No abstract

In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560 • C, 600 • C and 650 • C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560 • C and 600 • C. At 650 • C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.

Deleterious phases precipitation in superduplex stainless steels is the main concern in fabrication by welding and hot forming of this class of material. Sigma, chi and secondary austenite phases are considered deleterious phases because they produce negative effects on corrosion resistance. Besides, sigma and chi phases also promote strong decrease of toughness. In the present work, the precipitations of sigma, chi and secondary austenite under aging in the 800-950 °C interval were studied in two UNS S32750 steels with different grain sizes. The deleterious phases could be quantified by light optical microscopy, with no distinction between them. Scanning electron microscopy was used to distinguish the individual phases in various aging conditions. The results elucidate the influence of the aging temperature and grain size on the kinetics precipitation and morphology of deleterious phases. The kinetics of deleterious phases is higher in the fine grained material in the initial stage of aging, but the maximum amount of deleterious phases is higher in the coarse grained steel.

The magnetic properties and hardness of a Ni-Co-Mo-Ti maraging steel 300 grade were measured as function of aging temperature. The austenite and martensite phase quantifications in the different heat treatment conditions were carried out by X-ray diffraction using direct comparison method. The behavior of the hardening, magnetization saturation and coercive force against aging temperature and time were explained taking into account the variation of austenite volume fraction with aging time and temperature.

This work investigates the influence of deleterious phases precipitation on the corrosion resistance of two wrought superduplex stainless steels UNS S32750 with similar composition, but different grain sizes. Isothermal treatments were carried out to introduce different amounts of deleterious phases, such as chi (χ), sigma (σ) and secondary austenite (γ 2 ). The specimens were tested by double loop electrochemical potentiodynamic reactivation test (DL-EPR). The kinetics of precipitation in the two steels was different due to the difference of the grain sizes. The results show a correlation between the sensitization degrees, measured by DL-EPR, with the amount of deleterious phases precipitated.