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

This manuscript elucidates the influence of varying ferrite/tempered martensite (TM) content, morphology and orientation relationship (OR) between ferrite/TM with reverted austenite (gamma) 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 degrees C, 745 degrees C, and 765 degrees C, followed by austempering at 310 degrees 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 gamma reversion process in these samples led to the formation of acicular gamma at prior martensite blocks and laths while globular gamma was formed at prior gamma boundaries and packet boundaries. Notably, a lower TM content correlated with increased globular gamma formation. The acicular gamma exhibited a Kurdjumov-Sachs (K-S) OR with prior gamma. The globular gamma formed at prior gamma grain boundaries showed the K-S OR with the prior gamma where it grows. However, globular gamma formed at packet boundaries maintained the K-S OR with one of its surrounding blocks in prior gamma where it lies. In the subsequent austempering process, bainite formed from reversed gamma 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 alpha/gamma 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 gamma lamellae increased the alpha/gamma interfacial density for bainitic nucleation. Moreover, K-S oriented alpha/gamma interfaces exhibited a lower activation energy for bainite nucleation. In samples without cold rolling, both high density of alpha/gamma interfaces and K-S oriented alpha/gamma interfaces collectively facilitate the bainitic transformation.