Grey cast iron refers to cast iron with flake graphite, because fracture fracture was dark gray, called grey cast iron. The main component is iron, carbon, silicon, manganese, phosphorus, sulfur, iron is the most widely used, its output accounts for more than 80% of the total output of cast iron. The gray cast iron in molding and core making, will cause pores, trachoma, sand and other defects. So, how to prevent these defects appear? The following for you one by one analysis.


A.blowhole

Features and discovery methods:

local porosity of castings, holes appear on the surface of the relatively clean smooth individual pores or honeycomb pores.

By visual inspection, mechanical or magnetic testing can be found.

analysis of causes:

a.The pouring system is not set properly so that the exhaust is not smooth or produces eddy current and gets involved in the gas.

b.The compactness of the sand mold is too high to reduce air permeability.

c.Poor sand core venting or airway obstruction.

method of improvement:

1.The pouring system shall take into account the smooth flow of the cavity and the smooth flow into the mold.

2.Sand compaction requirements uniform, should not be too tight.

3.The sand core venting needs to be unblocked. When the box is closed, the gap between the core and the core must be sealed so as to prevent the iron liquid from drilling and blocking the air passage.

4.At the highest point of the casting can be provided with an air outlet or air film etc.

5.When lifting and repairing, it is not appropriate to brush too much water.

6.For large flat castings, lean casting can be adopted, and the vent holes should be slightly higher to facilitate venting.

7.Core support and cold iron must be clean, no rust.


B.voids

The characteristics and discovery method:

The casting cavity containing sand.

The inspection, mechanical or magnetic testing can be found.

analysis of causes:

1.The gating system position is not appropriate, such as straight on the sand core, or gate is too small, the molten iron scouring force. Destruction of local sand.

2.Because the design of the model structure is not good enough, the die sticking occurs while the sand mould is not repaired or the fillet of the casting is not rounded.

3.Wet type in the long residence time before pouring, the dry bath or protruding parts fall off.

4.Molding and closing of the sand, not clear sand clean.

method of improvement:

1.The position and size of the gating system are suitable.

2.Reasonable selection of the mold slope and fillet, manual molding can be extruded fillet. In batch production, the pattern should be painted parting agent, so as not to stick to the mold, and intended to repair the damaged parts.

3.Shorten the residence time in the wet type before pouring.

4.The physical gap between the core head and the core seat, so as not to crush the box together.

5.Before I close the box, I must clean the sand in the mold, carefully close the box and cover the riser in time so as not to fall into the sand again.

C. sand inclusion 

The characteristics and discovery method:

On the surface of the casting, a layer of sand sandwiched between a layer of iron and casting.

By visual inspection or mechanical processing can be found.

analysis of causes:

The molten iron into the sand. The surface layer of water inward migration, formation of high water from the surface of 3~5mrn. It greatly reduced strength, easy to cause the molten iron into, or because of the high temperature expansion stress of silicon sand on the surface layer muster, drill into the molten iron, the formation of sand. 

1.Through the hard sand compaction or uneven compaction 

2.Improper pouring position for large castings; pouring the horizontal plane, sometimes because somewhere in liquid iron cover a large plane and intermittent sand 

Method of preventing: 

1.Sand tightness should not be too tight, uniform requirements, and strengthen ventilation 

2.Hand molding, the local weak point, can be inserted to strengthen nails

3.Try to make the large plane down or on the side, reducing the baking area and baking time of the liquid iron on the upper plane

4.For large flat castings, the gating system can be dispersed, and the gate section shall be appropriately enlarged to shorten pouring time or to tilt pouring.

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2017年07月27日

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Grey cast iron analysis and method of preventing the main defects and the reasons caused by the molding and core making

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