Rotary kiln has been in existence for a hundred years and is widely used in building materials, metallurgy, steel, lime, environmental protection, energy and other industries. It is an indispensable equipment for dry process cement production. From the beginning, China has relied on imports to localize rotary kilns, and then exported to the international market in large quantities. Now it has become the country with the largest production, sales and usage in the world. The relevant standards for rotary kilns stipulate the requirements for the manufacture and installation of rotary kilns, but a rotary kiln that meets the standard requirements may not be a perfect rotary kiln.

1.1 Cylinder material

 The rotary kiln shell is one of the main components of the rotary kiln, and its steel consumption accounts for about half of the entire rotary kiln. The barrel of the rotary kiln is a long cylindrical barrel formed by rolling and welding steel plates, as shown in Figure 2. For a long time, the carbon structural steel Q235B (original grade A3) or the higher quality Q235C has been generally selected for the cylinder material. Low-alloy high-strength structural steel Q345D (original grade 16Mn) or boiler steel Q245R (original grade 20 g) is widely used due to its higher yield strength or lower content of harmful chemical components and higher comprehensive properties.

It is not uncommon for the rotary kiln to be broken and severely corroded due to the high harmful chemical composition of the materials used. Three precalcining kilns in a cement plant in Xinjiang have cylinder body cracks at the large gear respectively, and the kiln cylinder body cracks near the large gear in the transition zone. The kiln body breaks from the second gear wheel belt to the kiln tail section, and the kiln tail section falls off the kiln platform Several cracks in the kiln shell were found in a precalciner kiln of a cement plant in Jidong; after 2013, vicious accidents of kiln shell fragmentation occurred successively in many cement plants. With the stricter control of the harmful chemical components of Q245R and Q345D in national standards, in 2018, the low-alloy high-strength structural steel Q345D was replaced by Q355D with better comprehensive performance and less harmful chemical components corresponding to EU standards.

The reduction of harmful chemical components reduces the cold brittleness and hot brittleness of the steel plate, and improves the ductility, toughness, weldability and cold bendability, which further improves the rigidity, cold state and hot state performance of the rotary kiln cylinder, and effectively reduces the thermal conductivity of the cylinder. the possibility of local fragmentation of the state.

In view of the importance of the rotary kiln shell, the sensitivity of materials to cold brittleness and hot brittleness, and the particularity of the use environment, it is preferable to choose Q245R and Q355D materials with low content of harmful chemical components such as P and S to make the rotary kiln shell to prevent and avoid it. effective way of fragmentation. Material Q235C can be selected from the perspective of economics alone; Q245R should be preferred from the perspective of cold brittleness and hot brittleness sensitivity; both from the perspective of cold brittleness and hot brittleness sensitivity, and the mechanical properties of the material, the material should be preferred Q355D. The choice of the cylinder material directly affects the cost of the rotary kiln, and the owner should participate in the material selection according to the price affordability.

1.2 Shaft materials such as idler shafts

The standard stipulates that the shaft materials such as the rotary kiln support shaft should not be lower than the requirements of high-quality carbon structural steel [4] 45 steel. Due to the low cost of 45 steel, most manufacturers choose 45 steel for shaft materials such as idler shafts instead of alloy structural steel 40Cr steel.

The material price of 40Cr steel is 5% to 10% higher than that of 45 steel. The mechanical properties of 45 forged steel are roughly equivalent to 40Cr steel, the difference lies in the heat treatment performance. The quenched and tempered 40Cr steel has good comprehensive mechanical properties, good low temperature impact toughness, hardenability and low notch sensitivity. Due to the improved hardenability, the strength, hardness, impact toughness and other mechanical properties of 40Cr steel after quenching (or quenching and tempering) treatment are also significantly higher than those of 45 steel.

The hardenability and comprehensive mechanical properties of 40Cr steel after quenching and tempering are better than those of 45 steel, and it is easy to control the mechanical properties such as strength, hardness and impact toughness after quenching and tempering. Accidents of important shaft parts of rotary kiln breaking due to mechanical properties are not rare. Therefore, it is recommended that the material of important shafts such as supporting roller shaft, pinion shaft and retaining shaft should be at least 40Cr steel with excellent heat treatment performance and high cost performance.

For particularly important shaft parts (such as pinion shafts) that have high static strength and impact toughness, high fatigue limit, and work requirements under high load, better hardenability and superior performance than 40Cr steel can be selected. The alloy structural steel 35CrMo, 42CrMo and other materials.

1.3 Belt and idler blanks

The standard stipulates that the material of the rotary kiln wheel belt supporting roller is cast steel. For example, the current national standard GB/T32994-2016 "Rotary Kiln for Cement Industry" [7] stipulates that the material of the wheel belt is not less than ZG35Mn, and the material of the supporting roller is not low. in ZG35Cr1Mo. Large-scale alloy steel castings manufactured by conventional manufacturing methods are difficult to ensure that there are no casting defects, but casting defects are very harmful, and the damage caused by improper repair welding is also very large. From the point of view of improving product quality, forged wheels can be preferred according to the cost performance.

The performance of forgings is higher than that of castings, mainly because the low cooling rate of solidification of castings usually leads to coarse as-cast grains and intragranular structure, and it is difficult to avoid macro and micro segregation, and there are often defects such as looseness and concentrated distribution of impurities. In the forging process, the plastic deformation and flow of the metal material are caused by the force applied to the casting billet, and then the material structure is dense, the composition is uniform, the grains are refined, and the impurities are uniformly distributed, which significantly improves the mechanical properties compared with the conventional as-cast structure. .

According to the domestic forging capacity and forging level, the supply of the blank can be completed by the forging method for the tire supporting roller of the rotary kiln below Φ4 m, but there is a difference in the material price of the forgings.

For a rotary kiln with a size of Φ4 m or more, the price of the forged material for the tire support roller is about 30% higher than that of the cast material; for a rotary kiln with a size of Φ4 m, the material price of the tire support roller for casting and forging is the same, which is the balance point of the price. ; For a rotary kiln with a specification below Φ4 m, the price of the forged material of the wheel belt supporting roller is slightly lower than that of the cast material by about 5%. For small-scale rotary kilns, the time is basically ripe for the replacement of wheel support roller castings into forgings.