Microstructure, fluidity, and mechanical properties of semi-solid processed ductile iron

The effect of aluminum (0, 2, 4, and 6 wt. %) and copper (0, 2, 4, and 6 wt. %) on graphite morphology, microstructure and compressive behavior of ductile iron specimens manufactured by sand casting technique were investigated. The graphite morphology and microstructure were evaluated using optical microscopy (OM) and scanning electron microscopy (SEM) equipped image processing software. To study the mechanical properties, the compression test was conducted on the ductile iron specimens. The results indicated that the surface fraction and nodule count of graphite decreased when the amount of aluminum increased from 0 to 2 wt. % and after that from 2 to 6 wt. %. In addition, the nodularity of graphite increased with the increment of the aluminum amounts. By adding the amount of copper, the surface fraction and nodule count of graphite increased and nodularity of graphite decreased. The addition of aluminum and copper decreased the surface fraction of ferrite and increased the surface fraction of pearlite in the microstructure. By increasing the amounts of aluminum and copper, compressive stress vs. strain curves were shifted upwards, and modulus of elasticity, yield strength, maximum compressive stress, and fracture strain improved. In comparison with copper, aluminum had a greater influence on the mechanical properties of ductile iron.

There is continuous demand for local production of quality automobile spare parts in the country nowadays. The current economic condition of the nation has placed more challenges on the indigenous foundry to improve on their technology to make automobile spare components accessible and affordable to the nation. This study examined the possibility of converting the indiscriminately dumped sleeve scraps from state of waste to ductile iron useful products thereby adding value to it. The mechanical properties of ductile iron produced from the sleeve scraps using local technology were determined and found comparable to that of international standard (ISO). Sleeve scraps were charged with graphite and FeSi into a rotary furnace and melted. Nodularization was achieved ising MgFeSi as an indigenous technology of the production process. The hypotheses of the study were tested and compared with the ISO standard of the grade of ductile iron and the findings revealed that Ductile iron that corresponds with the standard was successfully produced from solid waste (automobile sleeves scraps). It was recommended among others that the effort be made towards the conversion of sleeve scraps to ductile iron automobile components, in order to minimize the menace of environmental pollution which leads to numerous health hazards and mortality. Also, making automobile spare parts available to the end users and reduce its importation

In this study, the effect of the cooling rate controlled by different castings thickness ranging from 3 to 25 mm on the mechanical and corrosion behavior of Si–Mo ductile iron (DI) castings was investigated. SEM-EBSD investigations demonstrated that the nodule counts, nodularity, and ferrite phase increased while the ferritic grain size decreased with decreasing in the casting wall thickness; this in turn led to improvements in the tensile properties of the DI. The corrosion behavior of Si–Mo DI castings was investigated after different immersion times in a 3.5 wt pct NaCl solution via electrochemical experiments. The electrochemical results showed that the corrosion resistance of Si–Mo DI decreased in the following order: 3 > 5 > 13 > 25 mm. The results indicated that all of the tested DI samples exhibited galvanic corrosion between the nobler carbide and ferritic matrix phases. SEM analysis showed that the dissolution of the ferrite phase with the pearlite phase exposed and accumulated on the DI’s surface, which led to intense corrosion acceleration. The higher corrosion rates that were observed for those samples with higher casting wall thicknesses could be associated with the higher contents of pearlite/carbide.

Characterization and modeling of the mechanical behavior of high silicon ductile iron

Effect of spheroidization of cementite in ductile cast iron

The main objective of the current work is to investigate the influence of different inoculation conditions on the microstructure and mechanical properties of heavy section ductile iron (DI) castings. Inoculation treatment was done via one step and double step treatments with different amounts of inoculants. The mechanical properties of the fabricated samples were evaluated. The best inoculation procedure in terms of graphite nodules characteristics and mechanical properties was double inoculation with 0.8% inoculants added at first and 0.2% in the late inoculation step. The presence of Sb in one of the cast alloys controlled the growth of graphite nodules in these heavy section ductile iron castings; however low impact toughness was recorded. The matrix structure of ductile cast iron showed a significant influence not only on the strength and impact properties but also on the fracture mode during testing.

The mechanical properties as well as microstructure of the ductile cast iron (DCI) are most likely affected by heat treatments. In this work, the mechanical properties of different heat treated of DCI alloy were investigated. Two heat treatment (HT) processes were conducted for DCI; austempering and quenching followed by lower tempering. The melted alloy of DCI was poured in Y-block, from which the specimens of the mechanical tests were prepared. Experimental tests were carried out to investigate the effect of these HT processes on the mechanical properties. A comparison between mechanical properties due to HT and as cast DCI are presented and discussed. The results showed that there is a difference in microstructure, homogeneity, wear rate and compression of DCI based on the conducted heat treatment.

High silicon and molybdenum (SiMo) ductile iron is commonly used for car exhaust systems, and its micro-structural dispersity depends on intrinsic parameters, which include alloy composition and inoculation efficiency, as well as extrinsic factors, such as casting wall thickness and molding material, which define cooling rate during solidification. Micro-structural dispersity is referred to as the degree of heterogeneity of sizes of structural constituencies within the microstructure. A variation in the micro-structural dispersity could impact the high temperature performance of SiMo ductile iron during static oxidation and transient thermo-mechanical loading conditions. In this study, static high temperature tests were performed on SiMo ductile iron solidified in a casting with varying wall thicknesses from 5 mm to 100 mm. The faster solidified specimens (taken from near chilled casting surfaces) had extremely high micro-structural dispersity as compared to the thicker section samples. After thermal exposure, each of the samples were characterized using 2D sections and 3D µCT images, and the results indicated an order of magnitude difference in graphite phase dispersity. The surface degradation was quantified after static oxidation experiments were implemented at temperature intervals between 650 °C and 800 °C. Non-destructive µCT 3D analysis and SEM/EDS were performed on cross sections and used to quantify the scale topology and structure. Carbon analysis was used to decouple the scale formation and decarburization phenomena that occurred within the samples. These methods enabled the quantification of the oxidation of the SiMo cast iron with different micro-structural dispersity levels after being exposed to high temperature static oxidation. Additionally, the complex material behavior during oxidation-assisted transient thermo-mechanical loading will be presented in a separate article.

This article begins with a description of the classes and grades of ductile iron. It discusses the factors affecting the mechanical properties of ductile iron. The article reviews the hardness properties, tensile properties, shear and torsional properties, compressive properties, fatigue properties, fracture toughness, and physical properties of ductile iron and compares them with other cast irons to aid the designer in materials selection. It concludes with information on austempered ductile iron.

The mechanical properties of ductile iron (DI) and compacted graphite iron (CGI) are measured and reported on standard machined specimens (as per ASTM). However, most castings retain most of the as-cast surface. This surface layer (the casting skin) includes both surface and subsurface features. Because of the casting skin, the mechanical properties of the part are typically significantly lower than those found on standard ASTM machined specimens.

The present investigation was carried out to study the effect of manganese and copper addition, singly as well as in combination, on the microstructure, micro-segregation, and mechanical properties of ductile irons. Alloy A (3.18C, 2.64Si, 0.45Mn), alloy B (3.35C, 2.51Si, 0.82Mn), alloy C (3.16C, 2.80Si, 1.08Mn, 0.56Cu), and alloy D (3.18C, 3.00Si, 1.04Mn, 1.13Cu) were melted and cast in the form of Y-block test pieces. The cast microstructures varied from ferrito-pearlitic in alloys A, B, and C to pearlitic in alloy D. However, on XRD analysis and SEM examination, the presence of martensite patches was also detected. There was a marginal decrease in nodule count in alloy B. In alloys C and D, nodule counts were higher, but the proportion of ferrite decreased drastically. Alloy D was found to be the strongest (UTS ≈ 800 MPa, El = 5%) with alloys A and C coming next in strength; while alloy B was weakest of the four. The presence of martensite patches in association with pearlite appears to be responsible for low toughness of these alloys. Microprobe analysis shows some silicon segregation near the graphite nodules and practically little segregation of manganese. Elemental mapping by FE-SEM does not indicate any manganese segregation.

High-nitrogen ductile iron (DI) was prepared by a new method of injecting nitrogen gas into molten iron and nodularizing treatment. The microstructure and mechanical properties of the as-prepared DI for different nitrogen gas injection periods were characterized. The graphite morphology gradually deteriorated with the increase in the nitrogen gas injection time. The maximum nitrogen and pearlite contents were obtained after 20 min of nitrogen gas injection, and the corresponding tensile strength and elongation of the DI were calculated as 492 MPa and 9.5%, respectively, which were 9.3% and 22% higher than those of the DI prepared without the nitrogen gas injection treatment, respectively.

Spheroidal Graphite Iron (SG Iron) is popularly known as ductile iron or nodular cast iron which is a special case of cast iron having carbon content of more than 3 wt% in volume and graphite is in the form of spherical tiny sized particles. Since the last three decades, the demand for SG Iron has been increasing due to its superior mechanical properties such as high strength and toughness, this nature leads to the usage of SG Iron in numerous industrial applications. From the earlier studies, it has been proved that addition of alloying elements to SG Iron leads to change in properties such as increased tensile strength and hardness. Heat-treatment of alloys is one of the valuable methods to achieve better properties. In the present study, the microstructures and mechanical properties of SG Iron were studied after various heat treatments beyond the limits, and tests were done to measure its mechanical properties like tensile strength, hardness, impact strength. Digital microphotographs, scanning electron microphotographs were analysed before and after the heat treatment. Results indicated great change in mechanical properties after the heat-treatment. From the results it can be concluded that the heat treatment of SG Iron results in changed composition of alloys, which also leads to economical growth of SG Iron.

Nodular cast iron has mechanical properties which make it superior to relatively brittle pig cast iron. As a matter of fact, by using appropriate heat treatment processes, the tensile strength of nodular cast iron can be improved to such a degree that its hardness corresponds to that of carbon steels. The main aims in this study are to find the most preferable heat treatment conditions which will yield high strength levels, and to clarify the temperature dependence of mechanical properties for nodular cast iron. The estimation of tensile strength from hardness is also discussed, since tensile tests at elevated temperatures are usually more expensive and time consuming than the hardness tests. Nodular cast irons, having four different microstructures were first prepared by performing the following heat treatments: (1) as-cast, (2) annealed, (3) normalized and (4) bainitized. Tensile property and hardness were then measured for the respective cast irons under elevated temperatures. The temperature dependence of the tensile strength as well as hardness was investigated. It was found that the dependence was well represented by an expression of σ=σ0exp(−BT). Thus results were discussed from a view-point of the reaction rate process. The correlation between tensile strength and hardness was also examined and a significant linearity was found between them. Based on this strict correlation, an estimation procedure of the tensile strength was finally proposed.

<div class="htmlview paragraph">The objective of this work is to test and develop monotonic tensile properties and strain controlled fatigue properties of a cast ductile iron. The test data and the related material constants will be used in conjunction with vehicle loading data to perform finite element stress-strain analysis and fatigue life prediction analysis to aid in the design of automotive components made from ductile iron. Currently, such material property data does not exist in the literature for this particular grade of ductile iron.</div> <div class="htmlview paragraph">Monotonic tension and fully reversed strain controlled fatigue tests were conducted by following ASTM E-8, ASTM E-606, and SAE J-1099 on samples machined from the cast ductile iron. Monotonic tensile properties were obtained, including Young's modulus, yield strength, ultimate tensile strength, elongation, reduction in area, strength coefficient K, and strain hardening exponent n. Fully reversed strain controlled fatigue properties were also obtained, including cyclic strength coefficient K′, cyclic strain hardening exponent n′, Young's Modulus as a function of fatigue cycles, mean stress as a function of cycles, fatigue strength coefficient, fatigue strength exponent, fatigue ductility coefficient, and fatigue ductility exponent. Four repetitive tests were conducted at each strain level. The ductile iron showed cyclic hardening behavior. Fatigue life scatter was about a factor of two for the five strain amplitude levels tested.</div> <div class="htmlview paragraph">Chemistry, microstructure, hardness, nodularity and nodule count was characterized for the ductile iron. Finally fractography analysis was done using a scanning electron microscope which revealed the fatigue region and transgranular cleavage fracture region on the fracture surface of tested samples. Fracture toughness of the ductile iron was estimated using the stress intensity factor equation of a surface crack in a solid round bar under tension and based on the fatigue crack depth at fast fracture and the maximum stress applied.</div> <div class="htmlview paragraph">In conclusion, monotonic tension and strain controlled fatigue tests were successfully performed. The fatigue performance of the material was characterized, and the material's fatigue properties will be used in durability design and fatigue life prediction.</div>