Haijiang Hu

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.

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.

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.

Kinetics model of bainitic transformation with stress

Thermal simulations were conducted on a Gleeble 3800 simulator. The main purpose is to investigate the effects of stress on the kinetics of bainitic transformation in a Fe-C-Mn-Si advanced high strength bainitic steel. Previous studies on modeling the kinetics of stress affected bainitic transformation only considered the stress below the yield strength of prior austenite. In the present study, the stress above the yield strength of prior austenite is taken into account. A new kinetics model of bainitic transformation dependent on the stress (including the stresses below and above the yield strength of prior austenite) and the transformation temperature is proposed. The new model presents a good agreement with experimental results. In addition, it is found that the acceleration degree of stress on bainitic transformation increases with the stress whether its magnitude is below or above the yield strength of austenite, but the increasing rate gradually slows down when the stress is above the yield strength of austenite.

Effects of Nb Addition on Transformation Kinetics and Microstructure Properties in Low-Carbon Bainitic Steels

Abstract To investigate the effects of Nb addition on transformation kinetics and microstructure properties in low-carbon bainitic steels, two C–Si–Mn bainitic steels were designed, one of which was added with 0.025 (wt%) Nb and the other was C–Si–Mn steel. Heat treatment experiments were carried out on thermal simulator. The microstructures were observed by scanning electron microscope. The results show that lath-like bainite is obtained in both two steels. Film-like retained austenite distributes between bainite laths. In addition, Nb retards bainite transformation owing to smaller parent austenite grains in steel with Nb addition. However, Nb improves the strength of the low-carbon bainitic steel by grain refinement. Moreover, transformation kinetics equations for two tested steels are established based on the experimental data. The experimental results are useful to clarify the function of Nb in low-carbon bainitic steels and provide the theoretical reference for the composition design of low-carbon bainitic steels.