Ductile iron block graphite defects are one of the common structural defects in thick and large-section ductile iron, high-silicon ductile iron and austenitic ductile iron parts. It has a great influence on the mechanical properties of castings, which are as follows:Tensile strength:Depending on the volume fraction of block graphite in the structure, the tensile strength of the casting will be reduced by 20%~40%. Elongation: will be reduced by 50%~80%. Impact toughness: will be reduced by 50%.
The main reasons for the formation of fragmented graphite are:
1.The nucleation ability of primary graphite or eutectic graphite is weak:
Due to poor cooling conditions, the graphite nucleus is precipitated and then dissolved. At this time, the supercooled iron liquid will preferentially precipitate primary austenite dendrites and grow up, and interconnected, narrow and long residual liquids are formed between the austenite dendrites. C atoms are supersaturated in these residual liquids. Under this condition, C atoms can only precipitate in the late solidification of the residual iron liquid. Due to the limitation of the surrounding solid austenite, it cannot precipitate as normal spherical graphite, but can only rapidly nucleate and grow freely, forming multi-branched fragmented graphite distributed along the austenite dendrites.
2.Excessive rare earth addition and harmful element content such as sulfur: usually appear when using high-purity furnace charge.
The microsegregation of certain elements is also considered to be an important factor in causing fragmented graphite. In particular, the segregation of the rare earth element Ce may lead to the formation of fragmented graphite at the interface between austenite and liquid phase when the eutectic reaction is close to the end point. However, the specific mechanism needs further study.
The following measures can be taken to prevent ductile iron from producing fragmented graphite defects:
1.Optimize the pouring temperature to avoid excessively high or low pouring temperatures that have adverse effects on the graphite morphology. Generally speaking, the pouring temperature should be controlled within an appropriate range to ensure that the graphite can be fully precipitated and form a good spherical shape.
2.Strictly control the composition of the molten iron, including carbon equivalent, silicon content, residual amount of rare earth and magnesium, etc., to ensure that they are within an appropriate range to avoid defects such as graphite floating and fragmented graphite.
3.Use a strong inoculant (silicon barium Ba4-6) and adopt an instantaneous delayed inoculation (silicon strontium Sr1-1.2) process, which can not only give the molten iron a large number of core particles, but also prevent inoculation decay, ensure that the ductile iron has enough graphite balls during eutectic solidification, and reduce graphitization defects.
4.Improve the compactness and uniformity of the sand mold, ensure that the casting mold has sufficient rigidity, and avoid graphitization defects caused by sand mold deformation.
5.Rationally design the pouring system and riser to ensure that the molten metal can smoothly fill the mold and solidify sequentially to reduce the occurrence of defects such as shrinkage cavities and shrinkage.
6.Heat treatment of castings, such as annealing or normalizing, to further improve the graphite morphology and matrix structure and improve the performance of castings.
For the treatment of thick and large-section castings, in addition to using heavy rare earth spheroidizers that are relatively resistant to recession, it is also necessary to use an appropriate amount of high-efficiency composite inoculants for multiple inoculations. The nucleation and growth abilities of graphite in molten iron can be improved to a certain extent to prevent its recession, which can effectively solve the problem of fragmented graphite.
