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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

Resumo: Os aços inoxidáveis duplex são materiais de alta resistência mecânica e alta resistência à corrosão, fatores que justificam a crescente aplicação destes materiais nos setores químico e petroquímico. As boas propriedades dos aços duplex são atribuídas à microestrutura de proporções praticamente iguais de ferrita e austenita, e às adições de elementos de liga como Cr, Mo e N. Entretanto, os processos de fabricação por conformação a quente e soldagem produzem alterações microestruturais importantes no material, que geralmente conduzem a prejuízos na tenacidade e resistência à corrosão. A formação de fase sigma é uma das transformações microestruturais indesejáveis que podem ocorrer durante a fabricação de equipamentos e componentes mecânicos. Neste trabalho foram investigados os efeitos de pequenas porcentagens de fase sigma na resistência à corrosão do aço inoxidável duplex UNS S31803. São também apresentados resultados sobre a detecção e quantificação de fase sigma por metalografia, medidas de dureza e medidas magnéticas com ferritoscópio. Palavras-chave: aços inoxidáveis duplex, resistência à corrosão, ferritoscópio.Abstract: Duplex stainless steels are high resistance materials with good corrosion resistance, which explain the large application of this class of materials on petrochemical and chemical sector. The excellent properties of these materials are attributed to the biphasic microstructure with equal parts of ferrite and austenite, and also to the Cr, Mo and N additions. However, the fabrication process by hot forming and welding produces important microstructural changes in the material, which generally decreases the toughness and corrosion resistance. The sigma phase formation is an undesirable microstructural transformation which happens during the equipments and mechanical components fabrication. At this work the effects of small percentages of sigma phase in the duplex UNS S31803 stainless steel corrosion resistance were investigated. Results concerning sigma phase detection and sigma phase quantification by metallography, hardness and magnetic measurements using the ferritoscope are also presented.Palavras-chave: duplex stainless steels, corrosion resistance, ferritoscope.

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