Formation of fully pearlitic microstructure in medium carbon steel

Abstract

Pearlitic microstructure can be obtained when austenite of eutectoid composition is slowly cooled from high temperature to below the Ae₃ temperature. It is also possible to induce such structure in hypo-eutectoid composition of austenite through proper heat treatment as well. However, in the current work, one hypo-eutectoid steel during very slow cooling only produced completely pearlitic microstructure which was not expected from a steel with such composition. The calculations carried out considering orthoequilibrium condition actually predicted the presence of about 37% ferrite in the microstructure. Further calculations considering different equilibrium modes and kinetics of transformation indicates that the composition and thermal condition of the steel under consideration was such that proeutectoid ferrite formation could not start before the composition reaches to the Negligible Partitioning Local Equilibrium phase boundary which further coincides with the area described by Hultgren extrapolation and thus, the steel completely transforms to pearlite even with hypo-eutectoid composition during very slow cooling.

Keywords:

• equilibrium
• thermodynamics
• kinetics
• concentration gradient
• spike width
• diffusion of solute

Acknowledgements

This work was carried out in Tata Steel Limited, India. The authors would like to acknowledge many discussions on the topic of the article with Dr Saurabh Kundu, Head, Product Research Group, Tata Steel Limited; Prof. H. K. D. H. Bhadeshia, University of Cambridge and the ThermoCalc helpdesk for helping in carrying out different calculations. It would have been difficult without their suggestion to proceed to the concluding part of the work. The authors are also grateful to Dr R. K. Ray, Visiting Scientist, Tata Steel Limited for his suggestions and to the management of R&D Division, Tata Steel Limited, India, for allowing us to carry out this work.