Influence of Ti and C on the Solidification Microstructure of Fe-10Al-5Cr Alloys
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PREVIOUS research[1] has shown that a weld overlay composition of Fe-10Al-5Cr exhibits good corrosion resistance in simulated low-NOx coal-fired boiler combustion environments compared to stainless steel or nickel-based alloys. In addition, the lack of nickel in the overlay has two main benefits. First, eliminating nickel reduces the raw material cost; second, iron, aluminum, and chromium promote ferrite formation,[2] which has been shown to eliminate residual microsegregation, which can compromise corrosion resistance.[3] Austenitic stainless steel and nickel-based overlay materials solidify as austenite, which limits the back-diffusion of solute elements. As a result, concentration gradients persist after solidification. The resulting residual microsegregation leads to localized corrosion at the alloydepleted dendrite cores, which can lead to cracking by a corrosion-fatigue mechanism.[4] In contrast, recent work has shown that Fe-Al-Cr weld overlays exhibit no concentration gradients after solidification, which is attributed to the higher diffusion rates in ferrite.[3] While the Fe-10Al-5Cr alloys provide excellent corrosion resistance, they are also susceptible to hydrogen embrittlement and associated cracking during welding.[3] It has been proposed that preheat and postweld heat treatments can alleviate the severity of the hydrogen cracking problem, but this is not feasible when overlaying large structures.[5] In previous work[3] the positive impact of (Cr,Fe)xCy and (Fe,Al)3C carbides on K.D. ADAMS, Senior Engineer, is with the Welding Technology group, Bechtel Marine Propulsion Corporation, West Mifflin, PA 15102. J.N. DUPONT, R.D. Stout Distinguished Professor, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Contact e-mail: [email protected] Manuscript submitted June 16, 2009. Article published online November 3, 2009 194—VOLUME 41A, JANUARY 2010
improving the weldability of Fe-Al-Cr weld overlays by acting as hydrogen trapping sites was observed. It has also been shown that some second-phase particles are more effective hydrogen trap sites than others.[6] Carbides such as TiC have been observed as effective hydrogen traps.[7,8] In particular, TiC has been of interest[9] in steels due to its high hydrogen binding energy, 87 to 98 kJ/mol.[9,10] Increasing fractions of hydrogen trap sites with high binding energies leads to an increased amount of hydrogen trapped at benign locations and, therefore, decreased amounts of hydrogen are available for embrittlement.[6] The amount of trapped hydrogen is expected to increase with an increased amount of TiC in the microstructure, thus providing a means for potentially improving the weldability (i.e., resistance to hydrogen assisted cracking) of these Fe-Al-Cr alloys. The Fe-rich corner of the Fe-Ti-C liquidus projection, shown in Figure 1, indicates the fields over which a particular primary phase will form during solidification (bcc ferrite, fcc austenite, TiC (titanium carbide), and Laves phase). This liquidus projection was
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