In contrast to previous suggestions of eutectoid σ phase formation in hyperduplex stainless steel (HDSS), our recent thermodynamic modeling of the multicomponent Fe-Cr-Mo-Mn-Ni [1] prove the σ phase formation to be monovariant. Further investigation by kinetic simulation [2] with Matcalc [3] using Mean Field approach allow us to find out that the formation of σ phase in hyperduplex stainless steel is most likely associated to the segregation of elements at grain boundaries. In order to further understand the mechanism of the early stage precipitation of σ phase in HDSS, the time-temperature-precipitation behavior of σ phase was analyzed by in-situ XRD was. It was found that the precipitation of σ phase occurs after only few seconds of aging and there is a very fast growth of the phase at the expense of ferrite. This was confirmed by short term annealing experiments using dilatometry, and subsequent microanalysis by FESEM-EDX. The role of moving boundary for the fast phase evolution kinetics in HDSS was tested with Dictra. It is seen that the mobility of elements plays a major role by enriching the grain boundaries of ferrite/austenite. Due to the accumulation of favorable elements for σ phase, its nucleation and growth along the boundaries is favored. Moreover, ferrite, as pool for σ phase stabilizing elements, feeds the σ phase evolution until its complete dissolution. Simulation shows appearance of σ phase within a couple of seconds. Its formation is already complete after 5 min at the nose temperature of T=900°C. [1] A. Jacob, E. Povoden-Karadeniz, Calphad 71 (2020) 101810. [2] A. Jacob, E. Povoden-Karadeniz, Calphad 71 (2020) 101998. [1] Kozeschnik, Ernst. (2022) "Mean-Field Microstructure Kinetics Modeling". In: Francisca G. Caballero (ed.), Encyclopedia of Materials: Metals and Alloys. vol. 4, pp. 521–526. Oxford: Elsevier.