When the amount of ferrite in the matrix increases, the elongation after fracture increases. The data · s] show that the tensile strength increases with the increase of pearlite content, and the pearlite content is the most significant in the range of less than 80%. When the pearlescent volume exceeds 80%, the strength growth is not obvious. When the pearlescent volume is more than 95%, the tensile strength increases sharply. This is due to less ferrite and fine pearlite structure. However, not 100% single matrix structure is better. Some studies have proved that the comprehensive mechanical properties of matrix structure with volume fraction of 90% pearlite + 10% ferrite are better than those with volume fraction of 100% pearlite. When the pearlite content is less than 50%, the elongation decreases linearly with the increase of pearlite volume, but when the pearlite content is more than 50%, the elongation decreases very little.
Production practice shows that it is easy to obtain nodular cast iron with ferrite pearlite mixed matrix in as cast state. As cast ductile iron composed of ferrite (QT400-15, qt450.10, etc.) and as cast ductile iron composed of pearlite (qt600.3, QT700-2) have been widely used. However, it is required to obtain nodular cast iron with 100% ferrite or 100% pearlite matrix in the as cast state, which will bring great difficulties to the production and increase the cost.
In actual production, ductile iron takes the required mechanical properties as the acceptance standard. It is qualified as long as the properties meet the requirements, regardless of the respective proportion of pearlite and ferrite in the matrix. The microstructure analysis of nodular cast iron can effectively help people improve material properties and reduce process production costs.
For the fatigue strength of nodular cast iron, the content of pearlite in the matrix, grain size and microstructure distribution have a great influence on it. The fatigue phenomenon is divided into two stages: the occurrence and development of cracks. Generally, fatigue cracks start in the matrix structure around graphite. Therefore, in addition to improving the spheroidizing effect and reducing stress concentration, the fatigue resistance of nodular cast iron can be optimized by improving the crack initiation work and propagation work of the matrix structure around graphite. For example, in the ox eye ferrite structure, the crack propagates in the ferrite after it is produced. When the crack expands to the ferrite area, it will produce passivation. When passing through the ferrite layer and entering pearlite, because the ductility of pearlite is lower than that of ferrite, the passivation at the crack tip is weakened and the propagation speed is accelerated, and then leads to fracture; For the pure pearlite matrix structure, the graphite is surrounded by pearlite. Due to the high strength of pearlite, the crack initiation and propagation at the junction with graphite are faster than that of bovine eye structure, that is, the fatigue life will be shorter.