I. Introduction to Lining Plates for Large Autogenous Mills

Large self-grinding mill shell liners are designed to protect the mill shell, shielding it from direct impacts and friction caused by grinding media and materials. At the same time, different types of liners can be used to optimize the movement patterns of the grinding media, enhancing their ability to crush the material effectively. This, in turn, helps improve the grinding efficiency of the self-grinding mill, boost production output, and reduce metal consumption.

In addition to protecting the mill shell, the liner plates of large autogenous mills also influence the movement patterns of the grinding media. To meet the requirements of various operating conditions—whether for coarse crushing or fine grinding—the shapes and materials of the liners differ accordingly. When crushing is the primary function, the liners need to provide strong lifting action for the grinding media while maintaining excellent impact resistance. Conversely, when fine grinding is the main goal, the liners feature smaller protrusions, exerting weaker lifting force on the media with reduced impact but enhanced grinding efficiency—thus demanding superior wear resistance from the liners.

II. Product Features

1. Massive and Heavy-Duty Design: Large autogenous mills typically feature a cylinder with a diameter exceeding 8 meters—often surpassing 12 meters. Consequently, each lining plate is sizable, measuring several meters in length, and remarkably thick (usually over 100 mm, sometimes even exceeding 200 mm)—resulting in weights that can reach several tons.

2. Withstanding Extreme Impact Loads: The autogenous mill uses large ore pieces themselves as the grinding media. After the ore is lifted to a high position, it freely falls, generating intense impact and chiseling action on the cylinder liner plates.

3. Enduring severe abrasive wear: Ore particles continuously tumble and slide within the cylinder, causing significant abrasive wear on the liner surface (primarily three-body wear).

4. Resistance to Corrosion and Wear: In wet autogenous mills, the chemical corrosion of the slurry intensifies wear. Dry autogenous mills may also experience some degree of oxidative corrosion.

5. Temperature fluctuations in the service environment: Heat generated during the grinding process causes the liner plate temperature to rise, while the temperature drops when the machine is shut down, leading to thermal stress.

6. Complex structural design: The liner plate shape must fulfill the dual function of efficiently lifting the ore (via lifters—either bars or corrugations)—and effectively protecting the cylinder body (through its flat sections). The design of the lifters (including height, angle, and spacing) directly influences the mill's efficiency.

7. High service life required: Replacing the lining plates demands lengthy downtime and incurs significant costs, making it essential for the plates to have the longest possible service life—typically aiming for 6 to 18 months or even longer.

8. Reliability requirements are extremely high: Once the liner plate fractures and fails, it could strike and damage the cylinder body or other components, leading to serious accidents and substantial economic losses. Therefore, the material must exhibit exceptional toughness, fatigue resistance, and manufacturing quality—every aspect is rigorously scrutinized.

II. Challenges in Casting Large Liners

1. Thick and Large Cross-Section Casting:

Slow solidification: The central region solidifies slowly, making it prone to forming coarse grains and segregation.

High susceptibility to shrinkage porosity and shrinkage cavities: A complex and extensive feeding system—featuring multiple large risers, chills, and feeder sleeves—is required to ensure sequential solidification and adequate feeding, thereby preventing internal defects.

Thermal stress is immense: During solidification and cooling, significant temperature differences exist between the inner and outer sections, generating substantial thermal stress that easily leads to thermal cracking.

Organizational non-uniformity: The surface cools rapidly, resulting in a fine and dense microstructure, while the core cools more slowly, leading to a coarse and larger-grained structure. Consequently, properties—especially toughness—are lower at the core compared to the surface.

2. Melting and casting requirements are stringent:

Large-capacity melting furnaces are required (such as arc furnaces, medium-frequency furnaces, etc.).

Strictly control the chemical composition of molten steel, especially elements harmful to toughness (P, S, O, H, N).

Out-of-furnace refining processes (such as the LF furnace) are commonly used for deoxidation, desulfurization, degassing, and precise adjustment of alloy composition.

Pouring temperature control must be precise—too high can easily lead to defects, while too low results in poor fluidity.

Pouring speed is steady, preventing gas entrapment and sand erosion.

3. High difficulty in heat treatment:

When quenching large liners, powerful quenching equipment—such as large water tanks or high-pressure spray quenching systems—is essential to ensure adequate cooling rates, enabling the achievement of the desired hardening depth and hardness. During quenching, significant temperature differences across the cross-section can lead to the叠加 of thermal stress and transformation stress, making cracking highly likely. Therefore, it’s crucial to meticulously control parameters like water inlet temperature, water temperature, cooling methods (e.g., first mist cooling followed by water cooling), and transfer time. Additionally, proper tempering must be carried out to attain the optimal balance between toughness and hardness. Both tempering temperature and duration need to be precisely managed. Moreover, large-scale heat treatment furnaces—such as carriage furnaces or pit furnaces—are indispensable equipment for this process.

III. Commonly Used Materials

The core of selecting liner materials for large autogenous mills lies in maximizing hardness and wear resistance—while ensuring sufficient toughness and fracture resistance (to withstand tremendous impact). Commonly used material systems include:

1. Chromium-molybdenum alloy steel liners (the mainstream choice)

Material Composition

• Basic components: Medium-to-high carbon (0.3%–0.6% C) + chromium (Cr: 1.5%–5%) + molybdenum (Mo: 0.3%–1.5%) + manganese (Mn), silicon (Si), and others.

◦ Typical grades: ZG42CrMo (low-cost general-purpose), ZG30Cr2Mo1 (high hardenability), ZG30CrNiMo (nickel-added for enhanced toughness, suitable for extreme operating conditions)

Core Advantages

1. Hardenability: Molybdenum (Mo) significantly enhances the hardening depth in thick sections (>200 mm), preventing soft spots in the core.

2. Toughness and Balance: Hardness HB 350–500 (surface) + Impact toughness ≥15 J/cm² (-20°C).

3. Anti-fatigue and Shock Resistance: High yield strength (≥650 MPa) resists deformation caused by ore impacts.

4. Heat Stability: Mo/Cr inhibits high-temperature tempering softening, maintaining hardness during service.

Heat Treatment Process

Tempering Treatment (Quenching + Tempering):

◦ Quenching: 880-950℃, water quenching or polymer quenching (to prevent cracking)

◦ Tempering: 400–600°C × 20–50 hours (thick sections require extra-long holding time to relieve internal stresses)

Key Controls:

◦ Quenching cooling rate ≥30°C/s (to prevent ferrite precipitation)

◦ Quenching followed by rapid cooling (to prevent Type II temper embrittlement)

2. Modified High-Manganese Steel (Specialized for High-Impact Areas)

Material upgrade of traditional high-manganese steel (ZGMn13): Defects include insufficient work hardening under low stress (hardness only HB 200) → resulting in poor wear resistance.

Applicable Scenarios

• Feeding-end lifting bars: Designed to withstand impact (ore drop height >5m), with a surface-hardened hardness of HB 500–600.

• Key manufacturing points:

◦ Water quenching treatment: 1100℃ × water quench → Results in a single austenite phase.

◦ Low-temperature tempering is prohibited (to prevent embrittlement caused by carbide precipitation).

3. High-chromium cast iron (for limited scenarios only)

Material Properties

• Ingredients: High-carbon (2.5-3.2%C) + high-chromium (15-28%Cr) + molybdenum/copper/nickel.

• Hardness: HRC 58–65 (ultra-high wear resistance).

• Fatal flaw: Low toughness (impact toughness ≤8 J/cm²), prone to fracture.

4. Material Selection Comparison Table