Precipitation and Assembling Behavior of Alloying Elements of Piercing-Plug Steel under High-Temperature C2H5OH–H2O Atmosphere

Abstract

A tungsten (W) and cobalt (Co) bearing high-alloy piercing-plug steel is designed to improve the service life by employing a two-stage oxidation–reduction surface treatment process under a C₂H₅OH–H₂O atmosphere. Morphology, chemical composition, and phase constituents of the oxide scale are studied by using scanning electron spectroscope, energy-dispersive X-ray spectroscopy, and X-ray diffraction, while the microstructure and hardness distribution of decarbonizing layer are studied with optical microscope and microhardness tester. It is shown that the W alloying element can precipitate at 1020 ºC as WC, WSi₂, and W₅Si₃ to strengthen the steel, and the precipitations aggregate and assemble into a cluster in the inner oxide scale as the steel matrix lattice is destroyed by oxidation of ferrum (Fe) atoms. It is found that Me₃O₄ and FeO phases are present together with Ni and WSi₂ with certain orientation relationship in the inner oxide scale, and both Co and nickel (Ni) exist as elementary substances in the oxide scale and show a cooperative precipitation behavior. It is noted that Co distribution in the outer oxide layer and W clustering with a hardness of 966.7 HV in the inner layer are the main factors for the improved service life of this steel piercing plug.