Mechanism Of Bainite Transformation Research Articles

The mechanism of bainite transformation in steels

Sympathetic nucleation and ledgewise growth are competitive with each other throughout the bainite formation in steels. Many evidences are provided to support this mechanism as follows: ultra-fine structures of bainite and its surface relief, and carbide nucleation inside austenite at the interface of ferrite/austenite, etc.

A Model for Surface Reliefs Formation in Bainite Transformation Mechanism

The most recent and important research results of our group on the microstructure and transformation mechanism of bainite in both ferrous and non-ferrous alloys are reviewed. The objective and popular existence of three-dimensional superledges and nanometre scale edges were confirmed. It was discovered by scanning tunneling microscopy (STM) that the bainitic plates were composed of ultra fine structural units, and their surface reliefs and those associated with Widmanstatten ferrite were tent-shaped. Carbides nucleated in the matrix and grow toward austenite and the model of carbide formation was provided. The sympathetic nucleation and ledgewise growth (SNLG) mechanism of bainite has further been described and applied to interpret the complicated multi-layer substructure. The model of multi-layer structure of bainite has been proposed.

Bainitic transformation in austempered ductile iron with reference to untransformed austenite volume phenomenon

Much interest has been focused on austempered ductile iron (ADI) because of its superior mechanical properties, which might be improved by further control of microstructure. It has so far been assumed that segregation of alloying elements in the intercellular region just delays bainitic reaction in these regions. However, the existence of bainite-free regions (UAV) even after 10,000 minutes at test temperature, e.g., 375 C, indicates something intrinsic to the mechanism of bainitic transformation. The bainitic transformation start (B{sub s}) temperature is a function of alloying elements; segregation of alloying elements can also alter the B{sub s} temperature. In other words, B{sub s} temperature in the region near graphite should be different from the intercellular region. Therefore, the intercellular region with higher concentration of alloying elements such as Mn should have a lower B{sub s} temperature, which leads to formation of UAV even after a long high-temperature austempering time (hereafter, this stable UAV will be named as the minimum UAV value). To examine this concept, theoretical and experimental procedures were employed.

Surface relief associated with martensite and bainite in a Cu–Zn–Al alloy measured by atomic force microscopy

Atomic force microscopy was employed to quantitatively study the surface relief accompanying martensite and bainite in a Cu–Zn–Al alloy. It is demonstrated that the surface relief angle associated with martensite is 14.3°, in good agreement with the theoretical result deduced from the phenomenal theory of martensite crystallography (PTMC). However, the surface relief angle associated with bainite is 2.0°–3.2°, which disagrees with the PTMC result. This indicates that the transformation mechanism of bainite is different from that of martensite. The fine structures of the surface relief associated with martensite and bainite are also investigated. The surface relief of martensite is composed of the small parallel relief caused by small martensite plates, and that of bainite is composed of small cells induced by subunits in bainite.

STM Study on Surface Relief, Ultra-Fine Structure and Transformation Mechanism of Bainite in Steels

The surface reliefs accompanying lower bainite transformation in steels have been studied by scanning tunneling microscopy (STM). With the exclusive vertical resolution of STM, we observed that the surface relief associated with bainite is a group of surface reliefs related to subplates, subunits and sub-subunits. From the bainite plate to the sub-subunit in it, the reliefs are in a tent shape, not of invariant plane strain (IPS) type. The fine structure of bainite in a steel has also been shown by STM and TEM that bainite plate is composed of subplates, subunits and sub-subunits. On the basis of the fine structure inside a bainitic ferrite plate observed under STM, sympathetic-ledgewise mechanism of bainite formation is proposed.

Metallographic observations of bainite transformation mechanism

Metallographic observation has directly revealed the continuous motion of the bainite/austenite interface in a manner consistent with the achievement of thermoelastic equilibrium. Further observation demonstrates that the shear deformation associated with the growth of bainite can induce mechanical twinning in the surrounding microstructure in appropriate circumstances. For the purposes of comparison, analogous observations for martensite are also reported.MST/3144

Metallographic observations of bainite transformation mechanism

Metallographic observation has directly revealed the continuous motion of the bainite/austenite interface in a manner consistent with the achievement of thermoelastic equilibrium. Further observation demonstrates that the shear deformation associated with the growth of bainite can induce mechanical twinning in the surrounding microstructure in appropriate circumstances. For the purposes of comparison, analogous observations for martensite are also reported.MST/3144

Growth kinetics and high-temperature tem in situ observation of bainite in a Cu-Zn alloy

A kinetics study within situ observation of the growth process of bainitic plates in a Cu-Zn alloy was conducted by means of high-temperature transmission electron microscopy (TEM). The lengthening and thickening rates of bainitic plate were measured and analyzed, respectively. It was pointed out that the measured lengthening rate of bainitic plate is not consistent with Trivedi’s model and that the thickening process is only macroscopically controlled by volume diffusion, although its thickening kinetics is inconsistent with diffusional ledge mechanism. It was found that the stacking-fault substructure exists just in the growing tip of fresh bainitic plate and so does the shear stress field in the matrix around the tip. Superledges in the interphase boundaries of bainite were found but their moving direction is not the same as that predicted by ledge mechanism. The degeneration of bainite to equilibriuma phase was also observed. The shear mechanism of bainitic transformation in Cu-Zn alloy is supported by this investigation.

Incomplete Reaction Phenomenon in High Strength Bainitic Steels

Bainite forms by a displacive transformation mechanism and exhibits an incomplete reaction phenomenon. The kinetics and mechanism of bainite transformation have been studied in high- strength Fe-C-Si-Mn and Fe-C-Si-Ni steels using high-speed dilatometry. The new experiments reported here confirm the incomplete reaction phenomenon, with the bainite transformation stop- ping well before paraequilibrium is achieved. These results show that bainite probably grows without diffusion, but soon afterwards, excess carbon is partitioned into the residual austenite.

Mechanism of bainitic transformation in compacted graphite cast irons

Samples of unalloyed compacted graphite cast iron (CGI) have been thermally treated to obtain a bainitic structure. Heat treatments consisting of various holding times at two different austenitizing temperatures and two different austempering temperatures have been carried out followed by metallographic observations of the resultant structures by scanning electron microscopy (SEM). The formation of bainite is discussed in terms of the temperature difference (undercooling) between the austenitizing temperature and austempering temperature, and the subsequent development of the phases present to bainite is related to the carbon concentration gradient caused by compacted graphite acting as a sink for carbon. This favors the final stages of the transformation. A hypothesis for the bainitic transformation mechanism in CGI's is thus proposed.

BAINITE : OVERALL TRANSFORMATION KINETICS

New experimental results on the overall transformation kinetics of the bainite reaction (in three different steels) are interpreted in terms of recent work on the transformation mechanism of bainite. The analysis is shown to be consistent with (a) a linear dependence of the activation energy for isothermal nucleation, on the transformation free energy change, (b) the sub-unit mechanism of bainitic transformation, (c) the variation of nucleation rate with the degree of transformation, (d) hard and soft impingement effectsand (e) alloying element effects. It is also suggested that the more conventional methods of analysing the overall transformation kinetics are inconsistent with the experimental results, and with the need to understand alloying element effects.

The bainite transformation in a silicon steel

An experimental silicon steel has been used in a detailed kinetic and structural study of the bainite transformation in an attempt to resolve some of the controversies concerning the reaction mechanism. Distinct reaction ‘C’ curves and transformation mechanisms were observed for the upper and lower bainite reactions. The observed set of three minima in transformation kinetics were found to be incompatible with the solute drag explanation of the kinetic Bs temperature. Transmission electron microscopy indicated the growth of both upper and lower bainite by the propagation of displacive subunits, with adjacent nucleation in the latter case. Definite evidence for carbon supersaturation was obtained for the lower bainitic ferrite. The results are best explained in terms of a shear mechanism for the ferritic component of bainite rather than a ledge mechanism (as is observed in Widmanstatten ferrite growth). Carbide precipitation events were also characterized and the evidence suggested that precipitation resulted from the aging of a supersaturated matrix in lower bainite. The evidence also suggests that carbide precipitation events are of secondary importance to the essence of bainite formation. It was further proven that the concept of a metastable equilibrium1 controlling the transition from upper to lower bainite was not applicable to the present steel and indeed, if any metastable equilibrium does exist in any other steel, it does not constitute a general phenomenon and hence is not essential to the bainite transformation mechanism.