In the field of industrial grinding, ball mills are critical equipment extensively used in mining, metallurgy, power generation, and building materials sectors. Grinding balls, as high-consumption wear parts, have an annual global demand exceeding millions of tons, making the selection and enhancement of ball materials economically and socially significant. Traditionally, grinding balls are produced via two primary methods: direct casting or forging/rolling of cast blanks. However, casting processes often introduce inherent defects such as residual stresses, shrinkage porosity, inclusions, and microstructural inhomogeneities, which can lead to premature failure, fragmentation, and loss of spherical shape during service. To address these limitations, recent advancements in horizontal continuous casting technology have enabled the production of dense and defect-free cast iron profiles, including various grades of white cast iron. This white cast iron exhibits superior strength, denser microstructure, and, intriguingly, a notable capacity for hot plastic deformation. Prior studies suggest that white cast iron can undergo significant shape change at elevated temperatures, where the interconnected carbide networks fracture and become isolated within the matrix, thereby potentially improving toughness. Leveraging these attributes, we propose a novel manufacturing route: producing high-quality grinding balls by hot skew rolling of horizontally continuous-cast white cast iron bars. This process promises to eliminate casting defects, enhance toughness through microstructural refinement, offer high production efficiency, and allow for direct utilization of forging heat for subsequent heat treatments. This article presents foundational research aimed at evaluating the plastic deformation behavior of manganese-alloyed white cast iron, serving as a precursor to developing this innovative ball-forming technology.
The experimental methodology was designed to simulate the conditions of horizontal continuous casting, aiming to obtain cast specimens with similar density and integrity. The simulation setup, illustrated conceptually, involved a horizontal continuous casting mold connected to a cooling water system. A保温 riser was added atop the mold to mimic the保温 funnel in actual continuous casting, providing molten metal feeding and maintaining a pressure head. The bottom utilized a graphite chill to establish a controlled temperature gradient, replicating the directional solidification characteristic of horizontal casting.
The charge materials for melting included Benxi pig iron, scrap steel, medium-carbon ferrochromium, and ferromanganese. Alloying and melting were conducted in a medium-frequency induction furnace, with composition adjustments made post-melting. The target chemical composition of the manganese white cast iron was as follows (in weight percent): Carbon (C): 2.8–3.2%, Manganese (Mn): 5.0–6.0%, Chromium (Cr): 1.5–2.0%, Boron (B): 0.005–0.015%, Silicon (Si): 0.8–1.2%, with Sulfur (S) and Phosphorus (P) kept below 0.05%. Two sizes of round bar specimens were cast: small-diameter bars (Ø20 mm) for impact toughness, hardness, and ball-forming samples, and medium-diameter bars (Ø40 mm) for upsetting tests.
The core of this investigation focused on assessing the hot workability of manganese white cast iron. The critical deformation rate, defined as the maximum permissible strain before crack initiation at a given temperature, was determined using the free upsetting test method. Cylindrical specimens with a specific height-to-diameter ratio were heated to predetermined temperatures and forged to varying degrees. The critical deformation rate ($\varepsilon_c$) was calculated using the formula:
$$ \varepsilon_c = \frac{H_0 – H_f}{H_0} \times 100\% $$
where $H_0$ is the initial height and $H_f$ is the final height at which the first crack appears. Specimens were batch-heated in a furnace. For the first temperature point, a soaking time of 30 minutes was applied; subsequent points did not involve holding to simulate practical forging conditions. Accounting for temperature drop during transfer, the actual furnace setpoint was 20°C higher than the target test temperature. At each temperature, 10–15 specimens were tested to accurately identify the critical strain. The results mapping critical deformation rate against temperature are summarized in Table 1 and graphically represented.
| Temperature Range (°C) | Critical Deformation Rate ($\varepsilon_c$) | Region Designation & Characteristics |
|---|---|---|
| < 600 | < 10% | Non-deformable Region: Microstructure consists of decomposed products of cementite and austenite; very low plasticity. |
| 600–800 | 10–40% | Blue Brittleness Region: Increased deformation possible but high resistance; cracking along grain boundaries; fracture exhibits blue tint. |
| 800–1000 | > 60% | Easy Deformation Region: Single-phase austenite matrix; significant plasticity enhancement; eutectic carbides undergo spheroidization and rounding. |
| 1000–1150 | > 60% (stable) | Stable Deformation Region: Critical deformation rate remains high and constant with temperature. |
| > 1150 | Near 0% | Fusion Region: Incipient melting occurs; material becomes extremely brittle. |
The relationship between temperature and critical deformation rate for this manganese white cast iron can be divided into five distinct regimes, as derived from the experimental data. The most promising window for hot working lies between 800°C and 1150°C, where the critical strain exceeds 60%. This indicates a broad and suitable temperature interval for plastic forming operations. Based on these findings, the recommended forging temperature range for this white cast iron is: start forging at 1050°C and finish forging no lower than 900°C.
To evaluate the effect of plastic deformation on mechanical properties, we conducted hardness and impact toughness tests on specimens subjected to different deformation processes and levels. The macrohardness (Rockwell or Brinell scale) showed minimal variation with increasing deformation, as illustrated in Table 2. This suggests that the hardness of this white cast iron is primarily governed by the high volume fraction of hard carbides, which remains relatively unchanged despite morphological alterations during deformation.
| Deformation Process | Deformation Rate (%) | Macrohardness (HRC) | Notes |
|---|---|---|---|
| As-cast (Reference) | 0 | 55–58 | Base material from Ø20 mm bar. |
| Free Upsetting | 40 | 56–59 | Specimen from Ø40 mm bar. |
| Free Upsetting | 60 | 57–60 | Specimen from Ø40 mm bar. |
| Longitudinal Forging (Drawing) | 50 | 56–59 | Specimen from Ø20 mm bar. |
| Longitudinal Forging (Drawing) | 70 | 57–60 | Specimen from Ø20 mm bar. |
In contrast, impact toughness exhibited a remarkable improvement following plastic deformation. Charpy impact tests were performed on notched specimens in various conditions. The results, detailed in Table 3, demonstrate that plastic deformation can more than double the impact energy absorption of this white cast iron. Notably, longitudinal forging (drawing) proved more effective in enhancing toughness compared to upsetting under equivalent strain levels. Furthermore, even when deformed specimens underwent subsequent quenching and tempering heat treatments, they retained significantly higher toughness than the as-cast state.
| Material State | Specimen ID | Impact Toughness (J/cm²) | Process Details |
|---|---|---|---|
| As-cast | AC-1, AC-2, AC-3 | 4.5 – 5.0 | Base reference from Ø20 mm bar. |
| Upset Deformed (40%) | UD-1, UD-2 | 8.0 – 8.5 | From Ø40 mm bar, free upset. |
| Longitudinally Forged (50%) | LF-1, LF-2 | 10.5 – 11.0 | From Ø20 mm bar, drawing process. |
| Longitudinally Forged (70%) | LF-3, LF-4 | 12.0 – 12.5 | From Ø20 mm bar, drawing process. |
| Deformed + Heat Treated | HT-1, HT-2 | 9.5 – 10.0 | Forged specimen, oil quenched from 950°C & tempered at 250°C. |
The substantial increase in impact toughness is intrinsically linked to the microstructural evolution induced by hot plastic deformation. In the as-cast condition, the microstructure of this manganese white cast iron comprises a continuous or semi-continuous network of eutectic carbides (primarily M3C type) embedded in a matrix that is predominantly martensitic/bainitic after cooling. This carbide network acts as a brittle skeleton, providing easy paths for crack propagation. During hot deformation within the austenitic region (above 800°C), the relatively ductile austenite matrix undergoes plastic flow. The brittle carbides cannot co-deform compatibly; instead, they fracture under the imposed stress. With increasing strain, the matrix material flows into these cracks, further fragmenting the carbides and isolating them into discrete blocks or particles. The degree of carbide network disruption can be qualitatively described by a fragmentation index, which correlates with strain. For longitudinal forging, the stress state is more conducive to breaking the carbide continuity compared to upsetting, explaining the superior toughness enhancement.
Scanning electron microscopy (SEM) examination of impact fracture surfaces provides compelling evidence. The as-cast fracture surface is characterized by large, smooth facets corresponding to cleavage through massive carbides or decohesion along carbide/matrix interfaces, with minimal signs of ductile tearing. Conversely, fracture surfaces of deformed white cast iron specimens display a much finer dimpled morphology, abundant tear ridges, and evidence of microvoid coalescence around isolated carbide particles. This indicates a shift in fracture mechanism from predominantly brittle interfacial failure to a mixed-mode involving significant plastic deformation of the matrix, thereby absorbing more energy.
The carbide morphology evolution can be modeled conceptually. Consider an initial carbide network with an average interconnectivity length scale $\lambda_0$. Under an applied true strain $\varepsilon$, the carbide fragmentation process reduces the effective length scale. A simple relation can be proposed:
$$ \lambda(\varepsilon) = \lambda_0 \cdot \exp(-k \cdot \varepsilon) $$
where $k$ is a material constant dependent on carbide type, matrix properties, and deformation mode. Higher strain $\varepsilon$ leads to smaller $\lambda$, promoting a more isotropic and tougher microstructure. For the studied white cast iron, longitudinal forging likely results in a higher effective $k$ value compared to upsetting.
Furthermore, the hot deformation process also influences the matrix itself. Dynamic recovery and possible recrystallization of the austenite during forging lead to a refined grain structure upon subsequent cooling, which further contributes to toughness improvement. The combined effect of carbide isolation and matrix refinement can be expressed as a synergistic contribution to the increase in impact energy ($\Delta W$):
$$ \Delta W = A \cdot (1 – \exp(-\alpha \varepsilon)) + B \cdot \varepsilon^{1/2} $$
where the first term represents the contribution from carbide network breakdown (saturating at high strain), and the second term represents the contribution from grain refinement, with $A$, $B$, and $\alpha$ being constants.
The practical formability of this manganese white cast iron was validated through various forging operations. Successful free upsetting, flat-anvil drawing, and even closed-die forging into spherical preforms (simulating grinding balls) were demonstrated. A key finding for die forging is the importance of appropriate billet dimensions. For instance, forging a Ø50 mm ball required a starting billet diameter of approximately Ø45 mm and a calculated length. Using a billet that was too short and stout led to unfilled die corners and cracking at the pole, while a properly sized billet produced sound, fully formed spheres. This underscores that, despite its good hot plasticity, white cast iron requires careful process design to avoid stress concentrations and defects inherent to its brittle carbide phase.
When performing drawing operations on white cast iron, caution is needed to prevent internal cracking, such as the classic “cross-shaped” crack. The interconnected carbide network presents planes of weakness. Therefore, a forging practice employing a “light-heavy-light” sequence of blows is recommended to gradually break down the structure without inducing excessive tensile stresses internally.
In summary, this comprehensive study establishes the feasibility of hot plastic forming for manganese-alloyed white cast iron. The material possesses a wide hot-working temperature window (800–1150°C) with critical deformation rates consistently above 60%, making it suitable for processes like rolling or forging. Plastic deformation, especially via longitudinal forging, dramatically improves impact toughness—from about 5 J/cm² in the as-cast state to over 12 J/cm²—while maintaining high hardness. This enhancement is directly attributable to the fragmentation of the continuous carbide network into isolated blocks within a refined matrix, altering the fracture mechanism. These findings provide a solid scientific foundation for developing a novel manufacturing route for grinding balls: horizontal continuous casting of white cast iron bars followed by hot skew rolling. This process chain has the potential to yield grinding media with superior combination of hardness, toughness, and integrity, offering significant economic and performance benefits across heavy industries. Future work should focus on optimizing the alloy composition for an even broader processing window, detailed thermomechanical processing maps, and full-scale pilot trials of the integrated casting-rolling process for white cast iron grinding ball production.