ARTICLES YOU MAY BE INTERESTED INEffect of bond coat and preheat on the microstructure, hardness, and porosity of flame sprayed tungsten carbide coatings AIP Conference Proceedings 1855, 030016 (2017) Abstract. Thermal-sprayed coatings offer practical and economical solutions for corrosion and wear protection of components or tools. To improve the coating properties, heat treatment such as preheat is applied. The selection of coating and substrate materials is a key factor in improving the quality of the coating morphology after the heat treatment. This paper presents the experimental results regarding the effect of preheat temperatures, i.e. 200ºC, 300ºC and 400ºC, on porosity and wear resistance of tungsten carbide (WC) coating sprayed by flame thermal coating. The powders and coatings morphology were analyzed by a Field Emission Scanning Electron Microscope equipped with Energy Dispersive Spectrometry (FE-SEM/EDS), whereas the phase identification was performed by X-Ray diffraction technique (XRD). In order to evaluate the quality of the flame spray obtained coatings, the porosity, micro-hardness and wear rate of the specimens was determined. The results showed that WC coating gives a higher surface hardness from 1391 HVN up to 1541 HVN compared to that of the non-coating. Moreover, the wear rate increased from 0.072 mm 3 /min. to 0.082 mm 3 /min. when preheat temperature was increased. Preheat on H13 steel substrate can reduce the percentage of porosity level from 10.24 % to 3.94% on the thermal spray coatings.

In this study, H13 tool steel and Cr-Mo-V steel were treated by two different types of surface treatments, i.e. double shot peening with nitriding and single shot peening. Samples were dipped into the molten aluminum alloy ADC12 as a simulation of the die casting process and held there for 0.5, 5, and 30 minutes. Several characteristics were analyzed, including surface hardness, microstructure observation, and identification of elements on the intermetallic layer formed. The results of the research showed that H13 steel treated by double shot peening with nitriding had higher surface hardness (1402 VHN) than when treated by shot peening only (536 VHN). A similar tendency emerged with the Cr-Mo-V steel, which had 1402 VHN and 503 VHN after treatment with double shot peening with nitriding and the single shot peening process. In addition, with a dipping time of 30 minutes, the H13 steel treated by double shot peening with nitriding produced a lower average thickness of the compact intermetallic layer. Moreover, double shot peening did not form a broken intermetallic layer, while single shot peening formed one (91.66 µm). Likewise, the Cr-Mo-V steel treated by double shot peening with nitriding produced a thinner compact intermetallic layer than single shot peening, 22.2 µm vs. 27.77 µm, as well as a lower average thickness of the broken intermetallic layer, 40.2 µm vs. 113 µm. This indicates that material treated by double shot peening with nitriding could minimize the occurrence of die soldering.

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Failure in the boiler tube is one of the major causes of the force outages in a coal-fired power plant. This paper presents a failure investigation on the high-strength alloy ASME SA213 T91 superheater tube of a 600 MW class coal-fired power plant due to coal switching. The research investigated the fuel quality and the materials using the material characterization method and mechanical testing. The potential slagging and fusibility were found based on coal properties. A wide-open rupture with thick and blunt edges is exposed in the tube failure. Several steps for failure investigation were performed. Microstructures of the failed tube show numerous creep voids estimated < 2.5 microns and decomposed into spheroids as evidenced by a decrease in the hardness number 165 HV nearest location of failure. Exposure to high temperatures in the long-term causes the formation of an oxide scale layer on the fireside tube. The thickness of the oxide scale on the inner side tube > 600 microns also accelerates tube failure and shortened creep life is shown by a time rupture value from 16.29 to 0.26 hours with higher operating hoop stress near to failure location. Changes in fuel characteristics are indicated by the value of fusibility of 1190°C and slagging index of 0.473 which can worsen the condition of overheating from the increase in metal temperature. Higher flue gas temperatures due to the coal switching characteristic and higher metal temperatures of tubes above 570°C were identified as the cause of overheating. The finding confirmed that the superheater tube failed by long-term overheating.