metals Article Effects of High-Temperature Tempering on Mechanical Properties and Microstructure of SA738 Gr.B Steel Yanmei Li 1,*, Shuzhan Zhang 1,*, Chunyao Zhao 1, Minghui Song 1 and Zaiwei Jiang 2 1 The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China; [email protected] (C.Z.); [email protected] (M.S.) 2 Nanjing Iron and Steel Co., Ltd., Nanjing 210035, China; [email protected] * Correspondence: [email protected] (Y.L.); [email protected] (S.Z.) Received: 4 August 2020; Accepted: 3 September 2020; Published: 9 September 2020 Abstract: In this paper, JMatPro thermodynamic software, OM, SEM, TEM, and EPMA were used to study the microstructure and mechanical properties of SA738 Gr.B nuclear power steel after tempering at 630–710 ◦C. When tempered within the range of 630–670 ◦C, a huge amount of M3C and MC carbides were dispersed and precipitated in the ferrite matrix, and the strength and toughness matched well; when the tempering temperature rose above 670 ◦C, hard and brittle plate-martensites formed at the grain boundary, leading to the tensile strength of the experimental steel increased, while the low-temperature impact toughness significantly decreased and the yield strength also declined due to the disappearance of the finely dispersed second phase particles in the matrix. Keywords: SA738 Gr.B; high-temperature tempering; mechanical properties; microstructure 1. Introduction In recent years, nuclear energy, as a clean energy source, has received considerable attention from countries in the context of increasingly tight world energy supplies and worsening environmental problems. Nuclear safety is an integral part of national security [1]. The typical nuclear power reactor is AP1000, designed by Westinghouse [2]. The structure of the AP1000 nuclear reactor is a double-layer envelope structure composed of steel and reinforced concrete, which is different from the traditional prestressed concrete envelope structure [3]. Therefore, the unconventional design of the AP1000 containment has put forward higher requirements of strength and low-temperature toughness. To meet the higher standard [4], the primary material of its containment is an SA738 Gr.B steel plate in the American Society of Mechanical Engineers (ASME) SA738/SA738 M standard [5], according to the material specification, SA738 Gr.B steel plates shall have a minimum impact energy of 27 J either at 45 °C in the quarter of the thickness direction. To obtain a better combination of strength and − low-temperature impact toughness, the typical heat treatment process of SA738 Gr.B is quenching and tempering [5]. For the quenching and tempering process of SA738 Gr.B steel, relevant scholars have carried out related research: Han et al. [6] and Sun [7] studied the ferrite matrix after quenching and tempering and found that the increase in tempering temperature coarsened the grains, and with the increase in the soft and ductile phase ferrite content, the strength of SA738 Gr.B steel decreased and the impact energy increased. Bi et al. [8] observed the precipitated phases after tempering at 630 ◦C by TEM; it was found that a large number of carbides with various shapes were dispersed and precipitated between the laths, hindering the growth of grains and inhibited the movement of dislocations. Zhang [9] et al. used the master curve (MC) method to modify the ductile–brittle transition zone and fracture toughness of the tempered SA738 Gr.B steel, which proves that it has sufficient toughness reserve at a low temperature. However, a systematic and comprehensive study on Metals 2020, 10, 1207; doi:10.3390/met10091207 www.mdpi.com/journal/metals MetalsMetals 20202020, 10,,10 x ,FOR 1207 PEER REVIEW 2 of2 11 of 12 and comprehensive study on the brittle temperature range and the mechanisms of strengthening and the brittle temperature range and the mechanisms of strengthening and toughening of SA738 Gr.B toughening of SA738 Gr.B steel is still lacking. steel is still lacking. In this paper, the effects of tempering temperature on the strength and toughness of SA738 Gr.B In this paper, the effects of tempering temperature on the strength and toughness of SA738 nuclear power steel are systematically studied by comparing the strength and low-temperature Gr.B nuclear power steel are systematically studied by comparing the strength and low-temperature impact energy at different tempering temperatures and clarifying the evolution of the microstructure impact energy at different tempering temperatures and clarifying the evolution of the microstructure combined with structural characterization methods such as SEM and TEM, and thermodynamic combined with structural characterization methods such as SEM and TEM, and thermodynamic software calculations. software calculations. 2. 2.Material Material and and Methods Methods TheThe material material used used in the in theresearch research was was rolled rolled SA738Gr.B SA738Gr.B steel, steel, and the and thickness the thickness was 46mm. was 46mm. Its chemicalIts chemical composition composition is shown is shown in Table in 1. Table Table1. 1 Table also 1lists also the lists chemical the chemical composition composition range of range SA738 of Gr.BSA738 steel Gr.B in the steel ASME in the standard ASME [5]. standard The steel [5]. plat Thee was steel cut plate in the was quarter cut in theof the quarter thickness of the direction, thickness 3 anddirection, several samples and several with samples dimensions with of dimensions 12 × 40 × 120 of mm 12 were40 120taken mm for3 heatwere treatment. taken for heatThe samples treatment. were austenitized at 900 °C for 30 min, followed by water× quenching× to room temperature, and then The samples were austenitized at 900 ◦C for 30 min, followed by water quenching to room temperature, tempered at 630, 650, 670, 690, and 710 °C for 60 min, respectively. Figure 1 is the SEM micrograph and then tempered at 630, 650, 670, 690, and 710 ◦C for 60 min, respectively. Figure1 is the SEM of micrographSA738 Gr.B of steel SA738 after Gr.B quenching, steel after through quenching, which through we can which conclude we can that conclude the microstructure that the microstructure of as- quenchedof as-quenched steel is lath steel martensite. is lath martensite. TableTable 1. 1.ChemicalChemical compositions compositions ofof the the steel steel SA738 SA738 Gr.B Gr.B in ASME in ASME standard standard and this and experiment this experiment (mass fraction, (mass %). fraction,%). Element C Mn P S Si Ni Cr Mo Nb V Ti Fe Element C Mn P S Si Ni Cr Mo Nb V Ti Fe Standard 0.2 0.9–1.5 0.008 0.005 0.15–0.55 0.6 0.3 0.3 Total 0.08 Bal. ≤ ≤ ≤ ≤ ≤ ExperimentalStandard ≤0.2 0.14 0.9–1.5 1.55 0.008 0.008 0.005 0.001 0.15–0.55 0.25 ≤ 0.550.6 ≤0.230.3 ≤0.280.3 Total Total ≤= 0.080.077 Bal. Bal. Experimental 0.14 1.55 0.008 0.001 0.25 0.55 0.23 0.28 0.02 0.04 0.017 Bal. FigureFigure 1. SEM 1. SEM image image of as-quenched of as-quenched SA738 SA738 Gr.B Gr.B steel. steel. AfterAfter heat heat treatment, treatment, Charpy Charpy V-notch V-notch specimen specimenss with with dimensions dimensions of of 10 10 mm mm × 1010 mm mm × 5555 mm mm3 3 × × werewere prepared. prepared. The The V-notch V-notch was was parallel parallel to tothe the rolling rolling direction. direction. Charpy Charpy impact impact tests tests were were performed performed at at−45 45°C ◦onC onan anInstron Instron Dynatup Dynatup 9250 9250 instrumented instrumented impact impact machine machine (Norwood, (Norwood, MA, MA, USA) USA) according according − to tothe the working working condition condition of ofthe the steel. steel. Three Three specimen specimenss were were tested tested for for each each heat heat treatment treatment process process in in orderorder to toguarantee guarantee reproducibility. reproducibility. Specimens Specimens for for the the strength strength tests tests with with a 5 a mm 5 mm diameter diameter and and 50 50mm mm gaugegauge length length were were prepared prepared according according to toGB/T GB/ T228–2010 228–2010 [10]. [10 ]. SamplesSamples for for metallographic metallographic analysis analysis were were prepared prepared by bywire-cut wire-cut electrical electrical discharge discharge machining. machining. TheThe microstructures microstructures were observedobserved by by ZEISS ZEISS ULTRA ULTRA 55 Field 55 EmissionField Emission Scanning Scanning Electron MicroscopyElectron Microscopy(FESEM, Carl (FESEM, Zeiss Carl AG, Jena,Zeiss Germany)AG, Jena, afterGermany) grinding, after polishing, grinding, andpolishing, etching and with etching 4% nital with solution. 4% nitalA Tecnaisolution. G2 A F20 Tecnai transmission G2 F20 transmission electron microscope electron microscope (TEM, FEI, (TEM, Hillsboro, FEI, OR,Hillsboro, USA) wasOR, USA) also used was to alsoobserve used to the observe dislocations the dislocations and phases. and The phases. distribution The distribution of alloy elements of alloy waselements analyzed was byanalyzed using JEOLby usingJXA-8530F JEOL JXA-8530F field emission field electronemission probe electron (EPMA, probe Jeol (EPMA, Co. Ltd., Jeol Kyoto, Co. Ltd., Japan). Kyoto, The Japan). crystal The structures crystal of structuresthe particles of the were particles identified were by Digitalidentified Micrograph by Digital software Micrograph (Gatan, software Pleasanton, (Gatan, CA, USA).Pleasanton, The diagram CA, USA). The diagram between mass fraction and temperature of all the phases and the trend of the Metals 2020, 10, x FOR PEER REVIEW 3 of 11 Metals 2020, 10, 1207 3 of 12 precipitation type and quantity of the carbides were calculated by JMatPro software (Version 6.0, Generalbetween Steel mass module, fraction Sente and temperatureSoftware Ltd., of Guildford, all the phases UK) and according the trend to the of the chemical precipitation composition type and of SA738quantity Gr.B.