The influence of ferrite content, ferrite-austenite morphology, and orientation relationship on bainite transformation in intercritically annealed bainitic steels

Abstract

This manuscript elucidates the influence of varying ferrite/tempered martensite (TM) content, morphology and orientation relationship (OR) between ferrite/TM with reverted austenite (γ) resulting at different intercritical temperatures on the subsequent bainite transformation behaviour. A comprehensive analysis was conducted using scanning electron microscopy, electron backscattered diffraction, and dilatometry. TM/bainite and ferrite/bainite steels were produced through intercritical annealing at 735 °C, 745 °C, and 765 °C, followed by austempering at 310 °C, using initially quenched and cold-deformed martensite, respectively. In samples without cold rolling, TM and bainite are alternately distributed at all intercritical temperatures. The γ reversion process in these samples led to the formation of acicular γ at prior martensite blocks and laths while globular γ was formed at prior γ boundaries and packet boundaries. Notably, a lower TM content correlated with increased globular γ formation. The acicular γ exhibited a Kurdjumov–Sachs (K-S) OR with prior γ. The globular γ formed at prior γ grain boundaries showed the K-S OR with the prior γ where it grows. However, globular γ formed at packet boundaries maintained the K-S OR with one of its surrounding blocks in prior γ where it lies. In the subsequent austempering process, bainite formed from reversed γ followed the K-S OR with TM. The TM and bainite shared the same variant in the same block. In samples with cold rolling, polygonal ferrite and bainite were distributed at all intercritical temperatures. Decreasing the ferrite content showed a coarsening of the microstructure. All polygonal samples revealed an irrational OR between bainite/martensite and ferrite. In samples with and without cold rolling, the bainitic formation rate decreased with increasing ferrite/TM, likely due to the high C and Mn content at the α/γ interface. Comparatively, the samples without cold rolling showed faster bainitic transformation than samples with cold rolling at similar ferrite/TM percentages, particularly noticeable at higher ferrite/TM content. The alternately arranged elongated TM and reversed γ lamellae increased the α/γ interfacial density for bainitic nucleation. Moreover, K–S oriented α/γ interfaces exhibited a lower activation energy for bainite nucleation. In samples without cold rolling, both high density of α/γ interfaces and K–S oriented α/γ interfaces collectively facilitate the bainitic transformation.