Papers by Author: Xin Lai He

Abstract: The dominant microstructures in low carbon bainite steels such as bainitic ferrite are non-equilibrium phases, which will tend to evolve into equilibrium phases when the steels are subjected to thermal disturbance. In-situ observation by optical and scanning electron microscopy was carried out in this investigation to track the evolution when the steels were isothermally held below A1 temperature. It is found that the primary polygonal ferrite grows slowly during isothermal holding, while bainitic ferrite changes rapidly into polygonal ferrite. Self-tempered bainitic ferrite would recover further and recrystallize. The lower the concentration product of carbon and niobium, the faster is the evolution.

Abstract: The relationship between the oxide inclusions and the Heat-affected-zone (HAZ) toughness of microalloying steels has been investigated. The low carbon steels are smelted with special oxide introduction technique and the properties of HAZ has been studied with thermo-simulation. The optical microscope and SEM were used to analyze the size, composition and distribution of the inclusions, the mechanical properties after thermo-simulation was also analyzed. The results show that the inclusions in steel are mainly Ti and Al oxide with MnS, these complex inclusions are well distributed and the size is less than 3 micron. Microstructure of HAZ consists of intragranular acicular ferrite (IAF), intergranular ferrite and small amount of lath bainite while the cooling time during the phase formation is short. After the thermo simulation with the cooling time between 800°C and 500°C (t8/5) increasing the toughness of HAZ decreased and the size of prior austenite grain increased. Inclusions which located near the prior austenite grain boundary couldn’t induce the nucleation of IAF, only the ones inside the prior austenite grain can promote IAF’s growth.

Abstract: Bainite in microalloyed steel possesses excellent synthetic mechanical properties. However, it will probably evolve towards equilibrium microstructure when it is subjected to thermal disturbance. In addition, bainite frequently undergoes more or less deformation during manufacture of steel structure. In the present investigation, cold deformation test, isothermal heat treatment, hardness measurement, optical microscopy and transmission electron microscopy were employed to detect evolution behavior of bainite isothermally held below A1 temperature. It was found that hardness of samples drops in generally during the isothermal heat treatment. Meanwhile, bainite evolves gradually into polygonal ferrite. Cold deformation enhances the initial hardness of samples while it largely accelerates softening and evolution of microstructure towards equilibrium one during heating. Almost same effects are produced by tensile strain and compressive strain. Higher solubility product of carbon and niobium results in higher thermal stability of bainite. Cold deformation cause dislocations in bainitic laths to distribute heterogeneously and most dislocations pile up along boundaries. During isothermal holding, dislocations redistribute further followed by extending of low dislocation density areas across lath boundaries. Finally, polygonal ferrite nucleates in those areas and grows gradually.

Abstract: Electrochemical measurement, metallographic observation and x-ray diffraction analysis were employed to investigate the further corrosion behaviors of low carbon bainitic steel in the environment containing Cl-, after its original rust layers had been damaged on different ways. It was found the damnification of rust layers on the low carbon bainitic steel (LCBS) and steels utilized as contrasts, i.e. low carbon ferrite steel (LCS) and a commercial weathering steel 09CUPCrNi (09Cu), could be rapidly self-repaired in the further corrosion process. When damnification degree and further corrosion time were same, the resistance of rust layers and the repair degree of damnification of the low carbon bainitic steel were higher than those of contrasts. The repair ratio of inside damnification is always higher than that of crossed damnification, due to faster formation of rust layer at damaged site, in which NaCl aqueous solution is reserved after dropping. Alloy elements such as Cu and Cr obviously enhance protection of rust layer newly formed at damaged sites. These results indicate that the alloy element content in weathering steel is not enough to improve obviously corrosion resistance of steel matrix, while it is sufficient to enhance protection of rust layer.