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Title: Phase stability in steels under electropulsing
Author: Lu, Wenjun
ISNI:       0000 0004 6061 5218
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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There are increasing interests to alternate the microstructure and hence the properties of steels that are applied in various environment conditions using a work-efficient and energy-saving manner. The desirable microstructure evolution is often not achievable by means of conventional thermo-mechanical processing and solid-state phase transition. This thesis has considered four fundamental engineering problems, namely (i) the possibility of anti-aging processing for the aged steels in service at high temperature, (ii) the recovery of the lost strength for the steels at high temperature, (iii) the suspension of crack initiation and propagation during cold-working of steels with eutectoid microstructures and (iv) the regaining of strength during tempering of a steel containing martensite. Phase stability in the processing environment is the primary concern in each of the list problems because it presents, in thermodynamically, the possibility to achieve the goals using the designed processing. Electropulsing processing has been considered and integrated with the conventional thermomechanical processing in the development of this PhD thesis. The so-called electropulsing treatment utilises electric current pulses with high peak current density and short pulse duration. Due to the nature of the short duration pulse, the energy consumption is very low. The high current density enables a very strong impact of electropulsing on the microstructure evolution and hence is work efficient. Following results have been obtained through the study: • Using the appropriate electropulsing parameters, the formed secondary phase (e.g. X-phase) by precipitation in 316L stainless steels at elevated temperature can be dissolved. Electropulsing processing can supress the precipitation and homogenize the alloying elements in the stainless steel. The stability of the secondary phases in the stainless steel has been changed by the imposed electropulse. • Electropulsing treatment is able to alternate the delta-ferrite phase transition. This has been proved in the treatment of 2205 duplex stainless steel. The new format of phase transition causes strengthening of the steel at high temperature. The stability of phases in the steel has been affected by the applied electropulsing treatment. • For the light steels containing high aluminium composition, electropulsing is able to affect the thermodynamic stability and grain morphology of κ-carbide. This leads to significant improvement of steel formability. • Application of electropulsing processing to dual-phase automotive steel changes the stability of martensite phase. The processing improve the mechanical properties and refined the microstructure of this steel. The fundamental understanding of the experimental observations has been developed based on the thermodynamic and kinetic analysis.
Supervisor: Qin, Rongshan ; Lin, Jianguo Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral