The effect of ageing on the magnetic properties of 316L stainless steel

Abstract

This thesis is composed of two major parts: the transformation characteristics of AISI 316L stainless steel under isothermal ageing conditions and the effect of ageing on the magnetic properties of the alloy. Commercial grade AISI 3 16L stainless steel specimens have been aged at temperatures in the range 500°C - 900°C for various periods from 24 hours up to 10000 hours. The time-temperature-precipitation (TTP) diagram of the alloy has been constructed. The types of precipitation are well defined by the ageing temperature. The transformation products below 650°C are M23C6 and laves-phase. Between 700°C to 800°C, the transformation sequence is formation of M23C6 and chi-phase, followed by the precipitation of laves-phase, and finally the formation of sigma-phase. Above 800°C, sigma precipitates predominantly after the formation of M23C6. The magnetic properties of the alloy were studied by using an a. c. susceptometer. Unaged alloy exhibits a paramagnetic-to-antiferromagnetic transition at 40K. It was observed that the transition temperature of bulk alloy varied systematically with ageing time and temperatures. The phenomenon was suggested to be related to the precipitation of various intermetallic phases. The intermetallic chi-phase was extracted electrochemically from the bulk alloy previously aged at 800°C for 500 hours. Magnetic susceptibility of the extracted residues was measured using various fields and driving frequencies. Magnetic transition was observed at around 1 lOK. Transition temperature of chi-phase was found to be field and frequency sensitive. A survey of literature shows that the magnetic properties of the chi-phase has never been studied. This work represents the first ever research effort on the magnetic properties of this intermetallic phase.

Date of Award	15 Oct 1999
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ki Leuk Joseph LAI (Supervisor)