Papers by Keyword: Bainite

Abstract: Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of grain boundary ferrite allotriomorph in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of the sintered steel, investigated in this study, with the aim of improved mechanical properties. The sintered steel, produced from pre-alloyed Fe-Cr-Mo powder mixed with 0.3 wt.% C, was modified by Ni addition and cooling rate. The alloy compositional change was performed additions of varied Ni contents of 1.0, 1.5, 2.0, 2.5 and 3.0 wt.%. The material processing variables were investigated by using two different cooling rates of 0.1 and 5.4 °Cs^-1. Under the cooling rate of 0.1 °Cs^-1, the sintered Fe-Cr-Mo-3C steel without Ni addition showed dual-phase microstructure consisting of ferrite (soft phase) and bainite (hard structure). With Ni additions, the dual-phase microstructure was replaced by bainitic structure. Microstructural heterogeneity was observed due to the presence of Ni-rich areas, which increased with increasing added Ni contents. Under the cooling rate of 5.4 °Cs^-1, the sintered Fe-Cr-Mo-3C steels with and without Ni additions showed bainitic structure. Microstructural heterogeneity was similar to that of slowly cooled steel. Ni additions not only modified the sintered steel microstructure but increased tensile strength and elongation. In general, Ni pushes the C-curve of pearlite transformation to the right hand side and lowers the martensite start temperature. The absence of ferrite from the sintered steel with only 1 wt.% Ni addition and slowly cooled at 0.1 °Cs^-1 suggests that Ni strongly suppresses the austenite → ferrite transformation. In another word, Ni promotes bainite formation in the sintered Fe-Cr-Mo-Ni-C steels.

Abstract: Microstructure consisting of ferrite and bainite in low C-Mn steels with different manganese contents (0.75-1.49 mass %) can be formed by accelerated cooling. Microstructure observation reveals that the transformation products of 0.75%Mn and 1.06%Mn steel mainly consist of equiaxed ferrite and pearlite when hot finish deformation was processed at the temperature of 800°C, while the transformed products mainly contained irregular ferrite and upper bainite when finish rolling temperature (FRT) increases to 850°C or 900°C. However, a large amount bainite can also be attained in 1.49%Mn steel when FRT decreased to 800°C. The strength increases by about 100MPa with the increase of manganese content from 0.75% to 1.49%. The 1.06%Mn steel exhibits a superior strength-elongation combination by deforming at FRT of 850°C and then accelerated cooling at the cooling rate of about 40°C/s to coiling temperature in the range of 490~510°C.

Abstract: The purpose of this investigation is focused on the direct quench and temper mechanisms of the high strength offshore steel. Microstructural analyses of martensite and retained austenite in the direct quenched steel, simulations of martensitic transformation temperatures, M_s/M_f, and morphologies of bainite and ferrite have been evaluated in the experiment. Additionally, carbide formation after temped at various temperatures and microhardness depth profiles after heat treating are also included in the study. The direct quenched steel is primarily comprised of martensite, bainite and a few retained austenite with Vickers microhardness above 300. Tempered martensite, iron carbide and bainite are widely observed from all tempered specimens.

Abstract: New trends focused on achieving higher performance steels has led to a so-called 3^rd Generation Advanced High Strength Steels (AHSS), in which the typical polygonal ferrite found in TRIP steels as a matrix phase is replaced by harder phases as Carbide-Free Bainite (CFB) and/or (tempered) martensite. Besides, large volume fractions of retained austenite (R.A.) with adequate stability are aimed for to improve the formability of the steels. Si containing steels are regarded as the most suitable to retard cementite formation and consequently reach high volume fractions of RA. In this work, CFB annealing schedules were applied to dilatometer samples of Fe-0.22C-2.0Mn-1.3Si. The overaging temperature T_B was varied between 390 oC and 480 oC, and other processing variables investigated were the austenitizing temperature T_aus, and the overaging holding time t_B. The annealed samples analyzed with LOM, FEG-SEM, EBSD and X-ray diffraction techniques show that markedly different complex microstructures made up of bainite, ferrite, MA phase and retained austenite (R.A) are accomplished depending on the specific thermal cycle. These results are described in detail and discussed in relation to the dilatometry measurements.

Abstract: The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac₁= 766 °C, while Ac₃ was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σ_f/ε) was directly proportional to temperature difference from pass to pass.

Abstract: The machinability of an experimental medium-carbon steel with a composition designed to promote rapid graphitisation during a high temperature anneal has been studied. The goal has been to explore alternative routes to a competitive free-cutting composition enabling less expensive steelmaking, manufacturing and recycling. Three starting microstructures prior to annealing have been considered; martensite, bainite and ferrite/pearlite. The microstructures and graphite dispersions formed have been characterised by optical and electron microscopy and the performance of the steel during machining compared with commercial free-cutting steel grades. A bench-top drill rig and metallographic techniques were used to evaluate relative machinability parameters, including surface roughness, tool wear and chip morphology. Thus it proved possible to rank the experimental steel graphitised from the three starting microstructural conditions and also against the commercial free-cutting steels.

Abstract: Processing bulk nanoscrystalline materials for structural applications still poses a significant challenge, particularly in achieving an industrially viable process. In this context, recent work has proved that complex nanoscale steel structures can be formed by solid reaction at low temperatures. These nanocrystalline bainitic steels present the highest strength ever recorded, unprecedented ductility, fatigue on par with commercial bearing steels and exceptional rolling-sliding wear performances. A description of the characteristics and significance of these remarkable structures in the context of the atomic mechanism of transformation is provided.

Abstract: New steel grades for forged components are designed to meet the requirements of the automotive industry in order to obtain excellent strength and toughness behavior as well as a high endurance limit. Beside precipitation hardened ferritic-pearlitic steels, bainitic steels have gained more and more importance. Basic considerations on the alloy design (C-, Si-, Cr-; B-content) are done using JMatPro-Calculations and by some experimental trials. Using the thermomechanical testing system Gleeble 3800, various cooling strategies have been applied and the kinetics of the bainite formation has been measured at different holding temperatures and times. A detailed microstructural characterization has been done with relation to the mechanical properties. The isothermal tests are compared to continuous cooling situations. Finally, forging trials are performed to find out the most suitable and robust production schedule to be used in practice. The actual findings support the increasing use of bainitic steels for forged parts, especially regarding material saving, independence of cross section and good fatigue performance.

Abstract: In the scope of the optimization of multi phase steels, e.g. for the automotive industry, control of the microstructure is essential to tailor the mechanical properties. In this study, two cold rolled steels varying in carbon content were annealed and cooled under different laboratory conditions. The microstructure is investigated using optical and electron microscopy and EBSD. The results are correlated to the mechanical properties obtained from tensile, hole expansion and bending test. It is found that tensile strength and elongation are mainly dependent on martensite volume fraction, while yield strength is less affected by chemical composition or annealing treatment. In contrast, hole expansion capacity and maximum bending angle are significantly improved by the homogenization of the microstructure which is independent of strength and elongation. The microstructure homogeneity is expressed by analyzing the Lorenz curves derived from the kernel average misorientation from EBSD measurements.

Abstract: The paper considers the effect of low-carbon pipeline steels initial condition on their mechanical properties and structure after quench hardening at different temperatures in the intercritical temperature range Ac₁–Ac₃ (ITR) followed by tempering at 600 °C. If the prior heat treatment is annealing or normalizing , which produces the ferrite-pearlite structure, then quenching from temperatures just above Ac₁ causes very strong embrittlement due to the formation of a high-carbon austenite film at ferrite/pearlite boundaries. Increasing quenching temperature in the intercritical range increases impact toughness and ductile fracture fraction for both types of prior treatment, though normalizing provides higher impact toughness than annealing. On the contrary, if the prior heat treatment is quench hardening, then the highest impact toughness is observed when the second quenching temperature lies a little above Ac₁. Impact toughness and ductile fraction for preliminarily quenched samples gradually decreases along with the increase in austenitization temperature in the intercritical range.