Magnetic properties and retained austenite quantification in SAE 4340 steel

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. INTRODUCTIONMagnetic materials are divided into two groups: soft and hard magnets. The latter must present high coercive field (H c ) and saturation magnetization (m s ) or induction (B s ). Soft magnetic materials must show high magnetic permeability and low H c , m s , B s . 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 M s was suggested. MATERIALS AND METHODSCylindrical 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

Influence of Austenitizing on the Mechanical Properties of Maraging 300 and Sae 4340 Steels - Comparative Study

“…Excessively coarse previous austenite grain size may promote intergranular quenching cracks in this family of steels 5 . Other effect of the coarse austenite grain size in low alloy Q&T steels is the increase of retained austenite, as observed in SAE 4340 steel 3 . Maraging steels are solution treated (or quenched) and aged in temperatures in the range of 450 o C to 650 o C depending on the microstructure and properties desired for the final product [6][7] .…”

“…The grain size of quenched and tempered (Q&T) steels is that of the previous austenite phase. This grain size is function of the austenitization temperature and time [2][3][4] . In Q&T low alloy steels, such as SAE 4340, 4140, 8640 and others, the austenitizing temperature range for commercial heat treatments must be high enough to homogenize the austenite, but low enough to avoid the austenite grain coarsening.…”

Crescimento Anormal Do Grão Austenítico E Sua Influência Na Fração Volumétrica De Austenita Retida Após a Têmpera De Um Aço Abnt 5135

“…Vários estudos [9,12,[16][17][18] relataram que em sistemas ferrosos o tamanho de grão austenítico afeta a temperatura M i pela redução da energia necessária para o cisalhamento se completar durante a transformação em função da eliminação de imperfeições na rede, devido à maior temperatura de austenitização. Da mesma forma, a nucleação da martensita pode ser impulsionada por um aumento nos aglomerados de lacunas, originando falhas no grão da austenita, devido a temperaturas de austenitização mais baixas [9].…”

“…Se por um lado vários trabalhos abordam a influência do tamanho de grão da austenita e sua composição química sobre a temperatura M i e sua cinética [15][16][17], poucas referências mencionam o efeito deste parâmetro sobre a fração de austenita retida [4,18]. Quando se menciona o efeito da granulação mista da austenita ou aquela originada do crescimento anormal de grão, nenhuma referência foi encontrada na literatura.…”

Mechanical and wear properties of pre-alloyed molybdenum P/M steels with nickel addition

“…The austenite phase becomes more stable with increasing nickel content and retained austenite amount increases. The amount of retained austenite was measured by using the magnetic saturation method, which is useful when the metallographic techniques give an imprecise result [19][20]. The results was 1,5, 0,5 and 6,9 % austenite phase, respectively, for specimens with 0, 2 and 4 % nickel.…”