1. Introduction
Bucket teeth are crucial components in coal mining machinery. They are typically composed of a tooth holder and a tooth tip, connected by a pin shaft. These bucket teeth are installed at the front end of the mining machine’s bucket and directly interact with rocks, ores, and other materials during operation. When cutting materials, significant positive extrusion forces are generated on the surface, resulting in substantial friction between the bucket teeth’s working surface and the materials, subjecting the bucket teeth to impact forces. Prolonged exposure to such impacts during coal mining leads to varying degrees of wear on the bucket teeth surface. A low impact value of bucket teeth can cause tooth breakage during work, resulting in early product failure. This article focuses on analyzing the reasons for the low impact value of bucket teeth and providing corresponding solutions.
2. Experimental Analysis of Bucket Teeth
2.1 Anatomical Scheme
When a sample of the coal mining bucket tooth tip was received, it had been cut off and the cross-section was blue. After removing the burned area, samples were taken from the remaining part. Three mechanical impact specimens were taken side by side in the middle cross-shaped part close to the tooth tip, named No. 1 (one side), No. 2 (middle), and No. 3 (the other side). Additionally, two cross-sectional specimens were taken for metallographic and chemical composition detection.
2.2 Chemical Composition
The chemical composition was tested as a blind sample as the client did not specify the material. The results are shown in Table 1.
| Element | Content (%) |
|---|---|
| C | 0.28 |
| S | 0.007 |
| Mn | 0.9 |
| P | 0.02 |
| Cr | 0.68 |
| Ni | 1.4 |
| Cu | 0.03 |
| V | 0.02 |
| AI | 0.03 |
| Mo | 0.28 |
The test results indicate that the material matches ZWC – ZG30CrMnSiMo, and the chemical composition range requirements for ZWC – ZG30CrMnSiMo are as follows: for carbon (C), 0.25% – 0.35%; for manganese (Mn), 0.6% – 1.6%; for silicon (Si), 0.5% – 1.8%; for chromium (Cr), 0.5% – 1.8%; and for molybdenum (Mo), 0.2% – 0.8%.
2.3 Mechanical Properties
2.3.1 Hardness
The HRC hardness test results and positions on the cross-section of the coal mining bucket teeth are shown in Figure 2. Most of the hardness values were between 49.1 and 52.1 HRC. According to the technical conditions T/CFA 02010204.7 – 2018 for excavator casting bucket teeth, the tooth tip hardness should be ≥ 46 HRC. The tested bucket teeth meet this requirement, but the center hardness is abnormally high, reaching 58.3 HRC.
2.3.2 Impact Test
The impact test was conducted on the specimens at a normal temperature of 20°C. The test results for specimens No. 1, No. 2, and No. 3 were 15.8 J, 17.3 J, and 14.8 J, respectively. The lowest impact value was at the side position of the cross-shaped part of the bucket tooth tip. According to the technical conditions T/CFA 02010204.7 – 2018, an impact value of ≥ 12 J is considered qualified, but the client specified a requirement of ≥ 18 J, so the tested bucket teeth are unqualified.
2.4 Metallographic Analysis
2.4.1 Specimen Morphology after Acid Immersion
The morphology of the specimen after acid immersion is shown in Figure 3. The specimen surface exhibited coarse cast crystal patterns with many small loose holes. The center showed segregation, especially on the hardness test piece inspection surface, where the segregation spot area size reached 4 mm.
2.4.2 Microscopic Structure Analysis
The No. 3 impact specimen was selected for microscopic structure inspection. The microscopic morphology is shown in Figure 4. In the polished state, large particles of foreign slag and holes could be observed (Figure 4a); there were also fragmented sulfide and oxide inclusions, and no large aggregations or large-sized endogenous inclusions were found (Figure 4b). After etching with a 4% nitric alcohol solution, the grain size was 7.0, and the tissue distribution was uneven, showing intergranular network and banded segregation (Figure 4c); the tissue was tempered martensite, with martensite in a needle-like and plate-like mixture, characteristic of typical medium-carbon martensite (Figure 4d).
2.4.3 Microscopic Fracture Test
The No. 3 specimen with the lowest impact value was selected for microscopic fracture analysis. The microscopic fracture morphology and energy spectrum are shown in Figure 5. The impact fracture could be observed at a relatively low magnification, and the microscopic fracture morphology was dimples (Figure 5a, 5b). There were densely distributed loose holes, pores, and inclusions (Figure 5c, 5d, 5f, 5g). The dimple obvious partition characteristics, with some areas showing shallow dimples and some showing deep dimples. The energy spectrum of the loose and pore regions detected the N element (Figure 5e), and the inclusions were mainly analyzed as aluminum oxide and manganese sulfide (Figure 5h and 5k).
3. Discussion on the Causes of Low Impact Value
3.1 Material Aspect
3.1.1 Chemical Composition
The chemical composition of the material conforms to ZWC – ZG30CrMnSiMo, which is a low-alloy wear-resistant steel and is suitable for bucket teeth.
3.1.2 Hardness and Segregation
The hardness in the center of the test block was abnormally high, exceeding 58 HRC, and the acid immersion test piece showed a center segregation defect. This was caused by the slower cooling of the central part during the solidification of the steel liquid, resulting in component segregation. If the impact specimen was selected from the center segregation area, an abnormal impact value would occur.
3.1.3 Inclusions and Loose Holes
In the routine inspection of inclusions, no aggregated or large-sized endogenous inclusions were found, only small inclusions, indicating relatively good material purity. However, on the impact fracture, microscopic inspection revealed densely distributed aluminum oxide and manganese sulfide inclusions. The bottom of the dimples on the impact fracture could expose more inclusions near the fracture surface, indicating poor material purity. A small number of small particle inclusions usually have little impact on impact performance, but when the number is large, it significantly reduces impact toughness and directly leads to a low impact value. Large loose holes also disrupting the continuity of the matrix, and the energy spectrum at the edge of the holes showed the presence of the N element. High nitrogen content in steel can cause metal lattice distortion due to nitride precipitation after long-term storage, generating large internal stresses and deteriorating the plasticity and impact toughness of the steel, making it brittle. The sources of nitrogen in steel mainly include molten iron, furnace gas in the hearth, and air in contact with the steel water. In general, the nitrogen content in steel should be reduced.
3.2 Heat Treatment Aspect
The microscopic structure of the material is tempered martensite, belonging to a quenched + low-temperature tempered, and the grain size is qualified. The microscopic structure distribution is uneven, showing intergranular network and banded segregation. The possible reasons for this are insufficient tempering of martensite or component segregation, which requires homogenization treatment. Regarding hardness testing, the technical condition requires a hardness of ≥ 46 HRC, and the actual tested hardness is mainly distributed between 49 HRC and 52 HRC. Generally, for materials with higher hardness and better wear resistance, their impact toughness is poorer. Appropriately reducing the hardness to below 50 HRC can improve impact toughness.
4. Conclusions and Recommendations
4.1 Conclusions
The high hardness value of bucket teeth is the main cause of the low impact toughness value. Excessive inclusions and loose holes in raw materials are another factor contributing to the low impact value.
4.2 Recommendations
4.2.1 Adjusting Heat Treatment
Increase the tempering temperature of bucket teeth to appropriately reduce the hardness.
4.2.2 Optimizing Smelting Process
Optimize the smelting process to minimize casting inclusions and loose hole defects.
4.2.3 Gas Content Detection
Conduct oxygen, nitrogen, and hydrogen gas content detection on bucket teeth. Since the material may have a high nitrogen content, it is recommended to control the nitrogen content during the melting process.
In conclusion, through a comprehensive analysis of the chemical composition, mechanical properties, metallographic structure, and impact fracture of bucket teeth, the causes of the low impact value have been identified, and corresponding solutions have been proposed. These measures can help improve the quality and performance of bucket teeth in coal mining, reducing the occurrence of early failure and improving production efficiency.