Study on the Control Method of Ferrite Grain Size of Nodular Cast Iron and Its Effect on Low-Temperature Impact Toughness

In this paper, the ferrite grain size was controlled by two annealing methods, which can affect the low-temperature toughness of ferrite nodular cast iron. The annealing temperature ranged from 720 to 750 °C, and one annealing method consisted of constant temperature heating with temperature controlled to ± 1 °C. The other method employed cyclic heating, with a temperature change of 19 ( + 14, − 5) °C and a period of 450 s. The change law of ferrite grain size under low temperature annealing and the effect of ferrite grain size on low-temperature impact toughness were studied. The conditions under which the superheated ferrite does not transform into austenite were determined. The mechanism by which coarse ferrite favors high toughness in nodular cast iron at low temperatures was discussed. Nodular cast iron with tensile strength of 392 MPa, yield strength of 206 MPa, elongation of 26% and impact energy of 13.5 J at − 70 °C was obtained.