Effects of Molybdenum on Continuous Cooling Transformation Behavior and Heat Treatment Characteristics for Low Alloyed Ductile Iron

The purpose of this study is to clarify the continuous cooling transformation behavior and the heat treatment characteristics of five (5) kinds of low alloyed ductile cast irons containing Mo between 0mass% and 2mass%. The specimens were austenitized at 1223K-900s, and the range of cooling time from Ac3 to 773K was about 3 to 4000s. Ferrite, pearlite, bainite and martensite transformation regions existed in CCT diagrams for ductile cast irons used. The ferrite transformation region in the CCT diagram shifted to a longer cooling time side with an increase in Mo content because of the impurity solute drag effect of Mo. The pearlite transformation region in CCT diagrams for ductile cast irons also shifted to a longer cooling time side. Movement to the long time side of this pearlite transformation region is considered owing to the behavior that Mo suppresses the growing speed of pearlite. The bainite transformation region existed even at a longer cooling time side in CCT diagrams when Mo was added. However, critical cooling time and transformation temperature for bainite in ductile cast iron containing Mo remained about the same, which would have originated from the decrease in the amount of ferrite and pearlite by Mo as well as the promoting action of the bainite transformation by Mo. The martensite transformation region existed even in the longer cooling time range with increasing Mo content. The Ms temperature of low alloyed ductile cast irons was not influenced by Mo, but the quenching hardness of 0Mo cast iron was 630HV10, and that of 2Mo cast iron was 700HV10. The difference in hardness is thought to have originated from the crystallized and precipitated carbide content. In conclusion, Mo has little effect on the hardenability for low alloyed ductile cast irons.