In this study, the effects of austempering on the microstructure, mechanical properties (micro hardness, impact energy and tensile strength) and corrosion behaviours of AISI 1018 low carbon steel were evaluated. The steel specimens were subjected to heat treatment by austenitizing at 830°C, maintained in this condition for 90 min before rapid cooling in a NaNO3 salt bath sustained at 300°C for isothermal transformation for additional 50 min and finally allowed to air cool to room temperature. The as-received and the austempered samples were tested for corrosion in both 0.5M aqueous acidic (HCl) and 0. 5M alkaline (NaOH) media. Microstructural investigation using scanning electron microscope (SEM) reveals transformation from ferrite/pearlite phases to bainite over the austempering process. Interestingly, significant improvements of 15.7% to 95.7% in the various mechanical properties (micro hardness, impact energy and tensile strength) and corrosion resistance in both media were observed.
This study is focused on the corrosive effects of varied salt solutions concentration on two mild steel specimens (API 5L X42 and API 5L X60). Gravimetric method was adopted in the evaluation of the weight loss and corrosion rate of the specimens. The hardness of the test specimens after dipping in corrosive solutions was examined. Potentiodynamic polarisation technique was also used to compare the most corrosive medium. API 5L X42 and API 5L X60 specimens exhibited highest corrosion rates of 9.593 mmpy and 9.698 mmpy respectively in solution A that contained 2.0 M NaCl, 3.0 M CaCl2 and 0.5 M Na2CO3. They also exhibited the lowest corrosion rates of 7.642 mmpy and 7.836 mmpy respectively, in solution H that contained 0.5 M NaCl, 3.0 M CaCl2 and 2.5 M Na2CO3. The API 5L X42 mild steel specimen in solution H exhibited the highest hardness value of 369 HV after immersion in the corrosive medium. The hardness of the corroded specimens decreased when compared to their hardness before immersion in the corrosive media, which indicates that the varied salt solutions impaired the surface morphology of the specimens’ thereby promoting indentation.
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