Discussion on common defects of steel castings in EPC and prevention measures

1. Carburization defect

Produces the main reason for the increased carbon defects: bubble shape material is mainly composed of carbon and hydrogen, sharp break down under the high temperature liquid steel, hydrogen and free of carbon, due to the strong affinity ability is carbon, hydrogen and oxygen gap between the high temperature decomposition of hydrogen with shape of oxygen in water vapour discharge, and the decomposition of a large number of free carbon residue in the mold, and the surface of the liquid steel immersion corrosion, resulting in a casting surface carburizing, carbonization phenomenon.Carburization mainly occurs on the surface of the casting, and almost no carburization in the core.There is no carburization near the inner gate, and the farther away from the inner gate, the more serious the carburization is.(1) The quality of high quality foam plastics directly affects the shape gasification speed and the morphology of cracking products in the pouring process.(2) choose a reasonable casting process The design of casting process of lost mold casting should be able to accelerate the gasification of die material, reduce and stagger the contact and reaction time between liquid and solid phases in the decomposition products and molten steel, and reduce or avoid the carburization of steel castings.(3) Taking advantage of the feature that the farther away from the inner sprue, the more serious the carburization is, the riser is set at the farthest end from the inner sprue or at the highest point of the casting, so that the liquid steel with serious carburization pollution entering the casting cavity first enters the riser, and the casting body obtains the purer liquid steel.

2. Stomatal defects

The pores produced by EPCC steel castings can be divided into the following four categories:(1) the turbulent flow in the pouring and filling process of the pores produced by the foam-like cracking products involved in the metal liquid, or in the process of gasification, part of the appearance is surrounded by the metal liquid and cracking, and the gas produced can not be discharged from the metal liquid, and the pores will be formed.Preventive measures: improve the process, so that the liquid steel smooth filling, no turbulence;Increase casting temperature and negative pressure degree;Improve air permeability of coating and molding sand.(2) foam appearance, poor coating drying caused by pores foam appearance, poor coating drying or foam appearance foaming agent content is too high, pouring will produce a lot of gas caused by pores.Preventive measures: fully dry foam appearance;Coating must be dry, strict control of the amount of foaming agent added.(3) too much shape binder caused by blowhole shape binder initiated a large amount, too much use, gasification is slow, easy to make gas involved in liquid steel to form pores.Preventive measures: choose low gas model binder;Under the premise of ensuring the stickiness, the less the stickiness, the better.(4) when pouring, the direct runner is not full, and it is easy to be involved in the air. If these gases can not be discharged in time, they are easy to cause pores.Preventive measures: through the design of a reasonable gating system, the metal liquid flow is stable when filling the mold, no air entrainment;When the closed gating system is used, there should be a certain amount of liquid metal in the sprue cup to ensure that the direct sprue is in a full state.The use of hollow straight runner mold, reduce the amount of air, is conducive to prevent the generation of air holes.(5) the porosity caused by the quality problem of molten steel melting carbon steel melting process contains gas, the need for pre-deoxidation treatment and final deoxidation treatment.Preventive measures: in the smelting process, strictly follow the smelting process, deoxidize before casting, purify the molten steel.

3. Slag inclusion defects

Slag inclusion defect refers to the defect formed by dry sand, coating and other inclusions as molten iron enters the casting during the pouring process.(1) choose the cast steel coating selected by the coating with good comprehensive performance, but also requires a series of properties such as high strength, high refractoriness, good explosive heat cracking resistance, coating hanging, suspension, non-flowing.First of all, high strength and fire resistance of coatings are required to prevent slag inclusion defects.It is required that the coating layer hanging on the surface of the white die should not crack during drying and transportation, that is, the coating should have sufficient room temperature strength.(2) standard packing operation packing, shape group placed on the sand at the bottom of the sand box should be stable, do not allow suspended placed when the start of sand vibration modeling, in order to avoid cracking the coating layer.Do not be on the face of violent sand, should first use flexible sand, to prevent the material layer rupture, to close the gate to avoid sand.(3) Reasonably set the casting head, temperature and time. The higher the casting head is, the greater the erosion of the casting system and the casting mold will be, and the greater the possibility of sanding caused by the damaged coating will be. The casting head of different sizes should be different.(4) Reasonably determine the size of negative pressure The casting process of EVC is generally carried out under vacuum conditions. The role of negative pressure is to tighten dry sand, accelerate exhaust and improve mold filling capacity.(5) Setting slag retaining, skimming and slag collecting riser gating system design, should consider setting slag retaining and skimming functions, and setting slag collecting riser on the casting is helpful to improve sand and slag inclusion defects.(6) the use of molten steel purification technology molten steel must pay attention to the problem of purification.

4. Jet

In the pouring process, due to the large amount of gas emitted by the thermal decomposition of the shape, and it can not be eliminated in time, the pressure in the cavity rises sharply, which is easy to cause fire or metal liquid spraying, leading to casting scrap.Prevention methods are: (1)EPS shape density control in 0.018 ~ 0.022g/cm3, shape to dry, dry after coating, reduce moisture content and gas.(2) choose good air permeability coating, adjust the coating thickness (0.5 ~ 1.0mm is appropriate) so that the appearance of the gas after cracking timely escape.(3) The air permeability and particle size of dry sand should be controlled. The mixed use of dry sand with different particle size should be avoided to reduce the air permeability.Control the negative pressure degree, so that the appearance in the vacuum hypoxia condition of gasification, and very few combustion, reduce the amount of gas.(4) control the pouring temperature and pouring speed, to ensure the shape of the heat of molten steel gasification, at the same time in the shape of a large number of gas production control pouring speed, so as not to pour too fast, to promote a large burst of cracking gas.(5) Design a reasonable gating system to ensure that the metal liquid is filled with the casting mold smoothly, evenly and rapidly, so as to ensure that the cracking gas of the mold can escape outside the cavity and be discharged.

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Post time: Feb-08-2021