The effect of porosity on the austenite to bainite transformation in powder metallurgy steels
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The effect of porosity on the kinetics of the austenite to bainite isothermal transformation in powder metallurgy steels was characterized using a high-speed quenching dilatometer. The measurements revealed that the presence of porosity shortens the incubation time as well as the overall isothermal transformation time. An Avrami-type equation was fitted to the measured data, and the effect of porosity on the nucleation rate of bainite was quantified. In addition, the activation energy for diffusion of carbon atoms during nucleation of bainite was calculated and was found to decrease with increasing porosity. I. INTRODUCTION
Despite numerous investigations, the effect of porosity on phase transformations in powder metallurgy steels is not yet certain. There is agreement that the presence of porosity in sintered steels causes a decrease in the Ac1 temperature, an increase in the Ar1 temperature, and, under isothermal conditions, shortening of the incubation time and the overall reaction time,1–3 yet a detailed quantitative understanding of the role that porosity plays in the kinetics of the transformations is still lacking. The present work focuses on the kinetics of the austenite to bainite transformation in powder metallurgy steels. In these experiments, typical powder metallurgy steel specimens of controlled porosity are austenitized and quenched at very high rates to various isothermal transformation temperatures in a high-speed quenching dilatometer. From the measured specimen dilatation, information on the bainite fraction formed over time is obtained and conclusions regarding the kinetics and the prevailing driving force for the austenite to bainite transformation are drawn. Particular focus of the work is placed on exploring the role that porosity plays in the transformation mechanism. Finally, the transformation kinetics is analyzed quantitatively, and the effect of porosity on the activation energy of diffusion of carbon atoms in austenite during the transformation is calculated. II. MATERIALS AND PROCEDURES
AUTOMET 4601 steel powder1 (Avanel Industries, Westbury, NY) was admixed with powdered graphite to yield 0.5-wt% carbon in the final product. Table I shows the chemical composition of the resultant powder. a)
Address all correspondence to this author. email: [email protected] DOI: 10.1557/JMR.2009.0379 J. Mater. Res., Vol. 24, No. 10, Oct 2009
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Bulk material was produced from this powder in three different densities corresponding to 90, 95, and 100% of theoretical density. To produce the 90% dense material, the powder was cold-compacted using 690 MPa pressure in a hydraulic press to produce green compacts that were then sintered at 1120 C for 30 min under a controlled atmosphere. To produce the 95% dense material, the powder was cold-compacted using 690 MPa pressure, but the green compacts were first presintered at 850 C for 30 min and then they were re-pressed using 690 MPa pressure and re-sintered at 1120 C for an additional 30
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