Mechanical spare parts, grey-cast iron parts
Grey cast iron has good casting performance, good vibration damping, good wear resistance, good cutting performance and low notch sensitivity.
① Effects of the chemical composition. Production mainly controls the quality fraction of carbon and silicon. The carbon and silicon quality fraction is too low, cast iron is easy to appear white mouth tissue, mechanical performance and casting performance are very low; when the carbon and silicon quality fraction is too high, the graphite sheet is too large and thick, and even graphite floating on the surface of iron water, reducing the performance and quality of the casting. Therefore, the content of carbon and silicon in gray cast iron is generally controlled at 2%~ 4%C; 1.0%~2.0%Si; 0.5% ~ 1.4% Mn.
② The influence of the cooling speed: under certain casting process conditions, the cooling speed of the casting has a great impact on the℃of graphitization. With the different wall thickness of the casting, the cooling speed slows down, carbon atoms have sufficient diffusion time, it is conducive to the full graphitization process, room temperature tissue is easy to form gray iron tissue; but the thin-walled parts in the cooling process is too fast, easy to form white iron tissue.
The heat treatment of gray cast iron can only change the organization of the matrix, not the shape of graphite, so it is impossible to significantly improve the mechanical properties of gray cast iron. The heat treatment of grey cast iron is mainly used to eliminate the internal stress and white mouth tissue, stabilize the size, improve the cutting performance, improve the surface hardness and wear resistance.
To eliminate the casting stress caused by the uneven cooling during the solidification process, and to prevent the deformation and crack of the casting. The process is to heat the casting to 500~600℃, insulation for a period of time with the furnace to 150~200℃ below the air cooling, sometimes the casting in the natural environment for a long time, so that the stress in the casting is relaxed, this method is called "natural aging", large gray cast iron can adopt this method to eliminate the casting stress [4].
In order to eliminate the white mouth tissue, reduce the hardness, improve the cutting performance. The method is to heat the casting to 850~900℃, keep warm for 2~5 hours, and then slow cooling with the furnace to 400~500℃, and then release the air cooling, so that the carburized body in the process of heat insulation and slow cooling decomposition to form graphite.
Improve the surface hardness and extend the service life. For example, the working surface of the guide surface of the machine tool and the working surface of the inner wall of the cylinder sleeve of the internal combustion engine is necessary to have high hardness and wear resistance, and the method of surface quenching can be used. The common methods are high (medium) frequency induction heating surface quenching and contact resistance heating surface quenching
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