With its excellent performance, ductile iron can sometimes replace expensive cast steel and forged steel in use, and ductile iron parts are widely used in the machinery manufacturing industry. Ductile iron parts produced by ductile iron casting technology have the advantages of high strength, high toughness and low price.

Surface defects of sand castings include castings produced with wet sand, water glass sand, resin sand and other sand types and cores. There are various surface defects of sand ductile iron parts, including: porosity, sand sticking, shrinkage and other defects.

I. Ductile Iron Casting Porosity Defects

Porosity is a casting defect that is difficult to analyze the cause of its formation and to find out the prevention method. This is because the formation of pores for many reasons, from the appearance and not easy to distinguish the pore is of that type. According to the mechanism of the occurrence of pores, can be divided into wrapped into, intrusion, precipitation and reaction four types of pores. Among them, wrap-in porosity is the air bubble wrapped in the metal liquid when pouring, with the liquid flow into the cavity and produce porosity defects. Intrusion pore is the casting surface solidification into a shell before the sand, sand core and other molding materials generated by heat gas intrusion into the metal liquid, the formation of air bubbles and pores ductile iron castings are often encountered in the pore defects are reaction pores and precipitation pores.

1, precipitation porosity

The atomic state of hydrogen and nitrogen gas elements dissolved in the metal liquid, with the metal temperature decreases and the solubility gradually decreases. When it drops to the crystallization temperature or solidification temperature, the solubility suddenly becomes smaller and hydrogen and nitrogen are precipitated in the molecular state to form bubbles, making the casting pores, called precipitation pores. The common precipitation porosity in factories producing cast iron is the ammonia-nitrogen porosity caused by the use of resin sand and sand core, and also the nitrogen porosity from the furnace charge and carbon enhancer.

2、Reaction porosity

The gas generated by the chemical reaction between the iron and sand, sand core, inclusions, etc., as well as the gas generated by the chemical reaction between the iron and dissolved compounds, is called reaction porosity. Generally, they are small holes of 1~3mm in diameter, distributed under the skin of the casting, so they are also called subcutaneous pores.

The subcutaneous pores of ductile iron parts are characterized by small pores of 1 to 3mm in diameter in bunches and clusters below the surface of the casting. The holes are spherical or teardrop-shaped, with smooth and shiny walls covered with a graphite film. The edge of the pores is often a metallic layer without graphite tissue. Some of the holes have very small openings and the walls are oxidized. A small part of the hole surface rough, containing Mg, Al, Mn, Si and other inclusions.

Second, the castings sticky sand defects

Sand casting table mu firmly sticky sand defects can be divided into two kinds of mechanical and chemical sticky sand. Mechanical sticky sand is caused by metal liquid drilling into the pores between the sand grains. Chemically sticky sand is sand SiO2 and metal oxides such as FeO and other chemical reaction to generate ferrous silicate (iron olivine) and castings stuck together. The chemical composition of ductile iron castings with high carbon content does not produce ferrous silicate, and the resulting sticky sand is only mechanical sticky sand.

1, ductile iron large pieces of sticky sand

The production of large parts with clay dry type, resin sand type, water glass sand type. Casting larger castings usually use zirconia powder, chromite powder, brown corundum powder, magnesium sand powder, graphite powder and other refractory aggregates of high-quality coatings. However, in order to save the production cost, the coating manufacturer may overmix certain by-product materials such as quartz powder, bauxite powder, coke powder, etc., which will reduce the effect of anti-sticky sand.

2、Sticky sand in wet medium and small ductile iron parts

Most of the cast iron parts contain coal powder in the sand, which produces a large amount of reducing gas when casting, and does not cause chemical sticky sand. The following respectively discuss the impact of various factors on the mechanical sticky sand.

(1) The degree of sand compactness

Manual modeling and shock pressure modeling of the degree of compactness is generally low, the sand surface of the sand is loose, metal liquid drilling into the sand between the feminine pores between the possibility of larger, sand cavity pits and corners of the local may also appear loose. The workers can use their fingers to tighten and use the tip of the hammer to punch tightly. Whether or not there is local looseness in high-density molding with high productivity depends on the fluidity of the sand, so many factories try to reduce the sand compacting rate to improve the fluidity of the sand. The use of micro-vibration in the sand addition and compaction process is very effective. It also depends on the setting of the compacting pressure of the moulding machine.

(2) Pouring temperature

The high temperature of the metal liquid and its good fluidity make it easy to penetrate into the pores between the sand grains and produce mechanical sand sticking and surface roughness. But from the castings to avoid porosity, cold compartment and other defects, pouring temperature can not be arbitrarily reduced. The production of complex thin-walled castings especially need a higher pouring temperature.

(3) sand particle size and permeability 

Wet type of sand grain thickness on the one hand to ensure that the exhaust after pouring, on the other hand, the permeability of the wet sand is not too high, so as to avoid easy penetration of metal liquid into the pores of the sand.