Ductile iron

Ductile iron, with its excellent properties, can sometimes replace expensive cast steel and forged steel in applications, and ductile iron parts are widely used in the machinery manufacturing industry. Ductile iron parts produced using ductile iron casting technology have advantages such as high strength, high toughness, and low price.

Surface defects in sand castings include those produced using sand molds and cores made with wet sand, water glass sand, resin sand, etc. There are various surface defects in sand-cast ductile iron parts, including porosity, sand adhesion, and shrinkage cavities.

I. Porosity Defects in Ductile Iron Castings

Porosity is a casting defect whose formation is difficult to analyze and whose prevention methods are difficult to find. This is because there are many causes of porosity, and it is not easy to distinguish the type of porosity from its appearance. According to the mechanism of porosity, it can be divided into four types: inclusion, intrusion, precipitation, and reaction porosity. Among them, inclusion porosity is a porosity defect caused by air bubbles being trapped in the molten metal during pouring, which enter the mold cavity with the liquid flow. Intrusion porosity occurs when gases generated by the heating of molding materials such as sand molds and cores penetrate the molten metal before the surface of the casting solidifies into a shell, forming bubbles. Common porosity defects encountered in ductile iron castings include reaction porosity and precipitation porosity.

1. Precipitation Porosity

The solubility of atomic hydrogen and nitrogen gaseous elements dissolved in molten metal gradually decreases as the metal temperature decreases. When the temperature drops to the crystallization temperature or solidification temperature, the solubility suddenly decreases, and hydrogen and nitrogen precipitate as molecular gaseous phases, forming bubbles, resulting in porosity in the casting, known as precipitation porosity. In cast iron production plants, common precipitation porosity is ammonia-nitrogen porosity caused by the use of resin sand molds and cores, and nitrogen porosity also originates from furnace charge and carburizing agents.

2. Reaction Porosity

Porosity generated by chemical reactions between molten iron and sand molds, cores, inclusions, etc., as well as by chemical reactions between molten iron and dissolved compounds, is called reaction porosity. Subcutaneous porosity generally refers to small pores, 1-3 mm in diameter, distributed beneath the surface of the casting, hence also called subcutaneous porosity.

The characteristics of subcutaneous porosity in ductile iron castings are: strings or clusters of small pores, 1-3 mm in diameter, located 1-3 mm below the casting surface. The pores are spherical or teardrop-shaped, with smooth, shiny walls covered by a graphite film. The edges of the pores are often a layer of metal without graphite structure. Some pores have very small openings, and the pore walls are oxidized. A small number of pores have rough surfaces and contain inclusions such as Mg, Al, Mn, and Si.

II. Sand Adhesion Defects in Castings

Sand adhesion defects, where molten metal adheres firmly to the surface of sand castings, can be divided into mechanical sand adhesion and chemical sand adhesion. Mechanical sand adhesion is caused by molten metal penetrating into the pores between sand grains in the sand mold. Chemical sand adhesion occurs when SiO2 in the sand mold reacts chemically with metal oxides such as FeO to form ferrous silicate (ferroolitic silicate), which adheres to the casting. Ductile iron castings have a high carbon content and do not produce ferrous silicate; the resulting sand adhesion is purely mechanical.

1. Sand Adhesion in Large Ductile Iron Castings

Large castings are typically produced using clay dry molds, resin sand molds, or water glass sand molds. For larger castings, high-quality coatings made from refractory aggregates such as zircon powder, chromite powder, brown corundum powder, magnesia powder, and graphite powder are commonly used. However, coating manufacturers may, to save production costs, excessively add certain low-value materials such as quartz powder, bauxite powder, and coke powder to the powder, thus reducing its anti-sand adhesion effect.

2. Sand Adhesion in Medium and Small Ductile Iron Castings in Wet Molds

Most casting sand contains coal powder, which generates a large amount of reducing gas during casting, preventing chemical sand adhesion. The following discusses the influence of various factors on mechanical sand adhesion.

(1) Sand Mold Compaction

Manual and vibratory molding generally result in lower compaction. The sand particles on the mold surface are relatively loose, increasing the likelihood of molten metal penetrating into the pores between the sand particles. Looseness may also appear in pits and corners of the mold cavity. Workers can tighten the mold by pressing with their fingers or using the tip of a hammer to locally compact it. Whether high-density molding with high productivity has localized looseness depends on the fluidity of the molding sand. Therefore, many factories try to reduce the compaction rate of the molding sand to improve its fluidity. Micro-vibration during sand addition and compaction is very effective. It also depends on the setting of the compaction pressure on the molding machine.

(2) Pouring Temperature

High molten metal temperatures result in better fluidity, making it easier for it to penetrate into the pores between sand particles, leading to mechanical sand adhesion and surface roughness. However, to avoid defects such as porosity and cold shuts in the casting, the pouring temperature should not be arbitrarily lowered. Higher pouring temperatures are especially necessary when producing complex thin-walled castings. (3) Particle size and permeability of molding sand

The particle size of wet molding sand should ensure that the air is vented smoothly after casting. On the other hand, the permeability of wet molding sand should not be too high, so as to prevent the molten metal from easily penetrating into the pores of the sand particles.