Papers by Author: Jian Qing Jiang

Abstract: Lamellar cementite will be spheroidized in drawn pearlitic steel wire during galvanization process. To understand the evolution of the microstructure in this process, effects of isothermal time on microstructure of drawn pearlitic steel wires were investigated by using scanning electron microscope (SEM), transmission electron microscope (TEM) and DSC Technique. Experimental results showed that the lamellar cementite would transform to spheroidized cementite during the isothermal treatment. During the heating process, no endothermic or exothermic peak existed in pearlitic strand, while an obvious exothermic peak appeared in cold drawn pearlitic wire at about 380°C. It results from the spheroidization of lamellar cementite. The dislocation density was very low in pearlitic strand, but the dislocation density increased shapely after drawing. During the isothermal treatment at 450°C, the high dislocation density zone disappeared and some cementite became spheroidized. The cementite spheroidization phenomena first began at the boundary of pearlitic blocks or grains, and then in the high dislocation density zone in pearlitic blocks.

Abstract: The present work has investigated the effect of aluminum on the solidification microstructure of M2 high speed steel. The results show that the as-cast structure is refined and the networks of M2C eutectic carbides are distributed more homogeneously with the addition of aluminum. Meanwhile, the morphology of M2C changes from the fibrous shape to the plate-like one. Despite of the same crystal structure, the microstructure differs greatly between the two carbides. Compared with the fibrous M2C, micro-twining is formed in the plate-like M2C, resulting in the orientation changes between different plates. It is expected that the solid/liquid interface structure of M2C may evolve from the non-faceted into the faceted due to the addition of aluminum.

Abstract: The present work has investigated the influence of calcium on the microstructure and properties of AISI M2 high speed steels. The results show that the as-cast structure consists of the iron matrix and networks of M2C eutectic carbides, which are greatly refined in the ingot modified by calcium. Meanwhile, the morphology of M2C evolves from the plate-like shape into the fibrous one. Compared with the plate-like M2C, the fibrous M2C is less stable at high temperatures, which promotes the spheroidization and refinement of carbides. Therefore, M2 steels modified by calcium obtain higher hardness and red-hardness after heat treatment than those without the addition of calcium.

Abstract: Continuous casting has been widely applied in the production of steels and other metals. However, it has been rarely used in producing high speed steels, which are still manufactured by the conventional method of mould-casting. Thus, little is known about the microstructure of high speed steels made by the continuous casting technology. In the present work, AISI M2 steel is produced by horizontal continuous casting and the difference of solidification microstructure of ingots by different casting technologies has been examined. The results show that the networks of M2C eutectic carbides are greatly refined in the ingot by continuous casting compared to that by mould casting. Meanwhile, the morphology of M2C eutectic carbides changes from the plate-like type to the fibrous one, due to the increasing cooling rates. Compared with the plate-like M2C, the fibrous M2C in continuous casting ingots is less stable and decomposes faster at high temperatures, spheroidizing obviously after heating and refining dimensions of carbides.

Abstract: During manufacturing Cu-30Ni alloy condenser tube, the defects such as crack were fatal to the successive processing operations. The cause of failure induced by manufacturing defect in Cu-30Ni alloy tube was investigated by employing nondestructive penetrative test technology, optical microscope, scanning electronic microscope with energy-dispersive spectrometer and tensile properties test. It was found that cracks including long continuous crack, short discontinuous crack and chevron crack were the main problem caused the manufacturing failure at the industrial production line. There were three principal evolutions of cracks, i.e., inter-granular, trans-granular and the combination of them. These cracks could decrease the elongation by about 10% in comparison with that of good tube, whereas it had no effect on yield ratio. From fractography around the crack, it was seen that there appeared to be different inclusions with different compositions. These inclusions were responsible for the manufacturing failure during the processing of the Cu-30Ni alloy tube.

Abstract: An electrolytic sulfurization at room temperature has been introduced in the present work, which process parameters are optimized to the quenched Cr12 substrate. Morphology and microstructure of the sulfide layers are investigated deeply, and their formation mechanism is discussed in detailed. According to optimum parameter of sulfurizing process ( assistant current density of 1.8A/dm2, the saturated rare-earth-halide solution containing 300g Na2S2O3, 60g KHSO3 and 1000ml H2O, 298K/20~40min), the ferrous sulfide coating was easily obtained on the quenched Cr12 substrate with a uniform thickness of above 10μm, no matter about the treated parts with the complex geometrical shapes. The tribological experiment results of a block-on-ring tribometer indicate that the sulfide coatings on hard substrate have a very low friction coefficient and could obviously reduce the wear extent of its counterpart. Compared with the traditional low-temperature process, this sulfurizing technology has such virtues as simple process, heatless, low cost, little deformation, without pollution question, etc., which could be employed for industrial solid lubricant due to the good friction-reducing characteristic of lamellar-structure FeS phase.