Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates

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 Table1, 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.

Metals 2016, 6, 35; doi:10.3390/met6020035 www.mdpi.com/journal/metals Metals 2016, 6, 35 2 of 11 Metals 2016, 6, 35 2 of 11 Metals 2016, 6, 35 2 of 11 punch‐shear process on brittleness. Figure 1 shows the dimensions of the specimen. Figure 2 shows punchThis research‐shear process uses a double on brittleness. loop-type Figure thin plate1 shows (t = the 0.78 dimensions mm) specimen of the to specimen. examine the Figure effects 2 shows of the a diagram of the austempering process. The heat treatment condition of the specimen was 1080 °C apunch-shear diagram of processthe austempering on brittleness. process. Figure The1 shows heat treatment the dimensions condition of the of specimen.the specimen Figure was2 1080shows °C (vacuum) held for 30 min for austenitic treatment (the austenite conditions of all specimens are the˝ (vacuum)a diagram held of the for austempering 30 min for austenitic process. treatment The heat treatment(the austenite condition conditions of the of specimen all specimens was 1080 are theC same), and then the specimen was immediately moved into a salt‐bath furnace for austempering. same),(vacuum) and held then for the 30 specimen min for austenitic was immediately treatment moved (the austenite into a salt conditions‐bath furnace of all for specimens austempering. are the There were two different conditions for the salt‐bath phase and thus two sets of specimens were Theresame), were and thentwo different the specimen conditions was immediately for the salt‐bath moved phase into and a salt-bath thus two furnace sets of for specimens austempering. were prepared. The first set underwent the same salt‐bath time (120 min) and different constant prepared.There were The two first different set underwent conditions forthe thesame salt-bath salt‐bath phase time and (120 thus min) two and sets ofdifferent specimens constant were temperatures (200, 300, and 400 °C), and were then quenched in the water. The second set underwent temperaturesprepared. The (200, ﬁrst set300, underwent and 400 °C), the and same were salt-bath then quenched time (120 min) in the and water. different The second constant set temperatures underwent the same salt‐bath ˝temperature (300 °C) and different time intervals (30, 60, 90, and 120 min), and the(200, same 300, salt and‐bath 400 temperatureC), and were (300 then °C) quenched and different in the time water. intervals The second (30, 60, set 90, underwent and 120 min), the same and were then quenched in the ˝water. Each austempering condition was called x °C‐y min, based on the weresalt-bath then temperature quenched in (300 the water.C) and Each different austempering time intervals condition (30, 60,was 90, called and x 120 °C min),‐y min, and based were on then the salt‐bath condition, such as 200 °C‐120 min [7,8]. ˝ saltquenched‐bath condition, in the water. such Each as 200 austempering °C‐120 min [7,8]. condition was called x C-y min, based on the salt-bath 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 C Mn Si P S V Cr Mo Fe Element C Mn Si P S V Cr Mo Fe ElementContent 0.75 C Mn1.00 1.00 Si 0.04 P 0.04 S 0.15 V 18.20 Cr 0.30 Mo Bal. Fe Content 0.75 1.00 1.00 0.04 0.04 0.15 18.20 0.30 Bal. Content 0.75 1.00 1.00 0.04 0.04 0.15 18.20 0.30 Bal.

Figure 1. ConfigurationConﬁguration of a double double-loop‐loop thin plate specimen. Figure 1. Configuration of a double‐loop thin plate specimen.

Figure 2. The tempering process with different salt‐bath temperatures. FigureFigure 2. TheThe tempering tempering process process with different salt salt-bath‐bath temperatures.

The characteristics of each austempered specimen were determined quantitatively by SEM The characteristics of each austempered specimen were determined quantitatively by SEM (Hitachi SU8000, Hitachi, Tokyo, Japan) and an image analyzer (Oxford Instrument, Abingdon, UK). (Hitachi SU8000, Hitachi, Tokyo, Japan) and an image image analyzer analyzer (Oxford (Oxford Instrument, Instrument, Abingdon, Abingdon, UK). UK). The hardness (HRA) and the tensile properties (tensile rate: 1 mm∙min−1) of each specimen were The hardness (HRA)(HRA) andand thethe tensiletensile propertiesproperties (tensile(tensile rate: rate: 11 mm mm¨ ∙minmin´−11)) of of each each specimen were evaluated. The structural phases were identified by XRD (Bruker AXS, Karlsruhe, Germany). The evaluated. TheThe structuralstructural phases phases were were identiﬁed identified by by XRD XRD (Bruker (Bruker AXS, AXS, Karlsruhe, Karlsruhe, Germany). Germany). The short The short (30 min) and long (120 min) tempering affected the precipitation of the tempering carbides, and short(30 min) (30 min) and longand long (120 (120 min) min) tempering tempering affected affected the the precipitation precipitation of of the the tempering tempering carbides,carbides, and Electron Spectroscopy for Chemical Analysis (ESCA, PHI 5000 V, ULVAC‐PHI, Inc., Kanagawa, Electron SpectroscopySpectroscopy for for Chemical Chemical Analysis Analysis (ESCA, (ESCA, PHI PHI 5000 5000 V, ULVAC-PHI, V, ULVAC‐ Inc.,PHI, Kanagawa, Inc., Kanagawa, Japan) Japan) was used to assess this. In addition, SEM and OM were used to observe the fracture surface Japan)was used was to used assess to assess this. In this. addition, In addition, SEM andSEM OM and were OM were used used to observe to observe the fracture the fracture surface surface and and subsurface of the specimen to clarify the tensile failure characteristics of thin plate specimens andsubsurface subsurface of the of specimen the specimen to clarify to clarify the tensile the tensile failure failure characteristics characteristics of thin of plate thin specimens plate specimens [11]. [11]. [11].

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3. Results Results and and Discussion Discussion ˝ Figure3 3 showsshows the the microstructures microstructures of of the the austempered austempered bainite bainite specimens specimens (1080 (1080 °C‐30 C-30min) min)with withdifferent different salt‐bath salt-bath conditions. conditions. The matrix The matrix is the feather is the feather bainite bainite structure. structure. Notably, Notably, the major the phases major phaseswere the were retained the retained austenite austenite phases combined phases combined with martensite, with martensite, and the particles and the were particles the carbides. were the In ˝ carbides.addition, Inthe addition, salt‐bath the specimen salt-bath at specimen 300 °C, became at 300 coarserC, became was coarser the austempering was the austempering duration increased duration increased(Figure 4). (Figure A fully4). Abainite fully bainite matrix matrix with withfiner ﬁnerprimary primary carbides carbides can can be be achieved achieved with with a longerlonger austempering time [[5,12–14].5,12–14]. However, However, if if the the tempering time was insufﬁcient,insufficient, then no featheryfeathery structure waswas seenseen inin thethe matrixmatrix afterafter etching.etching.

(a) (b)

(c)

Figure 3. MicrostructuralMicrostructural characteristics characteristics of the of theaustempered austempered specimens specimens (1080 (1080 °C‐30˝ min)C-30 with min) a withsalt‐ abath salt-bath time of time 120 of min 120 and min different and different salt‐bath salt-bath temperatures: temperatures: (a) 200 (a )°C, 200 (b˝)C, 300 (b )°C, 300 and˝C, (c and) 400 (c °C.) 400 ˝C.

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(a) (b)

Figure 4. MicrostructuralMicrostructural characteristics characteristics of the of theaustempered austempered specimens specimens (1080 (1080 °C‐30˝ min)C-30 with min) a withsalt‐ abath salt-bath temperature temperature of 300 °C of and 300 ˝differentC and different salt‐bath salt-bath times: (a) times: 30 min, (a ()b 30) 60 min, min, ( b(c)) 6090 min, and (c) 90 (d) min, 120 andmin. ( d) 120 min.

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 Figures5 and6. 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 Figure5 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 Figure6 shows that the austenite content of the specimen decreases as decreases as the austempering time increases. the austempering time increases.

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Figure 5. XRD of austempered specimens (1080 °C˝C-30‐30 min) min) with with a salt salt-bath‐bath time of 120 min and Figure 5. XRD of austempered specimens (1080 °C‐30 min) with a salt‐bath time of 120 min and different salt-bathsalt‐bath temperatures. different salt‐bath temperatures.

˝ ˝ Figure 6. XRD of austempered specimen (1080 °CC-30‐30 min) with a salt-bathsalt‐bath temperature of 300300 °CC and Figure 6. XRD of austempered specimen (1080 °C‐30 min) with a salt‐bath temperature of 300 °C and different salt-bathsalt‐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 77 isis thethe schematicschematic diagramdiagram ofof thethe tensiletensile testtest methodmethod inin thisthis studystudy wewe useuse twotwo pinspins toto insert into the double‐loop type thin plate specimen and vertical. Figure 8 shows a comparison of the insert into the double double-loop‐loop type thin plate specimen and vertical. Figure 88 showsshows aa comparisoncomparison ofof thethe tensile properties obtained with different salt‐bath temperatures. The tensile properties of the 300 ˝°C tensile properties obtained with different salt-bathsalt‐bath temperatures. The tensile properties of the 300 °CC specimen are significantly better than those of the other specimens. The main reason for this is that specimen are signiﬁcantlysignificantly better better than than those those of of the the other other specimens. specimens. The The main main reason reason for for this this is that is that the the austempering temperature of ˝200 °C is too low for complete austempering and thus the matrix theaustempering austempering temperature temperature of 200 of 200C is °C too is lowtoo low for complete for complete austempering austempering and and thus thus the matrix the matrix also also contains martensite, which increases the brittleness of the material [5,17,18]. The ˝400 °C specimen alsocontains contains martensite, martensite, which which increases increases the brittleness the brittleness of the of material the material [5,17 [5,17,18].,18]. The The 400 400C specimen °C specimen has has more retained austenite, which results in lower tensile mechanical properties. In the hardness hasmore more retained retained austenite, austenite, which which results results in lower in lower tensile tensile mechanical mechanical properties. properties. In the In hardness the hardness tests, tests, the˝ 200 °C specimen had greater hardness because of the martensite inside the matrix [2,19], tests,the 200 theC 200 specimen °C specimen had greater had greater hardness hardness because because of the martensite of the martensite inside the inside matrix the [2 ,matrix19], while [2,19], the while the other specimens had an HRA of about 76. whileother specimensthe other specimens had an HRA had of an about HRA 76. of about 76.

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FigureFigure 7. 7. TheThe schematic schematic diagram diagram of of the the tensile tensile test test method. method.

FigureFigure 8. 8. TheThe mechanical mechanical properties properties of of the the austempered austempered specimens specimens (1080 (1080 °C˝°CC-30‐‐3030 min) min) with with a a salt salt-bathsalt‐‐bathbath timetimetime of of 120 120 min min and and different different salt-bathsaltsalt‐‐bathbath temperatures.temperatures.

Figure 9 shows a comparison of the tensile properties for specimens obtained at different salt‐ Figure9 9 shows shows a a comparison comparison of of the the tensile tensile properties properties for for specimens specimens obtained obtained at different at different salt-bath salt‐ bath times (30–120 min, fixed salt‐bath temperature at 300 °C) after austenitizing at 1080 °C for 30 timesbath times (30–120 (30–120 min, ﬁxedmin, fixed salt-bath salt‐ temperaturebath temperature at 300 at˝ C)300 after °C) after austenitizing austenitizing at 1080 at 1080˝C for °C 30 for min. 30 min. The average tensile strength of the 90 min specimen was greater than that of the others (>1100 Themin. average The average tensile tensile strength strength of the of 90 the min 90 specimen min specimen was greater was greater than that than of that the othersof the others (>1100 (>1100 MPa), MPa), and the hardness of all four heat treatment conditions was above HRA 76 [17,18]. Therefore, andMPa), the and hardness the hardness of all fourof all heat four treatment heat treatment conditions conditions was abovewas above HRA HRA 76 [17 76,18 [17,18].]. Therefore, Therefore, the the condition of austenitizing at 1080 °C for 30 min, followed by austempering at salt‐bath conditionthe condition of austenitizing of austenitizing at 1080 at˝C 1080 for 30 °C min, for followed 30 min, by followed austempering by austempering at salt-bath temperature at salt‐bath of temperature of 300 °C for 90 min, led to the best tensile properties and hardness (and the composition 300temperature˝C for 90 of min, 300 led °C tofor the 90 bestmin, tensile led to propertiesthe best tensile and hardnessproperties (and and the hardness composition (and the of thecomposition resulting of the resulting microstructure was 68.8% bainite + 12.6% martensite + 15.5% austenite + 3.1% microstructureof the resulting was microstructure 68.8% bainite was + 12.6% 68.8% martensite bainite ++ 15.5%12.6% austenite martensite + 3.1% + 15.5% carbides). austenite The fracture+ 3.1% carbides). The fracture characteristics of the various specimens were then compared using a tensile characteristicscarbides). The offracture the various characteristics specimens of were the various then compared specimens using were a tensile then compared test. using a tensile test.test.

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FigureFigure 9. 9.The The mechanical mechanical properties properties of of thethe austemperedaustempered specimens specimens (1080 (1080 °C˝C-30‐30 min) min) with with a salt a salt-bath‐bath Figure 9. The mechanical properties of the austempered specimens (1080 °C‐30 min) with a salt‐bath temperaturetemperature of of 300 300˝ C°C and and different different salt-bath 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 timesFigure (30–120 1010 showsshows min, fixed the the strain-stress saltstrain‐bath‐stress temperature curve curve of theof at the300 specimens specimens°C) after produced austenitizing produced under atunder 1080 different °Cdifferent‐30 salt-bath min. salt There‐ timesbath ˝ ˝ times(30–120is no (30–120 necking min, fixedmin, characteristics fixed salt-bath salt‐ temperaturebath that temperature could be at observed, 300 at 300C) after°C)so it after austenitizingis the austenitizing brittle failure. at 1080 at 1080FigureC-30 °C 11‐30 min. shows min. There Therethe is isno nofracture necking necking surfaces characteristics characteristics of the specimens that that could could produced be be observed, observed, at 300 °C so so with it it is is different the the brittle brittle austempering failure. failure. Figure durations. 11 shows It can the ˝ fracturebe seen surfaces that there of arethe manyspecimens peel‐off produced structures, at 300and °CthisC with confirms different the characteristics austempering of durations. ductile‐brittle It can be failureseen that (Figure there 11a–c). are many Figure peel peel-off 11d‐off shows structures, the peel and ‐off this microstructure, confirms confirms the but characteristics some splitting of behavior ductile ductile-brittle‐ alsobrittle failureoccurred (Figure on the11a–c).11a–c). fractured Figure surface, 11d11d shows and this the the was peel peel-off due‐off tomicrostructure, the brittle fracture but some mode. splitting Figure 12 behavior shows the also occurredsubsurface on the of thefractured 300 °C surface, specimens and produced this was with due salt to the ‐bath brittle times fracture of 30 min mode. and 120Figure min. 12 When shows the the ˝ subsurfacematrix was of composedthe 300 °CC of specimens a fine feather produced structure with and salt salt-bath martensite‐bath times and of subjected 30 min toand a short 120 min. duration When (30 the min) of austempering, the brittleness became significant and caused the subsurface to become matrix was was composed composed of of a a fine fine feather feather structure structure and and martensite martensite and and subjected subjected to a to short a short duration duration (30 irregular (Figure 12a) [12,20]. Figure 12b shows the flat fracture characteristic that was obtained with min)(30 min) of austempering, of austempering, the the brittleness brittleness became became significant significant and and caused caused the the subsurface subsurface to to become a longer austempering time (120 min). Since the austempering duration affected carbide precipitation irregularirregular (Figure 1212a)a) [[12,20].12,20]. Figure 1212bb showsshows the the flat flat fracture fracture characteristic characteristic that that was was obtained obtained with with a and the tensile fracture behavior, electron spectroscopy chemical analysis (ESCA) was applied to the alonger longer austempering austempering time time (120 (120 min). min). Since Since the the austemperingaustempering durationduration affected carbide precipitation andaustempered the tensile fractureSUS440 to behavior, measure electronthe carbide spectroscopy contents (Figure chemical 13). With analysis the shorter (ESCA) austempering was applied time to the and(30 the min), tensile the fracture main carbides behavior, were electron the M23C6spectroscopy (M7C3) chemical precipitate analysis phases (ESCA) (Figure was 13a). applied As tothe the austempered SUS440 to measure the carbide contents (Figure 13). With the shorter austempering austemperedaustempering SUS440 time toincreased measure (Figure the carbide 13b), contentsthe content (Figure of M23C6 13). With (M7C3) the shorterprecipitate austempering (particle‐like) time time (30 min), the main carbides were the M23C6 (M7C3) precipitate phases (Figure 13a). As the (30decreased, min), the and main the amountcarbides of werestable theM7C3 M23C6 carbides (M7C3) increased precipitate [21]. phases (Figure 13a). As the austempering time time increased increased (Figure (Figure 13b),13b), the content of M23C6 (M7C3) precipitate (particle (particle-like)‐like) decreased, and the amount of stable M7C3 carbides increased [[21].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 strain-stress‐stress curve of the austempered specimens (1080 °CC-30‐30 min) with a salt salt-bath‐bath ˝ temperaturetemperature of 300 °CC and and different salt salt-bath‐bath times.

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(a) (b) (a) (b)

(c) (d) (c) (d) Figure 11. The fracture surfaces of the austempered specimens (1080 °C˝ ‐30 min) with a salt‐bath Figure 11. TheThe fracture surfaces of the austempered specimens (1080 °CC-30‐30 min) with a salt-bathsalt‐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 °CC and different salt-bathsalt‐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 subsurface of the austempered specimens˝ (1080 °C‐30 min) with a salt‐bath temperatureFigure 12. The of subsurface300 °C and of different the austempered salt‐bath specimenstimes: (a) 30 (1080 min, C-30and ( min)b) 120 with min. a salt-bath temperature temperatureof 300 ˝C and of different 300 °C and salt-bath different times: salt (‐abath) 30 min,times: and (a) (30b) min, 120 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 14 shows the mechanical properties of the corresponding microstructures at different salt‐ FigureFigure 14 14 shows shows thethe mechanical mechanical properties properties of the of the corresponding corresponding microstructures microstructures at different at different salt‐ bath conditions. The bainite (high‐retained austenite) with M7C3 coarsened as the salt‐bath salt-bathbath conditions. conditions. The The bainite bainite (high (high-retained‐retained austenite) austenite) with with M7C3 M7C3 coarsened coarsened as asthe the salt salt-bath‐bath temperature increased. As the austempering time rose, the salt‐bath condition of 300 °C˝ ‐90 min, temperaturetemperature increased. increased. AsAs thethe austemperingaustempering time time rose, rose, the the salt salt-bath‐bath condition condition of of300 300 °C‐90C-90 min, min, producedproduced the the specimen specimen with with the the highest highest fracturefracture strength. TheThe results results showed showed that that an an SUS440 SUS440 thin thin plateplate with with a suitable a suitable bainite bainite matrix matrix can can have have both both lower brittlenessbrittleness and and greater greater reliability reliability with with regard regard to tothe the tensile tensile mechanical mechanical properties. 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-bathsalt‐bath conditions: ( a) different salt‐bath temperatures, and (b) different salt‐bath times. saltsalt-bath‐bath temperatures, and (b) different salt-bathsalt‐bath times.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

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