The Precautions and Difficulties of Ductile Iron?

Ductile iron is gray cast iron with excellent mechanical properties. Generally, before pouring, a small amount of spheronization agent (usually rare earth magnesium ferrosilicon alloy) and inoculant (usually ferrosilicon) are added to the molten iron to solidify the molten iron to form spherical graphite. This kind of cast iron has higher strength and toughness than other cast irons, and sometimes can replace cast steel and malleable cast iron, and has been widely used in the machinery manufacturing industry. We need to pay attention to the following things.

(1) The chemical composition is strictly required. The carbon and silicon content required for the original molten iron is higher than that of gray cast iron, which reduces the content of manganese, phosphorus, and sulfur in ductile iron.

(2) The temperature of molten iron is higher than that of gray cast iron to compensate for the loss of molten iron temperature during spheronization and inoculation.

(3) Carry out spheronization treatment, that is, adding spheronization agent to the molten iron.

(4) Add inoculant for inoculation treatment. 

(5) Nodular cast iron has poor fluidity and large shrinkage, so higher pouring temperature and larger pouring system size are required, and the riser and chilled iron are used reasonably, and the principle of sequential solidification is adopted.

(6) Heat treatment.  

①Annealing.

Obtain ferrite matrix, improve plasticity, toughness, eliminate stress, and improve cutting performance. 

②Normalizing.

A pearlite matrix is obtained, which improves strength and wear resistance. 

③ Tempering.

Obtain the matrix structure of tempered sorbate and good comprehensive mechanical properties, such as main shaft, crankshaft, connecting rod, etc.

④ Austempering.

Make parts with complex shapes and high comprehensive performance requirements obtain the matrix structure of lower bainite, as well as comprehensive mechanical properties such as high strength, high hardness, and high toughness, to avoid cracking during heat treatment, such as spindles, crankshafts, gears, etc.

 Finally, I will introduce some difficulties in the production of ductile iron.

Due to the thick section of this type of casting, the cooling is slow, the solidification time of the liquid metal is long, and the inside of the casting is prone to shrinkage porosity. In the production of ferritic ductile iron, in order to obtain higher tensile strength, yield strength, and elongation, fertilizing heat treatment has been used in the past. The heat treatment temperature is based on whether there is free cementite or pearlite in the as-cast structure. , While using 900-950℃ high-temperature heat treatment. However, the high production cost, complex process, and long production cycle bring great difficulties to the production organization and delivery date, which requires that the ferrite matrix must be obtained in the as-cast state.

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