Abstract: Using the FormastorFⅡphase transformation thermal expansion instrument， the static CCT diagram and critical points Ac1 and Ac3 of 5Ni steels were investigated in order to provide the basis for the process plan of the heat treatment. Microstructures， Vickers microhardness and composition segregation were investigated by means of durometer， optical microscopy(OM)， scanning electron microscope(SEM) and electron probe microanalyzer(EPMA). It was found that with the increase of the cooling rate，microstructures of the specimens transformed from polygonal ferrite and pearlite to bainite and martensite. The CCT diagram revealed that the critical cooling rate need exceed 20 ℃/s to avoid ferrite and bainite transformation in quenching treatment. The composition segregations of Ni were found under different cooling rates that were beneficial to the formation of retained austenite during the tempering，which improved the toughness of 5Ni steels.

Key words: 5Ni steel, cooling rate, microstructure, CCT diagram, composition segregation