Effect of Different Heat Treatment Procedures on Mechanical Properties and Wear Behavior of Ductile Cast Iron

Abstract

The Ductile Cast Iron (also known as Nodular or Spheroidal Graphite Iron) was first manufactured in 1948. It came into being as a results of scientist’s effort to develop a particular type of cast iron which can be preferred to malleable cast iron. While malleable cast iron can be formed by a lengthy annealing process (known as malleabilization) of white cast iron, the ductile cast iron can be manufactured simply by adding some nodularizing agents like Mg or Ce (or both) into the liquid melt of gray cast iron. Due to the absence of any heat-treatment schedule of long duration, the production cost of ductile or spheroidal graphite (S.G) iron is much less than that of malleable iron. So the former is preferred to the latter even if the both are equal to each other as far as industrial use is concerned. The use of ductile (or nodular or S.G.) iron has been increasing day by day after its discovery in 1948. It is widely used as the material for windmill. Now it has completely replaced galvanized iron as the material for the water-supply pipes. Now research works are being conducted to examine whether this material can be used for the container for nuclear fuel waste. It has been found that S.G. Iron can be used as the material for the casks used for the transportation of the nuclear fuel wastes. As a result of all these developments extensive works are now being performed for the property development in the ductile cast iron. This can be done mainly by addition of alloying elements or by various heat treatment techniques. In this work the effect of different heat-treatment techniques on the microstructure and mechanical properties of ductile iron has been studied. A special importance has been given on the use of ductile cast iron as a material for the container of nuclear fuel wastes. 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%, were subjected to different types of heat treatment processes such as annealing, normalizing, quenching & tempering (Q&T), austempering and inter-critical austenitization (ICA) to develop dual matrix structure (DMS). The first objective is to study the microstructural features of as-cast and heat-treated samples. This was done by using an optical microscope (OM), transmission electron microscope, and X-ray diffraction (XRD) technique. From the results, it has been identified that each grade of DCI has graphite spheroids embedded in ferritic matrix 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 addition increases the ferrite volume percentage, while the concentration of Mg, Cu, Si, and Ce are found to raise nodules' quantity and nodularity. With a graphite nodule incorporated in each matrix, normalizing, Q&T, and austempering heat treatments obtained pearlitic, ferritic, tempered martensitic, and coarse upper bainitic matrix, respectively. On the other hand, the as-cast ferrite matrix is 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 increases the number of nodules in the matrix by restricting the transfer of carbon from the austenite to nearby graphite nodules. The second objective is to study different mechanical properties like hardness, uniaxial tension, compressive, and impact properties of differently heat treated specimens. The results show that greater hardness readings are 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 are obtained for annealed 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 has been identified that strength decrease and ductility increase with the increase in the strain rate and vice-versa. The higher strength and lower elongation are obtained for Q&T and austempered specimens, the lower strength and higher elongation are obtained for the annealed sample, and optimum strength and elongation were obtained for the ICA sample. The tensile testing exhibits 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 samples since it resists the tensile deformation by hard phases. The compression properties followed a similar trend like tensile properties. The bulk texture of deformed compressed samples shows that the high intensity ζ--fiber is formed due to increased plastic deformation of the annealed specimen. However, because of the less plastic deformation, less intensity of ζ-fiber combined with the formation of ϒ -fiber has been observed in the austempered and ICA samples. From the impact test results, it has been found 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 x temperatures, the impact strength of the ICA sample is near that of annealed specimens and good at sub-zero temperatures. The last objective is the study of wear behavior. 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 confirms 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. The 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 confirm 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.