Modern developments in vacuum furnace technologies have led to an increased interest in vacuum sintering of powder metallurgy (PM) steel components owing to the robustness, productivity and the possibility to integrate heat-treatment into the process at an attractive cost. In the present study, the potential for vacuum sintering of chromium-alloyed PM steels and the effect of the vacuum level on the final properties of the components are evaluated. The studies were performed on Fe-1.8 wt.% Cr powder and the sintering experiments were performed in a dilatometer. Four different vacuum levels: 10, 1, 10−2 and 10−4 mbar were studied and the effect of density and carbon content were also examined. The results indicate an efficient oxide reduction and process robustness at intermediate vacuum levels whereas high-temperature sintering in high vacuum brings the risk of Cr-sublimation. Furthermore, industrial trails were performed at 10 and 1 mbar vacuum levels and their mechanical properties were evaluated. Vacuum sintering proved to be an attractive alternative to process Cr-alloyed steel powders.
Water atomized steel powder particles are covered by heterogeneous surface oxide, formed by thin (~ 6 to 8 nm) iron oxide layer covering most of the powder surface, and particulate features formed by thermodynamically stable oxides containing, for example, Cr and Mn with surface coverage about 5%. Development of sufficiently strong interparticle necks requires as minimum full removal of the iron surface oxide layer that can be achieved by gaseous reducing agents as CO and H2 as well as by carbon typically admixed in the form of graphite. The study evaluates the effect of concentration of reactive components of the sintering atmosphere, with special focus on carbon monoxide, on the reduction/oxidation and carburization/decarburization processes taking place during the whole sintering process. Results of the thermal analysis, SEM analysis of oxide characteristics, metallographic, and chemical analysis of the sintered compacts were correlated with thermodynamic simulation of the oxide stability in applied sintering atmospheres. High oxidation potential of the CO‐containing atmospheres in case of Cr‐alloyed PM steels was detected during heating stage until ~1000°C. Oxidation potential is linearly increasing with the increasing content of the carbon monoxide in the processing atmospheres and rather severe oxidation is observed if CO content exceeds 1 vol%.
Thermodynamic evaluation of the reducing and carburizing activities of N 2 -based lean sintering atmospheres containing CO, H 2 , and small amounts of hydrocarbons with their total content not exceeding 5 vol% was performed with regard to sintering of chromium alloyed PM steels. Based on the thermodynamic evaluation, three sintering atmosphere compositions were chosen for experimental studies. For the chosen sintering atmospheres, the effect of graphite addition, sintering temperature, holding time and cooling rate on the microstructure, and final oxygen and carbon contents was studied. The compacts without admixed graphite exhibited a microstructure with pearlitic, and in some cases bainitic regions, with their content varying according to the temperature profile and increasing with increase in CO concentration in the atmosphere. These results are indicative of the carburizing ability of the atmospheres which was also supported by the results from chemical analysis. Pearlitic-bainitic microstructures were observed in the compacts admixed with graphite after sintering which suggests that the proposed atmospheres provide conditions sufficient for at least partial reduction of the surface iron oxide during sintering. The final oxygen content obtained after sintering in the proposed lean atmospheres was higher than that obtained in the case of nitrogen/hydrogen mix but still was within acceptable limits. The results obtained suggest that the proposed lean CO and H 2 containing sintering atmosphere compositions are promising candidates for robust sintering of chromium alloyed PM steels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.