1. Introduction to Large Autogenous Mill Shell Liners

Large autogenous mill shell liners are used to protect the shell, preventing direct impact and abrasion from grinding media and materials. They also adjust the movement state of the grinding media through different liner forms to enhance the grinding media's crushing effect on materials, helping to improve the grinding efficiency of the autogenous mill, increase output, and reduce metal consumption.

Besides protecting the shell, large autogenous mill shell liners influence the movement pattern of the grinding media. To meet different working conditions (crushing or fine grinding), the shape and material of the liners vary. For crushing, liners require strong lifting ability for the grinding media and good impact resistance; for fine grinding, liners have smaller protrusions, weaker lifting effect, less impact, stronger grinding action, and require good wear resistance.

2. Product Features

1. Huge size and heavy weight: The diameter of large autogenous mill shells is usually over 8 meters, sometimes exceeding 12 meters. Therefore, liner pieces are large (lengths can reach several meters), thick (usually over 100mm, even exceeding 200mm), and can weigh several tons.

2. Withstand huge impact loads: The autogenous mill uses large ore blocks as grinding media, which are lifted high and then fall freely, causing strong impact and chipping on the shell liners.

3. Withstand severe abrasive wear: Ore particles continuously roll and slide inside the shell, causing serious abrasive wear on the liner surface (mainly three-body wear).

4. Withstand corrosive wear: In wet autogenous mills, chemical corrosion from slurry accelerates wear. Dry autogenous mills may also experience some oxidation corrosion.

5. Service environment temperature changes: Heat generated during grinding raises liner temperature, which drops when stopped, causing thermal stress.

6. Complex structural design: The liner shape must fulfill dual functions of lifting ore (lifting bars/waves) and effectively protecting the shell (flat parts). The design of lifting bars (height, angle, spacing) directly affects mill efficiency.

7. High service life requirements: Replacing liners requires long downtime and high costs, so liners are required to have as long a service life as possible (usually targeted at 6-18 months or longer).

8. Extremely high reliability requirements: If a liner breaks or fails, it may damage the shell or other parts, causing serious accidents and huge economic losses. Therefore, toughness, fatigue resistance, and manufacturing quality of materials are strictly required.

2. Difficulties in Casting Large Liners

1. Thick and large section casting:

Slow solidification: The center solidifies slowly, prone to coarse grains and segregation.

High tendency for shrinkage porosity: Requires complex and large riser systems (multiple large risers, chillers, feeders) to ensure sequential solidification and sufficient feeding to prevent internal defects.

Huge thermal stress: During solidification and cooling, large temperature differences between inner and outer sections generate huge thermal stresses, easily causing hot cracks.

Uneven microstructure: Surface cools quickly with fine microstructure; core cools slowly with coarse microstructure, resulting in lower performance (especially toughness) than the surface.

2. High requirements for melting and pouring:

Requires large-capacity melting furnaces (arc furnace, induction furnace, etc.).

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

Commonly uses ladle refining (such as LF furnace) for deoxidation, desulfurization, degassing, and composition adjustment.

Precise control of pouring temperature (too high causes defects, too low reduces fluidity).

Smooth pouring speed to avoid air entrapment and sand inclusion.

3. Difficulties in heat treatment:

Large liners require powerful quenching equipment (such as large water tanks, strong spray quenching systems) to ensure sufficient cooling rate, achieving the required hardened layer depth and hardness. Large temperature differences during quenching cause combined thermal and structural stresses, easily leading to cracking. Strict control of water temperature, cooling method (e.g., mist cooling followed by water cooling), and transfer time is necessary. Tempering must be thorough to achieve the required toughness and hardness balance. Tempering temperature and time must be precisely controlled. Large heat treatment furnaces (car-type, pit-type) are essential.

3. Common Materials

The core of material selection for large autogenous mill liners is to ensure sufficient toughness and fracture resistance (to withstand huge impacts) while maximizing hardness and wear resistance. Common material systems include:

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

Material composition

• Basic components: medium-high carbon (0.3%-0.6%C) + chromium (Cr: 1.5%-5%) + molybdenum (Mo: 0.3%-1.5%) + manganese (Mn), silicon (Si), etc.

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

Core advantages

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

2. Strength-toughness balance: Hardness HB 350-500 (surface) + impact toughness ≥15 J/cm² (-20°C).

3. Fatigue and impact resistance: High yield strength (≥650 MPa) resists deformation from ore impacts.

4. Heat stability: Mo/Cr suppress high-temperature temper softening, maintaining hardness during service.

Heat treatment process

Quenching and tempering treatment (quenching + tempering):

◦ Quenching: 880-950℃ water quenching/polymer quenching (crack prevention)

◦ Tempering: 400-600℃ × 20-50 hours (extra long holding time for thick parts to relieve stress)

Key controls:

◦ Quenching cooling rate ≥30℃/s (to avoid ferrite precipitation)

◦ Rapid cooling after tempering (to prevent secondary temper brittleness)

2. Modified high manganese steel (special for high impact areas)

Material upgrade from traditional high manganese steel (ZGMn13) defect: insufficient work hardening under low stress (hardness only HB 200) → poor wear resistance.

Applicable scenarios

• Feed end lifting bars: withstand impact (ore drop >5m), surface hardened hardness HB 500-600.

• Process key points:

◦ Water toughening treatment: 1100℃ × water quenching → obtain single austenite.

◦ Low temperature tempering prohibited (to avoid carbide precipitation embrittlement).

3. High chromium cast iron (used in limited scenarios)

Material characteristics

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

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

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

4. Material selection comparison table