No abstract

The effect of initial microstructures of low-carbon steel on austenite formation behavior during intercritical annealing was investigated. Three types of hot-rolled sheet specimens with different microstructures were used; specimen P consisting of ferrite and pearlite, specimen B consisting of bainitic structures, and specimen M consisting of fully martensitic structures. After the hot rolling, these specimens were coldrolled, and subsequently heated to target temperature, and then water-quenched to room temperature. The martensite and/or bainite fraction corresponds to the fraction of austenite during intercritical annealing since the austenite transforms into martensite and/or bainite during the cooling process. The austenite fraction in specimen M was larger than that in specimens P and B below 730°C, whereas the order of specimens changed to P > B > M above 740°C. Below 730°C, austenite connected along the rolling direction was observed in specimens P and B, while the distribution of austenite in specimen M was uniform. In contrast, austenite was connected and elongated along the rolling direction in all the specimens above 740°C. The nucleation and growth of austenite can proceed under local equilibrium in specimens P and B, whereas that can proceed under paraequilibrium in specimen M below 730°C. Moreover, the austenite growth can progress under local equilibrium in all specimens above 740°C.

We investigated the microstructural evolution during intercritical annealing in Nb-added low-carbon steels, focusing on the synergistic effects of the addition of Nb and the ferrite (α) to austenite (γ) phase transformation on the recrystallization behavior of α. Two kinds of specimens, containing 0.02 and 0.05 mass% Nb, were prepared and annealed at the intercritical temperature (750°C) for a long time. The progress of recovery and α recrystallization was retarded by increasing the amount of Nb addition during intercritical annealing. Moreover, the progress of α recrystallization during intercritical annealing was mainly attributed to continuous recrystallization due to subgrain growth. The fraction of γ that formed during intercritical annealing increased owing to Nb addition, but it did not increase with increasing amount of Nb addition. These results suggest that the progress of recovery and α recrystallization was retarded during intercritical annealing by the addition of Nb, thereby causing the increase in γ fraction. Furthermore, the increase in γ fraction led to further retardation of α recrystallization and α refinement with the addition of Nb.

The effect of initial microstructures of low-carbon steel on austenite formation behavior during intercritical annealing was investigated. Three types of hot-rolled sheet specimens with different microstructures were used; specimen P consisting of ferrite and pearlite, specimen B consisting of bainitic structures, and specimen M consisting of fully martensitic structures. After the hot rolling, these specimens were cold-rolled, and subsequently heated to target temperature, and then water-quenched to room temperature. The martensite and/or bainite fraction corresponds to the fraction of austenite during intercritical annealing since the austenite transforms into martensite and/or bainite during the cooling process. The austenite fraction in specimen M was larger than that in specimens P and B below 730 °C, whereas the order of specimens changed to P > B > M above 740 °C. Below 730 °C, austenite connected along the rolling direction was observed in specimens P and B, while the distribution of austenite in specimen M was uniform. In contrast, austenite was connected and elongated along the rolling direction in all the specimens above 740 °C. The nucleation and growth of austenite can proceed under local equilibrium in specimens P and B, whereas that can proceed under paraequilibrium in specimen M below 730 °C. Moreover, the austenite growth can progress under local equilibrium in all specimens above 740 °C.

Background: It is important to investigate the mechanism for the hydrogen embrittlement of Ni-Ti superelastic alloy in acidic fluoride solutions so that the reliability and safety of the alloy as a dental device can be improved. The purpose of the present study is to investigate the effect of aging at room temperature on the hydrogen embrittlement behavior of Ni-Ti superelastic alloy immersed in acidic fluoride solution. Methods: Specimens were immersed separately in 50 mL of 0.2 % acidulated phosphate fluoride (APF) solution with pH 5.0 at room temperature for 16 h. The hydrogen-charged specimens were aged for various periods at room temperature in air to adjust the hydrogen distribution. Results: The specimen that was tested immediately after immersion in 0.2 % APF solution fractured near the critical stress for martensitic transformation without martensitic transformation. The tensile strength of the immersed specimen was partially recovered by aging at room temperature for 24 h. In addition, the tensile strength of the specimen immersed in 0.2 % APF solution was completely recovered by aging at room temperature for 240 h. Conclusions: After aging at room temperature for 24 h, the tensile strength of the immersed specimen was partially recovered. In addition, the tensile strength of the specimen immersed in 0.2 % APF solution was completely recovered by aging at room temperature for 240 h. These results indicate that the mechanism for the hydrogen embrittlement of Ni-Ti superelastic alloys aged at room temperature after immersion in 0.2 % APF solution is dependent on the aging time.