXS, Karlsruhe, Germany). The short short (30 min) and long (120 min) tempering affected the precipitation of the tempering carbides, and short (30 min) and long (120 min) tempering affected the precipitation of the tempering carbides, and (30 min) and long (120 min) tempering affected the precipitation of the tempering carbides, and Electron Spectroscopy for Chemical Analysis (ESCA, PHI 5000 V, ULVAC‐PHI, Inc., Kanagawa, Electron Spectroscopy for for Chemical Analysis (ESCA, V, ULVAC‐PHI, Inc., Kanagawa, Electron Spectroscopy Chemical Analysis (ESCA, PHIPHI 50005000 V, ULVAC-PHI, Inc., Kanagawa, Japan) Japan) was used to assess this. In addition, SEM and OM were used to observe the fracture surface Japan) was used to assess this. In addition, SEM and OM were used to observe the fracture surface was used to assess this. In addition, SEM and OM were used to observe the fracture surface and and subsurface of the specimen to clarify the tensile failure characteristics of thin plate specimens and subsurface of the specimen to clarify the tensile failure characteristics of thin plate specimens subsurface of the specimen to clarify the tensile failure characteristics of thin plate specimens [11]. [11]. [11].
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3. Results and Discussion 3. Results and Discussion Figure 3 shows the microstructures of the austempered bainite specimens (1080 ˝ C-30 min) Figure 3 shows the microstructures of the austempered bainite specimens (1080 °C‐30 min) with with different salt-bath conditions. The matrix is the feather bainite structure. Notably, the major different salt‐bath conditions. The matrix is the feather bainite structure. Notably, the major phases phases were the retained austenite phases combined with martensite, and the particles were the were the retained austenite phases combined with martensite, and the particles were the carbides. In carbides. In addition, the salt-bath specimen at 300 ˝ C, became coarser was the austempering duration addition, the salt‐bath specimen at 300 °C, became coarser was the austempering duration increased increased (Figure 4). A fully bainite with finer primary carbides achievedwith witha a longer (Figure 4). A fully bainite matrix matrix with finer primary carbides can can be be achieved longer austempering However, if if the the tempering tempering time time was was insufficient, then no austempering time time [5,12–14]. [5,12–14]. However, insufficient, then no feathery feathery structure was seen in the matrix after etching. structure was seen in the matrix after etching.
(a)
(b)
(c) Figure 3. Microstructural characteristics of the austempered specimens (1080 °C‐30 min) with a salt‐ Figure 3. Microstructural characteristics of the austempered specimens (1080 ˝ C-30 min) with bath time of 120 min and different salt‐bath temperatures: (a) 200 °C, (b) 300 °C, and (c) 400 °C. a salt-bath time of 120 min and different salt-bath temperatures: (a) 200 ˝ C, (b) 300 ˝ C, and (c) 400 ˝ C.
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(a)
(b)
(c)
(d)
Figure 4. Microstructural characteristics of the austempered specimens (1080 °C‐30 min) with a salt‐ Figure 4. Microstructural characteristics of the austempered specimens (1080 ˝ C-30 min) with bath temperature of 300 °C and different salt‐bath times: (a) 30 min, (b) 60 min, (c) 90 min, and (d) 120 a salt-bath temperature of 300 ˝ C and different salt-bath times: (a) 30 min, (b) 60 min, (c) 90 min, min. (d) 120 min. and
The XRD patterns of the specimens produced under different salt‐bath conditions after The XRD patterns of the specimens produced under different salt-bath conditions after austempering at 1080 °C‐30 min, are shown in Figures 5 and 6. All the specimens shown in the figure austempering at 1080 ˝ C-30 min, are shown in Figures 5 and 6. All the specimens shown in the have a bainite matrix, M7C3 carbide, and retained austenite combining martensite, while the figure have a bainite matrix, M7C3 carbide, and retained austenite combining martensite, while the specimen produced at an austempering temperature of 200 °C had more martensite phases [8,15,16]. specimen produced at an austempering temperature of 200 ˝ C had more martensite phases [8,15,16]. Figure 5 shows that the retained austenite content of the specimen produced at a salt‐bath Figure 5 shows that the retained austenite content of the specimen produced at a salt-bath temperature temperature of 400 °C is the highest, while Figure 6 shows that the austenite content of the specimen of 400 ˝ C is the highest, while Figure 6 shows that the austenite content of the specimen decreases as decreases as the austempering time increases. the austempering time increases.
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˝ C-30 min) Figure 5. 5. XRD 120 min min and and Figure XRD of of austempered austempered specimens specimens (1080 (1080 °C‐30 min) with with a a salt‐bath salt-bath time time of of 120 Figure 5. XRD of austempered specimens (1080 °C‐30 min) with a salt‐bath time of 120 min and different salt‐bath temperatures. different salt-bath temperatures. different salt‐bath temperatures.
˝ C-30
Figure 6. XRD of austempered specimen (1080 min) with a salt-bath temperature of 300 ˝ C and Figure 6. XRD of austempered specimen (1080 °C‐30 min) with a salt‐bath temperature of 300 °C and Figure 6. XRD of austempered specimen (1080 °C‐30 min) with a salt‐bath temperature of 300 °C and different salt-bath time. different salt‐bath time. different salt‐bath time.
Figure 7 is the schematic diagram of the tensile test method in this study we use two pins to Figure 7 is the schematic diagram of the tensile test method in this study we use two pins to Figure 7 is the schematic diagram of the tensile test method in this study we use two pins to insert into the double‐loop type thin plate specimen and vertical. Figure 8 shows a comparison of the insert into the double-loop type thin plate specimen and vertical. Figure 8 shows a comparison of the insert into the double‐loop type thin plate specimen and vertical. Figure 8 shows a comparison of the tensile properties obtained with different salt‐bath temperatures. The tensile properties of the 300 °C tensile properties obtained with different salt-bath temperatures. The tensile properties of the 300 ˝ C tensile properties obtained with different salt‐bath temperatures. The tensile properties of the 300 °C specimen are significantly better than those of the other specimens. The main reason for this is that specimen are significantly better than those of the other specimens. The main reason for this is that the specimen are significantly better than those of the other specimens. The main reason for this is that the austempering temperature of 200 °C is too low for complete austempering and thus the matrix austempering temperature of 200 ˝ C is too low for complete austempering and thus the matrix also the austempering temperature of 200 °C is too low for complete austempering and thus the matrix also contains martensite, which increases the brittleness of the material [5,17,18]. The 400 °C specimen contains martensite, which increases the brittleness of the material [5,17,18]. The 400 ˝ C specimen has also contains martensite, which increases the brittleness of the material [5,17,18]. The 400 °C specimen has more retained austenite, which results in lower tensile mechanical properties. In the hardness more retained austenite, which results in lower tensile mechanical properties. In the hardness tests, has more retained austenite, which results in lower tensile mechanical properties. In the hardness ˝ C specimen had greater hardness because of the martensite inside the matrix [2,19], while the tests, the 200 °C specimen had greater hardness because of the martensite inside the matrix [2,19], the 200 tests, the 200 °C specimen had greater hardness because of the martensite inside the matrix [2,19], while the other specimens had an HRA of about 76. other specimens had an HRA of about 76. while the other specimens had an HRA of about 76.
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Figure 7. The schematic diagram of the tensile test method. Figure 7. The schematic diagram of the tensile test method. Figure 7. The schematic diagram of the tensile test method.
Figure 8. The mechanical properties of the austempered specimens (1080 °C‐30 min) with a salt‐bath Figure 8. The mechanical properties of the austempered specimens (1080 °C‐30 min) with a salt‐bath Figure 8. The mechanical properties of the austempered specimens (1080 ˝ C-30 min) with a salt-bath time of 120 min and different salt‐bath temperatures. time of 120 min and different salt‐bath temperatures. time of 120 min and different salt-bath temperatures.
Figure 9 shows a comparison of the tensile properties for specimens obtained at different salt‐ Figure 9 shows a comparison of the tensile properties for specimens obtained at different salt‐ Figure 9 shows a comparison of the tensile properties for specimens obtained at different salt-bath bath times (30–120 min, fixed salt‐bath temperature at 300 °C) after austenitizing at 1080 °C for 30 bath times (30–120 min, fixed salt‐bath temperature at 300 °C) after austenitizing at 1080 °C for 30 times (30–120 min, fixed salt-bath temperature at 300 ˝ C) after austenitizing at 1080 ˝ C for 30 min. min. The average tensile strength of the 90 min specimen was greater than that of the others (>1100 min. The average tensile strength of the 90 min specimen was greater than that of the others (>1100 The average tensile strength of the 90 min specimen was greater than that of the others (>1100 MPa), MPa), and the hardness of all four heat treatment conditions was above HRA 76 [17,18]. Therefore, MPa), and the hardness of all four heat treatment conditions was above HRA 76 [17,18]. Therefore, and the hardness of all four heat treatment conditions was above HRA 76 [17,18]. Therefore, the the of at 1080 °C for 30 followed by at the condition condition of austenitizing austenitizing for 30 min, min, by austempering austempering at salt‐bath salt‐bath ˝ C 1080 condition of austenitizing at 1080 at for 30°C min, followed byfollowed austempering at salt-bath temperature of temperature of 300 °C for 90 min, led to the best tensile properties and hardness (and the composition temperature of 300 °C for 90 min, led to the best tensile properties and hardness (and the composition ˝ C for 90 min, led to the best tensile properties and hardness (and the composition of the resulting 300 of microstructure was 68.8% bainite + 12.6% martensite + austenite + of the the resulting resulting microstructure 68.8% bainite + + 15.5% 12.6% austenite martensite + 15.5% 15.5% austenite + 3.1% 3.1% microstructure was 68.8% bainite was + 12.6% martensite + 3.1% carbides). The fracture carbides). The fracture characteristics of the various specimens were then compared using a tensile carbides). The fracture characteristics of the various specimens were then compared using a tensile characteristics of the various specimens were then compared using a tensile test. test. test.
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Figure 9. The mechanical properties of the austempered specimens (1080 °C‐30 min) with a salt‐bath Figure 9. The mechanical properties of the austempered specimens (1080 ˝ C-30 min) with a salt-bath Figure 9. The mechanical properties of the austempered specimens (1080 °C‐30 min) with a salt‐bath temperature of 300 °C and different salt‐bath times. temperature of 300 ˝ C and different salt-bath times. temperature of 300 °C and different salt‐bath times.
Figure 10 shows the strain‐stress curve of the specimens produced under different salt‐bath Figure 10 strain-stress curve of the specimens produced under different salt-bath times times (30–120 min, fixed salt‐bath temperature at 300 °C) after austenitizing at 1080 °C‐30 min. There Figure 10 shows shows the the strain‐stress curve of the specimens produced under different salt‐bath ˝ C) after austenitizing at 1080 ˝ C-30 min. There is (30–120 min, fixed salt-bath temperature at 300 is no necking characteristics that could be observed, so it is the brittle failure. Figure 11 shows the times (30–120 min, fixed salt‐bath temperature at 300 °C) after austenitizing at 1080 °C‐30 min. There no necking characteristics that could be observed, so it is the brittle failure. Figure 11 shows the fracture surfaces of the specimens produced at 300 °C with different austempering durations. It can is no necking characteristics that could be observed, so it is the brittle failure. Figure 11 shows the fracture surfaces of the specimens produced at 300 ˝ C with different austempering durations. It can be seen that there are many peel‐off structures, and this confirms the characteristics of ductile‐brittle fracture surfaces of the specimens produced at 300 °C with different austempering durations. It can be failure (Figure 11a–c). Figure 11d shows the peel‐off microstructure, but some splitting behavior also seen that there are many peel-off structures, and this confirms the characteristics of ductile-brittle be seen that there are many peel‐off structures, and this confirms the characteristics of ductile‐brittle occurred on the fractured surface, and this was due to the brittle fracture mode. Figure 12 shows the failure (Figure 11a–c). Figure 11d shows the peel-off microstructure, but some splitting behavior also failure (Figure 11a–c). Figure 11d shows the peel‐off microstructure, but some splitting behavior also subsurface of the 300 °C specimens produced with salt‐bath times of 30 min and 120 min. When the occurred on the fractured surface, and this was due to the brittle fracture mode. Figure 12 shows the occurred on the fractured surface, and this was due to the brittle fracture mode. Figure 12 shows the matrix was composed of a fine feather structure and martensite and subjected to a short duration (30 subsurface of the 300 ˝ C specimens produced with salt-bath times of 30 min and 120 min. When the subsurface of the 300 °C specimens produced with salt‐bath times of 30 min and 120 min. When the min) was of austempering, brittleness became significant and caused the subsurface to become matrix composed of the a fine feather structure and martensite and subjected to a short duration matrix was composed of a fine feather structure and martensite and subjected to a short duration (30 irregular (Figure 12a) [12,20]. Figure 12b shows the flat fracture characteristic that was obtained with (30 min) of austempering, the brittleness became significant and caused the subsurface to become min) of austempering, the brittleness became significant and caused the subsurface to become a longer austempering time (120 min). Since the austempering duration affected carbide precipitation irregular (Figure 12a) [12,20]. Figure 12b shows the flat fracture characteristic that was obtained with a irregular (Figure 12a) [12,20]. Figure 12b shows the flat fracture characteristic that was obtained with and the tensile fracture behavior, electron spectroscopy chemical analysis (ESCA) was applied to the longer austempering time (120 min). Since the austempering duration affected carbide precipitation a longer austempering time (120 min). Since the austempering duration affected carbide precipitation austempered SUS440 to measure the carbide contents (Figure 13). With the shorter austempering time and the tensile fracture behavior, electron spectroscopy chemical analysis (ESCA) was applied to the and the tensile fracture behavior, electron spectroscopy chemical analysis (ESCA) was applied to the (30 min), the main carbides were the M23C6 (M7C3) precipitate phases (Figure 13a). As the austempered SUS440 to measure the carbide contents (Figure 13). With the shorter austempering austempered SUS440 to measure the carbide contents (Figure 13). With the shorter austempering time austempering time increased (Figure 13b), the content of M23C6 (M7C3) precipitate (particle‐like) timemin), (30 min), the main carbides were M23C6 (M7C3)precipitate precipitatephases phases(Figure (Figure 13a). 13a). As As the the (30 the main carbides were the the M23C6 (M7C3) decreased, and the amount of stable M7C3 carbides increased [21]. austempering time increased time increased (Figure (Figure 13b), 13b), the the content content of of M23C6 (M7C3) M23C6 (M7C3) precipitate precipitate (particle‐like) (particle-like) austempering decreased, and the amount of stable M7C3 carbides increased [21]. decreased, and the amount of stable M7C3 carbides increased [21].
Figure 10. The strain‐stress curve of the austempered specimens (1080 °C‐30 min) with a salt‐bath temperature of 300 °C and different salt‐bath times.
Figure 10. The strain-stress curve of the austempered specimens (1080 ˝ C-30 min) with a salt-bath Figure 10. The strain‐stress curve of the austempered specimens (1080 °C‐30 min) with a salt‐bath temperature of 300 ˝ C and different salt-bath times. temperature of 300 °C and different salt‐bath times.
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(a) (a)
(b) (b)
(c) (d) (c) (d) Figure 11. The fracture surfaces of the austempered specimens (1080 °C‐30 salt‐bath ˝ Figure 11. The fracture fracture surfaces surfaces of of the the austempered specimens (1080 (1080 °C‐30 C-30 min) min) with with a a salt-bath Figure 11. The austempered specimens min) with a salt‐bath temperature of 300 °C and different salt‐bath times: (a) 30 min, (b) 60 min, (c) 90 min, and (d) 120 min. temperature of 300 ˝ C and different salt-bath times: (a) 30 min, (b) 60 min, (c) 90 min, and (d) 120 min. temperature of 300 °C and different salt‐bath times: (a) 30 min, (b) 60 min, (c) 90 min, and (d) 120 min.
(a) (b) (a) (b) Figure 12. The subsurface of the austempered specimens (1080 °C‐30 min) with a salt‐bath Figure 12. The The subsurface of austempered the austempered specimens (1080 °C‐30 with temperature a salt‐bath Figure 12. subsurface of the specimens (1080 ˝ C-30 min) withmin) a salt-bath temperature of 300 °C and different salt‐bath times: (a) 30 min, and (b) 120 min. temperature of 300 °C and different salt‐bath times: (a) 30 min, and (b) 120 min. ˝ of 300 C and different salt-bath times: (a) 30 min, and (b) 120 min.
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Figure 13. ESCA of carbides: (a) 30 min, and (b) 120min. Figure 13. ESCA of carbides: (a) 30 min, and (b) 120min. Figure 13. ESCA of carbides: (a) 30 min, and (b) 120min.
Figure 14 shows the mechanical properties of the corresponding microstructures at different salt‐ Figure 14 shows the mechanical properties of the corresponding microstructures at different Figure 14 shows the mechanical properties of the corresponding microstructures at different salt‐ bath conditions. The The bainite (high‐retained austenite) with M7C3 coarsened as the salt‐bath salt-bath conditions. bainite (high-retained austenite) with M7C3 coarsened the salt-bath bath conditions. The bainite (high‐retained austenite) with M7C3 coarsened as as the salt‐bath ˝ temperature increased. As the austempering time rose, the salt‐bath condition of 300 °C‐90 min, temperature increased. time rose, rose,the thesalt‐bath salt-bathcondition condition 300 C-90 min, temperature increased. As As the the austempering austempering time of of 300 °C‐90 min, produced the specimen with the highest fracture strength. The results showed that an SUS440 thin produced the specimen with the highest fracture strength. The results showed that an SUS440 thin produced the specimen with the highest fracture strength. The results showed that an SUS440 thin plate with a suitable bainite matrix can have both lower brittleness and greater reliability with regard plate with a suitable bainite matrix can have both lower brittleness and greater reliability with regard plate with a suitable bainite matrix can have both lower brittleness and greater reliability with regard to the tensile mechanical properties. to to the tensile mechanical properties. the tensile mechanical properties.
(a) (b) (a) (b) Figure 14. Diagram of tensile mechanical properties with different salt‐bath conditions: (a) different Figure 14. Diagram of tensile mechanical properties with different salt-bath conditions: (a) different Figure 14. Diagram of tensile mechanical properties with different salt‐bath conditions: (a) different salt‐bath temperatures, and (b) different salt‐bath times. salt-bath temperatures, and (b) different salt-bath times. salt‐bath temperatures, and (b) different salt‐bath times.
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4. Conclusions (1) The brittleness of an SUS440 thin plate produced with a short austempering time will increase due to the martensite phases in the matrix. A longer austempering time reduces the strength of the SUS440 thin plate due to changes in the content of retained austenite and M7C3 precipitate. The 300 ˝ C-90 min specimen had the best mechanical properties. (2) The martensite and retained austenite of the bainite matrix have a close relationship with the tensile brittleness of an SUS440 thin plate. The distribution of tempering M7C3 precipitate phases affected the failure mechanism. Acknowledgments: The authors are grateful to The Instrument Center of National Cheng Kung University and 103-2221-E-006-066 for financial support for this research. Author Contributions: C.-Y.C. and F.-Y.H. designed the research and wrote the manuscript with help from the other authors; C.-Y.C. performed the experiments and analyzed the data; F.-Y.H. C.-S.L. and L.-H.C. gave the support and comments. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Bee, J.V.; Powell, G.L.F.; Bednarz, B. A substructure within the austenitic matrix of high chromium white irons. Scr. Metall. Mater. 1994, 31, 1735–1736. [CrossRef] Salleh, S.H.; Omar, M.Z.; Syarif, J. Carbide formation during precipitation hardening of SS440C steel. Eur. J. Sci. Res. 2009, 34, 83–91. Lin, Y.L.; Lin, C.; Tsai, T. Microstructure and mechanical properties of 0.63C-12.7Cr martensitic stainless steel during various tempering treatments. Mater. Manuf. Proc. 2010, 25, 246–248. [CrossRef] Salih, A.A.; Omar, M.Z.; Junaidi, S.; Sajuri, Z. Effect of different heat treatment on the SS440C martensitic stainless steel. Aust. J. Basic Appl. Sci. 2011, 5, 867–871. Bhadeshia, H. Martensite and bainite in steels: Transformation mechanism & mechanical properties. J. Phys. IV 1997, 7, 367–376. Lee, H.Y.; Yen, H.; Chang, H.; Yang, J. Substructures of martensite in Fe-1C-17Cr stainless steel. Scr. Mater. 2010, 62, 670–673. [CrossRef] Salleh, S.H. Investigation of microstructures and properties of 440C martensitic stainless steel. Int. J. Mech. Mater. Eng. 2009, 4, 123–126. Carpenter, S.D.; Carpenter, D. X-ray diffraction study of M7C3 carbide within a high chromium white iron. Mater. Lett. 2003, 57, 4456–4459. [CrossRef] Carpenter, S.D.; Carpenter, D.; Pearce, J.T.H. XRD and electron microscope study of an as-cast 26.6% chromium white iron microstructure. Mater. Chem. Phys. 2004, 85, 32–40. [CrossRef] Liu, C.; Zhao, Z.; Northwood, D.O.; Liu, Y. A new empirical formula for the calculation of MS temperatures in pure iron and super-low carbon alloy steels. J. Mater. Proc. Technol. 2001, 113, 556–562. [CrossRef] Chen, C.Y.; Hung, F.; Lui, T.; Chen, L. Microstructures and mechanical properties of austempering Cr-Mo (SCM 435) alloy steel. Mater. Trans. 2013, 54, 56–60. [CrossRef] Tsuzaki, K.; Maki, T. Some aspects of bainite transformation in Fe-based alloys. J. Phys. IV 1995, 5, 61–70. [CrossRef] Funatani, K. Low-temperature salt bath nitriding of steels. Met. Sci. Heat Treat. 2004, 46, 277–281. [CrossRef] Lee, B.-J. On the stability of Cr carbides. Calphad 1992, 16, 121–149. [CrossRef] Kwok, C.T.; Lo, K.; Cheng, F.; Man, H. Effect of processing conditions on the corrosion performance of laser surface-melted AISI 440C martensitic stainless steel. Surf. Coat. Technol. 2003, 166, 221–230. [CrossRef] Carpenter, S.D.; Carpenter, D.; Pearce, J.T.H. XRD and electron microscope study of a heat treated 26.6% chromium white iron microstructure. Mater. Chem. Phys. 2007, 101, 49–55. [CrossRef] Avishan, B.; Yazdani, S.; Nedjad, S.H. Toughness variations in nanostructured bainitic steels. Mater. Sci. Eng. A 2012, 548, 106–111. [CrossRef] Luo, Y.; Peng, J.; Wang, H.; Wu, X. Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel. Mater. Sci. Eng. A 2010, 527, 3433–3437. [CrossRef]
Metals 2016, 6, 35
19. 20. 21.
11 of 11
Yang, J.R.; Yu, T.H.; Wang, C.H. Martensitic transformations in AISI 440C stainless steel. Mater. Sci. Eng. A 2006, 438, 276–280. [CrossRef] Sajjadi, S.A.; Zebarjad, S.M. Isothermal transformation of austenite to bainite in high carbon steels. J. Mater. Proc. Technol. 2007, 189, 107–113. [CrossRef] Isfahany, A.N.; Saghafian, H.; Borhani, G. The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel. J. Alloys Compd. 2011, 509, 3931–3936. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
