Transformation and magnetic behaviour of duplex stainless steel

Abstract

This thesis is on the phase transformation and magnetic behaviour of duplex stainless steels at different temperatures (from 350 to 1300). Prior to thermal ageing, samples were differently pre-treated through plastic deformation and different solution treatments. The possibility of using duplex stainless steels for temperature measurement and temperature fluctuation monitoring has been explored. The microstructures of duplex stainless steels at different temperatures have been characterised by using optical metallography and electron microscopy. Below 550℃, the ferrite phase undergoes spinodal decomposition, which is followed by the precipitation of the G phase. Between 600 and 900, the main transformation ℃℃product is the intermetallic sigma phase. Between 900 and 1000, the domina℃℃nt precipitate phase is carbide. While plastic deformation accelerates the kinetics of precipitation, raising the solution treatment temperature has the opposite effect. The attendant changes in the room-temperature and cryogenic magnetic properties of duplex stainless steels after different thermal ageing treatments have been studied by using a vibrating sample magnetometer and an a.c. magnetic susceptometer. Below 550when the ferrite decomposes spinodally, the room-temperature a.c. magnetic susceptibility might be used for temperature measurement. But between 600 and 900 when the sigma phase forms, the possibility of using duplex stainless steels for temperature measurement seems to be low. The cryogenic magnetic behaviour of the intermetallic sigma phase of the Fe-Cr-Ni-Mo system has been systematically studied. The feasibility of using room-temperature a.c. magnetic susceptibility for ferrite content measurement has been examined. The growth of the spinodal domain in the ferrite phase has been studied by using cryogenic a.c. magnetic susceptibility measurement. And the scaling law so obtained has been compared with those available in the literature. The feasibility of using cryogenic magnetic susceptibility for monitoring spinodal decomposition has been discussed. The transformation behaviour of the sigma phase has been investigated by using differential thermal analysis (DTA). The viability of using DTA and duplex stainless steels for temperature measurement has been explored.

Date of Award	16 Jul 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ki Leuk Joseph LAI (Supervisor)