The effects of Al and Si additions on the microstructures of precipitates in low carbon steels containing Sn

The microstructures of low carbon steels with Sn additions were investigate d using scanning electron microscopy, electron probe microanalysis, transmi ssion electron microscopy and energy dispersive X-ray spectroscopy. Four st eels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effe cts of heat treatment and the level of alloying elements Al and Si on the p recipitation of Sn-rich phases were studied. After ageing at 1150 degrees C and 850 degrees C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was a lso found to affect the formation of Sn-rich compounds after heat treatment at 850 degrees C for 96 hours. In the lower Al or Al-free steels, a eta-Fe 2Nb3 phase, which dissolves a significant amount of Si, was observed. In th e higher Al steels, a Fe2Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic c onsiderations is described, which accounts for the experimental observation s. (C) 2000 Kluwer Academic Publishers.