Abstract: Based on the structural characteristics, chemical composition, and customer requirements of the coal mill roller sleeve substrate, this paper develops a scientific and reasonable casting technology. By utilizing casting simulation software, a comprehensive optimization analysis of the designed casting process is conducted to verify its feasibility during production. To ensure the smooth progression of the production process, countermeasures are predicted and formulated in advance to address a series of potential quality issues that may arise during the production of castings, ensuring that the performance of the castings fully meets the standard requirements of customers. The successful production of this coal mill roller sleeve has laid the foundation for the company’s subsequent production of such products.
Keywords: coal grinding roller sleeve; software simulation; pouring system; riser
1. Introduction
As an efficient grinding equipment, the coal mill can comprehensively and efficiently complete multiple processes such as material grinding, drying, and powder selection. It is a key equipment for re-grinding materials after they are crushed [1]. The roller sleeve of the coal mill is a critical wear and consumption part of the coal mill in power plants. Currently, a major trend in manufacturing coal mill roller sleeves is multi-method and multi-material composite production, such as bimetallic composite casting [2], cast-in or inlay composite forming [3], surface surfacing welding or overall surfacing welding treatment [4].
This paper mainly combines the manufacturing technical requirements of the ordered products in the enterprise, carefully plans the casting process for the ZGM95-Ⅱ roller-type coal mill roller sleeve substrate based on its structural characteristics, chemical composition, and quality standards, and uses casting simulation software to simulate and calculate the solidification process of the casting to predict the generation and distribution of casting defects. Finally, the casting process is optimized and improved based on the simulation results. The research results not only provide a solid theoretical foundation for the enterprise in the production of roller sleeve substrates but also provide powerful technical support for its practical operations.
2. Structure and Chemical Composition of the Casting
2.1 Casting Structure
The contour dimensions of the ZGM95-Ⅱ roller-type coal mill roller sleeve substrate are φ1521mm × 541mm, and the weight on the product drawing is 1687kg. The wall thickness at the upper and lower ends of the product is up to 116mm, and the middle wall thickness is 65mm. The overall shape of the casting resembles a begging bowl used by monks, with thicker flanges at the upper and lower ends forming two hot spots, and a thinner and curved middle wall. This structure results in the molten steel in the riser being unable to normally fill the lower hot spots. During the casting process design, special attention should be paid to the feeding channel of molten steel. According to the technical agreement, the flaw detection grade of the casting must reach Grade 2. After processing, the casting must not have defects such as cracks, shrinkage cavities, and slags. To meet these requirements, the casting process design is relatively strict. The specific structure of the casting is shown in Figure 1.
Figure 1. Casting Structure Diagram
2.2 Chemical Composition of the Casting
The material of the ZGM95-Ⅱ roller-type coal mill roller sleeve substrate is ZG20SiMn. This cast steel has good casting and welding properties and is generally used for casting large-section thick-walled heavy castings, such as large columns and working cylinders of hydraulic presses, runners and blades of water turbines, main shafts and flanges of power stations, and seat rings [5]. Its main chemical composition is listed in Table 1.
Table 1. Chemical Composition Requirements
| Element | Composition Requirement | Element | Composition Requirement |
|---|---|---|---|
| C | ≤0.23 | P | ≤0.025 |
| Si | ≤0.6 | S | ≤0.025 |
| Mn | 1.00 – 1.50 | Cr | ≤0.30 |
ZG20SiMn is a low-carbon steel with a very narrow crystallization temperature range. Therefore, the casting solidifies in a layer-by-layer manner. The solid-liquid coexistence zone in the solidification zone is small, and the solidification front is directly in contact with liquid metal. The advantage of this characteristic is that when liquid metal solidifies into a solid and undergoes volume contraction, liquid metal can continuously supplement the contraction area, making the material less prone to dispersed porosity and more prone to concentrated shrinkage porosity during the final stage of solidification [6].
3. Casting Process Design
3.1 Feeding Scheme Design
The ZGM95-Ⅱ roller-type coal mill roller sleeve is a low-carbon alloy steel casting. When designing the casting process, risers, chills, and subsidies should be set according to the design criteria and sequential solidification method for steel castings. This allows the casting to form a good temperature gradient from the riser to the casting during solidification, thereby maximizing the feeding effect of the riser. The ZGM95-Ⅱ roller-type coal mill roller sleeve has a rotational structure with uneven longitudinal wall thickness and large hot spots at the upper and lower ends. From the perspective of casting process design theory, the structure of this casting is not conducive to sequential solidification, and it is difficult to eliminate the lower hot spots directly with an open riser. It requires the use of chills and subsidies or the adoption of blind risers to eliminate defects.
ZG20SiMn is a low-carbon steel with a narrow crystallization temperature range, so the casting solidifies in a layer-by-layer manner. The solid-liquid coexistence zone during solidification is small, and the solidification front is directly in contact with liquid metal. This characteristic has the advantage that when liquid metal solidifies and becomes solid, causing volume contraction, liquid metal can continuously supplement the contraction area. This makes the material less prone to dispersed porosity and more prone to concentrated shrinkage porosity during the final stage of solidification.
Given the structural and material characteristics of the casting, measures such as chills and subsidies are adopted to achieve sequential solidification of the casting. Firstly, a circle of chills with a diameter of 60mm is placed at the bottom and side of the lower hot spot, and an open riser is set at the upper part of the casting. The size and number distribution of the risers should be determined through theoretical calculations. At the same time, subsidies are set at the lower end of the riser. Given that the middle part of the casting is arcuate, the conventional hot spot circle method cannot be used for subsidy design. The subsidy design method is fine-tuned by enlarging the hot spot circle to nine circles with a coefficient of 1.05 and then drawing three connected arcs based on this, ultimately forming a complete subsidy. This allows the casting to form a temperature gradient from top to bottom, thereby achieving sequential solidification. The process design for the casting’s risers, subsidies, and chills is detailed in Figure 2.
Figure 2. Design of Risers, Subsidies, and Chill Distribution in the Casting
3.2 Pouring System Design
Given the concentrated shrinkage porosity characteristic of low-carbon steel, special attention should be paid to enhancing the sequential solidification temperature gradient of the casting when designing the pouring system to ensure that the riser can fully exert its feeding capacity. The pouring system design is shown in Figure 3.
Figure 3. Pouring System Design
To achieve this goal, four ingates are set in the pouring system and placed below the riser, allowing molten metal to flow in from the subsidy of the casting, forming an ideal sequential solidification temperature gradient. The above design successfully constructs an efficient pouring system aimed at optimizing the casting process of ZGM95-Ⅱ roller-type coal mill roller sleeve substrate materials, reducing defects such as shrinkage porosity, and improving product quality.
3.3 Analysis of Solidification Simulation Results
After determining the casting process plan, a corresponding model was constructed using three-dimensional modeling technology, and the plan was comprehensively optimized and adjusted with the help of professional casting simulation software. The shrinkage porosity simulation result of the finally determined casting process plan is shown in Figure 4.
Figure 4. Shrinkage Porosity Simulation Result of the Casting Process
3.4 Shrinkage Porosity Simulation Result of the Casting Process
The simulation results show that the overall quality of the casting after pouring with this casting process is good, with no obvious shrinkage porosity defects. The riser in this process plays a good role in feeding the casting, and there is no technical quality risk in casting production.
4. Production Process Control
Based on the existing conditions of the enterprise, product patterns are typically crafted using the conventional method of horizontal parting at the largest parting surface, accompanied by vertical parting of the core box. However, when this method is applied to the ZGM95-Ⅱ roller-type coal mill roller sleeve substrate, it can lead to sand inclusion issues between the upper end of the sand core and the pattern solid sample. To address this, additional drafting subsidies are often required to facilitate the removal of the sand core, which is both time-consuming and labor-intensive, and may also compromise the dimensional accuracy of the pattern.
To ensure product quality and enhance efficiency, technical analysis and field trials were conducted to optimize the sand core parting method. The original vertical bipartition was adjusted to a horizontal bipartition, and the material removal method for the bevel surface was improved by adopting a movable block for material extraction, which allows for insertion and removal using steel bars from the outside of the core box. This innovative measure ensures structural stability and dimensional accuracy of the pattern, reduces the potential for sand inclusion, simplifies the operation process, and improves production efficiency. The structure of the improved pattern is illustrated in Figure 5.
By refining the sand core parting method and optimizing the material removal process, the enterprise has successfully overcome the challenges associated with pattern making for the ZGM95-Ⅱ roller-type coal mill roller sleeve substrate. This not only enhances the quality of the final product but also streamlines the production process, contributing to overall operational efficiency and cost-effectiveness.