Experimental Study of High Temperature Phase Equilibria in the Iron-Rich Part of the Fe-P and Fe-C-P Systems
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PHOSPHORUS is generally known as a harmful element in steel. In the continuous casting process, the strong segregation of P during solidification may lead to internal quality problems, e.g. hot tear formation. Increasing amounts of carbon and other alloying elements lower the solidification temperatures and possibly change the phase transformation path in the peritectic range. As a consequence, the limited diffusivity of P along the dendritic microstructure at decreased temperatures and the low solubility of P in austenite favor the interdendritic enrichment and cause an even higher risk of crack formation in the casting process.[1–4] However, despite the detrimental effects on the product quality, phosphorus is partly added in steelmaking MICHAEL BERNHARD, PETER PRESOLY, NORA FUCHS, and CHRISTIAN BERNHARD are with the Chair of Ferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria Contact e-mail: [email protected] YOUN-BAE KANG is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Kyungbuk 37673, Pohang, Republic of Korea. Manuscript submitted February 27, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
nowadays to improve various performance characteristics of steel: (i) P acts as the most effective solution-hardening element in the steel matrix. This strengthening mechanism is particularly used in the production of High-Strength Interstitial-Free (HSIF) steels.[5] (ii) P is added up to 0.10 mass pct. to structural steels in order to improve the atmospheric resistance.[6,7] (iii) In transformation induced plasticity (TRIP) steels, P stabilizes the retained austenite; amounts up to 0.10 wt pct are considered to show beneficial influence on the TRIP effect.[8] Reliable phase diagram data in the high-temperature range of the ternary Fe-C-P system are essential for developing a comprehensive understanding of solidification phenomena in P alloyed steels. In the present study, high-temperature phase transformations of 37 model alloys in the Fe-P and Fe-C-P systems were investigated using differential scanning calorimetry (DSC) and high-temperature laser scanning confocal microscopy (HT-LSCM). The DSC method has already been used in previous works[9,10] to characterize the influence of alloying elements on the peritectic range and determine melting equilibrium temperatures in Fe-C-X isoplethal sections. Linking the DSC method with HT-LSCM observations is a supportive and powerful
tool to gain additional information on present phase stabilities in the peritectic region and confirm the interpretation of the DSC signal.[9] The first part of this work deals with the reinvestigation of phase equilibria in the binary Fe-P subsystem in the range 0.025 to 9 mass percent P. Due to the lack of experimental literature data at low P compositions, special focus was placed on compositions < 1 mass percent P. In the second part, three isoplethal sections in the Fe-C-P system were investigated. Model alloys with Fe-C-0.10 wt pct P and Fe-0.20 wt pc
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