Magnetic properties and retained austenite quantification in SAE 4340 steel

Revista Matéria, v. 14, n. 3, pp. 993 – 999, 2009 http://www.materia.coppe.ufrj.br/sarra/artigos/artigo11069 ISSN 1517-7076 Magnetic properties and retained austenite quantification in SAE 4340 steel AJUS I, C.; TAVARES I, S.S.M.; SILVA II, M.R.; CORTE I, R.R.A. I Universidade Federal Fluminense – Departamento de Engenharia Mecânica, Rua Passo da Pátria, 156 – CEP 24210-240, Niterói, RJ, Brazil e-mail: , , II Universidade Federal de Itajubá - Instituto de Ciências, Departamento de Física e Química - Avenida B. P. S., 1303 Pinheirinho 37500-903 - Itajubá, MG, Brasil - Caixa-Postal: 50 e-mail: ABSTRACT Magnetic properties (coercive force and saturation magnetization) of SAE 4340 steel, heat treated in different ways, were measured in a Vibrating Sample Magnetometer. Four soaking temperatures were investigated: 860C, 960C, 1060C and 1160C. The oil-quenched samples were tempered at different temperatures between the range of 200C and 650C. The increase of the soaking temperature promotes the decrease of the saturation magnetization, which is mainly due to the retained austenite in the steel. This austenite transforms into bainite during tempering between 200C and 300C. The saturation magnetization measurements were used to quantify the amount of retained austenite in the different samples. The coercive force of the steel varied from 115.0 Oe (9151.7 A/m) to 133.0 Oe (10584.1 A/m), depending on the heat treatment condition. Keywords: Magnetic properties, retained austenite, SAE 4340 steel. 1 INTRODUCTION Magnetic materials are divided into two groups: soft and hard magnets. The latter must present high coercive field (Hc) and saturation magnetization (ms) or induction (Bs). Soft magnetic materials must show high magnetic permeability and low Hc, ms, Bs. The hysteresis loop must be high and thin. For some specific uses, there is a demand for high strength materials, with soft magnetic properties. This is a problem because, in general, materials that present high mechanical strength also present a greater magnetic hardness. One of the applications for materials with soft magnetic properties and high strength is in the rotors of ultra-high speed engines used in the nuclear industry. For this application maraging steels have been used in the aged condition [1-2]. However, maraging steels are high cost materials and, frequently, not available for nuclear power generation in developing countries. SAE 4340 steel has demonstrated interesting applications where good mechanical and magnetic properties are necessary, at a low cost. This steel presents a good hardening ability that promotes the improving of its mechanical properties when submitted to quenching and tempering. The behaviour of coercive force and saturation magnetization as a function of soaking and tempering temperatures were investigated in this paper. An equation to measure the amount of retained austenite as a function of Ms was suggested. 2 MATERIALS AND METHODS Cylindrical samples, with a diameter of 3.5mm, of SAE 4340 steel (chemical composition shown in table 1) were machined and oil quenched. Four soaking temperatures were investigated: 860C, 960C, 1060C and 1160C. After that, the cylindrical samples were cut into slices with a thickness of 0.3mm by wire electro-erosion. These samples were tempered for 1 hour at different temperatures from 200C to Autor Responsável: AJUS, C.; TAVARES, S.S.M. ; SILVA, M.R. Data de envio: 14/01/09 Data de aceite: 25/09/09 AJUS, C.; TAVARES, S.S.M.; SILVA, M.R.da; CORTE, R.R.A; Revista Matéria, v. 14, n. 3, pp. 993 – 999, 2009. 650C, at intervals of 50C. Magnetic measurements were carried out in a Vibrating Sample Magnetometer (VSM) with a maximum applied field of 0.5T, at a time constant of 10ms and a total measuring time of 20 minutes. The samples were etched, for microstructure analysis, with nital 2% (solution 1) or with a solution of picric acid and ferric chloride (solution 2) which reveals prior austenite grain boundaries. Microstructures were analyzed by optical microscopy and X-ray diffraction techniques. Vickers hardness measurements were also carried out for all the heat treatment conditions, with a load of 30kgf. Table 1: Chemical Composition of SAE 4340 steel %C %Si 0.410 0.240 Iron: remaining 3 %Mn 0.620 %S 0.016 %P 0.019 %Cr 0.740 %Ni 1.670 %Mo 0.270 RESULTS Figure 1 shows the behaviour of coercive force (Hc) versus tempering temperature as a function of the soaking temperature. It is observed in this figure that Hc increases slightly with the soaking temperature for the untempered samples. Figures 2(a) and 2(b) show the microstructures of quenched samples at 860C and 1060C. In these micrographs the prior austenite grain boundaries were revealed by solution 2. The average austenite grain sizes in the quenched samples were determined by quantitative metallography and listed in Table 2. Slightly higher values of coercive force were obtained in samples quenched from higher temperatures (1060C and 1160C). However, the influence of the soaking temperature on the Hc is a lot less important than expected when considering the differences of prior austenite grain sizes and retained austenite volume fractions. As observed by Battistini, and Benasciutti [5] in ferritic stainless steel, the coercive force, generally, tends to decrease with an increase of the grain size. This effect was not observed in the SAE 4340 steel. Figure 1: Coercive force behaviour as a function of tempering temperature The coercive force decreases as a function of tempering temperature and presents the lowest values between 300C and 500C. The smallest Hc value (115.0 Oe or 9151.7 A/m) was found in the sample quenched at 860oC and tempered at 450C. A significant increase of Hc was observed in the sample quenched at 860C, 1060C and 1160C, and tempered at 600C. Similar results were also presented by Berti Neto et al. [3], who observed a significant decrease of Hc between 200C and 300C and a small increase at 600C in samples quenched with a soaking temperature of 870C. 994 AJUS, C.; TAVARES, S.S.M.; SILVA, M.R.da; CORTE, R.R.A; Revista Matéria, v. 14, n. 3, pp. 993 – 999, 2009. A B Figure 2: (a) Austenite grain boundaries of quenched steel at 860C. 800 X; (b): Austenite grain boundaries of quenched steel at 1060C. 800 X. Table 2: Average grain size at different soaking temperatures Soaking Temperature (ºC) Average Grain Size (μm) Standard Deviation ((μm) 860 19 2 960 22 3 1060 36 14 1160 61 6 It is interesting to show the variation curves of mechanical hardness against tempering temperature and observe how the increase of soaking temperature above 860oC decreases the hardness, not only in the quenched samples but also in all the tempered conditions (Figure 3). A correlation between mechanical and magnetic hardness is always searched for in metallic materials because the hardening mechanism tends to impede the magnetic domains (...truncated)