Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates

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Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates 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 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]. Metals 2016, 6, 35 3 of 11 Metals 2016, 6, 35 3 of 11 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.
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