To design the process of nodular cast iron, in addition to selecting the correct casting method and process scheme, the process parameters such as shrinkage, weight tolerance and dimensional tolerance of nodular cast iron should also be selected according to the actual situation. Reasonable selection of process parameters is of great significance to improve the quality of nodular cast iron, reduce production cost and improve production efficiency.
(1) Dimensional tolerance of nodular cast iron
Dimensional tolerance refers to the difference between the two allowable limit dimensions of the nominal size of nodular cast iron. Within these two limit dimensions, nodular cast iron can meet the requirements of processing, assembly and use. The higher the dimensional accuracy of nodular cast iron, the smaller the dimensional tolerance and the higher the production cost. Therefore, the selection of dimensional tolerance of nodular cast iron must comprehensively consider the actual situation and ensure the highest quality of nodular cast iron within the appropriate cost range. The high strength and toughness automobile hub support material studied belongs to pearlite and ferrite nodular cast iron, and the blank size is 131 mm × 39 mm。 According to the national standard GB / T 6414-1999, the dimensional tolerance of hub support is determined according to ct9, and the value is 2.5 mm.
(2) Weight tolerance of nodular cast iron
Weight tolerance refers to the allowable variation range of the quality of nodular cast iron expressed as a percentage of the weight of nodular cast iron. The weight tolerance of nodular cast iron shall be selected corresponding to its dimensional tolerance grade. The dimensional tolerance of nodular cast iron for hub support is ct9, so the weight tolerance grade is GB / T 11351-1989-mt9. According to the quality tolerance value comparison table of nodular cast iron, taking the weight of the sample 6.9 kg as the nominal mass of nodular cast iron, the weight tolerance of hub support is determined to be 12%.
(3) Shrinkage
The shrinkage rate of nodular cast iron refers to the relative linear shrinkage of the casting from the linear shrinkage starting temperature to the room temperature. The complexity and size of nodular cast iron are the key factors affecting the shrinkage of castings. In order to ensure the dimensional accuracy of nodular cast iron, the appropriate linear shrinkage must be selected in combination with the actual situation. The high strength and toughness hub support studied in this paper is made of pearlite ferrite nodular cast iron. During solidification, the shrinkage of liquid metal is between free shrinkage and hindered shrinkage, but the wall thickness of nodular cast iron is large and the structure is uneven, so it is more inclined to free shrinkage. Therefore, according to the linear shrinkage of commonly used casting alloys in the casting manual, the shrinkage of nodular cast iron for hub support is determined to be 1.2%.