Effect of Intercritical Austenitization Temperatures in Normalizing Heat Treatment on Microstructure and Mechanical Properties of Ductile Iron

Abstract

In this investigation, the effect of normalizing heat treatments from annealing at intercritical austenitization temperatures on the microstructural evolution and mechanical properties of ductile iron was examined. Tensile specimens were prepared by turning from Y block castings made from unalloyed ductile iron. The specimens were air-cooled from different intercritical austenitization temperatures at (770°C to 820°C) for 60 minutes. Microstructural characterization was performed by optical microscope and scanning electron microscope (SEM). Mechanical properties were determined by tensile and hardness tests. Ductile iron containing different pearlite volume fractions (8 % to 75 %) can be produced by normalizing heat treatment from intercritical austenitization temperatures in ductile iron with a certain chemical composition. The pearlite volume fraction is significantly increased with increasing intercritical austenitization temperatures. Moreover, the hardness, yield, and tensile strength are significantly increased with increasing pearlite volume fraction. However, total elongation and reduction in area decreased with decreasing volume fraction of proeutectoid ferrite. With increasing volume fraction of pearlite from 8 % to 75 %; the hardness increased from 181 BHN to 259 BHN; yield strength increased from 348 MPa to 470 MPa; and the tensile strength significantly increased from 569 MPa to 790 MPa, while the total % elongation decreased from 12.6 % to 8.3 %. The fractographic analysis showed that the number of dimples zones decreased with increasing volume fraction of pearlite.