Development of Interparticle Bonding during Sintering of Metal Powders with the Addition of Carbon

Introduction. Publications on sintered metal powder parts consider interparticle bonding in hot-deformed materials and features of low-alloy structural steels, as well as the use of carbon-containing materials. The authors of the presented article have previously investigated sintering in relation to structural changes in the material, described changes in physical and mechanical properties, reduction of oxides, recrystallization, etc. This paper shows the relationship of mechanical properties of powder steels with the parameters of intracrystalline bonding. The kinetics of its development during sintering is demonstrated for the first time. The study objective is to find out how sintering affects the interparticle bonding and structure of powder alloys with iron and carbon. The task is to study the technological modes of sintering samples from alloyed and pure iron powder to achieve the best mechanical characteristics.

Materials and Methods. The powders of the Höganäs company were sintered at a temperature of 900-1150 ºС for 0.5–2.5 hours. The protective gas medium (dissociated ammonia) made it possible to prevent oxidative and other sintering reactions. For static cold pressing, a hydraulic press 2PG-125 with a maximum force of 1250 kN was used.

Results. For the first time, the presence of intracrystalline bonding mechanisms with different intensity during sintering has been experimentally established. The dependences of the increment of the relative area of the contact surface on the duration of the isothermal exposure were constructed. With an increase in the sintering temperature to 1150 ºС and a holding time of more than 80 minutes, the contact surface area gradually increased. It was shown that the samples from the powder grades under consideration formed an intracrystalline bonding on the entire contact surface at 1150 ºС. Therefore, this technology can be recommended for practical use. The addition of graphite to the charge slows down the growth of the contact surface. At the same time, the molds from pure powder ABC100.30 and from Distaloy HP-1 powder showed differences. In the first case, with the addition of graphite to the charge, the contact surface developed more intensively than in the second one. The obtained results were recorded in the photo and visualized in the form of graphs.

Discussion and Conclusion. According to the results of mechanical tests, it is possible to estimate the proportion of the contact section of the molding with intracrystalline bonding. Its feature is the structural correspondence of the interparticle surface of the splice and the intergrain boundary. The value of this boundary is determined by comparing the relative area of the contact section with the intracrystalline bonding and the relative area of the contact surface. The possibilities of improving the quality of bonding of powder steels by increasing the temperature and time of their exposure during sintering are determined.

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