Papers by Keyword: Vacuum Degassing

Abstract: Ti-47Al-2Cr-2Nb-0.15B alloy is a typical γ-TiAl alloy, and powder metallurgy (PM) as a near-net shape method was used to prepare it in this article. Clean pre-alloyed powders were prepared by argon gas atomization, and TiAl alloy was prepared by hot isostatic pressing (HIP) at 1150 °C and 1230 °C. However, surface contamination is inevitable due to chemical reactions with the residual O₂ in the vacuum chamber during gas atomization, or due to physical adsorption of O₂ and H₂O during storage of the powder at room temperature. Infrared spectrometry was used to study this process. We found that the adsorption of gases is mainly H₂O. The adsorbed gas in powders would deteriorate the performance of PM alloy, so a gas protection environment is suggested. Tensile properties of PM TiAl alloy were compared with as-cast alloy. Results showed that PM TiAl alloy had better strength which also had more fine and uniform microstructure.

Abstract: JSW Steel Limited, Salem Works (JSWSL), is an integrated steel plant, having a production capacity of 1.0 mtpa (million tons per annum) of high-grade automotive special steels. At JSWSL, hydrogen induced cracks and center unsoundness contribute more to UT (Ultrasonic testing) rejections in chrome-molybdenum (Cr-Mo) and high manganese grade steels. Hydrogen induced cracks was controlled by increasing argon flow rate from 3-4 Nm³ to 7-9 Nm³ during vacuum degassing. Vigorous purging led to a reduction in hydrogen levels from around 2 ppm (parts per million) to less than 1.5 ppm. Center unsoundness was controlled through optimising superheats in tundish. Data of trial heats revealed that, UT rejections were more in heats cast with superheat levels more than 35°C. Based on the data obtained from trial heats superheat was optimised to 25-35°C for both Cr-Mo and high manganese steels. The present paper discusses the measures taken to reduce UT rejections in these grades.KeywordsJSWSL, Cr-Mo, High Mn steels, UT rejections, superheat, hydrogen induced cracks, center unsoundness, vacuum degassing, continuous casting.

Abstract: Ni deposits were prepared from a nickel sulfamate type electrolyte bath under vacuum-degassing and temperature-gradient conditions without any additives. Morphology and microhardness of the deposits obtained at the current density of 1A/dm2, 3A/dm2, 5A/dm2 and 7A/dm2 were examined and analyzed. Experimental results showed that the deposits obtained under vacuum-degassing and temperature-gradient conditions exhibited fewer pinhole defects and finer grain size comparing with those formed under conventional deposition conditions, and that the microhardness of the deposits was greater than that from conventional deposition conditions without additives while lower than that from conventional deposition conditions with additives. Under the vacuum-degassing conditions, the electroforms had remarkably smoother surface with fewer void defects and finer grain size, and considerably microhardness.

Abstract: Pinhole defect in the deposit is one of the ticklers which have not yet been solved fundamentally when micro-electroforming nickel into high-aspect-ratio (HAR) micro-molds. To obtain void-free micro-components, a new micro-electroforming technique, which was carried out under the complex circumstance of vacuum-degassing and temperature-gradient, was developed in this paper. Mechanisms of this micro-electroforming process were introduced using graphic and simulation methods. To investigate the feasibility of the exploited technique, a series of experiments were performed using special equipment developed by authors, followed by the evaluating of surface morphology of nickel micro-electroforms using a scanning electron microscope (SEM). Experimental results showed that, comparing with the conventional electroforming practices, a significant reduction in pinhole defects of the samples electroformed by the novel process was achieved.