Phenomenological Analysis Of Surface Degradation Of Metallic Materials In Extreme Environment

The resistance to surface degradation in metallic alloys plays an important role for the lifetime of the components working in harsh environments. The mechanisms involved in degradation of metallic surface in a high-temperature aggressive gaseous atmosphere include the following: forming adherent to the surface multiphase oxide layer, partial spallation, and possible vaporization of formed compounds. The governing equation, which describes a parabolic growth of adherent layer, time-dependent vaporization, and cross-linked to instantaneous thickness of adherent layer spallation rate, was suggested and analyzed. The several relationships between the kinetic constants were defined from analysis of the governing equation. Design of routes for experimental procedures to determine the independent kinetic constants was discussed and an integrated simulator was used to calculate the kinetic constants based on experimental results. Two examples of high-temperature oxidation of heat-resistant Cr/Ni austenitic steel were used to illustrate the capability of the suggested method to determine the oxidation, spallation, and vaporization kinetic constants from a single experiment. The suggested methodology could be considered in future for the analysis of different types of surface degradation of solid materials in gaseous, liquid, or solid environments

Engineering Heterogeneous Nucleation During Solidification Of Multiphase Cast Alloys: An Overview

The theory of heterogeneous nucleation was initially developed as a part of condensed matter physics, and later it was used as an important engineering tool to design metallurgical processes. This success has led to wide applications of the theory in metallurgical practice. For example, engineering heterogeneous nucleation in ductile iron has been used to reduce shrinkage defects, suppress cementite formation, and modify the size and shape of microstructural constituencies. This demonstrates how theoretical knowledge could benefit industry practice. This overview aims to summarize the authors\u27 published studies in co-authorship with colleagues and students, which covers different aspects of engineering heterogeneous nucleation in multiphase cast alloys. Several approaches for engineering heterogeneous nucleation using thermodynamic simulation as well as practical methods for improving efficiency of nucleation using the co-precipitation technique and a local transient melt supersaturation are suggested. Automated scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis and high-resolution transmission electron microscopy (TEM) were used to verify the simulation predictions. Practical examples of controlling microporosity shrinkage in cast irons with spheroidal graphite are presented to illustrate the power of engineering heterogenous nucleation

Stochastic Model for High Temperature Oxidation of Cr–Ni Austenitic Steels Assisted by Spallation

Cr–Ni austenitic steels offer significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can experience intensive degradation because of scale spallation, which could be detrimental to the in-service life. To predict the effect of scale spallation on oxidation kinetics, a simulation was implemented using a stochastic model. The model considers topological parameters and intensity of spallation which can occur, while delivering a true oxidation constant. The experimental procedure identified the amount of formed spalled scale and topology of spallation based on the use of element mapping of the surface. This information was used to determine a true kinetic constant for a corresponding spallation intensity in oxidized Cr–Ni austenitic steel. To illustrate the capability of the stochastic model, a parametric analysis was performed. The model verified how the spallation parameters could change the oxidation processes from parabolic growth of an adhered oxide layer without spallation to a mixed linear-parabolic, or with a constant thickness of residual scale at high spallation intensity. The spallation model will be used in a separate article to characterize high temperature surface degradation of several Cr–Ni austenitic steels during harsh oxidation environments

Effect of Cr and Ni Concentrations on Resilience of Cast Nb-Alloyed Heat Resistant Austenitic Steels at Extreme High Temperatures

Austenitic Cr–Ni Alloyed Heat Resistant Steels with Nb Additions Are Used for Intensively Thermo-Mechanically Loaded Cast Components Working in Extreme High Temperature Oxidizing Environment. their Performance during Static Oxidation and Transient Thermo-Mechanical Loading Was Investigated to Recommend an Optimal Cost-Effective Cr/Ni Composition of Nb-Alloyed Austenitic Class Steels. the Static Oxidation and Transient Thermo-Mechanical Behavior of Three Austenitic Steels with Different Cr/Ni Alloying Levels Were Investigated and Compared for Variety of Working Conditions. Static Oxidation Was Performed between 900 °C and 1000 °C in Air for 400 H. the Critical Temperature Which Increases Spallation during Static Oxidation Was Determined for Each of the Steel Alloying Levels. in Addition, Thermal Cycling of a Constrained Specimen Was Done with Varying Upper Cycling Temperatures between 850 °C and 1000 °C. SEM and TEM Analyses Were Supported by Thermodynamic Simulation of the Phases Precipitated in the Metal Matrix and the Structure of Formed Oxide Layers. These Studies Were Used to Determine the Mechanisms of Degradation of Thermo-Mechanically Loaded Cr/Ni Austenitic Steels at Extreme High Temperatures. a Recommendation for a Cost-Effective Cr/Ni Alloying Level for Different Working Conditions Was Determined

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

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

Effect of Spallation on Oxidation Kinetics of Heat-Resistant Cr–ni Austenitic Steels on Air and Combustion Atmosphere

Thin-walled castings made from Cr–Ni austenitic steels offer a combination of light weight of a near-net component with significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can result in intensive surface degradation due to scale spallation. Scale spallation can decrease the wall thickness which could be detrimental to the in-service life of thin-walled castings. Experiments and stochastic simulations of spallation assisted oxidation were performed with three cast austenitic heat-resistant steels having different Cr–Ni concentrations at temperatures between 900 and 1000 ℃ on air and water vapor containing combustion atmosphere. The recorded specimen and spalled scale weight together with SEM and TEM analysis were used to predict the oxidation constant to form adherent layer and spallation intensity. Three oxidation modes, including oxidation controlled by diffusion with forming a strongly adherent to steel surface multi-layered scale, spallation assisted oxidation, and oxidation with additional partial vaporization of scale components in the water vapor environment were distinguished. It was revealed that the Cr and Ni concentrations moved temperature boundaries between these surface degradation mechanisms depending on the exposed oxidation environment. Our approach is aimed to alleviate an appropriate alloy selection for service conditions

ANALYSIS OF HETEROGENEOUS NUCLEATION IN DUCTILE IRON

Abstract A combination of an automated SEM/EDX analysis, a special 2D-3D converter of nodule size distribution, and adaptive thermal analysis were used for the study of heterogeneous nucleation in ductile iron. The special quenching technique was applied to increase probability to reveal non-metallic heterogeneous nuclei in small graphite nodules. Ternary diagrams were developed to compare statistics of oxide and sulfide compositions in graphite nodules and metal matrix. Thermodynamics of heterogeneous nucleation of graphite phase in Mg-treated cast iron is discussed based on the novel experimental data. Experimental and Simulation Methods Shape, size, quantity, and distribution of the graphite phase, developed during heterogeneous solidification in high carbon iron alloys (cast irons), are some of the most important microstructural parameters because the graphite phase influences the physical and mechanical properties of the final castings. In this article, developed approaches, including (i) an automated SEM/EDX analysis of graphite nodule heterogeneous nuclei chemistry 1-2 in quenched specimens and (ii) a special algorithm to convert two-dimensional to three-dimensional graphite nodule size distribution 3-5 , were used in combination with (iii) an adaptive thermal analysis 6 . Heterogeneous nucleation statistic. Heterogeneous nucleation plays an important role in stable graphite eutectic solidification to avoid metastable cementite formation and associated shrinkage defects 7 . A vast variety of FeSi-based inoculants are used in ductile iron industrial practices. The thermo-chemistry of heterogeneous nucleation formation during melt inoculation was described by author

Stochastic Model for High Temperature Oxidation of Cr-Ni Austenitic Steels Assisted by Spallation

Cr-Ni austenitic steels offer significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can experience intensive degradation because of scale spallation, which could be detrimental to the in-service life. To predict the effect of scale spallation on oxidation kinetics, a simulation was implemented using a stochastic model. The model considers topological parameters and intensity of spallation which can occur, while delivering a true oxidation constant. The experimental procedure identified the amount of formed spalled scale and topology of spallation based on the use of element mapping of the surface. This information was used to determine a true kinetic constant for a corresponding spallation intensity in oxidized Cr-Ni austenitic steel. To illustrate the capability of the stochastic model, a parametric analysis was performed. The model verified how the spallation parameters could change the oxidation processes from parabolic growth of an adhered oxide layer without spallation to a mixed linear-parabolic, or with a constant thickness of residual scale at high spallation intensity. The spallation model will be used in a separate article to characterize high temperature surface degradation of several Cr-Ni austenitic steels during harsh oxidation environments

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

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

Effect of Micro-Structural Dispersity of Simo Ductile Iron on Thermal Cycling Performance

High alloyed by silicon and molybdenum (SiMo) ductile iron is a common material 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 the cooling rate during solidification. Micro-structural dispersity refers to sizes of structural constituencies and space distribution within the micro-structure. A variation in the micro-structural dispersity can significantly affect high-temperature performance of SiMo ductile iron during static oxidation and transient thermo-mechanical loading conditions. In the first published part of this study, high-temperature static oxidation tests were performed on SiMo ductile iron solidified in a casting with varying wall thicknesses from 5 to 100 mm. In addition, the faster solidified specimens with extremely high micro-structural dispersity were taken from near the chilled casting surface. It was shown that above the critical temperature diapason, increasing micro-structural dispersity intensified the surface degradation due to intensive decarburization (deC). In this second part of the study, the specimens with different micro-structural dispersity were subjected to constrained thermal cycling by applying different cycle schedules to quantify interactions between thermal fatigue and oxidation. It was shown that the performance of SiMo ductile iron could be improved by optimizing the micro-structural dispersity for different transient thermo-mechanical conditions