Manufacturing Process of Super Large Mill Head (End Cover) Steel Castings 0. Preface Mining machinery plays a vital role in economic construction, technological advancement, and social development, serving as a pillar industry of the national economy. In recent years, with the rapid development of China's industry, the demand for mineral resources has significantly increased. Approximately 80% of mining machinery components are made of castings, which indirectly imposes substantial and high-level demands on large steel castings, especially high-performance ones. This has also brought broader market prospects for China's large steel casting industry. As a traditional grinding equipment, the mill has a history of over 100 years. It is an essential device for refining solid materials into powder and is widely used in metallurgy, chemical industry, cement, ceramics, construction, power, pharmaceuticals, and defense industries. Particularly in the mineral processing sector of the metallurgical industry, the operation of mills holds a crucial position. Mill castings mainly include the trunnion castings and feed/discharge end castings of ball mills, semi-autogenous mills, and autogenous mills. This article primarily introduces the manufacturing process and technical quality characteristics of a super-large ball mill head(end cover) steel casting. 1. Quality Requirements for Steel Castings The 3D model of the casting product described in this article is shown in Figure 1. The casting dimensions are φ10,600 × 983 (mm), with a net weight of 36 tons, a rough weight of 47 tons, a maximum wall thickness of 250 mm, and a minimum wall thickness of 125 mm. Figure 1: Super Large Mill Head(End Cover) Casting The casting material is ASTM A216 WCA. The specific chemical composition requirements are listed in Table 1, and the mechanical property requirements are listed in Table 2. Table 1: Chemical Composition Requirements (mass fraction, %) Index Tensile Strength / MPa Yield Strength / MPa Elongation / % Reduction of Area / % Hardness / HB Standard 448~586 ≥ 241 ≥ 20 ≥ 35 ≥ 130 Table 2: Mechanical Property Requirements Index Tensile Strength / MPa Yield Strength / MPa Elongation / % Reduction of Area / % Hardness / HB Standard 448~586 ≥ 241 ≥ 20 ≥ 35 ≥ 130 2. Manufacturing Process of Key Procedures 2.1 Casting Process This product may deform during casting, heat treatment, machining, welding, and other processes. Once deformed, it is difficult to correct. If dimensional welding repair is used, it is more prone to deformation. Therefore, based on the production experience of similar small mill castings and the deformation trend, machining allowances and non-machining surface process correction allowances are set at different positions of the casting. The machining surface is set at 20-50 mm, and the non-machining surface is set at 5-15 mm, which can effectively solve the problem, as shown in Figure 2. Additionally, this type of steel casting is prone to shrinkage porosity and shrinkage cavity defects. Computer 3D solidification simulation and traditional modulus calculation are used to verify and optimize the casting process design, ensuring a reasonable feeding gradient, effective feeding, and sequential solidification, thereby achieving a dense structure and extremely low defect rate. Figure 2: Machining Allowance Design Diagram 2.2 Molding Process The casting shape is simple, but its diameter is large, requiring a large sandbox for molding. The gating system is molded using a small sandbox template. After the gating system is molded and the box is flipped, the middle ring is molded. Due to the structural characteristics of the casting, a large space is formed between the slope of the casting and the sandbox plane. Filling this space entirely with sand would be a significant waste. Therefore, some support fillers are placed in this space to reduce sand usage. After the middle ring sand hardens, the upper box molding begins. The upper box sandbox is placed on the middle ring, and after the risers, chills, and other process measures are prepared, sand is poured. The facing sand is chromite sand, with a thickness of about 40-80 mm, and the backing sand is silica sand, as shown in Figure 3. Figure 3: Upper Box Molding 2.3 Smelting Process The pouring weight of this casting is 130 tons, and the smelting weight is 140 tons. The smelting process is EAF + LF + argon-protected pouring. The main difficulty in smelting this casting is controlling the Cr content. Scrap steel with a Cr content of less than 0.15% is selected, and corresponding pig iron is added to ensure the Cr content meets the requirements. Each process and node in the smelting process is strictly controlled, with real-time computer tracking. Chemical element content is tested using pipeline sampling + spectral analysis + gas analysis, and only after meeting the requirements is the process moved to the next step, effectively ensuring the chemical composition meets the standard requirements. The refining process involves continuous argon blowing. Before adding alloys, the oxidation level must be reduced to below 7×10⁻⁶. Silicon-aluminum-calcium-barium refining agents are used to spheroidize inclusions, allowing them to fully float. The soft blowing time before tapping is more than 10 minutes. During pouring, an argon ring is used to protect the pouring process, reducing oxidation. Additionally, an argon sliding gate is used to ensure automatic pouring, reducing the entry of oxides into the mold cavity caused by entrance, thereby ensuring the quality of the casting. 2.4 Heat Treatment Process The main difficulty in the heat treatment of this casting is the reasonable design of the chemical composition and achieving the desired mechanical properties and microstructure under the customer-specified normalizing heat treatment method. Additionally, preventing deformation during heat treatment is crucial. Once the casting deforms, it is difficult to correct through mechanical or thermal methods. Therefore, historical data from similar products and specialized STECAL heat treatment simulation software are used to optimize the simulation, ultimately determining a reasonable internal control composition. To achieve good mechanical properties and prevent deformation during heat treatment, a normalizing air cooling + tempering process is adopted. The normalizing temperature must be appropriately selected. The purpose of normalizing heat treatment is to fully dissolve the precipitates in the casting, form fine-grained austenite after austenitizing, and form a fine F+P structure after cooling. After normalizing, medium-high temperature tempering is performed to promote the precipitation of a small amount of carbon in the form of carbides and reduce residual stress in the casting, ensuring dimensional stability in later stages. To ensure temperature control during heat treatment, in addition to using a computer-controlled natural gas heat treatment furnace for temperature control, more than three thermocouples are attached to the casting to track the actual process temperature. 2.5 Welding Process Through visual and ultrasonic testing, defects found in the casting are primarily repaired using manual electrode arc welding. All easily deformable areas are welded using electrode arc welding. During welding preheating, natural gas pipe flame preheating is used to preheat the welding surface to 50-60°C, removing moisture from the casting surface. Symmetrical welding by zones is adopted, controlling each weld bead width to less than 30 mm and each layer thickness to less than 3 mm. The interpass temperature during welding is controlled to less than 100°C. For large welding volumes, a welding rod with a diameter of ¢4.0 mm is used, with welding current controlled between 120-160 A. During welding, dial indicators are installed on the back of the welding area and other parts of the casting for measurement and monitoring. If the deformation exceeds 0.5 mm, welding is stopped and resumed only after the dimensions are restored. The welding sequence is from the inside out. After the internal and external cavity dimensions and defect treatment are qualified, the entire casting is subjected to stress relief post-weld heat treatment. 3. Conclusion By analyzing and identifying the characteristics and difficulties of the casting product, optimizing the design of key processes, and strictly controlling the process, the product's dimensions, appearance, composition, mechanical properties, and NDT defects all meet the performance requirements. Additionally, it has strong competitiveness in production costs, delivery cycles, and process efficiency.

Manufacturing Process of Super Large Mill Head (End Cover) Steel Castings

Feb 26, 2025

Manufacturing Process of Super Large Mill Head (End Cover) Steel Castings

0. Preface
Mining machinery plays a vital role in economic construction, technological advancement, and social development, serving as a pillar industry of the national economy. In recent years, with the rapid development of China's industry, the demand for mineral resources has significantly increased. Approximately 80% of mining machinery components are made of castings, which indirectly imposes substantial and high-level demands on large steel castings, especially high-performance ones. This has also brought broader market prospects for China's large steel casting industry.
As a traditional grinding equipment, the mill has a history of over 100 years. It is an essential device for refining solid materials into powder and is widely used in metallurgy, chemical industry, cement, ceramics, construction, power, pharmaceuticals, and defense industries. Particularly in the mineral processing sector of the metallurgical industry, the operation of mills holds a crucial position.
Mill castings mainly include the trunnion castings and feed/discharge end castings of ball mills, semi-autogenous mills, and autogenous mills. This article primarily introduces the manufacturing process and technical quality characteristics of a super-large ball mill head(end cover) steel casting.
1. Quality Requirements for Steel Castings
The 3D model of the casting product described in this article is shown in Figure 1. The casting dimensions are φ10,600 × 983 (mm), with a net weight of 36 tons, a rough weight of 47 tons, a maximum wall thickness of 250 mm, and a minimum wall thickness of 125 mm.

Figure 1: Super Large Mill Head(End Cover) Casting
The casting material is ASTM A216 WCA. The specific chemical composition requirements are listed in Table 1, and the mechanical property requirements are listed in Table 2.
Table 1: Chemical Composition Requirements (mass fraction, %)

Index	Tensile Strength / MPa	Yield Strength / MPa	Elongation / %	Reduction of Area / %	Hardness / HB
Standard	448~586	≥ 241	≥ 20	≥ 35	≥ 130

Table 2: Mechanical Property Requirements

Index	Tensile Strength / MPa	Yield Strength / MPa	Elongation / %	Reduction of Area / %	Hardness / HB
Standard	448~586	≥ 241	≥ 20	≥ 35	≥ 130

2. Manufacturing Process of Key Procedures
2.1 Casting Process
This product may deform during casting, heat treatment, machining, welding, and other processes. Once deformed, it is difficult to correct. If dimensional welding repair is used, it is more prone to deformation. Therefore, based on the production experience of similar small mill castings and the deformation trend, machining allowances and non-machining surface process correction allowances are set at different positions of the casting. The machining surface is set at 20-50 mm, and the non-machining surface is set at 5-15 mm, which can effectively solve the problem, as shown in Figure 2. Additionally, this type of steel casting is prone to shrinkage porosity and shrinkage cavity defects. Computer 3D solidification simulation and traditional modulus calculation are used to verify and optimize the casting process design, ensuring a reasonable feeding gradient, effective feeding, and sequential solidification, thereby achieving a dense structure and extremely low defect rate.

Figure 2: Machining Allowance Design Diagram

2.2 Molding Process
The casting shape is simple, but its diameter is large, requiring a large sandbox for molding. The gating system is molded using a small sandbox template. After the gating system is molded and the box is flipped, the middle ring is molded. Due to the structural characteristics of the casting, a large space is formed between the slope of the casting and the sandbox plane. Filling this space entirely with sand would be a significant waste. Therefore, some support fillers are placed in this space to reduce sand usage. After the middle ring sand hardens, the upper box molding begins. The upper box sandbox is placed on the middle ring, and after the risers, chills, and other process measures are prepared, sand is poured. The facing sand is chromite sand, with a thickness of about 40-80 mm, and the backing sand is silica sand, as shown in Figure 3.

Figure 3: Upper Box Molding

2.3 Smelting Process
The pouring weight of this casting is 130 tons, and the smelting weight is 140 tons. The smelting process is EAF + LF + argon-protected pouring. The main difficulty in smelting this casting is controlling the Cr content. Scrap steel with a Cr content of less than 0.15% is selected, and corresponding pig iron is added to ensure the Cr content meets the requirements. Each process and node in the smelting process is strictly controlled, with real-time computer tracking. Chemical element content is tested using pipeline sampling + spectral analysis + gas analysis, and only after meeting the requirements is the process moved to the next step, effectively ensuring the chemical composition meets the standard requirements.
The refining process involves continuous argon blowing. Before adding alloys, the oxidation level must be reduced to below 7×10⁻⁶. Silicon-aluminum-calcium-barium refining agents are used to spheroidize inclusions, allowing them to fully float. The soft blowing time before tapping is more than 10 minutes. During pouring, an argon ring is used to protect the pouring process, reducing oxidation. Additionally, an argon sliding gate is used to ensure automatic pouring, reducing the entry of oxides into the mold cavity caused by entrance, thereby ensuring the quality of the casting.

2.4 Heat Treatment Process
The main difficulty in the heat treatment of this casting is the reasonable design of the chemical composition and achieving the desired mechanical properties and microstructure under the customer-specified normalizing heat treatment method. Additionally, preventing deformation during heat treatment is crucial. Once the casting deforms, it is difficult to correct through mechanical or thermal methods. Therefore, historical data from similar products and specialized STECAL heat treatment simulation software are used to optimize the simulation, ultimately determining a reasonable internal control composition.
To achieve good mechanical properties and prevent deformation during heat treatment, a normalizing air cooling + tempering process is adopted. The normalizing temperature must be appropriately selected. The purpose of normalizing heat treatment is to fully dissolve the precipitates in the casting, form fine-grained austenite after austenitizing, and form a fine F+P structure after cooling. After normalizing, medium-high temperature tempering is performed to promote the precipitation of a small amount of carbon in the form of carbides and reduce residual stress in the casting, ensuring dimensional stability in later stages. To ensure temperature control during heat treatment, in addition to using a computer-controlled natural gas heat treatment furnace for temperature control, more than three thermocouples are attached to the casting to track the actual process temperature.

2.5 Welding Process
Through visual and ultrasonic testing, defects found in the casting are primarily repaired using manual electrode arc welding. All easily deformable areas are welded using electrode arc welding. During welding preheating, natural gas pipe flame preheating is used to preheat the welding surface to 50-60°C, removing moisture from the casting surface. Symmetrical welding by zones is adopted, controlling each weld bead width to less than 30 mm and each layer thickness to less than 3 mm. The interpass temperature during welding is controlled to less than 100°C. For large welding volumes, a welding rod with a diameter of ¢4.0 mm is used, with welding current controlled between 120-160 A. During welding, dial indicators are installed on the back of the welding area and other parts of the casting for measurement and monitoring. If the deformation exceeds 0.5 mm, welding is stopped and resumed only after the dimensions are restored. The welding sequence is from the inside out. After the internal and external cavity dimensions and defect treatment are qualified, the entire casting is subjected to stress relief post-weld heat treatment.

3. Conclusion
By analyzing and identifying the characteristics and difficulties of the casting product, optimizing the design of key processes, and strictly controlling the process, the product's dimensions, appearance, composition, mechanical properties, and NDT defects all meet the performance requirements. Additionally, it has strong competitiveness in production costs, delivery cycles, and process efficiency.

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