Mingxing Zhou

Effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel

The effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel was investigated through metallography, X-ray diffraction and dilatometry. The geometrical relationships of the amount of bainite transformation and the volume fractions of retained austenite with deformation strains were studied. The results show that the degree of promotion of small strains on bainite transformation is nonlinear because of the dual effects of accelerated nucleation and retarded growth caused by ausforming. The transformed bainite fraction first increased and then decreased with increased small strains. It indicates that there is a maximum degree of the promotional function corresponding to a certain small strain at low temperature. Although small strains promote bainite transformation, a larger quantity of retained austenite exists at room temperature due to the suppressed martensite transformation during the cooling process after bainite transformation. The carbon content in retained austenite increases with the amount of baintie transformation, which contributes to the stability of austenite. Compared with the stabilizing effect due to carbon enrichment, mechanical stabilization caused by ausforming has a decisive effect on determining the volume fraction of retained austenite after isothermal bainite transformation.

Combined effect of the prior deformation and applied stress on the bainite transformation

There has been a continued interest over the past years in the effects of external stress or prior deformation on the bainite transformation. In this study, the combined effect of prior deformation and stress on the bainite transformation was investigated and the interaction between the effects of prior deformation and stress was discussed in detail. The results show that although single deformation and single stress promote the bainite transformation, their combination cannot promote the bainite transformation to a much larger degree. In addition, at the early stage of transformation, the promotion effects of prior deformation and stress on the amount of the bainite transformation are enhanced by each other. However, at the latter stage, the deformation weakens the promotion effect of the stress. Moreover, prior deformation at a low temperature accelerates the kinetics of the bainite transformation with stress, but it decreases the amount of the bainite transformation even if the deformation is small.

The effects of external compressive stress on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel

Abstract The effects of external compressive stress below the yield strength of austenite on the kinetics of low temperature bainitic transformation and microstructure in a superbainite steel were studied through dilatometry, scanning electron microscopy and X-ray diffraction analysis simultaneously. The results show that even under a relatively low isothermal temperature of 300°C, compressive stress less than yield stress can clearly accelerate bainitic transformation because of the increase in total driving force for bainitic transformation and anisotropic transformation strain. Bainite ferrite becomes longer, the orientations of bainite sheaves become more uniform and the transformation strain becomes anisotropic under external compressive stress because of variants selection. The amount of bainite increases while the amount of martensite decreases with the application of external stress.

Comprehensive analysis of the dilatation during bainitic transformation under stress

The thermal simulation experiments on bainitic transformation were conducted on Gleeble 3800. The microstructures of specimens were examined by scanning electron microscopy. The main purpose is to investigate the relationship between the dilatation of specimens and bainitic transformation under stress. Results show that the radial strain cannot represent the real amount of bainitic transformation under stress because the radial strain contains both shear and dilatational components of invariant plane strain. In this case, the volume strain, which eliminates the influence of the shear components, should be used to analyze the amount of bainitic transformation under stress. In addition, the radial strain should not be used for the investigation of the kinetics of bainitic transformation under stress. For bainitic transformation without stress, the radial strain of the specimen can represent the amount of bainite, and can be used for the analysis of the kinetics of bainitic transformation; so it is not necessary to measure the volume strain. The present study clarifies the relationship between dilatation and the bainitic transformation under stress, and provides a useful reference to the analysis of bainitic transformation under stress.

New Insights to the Promoted Bainitic Transformation in Prior Deformed Austenite in a Fe-C-Mn-Si Alloy

The varying trends of the amount and rate of bainitic transformation with strains at low temperature were investigated through metallography, X-ray diffraction and dilatometry. The results show that deformation at 573 K promotes bainitic transformation, whereas the promotion degree on bainite transformation by ausforming is nonlinear with strains. The amount of bainite in deformed austenite first increases and then decreases with the increase of strains. There exists a maximum value of the promotion effect corresponding to a critical small strain at a low temperature. Bainitic transformation rate can be increased by ausforming at low temperature, whereas a large strain weakens the acceleration effect. The amount of bainite in deformed materials is synthetically depended on the effect of enhanced nucleation and repressed growth. In addition, the volume fraction of retained austenite is not completely consistent with carbon content, indicating that ausforming plays a important role in determining the amount of austenite.

Effect of Undercooling and Austenitic Grain Size on Bainitic Transformation in an Fe-C-Mn-Si Superbainite Steel

Abstract Superbainite steel is a new high strength steel, widely used in mechanical machinery, structure, bridge engineering etc. The effects of transformation and austenization temperatures on bainitic transformation in an Fe–C–Mn–Si superbainite steel were investigated by the combination of high temperature metallography and dilatometry. The experimental results indicate that the overall kinetics of bainite transformation is promoted by higher austenization temperature and lower isothermal transformation temperature. In conclusion, the nucleation of bainite nuclei of the steel under current study is mainly controlled by carbon diffusion, whereas the growth of bainite nuclei is dominated by displacive mechanism. Moreover, isothermal transformation temperature has a far more significant effect on bainitic transformation than austenization temperature.

Dynamic observation of bainite transformation in a Fe-C-Mn-Si superbainite steel

The dynamic observations of bainitic transformation in a Fe-C-Mn-Si superbainite steel were conducted on a high temperature laser scanning confocal microscope. It is indicated that the mutual intersection of bainite sheaves often occurs during growth of bainite ferrite, resulting in an interlocked bainite microstructure. Moreover, bainite transformation is promoted by higher austenization temperature and the longer and finer bainite platelets are obtained. Further, The average growth rate of bainite after austenization at 1 100 ° is calculated as 5.8 µm·s−1. In situ observation investigation makes it possible to identify bainite transformation in real time during isothermal holding.

In Situ Observation of the Lengthening Rate of Bainite Sheaves During Continuous Cooling Process in a Fe–C–Mn–Si Superbainitic Steel

The evolution of lengthening rate of bainite sheaves during continuous cooling process in a Fe–C–Mn–Si superbainitic steel was investigated by in situ observation on high-temperature laser scanning confocal microscope. The lengthening rates of bainite sheaves in three temperature ranges were calculated. The results indicate that the lengthening rate of bainite sheaves continuously decreases with the decrease of temperature during continuous cooling process. The lengthening rate of bainite sheaf depends on undercooling, transformation temperature, the diffusion of carbon atoms and the carbon content in parent austenite etc. The lengthening rate at high temperature is large due to the favorable carbon diffusion, smaller carbon content and less plastic deformation in untransformed austenite. Additionally, the microstructures after different isothermal holding temperatures were analyzed, indicating that the larger lengthening rate of bainite sheaves due to the high isothermal transformation temperature does not mean more amount of bainite transformation. Lower bainitic transformation temperature results in more and finer bainite plates.

Effects of Austenitization Temperature and Compressive Stress During Bainitic Transformation on the Stability of Retained Austenite

The effects of stress and austenitization temperature on the carbon content and morphology of retained austenite (RA) have been investigated. The stability of RA has been analyzed. An interesting finding is that the increase in the amount of bainite is not accompanied by the increase in the carbon content in RA under the effect of stress. This does not match with what is expected from the bainitic transformation theory. The amount of bainitic transformation and stress both affect the carbon content in RA. In addition, the stress during bainitic transformation helps to increase the stability of RA by decreasing the amount of blocky RA, whereas it has little effect on the carbon content in RA. Moreover, the increase in austenitization temperature is beneficial to increasing the stability of RA, whereas it has no significant effect on the morphology of RA.

A Method to Reduce the Oxide Scale of Silicon-Containing Steels by Adjusting the Heating Route

A method to reduce the scale of silicon-containing steels by adjusting the heating route has been proposed in the present study. The influence of heating rate on the oxidizing behavior of steels containing Si was studied by simultaneous thermal analyzer with various heating rates under the condition of the fixed heating time and same heating temperature. The heating process was divided into two sections, i.e., fast heating stage and slow-heating stage. Three kinds of heating route with different end temperatures at fast heating stage and different heating rates at slow-heating stage were designed. Additionally, the solidification process of Fe2SiO4 at different cooling rates was observed by in situ observation. The results showed that the amount of Fe2SiO4 and oxidation mass gain both decreased with the decrease of end temperature at fast-heating stage. Likewise, the net-like distribution of Fe2SiO4 became less apparent with the decrease of end temperature at fast-heating stage and the increase of the heating rate at slow-heating stage. Therefore, it would be beneficial for descaling with a heating route of a lower end heating temperature at the fast-heating stage and a higher heating rate at slow-heating stage. Besides, the solidifying point of Fe2SiO4 decreased apparently at a higher cooling rate.