Bainite

About: Bainite is a research topic. Over the lifetime, 9520 publications have been published within this topic receiving 145305 citations.

On the synergy of diffusible hydrogen content and hydrogen diffusivity in the mechanical degradation of laboratory cast Fe-C alloys

Abstract: The present work investigates the influence of hydrogen on the mechanical properties of generic Fe-C alloys by tensile tests on notched samples. Different microstructures, such as pearlite, bainite and martensite are generated in a 0.2%C Fe-C alloy by an appropriate heat treatment. “Pure” iron is used as a reference material and a variation in the carbon content up to 0.4% is established for the bainitic grade. The effect of hydrogen is demonstrated by mechanical tests on both in-situ hydrogen charged and uncharged specimens. At high cross-head deformation speed (5 mm/min), the results indicate a considerable, though variable hydrogen effect for different microstructures. The bainitic and martensitic materials both show ductility drops of about 20%, whereas the pearlitic and ferritic grades display a higher sensitivity to hydrogen embrittlement (HE) with a ductility loss of approximately 50%. In order to evaluate the role of the diffusible hydrogen, tensile tests are performed at a lower cross-head deformation speed (0.05 mm/min) as well. Next to the correlation between the amount of diffusible hydrogen and HE, the distance over which hydrogen can diffuse during a tensile test, determined by hydrogen diffusion coefficient, seems to play a crucial role as well.

One-step model of the face-centred-cubic to body-centred-cubic martensitic transformation

Abstract: A `one-step' theory based on Pitsch distortion is proposed to explain the continuity between the orientations of the Kurdjumov–Sachs (KS), Nishiyama–Wassermann (NW) and Pitsch variants observed on the electron backscatter diffraction pole figures of martensitic alloys. The Pitsch distortion respects the hard-sphere packing of the iron atoms and implies the existence of a neutral line along the close-packed direction [110]γ = [111]α. Its principal strains are 0, −5.7 and 15.5%, well below the +12, +12 and −20% values of the Bain distortion. At the nucleation step, the distortion generates martensite that continuously deforms the austenitic matrix. Martensite continues to grow in Pitsch orientation inside the deformation field of the surrounding austenite to reach KS and NW orientations in reference to the bulk austenite. Some experimental results reported in the literature are revisited, such as the {225}γ habit planes, the sometimes observed `twins' at the midrib, the formation of butterfly martensite, and the effect of prior plastic deformation of austenite on the transformation temperature and on variant selection mechanisms.

Effect of B and B + Nb on the bainitic transformation in low carbon steels

Abstract: Development of new, advanced high and ultra-high strength bainitic steels requires the selection of the optimum balance of bainite promoting elements allowing the production of the desired bainitic microstructure over a wide range of cooling rates. The addition of boron or a combined addition of boron and niobium is well known to retard strongly the polygonal ferrite formation but very little knowledge has been acquired on the bainitic transformation. Therefore, the purpose of this study is to investigate the influence of boron and boron plus niobium on the bainite transformation kinetics, microstructural evolution and mechanical properties in a low carbon steel (Fe–0.05C–1.49Mn–0.30Si). Isothermal and continuous cooling transformation diagrams were determined and followed by a detailed quantitative characterisation of the bainite microstructure and morphology using complementary advanced metallographic techniques (FEG-SEM-EBSD, SIMS and TEM). The relationship between microstructure and hardness has been evaluated. Finally, results of SIMS and TEM analyses coupled with microstructural investigations enable to propose a mechanism to explain the effect of the synergy between boron and niobium on the bainitic transformation and the resultant microstructure.