Papers by Author: Kai Ming Wu

Abstract: Given the strong recent interest in quenching-partitioning-tempering processed steels, the Niobium-alloyed medium carbon steel was investigated here. The microstructural observations and hardness were analyzed by optical microscope, transmission electron microscope, X-ray diffraction and hardness test. Results show that when quenched at 210°C and partitioned at 450°C, the quenching partitioning-tempering process leads to ultra fine-grained microstructures of martensite, retained austenite and carbides. And the microstructure and hardness changed differently with the increase of partitioned time.

Abstract: The effect of heat input on toughness of coarse-grained heat-affected zone of an ultra low carbon acicular ferrite steel were investigated when the welding was conducted with high heat input. Microstructural observations, energy dispersive X-ray spectroscopy analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The microstructures of coarse-grained heat-affected zone consist of predominantly bainitic microstructure and a small proportion of acicular ferrite grains. The bainitic microstructures become coarsened with increasing heat input. The impact toughness of coarse-grained heat-affected zone remained at a higher level when the heat input ranged from 42 to 55 kJ/cm. It became not stable and dropped to a lower level when the heat input increased to 110150 kJ/cm. The enhancement in impact toughness was attributable to the MnS precipitation on the pre-formed Ti oxides as well as the formation of intragranular ferrite. When specimens were welded with higher heat input, the deterioration of impact toughness was caused by the coarsening of austenite grains.

Abstract: The characteristics and the mechanism of ferrite transformation in alloy steels which contain a carbide-forming element have attracted considerable attention for past decades. Since it is reported that the nucleation and growth of ferrite in Fe-C base alloys is accelerated by high magnetic field, the influence of a magnetic field of 12 Tesla on ferrite transformation was studied in a Fe-C- Mo alloy. Whereas a significant amount of expedition was observed at lower temperatures, the principal features of ferrite transformation, namely, a marked retardation of transformation at intermediate temperatures and premature cessation of transformation before it reaches the final equilibrium amount below the bay temperature were essentially retained. In contrast, the influence of magnetic field was much less at higher temperatures. These results are discussed in terms of the influence of magnetic field on the phase equilibrium and coupled-solute drag effects on the migration a/g phase boundary.

Abstract: A high strength low alloy wear-resistant steel was quenched at 900°C and tempered at varying temperatures. The microstructures were observed utilizing optical and electron microscope. Results show that microstructures consist of predominantly martensite and lots of bainite in the as-quenched specimens. When the specimen was tempered at 250°C, no obvious change in the microstructure was observed. It has an optimized strength and elongation in this condition of heat treatment. With the increase of tempering temperature, the lath or plate of martensite were coalescenced. The strength of the steel is thus greatly reduced and the elongation was accordingly increased.

Abstract: The macro structures, microstructures, textures and precipitates in non-oriented electrical steels were studied by means of optical microscopy, scanning electron microscopy (SEM), electron back scattering dispersion (EBSD) and transmission electron microscopy (TEM) in the specimens produced by conventional continuous casting process and thin slab continuous casting and direct rolling process. Results showed the macro structures in as-cast slabs and the microstructures in as-rolled strips were more fine-grained and uniform with regard to compact strip production (CSP) process, compared with that in conventional process. No obvious texture was observed in hot rolled strips produced by CSP process.

Abstract: Hot ductility of a high carbon steel 65Mn produced by CSP (Compact Strip Production) was tested on Gleeble-3500 simulator. Results show that the great reduction of hot ductility occurred between 700°C900°C. This is resulted from the formation of ferrite allotriomorphs along prior austenite grain boundaries and aluminum nitride precipitation along and near austenite grain boundaries. In order to prevent from surface cracking on thin slabs, it is suggested that the unbending temperature should be controlled above 950°C.

Abstract: The microstructures of multipass deposited metal for 9Cr-1Mo ferritic steels were investigated. Columnar martensites and a number of film austenites retained along the lath of martensite comprised primary microstructure. -ferrite was formed and the volume fraction of -ferrite increased as the chromium equivalent (Creq) of steel composition increased. Subjected to weld heating, the primary microstructure transformed. “Weld bead heat-affected zone (WBHAZ)” was thus formed. The morphology of -ferrite changed and columnar martensite grains were broken in sub-zone I and II of WBHAZ. The size and amount of -ferrite reduced because of austenitization and growth of austenite in sub-zone III of WBHAZ. Retained austenite morphology changed from film into island-like in sub-zone IV and martensite tempered to some degree in sub-zone V of WBHAZ. The precipitation and evolution of M23C6, MX and Laves phase by means of neutron diffraction is to be conducted in the future.

Abstract: The isothermal transformation of pearlite in an Fe-0.27%C-3.15%Mo (weight percent) alloy applying large compressive deformation was investigated by means of thermal simulation, optical and scanning electron microscopy. The pearlite transformation was promoted by large compressive deforming on austenite, which is attributed to the effect of large compressive deformation on nucleation of pearlite transformation.

Abstract: Mechanical properties of heat treated 30CrMo steel produced by CSP (Compact Strip Production) and conventional continuous casting process. Results showed that 30CrMo steel produced by CSP exhibits higher hardness than that produced by conventional continuous casting process. The higher hardness was attributed to fine-grained microstructures in the as-rolled CSP strips.