Evaluation of the Influence of Bainitic Transformation on some Mechanical Properties of Calcium Treated Cast iron

Abstract

This work has investigated the effect of austempering variables on hardness, strength and toughness of 0.48wt. % calcium treated cast iron. The iron was subjected to varying austenitizing and isothermal transformation temperatures and times. Samples were austenitized for 15, 30, 45 and 60 minutes at each of the temperatures 850, 875, 900, 925 and 950 degrees centigrade. Also for 950℃ and 60 minutes of austenitizing conditions, samples of the 0.48wt. % calcium treated irons were isothermally transformed at 300 330, 360 and 420℃ for 2 hours each. The results of this work show that austenitizing temperature enhances the hardness property of the austempered iron. The Brinell hardness values of austempered iron increased from 275 BHN to368 BHN after austenitizing for an hour between 850 and 950℃ respectively. The austenitizing time does not show any significant influence in the same temperature range. The tensile strength increased from 277.5MPa at the transformation temperature 300℃ to a maximum of about 430MPa at 374℃ and decreased afterwards. The percentage elongation on the other hand, reduced to a minimum of 2.7% at 360℃ and increased thereafter with increase in transformation temperature. The austempered microstructures were the accicular lower bainites that are fine for isothermal temperatures below 374℃ and coarse upper bainites for higher isothermal transformation temperatures.