According to the national standard for metallographic examination of ductile iron, the cross-sectional metallographic structure of the crankshaft hole is tested. There is a large amount of graphite around the pores, which is evenly distributed and has good roundness. The graphite spheroidization level is level 2, and the graphite ball diameter is level 6. Corrosion observation was conducted on the sample using a 5% nitric acid ethanol solution, and it was found that the tissue near the pores was composed of lamellar pearlite, a small amount of ferrite, and graphite, with a pearlite content of 95%; In addition, a layer of dark matter with a thickness of 5-10 μ m was also observed at the bottom of the hole. SEM morphology image of the crankshaft hole sample. Analyze the dark matter at the bottom of the hole using EDS, with a white cross in the center; EDS shows that the main elements of dark matter are C, O, and Fe. From the test results, it can be seen that the bottom of the hole is also covered with a layer of oxide, and no elements such as S or Mg were found. Moreover, there is no flake like graphite around the hole, indicating that there is no slag inclusion problem.
During the macroscopic inspection using a body mirror, scanning electron microscopy, and microscopic analysis, it was found that the hole at the fracture position of the seventh crank hole was located near the surface of the casting, had a large volume, an elliptical shape, a smooth hole wall surface, and the presence of oxides in the hole. After comprehensive analysis of these characteristics, it was determined that the hole at the fracture position of the seventh crank was also a pore and belonged to invasive pores. The presence of pores not only reduces the effective bearing area of the crankshaft material, but also causes local stress concentration, resulting in a decrease in the strength and fatigue ability of the material, leading to workpiece fracture Potential sources of fatigue. The casting defect at the fracture position of the handle was identified as invasive porosity during macroscopic and microscopic examination., Then the outer shell of the casting begins to cool and solidify, while the amount and pressure of gas continue to increase. Gas cannot flow outward due to the solidification of the outer shell, while the internal temperature of the casting is high, the metal fluidity is good, and the resistance is small, causing gas to flow inward.
The analysis results indicate that the production process of the crankshaft fully meets the requirements, and it is not recommended to make major process, operation methods, or material changes. However, in terms of long-term continuous improvement, the following suggestions are proposed:
(1) Strictly control the content and moisture of gas generating substances in the molding sand and core sand. The molding sand laboratory tests the volatile matter of the molding sand every day, and the ductile iron controls the volatile matter of the molding sand at 2.3-2.5. If it exceeds the upper limit, notify the molding sand control in a timely manner Adjust the sand ratio in the production room;
(2) Adjust the parameters of the automatic air hole punching machine, increase the depth of the air hole punching on the surface of the sand mold [10-11], 30mm away from the surface of the mold cavity, increase the exhaust capacity of the sand mold, and facilitate the rapid discharge of gases generated during pouring;
(3) Control the quality of furnace charge management, requiring that the furnace charge must be dry and have no rust on the surface; During melting, increase the static state of high-temperature molten iron, raise the temperature of the electric furnace to 1500-1520 ℃, and stop the power for 3-5 minutes; After spheroidization, the frequency of slag removal in the molten iron increased from 2 times to 3 times to reduce liquid slag in the molten iron.
Conclusion
Taking the four cylinder failed crankshaft as the research object, a comprehensive analysis of the crack source of the crankshaft is conducted using theoretical analysis, macroscopic detection, and microscopic analysis, providing a certain reference for similar crankshaft defect analysis.
(1) Through macroscopic inspection, it was found that the hole type casting defect at the junction of the crankshaft and the fourth connecting rod fillet of the four cylinder crankshaft is the crack source of the crankshaft fracture. A comprehensive analysis was conducted on the fracture crack area of the failed crankshaft using scanning electron microscopy, metallographic microscope, and energy dispersive spectrometer, and it was confirmed that the hole was an invasive pore. Dark substances were detected inside the hole, mainly composed of C, O, and Fe elements.
(2) Water, coal powder, and starch in the molding sand, as well as gases such as water vapor, carbon monoxide, and carbon dioxide formed by the combustion of organic matter such as resin and curing agents in the sand core, invade the crankshaft and the bottom dead center rolling groove edge of the fourth connecting rod
Elliptical intrusive pores are formed along the surface of the casting at the junction position.
(3) This article only focuses on improving the moisture content of molding sand and core sand, as well as the quality of furnace materials, without conducting in-depth research on the crankshaft process, operating methods, and materials, which is also the focus of the next research step.