Institute of Metals Division - Solubility of Oxygen in Alpha Iron: A Revision

Since the time this topic was originally treated in 1954, more recent French3,1 work has been published making it advisable to repeat the earlier oxygen solubility experiments, but using iron of a higher purity. Upon repeating oxygen saturation experiments with zone-refined iron, it was found that the oxygen solid solubility was at least an order of magnitude less than previously observed. SINCE the publication of an earlier paper1 (1954) on this subject by the present author, several contributions on this topic have appeared. Meijering2 in 1955 attempted to reconcile recent conflicting solubility data, without reaching any definite conclusion. sifferlen3 (1955) found that iron of high purity appeared to absorb a quantity of oxygen (0.005 to 0.025 pct) approximately proportional to the structural imperfections present, such as polygonization and grain boundary area. However, he also indicated that purity itself was a significant factor. He concluded that very pure iron nearly free of lattice imperfections would have essentially zero oxygen solubility. His second paper4 (1957) supported this contention since a zone-melted iron containing initially 0.0007 pct O analyzed only 0.0006 pct 0 after prolonged soaking at 850°C with an FeO layer on the surface. Because of the discrepancy between the results of Ref. 1 and those of Sifferlen, it seemed desirable to attempt to resolve this disagreement. EXPERIMENTAL PROCEDURE AND MATERIALS The experimental method used by sifferlen3,1 was essentially the same as that of Ref. 1. In this technique thin iron strip with a surface layer of FeO is soaked for prolonged periods at an elevated temperature in order to saturate the iron with oxygen. After two days at temperatures near 800° to 900° C, one would expect that the iron would be in equilibrium with the surface FeO. In the work reported in Ref. 1 there was evidence that two days holding at 8000to 900°C was more than sufficiently long for saturation. Sifferlen used 48 hr at 850°C. Because of the evidence presented by Sifferlen that the apparent oxygen solubility is a function of iron purity, three grades of iron were used as indicated in Table 1. The Bureau of Standards wire was used only for resistivity tests, while the other two irons were used for saturation-analysis tests of the type described above.