Improving High-Temperature Performance of High Si-Alloyed Ductile Iron by Altering Additions

Abstract

High alloyed Si ferritic ductile irons can offer potential benefits because they combine high strength, ductility at room temperature, and low oxidation rate at high temperature. However, there is one known drawback and that is these cast irons have limited performance during thermal cycling due to a significant drop of ductility at warm temperatures. This decrease in ductility has been linked to poisoning ferrite grain boundaries by Mg. Therefore, thermodynamic simulations were used to identify altering additions which were able to meditate this negative effect by forming intermetallic phases with Mg. To verify thermodynamic predictions, three alloys were cast including a base and two high Si ductile irons with additions of P and Sb. High-temperature performance of these alloys was experimentally verified including tensile properties at warm temperatures (350-550°C), oxidation in air at temperatures (700-800°C), and thermal cycling between 300 and 800°C. SEM and TEM analyses confirmed that the studied additions reacted with Mg forming different compounds which could prevent poisoning ferrite grain boundaries and improve high-temperature performance of high Si ductile iron.

Cyclic oxidation of high-silicon spheroidal graphite iron

“…4 for the micrographs of the cycled samples, while it is less apparent but still present on the isothermal sample. Such a feature could be seen on a micrograph in a study by Lekakh et al [21] (their Fig. 10) but was not commented by these authors.…”

Control of High-Temperature Static and Transient Thermomechanical Behavior of SiMo Ductile Iron by Al Alloying

“…A twozone horizontal quartz tube furnace, at ambient pressure, was used for these tests with ?/-1 °C temperature control. A special procedure, 8 for decoupling oxidation and deC by measuring weight gain and changes in C, wt.% were conducted. Specimens and crucibles were weighed before and after experiments to determine a total weight change (DG).…”

“…Thermodynamic simulations of oxidation were performed with assumptions that the local equilibrium condition could be reached between the gas components, formed oxide phases, and SiMo cast iron at different distances from surface. 8 This method was used to predict the phases that could be formed at low (inner scale layer) and high (outer scale layer) oxygen levels. Therefore, the local equilibrium conditions were simulated (FactSage 7.3 software with FSsteel, FToxid, and FactSPS databases 29 ) by adding oxygen stepwise into the alloy streams from the previous simulation step assuming an irreversible reaction.…”

“…6 Therefore, the possibilities of extending the usage temperature of the SiMo51 ductile iron were investigated by applying different approaches. These approaches included optimization of Si and Mo concentrations, 7 small altering micro-alloying additions, 8 macro-alloying [9][10][11] as well as surface modification of cast component. 12 It is well known that increasing the Si level can increase the temperature at which ferrite transforms to austenite.…”

Certain investigation on feasibility of developing riser less ductile iron castings

“…Studies were found on investigating various methods of enhancing the properties of ductile iron, including corrosion and wear resistance, high temperature performance, fatigue strength, and reducing internal porosity. Various works have focused on modifying the microstructure through alloying additions, heat treatment, and controlling casting parameters [5][6][7][8][9].…”