My journey into the realm of wear-resistant materials spans decades, where I have systematically explored the development and application of various alloys designed to withstand abrasive environments. The systematic study of wear-resistant materials has a history exceeding a century, progressing from high-manganese steels and alloy steels to nickel-hard cast irons, low and high-chromium cast irons, and ductile iron castings. Each of these materials has undergone a fundamental process of research, development, and continuous improvement in production techniques. In recent decades, significant achievements and substantial economic benefits have been realized in the systematic research and application of wear-resistant materials, including strengthened high-manganese steels, various alloy wear-resistant steels, low-chromium and high-chromium cast irons, and notably, ductile iron castings. Grinding balls for ball mills constitute a considerable proportion of wear-resistant materials. Over years of development, their production processes, equipment conditions, and quality control have greatly improved, substantially reducing the consumption and cost of wear parts for end-users.
In my extensive experience, casting grinding balls are immensely utilized wear products in non-ferrous and metallurgical mining, cement building materials, and power generation industries. Currently, the main cast grinding balls employed in mills within my purview include: high- and low-chromium iron grinding balls, ductile iron cast grinding balls, and other alloy cast balls. For historical reasons, the grinding balls used in domestic metallurgical mining, cement, and thermal power industries—especially cement and power—remain predominantly wear-resistant cast iron balls, accounting for over 90% of total demand. As ball mills and grinding ball diameters trend toward largescale development, the demand for grinding balls is becoming increasingly diversified.
Reflecting on the materials, processes, and performance of casting grinding balls, I have categorized them into several types. Low-chromium cast iron grinding balls, utilizing white cast iron with a chromium content typically between 1% to 3%, are generally produced via iron mold casting. Due to their simple production process, low alloy content, and low requirements for worker skills and production conditions, these balls are relatively inexpensive. Since the 1970s, low-chromium cast iron was a widely adopted material for cast grinding balls in my region. However, they retain a certain market share primarily because of their low cost, suitable for small mills with less stringent performance requirements. Their defects are notable: low hardness (HRC ≥ 45), poor toughness (impact toughness ≥ 2 J/cm²), and high wear loss. Statistical data from various concentrators indicate an average wear loss for low-chromium cast iron balls in non-ferrous copper ores of 0.8 kg/t to 1.0 kg/t, and in iron ores of 1.2 kg/t to 1.5 kg/t. They are prone to breakage and out-of-roundness, leading to clogging of mill grate plates and reduced grinding efficiency. Moreover, they contain chromium, a scarce element in my country, and their consumption results in the complete loss of chromium resources. As mill sizes in mining increase, demanding higher anti-breakage capability, lower wear loss, and energy efficiency, the issues with low-chromium cast iron balls become more pronounced, leading to a gradual reduction in their market share.
High-chromium cast iron grinding balls, made from high-chromium white cast iron with chromium mass fraction generally between 10% to 13%, are predominantly produced using iron mold sand-covered processes and undergo oil quenching + tempering heat treatment. The addition of chromium enhances hardenability and increases hardness. High-chromium balls are widely used in cement and metallurgical mining industries, gaining general recognition. Their surface hardness typically reaches above HRC 56, with impact toughness ≥ 3 J/cm². However, their cost is significantly higher due to the substantial chromium content. While high-chromium balls exhibit higher hardness and slightly better toughness than low-chromium ones, two major issues persist: chromium is a precious alloy element, and its loss through wear and tailings disposal is irrecoverable, posing resource sustainability concerns given my country’s reliance on imports. Additionally, their toughness, though improved, still falls short of the requirements for large-diameter balls in big ball mills and semi-autogenous mills regarding toughness and anti-breakage capability, thus limiting their market share.
In contrast, ductile iron castings for grinding balls have emerged as a promising alternative over the past decade. Based on research into global developments, I have focused on developing grinding balls using ductile iron materials, achieving remarkable results. Representative types include Si-Mn alloyed ductile iron grinding balls and austempered ductile iron (ADI) grinding balls. Ductile iron grinding balls can be produced via iron mold sand-covered processes, subjected to oil quenching and tempering, water-based quenching and tempering, or isothermal heat treatment. In ductile iron castings, carbon exists as spheroidal graphite, effectively avoiding stress concentration and providing excellent impact toughness. Through reasonable chemical composition, effective inoculation and spheroidization, and heat treatment processes to regulate the matrix structure, the comprehensive mechanical properties of the ductile iron matrix can be optimized.
The presence of numerous spheroidal graphite particles in ductile iron castings helps mitigate external impact on the matrix and absorbs substantial impact energy. Simultaneously, spheroidal graphite aids in terminating the propagation and extension of microcracks in the matrix structure, offering superior anti-breakage capability. Therefore, high spheroidization rates and grades are crucial for optimal performance. Si-Mn alloyed ductile iron grinding balls utilize silicon and manganese additions to adjust graphite spheroidization and hardenability, enabling high hardness and impact toughness at relatively low cost.
Currently, heat treatment media for ductile iron grinding balls include oil quenching, salt bath quenching, and water-based quenching. Oil and salt bath quenching pose environmental pollution risks, requiring significant investment in environmental protection equipment. Water-based quenching is more environmentally friendly. A comparative overview of various grinding balls used domestically is presented in Table 1.
| Grinding Ball Material | Main Composition (Mass %) | Current Heat Treatment Process | Surface Hardness (HRC) | Impact Toughness (J/cm²) | Cost Comparison | Environmental Equipment Investment |
|---|---|---|---|---|---|---|
| Low-Chromium White Iron | Cr: 1%~3% | Oil Quenching + Tempering | ≥ 45 | ≥ 2 | Low | Low |
| High-Chromium White Iron | Cr: 10%~13% | Oil Quenching + Tempering | ≥ 56 | ≥ 3 | High | High |
| CADI Ductile Iron | Cr: 1.0%~2.0%, Mn: 1.0%~3.0%, Mo: 0.5%~1%, Cu: 0.8%~1.0 | Salt Bath Isothermal Treatment | ≥ 56 | ≥ 10 | Very High | Very High |
| Si-Mn Ductile Iron | Si: 2.0%~3.5%, Mn: 1.0%~3.0% | Water-Based Medium Quenching + Tempering | ≥ 50 | ≥ 10 | Moderate | Low |
In my pursuit of cost-effective grinding ball solutions, I have identified key constraints in traditional austempered ductile iron (ADI or CADI) production for grinding balls. First, the addition of precious alloy elements like Mo and Cu is necessary to ensure hardenability and organizational uniformity for balls larger than 80 mm in diameter, escalating costs with increasing size. Second, salt bath isothermal heat treatment requires massive equipment investment, high energy consumption for salt melting and maintenance, limited production continuity, extended cycles, and environmental and health hazards from nitrate decomposition, necessitating additional环保 equipment. These factors impede widespread adoption.
To overcome these limitations, I explored alternative alloying strategies. Si-Mn alloyed ductile iron castings replace expensive Cu and Mo with affordable silicon and manganese. Manganese, an inexpensive alloying element, expands the austenite region, shifts the C-curve rightward, increases hardenability, and facilitates bainite or martensite formation. However, manganese is a graphitization inhibitor and tends to segregate, promoting carbide precipitation at eutectic cell boundaries, which increases hardness but reduces toughness. Silicon, another low-cost element, strongly promotes graphitization. Adding appropriate silicon content effectively suppresses carbide precipitation and toughness loss induced by manganese. Coupled with strict control of inoculation and spheroidization quality, Si-Mn alloyed ductile iron emerges as a novel grinding ball material. Its typical chemical composition is shown in Table 2.
| Element | C | Si | Mn | P | S | Re, Mg (Residual) |
|---|---|---|---|---|---|---|
| Content (Mass %) | 3.1~3.8 | 2.0~3.2 | 1.4~2.5 | ≤ 0.04 | ≤ 0.03 | ≥ 0.04 |
The heat treatment of ductile iron castings is critical for achieving desired properties. In my work, I emphasize the selection of quenching media. Ideal media should exhibit rapid cooling speed at high temperatures (to avoid pearlite transformation) and slow cooling at low temperatures (to reduce martensitic transformation stress and prevent cracking). Water offers fast cooling at high temperatures but excessive speed at low temperatures risks cracking. Oil provides slow cooling at low temperatures but insufficient speed at high temperatures. Water-based polymer quenchants, developed recently, dissolve infinitely in water and display cooling characteristics akin to water at high temperatures and oil at low temperatures. Data for a typical water-based quenchant are shown in Table 3.
| Performance Metric | Value |
|---|---|
| Maximum Cooling Rate (°C/s) | 146 |
| Temperature at Max Rate (°C) | 689 |
| Cooling Rate at 300°C (°C/s) | 31.88 |
| Time to 600°C (s) | 2.37 |
| Time to 400°C (s) | 5.2 |
| Time to 200°C (s) | 13.01 |
The cooling curve can be approximated by an exponential decay function, where the cooling rate $v(T)$ varies with temperature $T$:
$$ v(T) = A \cdot e^{-B(T – T_m)} + C $$
Here, $A$, $B$, $C$ are constants dependent on the quenchant concentration and temperature, and $T_m$ is the temperature at maximum cooling rate. For water-based quenchants, $A$ is large, ensuring rapid initial cooling, while $C$ moderates the low-temperature phase.
The heat treatment process for Si-Mn alloyed ductile iron castings involves heating, quenching, and tempering or isothermal stages. The heating curve, as I have optimized, includes a slow heating phase to ensure uniform temperature, austenitization at 870°C to 930°C for 1.5 to 3 hours, followed by quenching. The quenching time and temperature are controlled based on the CCT (Continuous Cooling Transformation) diagram. The tempering curve involves holding at 200°C to 260°C for 2 to 4 hours to relieve stresses and enhance toughness.
The microstructure of properly treated Si-Mn alloyed ductile iron castings consists of spheroidal graphite (grade I-II), a matrix of martensite and some bainite, with appropriate amounts of retained austenite and carbides. This uniform structure from surface to core ensures consistent mechanical properties. For instance, the hardness distribution across a φ100 mm grinding ball is shown in Table 4.
| Position | Hardness HRC (Measurement 1) | Hardness HRC (Measurement 2) | Average Hardness HRC |
|---|---|---|---|
| Surface | 54.1 | 53.2 | 53.65 |
| 0.5R | 53 | 52.5 | 52.75 |
| Core | 51.3 | 51.5 | 51.4 |
The overall average hardness is approximately HRC 52.6, with a small gradient, indicating good hardenability. Impact toughness values exceed 10 J/cm², and drop-ball tests show over 20,000 impacts without fracture. These ductile iron castings also exhibit work-hardening; during operation, surface retained austenite transforms to martensite under stress, increasing hardness to about HRC 58.
The relationship between hardness and composition can be modeled using regression equations. For Si-Mn ductile iron castings, hardness $H$ (in HRC) correlates with silicon and manganese content as:
$$ H = \alpha \cdot \%Si + \beta \cdot \%Mn + \gamma \cdot (\%C) + \delta $$
where $\alpha$, $\beta$, $\gamma$, $\delta$ are coefficients determined experimentally. Typically, silicon increases hardness moderately, while manganese enhances hardenability but must be balanced to avoid brittleness.
In my applications, Si-Mn alloyed ductile iron grinding balls have demonstrated excellent performance in large semi-autogenous mills. For example, in an 8.8-meter diameter mill at a gold mine, these ductile iron castings achieved low breakage rates and consistent grinding efficiency. Another trial in a φ10.37 m × 5.49 m semi-autogenous mill at an iron ore plant reported an average hardness of HRC 51.3, breakage rate of 1%, stable throughput of 950 t/h, and normal process indicators.
To facilitate mass production, I have developed automated heat treatment lines tailored for ductile iron castings. These systems include preheating and gas-protected heating furnaces, water-based quenching centers, variable-speed mesh-belt tempering furnaces, and automatic control systems. They enable continuous processing, precise temperature control, and reduced energy consumption compared to traditional methods. The lines can handle various ball sizes and materials, making them versatile for ductile iron castings production.
In conclusion, my research and practical experience affirm that ductile iron castings, particularly Si-Mn alloyed variants, offer a superior balance of hardness, toughness, and cost-effectiveness for grinding balls. By replacing expensive alloys with silicon and manganese, optimizing water-based quenching media, and implementing advanced heat treatment processes, these ductile iron castings meet the demands of modern large-scale mills. Their environmental friendliness, coupled with excellent performance, positions ductile iron castings as a leading choice for future wear-resistant applications. The continuous improvement in production technology and equipment will further enhance the viability of ductile iron castings in global industrial sectors.
Further mathematical modeling of wear loss $W$ (in kg/t) can be expressed as a function of hardness $H$ and impact toughness $K$:
$$ W = \frac{k_1}{H^{n_1}} + \frac{k_2}{K^{n_2}} $$
where $k_1$, $k_2$, $n_1$, $n_2$ are material constants. For ductile iron castings, high $H$ and $K$ yield low $W$, validating their efficiency. Additionally, the economic benefit $B$ per ton of ore processed can be estimated as:
$$ B = C_{\text{saved}} – C_{\text{ball}} \cdot W $$
where $C_{\text{saved}}$ is the cost saved from reduced downtime and improved grinding, and $C_{\text{ball}}$ is the cost per kg of grinding balls. Ductile iron castings often maximize $B$ due to their durable nature.
The future of ductile iron castings lies in further alloy design and process innovation. Research into nano-structured additives or hybrid heat treatments could unlock even greater potential. As industries strive for sustainability and efficiency, ductile iron castings will undoubtedly play a pivotal role in advancing wear-resistant technology.