In situ observation of bainite growth during isothermal holding

The advanced electron backscatter diffraction (EBSD) technique was used to examine the microstructure of a widely used A517GrQ low-carbon low-alloy steel after different heat treatments. Three distinguishable microstructures were studied. Slow cooling in the furnace after austenitization led to the formation of a granular structure that consisted of massive ferrite and randomly distributed M–A constituents. Medium rate cooling in air produced granular bainite that was composed of lath ferrite, and M–A constituents were distributed between the laths. Lath martensite was formed by fast cooling into ice brine. EBSD analysis revealed that, in one austenite grain, the massive ferrite in the granular structure and the lath ferrite in the granular bainite were predominately separated by high-angle boundaries, whilst the ferrite laths in the martensite were separated by low-angle boundaries. The specimens with granular bainite formed by medium rate cooling had higher strength (both yield strength and tensile strength), and also almost 5 times higher Charpy impact energy than that of the specimens containing granular structure obtained at the slow cooling. The strength of the specimens with lath martensite after quenching into ice brine was slightly higher than the granular bainite but were associated with much lower Charpy impact energy. The present work indicates that it is critical to control the cooling rate after austenitization in order to simultaneously achieve high strength and high toughness of low-carbon low-alloy steels.

Effect of cooling rate on microstructure and mechanical properties of a low-carbon low-alloy steel

The prebainitic phenomenon in several copper alloys and ferrous alloys was investigated by means of internal friction measurement and composition analysis. The internal friction peaks of zinc or carbon atoms diffusion inβ brasses or in austenite of ferrous alloys, respectively, were related to the zinc or carbon-depleted region formation within the bainitic incubation period observed by composition analysis. The possibility of bainite shear nucleation in solute atoms’ depleted regions, formed by diffusing of solute atoms to a certain level, was pointed out.

On the prebainitic phenomenon in some alloys

The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950 °C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}γ and {211}α reflections during bainitic transformation are discussed in detail.

/pdf/in-situ-investigation-of-the-bainitic-transformation-from-1uvc9yvzpv.pdf

In Situ Investigation of the Bainitic Transformation from Deformed Austenite During Continuous Cooling in a Low Carbon Mn-Si-Cr-Mo Steel

The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total © University of Pretoria iii absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy.

The role of Cr and Mo alloying element additions on the kinetics and effects of Upper Bainite formation in quench and tempered plate steels

Time-temperature-transformation (TTT) diagrams within the medium temperature range of some alloy steels have usually been found to be composed of three separate kinds of C-curves for low-carbon alloy steels. The microstructure associated with one of three kinds of C-curves is a granular structure which is nonbainite, upper bainite, or lower bainite, depending on the higher, middle, or lower location of the C-curve in the TTT diagrams. On the other hand, only two types of C-curves can be established for the alloy steels of middle and high carbon content, and the category of microstructures (upper bainite and lower bainite) corresponding to each C-curve also depends on the location of the C-curve. Meta-upper bainite, or alternatively, carbide-free bainite, and meta-lower bainite and/or the mixture of them can often be obtained in some alloy steels containing silicon. Each type of microstructure still possesses its own overall transformation activation energy, Q*, and morphological exponent, n. A considerably detailed analysis has suggested some differences among the transformation mechanisms of granular structure, upper bainite, and lower bainite.

/pdf/the-time-temperature-transformation-diagram-within-the-im3hmbn5v4.pdf

The time-temperature-transformation diagram within the medium temperature range in some alloy steels

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The time-temperature-transformation diagram within the medium temperature range in some alloy steels