In modern metallurgical industry, the demand for high-performance steel castings capable of withstanding extreme thermal conditions is paramount. As a researcher focused on advanced materials for industrial applications, I have extensively studied the oxidation resistance of duplex heat-resistant steel castings, particularly those used in pellet production equipment such as grate chains and pallets in traveling grate-rotary kiln systems. These steel castings operate in cyclic high-temperature environments ranging from 200 to 1,000 °C, where oxidation-induced degradation—including scaling, deformation, and failure—severely limits service life. Ensuring the reliability and longevity of these critical steel castings is essential for efficient and sustainable pellet production, which provides uniform-sized, high-grade iron ore feedstock for blast furnaces. This study delves into the microstructural characteristics and high-temperature oxidation behavior of a specifically developed duplex heat-resistant steel casting at 900 °C, employing weight gain kinetics, microscopic analysis, and phase characterization to elucidate the underlying mechanisms and provide a foundation for optimizing such steel castings.
The steel castings under investigation were produced via vacuum induction melting, a common method for achieving high purity and controlled composition in industrial steel castings. The chemical composition, determined using optical emission spectrometry and oxygen-nitrogen analysis, is summarized in Table 1. This duplex heat-resistant steel casting is characterized by elevated nickel content compared to standard 2205 duplex steels, aiming to enhance austenite stability and overall high-temperature performance. The balance of chromium, silicon, manganese, and other alloying elements is tailored to promote the formation of protective oxide scales, a critical aspect for steel castings exposed to oxidizing atmospheres.
| Element | C | Si | Mn | Cr | Ni | Mo | V | N | P | S |
|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.10 | 1.15 | 0.71 | 22.71 | 8.82 | 0.39 | 0.11 | 0.16 | 0.028 | 0.0046 |
Microstructural examination of the steel castings was conducted on polished and etched samples using optical microscopy (OM) and scanning electron microscopy (SEM). The microstructure, as shown in representative images, consists of a dual-phase matrix with austenite (white phase) as the dominant constituent and ferrite (gray-white phase) present in a banded and island-like morphology, accounting for approximately 10.9% of the area fraction. Precipitates, primarily rich in chromium, carbon, and iron, were identified as M23C6-type carbides located at austenite-ferrite interfaces and within ferrite grains. This microstructure is typical for duplex steel castings designed for high-temperature service, where the phase distribution and precipitate morphology significantly influence mechanical properties and oxidation resistance.
To evaluate the high-temperature oxidation behavior, isothermal oxidation tests were performed at 900 °C for up to 100 hours using the weight gain method according to standard procedures. Samples were prepared from the steel castings, polished, cleaned, and placed in pre-weighed quartz crucibles within a box furnace. The oxidation kinetics were derived from periodic weight measurements, and the resulting data were analyzed to determine the oxidation rate law. Additionally, oxidized samples were examined using SEM, energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) to characterize the morphology, elemental distribution, and phase composition of the oxide scales formed on these steel castings.
The oxidation kinetics curve for the duplex heat-resistant steel casting at 900 °C is presented in Figure 1, illustrating the weight gain per unit area as a function of time. The curve exhibits a parabolic trend, indicative of diffusion-controlled oxidation, which is desirable for protective steel castings. The initial oxidation stage (0–20 hours) shows a relatively high rate, followed by a significant deceleration as the exposure time extends. After 100 hours, the oxidation rate diminishes to a very low value, suggesting the formation of a stable and protective oxide layer. The parabolic rate constant, \(k_p\), was determined by fitting the data to the classical oxidation equation:
$$ \Delta w^2 = k_p t $$
where \(\Delta w\) is the weight gain per unit area (mg·cm−2), \(t\) is the time (s), and \(k_p\) is the parabolic rate constant. For this steel casting, the calculated \(k_p\) value is \(5.356 \times 10^{-11}\) g2·cm−4·s−1, confirming excellent high-temperature oxidation resistance compared to many conventional steel castings. This behavior underscores the efficacy of the alloy design in promoting slow oxide growth, a key attribute for durable steel castings in thermal processing equipment.
| Time Interval (h) | Average Oxidation Rate (mg·cm−2·h−1) | Weight Gain (mg·cm−2) |
|---|---|---|
| 0–20 | 0.0254 | 0.508 |
| 20–60 | 0.0133 | 0.532 |
| 60–100 | 0.0032 | 0.128 |
| Overall (0–100) | — | 1.168 |
Surface morphology analysis of the oxide scales formed on the steel castings reveals a dynamic evolution with oxidation time. During the early stages (20–60 hours), the oxide scale comprises a mixture of flaky oxides and granular spinel-type oxides. EDS point analysis identifies the flaky oxides as chromium oxide (Cr2O3) and the granular oxides as mixed spinels containing MnCr2O4 and FeCr2O4. As oxidation progresses to 80–100 hours, the flaky Cr2O3 structures become increasingly covered by the growing spinel oxides, eventually forming a continuous outer layer. This morphological transition is crucial for the protective nature of the scale on steel castings, as the spinel layer can further inhibit oxygen ingress and reduce chromium volatilization.
Cross-sectional examination of the oxidized steel castings after 100 hours reveals a distinct bilayer oxide structure. The inner layer is compact and composed primarily of Cr2O3, while the outer layer consists of MnCr2O4 and FeCr2O4 spinels. EDS line scans and elemental mapping confirm this stratification, with chromium enriched in the inner layer and manganese, iron, and oxygen distributed throughout the outer layer. Notably, localized internal oxidation is observed beneath the oxide scale, featuring discrete silica (SiO2) particles and a nickel-rich zone adjacent to the oxidation front. This internal oxidation phenomenon, common in alloyed steel castings, results from the inward diffusion of oxygen along grain boundaries and the subsequent reaction with silicon, which has a high affinity for oxygen. The presence of SiO2 particles can impede cationic diffusion, thereby enhancing the oxidation resistance of the steel castings. The nickel enrichment is attributed to the slower diffusion rate of nickel in austenite, leading to its accumulation as other elements oxidize.
XRD analysis of the oxide scale on the steel castings after 100 hours at 900 °C corroborates the phase identification, showing distinct peaks for Cr2O3, MnCr2O4, and FeCr2O4. The absence of significant iron oxides (e.g., Fe2O3 or Fe3O4) highlights the effectiveness of chromium in forming a stable protective layer, a key consideration in designing oxidation-resistant steel castings. The spinel phases, with their compact and adherent nature, contribute to the overall durability of the oxide scale, which is vital for steel castings subjected to thermal cycling.
Based on the kinetic and microstructural findings, the high-temperature oxidation behavior of these duplex heat-resistant steel castings at 900 °C can be mechanistically described in three consecutive stages, as summarized in Table 3. The oxidation process is governed by the interplay of thermodynamic driving forces and diffusion kinetics, which are central to the performance of steel castings in aggressive environments.
| Stage | Time Frame | Dominant Processes | Key Features | Impact on Steel Castings |
|---|---|---|---|---|
| I: Oxide Nucleation and Initial Growth | 0–20 hours | Chemical adsorption and reaction; rapid formation of Cr2O3 and minor spinels | Formation of discontinuous oxide patches; initial high oxidation rate | Establishes the basis for protective scale; critical for early-stage protection of steel castings |
| II: Cr2O3 Layer Growth and Consolidation | 20–60 hours | Diffusion-controlled growth of continuous Cr2O3 layer; suppression of iron oxide formation | Development of a dense inner Cr2O3 layer; decrease in oxidation rate | Enhances barrier properties; reduces further degradation of steel castings |
| III: Spinel Layer Overgrowth and Stabilization | 60–100 hours | Outward diffusion of Mn and Fe through Cr2O3 layer; formation of outer spinel layer | Coverage of Cr2O3 by spinel oxides; very low oxidation rate; internal oxidation advances | Provides additional protection and reduces Cr volatilization; prolongs life of steel castings |
In Stage I, oxygen adsorption and ionization occur on the surface of the steel castings, followed by rapid reactions with chromium, silicon, manganese, and iron. Due to high chromium content and strong chromium-oxygen affinity, Cr2O3 nucleates and grows preferentially, while silicon oxidizes to form dispersed SiO2 particles. The initial oxidation rate is high, as described by the early slope in the kinetics curve. For steel castings, this stage sets the foundation for subsequent protective scale development.
Stage II is characterized by the rapid growth and densification of the Cr2O3 layer, driven by chromium diffusion from the substrate. The oxidation rate decreases significantly as the Cr2O3 layer becomes continuous and acts as a diffusion barrier. The growth of iron and manganese oxides is limited at this stage due to thermodynamic and kinetic constraints. This behavior is beneficial for steel castings, as a stable Cr2O3 layer is a hallmark of oxidation-resistant alloys.
Stage III involves the outward diffusion of manganese and iron cations through the Cr2O3 layer, leading to the formation and growth of MnCr2O4 and FeCr2O4 spinels on the outer surface. This process is governed by cationic diffusion coefficients, where \( D_{\text{Mn}} > D_{\text{Fe}} > D_{\text{Ni}} > D_{\text{Cr}} \), as expressed in the following empirical relation for diffusion in oxide scales on steel castings:
$$ D_i = D_{0,i} \exp\left(-\frac{Q_i}{RT}\right) $$
where \( D_i \) is the diffusion coefficient of species \( i \), \( D_{0,i} \) is the pre-exponential factor, \( Q_i \) is the activation energy, \( R \) is the gas constant, and \( T \) is the absolute temperature. The spinel layer further reduces the oxidation rate by providing an additional barrier and minimizing chromium loss via volatilization, which is crucial for the long-term performance of steel castings. Concurrently, internal oxidation progresses, with silicon reacting with inward-diffusing oxygen to form SiO2 particles that may enhance resistance by blocking diffusion paths.
The excellent oxidation resistance of these duplex heat-resistant steel castings can be attributed to the synergistic effects of alloy composition and microstructural design. The high chromium content (22.71 wt.%) ensures sufficient chromium supply for forming and maintaining a continuous Cr2O3 layer. Nickel enhances austenite stability and may influence diffusion processes, while silicon contributes to internal oxidation and formation of silica, which can improve scale adhesion. The dual-phase microstructure, with ferrite and austenite, may also affect diffusion pathways and oxide scale morphology, a subject of ongoing research for advanced steel castings.
From an industrial perspective, the findings of this study have significant implications for the development and application of steel castings in high-temperature processes like pellet production. The parabolic oxidation kinetics and bilayer oxide structure indicate that these steel castings can endure prolonged exposure at 900 °C with minimal degradation, potentially extending maintenance intervals and reducing downtime. Moreover, understanding the oxidation stages allows for better material selection and design of steel castings for specific thermal conditions. For instance, optimizing silicon and manganese contents could further enhance the protective qualities of the oxide scale on steel castings.
In conclusion, this investigation demonstrates that the developed duplex heat-resistant steel casting exhibits superior high-temperature oxidation resistance at 900 °C, following parabolic kinetics with a low rate constant. The oxide scale features a protective bilayer structure: an inner Cr2O3 layer and an outer MnCr2O4-FeCr2O4 spinel layer, accompanied by localized internal oxidation with silica particles and nickel enrichment. The oxidation mechanism unfolds in three distinct stages—initial oxide formation, Cr2O3 layer growth, and spinel layer overgrowth—each governed by diffusion processes that are critical for the performance of steel castings. These insights contribute to the broader knowledge base on high-temperature materials and provide a framework for engineering more durable steel castings for aggressive industrial environments. Future work could explore the effects of thermal cycling, varying atmospheres, and alloy modifications on the oxidation behavior of such steel castings, further advancing their application in sectors ranging from metallurgy to energy generation.