Characterization of Solid State Phase Transformation in Continuously Heated and Cooled Ferritic Weld Metal

Abstract

Arc welding processes involve cooling rates that vary over a wide range (1–100 K/s). The final microstructure is thus a product of the heating and cooling cycles experienced by the weld in addition to the weld composition. It has been shown that the first phase to form under weld cooling conditions may not be that predicted by equilibrium calculations (Babu et al. 2002a). The partitioning of different interstitial/substitutional alloying elements at high temperatures can dramatically affect the subsequent phase transformations. In order to understand the effect of alloying on phase transformation temperatures and final microstructures time-resolved X-ray diffraction technique has been successfully used for characterization (Babu et al. 2002a, 2005; Stone et al. 2008; Babu 2002). The work by Jacot and Rappaz (1999) on pearlitic steels provided insight into austenitization of hypo-eutectic steels using a finite volume model. However there is very little work done on the effect of heating and cooling rates on the phase transformation paths in bainitic/martensitic steels and weld metals (Thiessen et al. 2007). Previous work on a weld with higher aluminum content, deposited with a FCAW-S process indicated that even at aluminum levels where the primary phase to solidify from liquid should be delta ferrite, non-equilibrium austenite was observed (Babu 2002). The presence of inhomogeneity in composition of the parent microstructure has been attributed to differences in transformation modes, temperatures and microstructures in dual-phase, TRIP steels and ferritic welds (Jimenez-Melero et al. 2009; Wang and Van Der Zwaag 2001; Babu et al. 2002b; Palmer and Elmer 2005).