Features: small holes filled with mold (core) sand inside or on the surface of defects are called sand holes. If the shape of the defect is irregular and the internal part is slag or inclusion, it is called slag hole.
Sand hole prevention measures:
(1) Improve the strength and compactness of sand mold (core), reduce the burr of sand core and sharp angle of sand mold, and prevent sand flushing.
(2) The floating sand on the surface of mold cavity and sand core shall be blown clean before mold closing, and pouring shall be carried out as soon as possible after mold closing. When cold core sand is used, the molten iron shall be dispersed as much as possible to avoid sand holes caused by scouring.
(3) Prevent dry out of sand core and long storage time.
(4) The pouring system should be designed reasonably to avoid too much scouring force of molten iron on the mold wall; the surface of the sprue cup should be smooth without floating sand.
Slag hole prevention measures:
(1) In order to raise the superheat temperature of molten iron, the number of slag raking should be increased for nodular iron, vermicular iron and alloy cast iron. If the temperature allows, the slag should be kept still for a period of time before pouring, so as to facilitate the slag floating up.
(2) To prevent the oxidation of molten iron, strictly control the amount of spheroidizing agent and inoculant (especially the amount of flow inoculation), and be careful when adopting flow inoculation.
(3) The pouring system should be designed reasonably, the filter screen should be placed to improve the slag retention capacity, and the slag retention system should be placed on the pouring package to keep continuous flow during pouring.
(4) In the process of casting and molten iron flowing in the mold cavity, the slag produced by the oxidation of molten iron or the reaction of various elements contained in the molten iron with the mold and sand core material is usually called “secondary slag” (to distinguish from the “primary slag” existing before casting). The slag inclusion defects formed by this slag can only be found on the fracture surface, and the processing surface of the finished casting often needs to be It can be found through magnetic particle inspection. This inclusion is mainly composed of oxides (MgO, SiO2, FeO…) And sulfides (MGS, FES, MNS…) And other inclusions.
Prevention measures for “secondary slag”:
① The residual magnesium content of molten iron should be strictly controlled (the general mass fraction should be 0.035% ~ 0.055%, the wall thickness should be controlled at the lower limit, and the wall thickness can be controlled at the upper limit).
② In order to reduce the sulfur content of the original molten iron, desulfurization treatment should be adopted if conditions permit, and the treatment temperature and pouring temperature should be increased. Desulfurization treatment can greatly reduce the sulfur content of the original liquid iron and effectively reduce the “secondary slag”.
③ Properly increasing the content of rare earth and decreasing the content of magnesium in the nodulizer will help to reduce the temperature of iron crust and secondary slag.