Navegando por Autor "Abdalla A.J."

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  • Artigo
    An in situ high-temperature X-Ray diffraction study of phase transformations in maraging 300 steel
    Reis A.G.D.; Reis D.A.P.; Abdalla A.J.; Couto A.A.; Otubo J. (2016)
    © 2016 Trans Tech Publications, Switzerland.An in situ high-temperature X-ray diffraction (HTXRD) study in maraging 300 steel was carried out to study the martensite to austenite transformation and effect of time of exposure in the austenite reversion below austenite start temperature. Solution annealed materials were subjected to controlled heating-holding cycles. The first sample was heated at a rate of 10°C/min from room temperature to 800°C, showing that the microstructure is completely martensitic (α110) until 600°C. From 650°C until 800°C, the microstructure is gradually changing from martensitic to austenitic, showed by the increasing peaks of λ111 and reducing peaks of α110. At 800°C the microstructure is completely austenitic (λ111). Another sample was heated at 10°C/min from room temperature to 600°C and held for 4 hours. At 600°C, at 0 h time of exposure, only a martensitic peak was observed. An austenite peak can be observed after some time of exposure at this temperature. The volume fraction of austenite increased with increasing time of exposure at 600°C, reaching 50/50 volume fraction after 4 hours of exposure. XRD diffraction patterns for the same sample that was held for 4 hours at 600°C and then cooled down in air to room temperature showed the same intensity of austenite and martensitic peaks found in situ at 600°C for 4 hours (retained austenite), with the volume fraction of 50/50 of austenite and martensite phases. The HTXRD technique can be used to identify and quantify martensite to austenite transformation and austenite retention.
  • Artigo de evento
    Hot tensile behavior and fracture characteristics of a plasma nitrided maraging 300 steel
    Reis A.G.; Reis D.A.P.; Abdalla A.J.; Otubo J.; Couto A.A.; Neto F.P. (2017)
    © 2017 Trans Tech Publications, Switzerland.The influence of plasma nitriding of a maraging 300 steel on mechanical properties at high temperature has been studied. Samples were tensile tested at 600°C in four conditions: solution treated (MAR-S), solution treated and aged (MAR-SA), solution treated and plasma nitrited (MAR-SP) and solution treated, aged and plasma nitrited (MAR-SAP). In the same sequence, the yield strength and ultimate tensile strength increased slightly respectively from 1073 to 1189 MPa and 1174 to 1301 MPa, an increase of about 10% due to plasma nitriding. All the samples presented similar values of elongation, around 18%, but the cross section area reduction decreased significantly by plasma nitriding from ~70% for MAR-S and MAR-SA to ~45% for MAR-SP and MAR-SAP, that is an decrease of 36% in average. This decrease is attributed to brittle fracture nucleated at 50 µm thick iron nitride layer. The inner fracture surface of the tensile tested specimens was predominantly ductile presenting characteristic microcavities.
  • Artigo de evento
    Short-term creep properties and fracture surface of 18 Ni (300) maraging steel plasma nitrided
    Dos Reis A.G.; Reis D.A.P.; Abdalla A.J.; Couto A.A.; Otubo J. (2017)
    © 2017 Universidade Federal de Sao Carlos. All rights reserved.Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 μm layer composed of ε-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45ºshear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.