Subcutaneous porosity is a common casting defect of nodular iron castings. The moisture in molding sand, resin in sand core, gas absorbed by molten iron in smelting process and refractories are all the sources of gas. The subcutaneous porosity found after casting is difficult to accurately determine its gas source. The effect of conventional improvement measures is often not ideal, and the probability of repeated subcutaneous porosity is high, which is a problem that every foundry may encounter.
In 2015, when a hot-selling brake caliper (called “693 calipers” in the company) was produced, it was found that there was air hole in the jaw part after processing, and the defect ratio suddenly increased from about 1.5% to 5% ~ 6%, while the air hole scrap rate of another model brake caliper (called “812 calipers” in the company) produced in the same period of time did not increase. In view of this phenomenon, the company has carried out a series of improvement tests to control the porosity scrap rate at about 0.3%, which greatly reduces the production cost while improving the production efficiency.
693 brake caliper has an outline size of 192 mm × 151 mm × 89 mm and a casting mass of 3.22 kg; 812 brake caliper has an outline size of 192 mm × 149 mm × 84 mm and a casting mass of 2.75 kg. The machining allowance of the claws of the two kinds of brake tongs is 2 mm, and the casting material grade is QT450-10; the tensile strength is required to be ≥ 450 MPa, the elongation is required to be ≥ 10%, and the Brinell hardness is 156-217 HB; the spheroidization rate is required to be ≥ 85%, the pearlite volume fraction is < 40%, and the carbide volume fraction is less than 5%; the casting is not allowed to have cracks, cold shuts, shrinkage defects, and the slag hole size is less than φ 2 mm × 1.5 mm. After machining, the surface is not allowed to have pores, slag holes and other defects.