Report IR-241029-BG1
Abstract
In ASTM F560-22, cobalt has been included as a new element for consideration, with a maximum content of 0.1% for R05400 and R05200. As material manufactured before 2022 typically has not been analyzed for cobalt, a study was conducted to clarify to what extent cobalt can be assumed to be present in amounts exceeding the allowed content. Based on this study, it is evident that cobalt content in material manufactured before 2022 is lower than 0.100% in the refined product, regardless of whether the material was tested for cobalt content. Concerning the chemical composition of tantalum manufactured according to ASTM F560-98 to ASTM F560-17, all will have cobalt content lower than 0.100% when manufactured according to the R05200 procedure.
Analysis
Tantalum grade ASTM F560 R05200 is purified by electron beam melting above the melting point of tantalum (3300 K). During this process, tantalum melts and has a relatively low vapor pressure, allowing metals with lower melting points than tantalum to evaporate from the melted tantalum. Through repeated melting cycles, the tantalum material is purified to above 99.9%. A comprehensive theoretical study conducted by Katia Vutova [1] supports this.
As shown Figure 1, tungsten (W) in its solid form (s) has a lower vapor pressure than tantalum and cannot be removed by electron beam melting. The other metals in liquid form (l) have a higher vapor pressure than tantalum and can be removed by electron beam melting. It can also be seen that niobium (Nb) and molybdenum (Mo) are the metals that are least easy to remove due to their relatively lower vapor pressures, so a relatively higher residual concentration may be expected after the refining process compared to elements with lower melting temperatures. Cobalt has a relatively low melting point (1768 K) and a boiling point (3143 K), both below the melting point of tantalum, and will therefore rapidly degas from the tantalum melt during the refining process. In conclusion, an insignificant amount of cobalt is expected to remain after refining…………
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