1. Introduction
Bucket teeth are crucial components on earth-moving machinery such as excavators and bulldozers and are also consumable parts. According to different processing techniques, bucket teeth can be divided into cast bucket teeth and forged bucket teeth. With the increasing emphasis on environmental protection and the call for low-carbon and green development, cast bucket teeth are gradually being phased out. Forged bucket teeth avoid defects such as carbon increase, sand sticking, wrinkling, slag inclusion, high gas content, and low density associated with the casting process and have a longer service life. Therefore, the use of forging processes to produce bucket teeth has become an inevitable trend in the development of the bucket teeth industry.
2. Forging Bucket Teeth Production Process and Performance Requirements
2.1 Forging Process
The forging process flow of bucket teeth is as follows: round steel cutting → heating → head forging molding → first funnel punching → second funnel punching (final shaping) → forging burr removal → pin shaft hole punching → marking → heat treatment. During the forging process, a large pressure is applied to the metal billet using a forging press, and it is extruded and formed at a high temperature to refine the grains in the forging, causing plastic deformation to obtain certain mechanical properties and improve its microstructure. Then, quenching + tempering heat treatment is carried out to ensure that the forged bucket teeth have good mechanical properties, wear resistance, and a longer service life.
2.2 Failure Modes
Bucket teeth directly contact materials such as ores, sand, and soil. During service, they not only have to withstand the impact of materials but also a certain bending moment. At the same time, the sliding of abrasives at the tip of the bucket teeth causes various furrows on the surface, resulting in wear or detachment. Among the failure modes of bucket teeth, 90% – 95% are wear failures, which lead to a reduction in the size of the bucket teeth until they are worn through and damaged and can no longer work; 5% – 10% are fracture and deformation failures, which generally occur due to improper material selection or extremely harsh working conditions, and some parts of the bucket teeth directly fracture and fail after a short period of service.
There are two main failure mechanisms in bucket teeth wear failure:
- Cutting mechanism:
- Under high-impact working conditions, when the bucket teeth first contact materials (such as rocks and ores), there is a large impact force, causing a certain plastic deformation at the tip of the bucket teeth and forming a plastic deformation furrow.
- When inserted into the material, hard abrasive grains move rapidly on the surface of the bucket teeth under the action of an external force, and the hard edges plow the surface of the bucket teeth into grooves, accompanied by the generation of microscopic cutting debris.
- The friction heat and deformation latent heat generated during service cause a sudden increase in the temperature of the chips, changing the internal microstructure of the chips, and in severe cases, local melting may occur.
- Fatigue peeling mechanism: Under low-impact working conditions, the abrasive grains (such as loess, limestone, and oil shale) are very soft. When the bucket teeth are inserted into the abrasive pile, the abrasive grains only slide on the metal surface of the bucket teeth, and the cutting effect on the metal surface is very small. At this time, the wear of the bucket teeth is mainly caused by the alternating fatigue stress of stress and strain in a local area for a long time.
2.3 Bucket Teeth Performance Requirements
The two mechanisms of bucket teeth wear failure require that bucket teeth have a good combination of hardness and toughness while having high wear resistance. The comprehensive requirements for bucket teeth material selection are as follows:
| Performance Requirement | Specification |
|---|---|
| Hardness | The matrix hardness value should not be lower than 46 HRC |
| Toughness | The impact absorption capacity value should be higher than 20 J |
| Hardenability | High hardenability is required to ensure a uniform microstructure of the bucket teeth |
| Microstructure | The austenite grain size should not be coarser than 5 levels |
3. Production Technology of Steel for Forging Bucket Teeth
3.1 Variety and Composition Design
Taking into account factors such as bucket teeth performance requirements, production costs, and production processes, the alloy elements of hot-rolled round steel for forging bucket teeth are mainly Si, Mn, and Cr, and a small amount of microalloying elements such as Mo, V, Ti, and Nb can be appropriately added to improve its comprehensive performance. Typical variety series can be divided into SiMn series, SiMnCr series, SiMnCrMo series, etc.
- Carbon content: The carbon content in steel mainly affects the morphology of the martensite structure obtained after quenching. Low-carbon steel (≤0.25%) obtains a lath martensite structure after quenching, which has high toughness and a certain hardness (40 – 50 HRC) but low wear resistance. High-carbon steel (≥0.60%) obtains a structure mainly composed of plate martensite after quenching, which has high hardness (≥60 HRC), high brittleness, and poor toughness. Medium-carbon steel (0.25%-0.60%) obtains a mixed martensite structure after quenching, which has sufficient toughness while ensuring high wear resistance. For steel used for bucket teeth, to obtain more lath martensite structure as much as possible while ensuring high wear resistance, the carbon content is determined to be 0.27%-0.40%.
- Silicon content: When silicon is used as an alloy element, it exists entirely in the form of a solid solution in ferrite, playing a role in solid solution strengthening, which can significantly improve the strength and hardness of steel and thus improve the wear resistance of steel. Moreover, silicon is a relatively inexpensive element, so it is a major alloy element for improving wear resistance in steel for bucket teeth. However, if the silicon content is too high, it will significantly reduce the plasticity and toughness of steel and increase the decarburization tendency during subsequent heat treatment processes. Therefore, the silicon content is determined to be 0.60%-1.60%.
- Manganese, chromium, and molybdenum content: These three elements can significantly improve the hardenability of steel and are essential elements for bucket teeth that require quenching treatment. However, the hardenability is related to the size of the bucket teeth and their service conditions. Therefore, the content ranges of manganese, chromium, and molybdenum should be reasonably designed according to user needs. Generally, the manganese content is 0.80%-1.50%, the chromium content is 1.10%-2.00% , and a trace amount of molybdenum element is added only when necessary, with a range of 0.10%-0.20%.
- Vanadium, titanium, and niobium content: Vanadium, titanium, and niobium are all elements that form strong carbides and nitrides, which can refine grains, improve toughness, and appropriately improve wear resistance. Therefore, one or a combination of these three elements can be appropriately added in a small amount. When a single element is added alone, the vanadium content is 0.05%-0.15%, the titanium content is 0.05%-0.10%, and the niobium content is 0.020%-0.070%.
3.2 Performance Indicators
Among the various performance indicators of hot-rolled round steel for bucket teeth, in addition to the mechanical properties and hardenability determination indicators that directly affect wear resistance, the strict control of other physical performance indicators also has a significant impact on the processing and service life of bucket teeth. Therefore, for hot-rolled round steel for bucket teeth, control requirements for low-magnification microstructure, grain size, non-metallic inclusions, and delivery hardness are also proposed.
- Low-magnification microstructure: The shape of the bucket teeth finished product is irregular, and the forging process is long and complex. More serious defects such as central porosity can lead to problems such as forging cracking during the forging process. Therefore, for hot-rolled round steel for bucket teeth, it is required that general porosity, central porosity, ingot segregation, and central segregation should all be ≤2.0.
- Grain size: The fine and uniform grains in steel are beneficial for improving the comprehensive performance of steel and making the heat treatment deformation more uniform. Therefore, it is required that the austenite grain size of hot-rolled round steel for bucket teeth should not be coarser than 6.0 levels.
- Non-metallic inclusions: Non-metallic inclusions represent the cleanliness of steel and directly affect the fatigue performance and service life of bucket teeth. Therefore, the non-metallic inclusions in steel should meet the requirements specified in Table 1.
- Delivery hardness: In the bucket teeth forging process, the round steel cutting process must be carried out. When the hardness of the hot-rolled round steel is too high, there are difficulties in sawing or shearing for cutting. Therefore, it is required that the delivery hardness of the round steel should not exceed 280 HBW. For steel for bucket teeth with a high alloy content, annealing treatment should be carried out to meet the requirements of delivery hardness.
3.3 Process Difficulty Control
The production process flow of hot-rolled round steel for bucket teeth is as follows: electric furnace smelting → LF refining → VD vacuum degassing → continuous casting → rolling → annealing (for some varieties). To ensure the service life of bucket teeth, during the smelting process, emphasis should be placed on cleanliness control. At the same time, for some high-alloy steels for bucket teeth, the casting billet heating process and annealing process should be well controlled.
- Cleanliness control: During the entire process of electric furnace smelting, a foaming slag is formed to ensure good oxidation boiling. The decarburization and heating rate are reasonably controlled to ensure the phosphorus removal effect at the steel-slag interface through the carbon-oxygen reaction. Before P reaches the required level, the carbon in the furnace should not be less than 0.15%, and the temperature should not exceed 1600°C. Before the electric furnace taps steel, the C content is controlled between 0.10%-0.12%, and the P content should not be greater than 0.012%. When the electric furnace taps steel, aluminum blocks are added according to 1.0 kg per ton of steel along with the steel flow for strong deoxidation, reducing the dissolved oxygen content to below 20X10^-6, ensuring the deep desulfurization conditions at the refining position. In LF refining, a carbon-increasing agent is used to adjust the slag, and a large amount of silicon carbide is prohibited from being used to adjust the slag to reduce the slag basicity, maintaining the appropriate fluidity of the slag and ensuring that the slag basicity is ＞3.0. Aluminum wire is fed to reduce the dissolved oxygen content and enhance the desulfurization effect, strictly controlling the S content to 0.010%. During the VD vacuum treatment process, the vacuum holding time is ensured to be ≥15 min, ensuring that the O content is 0.0015%. At the same time, continuous casting protection pouring control is carried out well.
- Heating process control: In some steels for bucket teeth, the alloy contents of Cr, Mo, etc. are relatively high, and the dendritic segregation phenomenon of the casting billet is relatively serious, and the thermal conductivity is poor. In the early stage of casting billet heating, if the heating speed is large, the temperature difference between the inside and outside of the casting billet is large, which will generate a large temperature stress. Coupled with the residual stress inside the casting billet, it is easy to cause the deformation or fracture of the casting billet. Therefore, the heating speed of the casting billet adopts a process system of slow heating in the preheating section (＜600°C) and rapid heating in the high-temperature section. At the same time, to ensure the uniform diffusion of alloy elements in the center of the casting billet, the holding time of the casting billet in the high-temperature section can be appropriately extended.
- Annealing process control: In some steels for bucket teeth, the alloy content is relatively high, and the hardness of the hot-rolled round steel in the hot-rolled state can reach about 330 HBW. To meet the requirements of delivery hardness, annealing treatment is required. An incomplete annealing process is adopted, and the holding time is 3 – 6 h, ensuring that the hardness of the annealed steel is controlled within the range of 210 – 260 HBW.
4. Conclusion
Through the analysis of the forging process, service conditions, failure modes, and performance requirements of bucket teeth, Laiwu Branch of Shandong Iron and Steel Co., Ltd. has reasonably designed the chemical composition, performance indicators, and production process key points of hot-rolled round steel for forging bucket teeth. The developed hot-rolled round steel for forging bucket teeth has high cleanliness, fine and uniform grains. Through different alloy composition designs, it is ensured that bucket teeth can meet the use requirements of different working conditions such as low impact and high impact, and the forged bucket teeth have high wear resistance and a good service life.
In the production of bucket teeth, continuous research and improvement are needed to further optimize the production process and improve the performance of bucket teeth to better meet the needs of the market and users. For example, further exploration of new alloy elements and their combinations may lead to better performance improvements. Additionally, advanced manufacturing technologies and quality control methods can be introduced to ensure the stability and reliability of product quality. The development of bucket teeth production technology is an ongoing process that requires continuous attention and investment.