Preventing methods of common casting defects of cast iron

1 Air hole

Characteristics: The pores in the casting are the smooth holes of different sizes that exist on or near the surface of the casting.

1.1 Intrusive stomata

The number of these pores is small and the size is large. They are pear shaped or spherical in some parts of the outer surface of the casting.

Preventive measures: (1) reduce the amount of gas: control the content of gas material in molding sand or core sand, the water content of wet molding sand should not be too high, and the amount of release agent and water should not be too much in molding and mold repair. The sand core should be ensured to be dried. The sand core after drying should not be stored for too long to prevent the sand core from absorbing moisture.(2) improve the air permeability of molding sand, select the appropriate cavity compactness, and arrange the air outlet position exhaust the air to ensure that the ventilation channel of the sand core is unblocked.(3) appropriately improve the pouring temperature, the exhaust hole and exhaust riser, so as to facilitate the invasion of the metal liquid gas floating discharge.

1.2 Exhalated stomata

This porosity is more and scattered, generally located on the surface of the casting, often the same batch of casting most are found.This porosity is mainly due to the absorption of liquid metals during the smelting process the gas cannot be completely precipitated out before solidification, and many disperse small pores are formed in the casting.

Preventive measures: (1) the use of clean and dry charge, limit the use of charge with more gas content.(2) ensure that the “three dry” : that is, the iron trough, the iron mouth, the bridge to the bottom of the drying.(3) water the package should be dried. It had better be scalded with liquid iron before use. It must be covered with agent on the surface of liquid iron.(4) all kinds of additives must be kept dry, when the humidity is high, to be used after drying.

1.3 Subcutaneous stoma

This kind of pores mainly appear in the upper surface layer of the casting subcutaneous 2 ~ 3mm, the diameter is about 1 ~ 3mm.

Preventive measures: (1) raise the pouring temperature appropriately and shorten the pouring time as far as possible.(2) The amount of inoculant should be controlled at 0.4% ~ 0.6%.(3) to prevent the oxygenation of liquid iron, appropriate supplement relay coke, strict control of air intake.(4) minimize the amount of spheroidizing agent on the premise of ensuring spheroidization.(5) the surface of molten iron is covered with crystal stone powder to prevent liquid iron oxygen during casting.(6) as far as possible to reduce the sand moisture.(7) improve the pouring speed.

2. Tracer hole and slag hole

Characteristics: the inner or surface of the defect filled with sand holes, called trachoma.If the defect shape is irregular, the interior is slag or inclusion, it is called slag hole.

Sand hole prevention measures: (1) improve the strength of sand and sand compactness, reduce the burr of the sand core and the acute Angle of the sand, to prevent sand washing.(2) Before forming, the surface of the cavity and sand core should be blown clean floating sand, after the mold to be poured as soon as possible.(3) prevent sand core drying and storage time is too long.(4) Reasonable design of the gating system to avoid excessive scouring force of molten iron on the mold wall;Sprue cup surface smooth, no sand floats.

Slag hole prevention measures: (1) To increase the overheating temperature of molten iron, the slag raking times of ductile iron, vermicular iron and alloy cast iron should be increased. If the temperature is allowed, the slag should be static for a period of time before pouring to facilitate the slag floating.(2) to prevent liquid iron oxygenation, strict control of the amount of nodulating agent, inoculant, nodular iron with the flow inoculation must be careful.(3) Reasonable design of the gating system, the placement of filter plate to improve slag retaining ability.(4) In the casting process of ductile iron and the flow of molten iron in the cavity, due to the oxygenation of molten iron, various elements contained in molten iron react with the casting mold and sand core materials of slag, usually called “secondary slag”.

Preventive measures of “secondary slag”: (1) Strictly control the amount of residual magnesium in molten iron.(2) To reduce the sulfur content of the original liquid iron, desulfurization should be adopted if possible, and the treatment temperature and pouring should be increased note the temperature.(3) Appropriately increasing the content of rare earth and decreasing the content of magnesium in the spheroidizer can reduce the temperature of molten iron crust.

3 Shrinkage cavity and porosity

Characteristics: in the thick section of the casting, the hot joint or the axis and other final solidification place to form a rough surface of the cavity, and more or less with dendritic crystallization.Having large and concentrated pores called shrinkage cavity, small and scattered called shrinkage.

Preventive measures: (1) according to the thickness of the casting wall to choose the appropriate chemical composition, ductile iron to strictly control the residual amount of magnesium, as far as possible to reduce the pouring temperature.(2) Reasonable design of riser and casting system, so that the casting can be fully fed.(3) Ingestation before furnace should not be excessive. Generally, the amount of inoculant added next time should be controlled at 0.4% ~ 0.6%.(4) to prevent liquid iron oxygenation, skyrocketing the iron oxide content of furnace slag should be less than 5%, and the molten iron of electric furnace should not be retained at high temperature for a long time in the furnace.(5) try to improve the mold stiffness.

4 Adhering sand

Features: The surface of the casting is bonded to a layer of molding material that is difficult to clean.Sticky sand is divided into mechanical sand and chemical sand, the difference is that the mechanical sand is high temperature metal liquid infiltration sand formed in the pores between grains;Chemical clay sand is a low melting point compound formed by metal oxidation and molding materials, which is formed by firmly bonding with castings.

Preventive measures: (1) select sand with high refractoriness, and the SiO 2 content of molding sand should be higher than 92%.(2) zirconium sand or chromite sand can be used for castings with higher requirements.(3) appropriately reduce pouring the injection temperature and increase the pouring speed, weaken the thermodynamic and chemical action of the metal liquid on the sand mold.(4) sand compactness should be high and even.

5 The crack

Features: Straight or curved cracks on the surface of the cast.Cracks are divided into hot crack and cold crack.The fracture surface of hot crack is zigzag and irregular

Pale black with a darker oxidized color.The crack of cold crack is straight, the surface of casting fracture has metallic luster and is relatively clean, and sometimes there is a slight oxidation color.The casting is cracked

The main reason is: cooling solidification shrinkage is hindered by the internal stress, when the internal stress is greater than the strength of the metal material, the casting will crack and form cracks.

Preventive measures: (1) strictly control the chemical composition of molten iron.The high sulfur energy causes the casting to produce “hot brittleness”, resulting in hot cracking, but it can not be too low, too low sulfur to affect the inoculation effect.(2) adjust the cooling rate of each part of the casting, avoid the local overheating of the casting, place the cold iron in the thick section or hot section of the casting, and the inner runner is properly dispersed, so that the temperature of each part of the casting is trending to the uniform.(3) after casting, do not open the type too early, do not use cold water pouring spray high temperature castings.(4) when conditions permit, change the structure of the casting to prevent casting cracking.

6 Deformation

Features: long castings are easy to produce deformation, such as machine tool bed, diesel engine cylinder block, cylinder head, because the casting wall thickness difference is too big, uneven cooling, easy to cause casting deformation.

Preventive measures: (1) for some castings prone to deformation, in addition to the appropriate increase in processing allowance, but also can make the mold into reverse deformation, to correct the deformation of the casting.(2) The casting is stress-free annealed to eliminate the internal stress of the casting.

7 Hardness is not uniform

Characteristics: the surface of the casting after processing, there are micro concave and convex, some local places and “bright” hard points, casting surface hardness difference is large, hard point parts of the hard point

The degree may exceed the standard.

Preventive measures: (1) improve the overheating temperature of liquid iron, in order to eliminate the influence of pig iron heredity.(2) prevent the content of C and Si from decreasing due to the oxygenation of liquid iron, and strictly control the addition of inoculants the dosage and the granularity of inoculant, and the inoculant should be added evenly.(3) it is best to use #20 below scrap steel, avoid the use of alloy steel, scrap steel is best before the use of rust removal treatment.(4)in the uneven thickness of the casting, ensure that all parts of the casting cooling uniform, especially thick large parts can be placed chiller or firebrick.

8 No spheroidization or poor spheroidization of ductile iron

Characteristics: the fracture of the casting is grayish black, and the mechanical properties are obviously low. The metallographic examination can find that the graphite is flabby, and the residual magnesium and rare earth content of the casting is too low. This situation is generally not spherical. The fracture of the casting is still silver gray, but there are scattered gray black spots, and the mechanical properties are low. The metallographic examination can find that a small part of the graphite is flakelike or worm-like, and the residual magnesium content and thinness of the casting the soil content is lower than normal, which is generally due to poor spheroidization.

Preventive measures: (1) according to the sulfur content of the original liquid iron and the content of magnesium and rare earth of the spheroidizer to decide the amount of spheroidizer added.(2) the spheroidizing treatment should ensure that there is enough spheroidizing agent and liquid iron reaction time.(3) minimize the sulfur content of the original liquid iron, such as the use of high carbon and low sulfur coke.(4) Strictly control anti-spheroidizing elements in pig iron.(5) to prevent oxidization of liquid iron, when treating nodular iron the temperature should be moderate.(6) The content of rare earth should be appropriately reduced in large section parts. When necessary, a small amount of antimony can be added to neutralize the rare earth to distort the nodular ink.

9 Spherical decay

Characteristics: in the castings cast in the same liquid iron coating, the spheroidization of the early castings is good, and the spheroidization of the late castings is bad or not.

Preventive measures: (1) the treated molten iron shall be poured as soon as possible, and the surface of molten iron shall be covered with insulation materials to avoid oxidation of the surface of molten iron.(2) Ensure that the molten iron has sufficient residual magnesium content, thick and large section the ductile iron parts can use the nodulating agent with strong declining ability.

10 Graphite Floating

Characteristics: A layer of clear and dense black spots can be seen on the upper surface of the casting fracture. Metallographic examination can find that the graphite balls gather at the top of the section, and sometimes at the lower part of the aggregation layer continuous or individual flowering graphite.Graphite floatation seriously weakens the mechanical properties of ductile iron, which reduces the strength, hardness, elongation and impact toughness.

Preventive measures: (1) strictly control carbon equivalent, which is the fundamental way to solve the graphite floating, carbon equivalent control in 4.3% ~ 4.7%.(2) accelerate the cooling rate of the casting in the thick section place chiller in place.(3) the rare earth content of the spheroidizer should not be too high.

11 Inverse chill

Characteristics: white mouth appears in the core of casting section, while the structure is normal in the outer part where cooling is relatively fast.The main reason for this defect is that the liquid iron of gray cast iron contains high sulfur, low manganese content.Sulphide and hydrogen are easy to segregate to the center of the casting, and are anti-graphitization elements, which hinder the growth of graphite and make the center of the casting too cold to the temperature of white mouth before crystallization. so that the center of the casting white mouth.

Preventive measures: (1) reduce the sulfur content of the original liquid iron, conditions can be taken desulfurization treatment.(2) to strictly control the amount of residual magnesium and rare earth in molten iron, according to the thickness of the casting wall appropriate carbon equivalent.(3) Strictly control the moisture content of raw and auxiliary materials and additives to ensure the “three dry” in place.(4) strengthen pre-furnace inoculation treatment or use compound inoculant to increase the crystalline core of molten iron.

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Post time: May-25-2021