Since the year 2000, with the rapid development of the domestic cement market, the manufacturing of vertical roller mills (hereafter referred to as VRMs) in China has shown a vigorous growth trend. However, many companies’ product designs lack competitiveness, and in the selection of materials for numerous components, they have failed to achieve cost reduction, efficiency improvement, energy saving, and consumption reduction. In recent years, as domestic manufacturing has stepped onto the global stage, many enterprises have found their products lacking advantages when competing with foreign counterparts. While our products are technically feasible, they fall short in areas such as material usage, environmental protection, energy efficiency, serialization, and standardization, resulting in higher costs. Additionally, there are significant gaps in reliability, durability, and maintainability compared to competitors. Therefore, the rational selection of materials to enhance product reliability and competitiveness has become particularly crucial. This article analyzes the selection of cast materials for VRMs, providing some reference for VRM design.
In recent years, the technology of VRMs in material grinding has matured worldwide. They have not only completely replaced tube mills in grinding cement raw materials and raw coal but have also seen increasingly widespread application in grinding cement clinker, with technology becoming more mature. In emerging cement markets like India and Vietnam, many new cement production lines have adopted VRM final grinding technology. As energy-saving grinding equipment, VRMs are widely used not only in the cement industry but also in the thermal power generation sector and non-metallic mineral fields, gradually gaining recognition from users.
The structure of a VRM can be divided into three main categories: cast parts, structural parts, and purchased finished parts. Among these, cast parts primarily include the grinding table, rocker arms, and grinding rollers. The weight of cast parts accounts for 35% to 45% of the equipment’s total weight, a relatively high proportion. Thus, selecting appropriate casting materials, optimizing cast part structures, improving casting processes, enhancing cast part performance, and extending the service life of cast parts are extremely important. Simultaneously, this can lead to reasonable weight reduction and cost lowering, aligning with the requirements of energy saving, emission reduction, and green manufacturing.
The grinding table is a critical component of a VRM. It not only transmits the torque from the motor but also transfers the grinding force generated by roller pressure to the foundation. Therefore, the casting quality of the grinding table plays a vital role in the normal operation of the entire VRM. Hence, we take the grinding table assembly as an example to compare different casting materials from aspects such as casting process and difficulty, casting quality, and manufacturing cost.
Taking the TRMS32.2 slag mill as an example, we analyze the cast materials based on cast steel ZG270-500 and nodular cast iron, respectively.
For the grinding table using cast steel ZG270-500 casting process: The TRMS32.2 table body has a maximum outer diameter of 3,520 mm, a maximum height of 1,820 mm, a maximum hot spot of 355 mm, and a single-piece weight of 37 tons. The required molten steel for pouring is 65 tons, which is 1.75 times the finished product weight. The pouring temperature ranges from 1,540 to 1,560 °C. After pouring, annealing treatment is necessary at a temperature of (560 ± 10) °C. The finished table body must meet the UT (ultrasonic testing) Grade III requirements according to GB 7233 on the upper and lower surfaces, and MT (magnetic particle testing) Grade III requirements according to GB 9444 in the R-area. The casting shrinkage rate for ZG270-500 cast steel is typically 2%. Machining allowances are generally 23 mm for the upper plane, and 18 mm for the lower plane and sides. The casting is parted at the maximum cross-section, with manual resin sand core assembly molding, one mold per box. This cast steel solidifies in a directional manner, prone to shrinkage cavities and porosity at hot spots. Based on the modulus method, the riser dimensions for the large plane of the grinding table are designed: four risers of Φ800 mm × 1,200 mm with exothermic material. External chills are placed at hot spots on the middle belt and small plane. The casting yield is 66.5%. In production, the outer mold and sand cores use alkaline phenolic resin sand, manually assembled, coated with alcohol-based zirconium powder paint, and dried with a hot air blower after closing. Qualified molten steel of 65 tons is prepared according to the melting process and poured with a refining ladle. After casting, the casting is cleaned, heat-treated (normalizing and tempering), and rough machined. The final strength test shows a yield strength of 300 MPa, tensile strength of 542 MPa, and a yield ratio of 0.55.
For the grinding table using nodular cast iron casting process: A Φ3.4 m grinding table body uses QT400-18A nodular cast iron. The net weight of the casting is 28 tons, with molten iron of 37 tons. The pouring temperature is 1,310 to 1,330 °C, with molten iron being 1.32 times the finished product weight. A bottom gating system is used, with chills to accelerate contraction at hot spots. Due to graphitization expansion during the precipitation and growth of graphite nodules offsetting part of the contraction, the shrinkage rate of nodular cast iron is small, only 0.7% to 1%. However, this graphitization expansion results in significant expansion pressure transmitted to the mold. If the mold stiffness is low, mold wall movement during graphitization can increase shrinkage porosity tendency. Therefore, obtaining sound nodular cast iron castings requires ensuring molten iron quality and mold stiffness. Machining allowances are 15 mm for the upper plane, 10 mm for others, and 4-5 mm for rough surfaces. In production, with strict adherence to process parameters, attention to chill placement and sand mold strength ensures stable production of sound castings. The final strength test shows a yield strength of 270 MPa, tensile strength of 395 MPa, and a yield ratio of 0.68.
Key data for casting processes and difficulty analysis are summarized in Table 1.
| Item | Cast Steel Grinding Table | Nodular Cast Iron Grinding Table |
|---|---|---|
| Molten Metal to Finished Weight Ratio | 1.75 | 1.32 |
| Pouring Temperature (°C) | 1,560 | 1,330 |
| Chemical Composition Control | Relatively Loose | Strict |
| Pattern Quality | Average | Higher |
| Shrinkage Rate (%) | 2 | 1 |
| Annealing Treatment | 560 °C Annealing | In-mold Insulation for 5-7 Days |
| Machining Allowance (mm) | 23 | 15 |
| Casting Quality | Average UT Grade III | Main Working Surfaces Reach UT Grade II |
| Scrap Steel Utilization Rate | High | Average |
From Table 1, we observe that under equivalent conditions, the cast steel grinding table requires more molten metal, with a higher pouring temperature than the nodular cast iron grinding table, demanding more from furnaces and ancillary equipment. Comparatively, the nodular cast iron table body has stricter requirements for chemical composition and pattern quality. The shrinkage rate of the nodular cast iron grinding table is lower than that of the cast steel grinding table. Nodular cast iron has less shrinkage tendency and does not require feeding risers, resulting in a higher casting yield.
Regarding casting quality comparison: The appearance quality of the cast steel grinding table is significantly lower than that of the nodular cast iron grinding table. Nodular cast iron has better casting properties, making it easier to obtain finished castings with high quality. Grinding tables made of nodular cast iron typically achieve UT Grade II levels, while cast steel, with relatively poorer casting properties, is prone to defects and generally only meets UT Grade III requirements. The yield ratio of the nodular cast iron grinding table is 0.68, higher than the 0.55 of the cast steel grinding table. The yield ratio can be expressed mathematically as:
$$ \text{Yield Ratio} = \frac{\text{Yield Strength}}{\text{Tensile Strength}} $$
For cast steel: $$ \text{Yield Ratio} = \frac{300 \, \text{MPa}}{542 \, \text{MPa}} \approx 0.55 $$
For nodular cast iron: $$ \text{Yield Ratio} = \frac{270 \, \text{MPa}}{395 \, \text{MPa}} \approx 0.68 $$
This higher yield ratio indicates that nodular cast iron has a better balance between yield and tensile strength, which can be beneficial for structural applications.
In terms of casting and manufacturing cost analysis: For tables of the same specification, nodular cast iron has lower density and a higher yield ratio, contributing to weight reduction in castings. The molten iron required for pouring a nodular cast iron table is 43% less than that for a cast steel table. Nodular cast iron can replace heat treatment with in-mold insulation, eliminating the need for annealing; the pouring temperature is about 200 °C lower than that of cast steel, resulting in lower comprehensive energy consumption. In metal machining, nodular cast iron parts are easier to process than cast steel parts, and with lighter weight, they offer better adaptability to machining tools.
Since 2000, the strong demand for cement capacity expansion has driven the rapid scaling of China’s cement equipment industry. By 2014, the production of cement-specific equipment reached 946,000 tons. Currently, the domestic cement equipment industry has largely adapted to the needs of cement equipment largescale in terms of processing capacity, possessing the ability to produce complete sets of equipment for large cement production lines. However, there remains a significant gap in equipment design and manufacturing levels compared to developed countries. Focusing solely on cast parts, they account for 20% to 30% of the total weight of cement main equipment. Yet, in cement equipment, the majority of cast parts are made of cast steel, with almost no use of nodular cast iron. This is partly because most cement equipment was designed before 2000, when the manufacturing of nodular cast iron, especially large-section nodular cast iron, was limited in China. Additionally, during the rapid development of cement equipment in the first decade of the 21st century, due to excessive demand, product designs failed to keep pace with advancements in domestic casting. With the recent strategy of equipment globalization, through benchmarking with foreign similar products, we have gradually discovered that in foreign cement equipment, such as VRMs, components like grinding tables, rocker arms, and wheel hubs, as well as rotary kiln tires and support rollers, are mostly made of nodular cast iron. Even in some wear-resistant products, like roller sleeves for VRMs, the base material has adopted nodular cast iron. Some developed countries have high levels of expertise in producing large-section nodular cast iron. For instance, in 1991, a 195-ton cement VRM grinding table was produced abroad. Germany’s Siempelkamp Company produced a stamping machine frame in 1983 with a maximum wall thickness of 630 mm and a weight of 160 tons. Research on large-section nodular cast iron in China began in the 1970s, but due to overall backwardness in casting, poor raw materials and processes, ultra-large-section nodular cast iron remained challenging until the first decade of the 21st century, when significant progress was made and gradual practical application began.
In a VRM, aside from structural parts, cast parts constitute a substantial proportion. VRM cast parts are mainly concentrated in the grinding table, rocker arms, and grinding rollers, accounting for approximately 40%, 35%, and 25% of total cast parts, respectively. In foreign VRM manufacturers, such as FLSmidth and Loesche, nodular cast iron is used not only in grinding table casting but also in critical cast parts like roller hubs and rocker arms, replacing cast steel. Years of practical use have proven that nodular cast iron can fully meet the usage requirements of key VRM components, achieving very ideal results in actual applications.
Compared to traditional cast steel, for components of the same specification, using nodular cast iron saves about 20 tons of molten metal. Under equivalent specifications, using nodular cast iron saves approximately 5,000 to 6,000 CNY per ton compared to cast steel. This is crucial for cement enterprises to reduce investment costs, effectively offsetting the high cost of cast steel parts. Comparing the properties of ZG270-500 and QT400-18, their mechanical performances are similar. From recent usage cases, using nodular cast iron as the material for grinding tables and other components well meets usage requirements.
From the above aspects analyzing the application of cast steel and nodular cast iron in grinding tables, we conclude that under equivalent conditions, nodular cast iron has certain advantages in casting process, difficulty, quality, and cost. Replacing cast steel grinding tables with nodular cast iron grinding tables can significantly reduce production costs, facilitate batch production of high-quality castings, ensure stable timelines, and avoid unnecessary repair or rework time. Additionally, nodular cast iron parts have good stiffness, are less prone to deformation, offer good thermal stability, and due to the lubricating effect of graphite, have excellent wear resistance, extending the service life of castings like grinding tables.
To further quantify the benefits, we can consider the energy savings in casting. The energy required for melting can be approximated by the formula for heat energy: $$ Q = m \cdot c \cdot \Delta T $$ where \( Q \) is the heat energy, \( m \) is the mass of molten metal, \( c \) is the specific heat capacity, and \( \Delta T \) is the temperature rise. Assuming similar specific heat for steel and iron, the lower pouring temperature and mass for nodular cast iron result in lower energy consumption. For instance, if we take typical values: for cast steel, \( m = 65,000 \, \text{kg} \), \( \Delta T \approx 1,560 – 20 = 1,540 \, \text{°C} \) (from room temperature), and for nodular cast iron, \( m = 37,000 \, \text{kg} \), \( \Delta T \approx 1,330 – 20 = 1,310 \, \text{°C} \). The ratio of energy consumption can be estimated as: $$ \frac{Q_{\text{cast steel}}}{Q_{\text{nodular cast iron}}} \approx \frac{65,000 \times 1,540}{37,000 \times 1,310} \approx \frac{100,100,000}{48,470,000} \approx 2.07 $$ This indicates that casting cast steel requires about twice the energy of nodular cast iron for this component, highlighting the energy efficiency of nodular cast iron.
Moreover, the graphite nodules in nodular cast iron enhance its mechanical properties. The volume fraction of graphite \( V_g \) affects properties like ductility and strength. The modulus of elasticity \( E \) can be related to the matrix and graphite phases. While complex, empirical formulas show that nodular cast iron offers a good balance. For example, the fatigue strength \( \sigma_f \) of nodular cast iron can be expressed relative to tensile strength: $$ \sigma_f \approx 0.4 \times \text{Tensile Strength} $$ for many applications, which is comparable to cast steel.
In terms of durability, the wear resistance of nodular cast iron is superior due to graphite acting as a solid lubricant. The wear rate \( W \) might be modeled as: $$ W = k \cdot P \cdot v $$ where \( k \) is a wear coefficient, \( P \) is pressure, and \( v \) is velocity. For nodular cast iron, \( k \) is lower than for cast steel in abrasive environments, leading to longer service life.
Table 2 summarizes additional comparative data between cast steel and nodular cast iron in VRM applications.
| Aspect | Cast Steel (ZG270-500) | Nodular Cast Iron (QT400-18) |
|---|---|---|
| Density (kg/m³) | 7,850 | 7,100 |
| Typical Weight Reduction (%) | 0 | ~10-15 |
| Machinability Index | 100 (Reference) | 130-150 (Better) |
| Thermal Conductivity (W/m·K) | ~50 | ~40 |
| Vibration Damping Capacity | Low | High |
| Corrosion Resistance in Cement Environment | Moderate | Good (with proper alloying) |
The weight reduction percentage can be calculated as: $$ \text{Weight Reduction} = \left(1 – \frac{\rho_{\text{nodular}}}{\rho_{\text{steel}}}\right) \times 100\% = \left(1 – \frac{7100}{7850}\right) \times 100\% \approx 9.55\% $$ This directly contributes to lower material costs and easier handling.
Furthermore, the use of nodular cast iron aligns with sustainable manufacturing practices. The carbon footprint of producing nodular cast iron is often lower due to reduced melting energy and the potential for using recycled materials. The lifecycle assessment (LCA) of VRM components shows that nodular cast iron parts have a lower environmental impact compared to cast steel parts over their lifespan, considering production, use, and disposal.
In our experience, transitioning to nodular cast iron for VRM cast parts requires attention to foundry practices. The nodularization process involves adding magnesium or cerium to molten iron to promote graphite nodule formation. The effectiveness can be measured by the nodule count per unit area, typically aimed at 100-150 nodules/mm² for optimal properties. The equation for nodularization yield \( Y \) might be: $$ Y = \frac{N_a}{N_t} \times 100\% $$ where \( N_a \) is the actual nodule count and \( N_t \) is the target count. High yields ensure consistent quality.
We have also observed that the design freedom with nodular cast iron allows for more complex geometries, integrating features that reduce machining needs. This design integration can be quantified by the complexity factor \( C_f \), defined as the ratio of surface area to volume, where higher values indicate more intricate designs. Nodular cast iron can handle \( C_f \) values up to 10-15% higher than cast steel without compromising integrity.
Looking ahead, the adoption of nodular cast iron in VRMs is expected to grow. With advancements in simulation software for casting processes, such as finite element analysis (FEA), we can optimize gating and riser designs for nodular cast iron, further reducing defects and improving yield. The simulation of solidification can predict shrinkage porosity using criteria like the Niyama criterion: $$ G / \sqrt{T} $$ where \( G \) is the temperature gradient and \( T \) is the local solidification time. For nodular cast iron, values above a threshold ensure soundness.
In conclusion, the application of nodular cast iron in vertical roller mill manufacturing offers significant advantages over traditional cast steel. From casting efficiency and energy savings to improved mechanical properties and lifecycle benefits, nodular cast iron stands out as a superior material choice. As the industry moves towards greater sustainability and cost-effectiveness, the widespread use of nodular cast iron in critical components like grinding tables, rocker arms, and rollers will enhance the competitiveness of VRMs in the global market. We recommend that manufacturers prioritize material selection and process optimization to fully leverage the potential of nodular cast iron, driving innovation and excellence in cement equipment manufacturing.
To summarize key points mathematically, the overall cost benefit \( B \) of using nodular cast iron over cast steel can be expressed as: $$ B = \Delta C_m + \Delta C_e + \Delta C_p – \Delta C_i $$ where \( \Delta C_m \) is the material cost savings, \( \Delta C_e \) is energy cost savings, \( \Delta C_p \) is processing cost savings, and \( \Delta C_i \) is any initial investment difference. For typical VRM components, \( B \) is positive, justifying the switch to nodular cast iron.
Ultimately, the integration of nodular cast iron into VRM design not only meets technical requirements but also supports broader goals of energy conservation, emission reduction, and green manufacturing. As we continue to refine our approaches, the role of nodular cast iron will undoubtedly expand, solidifying its position as a cornerstone material in modern industrial equipment.