Effect of Micro-Structural Dispersity of SiMo Ductile Iron on High Temperature Performance during Static Oxidation

Abstract

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.

Engineering Heterogeneous Nucleation during Solidification of Multiphase Cast Alloys: An Overview

“…The control of heterogeneous nucleation in combination with alloying by Si, Al and Mo were used to improve the high-temperature performance of SGI [18][19][20][21][22][23][24]. Additional melt treatment was used for the development of a dual graphite structure with controlled zones from the flake and spherical graphite shapes in one casting [25,26].…”

“…However, the following instabilities of freely growing interfaces produce morphological shape evolution. The factors that lead to shape instability include: (i) surface energy of a curved interface, (ii) atom attachment kinetics, (iii) trapping solute elements, (iv) mass transport of alloying elements, and finally, (v) possible The control of heterogeneous nucleation in combination with alloying by Si, Al and Mo were used to improve the high-temperature performance of SGI [18][19][20][21][22][23][24]. Additional melt treatment was used for the development of a dual graphite structure with controlled zones from the flake and spherical graphite shapes in one casting [25,26].…”

High Temperature Oxidation Behavior of High Al-Si Alloyed Vermicular Graphite Cast Iron for Internal Combustion Engine’s Hot-End Exhaust Components

“…In addition, silicon enhances high temperature oxidation resistance by forming a silicon-rich oxide layer. The high-temperature oxidation behavior of SiMo nodular cast iron has been studied in dry air, wet air (air + 10% H 2 O), and exhaust gas environments at elevated temperatures (650 to 1050 • C) by many researchers [14][15][16][17][18][19][20][21]. These studies showed that when SiMo cast iron was exposed to high temperatures in the presence of air and exhaust gases (synthetic diesel and gasoline), it formed a multilayered oxide scale.…”

“…The formation of the silica film was much more favorable at 800 • C than at 700 • C [26]. However, Lekakh et al [19,27] showed that the protective and continuous amorphous silica layer obtained in the SiMo cast iron was limited at 700 • C since the oxygen permeability of the amorphous silica layer increased above 750 • C and promoted a decarburization reaction near the interface. The high pressure of CO gas from decarburization caused deformation of the metal matrix, which led to delamination in the brittle silica oxide layer.…”

“…Since graphite flakes were almost interconnected to each other, they served as the primary core and pathway for oxidation. Furthermore, Lekakh et al [19,27] reported that metal matrix oxidation and decarburization interact with each other. Decarburization impacted the oxide morphology and oxidation rate, and at the same time, the presence of oxides could slow down the decarburization rate for SiMo ductile iron.…”