**1. Introduction**

Intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) are the main corrosion modes of stainless steels when exposed to an aggressive environment. They have been long recognized to be induced by the boundary sensitization, i.e., the existence of a chromium (Cr) depleted zone adjacent to boundaries [1–3]. Even though other underlying mechanisms [4–6] for the formation of a Cr-depleted zone are found, the precipitation of Cr-rich carbide and nitride at boundaries is certainly the major one [7,8]. Provided a sufficient chemical driving force for precipitation, this is conceivable since the interface energy for nucleation is comparatively large at boundaries and the subsequent growth would drain Cr atoms from neighboring areas alongside the boundaries [9]. Thus, from the kinetic perspective, the precipitation of Cr-rich precipitates would be easier in ferritic stainless steel (FSS) [10] or at the ferrite side of the interphase boundary in duplex stainless steel (DSS) [11] due to the lower solubility of C and N and the fast diffusivity of Cr in the ferrite phase.

In contrast to the well-investigated IGC and IGSCC, the loss in ductility and toughness caused by the Cr-rich precipitates at boundaries has drawn much less attention. Shankar et al. [12] attributed the deterioration of ductility in 316LN stainless steel to the Cr-rich precipitates at grain boundaries, their interaction with dislocations, and the associated stress buildup at the grain boundaries. Ghosh [13] found that the fracture mode changed from transgranular to intergranular with increasing formation

of grain boundary precipitates, and the ductility and fracture toughness decrease significantly. Hilders et al. [14] also related the decrease in toughness of 304L stainless steel to the increasing volume fraction of voids nucleated at the grain boundary precipitates formed during sensitization. Kumar Subodh and Shahi [15] revealed the detachment at heavily precipitated grain boundaries in the heat-affected zone of AISI 304L welds after post-weld thermal aging.

Considering the adverse effect on the in-use properties of stainless steels, investigation on the mechanism for the formation of Cr-rich precipitates would be of great importance. The present work focuses on two aspects, i.e., the austenite–ferrite transformation kinetics and the formation of Cr-rich precipitates at prior austenite/ferrite interphase boundaries in type 430 stainless steel. The experimental studies and DICTRA simulation have, for the first time, enabled the establishment of the correlation between these two physical metallurgical behaviors.
