25Cr-20Ni耐熱鋳鋼におけるσ相の同定

抄録

Some experiments were carried out by means of X-ray diffraction, electrolytic isolation and potentiostatic etching technique, in order to make clear the relation between precipitated carbides and σ-phase in 25Cr-20Ni heat-resistant cast steel.
The results obtained were summarized as follows:
(1) When electrolytic residues contain more than 5wt% of σ-phase, it can be identified by X-ray diffraction method.
(2) For the electrolytic isolation of σ-phase, 45wt% FeCl₃ aq. solution showed a favourable result as compared with 10vol% HCl-C₂H₅OH solution.
(3) Not only M₇C₃ is distinguishable from M₂₃C₆ carbide but also σ-phase from M₂₃C₆ by means of potentiostatic etching in 10 n-NaOH solution. At 200mV vs. Hg-HgO reference electrode M₂₃C₆ carbide is etched more rapidly than M₇C₃ carbide or σ-phase, at 500mV M₂₃C₆ and M₇C₃ carbides, or M₂₃C₆ and σ-phase are simultaneously etched, and at 600mV, σ-phase etched most rapidly.
(4) From these investigations it was found that σ-phase may be possible to precipitate even in the prolonged heat treated cast steel containing 0.35% carbon.