In my research, I focused on developing high-performance grinding rolls for medium-speed coal mills, specifically targeting the replacement of imported nickel-hard cast iron rolls with a more cost-effective and durable alternative based on high-chromium white cast iron. This white cast iron material is recognized as a third-generation wear-resistant alloy, offering superior hardness and toughness compared to earlier materials. The motivation stemmed from the widespread use of medium-speed coal mills in power plants, where grinding rolls are critical wear parts subjected to harsh conditions. By leveraging domestic resources and optimizing composition and processing, I aimed to create a white cast iron solution that enhances service life and reduces operational costs.
The working environment of grinding rolls in medium-speed coal mills involves a combination of abrasive wear, cyclic compressive stresses, and occasional impact loads. Coal particles, including hard impurities like pyrite and shale, continuously erode the roll surface through a “plowing and cutting” mechanism, while repeated deformation leads to fatigue spalling. To withstand these conditions, the white cast iron material must possess high hardness to resist abrasion, adequate toughness to prevent fracture, sufficient hardenability to achieve a martensitic matrix upon air cooling, and sound casting integrity to minimize defects. My analysis confirmed that high-chromium white cast iron, with its tailored microstructure, is ideal for this application.
Designing the chemical composition was a cornerstone of this project. I aimed to balance wear resistance and toughness by carefully selecting elements. Carbon content influences carbide volume; higher carbon increases hardness but reduces toughness. I targeted a range of 2.8–3.2% C to optimize this balance. Chromium is crucial for forming hard carbides and improving hardenability. A chromium-to-carbon ratio (Cr/C) of around 20:1 was chosen to promote the formation of M7C3 carbides rather than M3C, enhancing wear resistance. Molybdenum was added at 0.5–1.0% to boost hardenability, especially for thick-section castings, while manganese (0.5–1.0%) and silicon (0.5–1.0%) were controlled to avoid adverse effects on hardenability. Additionally, trace elements like titanium, vanadium, and boron were incorporated to refine grains and modify carbide distribution. The finalized composition range is summarized in Table 1.
| Element | Range | Role in White Cast Iron |
|---|---|---|
| C | 2.8–3.2 | Forms carbides for hardness; excess reduces toughness |
| Cr | 18–22 | Promotes M7C3 carbides; improves hardenability and corrosion resistance |
| Mo | 0.5–1.0 | Enhances hardenability; prevents pearlite formation |
| Mn | 0.5–1.0 | Increases hardenability; lowers Ms temperature |
| Si | 0.5–1.0 | Elevates Ms temperature; used with Mo and Cu |
| Ti, V, B | Trace | Refine grains; improve carbide morphology |
The casting process was meticulously designed to ensure dense and defect-free white cast iron rolls. Given the large size and thick walls of the rolls, I employed metal mold patterns with a shrinkage allowance of 2.0%. To handle the high fluidity and contraction characteristics of white cast iron, I used resin-coated sand for molds and woodchip-based sand for cores, incorporating coke particles to enhance permeability. A key innovation was adopting a three-riser gating system instead of a single riser, coupled with strategically placed chill rings around the mold surface. This approach ensured adequate feeding and reduced micro-shrinkage, as described by the solidification model: $$ V_{riser} \geq \frac{V_{casting} \cdot \beta}{\eta} $$ where \( V_{riser} \) is riser volume, \( V_{casting} \) is casting volume, \( \beta \) is shrinkage coefficient (approximately 4-6% for white cast iron), and \( \eta \) is feeding efficiency. The improved feeding led to a denser microstructure, boosting wear resistance by over 30% compared to single-riser designs.
Melting and pouring were critical to maintaining the integrity of the white cast iron. I used an oxidation-reduction melting process, with temperatures controlled between 1550°C and 1600°C to ensure homogeneous composition. The tapping temperature was around 1500°C, and pouring temperature was strictly maintained at 1420–1450°C to prevent sand burning and shrinkage defects. A bottom-pour ladle was employed for smooth filling: starting with a slow stream, accelerating during mid-pour, and slowing again when the mold was nearly full, followed by riser topping with insulating covers to promote directional solidification. This minimized gas entrapment and segregation in the white cast iron rolls.
Riser removal posed a challenge due to the hardness of high-chromium white cast iron. I developed a novel method using carbon arc air gouging to cut the risers, controlling speed and arc height to avoid thermal cracking. After cutting, the risers were broken off and ground with suspended wheels. Non-destructive testing confirmed no cracks or defects at the riser roots, ensuring the safety of the white cast iron components in service.
Heat treatment was optimized to achieve the desired microstructure and hardness in the white cast iron. Through laboratory experiments, I investigated the effects of quenching and tempering temperatures on hardness, as shown in Figure 1 (described textually). The hardness \( H \) can be modeled as a function of temperature \( T \): $$ H(T) = H_0 + k \cdot e^{-\frac{E_a}{RT}} $$ where \( H_0 \) is base hardness, \( k \) is a constant, \( E_a \) is activation energy, and \( R \) is the gas constant. For quenching, hardness increased with temperature up to 980°C, beyond which secondary carbide dissolution led to retained austenite and reduced hardness. Tempering below 400°C had minimal effect, but at 500°C, secondary hardening occurred due to precipitation of fine carbides. Based on this, I established the following heat treatment cycle for the white cast iron rolls: austenitize at 950–980°C for 4–6 hours (depending on wall thickness), air cool to room temperature, then temper at 480–520°C for 4–6 hours. This yielded a consistent hardness range of 62–65 HRC, with a microstructure comprising martensite, M7C3 carbides, and less than 10% retained austenite.
The developed high-chromium white cast iron rolls exhibited excellent surface quality and dimensional accuracy, as shown in Figure 2 (described textually). Non-destructive inspection revealed no cracks, porosity, or shrinkage defects. Metallographic analysis confirmed the targeted microstructure, with evenly distributed carbides in a martensitic matrix, which is typical for high-performance white cast iron. Hardness measurements across multiple rolls showed uniformity within the specified range, meeting the operational requirements.
Field trials were conducted in power plants to compare the performance of our white cast iron rolls against imported nickel-hard cast iron rolls. The wear behavior was evaluated after 6000 hours of operation. For the high-chromium white cast iron rolls, wear surfaces were smooth and even, indicating uniform material loss. In contrast, nickel-hard cast iron rolls exhibited wavy patterns, suggesting uneven wear and lower toughness. Wear depths were measured at specific locations (labeled A through E on the roll circumference), and wear rates were calculated using: $$ \text{Wear rate} = \frac{\Delta d}{t} $$ where \( \Delta d \) is wear depth (mm) and \( t \) is time (hours). Results are summarized in Table 2.
| Roll Material | Max Wear Depth (mm) | Average Wear Rate (mm/1000 h) | Relative Wear Resistance Improvement |
|---|---|---|---|
| High-Chromium White Cast Iron | 8.5 | 1.42 | Base (reference) |
| Nickel-Hard Cast Iron (Ni-Hard 4) | 15.2 | 2.53 | 44% lower than white cast iron |
The white cast iron rolls demonstrated a wear rate of approximately 1.42 mm/1000 hours, compared to 2.53 mm/1000 hours for nickel-hard cast iron. This translates to a wear life improvement of over 44% for the white cast iron material. Assuming an allowable wear depth of 100 mm, the estimated service life of the high-chromium white cast iron rolls exceeds 70,000 hours, significantly higher than that of nickel-hard counterparts. This enhancement is attributed to the superior hardness and optimized carbide structure of the white cast iron, which better resists abrasive and fatigue mechanisms.
In conclusion, my research successfully developed high-chromium white cast iron grinding rolls for medium-speed coal mills. The three-riser casting process ensured dense and sound castings, while the optimized heat treatment achieved a hard martensitic matrix with dispersed carbides. Field trials confirmed that this white cast iron material outperforms nickel-hard cast iron by over 40% in wear resistance, offering substantial economic benefits through extended service life and reduced reliance on imported alloys. Future work could explore further alloy modifications or surface treatments to enhance the performance of white cast iron in even more demanding applications. The versatility and durability of white cast iron make it a cornerstone material for industrial wear parts, and this study underscores its potential in energy sector applications.