Nuclear power is one of the main power sources for the entire world. Nuclear energy is always safe and environmentally friendly in comparison with thermal power. It produces electricity utilizing environmentally safe green energy. Nuclear fuels must be disposed of safely after production. Nuclear-spent fuel casks are used for the environmentally friendly and safe transportation and disposal of spent nuclear fuel. Managing nuclear fuel waste is undoubtedly a difficult issue. Prior to getting disposed of, these fuels are cooled in the reactor pool for minimum a year using robust containers called spent fuel casks. These casks are designed with steels in the sandwich manner in the olden days by placing the lead in between them, due to which the cost and time for the production of fuel casks increase. Ductile cast iron (DCI) was introduced due to its simplicity in casting and capacity to meet cask criteria. Researchers opted the DCI to store used fuel since it reduces the production cost and time.
In the current work, DCI or spheroidal graphite (SG) cast iron samples of four different grades (namely, SG1, SG2, SG3, and SG4) having carbon equivalent varies from 4.12% - 4.36%, underwent different types of heat treatment processes such as annealing, normalizing, quenching & tempering (Q&T), austempering and inter-critical austenitization (ICA) or dual matrix treatment (DMS). In the first objective, the microstructural features of as-cast and heat-treated samples were studied using an optical microscope (OM), transmission electron microscope, and X-ray diffraction (XRD) technique. From the results, it was identified that each grade of DCI has the ferritic matrix incorporated graphite spheroids in its as-received state. The nodules count in the graphite spheroids varies from 20-34 nodules/unit area, and they are Type-I (completely spheroids) nodules with a nodularity greater than 93%. Si incorporation increased the ferrite volume percentage, while the concentration of Mg, Cu, Si, and Ce were discovered to raise nodules' quantity and nodularity. With a graphite nodule incorporated in each matrix, normalizing, Q&T, and austempering heat treatments obtained pearlitic or ferritic, tempered martensitic, and coarse upper bainitic matrix, respectively. On the other hand, the as-cast ferrite matrix was transformed into a ferrite + martensite matrix after undergoing ICA heat treatment followed by quenching. Except for annealing, the greater cooling rate during the consequent quenching stages for all heat treatments increased the number of nodules in the matrix by restricting the transfer of carbon from the austenite to nearby graphite nodules.
In the second objective, the DCI heat-treated samples were subjected to investigate the mechanical characteristics such as hardness, uniaxial tension, compression, and impact properties using Vicker&rsquos microhardness tester, universal testing machine (UTM), and impact testing machine. The results show that greater hardness readings were obtained for Q&T, followed by austempering and normalizing for all grades of DCI specimens due to hard phases like martensite, bainite, and pearlite in their microstructures. The lower hardness values were obtained for as-cast followed by annealing samples due to the existence of soft phases like ferrite for all grades of DCI samples. On the other hand, the optimum hardness values were obtained for all grades of ICA samples due to the existence of dual phases (i.e., soft ferrite and hard martensite) in its microstructures. The uniaxial tension tests were carried out at different strain rates. From the tensile data, it was identified that strength decrease and ductility increase with the increase in the strain rate and vice-versa. The higher strength and lower elongation were obtained for Q&T and austempered specimens, the lower strength and higher elongation were obtained for the annealed sample, and optimum strength and elongation were obtained for the ICA sample. The tensile testing showed a substantial increase in texture intensity for both annealing and ICA specimens due to significant plastic deformation. On the other hand, the increase in texture intensity is less in the austempered and Q&T since it resists the tensile deformation by hard phases. The compression properties followed a similar trend of tensile properties. The bulk texture of deformed compressed samples showed that the high intensity &zetafiber is formed due to increased plastic deformation of the annealed specimen. However, because of the less plastic deformation, less intensity of &zeta-fiber combined with the formation of &upsih -fiber was observed in the austempered and ICA samples. From the impact test findings, it was identified that though, the annealed specimens possess the highest impact energy value at higher temperatures, their impact strength falls rapidly below 0oC. Moreover, even at higher temperatures, the impact strength of the ICA sample is near that of annealed specimens and good at sub-zero temperatures.
In the final objective, the heat-treated DCI samples were tested under a ball-on-plate type tribometer to study the wear behavior. The Taguchi optimization technique (L16) was initially applied to evaluate the influence of different process variables (load, time, heat treatment, and grade) during the ball-on-plate wear test. Meanwhile, the analysis of variance (ANOVA) method was adopted to know the significance of aforesaid process variables. ANOVA results confirmed that the heat-treatment process has the highest significance (54.76%) within all process variables. Among heat-treated specimens, austempered samples have outstanding wear resistance, while the ICA samples have lower wear resistance. In addition, overall utility values have been evaluated using individual utility values of weight loss and hardness. An obtained overall utility value gives the optimum combination for achieving higher wear resistance and hardness. Additionally, the morphology of wear surfaces was examined in a scanning electron microscope, and the micrographs confirmed the existence of inferior surfaces in terms of abrasive wear, adhesive wear, particle pullout, and delaminated sheets on the wear track. Enrichment of oxygen element has been observed on the worn path through energy-dispersive spectroscopy. XRD analysis confirms the existence of different compounds like iron and silicon oxides on the wear track surface which may improve its hardness.