Microstructural Evolution During Rapid Induction Heating and Hardening of Microalloyed 1045 Medium Carbon Steels: Effect of Initial Microstructure

The effects of starting microstructure (hot-rolled vs lamellar pearlite (LP)-annealed) and microalloying (Nb vs Al) on the induction hardening response of 1045 steels were investigated. The hot-rolled microstructures of the steels consisted of pearlite and pro-eutectoid ferrite, while the LP anneal produced ferrite/pearlite banded microstructures. The 1045Nb steel had a lower initial ferrite fraction than the 1045Al steel. All samples after simulated induction hardening contained non-martensitic transformation products (NMTP), consisting of ferrite, bainite, pearlite, and retained austenite, in the martensitic matrix. The NMTP fraction generally depended on the prior ferrite fraction and size. For a microstructure exhibiting a higher fraction of ferrite and larger size of ferrite islands, complete austenitization during heating was slower. More homogeneous, post-induction, martensitic microstructures were obtained when Al was replaced with Nb, which is associated with decreased ferrite fraction in the pre-induction microstructure for the 1045Nb steel, rather than the direct alloying influence. Hardenability for a given induction thermal cycle was also closely related to the ferrite amount/size in the pre-induction microstructure. It is therefore concluded that prior processing to reduce ferrite fraction/size in the starting microstructure can positively contribute to induction hardenability by efficiently homogenizing the martensitic microstructure during rapid induction hardening.