Development Of Spheroidal Graphite Cast Iron For Nuclear Fuel Transport Cask

Abstract

In the present work SG iron specimens with carbon equivalent (%CE) ranging from 4.12 - 4.36, has been subjected to annealing, normalizing, quench & tempering, austempering and DMS treatment to obtain different matrix microstructure and microconstituent. Optical microscopy is observed for microstructure, phase volume fraction, nodularity, and nodule count for each of the as-cast and heat treat specimens. XRD analysis is done to validate the phases present in each specimen as well as to determine the amount of carbon dissolution in respective phases. Mechanical properties such as tensile strength, % elongation, Vicker’s hardness, and impact strength are measured by conducting necessary test following ASTM standards. Failure mechanisms involved in static and dynamic loading are investigated observing the fracture surfaces after tensile and impact test respectively, under Scanning Electron Microscope. The corrosion behavior of as-cast and heat treated specimens, (in sea water) is also studied. Specimens are immersed completely in sea water at room temperature and pressure, and the weight difference is recorded at regular intervals, for twelve weeks. The mechanical properties showed a quite good relationship with microconstituents for respective as-cast and heat treated specimens. The as-cast specimens show graphite spheroids within ferritic matrix resulted in increased ductility and impact strength (with increasing ferrite volume fraction) caused by increasing Si content. Annealing treatment led to the presence of completely ferritic matrix for every alloy consequently increasing ductility and impact toughness as compared to as-cast matrices. On the other hand specimens which underwent quenching & tempering treatment show the highest strength and hardness due to the tempered martensitic matrix, among all other heat treatment processes. Strength and ductility values of normalized austempered and DMS-treated specimens are intermediate to those of lowest strength value for annealed specimen and higher elongation value and that of highest strength and lowest elongation value for the quenched & tempered specimens. It is observed that the elongation increases with increased nodularity can be attributed to increases amount of Mg and Ce, whereas nodule count increases the hardness of respective as-cast and heat treated specimens. The increase of hardness may be due to increase of Ni and Cr content which provides strength to ferrite via solid solution strengthening (for ferritic specimens). Normalizing treatment produced a ferritic/pearlitic matrix and showed increased UTS and hardness with increased pearlite content. Austempered heat treatment resulted in coarse upper bainitic matrix leading to suitable combination of strength and ductility, whereas matrix consists of martensite and ferrite are obtained by DMS treatment. The failure mode for the soft ferritic matrix is observed to be ductile in nature caused by microvoid coalescence, and that of other matrices are mostly brittle signified by the presence of low energy stress paths (River marking) and cleavage facets. Mechanical properties of SG iron alloys studied in current research, found to be well above the recommended properties for fabrication of nuclear fuel cask, (in as-cast as well as heat treated conditions) and hence can successfully be used for the desired purpose.