Bainite

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

Temperature Dependence of Austenite Nucleation Behavior from Lath Martensite

Abstract: The temperature dependence of austenite nucleation behavior was investigated in an ultralow carbon 13%Cr–6%Ni martensitic stainless steel. The martensitic structure was partially reversed to austenite by heat treatment at different temperature in (austenite+ferrite) two-phase region. With increasing the reversion temperature, the shape of austenite grains tend to be changed from acicular to granular, and their nucleation site is changed from lath boundaries to prior austenite grain boundaries. The transition of nucleation site was discussed in terms of energetics by considering the increases in interfacial energy and elastic strain energy by formation of an austenite nucleus. The calculation results suggested that lath boundary is more preferential nucleation site rather than prior austenite grain boundary because the increment of elastic strain energy is reduced with lowing the reversion temperature.

Bainitic transformation during the two-step quenching and partitioning process in a medium carbon steel containing silicon

Abstract: A study of 40SiMnNiCr steel subjected to a two-step quenching and partitioning process (Q&P) is presented. The result suggests that strength variation of Q&P steels during the two-step Q&P process was a cumulative effect of increase of retained austenite fraction, decrease of carbon supersaturation of virgin martensite, and particularly much of lower bainite formation. A trade-off between high strength and good ductility of two-step Q&P steels can be tailored and adjusted by controlling lower bainite fraction. The final amount of austenite was influenced by the transformation kinetics of lower bainite during the partitioning process.

Role of ɛ martensite in tensile properties and hydrogen degradation of high-Mn steels

Abstract: Effects of ɛ martensite on tensile properties and hydrogen degradation behaviors of a high Mn steel were investigated. For this purpose, a Fe–15Mn–2Cr–0.6C steel containing various amount of ɛ martensite was prepared and tensile tested at room temperature. Microstructures were examined by electron back scattered diffraction and transmission electron microscopy. Then, a series of electrochemical hydrogen pre-charging, slow strain rate tests, and thermal desorption spectrometry (TDS) analyses was conducted to examine the hydrogen degradation behaviors. Deformation of the steel without ɛ martensite (i.e. fully austenitic) was dominated by slip and mechanical twinning, but that of the steel containing ɛ martensite was mainly attributed to transformation induced plasticity in association with strain induced martensitic transformation during deformation, resulting in higher work hardening rate. However, tensile strength and elongation on the steel containing ɛ martensite were lower than those of the fully austenitic steel, since cracks were prone to be initiated and propagated at the region of ɛ martensite which is harder than austenite. Furthermore, it was found that ɛ martensite provided many diffusible hydrogen trapping sites. Consequently, the notch fracture stress of the steel containing ɛ martensite decreased significantly as the diffusible hydrogen content increased. The activation energy for hydrogen detrapping from its trapping sites was also calculated by means of the TDS analyses, ∼22 kJ/mol for the γ/ɛ interfaces, and ∼37 kJ/mol for dislocations/γ grain boundaries.

Coupled-solute drag effects on ferrite formation in Fe-C-X systems

Abstract: The influence of X upon proeutectoid ferrite/microstructurally defined bainite formation in Fe-C-X alloys, where X is Co, Cr, Cu, Mn, Mo, Ni, Ni, Si, or V, is examined in terms of the competing influences of the coupled-solute drag effect (C-SDE) and the shifting in the paraequilibrium Ae3 curve. The relative strength of the C-SDE was estimated primarily in terms of the influence of X upon the Wagner interaction parameter for C-X interaction in austenite, e 12 γ . Changes in the W s (Widmanstatten-start temperature) with X additions at a constant pct C are ascribed almost entirely to shifts in the paraequilibrium Ae3. Influence of X upon the steady-state nucleation rate of ferrite allotriomorphs at austenite grain faces is largely explicable upon the same basis, though additional effects appear to be exerted by Mn and Ni upon relative values of the interfacial energies involved in the nucleation process. Development of a bay in the TTT curve for initiation of ferrite formation occurs more readily with increasingly negative e 12 γ and increasing carbon concentration. When e 12 γ > 1, considerably larger X and C concentrations are required to form what may sometimes be described as a “virtual bay,” only the lower portion of which is experimentally detectable. As e 12 γ becomes increasingly negative (but not as much when e 12 γ is increasingly positive), the influence of paraequilibrium Ae3 shifts is much diminished. Widmanstatten sideplate formation is also increasingly suppressed and replaced by grain boundary and twin boundary allotriomorphs at temperatures between the upper nose and the bay of the ferrite-start TTT curve. Changes in carbide precipitation patterns and replacement of (Fe, X)3C by alloy carbides directly follow from these alterations in ferrite morphology. When e 12 γ is sufficiently negative, incomplete transformation occurs below the bay temperature. Much higher proportions of X and C are required to produce this effect when e 12 γ > 1. Still more negative values of e 12 γ may be required to develop the degenerate ferrite microstructures below the bay that result from increasing C-SDE restrictions on growth and the consequent frequently repeated sympathetic nucleation.

Prediction of Martensite Start Temperature in Alloy Steels with Different Grain Sizes

Abstract: A new equation to predict the martensite start (M s) temperature was proposed considering the effects of the chemical composition and grain size of austenite in alloy steels. The calculation results demonstrated that the addition of an alloying element decreased the M s temperature, and austenite with smaller grain sizes resulted in a remarkable reduction of the M s temperature. The variation of the M s temperature calculated by the proposed equation was in good agreement with experimental data of alloy steels with various grain sizes.