Elucidating the Effect of Alloying Elements on the Behavior of Austenitic Stainless Steels at Elevated Temperatures
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e grain size has a significant effect on determining mechanical properties of austenitic stainless steels and these materials often are used at elevated temperatures, investigating the grain growth behavior, its kinetics, and microstructural evolution might be of interest for many practical or potential applications. There are some reports dealing with this subject[1–3] but there are very few systematic investigations on the effect of alloying elements such as carbon and molybdenum on the high-temperature behavior of stainless steels. In fact, the effect of these elements on static recrystallization[4,5] or dynamic recrystallization[6,7] behavior of steels or stainless steels has been investigated. But, their effect on the grain growth behavior remained unclear. The present work aims to deal with this subject. For investigating the effect of carbon and molybdenum on the high-temperature behavior of austenitic stainless steels, AISI 304, 304L, and 316L stainless steels
MEYSAM NAGHIZADEH, Master Student, and HAMED MIRZADEH, Assistant Professor, are with the School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted March 18, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
were considered in this work. The chemical compositions of these materials are shown in Table I. It can be seen that the main difference between AISI 304 and AISI 304L is in the amount of carbon and between AISI 304L and AISI 316L is in the amount of molybdenum. The average grain size of the as-received materials were found to be ~23, 14, and 14 lm for 304L, 304L, and 316L stainless steels, respectively. The samples were exposed to elevated temperatures in the range of 923 K and 1323 K (650 C and 1050 C) for soaking times between 0.5 and 8 hours. To reveal the grain boundaries, as shown in Table II, electroetching was carried out in a HNO3 solution[8] preceded by electrolytic polishing.[9] Optical microscopy, scanning electron microscopy equipped with EDS analysis (using a CamScan MV 2300 SEM), and X-ray diffraction (using a Philips PW-3710 diffractometer with Cu-Ka radiation) were used for microstructural, elemental, and phase analysis. Figure 1 shows the obtained optical micrographs, which reveals that there is no significant grain coarsening at 923 K (650 C) for all examined materials because the bulk diffusion is not pronounced. However, the grain boundaries of AISI 304 stainless steel soaked at 923 K (650 C) for 4 hours are decorated by a dark phase, which is not present in the low-carbon grades. As can be deduced from the EDS map shown in Figure 2, the grain boundaries have high amount of carbon (red color). While XRD pattern shown in Figure 3 does not reveal any other phase except austenite and martensite, the inlay figure shown in Figure 3[10] demonstrates that after annealing at 923 K (650 C) for 4 hours, the chromium carbide forms in stainless steel with 0.042 wt pct C. Therefore, this carbide phase should be present in t
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