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Title: Electropulsing of pearlitic steels
Author: Elliott-Bowman, Bernadette
ISNI:       0000 0004 6061 6763
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Electropulsing is a process characterised by the application of short duration (~100μs) electrical current pulses. The process is claimed to be capable of stimulating microstructure change in a variety of metals at low temperatures and energy levels. In this thesis, electropulsing is applied to examine its potential to produce nanostructured, strong yet ductile pearlitic steel. Electropulsing of cold-rolled 0.92wt%C pearlitic steel plate was conducted at room temperature across a range of variables (J ~ 10⁷ - 10⁹ A·m-2, f ~ 1 - 100Hz, d ~ 80 - 160μs, N ~ 1 - 90000), in order to identify the parameters required to induce microstructure change. A microstructure of equiaxed ferrite and spheroidised cementite was obtained under high frequency, current density and pulse duration electropulsing. Martensitic transformation and Widmanstätten structures were also observed. No microstructure change was produced by electropulsing treatments of low current density, frequency and pulse duration. Elevated temperature electropulsing tests on 0.73wt%C pearlitic rod, which combined pulsed current treatments (J ~ 10⁷ A·m-2) with furnace annealing (660-740°C), were conducted to attempt to overcome any initial energy barrier threshold, in order to highlight any current-based athermal effects. Grain size analysis did not reveal any variation between electropulsed and furnace treated samples. Secondary, transmission and backscatter electron microscopy were used for microstructure characterisation, and mechanical and electrical properties were also experimentally determined. Atom probe tomography was applied to select samples to analyse the steel composition after electropulsing. Resistive heating-based temperature effects were calculated and compared with similarly treated Gleeble-processed samples, to understand the impact of heating rate on the microstructures observed. Electropulsing was determined to have influenced the microstructure of pearlitic steel through resistive heating. An athermal mechanism could not be clearly discerned.
Supervisor: Dye, David ; Qin, Rongshan Sponsor: Defence Science and Technology Laboratory
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral