The SiO2 content (mass fraction) of the molding sand should be higher than 92%, preferably higher than 95%. (2) Zirconium sand (ZrSiO4) or chromite sand (FeCr2O4) can be used for castings with higher requirements, which can achieve better results. (3) Properly reduce the pouring temperature and increase the pouring speed, and weaken the thermodynamic and chemical effects of the molten metal on the sand mold. (4) The compactness of the sand mold should be high (the B-type hardness tester is higher than 85, preferably higher than 90), and it should be uniform. The repair of the sand core should be in place, and there should be no local loosening, and at the same time, the coating should be prevented from peeling. 5 Crack characteristics: There are straight or curved cracks on the surface of the cast casting. Cracks are divided into hot cracks and cold cracks. The cracks of hot cracks are mostly tortuous and irregular in shape, and the surface of the fracture is light black with a deep oxidation color.
Cold cracks are straight, and the fracture surface of the casting has a metallic luster and is relatively clean, sometimes with a slight oxidation color. The main reason for the cracks in the castings is that the internal stress is generated when cooling and solidification shrinks. When the internal stress is greater than the strength of the metal material, the casting cracks to form cracks. Preventive measures: (1) strictly control the chemical composition of molten iron. Among them, the high sulfur energy can cause the "hot brittleness" of the casting and cause thermal cracking. Therefore, the w (S) in gray cast iron is preferably less than 0.12%, but it should not be too low (not less than 0.05%). Too low sulfur will affect the inoculation effect , The most suitable w (S) is 0.05% ~ 0.12%. Phosphorus high energy can cause "cold brittleness" of castings and cause cold cracking. Therefore, w (P) in gray cast iron is preferably less than 0.15%, and w (P) in ductile iron is preferably less than 0.08%. (2) Adjust the cooling rate of each part of the casting to avoid local overheating of the casting. Place cold iron on the thick section or hot joint of the casting. The inner runners are properly dispersed to make the temperature of each part of the casting tend to be uniform. (3) After the casting is cast, the mold opening should not be too early, and do not use cold water to spray high-temperature castings. The appropriate mold opening time is when the temperature of the casting in the mold is less than 600 ℃. (4)
When conditions permit, change the structure of the casting to prevent the casting from cracking. If reinforcing ribs are provided, the junction of the two sections will be changed from right angle to rounded corner to reduce stress concentration. 6 Deformation characteristics: Long wholesalers Copper fittings https://www.bronzecast.net/product/air-conditioner-parts-series/coupling/coupling-8.html castings are more prone to deformation, such as machine tool bed, cylinder block and cylinder head of diesel engine. Due to the difference in wall thickness of castings and uneven cooling, castings are easily deformed. Some castings are deformed after being stored for a period of time after being processed. Preventive measures: (1) For some castings that are prone to deformation, in addition to appropriately increasing the processing allowance, the mold can also be made into reverse deformation (such as making the mold into reverse bending) to correct the deformation of the casting. (2) Stress relief annealing of the casting to eliminate the internal stress of the casting; when conditions permit, aging treatment may be adopted. Do not prematurely open the mold, and do not stack the castings after sanding.
Cold cracks are straight, and the fracture surface of the casting has a metallic luster and is relatively clean, sometimes with a slight oxidation color. The main reason for the cracks in the castings is that the internal stress is generated when cooling and solidification shrinks. When the internal stress is greater than the strength of the metal material, the casting cracks to form cracks. Preventive measures: (1) strictly control the chemical composition of molten iron. Among them, the high sulfur energy can cause the "hot brittleness" of the casting and cause thermal cracking. Therefore, the w (S) in gray cast iron is preferably less than 0.12%, but it should not be too low (not less than 0.05%). Too low sulfur will affect the inoculation effect , The most suitable w (S) is 0.05% ~ 0.12%. Phosphorus high energy can cause "cold brittleness" of castings and cause cold cracking. Therefore, w (P) in gray cast iron is preferably less than 0.15%, and w (P) in ductile iron is preferably less than 0.08%. (2) Adjust the cooling rate of each part of the casting to avoid local overheating of the casting. Place cold iron on the thick section or hot joint of the casting. The inner runners are properly dispersed to make the temperature of each part of the casting tend to be uniform. (3) After the casting is cast, the mold opening should not be too early, and do not use cold water to spray high-temperature castings. The appropriate mold opening time is when the temperature of the casting in the mold is less than 600 ℃. (4)
When conditions permit, change the structure of the casting to prevent the casting from cracking. If reinforcing ribs are provided, the junction of the two sections will be changed from right angle to rounded corner to reduce stress concentration. 6 Deformation characteristics: Long wholesalers Copper fittings https://www.bronzecast.net/product/air-conditioner-parts-series/coupling/coupling-8.html castings are more prone to deformation, such as machine tool bed, cylinder block and cylinder head of diesel engine. Due to the difference in wall thickness of castings and uneven cooling, castings are easily deformed. Some castings are deformed after being stored for a period of time after being processed. Preventive measures: (1) For some castings that are prone to deformation, in addition to appropriately increasing the processing allowance, the mold can also be made into reverse deformation (such as making the mold into reverse bending) to correct the deformation of the casting. (2) Stress relief annealing of the casting to eliminate the internal stress of the casting; when conditions permit, aging treatment may be adopted. Do not prematurely open the mold, and do not stack the castings after sanding.
コメント