1. Introduction
1.1 Background and Significance
Excavators play a crucial role in various engineering projects. Bucket teeth, as an important part of excavators, are constantly in contact with sand, soil, and rocks, resulting in severe wear and failure. Therefore, improving the wear resistance of bucket teeth has become a research hotspot. This study focuses on the heat treatment process and microstructure of 30Cr2MnSi steel for bucket teeth to enhance its wear resistance.
1.2 Research Objectives
The main objectives of this research are as follows:
- To analyze the wear resistance of different bucket teeth materials through testing and comparison.
- To study the influence of heat treatment methods on the microstructure and properties of bucket teeth materials.
- To optimize the heat treatment process to improve the wear resistance of 30Cr2MnSi steel for bucket teeth.
2. Experimental Materials and Methods
2.1 Materials
- Bucket Teeth Samples: Samples of bucket teeth from Komatsu (including genuine products, imitations, and cheap products), as well as forged bucket teeth from Wanxin, were used for testing.
- 30Cr2MnSi Steel: This steel was used for further research on heat treatment improvement. Its chemical composition was analyzed, and it was initially composed of pearlite and ferrite.
2.2 Testing Methods
- Mechanical Property Tests:
- Toughness Test: Samples were prepared and tested using a specific process and equipment to measure toughness at -40°C.
- Tensile Property Test: Round bar samples were taken and processed, and tensile properties were tested using a servo universal testing machine.
- Hardness and Wear Performance Test: Samples were prepared, and hardness was measured using a hardness tester. Wear tests were carried out using a wear testing machine under specific conditions.
- Microstructure Analysis:
- Metallographic Sample Preparation: Samples were cut, ground, and polished for microstructure analysis.
- Component Analysis: The composition of samples was detected using a spectrometer.
- Optical Microscope Analysis: The microstructure was observed using an optical microscope.
- Scanning Electron Microscope Analysis: The microstructure and fracture morphology were analyzed using a scanning electron microscope, and the composition of the fracture surface was analyzed using an energy dispersive spectrometer.
3. Comparison of Wear Resistance of Different Bucket Teeth
3.1 Composition Comparison
The chemical compositions of different bucket teeth were detected. The results showed that the alloy element content in Komatsu genuine products was relatively low, belonging to low-alloy steel, and the carbon content was also low, belonging to low-carbon steel. In contrast, other bucket teeth had higher carbon and alloy element contents. The nickel content in Komatsu genuine products was relatively high, which could improve the toughness and wear resistance of the material.
3.2 Microstructure Comparison
- Komatsu Genuine Products: The microstructure of Komatsu genuine bucket teeth showed obvious segregation under an optical microscope, with dendritic and cellular structures. The main microstructure was tempered lath martensite, and the lath size affected the toughness.
- Komatsu Imitations: Similar to genuine products, there was also segregation, but the dendritic diameter was larger. The microstructure was mainly lath martensite, but the lath was coarser.
- Wanxin Forged and Komatsu Cheap Products: The microstructure of Wanxin forged products was uniform, with no obvious segregation, while the Komatsu cheap products had obvious segregation. The microstructure of Wanxin forged products showed finer martensite laths, while the Komatsu cheap products had longer and coarser martensite laths, and some had tempered acicular martensite due to higher carbon content.
3.3 Mechanical Properties and Wear Resistance
- Hardness and Tensile Strength: The hardness of most bucket teeth was around 50HRC, except for one sample with a lower hardness due to its different alloy element content. The tensile strength of most bucket teeth was around 1700MPa. The hardness and tensile strength were generally positively correlated, but factors such as segregation and microstructure also affected the results.
- Impact Toughness: The impact toughness of different bucket teeth varied significantly. In Komatsu genuine products, the toughness of some samples was lower due to segregation and larger martensite laths, while others had better toughness. The Wanxin forged products had good toughness due to uniform microstructure and fine grains. The Komatsu imitations had lower toughness due to higher carbon equivalent and coarse microstructure.
- Fracture Morphology: The fracture morphology of most bucket teeth was mainly quasi-cleavage fracture, with dimples, tearing edges, and cleavage platforms. The size and distribution of second-phase particles affected the dimple size. The Wanxin forged products had a more uniform fracture morphology with smaller dimples, indicating higher toughness.
- Wear Resistance and Wear Morphology: The wear resistance of different bucket teeth was evaluated by measuring the wear amount. In Komatsu genuine products, some samples had better wear resistance due to less segregation and finer martensite laths, while others had poorer wear resistance. The wear mechanisms included abrasive wear, adhesive wear, and fatigue wear. The wear morphology showed ploughing, adhesion, and fatigue regions. The Wanxin forged products had better wear resistance due to uniform microstructure and fine grains, while the Komatsu cheap products had poorer wear resistance due to coarse microstructure and obvious segregation.
3.4 Summary
- The wear resistance of bucket teeth materials is related to toughness. When the hardness is similar, higher toughness leads to better wear resistance.
- Segregation in the microstructure reduces the toughness and wear resistance of bucket teeth materials.
- The wear mechanism and morphology of bucket teeth materials are affected by their microstructure and mechanical properties.
4. Secondary Normalizing Heat Treatment Process
4.1 Experimental Scheme
Based on the analysis of the influence of segregation on the wear resistance of Komatsu bucket teeth, a secondary normalizing process was added to the heat treatment process of Komatsu cheap products. Different high-temperature normalizing holding temperatures and times were set to analyze the influence on the microstructure and wear resistance of the material.
4.2 Influence on Microstructure
- Before secondary normalizing, the sample had serious segregation and coarse martensite laths.
- After secondary normalizing at different temperatures and times, the segregation was significantly reduced. At 1020°C for 5h, the segregation could be effectively eliminated, and the microstructure became more uniform. At higher magnifications, it could be seen that the material was mainly composed of tempered lath martensite, and the lath became shorter and the martensite bundle became smaller compared to the sample without secondary normalizing. The martensite laths were the finest when the holding time was 1h, and they became coarser with the increase in holding time.
4.3 Influence on Hardness, Toughness, and Wear Resistance
- Hardness: The hardness of the sample increased slightly after secondary normalizing, mainly due to the refinement of grains and the strengthening effect of alloy elements.
- Toughness: The impact toughness of the sample at -40°C was tested. The results showed that adding a suitable secondary normalizing process could improve the toughness of the material. The toughness increased significantly when the holding temperature was 1020°C, and it was affected by factors such as the dissolution of carbides and the uniformity of the microstructure.
- Fracture Morphology: The fracture morphology of the sample changed significantly after secondary normalizing at 1020°C. The cleavage plane disappeared, and more and larger dimples appeared, indicating an increase in toughness. At higher temperatures and longer holding times, the fracture morphology showed intergranular fracture or a large number of tearing edges, indicating a decrease in toughness.
- Wear Resistance and Wear Morphology: The wear resistance of the sample was tested. The results showed that the wear amount of the sample after secondary normalizing at 1020°C was significantly lower than that of the sample without secondary normalizing. The wear mechanism was mainly abrasive wear and fatigue wear, and the ploughing density was smaller and the bulge was more obvious compared to the sample without secondary normalizing. At higher temperatures and longer holding times, the wear resistance decreased, and the wear morphology showed pitting and less obvious bulges.
4.4 Summary
- The secondary normalizing process can effectively eliminate segregation, refine grains, and optimize the microstructure of bucket teeth materials.
- The optimal secondary normalizing condition is 1020°C for 3h, which can significantly improve the wear resistance of the material.
- The wear mechanism and morphology of the material change with different secondary normalizing conditions.
5. Influence of Tempering Temperature on Wear Resistance
5.1 Experimental Scheme
Based on the optimal secondary normalizing and quenching conditions obtained in the previous chapter, different tempering temperatures were set to analyze the influence on the microstructure, mechanical properties, and wear resistance of the material.
5.2 Influence on Microstructure
The microstructure of the material changed with different tempering temperatures. At 120°C and 220°C, the microstructure was mainly tempered lath martensite. At 320°C, the lath became blurred due to the precipitation of carbide flakes. At 420°C, most of the martensite structure had decomposed, and at 520°C and 620°C, the microstructure had completely transformed into tempered troostite and tempered sorbite, respectively.
5.3 Influence on Hardness, Toughness, and Wear Resistance
- Hardness and Toughness: The hardness of the material decreased with the increase in tempering temperature. At 420°C, the hardness was 44HRC, indicating partial decomposition of the martensite structure. The toughness of the material decreased at 320°C due to the appearance of the first type of tempering brittleness, and it increased with the increase in tempering temperature above 320°C.
- Fracture Morphology: The fracture morphology of the material changed with different tempering temperatures. At 120°C and 220°C, the fracture morphology showed more river-like patterns and fewer dimples. At 320°C, the fracture morphology had dimples and a smooth plane. At 420°C, the fracture morphology had larger and deeper dimples, indicating better toughness.
- Wear Resistance and Wear Morphology: The wear resistance of the material was tested. The results showed that the wear amount of the material first decreased and then increased with the increase in tempering temperature. The wear amount was the lowest at 420°C, indicating the best wear resistance. At 120°C and 220°C, the wear mechanism was mainly micro-cutting, and the wear amount was large. At 320°C, the wear surface showed ploughing and fatigue regions, and the wear amount decreased. At 420°C, the wear surface showed obvious plastic deformation, and the wear debris adhered to the surface, reducing the wear amount. At 520°C and 620°C, the wear surface showed wide and deep ploughing, and the wear amount increased.
5.4 Summary
- The tempering temperature has a significant influence on the microstructure, mechanical properties, and wear resistance of bucket teeth materials.
- The optimal tempering temperature is 420°C, which can improve the wear resistance of the material.
- The wear mechanism and morphology of the material change with different tempering temperatures.
6. Conclusion
6.1 Summary of Research Results
- Through the analysis and comparison of different bucket teeth materials, it was found that the wear resistance of bucket teeth materials is related to toughness. When the hardness is similar, higher toughness leads to better wear resistance. Segregation in the microstructure reduces the toughness and wear resistance of bucket teeth materials.
- The secondary normalizing process can effectively eliminate segregation, refine grains, and optimize the microstructure of bucket teeth materials. The optimal secondary normalizing condition is 1020°C for 3h, which can significantly improve the wear resistance of the material.
- The tempering temperature has a significant influence on the microstructure, mechanical properties, and wear resistance of bucket teeth materials. The optimal tempering temperature is 420°C, which can improve the wear resistance of the material.
6.2 Significance and Application Prospects
The research results of this study have important theoretical and practical significance. The optimized heat treatment process can improve the wear resistance of 30Cr2MnSi steel for bucket teeth, which can effectively reduce the wear and failure of bucket teeth, improve the service life of excavators, and reduce maintenance costs. This research can also provide a reference for the development and application of other wear-resistant materials. In the future, further research can be carried out on the optimization of heat treatment processes for different materials and the exploration of new wear-resistant materials to meet the needs of different engineering applications.
| Table 1. Chemical Compositions of Different Bucket Teeth (%) | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sample | C | Si | Mn | Cr | Ni | Cu | Mo | Ti | V | P | S | Ceq |
| G1 | 0.27 | 0.90 | 0.91 | 1.87 | 0.26 | 0.12 | 0.27 | 0.01 | 0.03 | 0.2 | 0.02 | 0.91 |
| G2 | 0.28 | 0.84 | 0.84 | 1.78 | 0.28 | 0.10 | 0.27 | 0.01 | 0.04 | 0.02 | 0.01 | 0.89 |
| G3 | 0.28 | 0.82 | 0.91 | 1.69 | 0.26 | 0.04 | 0.26 | 0.01 | 0.04 | 0.02 | 0.01 | 0.88 |
| G4 | 0.28 | 0.81 | 0.90 | 1.71 | 0.26 | 0.04 | 0.25 | 0.01 | 0.04 | 0.02 | 0.01 | 0.88 |
| 11 | 0.30 | 1.26 | 1.17 | 1.79 | 0.02 | 0.02 | 0.14 | 0.01 | 0.02 | 0.03 | 0.01 | 0.94 |
| 12 | 0.32 | 1.37 | 1.12 | 1.84 | 0.03 | 0.02 | 0.17 | 0.01 | 0.02 | 0.02 | 0.01 | 0.98 |
| 13 | 0.27 | 1.23 | 1.04 | 1.79 | 0.02 | 0.02 | 0.20 | 0.00 | 0.02 | 0.02 | 0.01 | 0.91 |
| 14 | 0.32 | 0.54 | 1.18 | 0.85 | 0.02 | 0.01 | 0.02 | 0.01 | 0.01 | 0.02 | 0.01 | 0.72 |
| W | 0.33 | 1.16 | 0.93 | 1.29 | 0.07 | 0.12 | 0.03 | 0.01 | 0.01 | 0.01 | 0.00 | 0.80 |
| C1 | 0.31 | 1.29 | 0.90 | 1.84 | 0.23 | 0.02 | 0.22 | 0.01 | 0.02 | 0.02 | 0.01 | 0.95 |
| C2 | 0.32 | 1.36 | 0.95 | 1.86 | 0.23 | 0.03 | 0.24 | 0.01 | 0.02 | 0.02 | 0.01 | 0.98 |
| Table 2. Mechanical Properties of Different Bucket Teeth | |||
|---|---|---|---|
| Sample | Hardness (HRC) | Tensile Strength (MPa) | Impact Toughness () |
| G1 | – | – | Around 40 |
| G2 | – | – | Over 50 |
| G3 | – | – | Around 40 |
| G4 | – | – | Over 50 |
| 11 | – | – | – |
| 12 | – | – | – |
| 13 | – | – | – |
| 14 | 42.01 | – | – |
| W | – | – | – |
| C1 | – | – | – |
| C2 | – | – | – |