As urbanization accelerates, the volume of municipal solid waste has surged dramatically, leading many cities to face the challenge of “waste besieging.” Waste incineration for power generation, as the optimal method for the “reduction, harmlessness, and resource recovery” of生活垃圾, bears the responsibility of ecological environmental protection. Heat-resistant steel castings are core components in incinerators, and their material quality and performance directly impact the reliability and combustion efficiency of the furnace. In my investigation, after a continuous service of seven months in a mechanical grate-type waste incinerator at a power plant, significant damage was observed in the cast steel guard plates of the forward-push section. These guard plates, made of ZG30Cr20Ni10 heat-resistant steel castings, protect the furnace’s steel structure alongside cover plates. The failure of these steel castings not only affects the normal operation of the incinerator and power generation but also poses significant safety risks. If a furnace leak occurs due to corrosion, it could threaten equipment and personal safety, leading to more severe consequences. To identify the root causes of the corrosion failure and prevent recurrence, I conducted an on-site inspection during a maintenance period, collected samples from the damaged guard plates, and performed a series of tests. Based on the findings, I will discuss the corrosion mechanisms and propose improvement measures, emphasizing the critical role of steel castings in such harsh environments.
Waste incineration involves complex chemical and thermal processes, where steel castings are exposed to aggressive conditions. The forward-push section of the incinerator, where the damage occurred, has high humidity and concentrated leachate, leading to pronounced chemical corrosion. In contrast, the reverse-push section, where steel castings remained intact, primarily experiences high-temperature oxidation. This disparity highlights the dual challenges faced by steel castings in waste incinerators: chemical attack from chlorides and other corrosive agents in leachate, and oxidative degradation at elevated temperatures. In this analysis, I will delve into the material properties, microstructural features, and environmental factors contributing to the failure of these steel castings, with a focus on optimizing their design and manufacturing for enhanced durability.
Field Investigation and Service Conditions
During my visit to the power plant, I examined a two-stage mechanical grate incinerator. The upper stage consists of a forward-push reciprocating grate, where waste is dried and ignited. Due to the moisture content in raw waste, this section has relatively high humidity and leachate concentration, making it prone to chemical corrosion. The lower stage is a reverse-push reciprocating grate, where waste is fully combusted through stirring and air exposure. Here, leachate is largely decomposed, so corrosion is minimal. The guard plates and cover plates in the forward-push section showed severe damage, especially on the upper parts, while similar steel castings in the reverse-push section, including grates and feed inlets, remained unaffected. This suggests that the combination of chemical and thermal stresses is critical. The guard plates, as protective steel castings, are essential for structural integrity, and their failure can compromise the entire system. I collected samples from the heavily damaged upper sections for further laboratory analysis, focusing on material composition and microstructure.
Sample Analysis Methodology
To understand the failure mechanisms, I performed comprehensive tests on the retrieved steel castings samples. The analysis included chemical composition assessment, mechanical property testing, and microstructural observation. These steps are crucial for evaluating the quality and performance of steel castings in service environments. I used advanced equipment such as a SPECTRO direct reading spectrometer for chemistry, a SHT4605 tensile tester for mechanical properties, and an OLYMPUS GX71 metallurgical microscope for microstructure. The goal was to correlate material characteristics with corrosion behavior, providing insights for improving steel castings design.
Chemical Composition Analysis
The chemical composition of steel castings determines their phase stability, corrosion resistance, and mechanical strength. For heat-resistant steel castings like ZG30Cr20Ni10, elements such as chromium and nickel enhance oxidation resistance, while carbon content influences carbide formation. I analyzed the guard plate samples, and the results are summarized in Table 1. Compared to the standard specifications, the carbon content exceeded the upper limit, which can lead to excessive carbide precipitation. This deviation is significant because carbon acts as a carbide former, reducing ductility and corrosion resistance in steel castings.
| Element | Measured Value | Standard Range (GB/T 8492-2014) |
|---|---|---|
| C | 0.48% | 0.20–0.40% |
| Si | 1.54% | ≤2.0% |
| Mn | 0.97% | ≤2.0% |
| P | 0.035% | ≤0.04% |
| S | 0.02% | ≤0.04% |
| Cr | 20.24% | 18.0–23.0% |
| Ni | 8.89% | 8.0–12.0% |
The elevated carbon content, at 0.48%, is particularly concerning for steel castings intended for corrosive environments. In high-chromium steel castings, carbon tends to form chromium carbides along grain boundaries, depleting chromium from the matrix and reducing localized corrosion resistance. This effect can be described by the carbide precipitation kinetics, where the rate of carbide formation depends on carbon diffusion and temperature. For instance, the driving force for carbide nucleation can be expressed as:
$$\Delta G = \frac{16\pi\gamma^3}{3(\Delta G_v)^2}$$
where $\Delta G$ is the Gibbs free energy change for nucleation, $\gamma$ is the interfacial energy, and $\Delta G_v$ is the volume free energy change. Higher carbon content increases $\Delta G_v$, promoting carbide formation in steel castings. Additionally, the presence of chromium carbides, such as M23C6, can be predicted using thermodynamic models like the CALPHAD approach, but in practice, excessive carbon leads to brittle networks that weaken steel castings.
Mechanical Properties Testing
Mechanical properties are vital for assessing the integrity of steel castings under stress. I conducted tensile and hardness tests on samples from different locations of the guard plate. The results, shown in Table 2, indicate that the steel castings had high hardness and strength but low ductility, failing to meet the elongation requirement. This brittleness increases susceptibility to stress corrosion cracking, especially in aggressive environments.
| Sample Location | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|---|
| Edge | 869 | 1080 | 10.0 | 321 |
| Center | 861 | 1060 | 11.0 | 314 |
| Standard Requirement | ≥235 | ≥490 | ≥23 | ≤255 |
The high hardness, above 300 HBW, suggests over-strengthening due to carbon and carbide presence. For steel castings, a balance between strength and toughness is essential to withstand thermal cycling and corrosion. The reduced elongation, below 23%, indicates embrittlement, which can be quantified by the ductility loss factor:
$$D_f = \frac{\epsilon_0 – \epsilon}{\epsilon_0} \times 100\%$$
where $\epsilon_0$ is the standard elongation and $\epsilon$ is the measured elongation. In this case, $D_f$ exceeds 50%, highlighting poor toughness. Such mechanical degradation in steel castings accelerates failure under combined chemical and thermal loads.
Microstructural Observation
Microstructure plays a key role in the corrosion behavior of steel castings. After polishing and etching with ferric chloride hydrochloride solution, I examined the samples under a microscope. The steel castings exhibited significant internal porosity, indicating poor densification during casting. This defect acts as a pathway for corrosive agents to penetrate deeper. Moreover, the matrix consisted of austenite with abundant primary carbides along grain boundaries, often in a network pattern. Secondary carbides and precipitates were dispersed within grains and boundaries, as shown in Figure 2. Microcracks were prevalent, especially near the surface, and intergranular corrosion was evident, following the carbide networks.
The microstructure of steel castings can be described in terms of phase fractions. For example, the volume fraction of carbides, $V_c$, can be estimated from carbon content using the lever rule in the Fe-Cr-C system:
$$V_c \approx \frac{C – C_{\gamma}}{C_{carb} – C_{\gamma}}$$
where $C$ is the total carbon content, $C_{\gamma}$ is the carbon solubility in austenite, and $C_{carb}$ is the carbon content in carbides. With high carbon, $V_c$ increases, leading to continuous carbide networks. In these steel castings, the carbides, primarily chromium-rich types, served as preferential sites for corrosion initiation. The intergranular attack can be modeled by corrosion propagation along boundaries, with a rate dependent on chloride ion concentration and temperature:
$$v_{corr} = k_0 \cdot [Cl^-]^n \cdot \exp\left(-\frac{Q}{RT}\right)$$
where $v_{corr}$ is the corrosion rate, $k_0$ is a pre-exponential factor, $[Cl^-]$ is chloride concentration, $n$ is an exponent, $Q$ is activation energy, $R$ is the gas constant, and $T$ is absolute temperature. In waste incinerators, leachate with high $[Cl^-]$ exacerbates corrosion in steel castings, especially at temperatures around 600–900°C.
Discussion on Corrosion Mechanisms
The failure of steel castings in waste incinerators results from synergistic chemical corrosion and high-temperature oxidation. Leachate from waste contains chlorides, organic acids, and other corrosive species that attack the metal surface. At the same time, temperatures up to 1000°C promote oxidation, forming scales that can be disrupted by chlorides. For steel castings, this dual attack is particularly severe due to microstructural inhomogeneities.
Chemical corrosion involves electrochemical reactions at the steel castings surface. In the presence of chlorides, pitting and intergranular corrosion occur. The anodic dissolution of iron and chromium can be represented as:
$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$$
$$\text{Cr} \rightarrow \text{Cr}^{3+} + 3e^-$$
Chloride ions adsorb on the surface, breaking down passive films and accelerating dissolution. In steel castings with high carbon, chromium carbides deplete chromium from adjacent areas, creating galvanic cells. The carbide-matrix interfaces act as cathodic sites, driving corrosion along grain boundaries. This intergranular corrosion reduces cohesion, leading to crack initiation and propagation.
High-temperature oxidation follows parabolic kinetics, where oxide scale growth is diffusion-controlled:
$$\frac{dx}{dt} = \frac{k_p}{x}$$
where $x$ is scale thickness, $t$ is time, and $k_p$ is the parabolic rate constant. For chromium-rich steel castings, $k_p$ is low due to protective Cr2O3 formation. However, chlorides from leachate can volatilize chromium oxides, forming chromium chlorides:
$$\text{Cr}_2\text{O}_3 + 4\text{Cl}_2 + 3\text{O}_2 \rightarrow 2\text{CrCl}_4 + 3\text{O}_2$$
This process disrupts the scale, allowing oxygen ingress and further oxidation. The combination of chemical and oxidative attacks leads to rapid degradation of steel castings.
Internal defects like porosity worsen the situation. Porosity increases the surface area exposed to corrosion and acts as stress concentrators. The presence of microcracks, as observed, accelerates failure by providing easy paths for corrosive media. For steel castings, densification is crucial to minimize such defects. The quality of steel castings directly impacts their lifespan in aggressive environments.
Improvement Strategies for Steel Castings
To enhance the performance of steel castings in waste incinerators, I propose improvements in material composition and manufacturing processes. These strategies aim to reduce corrosion susceptibility and improve microstructural homogeneity.
Optimization of Chemical Composition
Lowering carbon content is paramount for steel castings. By reducing carbon to the standard range (0.20–0.40%), carbide formation can be minimized, preserving chromium in the matrix for corrosion resistance. Additionally, alloying elements like molybdenum, copper, and titanium can be incorporated. Molybdenum improves pitting resistance by stabilizing passive films, copper enhances general corrosion resistance, and titanium forms fine carbides that pin grain boundaries, reducing intergranular attack. Increasing nickel content ensures austenite stability and toughness. The modified composition for steel castings could be, for example, ZG25Cr20Ni12Mo2Cu1Ti, with approximate ranges as shown in Table 3.
| Element | Target Range | Rationale |
|---|---|---|
| C | 0.20–0.30% | Reduce carbides, improve ductility |
| Cr | 20–22% | Maintain oxidation resistance |
| Ni | 12–14% | Enhance austenite stability |
| Mo | 1.5–2.5% | Improve chloride resistance |
| Cu | 0.5–1.5% | Increase general corrosion resistance |
| Ti | 0.1–0.3% | Form fine carbides, refine grains |
| Si, Mn, P, S | As per standards | Control impurities |
The effectiveness of alloying can be assessed using corrosion indices. For instance, the pitting resistance equivalent number (PREN) for steel castings can be calculated as:
$$\text{PREN} = \%\text{Cr} + 3.3 \times \%\text{Mo} + 16 \times \%\text{N}$$
Although nitrogen is not a primary addition here, molybdenum boosts PREN, indicating better performance in chloride environments. For steel castings, a PREN above 40 is desirable for waste incineration conditions.
Advanced Casting and Heat Treatment Processes
Manufacturing improvements are essential for producing high-quality steel castings. I recommend using lost foam casting or investment casting to achieve better dimensional accuracy and surface finish. These methods reduce gas entrapment and shrinkage porosity, enhancing densification. Additionally, post-casting treatments like homogenization and solution annealing can dissolve carbides and improve microstructural uniformity. For example, a heat treatment at 1150°C for 2 hours followed by rapid cooling can homogenize the alloy and minimize carbide networks in steel castings.
The solidification of steel castings should be controlled to avoid segregation. Using Chvorinov’s rule, the solidification time $t$ can be estimated as:
$$t = B \left( \frac{V}{A} \right)^2$$
where $B$ is a mold constant, $V$ is volume, and $A$ is surface area. By optimizing gating and riser design, $V/A$ can be adjusted to promote directional solidification, reducing internal defects in steel castings. Furthermore, non-destructive testing methods, such as ultrasonic inspection, should be employed to ensure the integrity of steel castings before service.
To mitigate high-temperature oxidation, surface coatings or aluminizing treatments can be applied to steel castings. These form protective alumina scales that resist chloride attack. However, the base material must first have adequate toughness and corrosion resistance, which is achieved through compositional and process optimizations.
Conclusion
In summary, the corrosion failure of heat-resistant steel castings in waste incinerators is a multifaceted issue driven by chemical corrosion from leachate and high-temperature oxidation. The analyzed guard plates exhibited high carbon content, leading to excessive carbide formation and intergranular corrosion, along with internal porosity that accelerated degradation. Through comprehensive testing and analysis, I have identified key factors: carbon control, alloying additions, and improved casting processes are critical for enhancing the durability of steel castings.
By lowering carbon and adding elements like molybdenum, copper, and titanium, steel castings can achieve better corrosion resistance and mechanical properties. Advanced manufacturing techniques, such as lost foam casting and proper heat treatment, will reduce defects and ensure microstructural homogeneity. These improvements will extend the service life of steel castings in harsh incineration environments, contributing to safer and more efficient waste-to-energy systems. Future work should focus on long-term performance validation and the development of predictive models for corrosion in steel castings under combined thermal and chemical loads. Ultimately, the reliability of waste incinerators hinges on the quality of their steel castings, making continuous material innovation essential.