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: 860C, 960C, 1060C and 1160C. The oil-quenched samples were tempered at different temperatures between the range of 200C and 650C. 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 200C and 300C. 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. INTRODUCTION Magnetic 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 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: 860C, 960C, 1060C and 1160C. 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 200C t