1. Introduction
The crankshaft is a crucial component in engines, especially in mining vehicles where it endures high – stress and heavy – load working conditions. The QT900 – 5 ductile iron crankshaft, known for its good mechanical properties such as high strength and toughness, is widely used. However, the occurrence of defects in these crankshafts can lead to serious consequences, including engine failure and equipment downtime. This article aims to comprehensively analyze the defects in a QT900 – 5 ductile iron crankshaft that fractured during operation in a mining vehicle, and propose effective improvement measures.
2. Background of the Problem
In July 2021, a mining excavator’s four – cylinder crankshaft suddenly broke after operating for 1269 hours. After disassembly, it was found that the crankshaft fractured into three parts at the 7th and 8th cranks. Additionally, the cylinder block’s rear end cracked, and the 4th connecting rod bent. The failed crankshaft was made of QT900 – 5 ductile iron and had undergone strengthening processes like fillet grooving rolling and journal medium – frequency induction quenching. Understanding the cause of this failure is essential to prevent similar issues in the future.
3. Experimental Methods and Instruments
3.1 Experimental Methods
To determine the cause of the crankshaft failure, a series of tests were carried out. Chemical composition analysis was conducted to check if the material met the specified requirements. Macro – analysis of the fracture surface was used to identify the crack source and the fracture propagation path. Scanning electron microscopy (SEM), optical microscopy, and energy – dispersive spectroscopy (EDS) were employed to analyze the surface morphology, microstructure, and chemical composition of the defect area. Metallographic analysis was performed on samples taken from the fracture area to understand the material’s internal structure.
3.2 Experimental Instruments
- HX – HW8B High – Frequency Infrared Carbon – Sulfur Analyzer: This instrument was used to analyze the carbon and sulfur content in the crankshaft material according to the national standard GB 20123 – 2006.
- Plasma 3000 ICP Spectrometer: It was used to analyze other elements in the material following the national standard GB/T 20125 – 2006.
- Hitachi S 3400N Scanning Electron Microscope: To observe the surface morphology of the defect area at a micro – level.
- JED – 2200 Energy – Dispersive Spectrometer: For analyzing the chemical composition of the substances in the defect area.
- Metallographic Microscope: To study the microstructure of the crankshaft material.
- E45 – 305 Electronic Universal Material Testing Machine: Used for mechanical property testing.
- THBS – 300D Brinell Hardness Tester: To measure the hardness of the material.
4. Experimental Results and Analysis
4.1 Chemical Composition Analysis
The chemical composition of the failed crankshaft was analyzed, and the results are shown in Table 1.
| Element | Found | Technical requirement |
|---|---|---|
| C | 3.64 | 3.60 – 3.90 |
| Mn | 0.5 | 0.30 – 0.50 |
| Si | 2.16 | 1.90 – 2.40 |
| P | 0.017 | ≤0.060 |
| S | 0.004 | 0.004 – 0.02 |
| Cr | 0.04 | ≤0.10 |
| Mg | 0.04 | 0.02 – 0.06 |
| Ce | 0.005 | 0.025 |
| Cu | 0.55 | 0.40 – 0.60 |
By comparing the measured values with the technical requirements, it can be seen that the chemical composition of the crankshaft meets the specified standards. This indicates that the chemical composition is not the direct cause of the crankshaft failure.
4.2 Macro – analysis
The fracture surface of the failed workpiece can provide important information about the failure mechanism. For the QT900 – 5 ductile iron crankshaft, the fatigue source of the fillet area is usually affected by the strengthening process. In this case, the crack source of the 7th crank was located at the intersection of the 7th crank and the 4th connecting rod’s lower dead – point rolling groove edge, near the surface of the 7th crank. The crack propagated at an approximately 45° angle to the axial direction of the crankshaft and finally led to instantaneous fracture near the 4th main journal fillet rolling groove edge.
The fracture surface of the 7th crank showed distinct regions of the crack source, propagation area, and instantaneous fracture area. The crack source was in the shape of an elliptical hole, with a size of approximately 3mm × 7mm. The hole was larger inside and smaller outside, and there were no obvious plastic deformations around it. The fracture surface of the 7th crank was smooth, with few friction and impact marks, indicating that it fractured rapidly under overload.
The 8th crank’s fracture surface showed that the fatigue crack originated from the 4 – connecting – rod fillet area. The fracture propagated from the 4 – connecting – rod side to the 5th main journal direction. The middle part of the 8th crank’s fracture surface had a mixed morphology, and the final instantaneous fracture area was near the 5th main journal. The 8th crank also fractured rapidly, with few friction marks on the fracture surface.
Based on the macro – analysis, it can be inferred that the 7th crank was the first to fracture, and its fracture led to the instantaneous overload and subsequent fracture of the 8th crank.
4.3 Scanning Electron Microscopy (SEM) Analysis
Under the SEM, the hole in the 7th crank was composed of two small holes, with smooth hole walls. There were several small cracks around the hole, and a small amount of friction marks and fatigue stripes were observed on the hole edge. The bottom area of the hole was dark in color. EDS analysis of the dark – colored substance at the bottom of the hole showed that the main elements were C, O, and Fe, indicating the presence of oxidation in the hole.
The raised area of the 7th crank showed severe wear, with most of the area covered by friction marks. The 8th crank near the 4 – connecting – rod fillet had a “river – like pattern” under the SEM, which is a characteristic of cleavage fracture. The middle area of the 8th crank’s fracture surface had both cleavage and dimple features, indicating that the fracture was caused by instantaneous overload.
4.4 Microstructure Analysis
Samples were taken from the crack source area of the 7th crank arm for metallographic and SEM analysis. The microstructure around the hole in the 7th crank showed that the graphite was evenly distributed, with a good degree of roundness. The graphite nodularity grade was 2, and the graphite sphere diameter grade was 6. The area near the hole was composed of lamellar pearlite, a small amount of ferrite, and graphite, with a pearlite content of 95%. A layer of dark – colored substance with a thickness of 5 – 10μm was also observed at the bottom of the hole.
EDS analysis of the dark – colored substance at the bottom of the hole in the 7th crank and the raised area showed that the main elements were C, O, and Fe, indicating the presence of oxides. The hole in the raised area of the 7th crank was identified as an invasive gas hole based on its characteristics, such as being connected to the surface by small worm – like holes and containing oxides. The hole in the 7th crank’s fracture area was also determined to be an invasive gas hole, which reduced the effective bearing area of the crankshaft material, caused local stress concentration, and became a potential fatigue source.
4.5 Performance Testing
Tensile test bars were cut from the 6th crank, and hardness samples were taken from the large – disk position. The mechanical property test results are shown in Table 2.
| Item | Yield strength/MPa | Tensile strength/MPa | Elongation percentage/% | (HBW) Hardness |
|---|---|---|---|---|
| measured value | 521 | 916 | 7.4 | 302 |
| technical specification | ≥460 | ≥820 | ≥4.5 | 250 – 320 |
The test results show that the mechanical properties of the crankshaft meet the technical requirements, indicating that the overall performance of the material is satisfactory.
5. Analysis of Defect Formation and Crankshaft Failure
5.1 Formation of Invasive Gas Holes
In this case, the gas entering the molten metal first formed an elliptical gas hole at the subsurface of the intersection of the 7th crank and the 4th connecting rod’s lower dead – point rolling groove edge. As the casting shell began to cool and solidify, the amount of gas and gas pressure continued to increase. Since the gas could not flow out due to the solidification of the shell, and the internal temperature of the casting was high with good metal fluidity and low resistance, the gas flowed into the interior of the casting. According to the principle of “gas always flowing along the path of least pressure” in casting defect analysis, the gas expanded inside the metal, forming an inner – large and outer – small shape.
The sources of the gas were mainly water, organic matter combustion in the sand mold and sand core, such as steam, carbon monoxide, and carbon dioxide. These gases invaded the surface layer of the casting at the intersection of the 7th crank and the 4th connecting rod’s lower dead – point rolling groove edge, resulting in the formation of invasive gas holes.
5.2 Impact of Invasive Gas Holes on Crankshaft Performance
Invasive gas holes had a significant negative impact on the performance of the crankshaft. Firstly, they reduced the effective bearing area of the crankshaft material, which directly affected the load – bearing capacity of the crankshaft. Secondly, the presence of gas holes caused local stress concentration, which greatly reduced the fatigue strength of the material. As a result, the crankshaft was more likely to form fatigue cracks under stress.
5.3 Crankshaft Failure Process
When the mining vehicle was working at full load, the bending and torsional stresses near the crankshaft journal fillet were excessive. The stress exceeded the fatigue limit of the area with invasive gas hole defects, which promoted the rapid formation and propagation of fatigue cracks. As a result, the 7th crank fractured instantaneously. After the 7th crank’s fatigue fracture, the crankshaft continued to rotate due to inertia, causing the 8th crank to be instantaneously overloaded. This led to the instantaneous failure and fracture of the 8th crank, as well as damage to other components such as the cylinder block, connecting rod, and camshaft.
6. Improvement Measures
6.1 Controlling the Content of Gas – Generating Substances and Moisture in Molding Sand and Core Sand
The content of gas – generating substances and moisture in molding sand and core sand should be strictly controlled. The molding sand laboratory should test the volatile content of the molding sand daily. For ductile iron, the volatile content of the molding sand should be controlled within 2.3 – 2.5. If it exceeds the upper limit, the molding sand control room should be notified in a timely manner to adjust the molding sand ratio. This can effectively reduce the amount of gas generated during the casting process and prevent the formation of invasive gas holes.
6.2 Adjusting the Parameters of the Automatic Gas – Hole – Drilling Machine
The parameters of the automatic gas – hole – drilling machine should be adjusted to increase the depth of the gas holes on the sand mold surface to 30mm from the cavity surface. This can improve the exhaust capacity of the sand mold, allowing the gas generated during pouring to be discharged quickly. By enhancing the exhaust effect, the pressure of the gas inside the casting can be reduced, reducing the likelihood of gas invasion and the formation of gas holes.
6.3 Controlling the Quality of Furnace Charge Management
The quality of furnace charge management is crucial. The furnace charge must be dry and free of surface rust. During smelting, the high – temperature molten iron should be left to stand. The electric furnace should be heated to 1500 – 1520°C, and the power should be turned off for 3 – 5 minutes. After spheroidization, the number of slag – skimming operations on the molten iron should be increased from 2 to 3 times to reduce the amount of liquid slag in the molten iron. This can improve the quality of the molten iron and reduce the possibility of gas and slag inclusions in the casting.
7. Conclusion
In this study, through comprehensive analysis of a failed QT900 – 5 ductile iron crankshaft, it was found that the invasive gas hole at the intersection of the 7th crank and the 4th connecting rod’s lower dead – point rolling groove edge was the root cause of the crankshaft failure. The gas hole was formed by the invasion of gases generated from the water and organic matter in the sand mold and core. It reduced the fatigue strength of the crankshaft, leading to the fracture of the 7th crank under the action of stress, and then causing the fracture of the 8th crank due to instantaneous overload.
To improve the quality of castings, corresponding improvement measures were proposed, including controlling the content of gas – generating substances and moisture in molding sand and core sand, adjusting the parameters of the gas – hole – drilling machine, and strengthening furnace charge management. Although this study focused on the improvement of molding sand, core sand, and furnace charge, further research on the crankshaft process, operation methods, and materials is needed in the future to comprehensively improve the quality and reliability of crankshafts.
In summary, understanding the defect mechanism and taking effective improvement measures can help prevent similar crankshaft failures, improve the performance and service life of engines, and reduce production costs and equipment downtime.
