Manufacturing Technology of Large Steel Girth Gears for Mine Mills
Manufacturing Technology of Large Steel Girth Gears for Mine Mills
Jan 02, 2025
Manufacturing Technology of Large Steel Girth Gears for Mine Mills
Introduction to Domestic and International Situations
Large girth gears are the core transmission components of mine milling machines, and their reliability is related to the safety and stability of the entire equipment's operation. With the continuous increase in equipment specifications and transmission power, large gears are also developing towards larger sizes, with diameters exceeding 10 meters and weights approaching hundreds of tons. To meet customers' requirements for high lifespan, high reliability, and high efficiency of mine milling machines, the requirements for non-destructive testing and mechanical properties of large girth gears are also increasing. Both domestic and foreign manufacturers have spent a huge cost to optimize the production process of large girth gears. At present, there are four main types of large girth gears for mine milling machines: cast steel large girth gears, welded large girth gears, ductile iron large girth gears, and sectional large girth gears. The production process of cast steel large girth gears is relatively mature, and the recognition of cast steel large girth gears by domestic and foreign mine mill equipment users is relatively high. However, cast steel parts require a large amount of solidification feeding, need to set larger risers, have low steel utilization rate, long production cycles, and it is difficult to completely eliminate the micro-porous defects on the tooth surface after hobbing. Welded large girth gears are generally made by welding alloy steel rim and low carbon steel plate, web plate together. Since the rim is a rolled steel plate, the problem of rim density is solved. However, due to the welding of different steels, the welding technology is more difficult, especially for the tooth circle with a hardness requirement of more than 300HB. How to overcome the poor weldability of materials and ensure the welding quality is the key to obtaining qualified welded girth gears. In addition, according to customer site understanding, welded large girth gears are more rigid than cast steel large girth gears, easy to deform, and difficult to install and debug. Relevant literature shows that welded structure girth gears are generally used for medium and small girth gears with a diameter of 900mm-5000mm. Ductile iron large girth gears have better forming performance than cast steel girth gears. Ductile iron has less shrinkage than steel, which can eliminate the large risers of cast steel girth gears, has a high yield rate, and low cost. Some foreign foundries have already produced ductile iron large girth gears with a diameter greater than 10m. However, the casting technology of ductile iron large girth gears with a wall thickness greater than 200mm and iron quantity of hundreds of tons is difficult, mainly because the production process of large section ductile iron parts, due to the large amount of iron, long cooling time, slow solidification speed, and fast degradation, is prone to graphite floating and graphite variation. At the same time, due to the slow cooling speed, the matrix organization is prone to precipitate a large amount of ferrite during the eutectoid transformation, which reduces the strength and hardness of the casting. In addition, the difficulty of welding and repairing defects in ductile iron castings is large, and these series of problems are important obstacles to its promotion and application in large girth gears. Sectional large girth gears have solved the problem of local failure and section replacement of girth gears well, and at the same time, they have got rid of the dependence on large casting and processing equipment in the manufacturing process. However, due to the large number of sections, the integrity of the casting structure and the uniformity of material properties are not as good as the above three forms. Considering the impact of the cumulative processing error caused by too many joint surfaces on the gear transmission, the manufacturing and installation accuracy of each section is required to be very high, which also limits its application in large sizes. The four types of large girth gears all have their own advantages, but also their own shortcomings. The choice of which form depends on the technical maturity of the designer and manufacturer in this field.
Material Selection and Standards
Cast steel large girth gears use different materials according to different product types, specifications, and working conditions. Common materials include ZG42CrMo, ZG45CrMo, ZG34Cr2Ni2Mo, ZG35CrNiMo, ZG40CrNi2Mo. The design and manufacturing of large girth gears comply with international standards such as AGMA, ISO, ASTM, AWS, DIN. Among them, ultrasonic non-destructive testing is carried out in accordance with ASTM A609, and the area division is shown in Figure 1. Zone A: The area 25.4mm below the gear outer circle to the root of the tooth, accepted according to ASTM A609 Level 1. Zone B: Except for Zone A, the rim part (Figure 1 B1 area) and the inner flange surface (Figure 1 B2 area), accepted according to ASTM A609 Level 2. Magnetic particle testing of the machined surface is carried out according to ASTM E709 standard, with linear and non-linear ≤5mm accepted. At present, cast steel large girth gears can all meet the above requirements and reach the international advanced level. Figure 1 Ultrasonic Testing Area Division of Cast Steel Large Gear
Casting Technology
According to the size of the gear, the design structure of cast steel large girth gears is divided into whole circle structure, 1/2 structure, 1/4 structure. Casting can be carried out according to the size of the molding pit, smelting capacity, and on-site lifting equipment capacity, using whole circle casting and sectional casting. 3.1 Riser Design The design of risers for cast steel parts must meet two requirements: one is that the riser must solidify later than the casting, and the other is to have enough feeding steel. As we all know, the weight of the riser for steel castings is about 50%-100% of the weight of the casting, which means that 1/3-1/2 of the steel liquid produced by the cast steel workshop is consumed by the riser. To meet the use requirements of large girth gears, the riser design must ensure the quality of the gear tooth area and the inner circle connecting flange, and meet the non-destructive testing requirements after rough machining. The risers for large girth gears are mainly arranged on the outer rim and the inner circle flange. The size of the riser is determined according to the hot node circle and modulus, and the riser is adjusted and optimized by solidification simulation using the MAGMA simulation software. Through many years of production research, the design form of the riser is also constantly optimized. To ensure the quality of the tooth area, the outer rim riser has been optimized from the early scattered riser to the annular riser. The form of the riser is shown in the following figures: Figure 2 Traditional Scattered Riser Design Scheme Figure 3 New Annular Riser Design Scheme 3.2 Subsidy and Chill Design In the casting process of cast steel parts, casting subsidies are often designed. There are usually two functions of subsidies. First, the structure of castings is complex and diverse, and they often cannot achieve sequential solidification. In order to achieve sequential solidification requirements, the local shape of the casting needs to be changed. By increasing the thickness at the riser, the wall thickness near the riser is thick, and the wall thickness far from the riser is thin, so that the solidification time near the riser can be extended. Second, setting subsidies in steel castings can increase the expansion angle of the feeding channel, increase the riser feeding channel, and facilitate the smooth flow of the feeding steel from the riser to the casting feeding place, improving the feeding efficiency of the riser. The casting subsidies for large girth gears are generally designed on the outer rim, and there are two forms of subsidies: external subsidies and internal subsidies. External subsidies can be removed by thermal cutting and processing later, but the design on the outer circle increases the thickness of the outer circle. After hobbing, the tooth area is located in the center of the casting, the organization is not dense, and the quality is difficult to ensure. At the same time, external subsidies are also not conducive to the layout of process chills. Therefore, internal subsidies are now mostly used for casting large girth gears. The comparison of MAGMA temperature field solidification simulation with and without internal subsidies for large girth gears is shown in Figures 4 and 5. It can be seen that reasonable subsidies make the casting solidification feeding more sufficient and the temperature field more reasonable. Figure 4 Schematic Diagram of Internal Subsidy on the Rim of Cast Steel Large Gear Without Subsidy With Subsidy Figure 5 Comparison of Temperature Field with and without Subsidy for Cast Steel Large Gear Chills have a cooling effect, and the setting of chills can increase the thickness of the dense layer on the surface of the casting. Chills are often set in key areas of part use. The outer circle tooth area of the large gear is a key meshing area during use and is also an area where fatigue failure is likely to occur. To ensure the density of the tooth area organization after hobbing, chills are often set on the outer circle in the gear casting process. The layout of chills for large girth gears is shown in Figure 6. Figure 6 Schematic Diagram of Chill Layout on the Outer Rim of Cast Steel Large Gear 3.3 Pouring Scheme Design According to the height of the gear, one or two layers of pouring systems are selected. The lower pouring system uses the bottom return form, which can ensure the steady rise of the pouring steel, avoid the occurrence of turbulence, and is conducive to ensuring the internal quality of the casting. To achieve rapid pouring and reduce the high-temperature radiation time of the mold cavity by the molten steel, while ensuring the steady filling of the steel, it is required that the pouring system settings must be fully open when the steel gate, straight pouring channel, cross pouring channel, and internal pouring channel are set. 3.4 Model Making and Molding The diameter of the large gear for mine milling machines is relatively large. To ensure the size of the mold cavity and the quality of the casting, the molding method of the large gear adopts the form of real sample + group core. The surface of the sand core is selected with high refractoriness chromite sand, and the surface of the sand mold is coated with alcohol-based zirconia powder paint.
Smelting Technology
The smelting of steel is carried out by electric arc furnace primary smelting + LF refining. To improve the purity of the steel, reduce harmful inclusions, and achieve the purpose of refining the grains, the following measures are mainly taken in the steel smelting process:
Internal control of chemical composition + micro-alloying to ensure the uniformity and stability of casting performance.
Strict control of harmful elements such as S, P (S≤0.010%, P≤0.015%) to minimize the impact of S, P elements on casting quality.
The steel is treated by vacuum degassing (VD) to improve the purity of the steel.
Argon blowing in the mold cavity before pouring, and argon blowing protection at the steel ladle pouring mouth during pouring to reduce the secondary oxidation of the steel during pouring.
Heat Treatment Technology
The heat treatment of cast steel large girth gears is mainly divided into post-casting heat treatment (i.e., pre-heat treatment) and performance heat treatment. Post-casting heat treatment is a heat treatment process of high-temperature annealing or normalization + tempering carried out in the rough cleaning stage of the casting blank. The main purpose of post-casting heat treatment is to refine the grains, uniform the organization, eliminate casting stress, adjust the hardness, improve the cutting performance, and prepare the organization for later quenching. In addition, because the large gear needs to undergo ultrasonic testing after rough turning, the purpose of post-casting heat treatment is also to improve the ultrasonic transmission of the casting to provide conditions for ultrasonic testing. Performance heat treatment is carried out after rough machining, that is, quenching + high-temperature tempering. Performance heat treatment can bring the material performance into full play. Quenching makes the cooling speed of each part of the casting uniform, the hardening layer deep, and the hardness gradient small. High-temperature tempering can fully eliminate internal stress and ensure dimensional stability. After tempering, the cast steel large gear obtains uniform mechanical properties, and the hardness difference of the casting can be kept within 30HB, and the heat treatment deformation can be controlled within 10mm. The metallographic organization after post-casting heat treatment and performance heat treatment is shown in Figure 7. Figure 7 Metallographic Organization After Post-casting and Performance Heat Treatment
Conclusion
Advanced and strict steel smelting technology and casting process technology are the effective guarantee for the internal quality of cast steel large girth gears for mine milling machines.
Post-casting heat treatment and performance heat treatment are more conducive to the improvement of the comprehensive performance of materials and are the effective guarantee for the cast steel large girth gears of mine milling machines to meet the use performance.
Cast steel large girth gears for mine milling machines are precision transmission components. The guarantee of quality requires the support and connection between casting, smelting, and heat treatment professionals. At the same time, it is also necessary to feedback the installation, use, and maintenance situation of the customer site to the manufacturing factory to accumulate data for continuous improvement.
