Papers by Author: Sung Kang Hur

Abstract: μWC-metal powder (powder) was coated onto the surface of Inconel 718 (IN718) using HVOF spraying for the improvement of surface properties. The coating was heat-treated by CO₂(g) laser for further improvement of the properties. Optimal coating process was obtained from surface properties of 9 coatings prepared by Taguchi program. Surface property improvement by HVOF coating and laser-heating were investigated. The surface hardness of IN718 was increased by coating from 399 Hv to 1260±30 Hv, and further increased to 1860±100 Hv by laser-heating. Porosity of the coating was decreased by laser-heating from 2.2±0.3% to 0.4±0.1%. The friction coefficient of the surface was decreased by the coating from 0.48±0.06 to 0.33±0.02 at room temperature. Friction coefficients of IN718 and the coating were decreased with increasing sliding surface temperatures from 25°C to 450°C. According to reciprocating sliding wear tests, wear depth and trace of the coating were smaller than IN718 at both 25°C and 450°C. The coating had better wear resistance compared to IN718, creating smaller wear traces. At 450°C, wear depth of coating was much smaller than IN718: 50-µm-deep groove compared to 75 µm for IN718.

Abstract: P92 steels as well as P91 are widely used as boiler tube materials of ultra super critical (USC) power plants these days. And thus embrittlement is very important for structural integrity of the USC plants. The embrittlement was observed when P92 (Modified 9Cr-1.8W-0.5Mo-V-Nb) steels were quenched to and held at the temperature of 320 to 350°C, which were the temperatures intermediate between Ms and Mf, and then air-cooled. Nearly same kind of the embrittlement had been observed with the T/P91 steels and a theory had been proposed to explain the mechanism of the embrittlement by us. From the theory, the embrittlement might be caused by the brittle martensite which is freshly formed during air-cooling. We tried to apply the theory for the embrittlement of the T/P92 steel. The behaviors of the embrittled T/P92 steel were explained well by the theory.

Abstract: A modified sol-gel method is proposed for the preparation of silicon carbide nano-particles and nano-whisker. Tetraethoxysilane (TEOS), SiO2 bead, and PMMA as precursor of carbon source were used for preparing a ternary carbonaceous silicon xerogel, and morpholine is employed in the sol-gel process as a catalyst reagent. SiC nano-particles and nano-whisker were obtained from carbothermal reduction and vapor-solid (VS) reaction of the ternary xerogel at 1300°C for 9h in a dynamic argon atmosphere (flow rate; 600 cm3/min), and then purified by removing excess silica, residual carbon, and other impurities. The purified SiC sample was characterized by TG/DTA, XRD, SEM, and HRTEM. The nano-particles and the nano-whisker were synthesized by different growth mechanisms, with an initial nucleation via a VS reaction between SiO vapor and carbon precursor (PMMA) and a partially subsequent growth via a vapor-vapor (VV) reaction between SiO and CO, respectively.

Abstract: The evolution of microstructure and its effects on the mechanical properties of modified 9%Cr-1%Mo steel during heat-treating at 1050°C for 15 min and then isothermal heat treatment at 380~760°C with subsequent air-cooling have been investigated. For the microstructural and mechanical property analyses, OM, SEM, EDS, XRD, hardness and impact tests were used. In accordance with the severity of the heat-treatment, the microstructure evolved from the untransformed martensite to the partially transformed dual phases of martensite and ferrite, and then fully transformed to ferrite. Impact values at ambient temperature for specimens isothermally heat-treated at 320 - 380°C, predominantly at about 350°C were lower than others’ with similar martensitic structure. The partially transformed specimens with dual phases of martensite and ferrite also showed lower impact values than samples with untransformed with martensitic, and transformed with ferritic structures.