Frequent maintenance and replacement of Semi-Autogenous Grinding (SAG) mill lining plates significantly impact operational efficiency and cost in mineral processing plants. While various materials like high-manganese steel and high-chromium cast iron are employed, medium-carbon chromium-molybdenum (Cr-Mo) steel offers a promising balance of wear resistance and toughness, particularly under low-stress conditions. Achieving optimal performance in these critical lining plate components hinges crucially on precise heat treatment. This study systematically investigates the influence of quenching and tempering temperatures on the microstructure and resultant mechanical properties of large Cr-Mo steel SAG mill cylinder lining plates, aiming to identify the optimal heat treatment regimen for extended service life.
The lining plate material, a medium-carbon Cr-Mo steel with the composition detailed in Table 1, was prepared using medium-frequency induction melting and lost foam casting. To simulate the thick sections typical of large lining plates, test blocks (200 mm x 200 mm x 200 mm) were cast, and samples were extracted from their center (Fig. 1). The heat treatment protocol involved varying quenching and tempering temperatures:
Quenching: Blocks were heated in a box-type resistance furnace to target temperatures (900 °C, 920 °C, 940 °C) at a rate of ~230 °C/h, held for 2 hours for austenitization, and then quenched in a 1:7 water-based polymer solution.
Tempering: Quenched blocks were reheated to target temperatures (480 °C, 520 °C, 560 °C, 600 °C) at ~280 °C/h, held for 3 hours, and air-cooled.
Mechanical properties were evaluated using hardness (HRS-150 Rockwell hardness tester, HRC scale, average of 5 readings) and Charpy V-notch impact toughness (JB-300B impact tester, unnotified 10x10x55 mm specimens, average of 3 tests). Microstructural analysis was performed on polished and etched (4% Nital) samples using optical microscopy and field-emission scanning electron microscopy (FE-SEM, UItraPlus), including examination of impact fracture surfaces.
Table 1: Chemical Composition of the Cr-Mo Steel Lining Plate Material (wt.%)
| Element | C | Si | Mn | P | S | Cr | Mo | Ni | Cu | Ti | V | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.65 | 0.50 | 0.80 | 0.03 | 0.03 | 2.00 | 0.40 | 0.35 | 0.15 | 0.05 | 0.05 | Bal. |
The microstructure after quenching exhibited a characteristic martensitic morphology. Quenching at 900 °C resulted in a mixture of fine and coarse lath martensite with visible prior austenite grain boundaries. Increasing the quenching temperature to 920 °C refined the martensitic structure significantly, yielding uniformly distributed, fine lath martensite. This refinement is attributed to the more complete dissolution of alloy carbides at the higher temperature, increasing the driving force for martensite nucleation and suppressing prior austenite grain growth initially. However, further increasing the quench temperature to 940 °C led to coarsening of both the prior austenite grains and the martensite laths. Elevated temperature reduces the pinning effect of dissolved carbides, allowing grains to grow, expressed by the grain growth kinetics relationship:
$$D^n – D_0^n = K t \exp\left(-\frac{Q}{RT}\right)$$
where $D$ is the final grain size, $D_0$ is the initial size, $n$ is the growth exponent, $K$ is a constant, $t$ is time, $Q$ is the activation energy for grain growth, $R$ is the gas constant, and $T$ is the absolute temperature.
This microstructural evolution directly influenced the as-quenched mechanical properties (Table 2). The hardness and impact toughness both peaked at the 920 °C quench temperature. The superior hardness (65.5 HRC) at 920 °C stems from the finer martensite laths, creating a higher density of boundaries acting as barriers to dislocation movement. The slight toughness improvement (5.3 J/cm² vs 4.8 J/cm² at 900 °C) is also linked to finer microstructure, offering a more tortuous path for crack propagation. Coarsening at 940 °C degraded both properties.
Table 2: Effect of Quenching Temperature on Mechanical Properties of Lining Plate Steel
| Quenching Temp (°C) | Hardness (HRC) | Impact Toughness (J/cm²) | Dominant Microstructure |
|---|---|---|---|
| 900 | 62.3 | 4.8 | Mixed Fine/Coarse Lath Martensite |
| 920 | 65.5 | 5.3 | Fine, Uniform Lath Martensite |
| 940 | 61.0 | 4.2 | Coarse Lath Martensite |
Subsequent tempering of the 920 °C quenched samples induced significant microstructural changes. Tempering between 480 °C and 600 °C transformed the martensite into tempered microstructures. Optical microscopy revealed a dark-etching matrix typical of tempered martensite/sorbite. SEM analysis confirmed the decomposition of martensite: fine carbide particles precipitated within the ferrite matrix. As tempering temperature increased, these carbides coalesced and coarsened (Ostwald ripening), and the ferrite matrix underwent recovery and recrystallization, evolving towards a classical tempered sorbite structure consisting of fine, spheroidized carbides in a ferrite matrix at higher temperatures (560 °C, 600 °C). The coarsening kinetics can be described by the Lifshitz-Slyozov-Wagner theory:
$$\bar{r}^3 – \bar{r_0}^3 = \frac{8\gamma DC_e V_m}{9RT}t$$
where $\bar{r}$ is the average particle radius at time $t$, $\bar{r_0}$ is the initial average radius, $\gamma$ is the interfacial energy, $D$ is the diffusion coefficient, $C_e$ is the equilibrium solute concentration, $V_m$ is the molar volume, $R$ is the gas constant, and $T$ is the absolute temperature.
The tempering temperature profoundly impacted the final mechanical properties of the lining plate material (Table 3). Hardness decreased monotonically with increasing tempering temperature due to martensite decomposition, carbide coarsening, and ferrite softening. Conversely, impact toughness exhibited a distinct peak. At 480 °C, toughness was relatively low (14.8 J/cm²) despite high hardness (52 HRC). Toughness dramatically increased to a maximum of 45.2 J/cm² at 560 °C, coinciding with a hardness of 45 HRC. Further tempering to 600 °C caused a decrease in toughness (34.5 J/cm²) while hardness continued to drop.
Table 3: Effect of Tempering Temperature (after 920°C Quench) on Mechanical Properties of Lining Plate Steel
| Tempering Temp (°C) | Hardness (HRC) | Impact Toughness (J/cm²) | Dominant Microstructure |
|---|---|---|---|
| 480 | 52.0 | 14.8 | Tempered Martensite (Early Stage) |
| 520 | 48.5 | 32.7 | Tempered Martensite / Sorbite |
| 560 | 45.0 | 45.2 | Tempered Sorbite (Optimal) |
| 600 | 38.0 | 34.5 | Coarsened Tempered Sorbite |
Fractography provided insights into the toughness variations. The impact fracture surface of the 480 °C tempered lining plate sample revealed a predominantly quasi-cleavage morphology. Features included fine, dense tear ridges, river patterns, and shallow dimples. Crucially, numerous second-phase particles were visible within the dimples and on the fracture surface. These particles act as stress concentrators, facilitating void nucleation and reducing the energy required for crack propagation, thereby limiting toughness. In contrast, the fracture surface of the 560 °C tempered sample exhibited a significantly higher density of deep, equiaxed dimples, characteristic of microvoid coalescence, a ductile fracture mode. Notably, second-phase particles within dimples were scarce. The elimination of these brittle particles and the finer, more homogeneous carbide distribution achieved at 560 °C tempering are key factors enabling the superior toughness. The relationship between dimple size ($d$), particle spacing ($\lambda$), and fracture toughness ($K_{IC}$) can be approximated by:
$$K_{IC} \propto \sigma_y \sqrt{\pi d} \propto \sigma_y \sqrt{\pi \lambda}$$
where $\sigma_y$ is the yield strength. The optimal microstructure minimizes $\lambda$ (fine, uniform carbide dispersion) without introducing large, brittle particles. At 600 °C, while particles might still be absent within dimples, excessive carbide coarsening increases $\lambda$ and may promote inter-carbide fracture or reduce the matrix’s ability to deform plastically, leading to the observed toughness decrease. The formation of elongated carbides can also hinder dislocation glide, promoting dislocation pile-ups and subsequent microcrack nucleation.
This study demonstrates that the heat treatment parameters critically govern the microstructure and hence the service-critical hardness and toughness of Cr-Mo steel SAG mill lining plates. Quenching at 920 °C produces a fine, uniform lath martensite structure, providing the optimal starting point for tempering. Tempering this structure at 560 °C results in a fine tempered sorbite microstructure characterized by uniformly dispersed, fine carbides within a soft ferrite matrix. This specific microstructure delivers the ideal balance of properties for demanding lining plate applications: sufficient hardness (45 HRC) to resist abrasive wear coupled with high impact toughness (exceeding 45 J/cm²) to withstand impact loads encountered in SAG milling operations. The elimination of detrimental brittle particles during this tempering stage is crucial for maximizing toughness. Implementing this optimized 920 °C quenching followed by 560 °C tempering process offers a significant pathway to enhance the durability and lifespan of large SAG mill cylinder lining plates, directly addressing the challenge of frequent maintenance and replacement. Optimizing the lining plate production through controlled heat treatment is thus paramount for improving mill availability and operational efficiency.