In the cement industry, the use of alloyed white cast iron for grinding balls has become widespread due to its superior wear resistance compared to traditional materials. However, a significant challenge persists: the high fracture rate of these grinding balls during operation. Fracture not only reduces efficiency but also poses safety risks and increases maintenance costs. Based on my investigation into the failure mechanisms of grinding balls, I identified that fracture in chromium-molybdenum-copper white cast iron grinding balls is primarily attributed to casting defects such as shrinkage cavities, porosity, and the continuous network of carbides that embrittle the matrix. To address this, I proposed and implemented a shift from sand casting to metal mold casting, combined with an optimized heat treatment process. This approach aimed to eliminate defects, modify carbide morphology, and enhance the matrix structure, thereby reducing fracture rates and improving wear resistance. The results demonstrated a substantial decrease in fracture incidence and wear loss, leading to notable economic and social benefits. This article details the experimental methodology, results, and discussions underpinning these improvements, with a focus on the behavior of white cast iron under different processing conditions.
The white cast iron studied in this work has a specific chemical composition designed for grinding ball applications. The alloy was melted in a medium-frequency induction furnace using raw materials including pig iron, ferrochromium, scrap steel, copper, ferromolybdenum, and ferromanganese. The chemical composition is critical for achieving the desired microstructure and properties in white cast iron. Table 1 summarizes the nominal composition.
| Element | C | Si | Mn | Cr | Mo | Cu | P | S |
|---|---|---|---|---|---|---|---|---|
| Content | 2.8 – 3.2 | 0.5 – 0.8 | 0.6 – 1.0 | 1.8 – 2.2 | 0.8 – 1.2 | 0.8 – 1.2 | < 0.1 | < 0.05 |
To understand the solidification characteristics, I conducted experiments comparing sand casting and metal mold casting for grinding balls with a diameter of 100 mm. Temperature-time curves were recorded using thermocouples placed at the center, mid-radius, and near the surface of the ball mold. The setup involved connecting thermocouples to an automatic balancing recorder, and pouring temperature was measured with a portable thermometer. Simultaneously, thermal analysis cups were poured from the same melt to determine the liquidus and solidus temperatures of the white cast iron. The cooling curves and derived temperature fields provided insights into the solidification behavior. For sand casting, the temperature gradient was relatively flat throughout solidification, indicating a volumetric solidification mode. In contrast, metal mold casting exhibited a steeper temperature gradient, leading to an intermediate solidification mode. This difference significantly affects the formation of defects in white cast iron.
The solidification process can be modeled using heat transfer equations. The temperature distribution in a casting can be described by the heat conduction equation:
$$
\frac{\partial T}{\partial t} = \alpha \nabla^2 T
$$
where \( T \) is temperature, \( t \) is time, and \( \alpha \) is thermal diffusivity. For white cast iron, the latent heat release during solidification complicates this, but the general principle shows that faster cooling in metal molds reduces the time available for defect formation. From the experimental curves, I plotted dynamic solidification curves, showing that in sand casting, the dendritic austenite network reaches the center quickly but remains in a mushy state for an extended period (approximately 20 minutes), promoting shrinkage and porosity. In metal mold casting, the solid shell forms rapidly (within about 5 minutes), and complete solidification occurs in about 15 minutes, minimizing defects. The macrostructures of the cast balls revealed a distinct difference: sand-cast white cast iron balls showed coarse grains and potential defect sites, while metal-mold-cast balls exhibited finer grains and a more homogeneous structure.
Heat treatment is essential for optimizing the microstructure and mechanical properties of white cast iron. In the as-cast state, the matrix is primarily pearlitic, and carbides form a continuous network, which embrittles the material. I subjected impact and bend test specimens to various austenitizing temperatures: 900°C, 950°C, 1000°C, 1050°C, and 1100°C, each held for 3 hours followed by air quenching. The properties evaluated included macrohardness (HRC), microhardness of the matrix and carbides, impact toughness, bend strength, and deflection. The results are summarized in Table 2, illustrating the effect of heat treatment on white cast iron.
| Heat Treatment Condition | Impact Toughness (J/cm²) | Macrohardness (HRC) | Matrix Microhardness (HV) | Carbide Microhardness (HV) | Bend Strength (MPa) | Deflection (mm) | Relative Toughness Index |
|---|---|---|---|---|---|---|---|
| As-cast | 3.5 | 48 | 450 | 1200 | 850 | 1.2 | 4.2 |
| 900°C, 3h, air quench | 4.8 | 56 | 580 | 1350 | 920 | 1.5 | 5.3 |
| 950°C, 3h, air quench | 5.6 | 62 | 650 | 1400 | 980 | 1.8 | 6.1 |
| 1000°C, 3h, air quench | 6.2 | 65 | 700 | 1450 | 1050 | 2.1 | 6.8 |
| 1050°C, 3h, air quench | 5.9 | 63 | 680 | 1420 | 1020 | 2.3 | 6.5 |
| 1100°C, 3h, air quench | 5.0 | 58 | 600 | 1380 | 950 | 2.5 | 5.5 |
The data indicates that heat treatment significantly enhances the properties of white cast iron. The optimal austenitizing temperature appears to be around 1000°C, where peak values for hardness, strength, and toughness are observed. This improvement is due to microstructural changes: at 950-1000°C, secondary carbides precipitate in a dispersed manner within the matrix, and the continuous eutectic carbide network becomes discontinuous. The matrix transforms from pearlite to martensite, increasing hardness and strength. The relationship between heat treatment temperature and properties can be expressed using empirical formulas. For instance, the macrohardness \( H \) as a function of austenitizing temperature \( T_a \) (in °C) might follow:
$$
H = H_0 + k \cdot (T_a – T_0)^2
$$
where \( H_0 \) is the as-cast hardness, and \( k \) and \( T_0 \) are constants derived from experimental data. Similarly, the impact toughness \( K \) shows a peak, indicating a trade-off between hardness and toughness in white cast iron.
Microstructural analysis revealed that after heat treatment at 1000°C, the white cast iron exhibited a martensitic matrix with finely dispersed secondary carbides and broken eutectic carbides. This structure enhances both wear resistance and fracture toughness. In contrast, as-cast white cast iron had a pearlitic matrix with a continuous carbide network, which acts as stress concentrators and crack initiation sites. The microhardness measurements corroborate this: carbides in heat-treated samples showed higher hardness due to alloy element redistribution, contributing to better wear performance. To quantify the effect on fracture resistance, I calculated a relative toughness index, defined as the product of impact toughness and deflection normalized by hardness. This index highlights the balance between strength and ductility in white cast iron.
Wear tests were conducted using a wet sand rubber wheel abrasion tester on specimens from the same melts. Samples were tested under loads of 50 N, 100 N, and 150 N, with a wheel speed of 240 rpm. Both as-cast and heat-treated (1000°C, 3h, air quench) white cast iron specimens were evaluated. The wear loss was measured after 1000 and 2000 revolutions. Table 3 presents the wear test results, showing the influence of load and heat treatment on the abrasion resistance of white cast iron.
| Sample Condition | Load (N) | Initial Weight (g) | Weight after 1000 rev (g) | Weight after 2000 rev (g) | Total Wear Loss (g) |
|---|---|---|---|---|---|
| As-cast | 50 | 50.12 | 49.95 | 49.78 | 0.34 |
| 100 | 50.08 | 49.75 | 49.42 | 0.66 | |
| 150 | 50.15 | 49.50 | 48.85 | 1.30 | |
| Heat-treated | 50 | 50.10 | 49.90 | 49.70 | 0.40 |
| 100 | 50.05 | 49.80 | 49.55 | 0.50 | |
| 150 | 50.12 | 49.70 | 49.28 | 0.84 |
The wear behavior of white cast iron is complex and depends on the microstructure. Under low loads (50 N), the as-cast white cast iron showed slightly lower wear loss than the heat-treated version, likely due to its harder surface layer and continuous carbides resisting abrasive penetration. However, at higher loads (150 N), the heat-treated white cast iron outperformed, with significantly lower wear loss. This is attributed to the martensitic matrix and dispersed carbides, which provide better support against abrasive particles and reduce micro-cutting. The wear rate \( W \) can be modeled as a function of load \( L \) and hardness \( H \):
$$
W = k \cdot L^n \cdot H^{-m}
$$
where \( k \), \( n \), and \( m \) are material constants. For white cast iron, heat treatment increases \( H \) and modifies \( n \) and \( m \), leading to improved wear resistance under service conditions. The scanning electron microscopy images of worn surfaces confirmed that heat-treated white cast iron had a more uniform wear morphology with fewer deep grooves, indicating enhanced durability.
Service trials were conducted in cement plants to validate the laboratory findings. Initially, sand-cast white cast iron grinding balls without heat treatment were used, resulting in a wear loss of approximately 800 g/ton and a fracture rate of about 10%. After switching to metal mold casting with insulated risers and heat treatment at 1000°C for 3 hours followed by air quenching, the wear loss dropped to around 500 g/ton, and the fracture rate reduced to about 2%. Additionally, the grinding balls showed minimal uneven wear, and the process yield increased by 15-20%. Over three years, this translated to cumulative economic benefits exceeding 400,000 USD, demonstrating the effectiveness of the proposed工艺 for white cast iron grinding balls.
In conclusion, the fracture resistance of chromium-molybdenum-copper white cast iron grinding balls can be significantly improved by altering the casting method and applying optimized heat treatment. Metal mold casting changes the solidification mode from volumetric to intermediate, reducing defects like shrinkage and porosity in white cast iron. Heat treatment at 1000°C transforms the matrix to martensite, disperses secondary carbides, and breaks the continuous carbide network, enhancing both toughness and wear resistance. The combined approach lowers fracture rates from ~10% to ~2% and reduces wear loss, yielding substantial economic gains. This study underscores the importance of microstructure control in white cast iron for demanding applications like grinding media. Future work could explore further alloy modifications or advanced processing techniques to push the limits of white cast iron performance.