The effect of sigma phase precipitation was revealed on toughness and corrosion resistance of SUS329J4L duplex stainless steel cooled under continuous and inconsistent speed．The prototype vacuum furnace was used to produce the practical heat treatment specimens through adjusting the heat transfer coefficient at interface between steel and nitrogen gas，which was controlled with fan rotating speed．The time constant （τ） derived from the measured temperature profile well corresponded to the calculated one by using the heat transfer coefficient and the physical properties of specimens as well as their characteristic length．The absorbed energy can be sorted according to time constant，i.e．, the same as water quenched one at less than 279s，rapidly decrease between 279~573s，and gradually decrease at more than 573s．These transitions were caused by the increase in cleavage fracture due to sigma phase precipitation. Corrosion in 6% FeCl3 solution at 313K also became sever with the time constant increase，but the surge of corrosion rate happened at larger time constant than that of the absorbed energy measured in impact test．
