UM E-Theses Collection (澳門大學電子學位論文庫)
Title
Effect of heat treatment and laser surface treatment on the corrosion behavior of stainless steels
English Abstract
In the present study, the effect of heat and laser treatment on intergranular corrosion (IGC) and pitting corrosion behavior of various austenitic stainless steels ASSs (UNS S30400, S31603, S32100, S34700, FeCrMn), duplex stainless steels DSSs (S31803 and S32950) and super duplex stainless steel SDSS (S32760) were investigated. Double loop electrochemical potentiokinetic reactivation (DL-EPR) test was performed by using a potentiostat on aged ASSs in 0.5 M H2SO4 + 0.01 M KSCN solution at 25 °C whereas for aged DSSs and SDSS, a more aggressive solution (2 M H2SO4, 0.01 M KSCN and 0.5 M NaCl at 25 °C) was used. SEM examination revealed that intergranular attack occurs at the Cr-depleted grain boundaries for the aged ASSs while attack takes place at the phase boundaries of the intermetallic  phase and metallic phase (2) for the aged DSSs. Cyclic potentiodynamic polarization was also carried out for investigating the pitting corrosion behavior of various aged stainless steels in 3.5 wt% NaCl solution at 25°C. For FeCrMn, increase in the aging time significantly deteriorates the pitting resistance and the specimens even do not passive when the aging time is longer than 50 h. For aged S31803 and S32950, the pitting resistance is also deteriorated as evidenced by the decrease in pitting potential. Similar results are obtained for aged S32760, but higher repassivation ability was observed mainly due to the present of more passivable alloying elements. Laser surface melting (LSM) was done on various stainless steels by a 2.5-kW CW Nd:YAG laser and a 2.3-kW diode laser. Different laser processing parameters were applied to melt the surface of FeCrMn and S32950 for optimizing the LSM conditions for corrosion resistance. The pitting resistance of the aged S32950 after LSM with a higher scanning speed and lower power of the laser is higher than that of the ones fabricated with a lower processing speed or higher power. In addition, the aged ASSs were essentially austenitic with some  (except FeCrMn) but the chromium carbides were completely removed after LSM. For the aged DSS and SDSS after LSM,  became the major phase and the  phase balance was disturbed but the  and 2 phases were eliminated. After LSM, the IGC resistance of the aged ASSs was found to be considerably improved as reflected by the reduction in degree of sensitization (DOS). This could be attributed to a more homogenous microstructure and the redissolution of chromium carbides. The IGC resistance of aged DSSs and SDSS was also significantly enhanced as indicated by the decrease in DOS. This could be attributed to a more homogenous microstructure and to the redissolution of  and 2 phases. Furthermore, the effects of re-aging after LSM were also investigated on FeCrMn and S32950. Both of them showed higher pitting resistance as compared with that of the aged specimens
Issue Date
2011
Author
Chan, Weng Kin
Faculty
Faculty of Science and Technology
Department:
Department of Electromechanical Engineering
Degree
M.Sc.
Subject
Electromechanical Engineering -- Department of Electromechanical Engineering
Heat -- Transmission
Mechanical engineering
Biomedical materials -- Effect of lasers on
Stainless steel -- Effect of lasers on
Supervisor
Kwok Chi Tat
Library URL
b2493669
Files In This Item:
TOC & Abstract
Full-text
Location
1/F Zone C
Supervisor
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