Common defects and how to prevent them?
Common defects in brake disc production: air hole, shrinkage porosity, sand hole, etc; The medium and type graphite in the metallographic structure exceed the standard, or the carbide quantity standard; Too high Brinell hardness leads to difficult processing or uneven hardness; The graphite structure is coarse, the mechanical properties are not up to standard, the roughness is poor after processing, and the obvious porosity on the casting surface also occurs from time to time.
1. Formation and prevention of air holes: air holes are one of the most common defects of brake disc castings. Brake disc parts are small and thin, cooling and solidification speed is fast, and there is little possibility of precipitation air holes and reactive air holes. The fat oil binder sand core has a large gas generation. If the mold moisture content is high, these two factors often lead to invasive pores in the casting. It is found that if the moisture content of molding sand exceeds, the porosity scrap rate increases significantly; In some thin sand core castings, choking (choking pores) and surface pores (shelling) often appear. When the resin coated sand hot core box method is used, the pores are particularly serious due to the large gas generation; Generally, the brake disc with thick sand core rarely has air hole defects;
2. Formation of air hole: the gas generated by the disc sand core of brake disc casting at high temperature shall flow outward or inward horizontally through the core sand gap under normal conditions. The disc sand core becomes thinner, the gas path becomes narrow and the flow resistance increases. In one case, when the molten iron quickly submerges the disc sand core, a large amount of gas will burst out; Or high-temperature molten iron contacts with high water content sand mass (uneven sand mixing) at some place, causing gas explosion, choking fire and forming choking pores; In another case, the formed high-pressure gas invades the molten iron and floats up and escapes. When the mold cannot discharge it in time, the gas will spread into a gas layer between the molten iron and the lower surface of the upper mold, occupying part of the space on the upper surface of the disc. If the molten iron is solidifying, or the viscosity is large and loses fluidity, the space occupied by the gas cannot be refilled, Will leave surface pores. Generally, if the gas generated by the core cannot float up and escape through the molten iron in time, it will stay on the upper surface of the disc, sometimes exposed as a single pore, sometimes exposed after shot blasting to remove the oxide scale, and sometimes found after machining, which will cause a waste of processing hours. When the brake disc core is thick, it takes a long time for molten iron to rise through the disc core and submerge the disc core. Before submerging, the gas generated by the core has more time to flow freely to the upper surface of the core through the sand gap, and the resistance to flow outward or inward in the horizontal direction is also small. Therefore, surface pore defects are rarely formed, but individual isolated pores may also occur. That is to say, there is a critical size to form choking pores or surface pores between the thickness and thickness of the sand core. Once the thickness of the sand core is less than this critical size, there will be a serious tendency of pores. This critical dimension increases with the increase of the radial dimension of the brake disc and with the thinning of the disc core. Temperature is an important factor affecting porosity. The molten iron enters the mold cavity from the inner sprue, bypasses the middle core when filling the disc, and meets opposite the inner sprue. Due to the relatively long process, the temperature decreases more, and the viscosity increases accordingly, the effective time for the bubbles to float up and discharge is short, and the molten iron will solidify before the gas is completely discharged, so pores are easy to occur. Therefore, the effective time of bubble floating and discharging can be prolonged by increasing the molten iron temperature at the disc opposite the inner sprue.
