M. N. NGUYEN, N. H. VU: INFLUENCE OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND ... 553–556 INFLUENCE OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND ULTIMATE TENSILE STRENGTH OF 28Cr3SiNiMoWV STEEL VPLIV TEMPERATURE POPU[^ANJA NA MIKROSTRUKTURO IN NATEZNO TRDNOST JEKLA VRSTE 28Cr3SiNiMoWV Minh Ngoc Nguyen 1* , Ngan Hanh Vu 2 1 School of Materials Science and Engineering, Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet street, Hanoi, Vietnam 2 Center of Materials and Failure Analysis, Institute of Materials Science, No. 18, Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Prejem rokopisa – received: 2023-02-03; sprejem za objavo – accepted for publication: 2023-08-27 doi:10.17222/mit.2023.787 In this research, the influences of the tempering temperature on the microstructure and ultimate tensile strength of 28Cr3SiNiMoWV steel were studied. The microstructure and ultimate tensile strength were investigated after tempering 28Cr3SiNiMoWV steel at different temperatures, ranging from 280 °C to 440 °C for 2 h. The results show that after tempering it at different temperatures, the microstructure of 28Cr3SiNiMoWV steel was tempered martensite. During the tempering pro- cess, the alloy carbides precipitated in the martensite matrix. Precipitation of alloy carbides in the microstructures of different specimens is the cause for an increase in the ultimate tensile strength. With the increasing tempering temperature, the ultimate tensile strength initially increases from 1390 MPa to 1601 MPa, and then decreases to 1466 MPa, with its maximum value at 280 °C. Keywords: 28Cr3SiNiMoWV steel, ultimate tensile strength, tempering temperature Povzetek: v pri~ujo~em ~lanku avtorji opisujejo raziskavo vpliva temperature popu{~anja na mikrostrukturo in natezno trdnost jekla vrste 28Cr3SiNiMoWV. Mehanske lastnosti in mikrostrukturo preiskovanega jekla so dolo~ili po kaljenju pri 920 °C in po dveurnem popu{~anju pri temperaturah med 280 °C in 440 °C. Po popu{~anju je v vseh primerih jeklo imelo mikrostrukturo popu{~enega martenzita. Med popu{~anjem so se z nara{~ajo~o temperaturo popu{~anja iz martenzitne matrice izlo~ali ustrezno veliki karbidni izlo~ki zlitinskih elementov, kar je povzro~ilo proporcionalno spremembo natezne trdnosti jekla. V izhodnem stanju je imelo preiskovano jeklo natezno trdnost 1390 MPa. Pri najni`ji izbrani temperaturi popu{~anja 280 °C je imelo jeklo najvi{jo natezno trdnost 1601 MPa in pri najvi{ji temperaturi popu{~anja 440 °C najni`jo natezno trdnost 1466 MPa. Klju~ne besede: jeklo vrste 28Cr3SiNiMoWV, natezna trdnost, temperatura popu{~anja 1 INTRODUCTION High-strength steel has been widely used for produc- ing landing gear, high stress joints, wing beams and bolts in the aviation industry because of its many distin- guished advantages, such as high strength, high hard- ness, good fracture toughness, etc. In order to obtain these advantages, steel needs to have an appropriate microstructure. In recent years, many studies on the con- trol of steel microstructures were carried out by research- ers. Related research on, for example, the grain size control 1,2 , control of carbide precipitation within a very small range, 3 twin martensite effect, 4,5 control of the phase ratio by a heat treatment process, 6,7 etc., was done. Although many important discoveries have been made, the microcosmic understanding of different steel types is still in the process of continuous development. Espe- cially the studies on the precipitation of alloy carbides in tempered martensite under heat treatment conditions still attract a lot of attention from researchers. 8,9 This is be- cause an addition of certain alloying elements to a com- position can improve the mechanical properties of steel, but the production cost increases. In order for the re- quired properties of steel to be obtained, the most eco- nomical and suitable steel type should be selected. Heat treatment is one of the most economical and feasible methods for adjusting the mechanical properties. 10 This method, consisting of quenching and tempering, is known as the most common application in mechanical manufacturing. It helps to create fine carbide particles during a tempering process, providing a major strength- ening effect in many steel types. 11 For this purpose, the tempering treatment is an important necessary stage for improving the ultimate tensile strength (UTS) of steel. However, the tempering temperature range allowing the suitable precipitation of alloy carbides often depends on the chemical composition of individual steels. 12 There- fore, it is necessary to study and optimize specific tem- perature ranges, thereby optimizing the mechanical prop- erties of steel. In this study, the effects of the tempering tempera- tures of 280–440 °C on the microstructure and UTS of 28Cr3SiNiMoWV steel are discussed. The aim of this Materiali in tehnologije / Materials and technology 57 (2023) 5, 553–556 553 UDK 669.15'24'26-194:621.785.796 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)553(2023) *Corresponding author's e-mail: minh.nguyenngoc@hust.edu.vn (Minh Ngoc Nguyen) study is to select the best tempering temperature range allowing a significant increase in the UTS of 28Cr3SiNiMoWV steel. 2 EXPERIMENTAL PART The chemical composition of the 28Cr3SiNiMoWV steel was analysed using a SPECTROMAXx (SPECTRO, LMX10). Samples for the microstructural analysis were prepared, having a cubic shape and dimen- sions of (10 × 10 × 10) mm while samples for the tensile testing were made according to the ISO 7500-1 standard. All the samples were quenched in an oil quenching me- dium before being subjected to next tempering stage. In order to select the appropriate quenching temperature, the Thermo-Calc program (Thermo-Calc, 2015b, TCFE7) was used to calculate and build the phase dia- gram for the steel composition shown in Table 1. Table 1: Chemical composition of the 28Cr3SiNiMoWV steel Element C Si Cr Ni Mo Mn W V P S Fe w/% 0.29 0.95 2.83 1.01 0.35 0.56 0.81 0.06 0.01 0.01 Bal. The binary diagram is shown in Figure 1. The selec- tion of the quenching temperature based on the phase di- agram must ensure that the microstructure of the steel is completely austenitic before cooling. However, the tem- perature should not be too high so that the fine austenite grains can be maintained. For this reason, the quenching temperature was selected to be 920 °C. To evaluate the effect of the tempering temperature, the quenched sam- ples were tempered at (280, 360 and 440) °C for2hb e - fore the microstructural observation and tensile testing. All sample surfaces were protected to prevent oxidation and decarburization during heat treatment processes. For the observation and evaluation of the micro- structure, the cubic samples were cross-sectioned, ground, polished and etched by the Villela solution be- fore a scanning electron microscope coupled with an en- ergy-dispersive spectrometer (SEM, JEOL JSM-6500F, EDS INCA X-SIGHT LH2-type detector, INCA EN- ERGY 450) was used to analyse the chemical composi- tions of the phases. In addition, the crystal structures of the phases were also characterized with X-ray diffraction (XRD, ARL EQUINOX 5000), with the Cu K radiation operating at 60 kV , 60 mA. The measurements were re- corded in a 2 range of 20–90° at a scanning speed of 2° per min and a 2 step of 0.015°. The values of the UTS were determined using a ten- sile testing machine (WE-1000B, New Luda). Before the tensile tests of the specimens, the surfaces of the samples were cleaned to obtain effective cross-sections and pre- cise residual mechanical properties. The tensile tests were conducted at room temperature using a 500 kN load cell with a displacement rate of 2 mm/min. The tests were recorded by the software, which automatically calculated the data. 3 RESULTS AND DISCUSSION The microstructures of the as-received, quenched and tempered samples are shown in Figure 2. In the as-re- ceived sample (Figure 2a), the shape of the crystal grains is slightly flattened in a certain direction. This in- M. N. NGUYEN, N. H. VU: INFLUENCE OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND ... 554 Materiali in tehnologije / Materials and technology 57 (2023) 5, 553–556 Figure 1: Phase diagram of the 28Cr3SiNiMoWV steel calculated using Thermocalc software dicates that the used steel has probably undergone a pre- vious hot rolling process. Figure 2b shows a fully martensitic structure of the quenched sample with lath-shaped features. 13 This proves that the quenched temperature and quenching medium selected above are completely suitable. In the tempered samples (Figures 2c to 2e), the tempered martensite was obtained through tempering in a temperature range of 280–440 °C. The tempered martensite was similar to that of the quenched sample, but the difference was that there was precipita- tion of the fine spherical carbides in the lath regions after the tempering, as shown by the black arrows (Figures 2c to 2e). In addition, it can also be seen that the particle size of carbides in the sample tempered at a low temper- ature (Figure 2c) is smaller than in the sample tempered at a higher temperature (Figure 2e). This is because with an increase in the tempering temperature, martensite de- composed faster, allowing carbon and alloying atoms to move out of the martensite easily to form carbides. This led to an increase in the particle size of carbides. Fig- ure 2f shows the EDS result for the alloy carbide particle distribution in the tempered-martensite matrix. Figure 3 shows X-ray diffraction patterns of the sam- ples, and the presence of the highest intensity peaks re- lated to the ferrite phase (the as-received sample) and martensite phase (quenched, tempered samples). A com- parison between the position of the main peak of the as-received sample (Figure 3a) and the main peak of the quenched sample (Figure 3b) shows that the latter tends to shift to the left in the direction of a smaller 2 angle. This is because during the austenitizing process, carbon and alloying elements were more soluble in the solid so- lution. Thereby, the lattice constant was increased, re- sulting in a decrease in the 2 angle. For all of the tempered samples (Figures 3c to 3e), the tendency to shift the positions of the main diffraction peaks occurs in the opposite direction. At the higher tem- pering temperature (Figure 3e), the 2 angle shifts more to the right, with a larger diffraction angle. This is due to the fact that at a higher tempering temperature, the de- composition rate of quenched martensite is faster. This reduces the lattice constant, resulting in an increase in the 2 diffraction angle. No diffraction peaks of the alloy carbide were observed. This may be because the density of the carbide particles is not high and the size of the car- M. N. NGUYEN, N. H. VU: INFLUENCE OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND ... Materiali in tehnologije / Materials and technology 57 (2023) 5, 553–556 555 Figure 3: XRD patterns of the samples: a) as-received, b) quenched, c) quenched and tempered at 280 °C, d) quenched and tempered at 360 °C, (e) quenched and tempered at 440 °C Figure 2: SEM images and EDS analysis of the samples: a) as-received, b) quenched, c) quenched and tempered at 280 °C, d) quenched and tem- pered at 360 °C, e) quenched and tempered at 440 °C, f) EDS result of spectrum 1 bide particles is very small, so it is difficult to recognize XRD. Thus, the types of the alloy carbides could not be detected with XRD measurements due to their small size and low content. However, based on the alloy composi- tion (Table 1), the tempering temperatures and the phase diagram shown in Figure 1, it is possible to predict that the alloy carbides are in the forms of MC and M 3 C (cementite). Figure 4 shows the load-time relationship for the ten- sile test, and the UTS of the samples is shown in Table 2. The UTS values for the samples after tempering (Fig- ures 4b to 4d) are increased in comparison with the as-received sample (Figure 4a), proving that the precipi- tation of alloy carbides during the tempering process im- proves the UTS of the steel. However, with an increase in the tempering temperature, the tempered martensite is quickly further decomposed into ferrite and carbides. Compared with the tempering at a lower temperature, the carbides that precipitate on the matrix become bigger and coarser. In addition, the recovery rate of the matrix increases significantly, making the dislocation density decrease. All of these developments reduce the UTS of the steel. Table 2: Ultimate tensile strength values for the samples Type of sample Sample a Sample b Sample c Sample d UTS (MPa) 1390 1601 1570 1466 4 CONCLUSIONS The tempering temperature can be used to control the microstructure and ultimate tensile strength of 28Cr3SiNiMoWV steel. In a tempering temperature range of 280–440 °C, with an increase in the tempering temperature, the tempered martensite was quickly further decomposed into ferrite and carbides while the ultimate tensile strength showed a decreasing trend. The highest value of the ultimate tensile strength was 1601 MPa, cor- responding to a tempering temperature of 280 °C. Acknowledgment This research was funded by the Hanoi University of Science and Technology (HUST) under project number T2022-PC-084. 5 REFERENCES 1 L. Lan, C. Qiu, L. Du, Effective grain size dependence of crack prop- agation resistance in low carbon steel, Theor. Appl. Fract. 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Mat. Fin., 2 (2017), 71–106, doi:10.1016/B978-0-12- 803581-8.09187-6 M. N. NGUYEN, N. H. VU: INFLUENCE OF TEMPERING TEMPERATURE ON THE MICROSTRUCTURE AND ... 556 Materiali in tehnologije / Materials and technology 57 (2023) 5, 553–556 Figure 4: Load-time record for the tensile test: a) as-received – sam- ple a, b) quenched and tempered at 280 °C – sample b, c) quenched and tempered at 360 °C – sample c, d) quenched and tempered at 440 °C – sample d