magnanese phosphorus and sulfur content of nodular cast iron

In the process of solid phase transformation, the diffusion of manganese atoms is more difficult than that of carbon atoms, and the diffusion rate is much lower than that of carbon atoms. This leads to an increase in the phase transition resistance of castings with higher manganese content, and hinders the diffusion of iron atoms, slowing down the decomposition rate of austenite Increase the degree of eutectoid transformation supercooling, lengthen the transformation incubation period, and reduce the critical cooling rate to avoid pearlite transformation.At a lower eutectoid temperature, the eutectoid transformation rate is accelerated, and the migration of carbon atoms in austenite becomes difficult. Therefore, manganese inhibits the formation of ferrite, thereby increasing the volume fraction of pearlite in the matrix. The ratio of ferrite and pearlite content in the matrix can be changed by adjusting the amount of manganese, and the properties of the casting can be changed. Manganese significantly reduces the eutectoid temperature. For every 1% increase of manganese in nodulalr cast iron, the eutectoid transformation temperature will drop by about 20 degrees.Manganese reduces the carbon content of the eutectoid structure. When the manganese content of the matrix is not too high, every 1% of manganese can reduce the carbon content of the eutectoid structure by 0.05%-0.06%. Add manganese to move the eutectoid point to the lower left of the phase diagram. The manganese atoms in the eutectoid transformation of austenite contribute to the hardening of thick nodulalr cast iron castings.

In some chemical composition standards of nodulalr cast iron, the mass fraction of phosphorus is less than or equal to 0.07%. But for thick-section as-cast ferritic nodulalr cast iron castings, when the phosphorus content in the molten iron is 0.04%, phosphorus eutectic has already appeared, and the plasticity and impact toughness of the material have decreased significantly.When the mass fraction of phosphorus (%) starts to increase from 0.06%, the tensile strength and hardness increase accordingly. The shrinkage rate at break dropped from 15% at 0.07% phosphorus to 5% at 0.1 phosphorus. Therefore, with the presence of phosphorus eutectic, large intergranular volume shrinkage occurs during solidification.This shrinkage is difficult to be supplemented by the feeding system of the casting. Therefore, it is easy to form microscopic porosity in the casting. For die castings, these pores often become defects that are difficult to repair. The phosphorus in nodulalr cast iron mainly comes from the charge. It is currently not possible to reduce the phosphorus content in iron during the smelting process. However, for ferritic nodulalr cast iron castings with high plastic toughness requirements, high-temperature annealing can be used to decompose the carbides in the phosphorus eutectic. Although the carbides in the phosphorus eutectic are relatively stable, after high-temperature annealing, The adverse effects of phosphorus eutectic can be reduced.

Sulfur is soluble in molten iron, but not soluble in Alpha iron and Gama iron. Sulfur combines with iron to form iron sulfide. The melting point is slightly higher than the eutectic temperature (1193 degrees Celsius). Iron sulfide and iron form a eutectic (melting point 985 degrees Celsius).  Two kinds of eutectic exist in the eutectic boundary after the casting is solidified, which destroys the continuity of the base metal and reduces the mechanical properties of nodulalr cast iron.Generally, the weight of sulphur is reduced to about 0.02% after pre-treatment of nodulalr cast iron parts. Normalized castings can generally meet this standard. For nodulalr cast iron parts used at low temperatures, in order to maintain low temperature plastic toughness, the sulfur mass fraction should not exceed 0.01%. Sulfur interferes with graphite spheroidization. Sulfur has a strong affinity with spheroidizing elements (RE, magnesium, lanthanum). The spheroidizing elements added to molten iron during spheroidizing process produce magnesium sulfide (manganese sulfide) or RE sulfide (CeS Ce2S or LaS). After most of the sulfur in the molten iron is combined with magnesium or cerium, it loses its interference effect. In addition, the compound of trace sulfur, magnesium and RE is the main component of the crystalline core of spherical graphite, which helps the nucleation of spherical graphite and increases the number of graphite spheres.